WO2012172313A1

WO2012172313A1 - Lipid metabolism

Info

Publication number: WO2012172313A1
Application number: PCT/GB2012/051305
Authority: WO
Inventors: John SCHWABE; Ben GOULT; Louise FAIRALL; Peter J. Tontonoz; Zhang Li
Original assignee: University Of Leicester
Priority date: 2011-06-14
Filing date: 2012-06-08
Publication date: 2012-12-20
Also published as: GB2497719A; GB201109913D0

Abstract

The invention relates to lipid metabolism, and to the uptake of lipoproteins via the Low-Density Lipoprotein receptor (LDLR), the Very Low Density Lipoprotein Receptor (VLDLR) and/or the Low density lipoprotein receptor-related protein 8 (apoER2) receptor. The invention provides novel biological targets associated with the degradation of these receptors, and pharmaceutical compositions, medicaments and methods of treatment for use in preventing, ameliorating or treating symptoms of hypercholesterolaemia and cardiovascular disease.

Description

LIPID METABOLISM

The present invention relates to lipid metabolism, and in particular to the uptake of lipoproteins via the Low-Density Lipoprotein receptor (LDLR), the Very Low Density Lipoprotein Receptor (VLDLR) and/or the Low density lipoprotein receptor-related protein 8 (apoER2) receptor. The invention provides novel biological targets associated with the degradation of these receptors, and pharmaceutical compositions, medicaments and methods of treatment for use in preventing, ameliorating or treating symptoms of hypercholesterolaemia and cardiovascular disease. Plasma Low-Density Lipoprotein (LDL) cholesterol levels are strongly linked to cardiovascular disease risk. The LDL receptor (LDLR) is a cell membrane protein that mediates uptake of LDL cholesterol and is a major determinant of plasma lipoprotein levels. Loss of function LDLR mutations in humans reduce LDL clearance, elevate plasma LDL levels and are associated with accelerated

atherosclerosis. The most effective drugs for lowering circulating cholesterol are called HMG-CoA reductase inhibitors or "statins", which cause the liver to produce more LDL receptors, which bring the cholesterol into the liver cells where it is metabolised. However, although statins are often very effective, in io% of patients they are associated with undesirable side effects, such asrhabdomyolysis. Thus, despite the widespread use of statin drugs, there still remains an urgent need for additional therapeutic strategies to modulate human lipid levels, which avoid these side effects. Understanding molecular mechanisms involved in the control of LDL uptake and processing will have important implications for treatment of human cardiovascular disease.

In eukaryotic cells, the degradation of many proteins is carried out by the ubiquitin system. In this pathway, proteins are targeted for degradation through the covalent conjugation of the 76-amino acid polypeptide ubiquitin. Conjugation proceeds via a three-step mechanism involving three enzymes, El, E2 and E3. To initiate the process, a ubiquitin molecule is activated by the ubiquitin-activating enzyme, El, to form a high-energy intermediate with El. The activated ubiquitin molecule is then transferred to a ubiquitin-conjugating enzyme, E2, to form an intermediate with the E2. Finally, association of this ubiquitin-charged E2 with an E3 ligase facilitates the conjugation of the ubiquitin molecule to the target protein. Specificity in

ubiquitination pathways derives from the ability of individual E3 ligases to recognize a discreet set of target proteins. There are two major categories of E3 ligases: HECT domain and RING domain E3 ligases. HECT domain E3 ligases mediate the conjugation of ubiquitin by formation of a HECT-ubiquitin intermediate, whereas RING domain E3 ligases facilitate the direct transfer of ubiquitin from the E2 to the substrate. The ubiquitin system is organized into a hierarchical structure: a single El can transfer ubiquitin to several species of E2 enzymes, and each E2 acts in concert with either one or several E3 enzymes. Upon the completion of ubiquitin conjugation, the proteolysis of ubiquitinated proteins can be conducted in either the proteasome or the lysosome. As discussed above, the LDL receptor (LDLR) is a cell membrane protein essential for the uptake of LDL cholesterol and the regulation of plasma lipoprotein levels. Loss of function LDLR mutations in humans reduce hepatic LDL clearance, elevate plasma LDL levels and accelerate atherosclerosis. The abundance of the LDLR is regulated by both transcriptional and post-transcriptional mechanisms in response to cellular cholesterol levels. The primary transcriptional regulator for LDLR is the SREBP-2 transcription factor. A reduction in the cholesterol levels in the endoplasmic reticulum (ER) triggers the processing of SREBPs to their mature nuclear forms and consequently activates the expression of genes important for the synthesis and uptake of cholesterol.

The RING domain E3 ubiquitin ligase, IDOL, has recently been identified as an additional post-transcriptional regulator of the LDLR pathway (Zelcer et al. 2009). Expression of the IDOL gene is induced by the sterol-activated transcription factors LXRoc and ΙΧΈβ. Increased IDOL expression triggers the ubiquitination of the LDLR, leading to its internalization and degradation, thereby increasing plasma cholesterol levels. Although it is clear that increased expression of the E3 ligase, IDOL, leads to ubiquitination of the LDLR on its cytoplasmic domain and subsequent degradation, the mechanism by which this is accomplished still remains to be elucidated. In particular, IDOL is unusual among E3 ligases in that it appears to affect the degradation of a very small number of proteins. Furthermore, although it is postulated that IDOL acts directly on the LDLR itself, this has also never been formally established. Since the expression of the IDOL gene is not regulated by SREBPs, the LXR-IDOL pathway represents an independent mechanism for feedback inhibition of the LDLR by cellular cholesterol levels. Finally, the ubiquitination and subsequent degradation of the LDLR is presumed to depend on a cascade of ubiquitin transfer reactions carried out by Ει, E2, and E3 enzymes. However, although IDOL has been identified as the E3 ligase, the identity of the specific E2 that is involved in the cascade has remained elusive.

It is therefore an aim of embodiments of the present invention to target the mechanisms of activity of IDOL in order to upregulate levels of the LDLR, and hence lower cholesterol concentrations circulating in the plasma. Such improved therapeutics and methods which result in the upregulation of LDLR can be used for treating symptoms of hypercholesterolemia and cardiovascular disease.

To effectively target the ability of IDOL to degrade the LDLR, the inventors conducted a series of structural, biophysical and cell-based assays to understand, in molecular detail, the interaction between IDOL and the LDLR. As a result of these studies, the inventors now have a detailed understanding of the mechanism by which IDOL degrades not only the LDLR, but also two closely-related receptors, the Very Low Density Lipoprotein Receptor (VLDLR) and the low density lipoprotein receptor-related protein 8 (apoER2), both of which can also be targeted by IDOL. Accordingly, the inventors have demonstrated that agents, which are capable of preventing the interaction between specific regions of IDOL, the target receptor (i.e. LDLR, VLDLR and/or apoER2) and/or the specific E2 enzyme that is involved in ubiquitination, can be effectively used to inhibit receptor degradation, and thereby promote lipoprotein uptake, resulting in reduced plasma cholesterol levels. Hence, according to a first aspect of the invention, there is provided an agent capable of:

(a) inhibiting binding or interaction between a sub-domain of the FERM domain of IDOL, the sub-domain being represented by amino acid residues 183-344 of SEQ ID No:i, or a functional fragment or variant thereof, and:

(i) a Low-Density Lipoprotein receptor (LDLR),

(ii) a Very Low Density Lipoprotein Receptor (VLDLR) and/or

(iii) a Low density lipoprotein receptor-related protein 8

(apoER2);

(b) inhibiting binding or interaction between IDOL and a

K/RNWXXKNXXSI/MXF motif present in the LDLR, VLDLR apoER2; (c) inhibiting interaction or binding between IDOL and a member of the ubiquitin-conjugating enzyme (UBE2D) family;

(d) inhibiting or preventing binding of iron ions with IDOL; or

(e) inhibiting or preventing the dimerisation of IDOL,

for use in inhibiting LDLR, VLDLR and/ or apoER2 degradation and/or promoting lipoprotein uptake.

According to a second aspect, there is provided an agent capable of:

(i) a Low-Density Lipoprotein receptor (LDLR),

(ii) a Very Low Density Lipoprotein Receptor (VLDLR) and/or (iii) a Low density lipoprotein receptor-related protein 8

(apoER2);

(b) inhibiting binding or interaction between IDOL and a

K/RNWXXKNXXSI/MXF motif present in the LDLR, VLDLR and/or apoER2;

(c) inhibiting interaction or binding between IDOL and a member of the ubiquitin-conjugating enzyme (UBE2D) family;

(d) inhibiting or preventing binding of iron ions with IDOL; or

(e) inhibiting or preventing the dimerisation of IDOL,

for use in the treatment, prevention or amelioration of hypercholesterolaemia or cardiovascular disease.

According to a third aspect, there is provided a method of inhibiting LDLR, VLDLR and/or apoER2 degradation and/or promoting lipoprotein uptake in a subject, the method comprising administering, to a subject in need of such treatment, a therapeutically effective amount of an agent capable of:

(i) a Low-Density Lipoprotein receptor (LDLR),

(apoER2);

(b) inhibiting binding or interaction between IDOL and a

K/RNWXXKNXXSI/MXF motif present in the LDLR, VLDLR and/or apoER2; or

(d) inhibiting or preventing binding of iron ions with IDOL; or

(e) inhibiting or preventing the dimerisation of IDOL,

to inhibit LDLR, VLDLR or apoER2 degradation and/or promote lipoprotein uptake in the subject.

According to a fourth aspect, there is provided a method of treating, preventing or ameliorating hypercholesterolaemia or cardiovascular disease in a subject, the method comprising administering, to a subject in need of such treatment, a therapeutically effective amount of an agent capable of:

(i) a Low-Density Lipoprotein receptor (LDLR),

(ii) a Very Low Density Lipoprotein Receptor (VLDLR) and/or

(iii) a Low density lipoprotein receptor-related protein 8 (apoER2);

(b) inhibiting binding or interaction between IDOL and a

K/RNWXXKNXXSI/MXF motif present in the LDLR, VLDLR and/or apoER2; or

(d) inhibiting or preventing binding of iron ions with IDOL; or

(e) inhibiting or preventing the dimerisation of IDOL,

to treat, prevent or ameliorate hypercholesterolaemia or cardiovascular disease in the subject.

Advantageously, blocking the action of IDOL by inhibiting or preventing binding or interaction between IDOL and either one of the above-mentioned receptors or a UBE2D protein presents a novel strategy for increasing levels of the LDLR, VLDLR and/or apoER.2 receptors, and hence lowering circulating cholesterol in the subject being treated. Such an approach should provide an alternative and/or

complementary therapy to treatment with statins. The inventors have determined the structure of IDOL, and its interactions with the LDL receptor and E2 ligase, using NMR and/or X-ray crystallographic approaches, and a range of biochemical and cell-based interaction assays. Together these experiments have provided a detailed understanding of the structure and function of IDOL, and its mechanism of interaction with both the LDLR, VLDLR and/or apoER2 receptors, as well as with the ubiquitin-conjugating enzyme UBE2D family of proteins (i.e. UBE2D1, UBE2D2, UBE2D3 and UBE2D4), as the E2 enzymes that collaborate with IDOL in receptor ubiquitination. The inventors have also

successfully obtained the crystal structure of the IDOL RING domain-UBE2D complex. Based on the information provided by this structure, the inventors have surprisingly demonstrated that disruption of the interaction interface between IDOL and UBE2D prevents LDLR, VLDLR and/or apoER2 receptors from being degraded by IDOL. These results provide a much better understanding of the molecular mechanism underlying the sterol-dependent regulation of protein levels of the LDLR, VLDLR and/or apoER2 receptors.

Surprisingly, the data also suggest that the closely-related family members LDLR, VLDLR and apoER2 are the only proteins targeted by IDOL. The basis for this remarkable specificity was previously unknown, and so the inventors have now defined the molecular basis for IDOL target recognition, and provide clear evidence that specific targeting of membrane receptors by binding of the IDOL FERM domain underlies an evolutionarily conserved, post-translational mechanism for the regulation of lipoprotein uptake.

The agent may be used for the treatment, amelioration or prevention of a

cardiovascular disease selected from a group consisting of disorders of the heart and vascular system, such as congestive heart failure; myocardial infarction; ischemic diseases of the heart; ischemic cardiomyopathy; myocardial disease; all kinds of atrial and ventricular arrhythmias; hypertensive vascular diseases; peripheral vascular diseases; atherosclerotic coronary artery disease; heart failure; hypertrophic cardiomyopathy; restrictive cardiomyopathy; congestive heart failure; cardiogenic shock; and hypertension. As illustrated in Figure 13A, IDOL is 445 amino acids in length, and is divided into an N-terminal FERM domain (344 amino acids in length), a 22 amino acid linker region and a C-terminal RING domain (77 amino acids in length). The amino acid sequence of human IDOL is provided herein as SEQ ID No:i, as follows:

MLCYVTRPDAVLMEVEVEAKANGEDCLNQVCRRLGI IEVDYFGLQFTGSKGESLWLNLRN RISQQMDGLAPYRLKLRVKFFVEPHLILQEQTRHIFFLHIKEALLAGHLLCSPEQAVELS ALLAQTKFGDYNQNTAKYNYEELCAKELSSATLNSIVAKHKELEGTSQASAEYQVLQIVS AMENYGIEWHSVRDSEGQKLLIGVGPEGISICKDDFSPINRIAYPWQMATQSGKNVYLT VTKESGNSIVLLFKMISTRAASGLYRAITETHAFYRCDTVTSAVMMQYSRDLKGHLASLF LNENINLGKKYVFDIKRTSKEVYDHARRALYNAGWDLVSRNNQSPSHSPLKSSESSMNC SSCEGLSCQQTRVLQEKLRKLKEAMLCMVCCEEEINSTFCPCGHTVCCESCAAQLQSCPV CRSRVEHVQHVYLPTHTSLLNLTVI

SEQ ID No:l

The FERM domain itself is sub-divided into three discrete sub-domains, denoted herein as "Fi", "F2" and "F3". Sub-domain Fi is defined by amino acid residues 1-85 of SEQ ID No:i, sub-domain F2 is defined by amino acid residues 86-182 of SEQ ID No:i, and sub-domain F3 is defined by amino acid residues 183-344 of SEQ ID No:i Accordingly, the agent is preferably capable of inhibiting binding or interaction between the receptor (i.e. LDLR, VLDLR and/or apoER2) and the F3 sub-domain of the FERM domain of IDOL, wherein the F3 sub-domain is defined by amino acid residues 183-344 of SEQ ID No:i, or a functional fragment or variant thereof. As shown in Figure 13A, sub-domain F3 of the FERM domain is itself sub-divided in three sub-domains denoted herein as "F3a", "F3b", and "F3C". Sub-domain F3a is defined by amino acid residues 183-214 of SEQ ID No:i, sub-domain F3b is defined by amino acid residues 215-272 of SEQ ID No:i, and sub-domain F3C is defined by amino acid residues 273-344 of SEQ ID No:i. Accordingly, the agent may be capable of inhibiting binding or interaction between the receptor (i.e. LDLR, VLDLR and/ or apoER2) and an F3a, F3b or F3C sub-domain of the FERM domain of IDOL, wherein sub-domain F3a is defined by amino acid residues 183-214 of SEQ ID No:i, or a functional fragment or variant thereof, sub-domain F3b is defined by amino acid residues 215-272 of SEQ ID No:i, or a functional fragment or variant thereof, and sub-domain F3C is defined by amino acid residues 272-344 of SEQ ID No:i, or a functional fragment or variant thereof.

In one preferred embodiment, however, the agent may be capable of inhibiting binding or interaction between an F3b sub-domain of the FERM domain of IDOL and LDLR, VLDLR and/ or apoER2, the F3b sub-domain being represented by amino acid residues 215-272 of SEQ ID No:i, or a functional fragment or variant thereof. Preferred amino acid residues in the F3I} sub-domain of IDOL, which may be targeted by the agent to prevent binding or interaction with the receptor, may be selected from a group of residues consisting of residues: 232; 265; and 269 of SEQ ID No:i.

In another preferred embodiment, the agent may be capable of inhibiting binding or interaction between an F3C sub-domain of the FERM domain of IDOL and LDLR, VLDLR and/or apoER2, the F3C sub-domain being represented by amino acid residues 273-344 of SEQ ID No:i, or a functional fragment or variant thereof.

Preferred amino acid residues in the F3C sub-domain of IDOL, which may be targeted by the agent to prevent binding or interaction with the receptor, may be selected from a group of residues consisting of residues: 285, 323, 327 and 342 of SEQ ID No:l.

The agent may be capable of inhibiting binding or interaction between a sub-domain of the FERM domain of IDOL and amino acid residues conserved between (i) the LDLR, (ii) the VLDLR, and (iii) the apoER2 receptors. Figure 15A shows a sequence alignment of the tail portion of human LDLR, VLDLR and apoER2 receptors. The amino acid sequence of the tail portion of human LDLR (i.e. residues 810-860) is provided herein as SEQ ID No: 2, as follows:

⁸¹⁰WKNWRLKNINS INFDNPVYQKTTEDEVHICHNQDGYSYPSRQMVSLEDDVA⁸⁶⁰ SEQ ID No: 2

As can be seen, the alignment illustrates the presence of a conserved motif of ^8llK/RNWXXKNXXSI/MXF⁸²3 between amino acids 811 and 823 of SEQ ID No:2, to which IDOL binds, where K is lysine, R is arginine, N is asparagine, W is tryptophan, S is serine, M is methionine, I is isoleucine, F is phenylalanine and X may be any amino acid. For example, the X amino acid at residue 14 may be arginine (R), glutamine (Q) or lysine (K), and the X amino acid at residue 15 may be lysine (L), histidine (H) or arginine (R). The X amino acid at residue 18 may be isoleucine (I), methionine (M) or threonine (T), the X amino acid at residue 19 may be asparagine (N) or lysine (K), and the X amino acid at residue 22 may be asparagine (N). The agent may be capable of inhibiting binding or interaction between IDOL and a SI/MXF motif present in the LDLR, VLDLR and/or apoER2, as represented in SEQ ID No:2. The motif may be represented by amino acid residues 820 and 823 of SEQ ID No:2, i.e. ⁸²⁰SI/MXF⁸²3, to which IDOL binds. Inspection of the alignment shown in Figure 15A highlights other conserved regions between LDLR, VLDLR and/ or apoER2 to which IDOL may bind. Therefore, in another embodiment, the agent may be also capable of inhibiting binding or interaction between IDOL and a ^8l0WKNW⁸¹³ motif represented in SEQ ID No: 2, the motif being present in the LDLR, VLDLR and/or apoER2.

In addition, the agent may be capable of inhibiting binding or interaction between IDOL and a ^8l6KN⁸¹⁷ motif represented in SEQ ID No: 2, the motif being present in the LDLR, VLDLR and/or apoER2. Also, the agent may be capable of inhibiting binding or interaction between IDOL and a ^DNPVY⁸²⁸ motif represented in SEQ ID No:2, the motif being present in the LDLR, VLDLR and/or apoER2.

In a preferred embodiment, the agent may be capable of inhibiting binding or interaction between IDOL and one or more of the binding motifs described herein, which are present in LDLR, VLDLR and/or apoER2, and preferably all of these motifs.

Accordingly, in embodiments of the invention, the agent preferably prevents binding or interaction between the FERM domain of IDOL and one or each motif represented in SEQ ID No: 2. In a preferred embodiment, the agent may prevent binding or interaction between one or each motif and the F3 sub-domain of the FERM domain of IDOL, wherein the F3 sub-domain is defined by amino acid residues 183-344 of SEQ ID No:i, or a functional fragment or variant thereof. In another preferred embodiment, the agent may prevent binding or interaction between one or each motif and the F3b sub-domain of the FERM domain of IDOL, wherein the F3b sub- domain is defined by amino acid residues 215-272 of SEQ ID No:i, or a functional fragment or variant thereof. In another preferred embodiment, the agent may prevent binding or interaction between one or each motif and the F3C sub-domain of the FERM domain of IDOL, wherein the F3b sub-domain is defined by amino acid residues 273-344 of SEQ ID No:i, or a functional fragment or variant thereof. The agent may be capable of inhibiting interaction or binding between the RING domain of IDOL and a member of the ubiquitin-conjugating enzyme (UBE2D) family, which member may be UBE2D1, UBE2D2, UBE2D3 or UBE2D4. The agent may be capable of inhibiting interaction of binding between the member of the ubiquitin-conjugating enzyme (UBE2D) family and one or more amino acid residues of the RING domain of IDOL selected from the group of residues consisting of:

GIU383; Val389; Leu4i5 and Pro4i9 of SEQ ID No:i.

In one embodiment, an amino acid sequence of human ubiquitin-conjugating enzyme (UBE2D1) is provided herein as SEQ ID No:3, as follows:

MALKRIQKELSDLQRDPPAHCSAGPVGDDLFHWQATIMGPPDSAYQGGVFFLTVHFPTDY PFKPPKIAFTTKIYHPNINSNGSICLDILRSQWSPALTVSKVLLSICSLLCDPNPDDPLV PDIAQIYKSDKEKYNRHAREWTQKYAM SEQ ID No:3

The agent may be capable of inhibiting interaction of binding between IDOL and one or more amino acid residues of the member of the ubiquitin-conjugating enzyme (UBE2D) family selected from the group of residues consisting of: Lys8; Argis;

Pro6i; Phe62 and Pro95 of SEQ ID No:3.

Preferably, the agent is capable of inhibiting interaction of binding one or more amino acid residues of the RING domain of IDOL selected from the group of residues consisting of: GIU383; Val389; Leu4i5 and Pro4i9 of SEQ ID No:i and one or more amino acid residues of the member of the ubiquitin-conjugating enzyme (UBE2D) family selected from the group of residues consisting of: Lys8; Argis; Pro6i; Phe62 and Pro95 of SEQ ID No:3.

As described in Example 7, and as shown in Figure 7, the inventors were very surprised to observe that IDOL is an iron-binding protein. They have determined at least three cysteine residues at the N-terminal end of the RING domain of IDOL, which together define a pocket into which iron ions can bind. Thus, the inventors believe that blocking binding of iron ions with IDOL can prevent ubiquitination of the LDLR, VLDLR and/or apoER2 receptor, and thus be used to reduce plasma cholesterol levels. Hence, the agent may be capable of inhibiting or preventing binding of iron ions with IDOL, preferably the RING domain thereof, and most preferably at the N-terminal of the RING domain. The agent may be capable of inhibiting or preventing binding of iron ions with amino acid residue C360, C363 and/or C383 of SEQ ID No:i.

As described in Examples 4 and 6, and as illustrated in Figures 6E and 6G, the inventors have demonstrated for the first time that IDOL dimer formation seems to be important for its biological function, because the dimer-defective mutant

V431R/L433R was unable to promote LDLR degradation and was resistant to auto- catalyzed degradation. They therefore believe that any agent, which can prevent or inhibit IDOL dimerisation, would be very useful for treating the diseases described herein.

As described in Example 14 and Figure 18, the inventors have also surprisingly shown that the cell membrane is involved in IDOL-dependent degradation of the LDLR, VLDLR and/or apoER2 receptors. Thus, the inventors believe that target recognition by IDOL involves a tripartite interaction between (i) the FERM domain of IDOL, (ii) the lipoprotein receptor tail, and (iii) phospholipids present in the cell membrane. Accordingly, the agent may also be capable of inhibiting or preventing binding of membrane phospholipids with IDOL, preferably the FERM domain thereof. Membrane-facing amino acid residues in the FERM domain of IDOL which have been shown to be involved in the interaction with membrane phospholipids, and which may also be targeted by the agent, to prevent binding or interaction with membrane phospholipids, may be selected from the group of residues including 73; 75; 193; 199; 259; 137; and 146 of SEQ ID No:i.

It will be appreciated that once the skilled person has knowledge of the target amino acid residues present (i) in IDOL (either the FERM or RING domain), (ii) in the member of the ubiquitin-conjugating enzyme (UBE2D) family (i.e. UBE2D1, UBE2D2, UBE2D3 or UBE2D4) and/or (iii) in the receptor (i.e. LDLR, VLDLR and/ or apoER2 receptor), then it will be a straightforward task to design a suitable agent, which is capable of inhibiting binding or interaction between IDOL (FERM or RING domain) and receptor; or IDOL (FERM or RING domain) and the member of the ubiquitin-conjugating enzyme (UBE2D) family (i.e. UBE2D1, UBE2D2, UBE2D3 or UBE2D4) and/or IDOL (FERM or RING domain) and membrane phospholipids. As described in the examples, the residues conserved between LDLR, VLDLR and apoER2 are important for IDOL recognition. It has been determined that the F3b and F3C sub-domains of the FERM domain of IDOL are especially important for interaction with the tail ends of these receptors at the ^8llK/RNWXXKNXXSI/MXF⁸²3 motif, as well as other conserved motifs. Furthermore, the inventors have shown that specific residues in the FERM domain are required for an interaction with membrane phospholipids, and that certain residues in the RING domain are required for binding to the UBE2D1-4 protein, and for binding iron ions. Disruption of any of these interactions using the agent would inactivate the IDOL-receptor degradation pathway via ubiquitination.

By way of example, the agent may comprise a competitive polypeptide, or a derivative or analogue thereof, or a peptide-like molecule or a small molecule. For example, the agent may be an antibody or a immunologically active fragment thereof.

The term "derivative or analogue thereof can mean a polypeptide within which amino acids residues are replaced by residues (whether natural amino acids, non- natural amino acids or amino acid mimics) with similar side chains or peptide backbone properties. Additionally, either one or both terminals of such peptides may be protected by N- and C-terminal protecting groups, for example groups with similar properties to acetyl or amide groups. It will be appreciated that the amino acid sequenced may be varied, truncated or modified once the final polypeptide is formed or during the development of the peptide. Design of such peptide inhibitors, based on the sequence of the natural protein partners has been successfully used previously. In the case of BCL6, peptides based on the BCOR protein bind BCL6 and blocks SMRT from interacting at the same site and in doing so blocks BCL6-mediated transcriptional repression and kills lymphoma cells (Ghetu et al 2008). Likewise, the design of a synthetic, cell-permeable, stabilised peptide that targets the protein-protein interface in the NOTCH transactivation complex has been successfully used in leukaemic cells in culture. In a similar way, a wildtype peptide corresponding to the LDLR tail sequence, SEQ ID No: 2, would compete for binding to the IDOL FERM and prevent degradation of the LDLR Receptor. Modifications/optimisation of the peptide sequence could be made to increase the affinity so that it is tighter than the wildtype which is naturally a relatively weak and short-lived interaction. The inclusion of a ubiquitination motif, as in the LDLR tail, in the synthetic peptide would serve a secondary function; as well as binding the FERM domain it could be possible for it to be ubiquitinated reducing the available pool of active E2 for ubiquitinating endogenous LDLR

It will be appreciated that such inhibitory peptides, peptide mimics or small molecules will exploit the inventor's knowledge of the LDLR FERM interaction and be based upon the sequences that have been identified as being important to that interaction. The agent would bind tightly and specifically to the FERM domain preventing interaction and hence degradation of the LDL receptor.

In the sections below, the IDOL-LDLR interaction is used as an example as to how an agent may be developed, though it will be appreciated that similar methods may be used to develop agents that are capable of inhibiting any of the other interactions (i.e. the interaction between the FERM domain and membrane phospholipids, the interaction between the RING domain and UBE2D1-4 protein, and the ability of the RING domain to bind iron ions) described herein. The IDOL-LDLR recognition sequence can be used as the basis for screens aimed at identifying small molecules that specifically disrupt IDOL-LDLR interaction, e.g. by targeting this region of LDLR. Accordingly, in certain embodiments, screening systems are contemplated that screen for the ability of test agents to bind the F3bc sub-domains of the FERM domain of IDOL and/or to bind/interact with the region of LDLR that interacts with IDOL (e.g.. the SI/MXF motif) and/or that inhibit the interaction of IDOL and LDLR. Methods of screening for agents that bind the F3bc sub-domains of the FERM domain of IDOL or that bind to the LDLR region (e.g. the SI/MXF motif) that interacts with IDOL are readily available to the skilled technician (Colas 2008).

For example, in one embodiment, the F3bc sub-domains of the IDOL FERM domain and/or the LDLR domains are immobilized and probed with test agents. Detection of the test agent (e.g., via a label attached to the test agent) indicates that the agent binds to the target moiety and is a good candidate modulator of IDOL/LDLR interaction. In another embodiment, the association of LDLR and IDOL or a FERM domain of IDOL in the presence of one or more test agents is assayed. This can be accomplished using, for example, a fluorescence resonance energy transfer system (FRET) comprising a donor fluorophore on one moiety (e.g., LDLR) and an acceptor fluorophore on the IDOL molecule. The donor and acceptor quench each other when brought into proximity by the interaction of LDLR and IDOL. When association is reduced or prevented by a test agent, the FRET signal decreases indicating that the test agent inhibits interaction of LDLR and IDOL. These assays are illustrative and not limiting. Using the teaching provided herein, numerous binding and/or

LDLR/IDOL interaction assays will be available to the skilled technician. In certain embodiments, cells, tissues, and/or animals are provided that are transfected with an IDOL-encoding construct so they overexpress IDOL. In other embodiments, cells, tissues, and/or animals in which IDOL is "knocked out" are provided. It will be appreciated that one or both of these constructs may be used in screens for suitable agents of the invention for inhibiting any of the IDOL interactions.

For example, in certain embodiments, test agent(s) (e.g., small molecules) may be screened for their effect on the IDOL pathway based on the IDOL-/- cells, tissues or animals. WT and IDOL-/- are screened for response to candidate small molecules. The effect of IDOL- specific small molecules will be lost in the IDOL-/- cells. These screening methods can be used, for example, in conjunction with the cell-based reporter screens described herein. In certain embodiments, knockout IDOL animals may be used in screens for suitable agents. The assays of this invention have immediate utility in screening for agents that inhibit IDOL activity in a cell, tissue or organism. The assays of this invention can be optimized for use in particular contexts, depending, for example, on the source and/or nature of the biological sample and/or the particular test agents, and/or the analytic facilities available. Thus, for example, optimization can involve determining optimal conditions for binding assays, optimum sample processing conditions (e.g. preferred PCR conditions), hybridization conditions that maximize signal to noise, protocols that improve throughput, etc. In addition, assay formats can be selected and/or optimized according to the availability of equipment and/or reagents. Thus, for example, where commercial antibodies or ELISA kits are available it may be desired to assay protein concentration. Conversely, where it is desired to screen for modulators that alter transcription of IDOL gene, nucleic acid based assays are preferred.

It will be appreciated that agents according to the invention may be used in a medicament which may be used in a monotherapy (i.e. use of only an agent which inhibits binding between IDOL, the target receptor and/ or the UBE2D), for treating, ameliorating, or preventing hypercholesterolaemia or cardiovascular disease.

Alternatively, modulators according to the invention may be used as an adjunct to, or in combination with, known therapies for treating, ameliorating, or preventing hypercholesterolaemia or cardiovascular disease. For example, agents of the invention may be used in combination with known agents for treating

hypercholesterolaemia or cardiovascular disease, such as statins.

The agents according to the invention may be combined in compositions having a number of different forms depending, in particular, on the manner in which the composition is to be used. Thus, for example, the composition may be in the form of a powder, tablet, capsule, liquid, ointment, cream, gel, hydrogel, aerosol, spray, micellar solution, transdermal patch, liposome suspension or any other suitable form that may be administered to a person or animal in need of treatment. It will be appreciated that the vehicle of medicaments according to the invention should be one which is well-tolerated by the subject to whom it is given.

Medicaments comprising agents according to the invention may be used in a number of ways. For instance, oral administration may be required, in which case the agents may be contained within a composition that may, for example, be ingested orally in the form of a tablet, capsule or liquid. Compositions comprising agents of the invention may be administered by inhalation (e.g. intranasally). Compositions may also be formulated for topical use. For instance, creams or ointments may be applied to the skin, for example, adjacent the heart. Agents according to the invention may also be incorporated within a slow- or delayed-release device. Such devices may, for example, be inserted on or under the skin, and the medicament may be released over weeks or even months. The device may be located at least adjacent the treatment site, e.g. the heart. Such devices may be particularly advantageous when long-term treatment with agents used according to the invention is required and which would normally require frequent

administration (e.g. at least daily injection). In a preferred embodiment, agents and compositions according to the invention may be administered to a subject by injection into the blood stream or directly into a site requiring treatment. For example, the medicament may be injected at least adjacent heart. Injections may be intravenous (bolus or infusion) or subcutaneous (bolus or infusion), or intradermal (bolus or infusion).

It will be appreciated that the amount of the agent that is required is determined by its biological activity and bioavailability, which in turn depends on the mode of administration, the physiochemical properties of the modulator and whether it is being used as a monotherapy or in a combined therapy. The frequency of

administration will also be influenced by the half-life of the agent within the subject being treated. Optimal dosages to be administered may be determined by those skilled in the art, and will vary with the particular agent in use, the strength of the pharmaceutical composition, the mode of administration, and the advancement of the hypercholesterolaemia or cardiovascular disease. Additional factors depending on the particular subject being treated will result in a need to adjust dosages, including subject age, weight, gender, diet, and time of administration. Generally, a daily dose of between o.o^g/kg of body weight and soomg/kg of body weight of the agent according to the invention may be used for treating, ameliorating, or preventing hypercholesterolaemia or cardiovascular disease, depending upon which agent is used. More preferably, the daily dose is between o.oimg/kg of body weight and 400mg/kg of body weight, more preferably between o.img/kg and 200mg/kg body weight, and most preferably between approximately lmg/kg and loomg/kg body weight.

The agent may be administered before, during or after onset of

hypercholesterolaemia or cardiovascular disease. Daily doses may be given as a single administration (e.g. a single daily injection). Alternatively, the agent may require administration twice or more times during a day. As an example, agents may be administered as two (or more depending upon the severity of the

hypercholesterolaemia or cardiovascular disease being treated) daily doses of between 2smg and 7000 mg (i.e. assuming a body weight of 70 kg). A patient receiving treatment may take a first dose upon waking and then a second dose in the evening (if on a two dose regime) or at 3- or 4-hourly intervals thereafter. Alternatively, a slow release device may be used to provide optimal doses of agents according to the invention to a patient without the need to administer repeated doses.

Known procedures, such as those conventionally employed by the pharmaceutical industry (e.g. in vivo experimentation, clinical trials, etc.), may be used to form specific formulations comprising the agents according to the invention and precise therapeutic regimes (such as daily doses of the agents and the frequency of administration). The inventors believe that they are the first to suggest an anti- cardiovascular disease composition or an anti-hypercholesteroaemia composition, based on the use of the agents of the invention.

Hence, in a fifth aspect of the invention, there is provided a hypercholesteroaemia or cardiovascular disease treatment composition comprising a therapeutically effective amount of an agent capable of:

(i) a Low-Density Lipoprotein receptor (LDLR),

(ii) a Very Low Density Lipoprotein Receptor (VLDLR) and/ or

(iii) a Low density lipoprotein receptor-related protein 8 (apoER2);

(b) inhibiting binding or interaction between IDOL and a

K/RNWXXKNXXSI/MXF motif present in the LDLR, VLDLR and/or apoER2;

(d) inhibiting or preventing binding of iron ions with IDOL; or

(e) inhibiting or preventing the dimerisation of IDOL,

and optionally a pharmaceutically acceptable vehicle.

The term "anti-cholesterolaemia composition" or "cholesterolaemia treatment composition" can mean a pharmaceutical formulation used in the therapeutic amelioration, prevention or treatment of hypercholesterolaemia in a subject. The term "anti-cardiovascular disease composition" or "cardiovascular disease treatment composition" can mean a pharmaceutical formulation used in the therapeutic amelioration, prevention or treatment of a cardiovascular disorder in a subject, such as myocardial infarction.

The invention also provides in a sixth aspect, a process for making the composition according to the fifth aspect, the process comprising contacting a therapeutically effective amount of an agent capable of:

(i) a Low-Density Lipoprotein receptor (LDLR),

(ii) a Very Low Density Lipoprotein Receptor (VLDLR) and/or

(iii) a Low density lipoprotein receptor-related protein 8 (apoER2);

(b) inhibiting binding or interaction between IDOL and a

K/RNWXXKNXXSI/MXF motif present in the LDLR, VLDLR and/or apoER2;

(d) inhibiting or preventing binding of iron ions with IDOL; or

(e) inhibiting or preventing the dimerisation of IDOL,

with a pharmaceutically acceptable vehicle.

The agent may be a polypeptide, peptide or peptide-like molecule, for example an antibody.

A "subject" may be a vertebrate, mammal, or domestic animal. Hence, compositions and medicaments according to the invention may be used to treat any mammal, for example livestock (e.g. a horse), pets, or may be used in other veterinary

applications. Most preferably, however, the subject is a human being.

A "therapeutically effective amount" of agent is any amount which, when

administered to a subject, is the amount of drug that is needed to treat the cardiovascular disorder or hypercholesterolaemia disorder, or produce the desired effect. For example, the therapeutically effective amount of modulator used may be from about o.oi mg to about 8oo mg, and preferably from about o.oi mg to about 500 mg. It is preferred that the amount of modulator is an amount from about 0.1 mg to about 250 mg, and most preferably from about 0.1 mg to about 20 mg.

A "pharmaceutically acceptable vehicle" as referred to herein, is any known compound or combination of known compounds that are known to those skilled in the art to be useful in formulating pharmaceutical compositions. In one embodiment, the pharmaceutically acceptable vehicle may be a solid, and the composition may be in the form of a powder or tablet. A solid pharmaceutically acceptable vehicle may include one or more substances which may also act as flavouring agents, lubricants, solubilisers, suspending agents, dyes, fillers, glidants, compression aids, inert binders, sweeteners, preservatives, dyes, coatings, or tablet- disintegrating agents. The vehicle may also be an encapsulating material. In powders, the vehicle is a finely divided solid that is in admixture with the finely divided active agents according to the invention. In tablets, the active agent (e.g. the peptide or antibody) may be mixed with a vehicle having the necessary compression properties in suitable proportions and compacted in the shape and size desired. The powders and tablets preferably contain up to 99% of the active agents. Suitable solid vehicles include, for example calcium phosphate, magnesium stearate, talc, sugars, lactose, dextrin, starch, gelatin, cellulose, polyvinylpyrrolidine, low melting waxes and ion exchange resins. In another embodiment, the pharmaceutical vehicle may be a gel and the composition may be in the form of a cream or the like.

However, the pharmaceutical vehicle may be a liquid, and the pharmaceutical composition is in the form of a solution. Liquid vehicles are used in preparing solutions, suspensions, emulsions, syrups, elixirs and pressurized compositions. The active agent according to the invention may be dissolved or suspended in a pharmaceutically acceptable liquid vehicle such as water, an organic solvent, a mixture of both or pharmaceutically acceptable oils or fats. The liquid vehicle can contain other suitable pharmaceutical additives such as solubilisers, emulsifiers, buffers, preservatives, sweeteners, flavouring agents, suspending agents, thickening agents, colours, viscosity regulators, stabilizers or osmo-regulators. Suitable examples of liquid vehicles for oral and parenteral administration include water

(partially containing additives as above, e.g. cellulose derivatives, preferably sodium carboxymethyl cellulose solution), alcohols (including monohydric alcohols and polyhydric alcohols, e.g. glycols) and their derivatives, and oils (e.g. fractionated coconut oil and arachis oil). For parenteral administration, the vehicle can also be an oily ester such as ethyl oleate and isopropyl myristate. Sterile liquid vehicles are useful in sterile liquid form compositions for parenteral administration. The liquid vehicle for pressurized compositions can be a halogenated hydrocarbon or other pharmaceutically acceptable propellant.

Liquid pharmaceutical compositions, which are sterile solutions or suspensions, can be utilized by, for example, intramuscular, intrathecal, epidural, intraperitoneal, intravenous and particularly subcutaneous injection. The agent may be prepared as a sterile solid composition that may be dissolved or suspended at the time of administration using sterile water, saline, or other appropriate sterile injectable medium. The agents and compositions of the invention may be administered orally in the form of a sterile solution or suspension containing other solutes or suspending agents (for example, enough saline or glucose to make the solution isotonic), bile salts, acacia, gelatin, sorbitan monoleate, polysorbate 8o (oleate esters of sorbitol and its anhydrides copolymerized with ethylene oxide) and the like. The agents used according to the invention can also be administered orally either in liquid or solid composition form. Compositions suitable for oral administration include solid forms, such as pills, capsules, granules, tablets, and powders, and liquid forms, such as solutions, syrups, elixirs, and suspensions. Forms useful for parenteral

administration include sterile solutions, emulsions, and suspensions.

It will be appreciated that the invention extends to any nucleic acid or peptide or variant, derivative or analogue thereof, which comprises substantially the amino acid or nucleic acid sequences of any of the sequences referred to herein, including functional variants or functional fragments thereof. The terms "substantially the amino acid/nucleotide/peptide sequence", "functional variant" and "functional fragment", can be a sequence that has at least 40% sequence identity with the amino acid/nucleotide/peptide sequences of any one of the sequences referred to herein, for example 40% identity with the sequence identified as SEQ ID No:i (i.e. IDOL protein) or its encoding nucleotide, or 40% identity with the polypeptide identified as SEQ ID No: 2 (i.e. the tail portion of human LDLR) or its encoding nucleotide, and so on. Amino acid/polynucleotide/polypeptide sequences with a sequence identity which is greater than 50%, more preferably greater than 65%, 70%, 75%, and still more preferably greater than 80% sequence identity to any of the sequences referred to are also envisaged. Preferably, the amino acid/polynucleotide/polypeptide sequence has at least 85% identity with any of the sequences referred to, more preferably at least 90%, 92%, 95%, 97%, 98%, and most preferably at least 99% identity with any of the sequences referred to herein.

The skilled technician will appreciate how to calculate the percentage identity between two amino acid/polynucleotide/polypeptide sequences. In order to calculate the percentage identity between two amino acid/polynucleotide/polypeptide sequences, an alignment of the two sequences must first be prepared, followed by calculation of the sequence identity value. The percentage identity for two sequences may take different values depending on:- (i) the method used to align the sequences, for example, ClustalW, BLAST, FASTA, Smith-Waterman (implemented in different programs), or structural alignment from 3D comparison; and (ii) the parameters used by the alignment method, for example, local vs global alignment, the pair-score matrix used (e.g. BLOSUM62, PAM250, Gonnet etc.), and gap-penalty, e.g.

functional form and constants.

Having made the alignment, there are many different ways of calculating percentage identity between the two sequences. For example, one may divide the number of identities by: (i) the length of shortest sequence; (ii) the length of alignment; (iii) the mean length of sequence; (iv) the number of non-gap positions; or (iv) the number of equivalenced positions excluding overhangs. Furthermore, it will be appreciated that percentage identity is also strongly length dependent. Therefore, the shorter a pair of sequences is, the higher the sequence identity one may expect to occur by chance.

Hence, it will be appreciated that the accurate alignment of protein or DNA sequences is a complex process. The popular multiple alignment program ClustalW (Thompson et ah, 1994, Nucleic Acids Research, 22, 4673-4680; Thompson et ah, 1997, Nucleic Acids Research, 24, 4876-4882) is a preferred way for generating multiple alignments of proteins or DNA in accordance with the invention. Suitable parameters for ClustalW may be as follows: For DNA alignments: Gap Open Penalty = 15.0, Gap Extension Penalty = 6.66, and Matrix = Identity. For protein alignments: Gap Open Penalty = 10.0, Gap Extension Penalty = 0.2, and Matrix = Gonnet. For DNA and Protein alignments: ENDGAP = -1, and GAPDIST = 4. Those skilled in the art will be aware that it may be necessary to vary these and other parameters for optimal sequence alignment.

Preferably, calculation of percentage identities between two amino

acid/polynucleotide/polypeptide sequences may then be calculated from such an alignment as (N/T)*ioo, where N is the number of positions at which the sequences share an identical residue, and T is the total number of positions compared including gaps but excluding overhangs. Hence, a most preferred method for calculating percentage identity between two sequences comprises (i) preparing a sequence alignment using the ClustalW program using a suitable set of parameters, for example, as set out above; and (ii) inserting the values of N and T into the following formula:- Sequence Identity = (N/T)*ioo.

Alternative methods for identifying similar sequences will be known to those skilled in the art. For example, a substantially similar nucleotide sequence will be encoded by a sequence which hybridizes to any sequences referred to herein or their complements under stringent conditions. By stringent conditions, we mean the nucleotide hybridises to filter-bound DNA or RNA in 3x sodium chloride/ sodium citrate (SSC) at approximately 45°C followed by at least one wash in o.2x SSC/o.i% SDS at approximately 20-65°C. Alternatively, a substantially similar polypeptide may differ by at least l, but less than 5, 10, 20, 50 or 100 amino acids from the sequences shown in SEQ ID No: 1-3.

Due to the degeneracy of the genetic code, it is clear that any nucleic acid sequence described herein could be varied or changed without substantially affecting the sequence of the protein encoded thereby, to provide a functional variant thereof. Suitable nucleotide variants are those having a sequence altered by the substitution of different codons that encode the same amino acid within the sequence, thus producing a silent change. Other suitable variants are those having homologous nucleotide sequences but comprising all, or portions of, sequence, which are altered by the substitution of different codons that encode an amino acid with a side chain of similar biophysical properties to the amino acid it substitutes, to produce a conservative change. For example small non-polar, hydrophobic amino acids include glycine, alanine, leucine, isoleucine, valine, proline, and methionine. Large non- polar, hydrophobic amino acids include phenylalanine, tryptophan and tyrosine. The polar neutral amino acids include serine, threonine, cysteine, asparagine and glutamine. The positively charged (basic) amino acids include lysine, arginine and histidine. The negatively charged (acidic) amino acids include aspartic acid and glutamic acid. It will therefore be appreciated which amino acids may be replaced with an amino acid having similar biophysical properties, and the skilled technician will know the nucleotide sequences encoding these amino acids.

All of the features described herein (including any accompanying claims, abstract and drawings), and/or all of the steps of any method or process so disclosed, may be combined with any of the above aspects in any combination, except combinations where at least some of such features and/or steps are mutually exclusive.

For a better understanding of the invention, and to show how embodiments of the same may be carried into effect, reference will now be made, by way of example, to the accompanying diagrammatic drawings, in which :- Figure l shows that embodiments of ubiquitin-conjugating enzyme family members (UBE2Ds) of the invention are specific partners for IDOL autoubiquitination. A) Immunoblot of a panel of 19 human UBE2 enzymes that were expressed in E. coli as fusion proteins with 6xHis tags on their N-termini. B) Autoubiquitination of IDOL induced by UBE2D family proteins in an in vitro autoubiquitination assay.

Immunoprecipitated TAP-IDOL was incubated with UBEl, HA-ubiquitin and the indicated UBE2 proteins. IDOL ubiquitination was detected by immunoblotting for HA-tagged ubiquitin associated with IDOL. C) Autoubiquitination of IDOL induced by UBE2D requires an active RING domain. Immunoprecipitated TAP-IDOL and TAP-IDOL C387A were incubated with UBEl, HA-ubiquitin and the indicated UBE2 proteins. IDOL ubiquitination was detected by immunoblotting for HA-tagged ubiquitin associated with IDOL. D) Autoubiquitination induced by UBE2D is specific to IDOL. Immunoprecipitated TAP-EGFP, TAP-IDOL and TAP-IDOL C387A were incubated with UBEl, HA-ubiquitin and UBE2D proteins. IDOL ubiquitination was detected by immunoblotting for HA-tagged ubiquitin associated with IDOL. The amounts of TAP-tagged proteins in the in vitro autoubiquitination assay were determined by immunoblotting using anti-FLAG antibody. E) UBE2D family proteins have similar capacity for inducing IDOL autoubiquitination. UBED1-4 protein levels used in the IDOL autoubiquitination assay were normalized by Coomassie staining of SDS-PAGE gels. Immunoprecipitated TAP-IDOL and TAP- IDOL C387A were incubated with UBEl, HA-ubiquitin and the indicated UBE2D family proteins. IDOL ubiquitination was detected by immunoblotting for HA-tagged ubiquitin associated with IDOL. F) IDOL autoubiquitination is not exclusively dependent on the Lysii, Lys48 or the Lys63 linkage of ubiquitin.

Immunoprecipitated TAP-IDOL and TAP-IDOL C387A were incubated with UBEl, UBE2D2 and HA-tagged ubiquitin with indicated lysine mutations. IDOL ubiquitination was detected by immunoblotting;

Figure 2 shows that UBE2D family proteins are the E2 enzymes for LDLR ubiquitination. A) Ubiquitination of the LDLR by UBE2D and IDOL in an in vitro ubiquitination assay. Membrane preparation of 293 cells expressing LDLR-GFP or GFP alone were incubated with UBEl, UBE2D2, tandem affinity purified IDOL or IDOL C387A, and HA-ubiquitin. LDLR was then immunoprecipitated with an anti- GFP antibody. The ubiquitination of LDLR was detected by immunoblotting for HA- tagged ubiquitin associated with LDLR. B) Expression of dominant negative

UBE2D2 inhibits the degradation of LDLR. Immunoblot analysis of protein levels in 293 cells transfected with WT or dominant negative UBE2D or UBE2H, in addition to LDLR and IDOL. C). Immunoblot analysis of IDOL and LDLR expression in response to inhibition of proteasomal or lysosomal degradation pathways. 293 cells were transfected for 24 h with expression vectors for WT or C387A mutant IDOL and LDLR. Cells were treated with MG-132 (25 uM) or bafilomycin (BFL, 100 nM) as indicated 4 h prior to harvest;

Figure 3 shows NMR chemical shift mapping of the IDOL RING domain with UBE2D1. A) Ή,^Ν HSQC spectra of 150 μΜ ^N-labelled IDOL RING domain in the absence (blue) and presence of UBE2D1 (green) at an equimolar ratio. B) Weighted shift map obtained from the ^H^NJ-HSQC spectra of IDOL RING domain with the addition of UBE2D1. C) Ribbon representation of the crystal structure of the IDOL RING domain. D) Surface representation of the IDOL RING domain with the most significant shifts (>o.05ppm) shown in orange and smaller perturbations

(>o.025ppm) shown in yellow; Figure 4 shows the crystal structure of the IDOL RING domain dimer complexed with UBE2D1. A) Cartoon representation with the IDOL RING shown in purple and the UBE2D1 in grey. B) The RING domain dimer interface. C) Close up of the RING domain dimer interface. D) The interface of the IDOL RING domain with UBE2D1. E) Close up of the IDOL RING-UBE2D1 complex interface;

Figure 5 shows specificity determinants for the IDOL- RING: UBE2D interaction. A) Electrostatic potential of the interface between the IDOL RING domain (left) and UBE2D1 (right). Note that the main interaction surface on the E2 is highly basic and the complementary surface on the E3 is acidic. Argis in UBE2D1 provides a basic pocket to accommodate GIU383 from IDOL. In non-complementary E2s such as UBE2E3 (insert) the residue in this position is neutral or acidic and disfavors interaction. B) Some E2s that are non-complementary with IDOL have a basic residue in position 15, but an important serine at the interface (Ser94 in UBE2D1) is substituted with other amino acids, such as lysine in UBE2L3. The serine makes an important backbone contact that could not be formed by the alternative residues. C) Alignment of key regions of various E2 ligases. Only members of UBE2D family have both a basic residue and a serine to support appropriate interactions with the IDOL RING;

Figure 6 shows disruption of the IDOL-UBE2D interaction blocks LDLR

degradation. A) Mutations in the IDOL RING domain-UBE2D interaction interface inhibit LDLR degradation. Immunoblot analysis of protein levels in 293 cells transfected with LDLR and WT or mutant IDOL expression vectors. B) UBE2D is unable to catalyze the autoubiquitination of mutant IDOL with a disrupted IDOL RING domain-UBE2D interaction. Immunoprecipitated TAP-IDOL, TAP-IDOL C387A and TAP-IDOL V389R were incubated with UBEl, UBE2D2 and HA- ubiquitin. IDOL ubiquitination was detected by Western blot for HA-tagged ubiquitin associated with IDOL. C) UBE2D2 mutated at the interface with IDOL is unable to catalyze IDOL autoubiquitination. Immunoprecipitated TAP-IDOL and TAP-IDOL C387A were incubated with UBEl, WT or P61A/F62R UBE2D2 and HA-ubiquitin. IDOL ubiquitination was detected by Western blot for HA-tagged ubiquitin associated with IDOL. D) Mutation of UBE2D2 residues predicted to be involved in IDOL specificity determination reduces the ability of UBE2D2 to support IDOL autoubiquitination. Immunoprecipitated TAP-IDOL and TAP-IDOL C387A were incubated with UBEl, WT or R15E, S94K, or K8E UBE2D2 and HA-ubiquitin. IDOL ubiquitination was detected by Western blot for HA-tagged ubiquitin associated with IDOL. E) IDOL forms a dimer in vivo. 293 cells were transfected with vectors expressing TAP-IDOL and V5-IDOL. V5-IDOL in the cell lysate was

immunoprecipitated with an anti-Vs antibody. The TAP-IDOL that co- immunoprecipitated with V5-IDOL was detected by immunoblotting using an anti- FLAG antibody. F) Structure-based mutations predicted to disrupt dimer formation prevent the co-immunoprecipitated of TAP-IDOL and V5-IDOL. 293 cells were transfected with indicated combination of expression vectors. V5-IDOL and V5- mutant IDOL in the cell lysate were immunoprecipitated with an anti-Vs antibody. The co-immunoprecipitated TAP-IDOL was detected by immunoblotting using anti- FLAG. G) A dimer-defective IDOL mutant is unable to induce LDLR degradation. Immunoblot analysis of protein levels in 293 cells transfected with LDLR and WT or mutant IDOL expression vectors. H) IDOL harboring a mutation in the IDOL RING domain-UBE2D interaction interface functions as a dominant negative in LDLR degradation assays. Immunoblot analysis of protein levels in 293 cells transfected with increasing amount of V5 -tagged WT or mutant IDOL, in addition to constant levels of LDLR and TAP-IDOL; Figure 7 shows that IDOL is an iron-binding protein. A) Schematic diagram of the domain structure of IDOL. B) Alignment of IDOL sequences, hs homo sapiens, cf cards familaris, mm m musculus, xl xenopus laevis, gg gallus gallus. The three conserved Cys residues N-terminal to the RING domain are highlighted in yellow. The Cys zinc ligands are highlighted in orange and the His zinc ligand is highlighted in blue. C) Results from Atomic Absorption Spectroscopy. D) Photograph of protein samples of IDOL constructs eluted from Glutathione Sepharose by cleavage with TEV protease. E) Coomassie-stained SDS-PAGE gel of IDOL constructs eluted from Glutathione Sepharose by cleavage with TEV protease. F) and G) Disruption of the putative iron-binding cysteine residues alters IDOL stability and LDLR degradation. Immunoblot analysis of protein levels in 293 cells transfected with LDLR and WT or mutant IDOL expression vectors. H) Effect of IDOL interaction mutants on the ability of IDOL to inhibit LDL uptake. 293 cells were transfected with LDLR and WT or mutant IDOL expression vectors and then incubated for 4I1 with Dil-labeled LDL. Cells were washed and cellular LDL associated quantified by fluorescence. Results are presented as % WT IDOL inhibitory activity in LDL uptake assays. The inhibitory activity of WT IDOL was defined as 100% and that of the inactive RING mutant (C387A) was defined as o. **P <o.ooi, *P <o.os;

Figure 8 shows degradation of LDLR by TAP-tagged IDOL. (A) Immunoblot analysis of protein levels in 293 cells transfected with vectors encoding LDLR and indicated TAP-tagged proteins. (B) Autoubiquitination of IDOL induced by UBE2D requires an active RING domain. Purified TAP-IDOL and TAP-IDOL C387A were incubated with UBEi, UBE2D2 and HA-ubiquitin. Ubiquitinated IDOL was detected by Western blot for IDOL associated with HA-tagged ubiquitin;

Figure 9 is a schematic diagram of the in vitro autoubiquitination assay of IDOL; Figure 10 is a schematic diagram of the in vitro ubiquitination assay of LDLR;

Figure 11 shows the structure of the IDOL RING-UBE2D1 complex. A) IDOL RING complexed with UBE2D1 colored by B-factors. This shows that the interface of UBE2D1 with the IDOL RING domain is well ordered and that the N-terminal helix of the IDOL RING domain is not as well ordered. B) Superposition of IDOL RING complexed with UBE2D1 (cyan) with the structure of the IDOL RING alone

(magenta). This shows that there are some small conformational changes in the IDOL RING domain upon binding UBE2D1. The first loop and the first zinc are less well- ordered in the structure of the IDOL RING domain alone. (C) The two Pro434 residues within the RING dimer move 4.3 A towards each other, tightening the homodimeric interface, in the complex with UBE2D1 compared to the RING domain alone; Figure 12 shows that the IDOL FERM domain directly interacts with lipoprotein receptor cytoplasmic tail. (A) Immunoblot analysis of HEK293T whole cell lysates following overnight co-transfection with LDLR and FLAG-IDOL WT, RING mutant (C389A) or mutants deleted after the indicated residue. (B) Fluorescence polarization assay of the binding of BODIPY-labeled VLDLR 820-842 or control peptide to Hise- tagged IDOL constructs 1-273. The binding curves were analyzed using GraphPad Prism. Dissociation constants were 15UM and >250uM;

Figure 13 shows that the FERM 3b subdomain of IDOL is critical for LDLR recognition. (A) Domain structure of IDOL and potential configurations of FERM F3 domain; residue numbers indicate domain boundaries (top); Computer generated 3D modeling of IDOL denoting surface residues available for target interaction in either conformation based on Talin interaction with integrin (middle). (B) Immunoblot of HEK293T whole cell lysates following overnight co-transfection with LDLR and IDOL WT or F3b or F3C subdomain domain mutants as indicated. The ratio of IDOL: LDLR expression plasmid was varied while keeping the total amount of DNA transfected constant;

Figure 14 shows the key residues in the F3b subdomain required for IDOL regulation of the LDLR pathway. (A) Immunoblot analysis of HEK293T cell surface protein isolated by biotinylation following overnight co-transfection with LDLR and IDOL WT or F3b subdomain mutant constructs as indicated. (B) IDOL-dependent inhibition of Dil-LDL uptake following overnight co-transfection of HEK293T cells with LDLR and IDOL WT, ring mutant (C387A) or F3b subdomain mutants as indicated. Cells were maintained in 10% LPDS overnight prior to incubation with Dil- LDL ^g/mL) for 1 hour at 37°C. Data represented as % inhibition and expressed as mean ± SEM, performed in triplicate. The inhibitory activity of WT IDOL was assigned a value of 100% and the inactive RING MUT was defined as 0% activity. *p<0.05, **p<o.oi vs WT IDOL. (C) Immunoblot analysis of whole cell lysates from IDOL-/- mouse embryonic fibroblasts (MEFs) stably expressing retroviral IDOL WT or F3b subdomain mutant constructs as indicated. Cells were cultured in 10% lipoprotein deficient serum (LPDS) overnight. (D) IDOL-dependent inhibition of Dil- LDL uptake in IDOL -/- MEFs stably expressing retroviral IDOL WT or F3b subdomain mutant constructs as indicated. Cells were maintained in 10% LPDS overnight prior to incubation with Dil-LDL (4μg/mL) for 1 hour at 37°C. Data represented as % inhibition and expressed as mean ± SEM, performed in triplicate. The inhibitory activity of WT IDOL was assigned a value of 100% and the inactive RING MUT was defined as 0% activity. ***p< 0.001 vs WT IDOL. (E) Analysis of ubiquitinated LDLR in HEK293T cell lysates following overnight co-transfection with GFP-LDLR, HA-ubiquitin and IDOL expression plasmids as indicated. Proteins were immunoprecipitated overnight with anti-GFP antibody followed by immunoblotting for HA-ubiquitin. (F) Immunoblot analysis of HEK293T whole cell lysates following overnight co-transfection with LDLR and TAP-IDOL constructs with mutations in the F3b subdomain as indicated;

Figure 15 shows that IDOL recognizes a conserved "SI/MxF" motif in its lipoprotein receptor targets. (A) Sequence alignment of the cytoplasmic tail of the three IDOL targets with key residues for IDOL recognition and ubiquitination (Ub) highlighted; homologous residues are also shaded in grey. (B) 3-dimensional model of

IDOL/LDLR interaction highlighting critical residues in the LDLR tail; IDOL is colored by element with red: acidic residue - blue: basic residue and yellow: sulphur (C). 3-dimensional model of IDOL/LDLR interaction highlighting critical residues in the IDOL F3b domain; green residues indicate those predicted to be most important; orange residues indicate those predicted to be somewhat important; IDOL is colored by element with red: acidic residue - blue: basic residue and yellow: sulphur. (D) Immunoblot analysis of HEK293T whole cell lysates following overnight co- transfection with IDOL and LDLR WT or cytoplasmic tail mutants as indicated; * denotes mutations that affect IDOL-mediated degradation of the LDLR. (E) IDOL- dependent inhibition of Dil-LDL uptake in HEK293T cells transfected with IDOL and LDLR WT or cytoplasmic domain mutants overnight as indicated prior to Dil-LDL (4 μg/mL) uptake for 1 hour at 37°C. Data represented as % inhibition and expressed as mean ± SEM, performed in triplicate. The inhibitory activity of WT IDOL on WT LDLR was assigned a value of 100%. ***p<o.ooi vs WT LDLR. (F) Analysis ubiquitinated LDLR in HEK293T cell lysates following overnight co-transfection with HA-ubiquitin, FLAG-IDOL and GFP-LDLR WT or cytoplasmic domain mutants. Proteins were immunoprecipitated overnight with anti-GFP antibody or IgG as indicated, followed by immunoblotting for HA-ubiquitin. (G) Immunoblot analysis of HEK293T whole cell lysates following overnight co-transfection with IDOL and V5- VLDLR WT or cytoplasmic tail mutants as indicated. (H) Immunoblot analysis of HEK293T whole cell lysates following overnight co-transfection with IDOL and V5- ApoER2 WT or cytoplasmic tail mutants as indicated;

Figure 16 shows that IDOL- LDLR structure function relationships are conserved in insect orthologs. (A) Sequence alignment of the F3b domain of IDOL and its insect homolog, DNRi, demonstrating conservation of key residues across species;

homologous residues are shaded in gray. (B) Immunoblot analysis of HEK293T whole cell lysates following overnight co-transfection with LDLR and WT, ring mutant (C387A) or F3 mutant FLAG-DNRi constructs as indicated. (C) IDOL- dependent inhibition of Dil-LDL uptake in HEK293T cells transfected with LDLR and DNRi constructs as indicated. Cells were maintained in 10% LPDS overnight prior to incubation with Dil-LDL (4 μg/mL) for 1 hour at 37°C. Data represented as % inhibition and expressed as mean ± SEM, performed in triplicate. The inhibitory activity of WT DNRi on WT LDLR was assigned a value of 100%. *p<0.05,

***p< 0.001 vs WT DNRi. (D) Sequence alignment of the cytoplasmic tails of the LDLR and its insect homolog, lipophorin (LpR), with key residues for IDOL recognition and ubiquitination highlighted; homologous residues are shaded in gray. (E) Immunoblot of HEK293T whole cell lysates 48 h following co-transfection with IDOL and FLAG- LpR WT or cytoplasmic tail mutants as indicated;

Figure 17 shows that the FERM 3c subdomain of IDOL is required for

autoubiquitination. (A) Sequence alignment of the F3C sub domain of IDOL/DNRi demonstrating conservation of key lysine residues across species; homologous residues are shaded in grey. (B) Immunoblot analysis of HEK293T whole cell lysates following overnight co-transfection with LDLR and TAP-IDOL WT, ring mutant (C387A) or F3C single lysine mutants as indicated. (C) Immunoblot analysis of HEK293T whole cell lysates following overnight co-transfection with LDLR and TAP- IDOL WT, ring mutant (C387A) or F3C multiple lysine mutants as indicated. (D) Analysis of IDOL autoubiquitination in HEK293T cell lysates following overnight transfection with TAP-IDOL WT, ring mutant (C387A) or F3C multiple lysine mutants and HA-ubiquitin. Cells were incubated with MG-132 for 5 hours prior to harvest. TAP-IDOL was immunoprecipitated overnight with streptactin beads followed by immunoblotting for HA-ubiquitin. (E) Immunoblot analysis of HEK293T whole cell lysates following co-transfection with LDLR and FLAG-DNRi WT, ring mutant (C579A) or F3C lysine mutants as indicated;

Figure 18 shows that the membrane is required for IDOL-dependent LDLR degradation. (A) Assay of IDOL association with the LDLR in membrane fractions. HEK293T cells were transfected overnight with vector or LDLR and TAP-IDOL WT or F3C subdomain mutants as indicated. Membrane fractions were obtained following permeabilization with digitonin (0.05%). Immunoblot analysis of whole cell lysate inputs is shown at top. Analysis of proteins in membrane pellets is shown at bottom. (B) 3-dimensional modeling of the electrostatic surface of the IDOL FERM domain denoting key residues of the F3 domain involved in membrane interaction; the basic surfaces are shown in blue and the red indicates acidic surfaces.

(C) The IDOL FERM domain interacts with negatively charged membrane phospholipids. IDOL 1-273 (o.ismg / ml) was mixed with vesicles (0.5 mg / ml) consisting of phosphatidylcholine (PC), phosphatidylserine (PS), or a 4:1 ratio of PC: PS and then centrifuged. Talin 196-400 which binds tightly to negatively charged lipids (Anthis et al. 2009 EMBOJ) was used as a positive control and Talin 1655-1822 which does not bind lipids was used as a negative control. (D) Mutations of the basic surface on F3 abolish the interaction of the IDOL FERM domain with these vesicles. (E) Immunoblot analysis of HEK293T whole cell lysates following overnight co- transfection with LDLR and TAP-IDOL WT, ring mutant (C387A) or IDOL mutants in which basic residues on membrane-facing surfaces of the Fi (R73/K75), F2 (K137/ 146) or F3 (R193/K199/R259) subdomains were mutated to glutamic acid;

Figure 19 (A) shows a sequence alignment of human IDOL with the FERM domain of Talin. * denotes 100% homology; : denotes conserved substitutions; . denotes semi-conserved substitutions. (B) shows a sequence alignment of human IDOL with the FERM domain of Moesin, and Figure 19C shows a sequence alignment of human IDOL with the FERM domain Radixin; Figure 20 (A) shows immunoblot analysis of HEK293T whole cell lysates following overnight co-transfection with IDOL and LDLR WT or cytoplasmic tail mutant constructs as indicated; C839A indicates deletion of residues downstream of cysteine 839. (B) shows immunoblot analysis of HEK293T whole cell lysates following co- transfection with IDOL and WT LDLR or truncated LDLR constructs; Δ indicates deletion of cytoplasmic tail downstream of denoted residue. Figure 20C shows IDOL- dependent inhibition of Dil-LDL uptake in LDLR-/- MEFs transfected with WT and mutant LDLR constructs as indicated. Cells were maintained in 10% LPDS overnight prior to incubation with Dil-LDL (4 μg/mL) for 1 hour at 37°C. Data represented as % inhibition and expressed as mean ± SEM, performed in triplicate. The degree inhibition of WT LDLR uptake by WT IDOL was assigned a value of 100%. *p<0.05 vs WT LDLR;

Figure 21 shows immunoblot analysis of HEK293T whole cell lysates following overnight co-transfection with LDLR and TAP-IDOL WT or F3C multiple lysine mutants. Cells were incubated with the proteasomal inhibitor MG-132 (25 μΜ) for 5 hours prior to harvest as indicated;

Figure 22 shows immunoblot of HEK293T whole cell lysate following overnight transfection with TAP-IDOL and WT LDLR, V5-EGFP or V5-EGFP-LDLR (C- terminal domain) chimera expression plasmids as indicated; and

Figure 23 is a schematic drawing showing the postulated interactions between IDOL, E2 ligase, LDLR and the phospholipid membrane.

Materials and Methods

Plasmids and constructs

pSA2-N-TAP plasmid that contains the 3xFLAG-Strep tag and the pcDNA-Vs-Dest plasmid were kindly provided by Dr. Enrique Saez (Scripps). pD0NR22i and pET300N-Dest plasmids were purchased from Invitrogen. The DNA sequence of the human Idol gene was amplified from a pcDNA-Vs::hIdol construct as previously reported (Zelcer et al. 2009), and was then subcloned into pSA2-N-TAP plasmid. The IDOL mutations for the pcDNA-Vs::hIdol and the pSA2-N-TAP::hIdol constructs were introduced by site-directed mutagenesis. The human E2 genes were cloned from HEK293 cell cDNA and were then sequentially subcloned into pD0NR22i and pET300N-Dest using the Gateway technology (Invitrogen). In addition, the human Ube2d2 and Ube2h genes in the pD0NR22i::hUbe2d2 and pD0NR22i::hUbe2h constructs were subcloned into pcDNA-Vs-Dest plasmid using the Gateway technology, and the UBE2D2 C85A and the UBE2H C87A mutations for the pcDNA- V5::hUbe2d2 and the pcDNA-Vs::hUbe2h constructs, respectively, were introduced by site-directed mutagenesis.

Antibodies

Rabbit anti-hLDLR antibody was purchased from Cayman Chemicals. Rabbit anti- actin and mouse anti-FLAG M2 antibodies were purchased from Sigma. Mouse anti- V5 antibody, HRP-conjugated goat anti-mouse IgG and goat anti-rabbit IgG were purchased from Invitrogen. Rabbit anti-Vs antibody was purchased from Abeam. Rabbit anti-GFP antibody was purchase from Clontech. Mouse anti-HA antibody was purchased from Covance. All commercially available antibodies were used according to the manufacturers' instructions.

Cell culture and transfection

(1) HEK293 cells were maintained in D-MEM (Invitrogen) supplemented with 10% fetal bovine serum (FBS; Omega), 2 mM L-glutamine (Invitrogen), 50 U/ml penicillin (Invitrogen), and 50 μg/ml streptomycin (Invitrogen). Cells were grown in a humidified incubator at 37 °C and 5% C0₂ atmosphere. HEK293 cells were transfected using FuGENE 6 reagents (Roche) according to the manufacturer's instructions. Clonal stable cell lines expressing IDOL were established by serial dilution selection with 2 μg/ml puromycin (Clontech). (2) IDOL-/- and LDLR-/- mouse embryonic fibroblasts (MEFs) were immortalized by stable expression of the SV40 Large T antigen retrovirus and subsequent selection with hygromycin B. Stable expression of control retrovirus (pBabe) or wild type or mutant IDOL or LDLR constructs was performed as previously described (Zelcer, Science) and selected with puromycin. Cells were maintained in DMEM

supplemented with 10% FBS and MEM non-essential amino acids (Gibco) unless otherwise specified.

Immunoblotting

Proteins were resolved on 4%-i2% gradient SDS-PAGE (Invitrogen) using standard protocols. The protein was electrophoretically transferred to nitrocellulose membranes (Amersham Biosciences) and blocked with milk solution (150 mM NaCl, 20 mM Tris, 5% milk, 0.2% Tween, pH 7.5) to quench nonspecific protein binding. The blocked membranes were probed sequentially with primary and secondary antibodies diluted in the milk solution, and the bands were visualized with the ECL kit (Amersham Biosciences). IDOL autoubiquitination assay

To prepare the E. coli lysates containing human UBE2 proteins, the BL2i(DE3) strain (New England Biolabs) of E. coli containing various pET300N::hUbe2 constructs were cultured in LB broth (Sigma) at 37 degree overnight. The bacteria cultures were then diluted 1:10 in LB broth and cultured at 37 degree for another 1 to 2 h until OD600 reached approximately 0.8, at which point a final concentration of 1 mM

IPTG was added to induce the expression of the UBE2 proteins. 2 h after the addition of IPTG, bacteria were collected in eppendorf tubes, washed with PBS and then sonicated using a thin -tip sonicater (Misonix). Crude lysate was cleared by centrifugation at 12,000 g for 10 min and the supernatant was collected for the in vitro autoubiquitination assays.

3xFLAG-Strep tagged human IDOL, IDOL C387A, IDOL V389R and EGFP were expressed in HEK293 cells. Cells were lysed in RIPA buffer (Boston BioProducts, Inc.) supplemented with the Complete protease inhibitor cocktail (Roche). Cell lysate was cleared by centrifugation at 12,000 g for 10 min and the supernatant was then incubated with Streptactin beads (IBA GmbH) at 4 degree for 2 h. The beads were then extensively washed with RIPA buffer before the in vitro autoubiquitination assays. For each in vitro autoubiquitination assay, 25 μΐ of IDOL or EGFP bound Streptactin beads were mixed with 5 μΐ E. coli lysate containing UBE2, 50 ng recombinant rabbit UBEi (Calbiochem) and 10 μg recombinant HA-ubiquitin (Boston Biochem).

Reaction buffer contains 50 mM Tris-HCl (pH 7.4), 5 mM MgCl₂, 2 mM ATP and 25 μΜ MG132 (Sigma). The reaction mixture was incubated at 37 °C for 1 h. After the reaction, the Streptactin beads were separated and then extensively washed with RIPA buffer before the proteins on the beads were eluted by heated protein loading buffer (Invitrogen). Ubiquitination status was analyzed by immunoblotting using an anti-HA antibody. LDLR ubiquitination assay

HEK293 cells expressing LDLR-GFP or GFP control were permeabilized and the cytosolic proteins were removed according to a protocol previously published (Song and DeBose-Boyd 2004). IDOL and IDOL C387A stably expressed in HEK293 cells were purified using a tandem affinity purification protocol (Gloeckner et al. 2009). For each in vitro ubiquitination assay, 25 μΐ of pelleted permeabilized cells were mixed with 2 μΐ purified IDOL, 2 μΐ E. coli lysate containing UBE2, 50 ng

recombinant rabbit UBEl and 10 μg recombinant HA-ubiquitin. Reaction buffer contains 50 mM Tris-HCl (pH 7.4), 5 mM MgCl₂, 2 mM ATP and 25 μΜ MG132 (Sigma). The reaction mixture was incubated at 37 degree for 1 h. After the reaction, the permeabilized cells were separated and lysed in RIPA buffer supplemented with the Complete protease inhibitor cocktail. The cell lysate was then cleared by centrifugation at 12,000 g for 10 min. LDLR in the lysate was immunoprecipitated with a rabbit anti-GFP antibody and Protein G beads (Santa Cruz), and the ubiquitination status of LDLR was analyzed by immunoblotting using an anti-HA antibody. NMR Spectroscopy

For the NMR experiments ¹⁵N ^C His-tagged IDOL 369-445 was purified on Ni-NTA (Qiagen) and after TEV-cleavage of the tag purified further on a Resource-Q column (GE Healthcare). The protein was transferred into 20 mM sodium phosphate, 150 mM NaCl and 0.25 mM TCEP using a PD10 (GE Healthcare) and concentrated to o.6mM immediately prior to collection of NMR spectra. NMR experiments for the resonance assignment of IDOL 369-445 were carried out with 0.6 mM protein in 20 mM sodium phosphate, pH 6.5, 100 mM NaCl, 10% (v/v) Ή2Ο. NMR spectra of all the proteins were obtained at 298 K using Bruker AVANCE DRX 600 or AVANCE DRX 800 spectrometers both equipped with CryoProbes. Proton chemical shifts were referenced to external 2,2-dimethyl-2-silapentane- 5-sulfonic acid, and ¹⁵N and ¹³C chemical shifts were referenced indirectly using recommended gyromagnetic ratios (Wishart et al. 1995). Spectra were processed with TopSpin (Bruker Corp.) and analyzed using Analysis (Vranken et al. 2005). Three-dimensional HNCO,

HN(CA)CO, HNCA, HN(CO)CA, HNCACB, and HN(CO)CACB experiments were used for the sequential assignment of the backbone NH, N, CO, CA, and CB resonances.

Crystallization and X-ray structure determination

For the crystallisation experiments GST-tagged IDOL 369-445 was purified using glutathione-sepharose resin (GE Healthcare), eluted by TEV cleavage and purified further on a Resource-Q (GE Healthcare). His-tagged UBE2D1 was purified on Ni- NTA (Qiagen) and, after TEV-cleavage of the tag, on a Superdex-75 column (GE Healthcare). IDOL 369-445 alone was concentrated to 7.5 mg/ml in a buffer containing 50 niM Tris pH 8, 100 niM NaCl and 0.5 niM TCEP. IDOL RING domain alone was crystallized from 0.1M sodium acetate pH 7-8, 16-20% MPD in the spacegroup 1 1 2 1. For the complex crystals the two proteins were concentrated independently, mixed at equimolar concentrations and crystallized from 0.1 M sodium citrate pH 5.5, 0.2 M sodium acetate, 10% PEG 4000 in the space group P 1 21 1. Data was collected at the synchrotron at ESRF on ID23-1 to 3.0 A for the RING domain alone and at Diamond on I04 (to 2.1 A) for the complex. The data was processed using MOSFLM (Leslie 2006) and both structures were solved by molecular replacement using Phaser (McCoy et al. 2007). The model for the RING domain alone was taken from the cIAP RING structure (3EB5) (Mace et al. 2008). The complex was solved by using the 3A IDOL 369-445 domain structure and the UBE2D2 structure from 3EB6 (Mace et al. 2008). The UBE2D2 complexed with cIAP (3EB6) was not a successful search model as the two proteins have moved with respect to each other in the IDOL 369-445 UBE2D1 structure. Model building and refinement were performed using Coot, REFMAC and Phenix (CCP4 1994; Adams et al. 2010; Emsley et al. 2010). The crystallographic statistics are shown in Table 1.

Atomic absorption spectroscopy

For the atomic absorption spectroscopy GST tagged IDOL 358-445 and GST tagged IDOL 369-445 were purified as described above. Zinc and iron standards were used. The zinc concentration for the brown IDOL 358-445 was 0.3 mM and the iron concentration was 0.06 mM. The zinc concentration for the clear IDOL 369-445 was 0.47 mM and the iron concentration was 0.012 mM.

Accession numbers

Coordinates and structure factors for the IDOL RING domain and the IDOL RING domain-UBE2Di complex crystal structures have been deposited in the Protein Data Bank (ID codes 2YHN and 2YHO respectively), and the lH, 15N, & 13C NMR chemical shifts for the IDOL RING domain have been deposited in the

BioMagResBank database (accession code 17550).

Expression Constructs and Transfections

Human ApoER2, drosophila melanogaster DNRi and LpR (Open Biosystems) and human VLDLR (Hong, JBC) were cloned into the gateway plasmid, pD0NR22i (Invitrogen). Constructs were sub-cloned into tagged destination vectors using gateway technology (Invitrogen). The FLAG destination plasmid was a kind gift of Dr James Wohlschlegel (UCLA) and the V5 destination plasmid was a kind gift of Dr Tom Vallim (UCLA). pSA2-N-TAP plasmid that contains the 3xFLAG-Strep tag was kind gift of Dr Enqrique Saez (Scripps). Truncated LDLR and IDOL constructs were amplified from appropriate wild type constructs using Platinum pfx (Invitrogen) and introduced into pDONR.221. The EGFP-LDLR chimera was made using traditional cloning techniques and was made up of an N-terminal EGFP, a 10 amino acid linker, and the cytoplasmic tail of LDLR (amino acids 811-860). All other constructs were obtained as previously described (Zelcer, Science). Mutations were introduced using the Quickchange site-directed mutagenesis kit (Stratagene). DNA sequencing was used to verify mutant constructs. Transfections were performed using Fugene (Roche Diagnostics) according to the manufacturers instructions with an

LDLR/VLDLR/ApoER2/LpR:IDOL/DNRi ratio of 4:1 or 2:1 unless otherwise stated. Cells were harvested approximately 24-48 hours following transfection. When indicated, the proteasomal inhibitor, MG-132 (25 μΜ), was added approximately 5 hours prior to harvest.

Immunoblotting, Biotinylation, Immunoprecipitation and Fractionation

HEK293T cells were washed with PBS then harvested in RIPA buffer (Boston

Bioproducts; Tris-HCl somM, pH 7.4, NaCl isomM, NP-40 1%, Sodium deoxycholate 0.5%, SDS 0.1%) supplemented with protease inhibitors (Roche Diagnostics). Lysates were clarified by centrifugation then quantified using the Bradford assay (Biorad) with BSA as a reference. Proteins were separated on Nupage Bis-Tris gels then transferred to PVDF (GE Osmonics). Membranes were probed with antibodies against the following, LDLR (Cayman Chemical Company), V5 (Invitrogen), FLAG (Sigma), HA (Covance), oc-tubulin (Calbiochem), β-actin (Sigma) and pan-cadherin (Santa Cruz). Appropriate secondary HRP-conjugated antibodies were used

(Invitrogen, Biorad) and visualized with chemiluminescence (Amersham). To assess cell surface expression, samples were biotinylated (Thermo Scientific; 250g/mL) for 30 minutes at 4°C then subsequently quenched and washed with PBS prior to harvesting in RIPA buffer with protease inhibitors. Equal amounts of clarified lysate were incubated with neutravidin agarose resin (Pierce) overnight with rotation then washed and heated to 95C with 2x sample buffer for 5 minutes. For LDLR-GFP immunoprecipitation, equal amounts of clarified lysate were incubated with anti-GFP antibody (Abeam) or IgG control overnight with rotation followed by the addition of protein G beads (Santa Cruz). For TAP-IDOL immunoprecipitation, equal amounts of clarified lysate, which had previously been treated with MG-132 (25 μΜ) for 5 hours prior to harvest, was incubated with streptactin beads (IBA GmbH) overnight with rotation. Samples were washed then heated to 70°C with 2x sample buffer for 20 minutes prior to immunoblotting. Cell membrane fractionation was performed by incubating cells with digitonin (0.05%) at 4°C for 1 hour with rotation and subsequent pelleting by centrifugation at 3000g for 1 minute prior to

immunoblotting.

Dil-LDL Uptake

LDL uptake was performed cells following overnight transfection with LDLR and IDOL constructs (HEK293T cells), overnight treatment with lipoprotein deficient serum (LPDS; IDOL -/- MEFs) or overnight treatment with LPDS in the absence or presence of GW3965 (ιμΜ; LDLR-/- MEFs). Cells were incubated with Dil-LDL (Invitrogen; 4g/mL) for 1 hour at 37°C then washed with PBS and harvested in RIPA buffer supplemented with protease inhibitors. Samples were clarified by

centrifugation then moved to a 384-well plate in triplicate and measured on a Typhoon apparatus (Amersham).

Modeling

The amino acid sequence of human IDOL (Q8WY64) was compared with the FERM domains of; mouse talin-i (P26039) residues 86-405 (delta 139-168), human radixin (P35241) residues 1-295 and human moesin (P26038) residues 1-295. Sequence alignments were carried out using T-coffee (Notredame et al. 2000). Structural homology models were generated using PHYRE (Kelley and Sternberg 2009)) using the IDOL sequences 1-276 and 1-344 (Δ215-272). Docking of the LDLR peptide was achieved using comparison of talin in complex with layillin, ΡΙΡΚΙ-γ and integrin- βιϋ (PDB IDs: 2koo, 2G35 and 3G9W respectively) and DABi in complex with the apoER2 cytoplasmic tail (PDB ID: lNTV). Electrostatic surfaces were calculated using the APBS pymol plugin (Baker et al. 2001). Phospholipid cosedimentation assays

Large multilamellar vesicles were prepared essentially as described earlier (Anthis et al 2009). Briefly, films of dried phospholipids (Sigma) were swollen at 5 mg/ml in 20 mM Hepes pH 7.4, 0.2 mM EGTA for 3I1 at 42°C. The vesicles were then centrifuged (20,000 g for 20 min at 4°C), and the pellet was resuspended in the same buffer at 5 mg/ml. Protein samples were diluted into 20 mM Tris/ HCl (pH 7.4), 0.1 mM EDTA, 15 mM β-mercaptoethanol. After centrifugation (20,000 g for 20 min at 4°C) proteins (0.15 mg/ml) were incubated (30mm, 25°C) in the absence or presence of phospholipid vesicles (0.5 mg/ml), 200 μΐ total volume, followed by centrifugation (25,000 g for 20 min at 4°C). Pellet and supernatant fractions were analyzed on a 10- 20% gradient gel (Expedeon) and proteins detected by Coomassie-blue staining.

Fluorescence Polarization Assays

VLDLR peptides with an amino-terminal cysteine residue were synthesized by BioMatik. Peptide stock solutions were made in PBS containing imM TCEP and then coupled via the amino-terminal cysteine to the thiol-reactive BOPIDY TMR dye (Invitrogen) in accordance with the manufacturers instructions. Unreacted dye was removed by gel filtration using a PD-10 column (GE Healthcare) and the labeled peptide was concentrated to a final concentration of imM using a centricon with 3K MWCO. Fluorescence polarization experiments were performed in a black 96 well assay plate (Corning). Multiple titrations were performed using a fixed concentration of LDLr peptide of 5μΜ with increasing concentrations of IDOL protein, in a final volume of Ιθθμΐ of assay buffer (PBS, 0.01% (v/v) Triton X-100, o.img/ml BSA). The plate was mixed by shaking for 1 min, and measurements were taken using a Victor X5 plate reader (Perkin Elmer) at room temperature with an excitation wavelength of 531 nm and an emission wavelength of 595 nm. Experiments were performed in triplicate and data were analyzed using GraphPad Prism (version 4.0, GraphPad Software, Inc.). Kd values were calculated by nonlinear curve fitting using a one site binding (hyperbola) model (Y = Bmax *X/(Kd + X).

Results

Example 1 - Identification of the E2 for IDOL autoubiquitination

Because it was not known whether IDOL could directly ubiquitinate the LDLR, the inventors established an in vitro IDOL autoubiquitination assay in order to identify IDOL-interacting E2 enzymes. They hypothesized that, similar to other E3 ligases, IDOL might employ the same E2 partner for both autoubiquitination and target (LDLR) ubiquitination. Autoubiquitination is characteristic of RING-type E3 ligases (Yang et al. 2000), and can be evaluated in vitro by incubating the E3 with its cognate E2 and the other factors required for ubiquitination. To establish the autoubiquitination assay, the inventors first immunoprecipitated IDOL from

HEK293 cells stably expressing an IDOL protein tagged with 3xFLAG and Strep on its N-terminal end (TAP-IDOL) . The efficacy of TAP-IDOL at degrading the LDLR was confirmed in cotransfection assays (see Figure 8A). According to the HUGO Gene Nomenclature Committee, 38 E2 genes have been documented in the human genome (Bruford et al. 2008). Recent systematic studies have defined a subgroup of these E2 enzymes that preferentially participate in ubiquitination mediated by RING-type E3 ligases (Markson et al. 2009; van Wijk et al. 2009). The inventors therefore screened a representative panel of the 19 E2 proteins belonging to this category. They expressed His-tagged E2 proteins in E. coli (see Figure lA), and then combined the crude lysates with immunoprecipitated IDOL, recombinant human UBEl, recombinant HA-tagged ubiquitin and the ATP- generating system (see Figure 9). Polyubiquitinated IDOL was detected in the presence of the four closely related members of the UBE2D family (UBE2D1-4), but not in the presence of any other E2 protein screened (see Figure lB). This reaction was dependent on IDOL E3 activity, because UBE2D proteins failed to ubiquitinate an IDOL mutant harboring a cysteine mutation in the RING domain (C387A) (see Figure lC and Figure 8B). This point mutation disrupts the function of the RING domain and abolishes the ability of IDOL to degrade itself or the LDLR (Zelcer et al. 2009). Furthermore, the ubiquitination reaction specifically targeted IDOL, because UBE2D proteins failed to induce the polyubiquitination of a TAP-EGFP protein immunoprecipitated in parallel with IDOL (see Figure lD).

The inventors then addressed the relative efficacy of the individual members of the UBE2D family in supporting IDOL autoubiquitination. They expressed UBE2D1, UBE2D2, UBE2D3 and UBE2D4 proteins in the same batch of E. coli cells, and used the same amount of each protein in autoubiquitination assays. They found that individual members of the UBE2D family members exhibited similar capacity for forming polyubiquitinated IDOL (see Figure lE). Polyubiquitin chains are usually formed via linkage on Lys48, Lysii or Lys63 residues of ubiquitin (Pickart 2001). They sought to determine whether the autoubiquitination of IDOL was dependent on the Lys48 or Lys63 linkage. They therefore provided exclusively wild-type ubiquitin, K11R ubiquitin, K48R ubiquitin, or K63R ubiquitin in the IDOL autoubiquitination assays. Interestingly, none of the mutant ubiquitins inhibited the autoubiquitination of IDOL (see Figure lF), suggesting that the IDOL autoubiquitination catalyzed by the UBE2D enzymes did not exclusively utilize either the Lys48, Lys63 or Lysii linkage.

Example 2 - UBE2D family proteins are the E2 enzymes for LDLR ubiquitination In order to test the ability of IDOL to ubiquitinate the LDLR in a cell-free system and to determine whether the UBE2D family proteins are the E2 enzymes for LDLR ubiquitination, the inventors sought to reconstitute an in vitro system where IDOL, together with UBEl and UBE2D enzymes, could mediate the transfer of ubiquitin to the LDLR. To this end, they expressed an LDLR-GFP fusion or GFP control in HEK293 cells and prepared membrane fractions by permeabilizing the plasma membrane and removing cytosolic proteins (Song and DeBose-Boyd 2004). They then mixed the membrane preparation with recombinant UBEl, crude lysate of E. coli expressing UBE2D2, tandem affinity purified IDOL, recombinant HA-tagged ubiquitin and the ATP-generating system (see Figure 10). After the in vitro ubiquitination reaction, the membrane preparation was disrupted and the LDLR was immunoprecipitated. Ubiquitination was then assayed by immunoblotting.

Remarkably, the inventors found that polyubiquitinated LDLR was formed in the presence of UBE2D2 and IDOL, but not in the absence of UBE2D2, or in the presence of RING domain mutant IDOL (C387A; see Figure 2A).

In order to demonstrate that the UBE2D family proteins are the E2 enzymes that catalyze LDLR ubiquitination in vivo, the inventors employed a dominant negative version of UBE2D2 lacking a critical cysteine residue within its catalytic domain (C85A) (Gonen et al. 1999). Expression of the dominant negative UBE2D2 in

HEK293 cells markedly inhibited IDOL-dependent LDLR degradation (see Figure 2B). By contrast, the expression of a dominant negative mutant of an unrelated E2, UBE2H (C87A), did not inhibit LDLR degradation. Taken together, these results indicate that the UBE2D family proteins participate in the endogenous IDOL- LDLR ubiquitination cascade.

To provide further insight into the functional effects of IDOL and LDLR

ubiquitination in cells, the inventors treated cells expressing WT or RING MUT IDOL and LDLR with inhibitors of protein degradation. Transfection of RING MUT IDOL expression vector gave rise to markedly increased protein levels compared to WT IDOL expression vector, consistent with loss of autoubiquitination and degradation (see Figure 2C). Addition of the proteasomal inhibitor MG-132, but not the lysosomal inhibitor bafilomycin, stabilized WT IDOL protein levels, consistent with the hypothesis that ubiquitinated IDOL is degraded in the proteasome. By contrast, IDOL-dependent LDLR degradation was blocked by bafilomycin, but not by MG-132. These results strongly suggest that IDOL-dependent LDLR ubiquitination and IDOL autoubiquitination have distinct functional consequences and lead to distinct degradation pathways.

Example 3 - The IDOL RING domain interacts directly with UBE2D1

To further investigate IDOL-UBE2D interaction, the inventors employed NMR spectroscopy. The IDOL RING domain protein (residues 369-445) was expressed in E. coli and readily purified. The Ή,¹⁵Ν HSQC NMR spectrum indicated a stable and well-defined protein fold. However, the line widths suggested a molecular weight higher than would be expected for a 9 kDa protein. The interaction between the IDOL RING domain and UBE2D1 was studied by collecting Ή,^Ν HSQC spectra of ¹⁵N-labeled RING domain in the presence of increasing concentrations of unlabeled UBE2D1 (see Figure 3A). A number of resonances showed progressive changes in chemical shift indicative of a direct interaction. To further analyze this, chemical shift data we used ^C-^N labeled protein IDOL RING domain (residues 369-445) to complete the backbone assignment. The weighted Ή,¹⁵Ν chemical shifts in IDOL RING induced by UBE2D1 were plotted as a function of residue number (see Figure 3B) and this analysis showed that the interaction is specific and involves residues M388, V389, C390, C391 and C411 of the IDOL RING domain, with chemical shift perturbations greater than o.osppm δΔ ppm (see Figure 3B). Thus, NMR analysis confirmed the direct interaction between IDOL and UBE2D1 suggested by the inventors' in vitro screen.

Example 4 - Structure of the IDOL RING-UBE2D1 complex

In addition to the NMR chemical shift titrations, the inventors determined the crystal structure of the IDOL RING domain (residues 369-445), both alone (see Figure 3C and D) and in complex with UBE2D1 (see Figure 4A-E, Figure 11, and Table 1).

Table 1 - Data collection and refinement statistics

IDOL (369-445) IDOL (369-445)

UBE2D1

63.75

,

90.OO 90.00

39.19-3.OO (3.16- 55.91-2.10 (2.21-2.1)

3.00)

13.8 (34-4) 8- 9 (35-5)

12.2 (7.4) 9- 0 (3.2)

99.2 (100.0) 99-6 (99-8) 3-9 (4-0) 3-5 (3-6)

22.84 (29.7) 18.56 (23.33)

880 6766

406

36

21.88 31.26

33-30

0.019 0.008

0.842 1.158

It was not immediately apparent from the protein sequence which residues of the IDOL RING domain would chelate zinc, due to the presence of multiple cysteine and histidine residues in addition to those normally observed in RING domains.

However, the structure reveals that the IDOL RING domain employs seven cysteines and one histidine to coordinate two zinc ions in a conventional pattern (Barlow et al. 1994), with the protein structure interleaved around the zinc ions (see Figure 3C). An amino-terminal helix precedes the IDOL RING domain. The whole structure forms a homodimer in the crystal lattice. This is mediated, in part, by the amino-terminal helix, but mainly through a tight interface between the RING domains that have a highly complimentary shape, such that the buried surface area of the dimerization interface is 1862 A² (see Figure 4B). The IDOL RING dimerization interface is one of the most ordered parts of the IDOL UBE2D1 complex structure (see Figure 11A), with multiple non-polar amino-acids (Val43i, Leu433, Ile395, Pro434) at the interface (see Figure 4C). In addition to the hydrophobic interactions, there are backbone interactions between Tyr432 and Gly403'. Tyr432 is also involved in a stacking interaction with the histidine ring of His 404' (see Figure 4C). The side chain of Gln429 makes a hydrogen bond to the backbone of Pro40i'. Three leucine residues (Leu374, Leu378, Leu38i) in the helix preceding the zinc-binding domain also appear to contribute to dimerization, but this part of the structure is less well ordered (see Figure 11A).

In the structure of the complex, the IDOL-UBE2D1 interface is well ordered and, like the dimerization interface, is predominantly hydrophobic (see Figures 4D and E). The core of the interface consists of amino acids Val389, Leu4i5 and Pro4i9 of IDOL packing on Phe62, Pro6i and Pro95 on UBE2D1. UBE2D1 side chains Lys4, Arg5 and Ser94 make hydrogen bonds to the side chain of GI11392 and the backbone of Met388 and Pro4i9 of IDOL respectively. Arg422 of IDOL makes hydrogen bonds to the backbone of Gln92. The interface observed in the crystal structure is consistent with the NMR chemical shift mapping. Interestingly, the RING: RING dimer interface is somewhat rearranged in the complex with UBE2D1 so as to form a tighter interface compared with the RING dimer alone (see Figure 11B). The two Pro434 residues within the RING dimer move 4.3 A towards each other, tightening the homodimeric interface, in the complex with UBE2D1 compared to the RING domain alone (see Figure 11C). The inventors believe that there may be some cooperative

rearrangement on binding the E2, though the influence of crystal packing cannot be ruled out.

The IDOL RING UBE2D1 structure is similar to the structure of UBE2D2 in complex with the cIAP2 RING (Mace et al. 2008) and explains why IDOL can interact with all members of the UBE2D family of E2 enzymes. The two structures vary however, in the orientation of the helix preceding the zinc-binding RING domain. The interface between the IDOL RING and UBE2D1 is not as extensive as the RING: RING dimer interface and buries only 1140 A², which would suggest that the complex may be rather transient in nature (see Figures 4D and E).

Example 5 - Stereochemical basis of the specificity of IDOL for UBE2DS

To understand why IDOL requires UBE2DS and does not degrade the LDLR in combination with other E2 ligases, the inventors carefully examined residues at the interface of the complex. It appears that residues in two positions in the interface play an important role in determining specificity. Argis is conserved in UBE2D1-4. Together with Lys8, Argis provides a basic pocket that accommodates the acidic sidechain of GIU383 in the IDOL RING (see Figure 5A). In nearly all the E2s that do not support IDOL-mediated degradation of the LDLR, the residue at this position is either uncharged or acidic. Due to the close proximity of Aspi6, either a neutral or acidic residue at position 15 results in an acidic surface that would perturb interaction of the E2 with the IDOL RING domain, as is the case for UBE2E3 (see inset in Figure 5A). All of the E2 ligases that do have an Arginine or Lysine equivalent to Argis ^m UBE2D1, are lacking a key serine residue (Ser94 in UBE2D1) at the other end of the interface (see Figures 5B and C). This serine sidechain makes a critical hydrogen bond to the backbone carbonyl oxygen of Pro4i9 in IDOL and this in turn brings about a tight stacking of the rings of Pro95 in UBE2D1 and Pro4i9 in IDOL. In UBE2L3, UBE2G1, and UBE2T, the serine is substituted by much larger sidechains (Lys, Leu and Arg respectively), which could not be accommodated at the interface with IDOL.

Example 6 - Disruption of the IDOL RING domain-UBE2D interaction inhibits LDLR degradation

Based on their structural data, the inventors generated targeted mutations to further interrogate the ID0L-UBED2 interaction. The structure suggested that Val389 and Leu4i5 were potentially critical IDOL residues mediating hydrophobic interactions with UBE2D1 (see Figure 4E). In order to validate these predictions, they expressed LDLR together with native or tagged IDOL mutants in HEK293 cells. Compared to WT IDOL, the IDOL mutants V389R and L415E exhibited reduced capacity for LDLR degradation (see Figure 6A). Interestingly, the auto-degradation of IDOL was also clearly inhibited by the introduction of the V389R and L415E mutations. The inventors also performed an in vitro autoubiquitination assay using IDOL V389R. Consistent with the cellular results, UBE2D2 was unable to efficiently catalyze the polyubiquitination of the IDOL V389R mutant (see Figure 6B). Furthermore, introduction of mutations in Pro6i and Phe62 in UBE2D2 (P61A, F62R), residues which are buried at the hydrophobic E2-E3 interface, also inhibited the ability of UBE2D2 to support IDOL autoubiquitination (see Figure 6C).

Mutational analyses also validated our model for IDOL-UBE2D specificity outlined in Figure 5. Residue GIU383 in IDOL interacts with a basic pocket formed by Argis and Lys8 of UBE2D. Mutation of these basic residues in UBE2D2 (R15E, K8E) reduced the ability of UBE2D2 to support IDOL autoubiquitination in vitro (see Figure 6D). The model further predicts that Ser94 of UBE2D, which makes an important hydrogen bond with the IDOL backbone, is a key determinant of specificity. UBE2L3, which is unable to pair with IDOL, has a lysine residue in this position (see Figures 5B and C). In support of the model, a UBE2D2 S94K mutant exhibited markedly reduced ability to catalyze IDOL autoubiquitination.

As the crystal structure of the IDOL RING domain-UBE2D complex revealed that IDOL could form a dimer via the residues within the RING domain, the inventors performed co-immunoprecipitation experiments to validate the physiological relevance of this finding. They found that when co-expressed with a Vs-tagged IDOL in HEK293 cells, TAP-IDOL could be co-immunoprecipitated with the Vs-tagged IDOL (see Figure 6E). These data indicated that the two different tagged versions of IDOL could form a complex in the cell. Furthermore, introduction of structure- guided mutations predicted to disrupt dimer formation (V431R/L433R) abolished the ability of TAP-IDOL and V5-IDOL to be co-immunoprecipitated from cells (see Figure 6F). Moreover, IDOL dimer formation appears to be essential for its biological function, because the dimer-defective mutant V431R/L433R was unable to promote LDLR degradation and was resistant to auto-catalyzed degradation (see Figure 6G).

Based on these results, the inventors reasoned that overexpression of IDOL mutants not capable of interacting with its cognate E2 should interact with and sequester wild-type IDOL molecules, thereby preventing them from participating in ubiquitin transfer. Such mutant IDOL proteins should therefore function as dominant negatives. To test this hypothesis, they co-expressed increasing amounts of IDOL V389R with a predetermined amount of WT IDOL and LDLR in HEK293 cells.

Indeed, expression of IDOL V389R inhibited the degradation of LDLR by the WT IDOL in a dose-dependent manner (see Figure 6H). Furthermore, the auto- degradation of WT IDOL was also inhibited by the expression the V389R mutant.

Example 7 - IDOL is an iron-binding protein

Immediately amino-terminal to the crystallized IDOL RING construct there are three cysteine residues (Cys36o, Cys363 and Cys368; see Figures 7A and B). Expression and purification of an extended RING domain containing residues 358-445 yields a brown protein (see Figures 7C, D and E). Other constructs of IDOL containing this region, including the full-length protein, are also brown (data not shown). Atomic absorption spectroscopy was performed on both IDOL RING constructs to measure the metal content in comparison to zinc and iron standards. For the shorter 369-445 construct, a 0.24 mM protein sample gave a zinc concentration of 0.47 mM and an iron concentration of 0.012 mM; for the longer 358-445 construct a 0.15 mM protein sample gave a zinc concentration of 0.3 mM and an iron concentration of 0.06 mM (see Figure 7C). These concentrations correspond to two zinc ions per protein molecule and one iron ion per dimer for the longer protein. Protein disorder prediction of IDOL using RONN (Yang et al. 2005) suggests that most of IDOL is ordered but that amino-acids 332-371 is inherently disordered. It is therefore possible that this region contributes to the dimerization interface when folded around an iron ion.

In order to address the potential functional significance of this iron-binding region, the inventors introduced mutations into each of the 3 cysteine residues, alone or in combination (see Figure 7F). Unexpectedly, these mutations led to increased LDLR degradation activity compared to WT IDOL. Analysis of TAP-IDOL expression revealed that the increased LDLR degradation in these experiments likely resulted from increased IDOL stability. Levels of IDOL protein expression were increased in an additive manner with mutation of C360, C363 and C368 (see Figure 7F), with the most prominent effects observed with the triple mutant (IDOL AAA) (see Figure 7G). The data suggest that the structure of the IDOL protein in the presence of iron may be more conducive to autoubiquitination and degradation. Finally, LDL uptake studies confirmed the functional consequences of the

dimerization, E2-interaction and iron-binding mutations. The E2-interaction mutants (V389R and L415E) and the dimerization mutant (L433R/V431R) reduced IDOL activity, whereas the iron-binding mutant (IDOL AAA) actually increased IDOL's ability to block LDL uptake (see Figure 7H). Thus, the combination of biochemical and structural analyses has defined molecular interactions critical for the LXR-IDOL-LDLR sterol regulatory pathway.

Discussion

In Examples 1 to 7, the inventors have identified the ubiquitin-conjugating enzyme E2D family proteins (UBE2D1-4) as the E2 ubiquitin carrier proteins involved in IDOL-dependent LDLR ubiquitination. The results provide strong evidence that IDOL directly facilitates the transfer of ubiquitin to LDLR by acting in a complex with UBE2D. The inventors also successfully carried out a biochemical and structural characterization of the E2-E3 complex and demonstrated that disruption of UBE2D activity or the interaction interface between UBE2D and IDOL inhibits the degradation of the LDLR. These results provide a better understanding of the molecular mechanism underlying the sterol-dependent regulation of LDLR protein levels. Since the LDLR is a membrane protein, it is challenging to study IDOL- LDLR interaction in a cell free system. The available assays for IDOL-dependent LDLR ubiquitination were not amenable to the screening of potential E2 enzymes. They therefore employed an alternative approach that assayed the auto-ubiquitination of IDOL in vitro. Auto-ubiquitination is characteristic of RING-type E3 ligases. It is achieved via the same chemical reaction as the ubiquitin-substrate ligation and mediated by the same E2 protein (Yang et al. 2000). The inventors screened 19 candidate E2 enzymes previously identified as preferentially interacting with the RING-type E3 ligases (Markson et al. 2009; van Wijk et al. 2009). Of these, proteins in the UBE2D family were the only ones that were able to catalyze IDOL auto- ubiquitination in vitro. Although the inventors initially identified the IDOL-UBE2D interaction based on IDOL autoubiquitination, several lines of evidence indicate that the UBE2D family enzymes also mediate the ubiquitination and degradation of the LDLR. The inventors showed that UBE2D2, together with recombinant El and purified IDOL, was able to induce ubiquitination of LDLR in cell-free membrane preparations in vitro. In addition, they demonstrated that the inhibition of UBE2D activity by over- expressing a dominant negative UBE2D enzyme inhibited the ability of IDOL to degrade the LDLR in cells. These dominant negative enzymes are postulated to function by interacting with the E3 enzyme and consequently prevent it from associating with endogenous E2 (Gonen et al. 1999).

The inventors also successfully obtained the crystal structure of the IDOL RING domain-UBE2D complex. The E2 ubiquitin-conjugating enzymes are structurally related and they share a conserved core domain with about 150 amino acids harboring the cysteine residues required for the formation of the ubiquitin-E2 thioester intermediate (Zheng et al. 2000). Binding of an E2 to a RING-type E3 is dependent on the E3 RING finger domain, which contains one histidine and seven cysteine residues that coordinate with two zinc ions (Joazeiro and Weissman 2000). The RING-based E3S share many structural similarities in their RING domains, as do different E2s in their E2 core domains. Consequently, the biophysical basis for the specific functional pairings between E2s and E3S in E3 auto-ubiquitination as well as the ubiquitination of substrates has been a long-standing puzzle. Careful

examination of the structure of the complex and comparison of the sequences of the E2s that do and do not support IDOL activity has enabled the inventors to identify residues at two key positions at the interface that appear to determine specificity.

However, it is important to note that specificity may not only be dependent upon the stereochemistry of the interface, but may also require optimal dynamics of association /dissociation. This is important because it is well-established that the El and E3 binding surfaces on the E2 are overlapping and that binding is mutually exclusive. This fits well with the observation that the interface between the IDOL

RING and UBE2D1 is relatively small (1140 A2), which is consistent with the reported dissociation constants of interaction of RING domains with E2s typically greater than 100 μΜ (Ozkan et al. 2005; Das et al. 2009). Thus, the dynamics of E2:E3 and E2:Ei interactions play a role in controlling ubiquitination of the target protein (van Wijk and Timmers 2010). Too tight an E2:E3 complex would block the E2:Ei interaction and vice versa.

The finding that IDOL is an iron-binding protein raises the question as to whether the iron is regulating the activity of the protein. It is provocative to note that iron has been implicated in heart disease (Sullivan 1996) and that studies of iron depletion show a lowering of LDL-cholesterol (Facchini and Saylor 2002).

Based on the information provided by the crystal structure of the UBE2D-IDOL complex, the inventors found that disruption of the interaction interface between IDOL and UBE2D not only inhibited the autoubiquitination of IDOL in an in vitro assay, but also prevented functional ubiquitination and degradation of the LDLR mediated by IDOL in cells. It has been generally assumed that the binding between an E2 and an E3 is the primary determinant of a functional E2-E3 pair. However, it has been shown that although c-Cbl and UbcH7 form a complex, UbcHsB, rather than UbcH7 appears to be the functional E2 for the c-Cbl-mediated ubiquitination (Huang et al. 2009). It has also been reported that the BRCA1/BARD1 E3 heterodimer can interact with UbcHsC and UbcH7 with similar affinity, but only UbcH5C is active in ubiquitination assays (Brzovic et al. 2003). Results from these studies suggest that the physical binding between E2-E3 pairs is not sufficient to infer biological function, highlighting the importance of complementary structural and functional assays of E2-E3 interactions.

Summary

The inventors have previously identified the E3 ubiquitin ligase IDOL as a sterol- dependent regulator of the LDL receptor (LDLR). The molecular pathway underlying IDOL action, however, remains to be determined. The inventors have now identified compelling biochemical and structural characterization of an E2-E3 ubiquitin ligase complex for LDLR degradation. They identified the UBE2D family (UBE2D1-4) as E2 partners for IDOL that support both auto-ubiquitination and IDOL-dependent ubiquitination of the LDLR in a cell-free system. NMR chemical shift mapping and a 2.1A crystal structure of the IDOL-RING domain-UBE2Di complex revealed key interactions between the dimeric IDOL protein and the E2 enzyme. Analysis of the IDOL-UBE2D1 interface also defined the stereochemical basis for the selectivity of IDOL for UBE2Ds over other E2 ligases. Structure-based mutations that inhibit IDOL dimerization or IDOL-UBE2D interaction block IDOL-dependent LDLR ubiquitination and degradation. Furthermore, expression of a dominant-negative UBE2D enzyme inhibits the ability of IDOL to degrade the LDLR in cells. These results identify the IDOL-UBE2D complex as an important determinant of LDLR activity and provide insight into molecular mechanisms underlying the regulation of cholesterol uptake.

Example 8 - The IDOL FERM domain binds directly to its targets

To improve the understanding of the mechanism whereby IDOL triggers degradation of LDLR, apoER2 and VLDLR, the inventors performed a series of structure-function analyses. IDOL contains two distinct domains: a C-terminal really interesting new gene (RING) domain, defining it as a member of the RING E3 ligase family; and an N-terminal FERM (Band 4.1, ezrin-radixin-moesin) domain, a putative protein- protein interaction motif. The IDOL FERM domain is comprised of a classic tri- domain structure common to FERM proteins comprising three independently folded domains F1-3 with the F3 domain having a structure similar to that of a PTB

(phospho/tyrosine binding) domain. This PTB-like domain is typically involved in the interaction with cytoplasmic tails of plasma membrane proteins, commonly via an NPxY motif (with additional N-terminal sequences enhancing the affinity and specificity. Despite the name, PTB domains can have specificity for either

phosphorylated or non-phosphorylated tyrosine. Sequence alignments of IDOL with other FERM domain-containing proteins, including Talin, Radixin and Moesin, revealed that, although the FERM domain would normally be expected to be around 290 residues, the sequence homology extends to residue 344 of IDOL, with an apparent insertion within the F3 domain (residues 215-272, designated subdomain F3b) based on the alignment with Talin (see Figures 19A-C). Further examination and secondary structure prediction suggested that there might be a duplication of the C-terminal portion of the F3 PTB domain; i.e. the F3b and F3C subdomains share significant homology.

Functional analysis indicated that each of the FERM subdomain regions is required for IDOL-mediated degradation of the LDLR, as deletion of any of them abrogated the ability of FLAG-tagged IDOL to promote LDLR degradation in an HEK293T cell co-transfection assay (Figure 12A). These data indicate that, in addition to the RING E3 ligase domain, the FERM domain plays an essential role in the IDOL mechanism of action. The inventors hypothesized that the FERM domain in IDOL might function as a mechanism for the specific recognition of targets for IDOL-dependent degradation. To directly test whether a direct interaction occurs between the IDOL FERM domain and the cytoplasmic C-terminal tail of its target proteins, the inventors used a fluorescence polarization (FP) assay to monitor binding of the IDOL FERM domain to a fluorescently-labeled synthetic peptide, based on the VLDL receptor (residues 820-842). VLDLR was chosen over LDLR as the model peptide since LDLR contains a cysteine residue in the tail and the peptide is labeled by means of conjugation to a terminal cysteine residue. The inventors observed clear binding to VLDLR peptide but not to control non-specific peptide in this assay and the data fit a single-site binding model (see Figure 12B). The dissociation constant for the interaction between the IDOL FERM domain (1-273) and the VLDLR peptide was 15.8 μΜ +/- 0.5. Although this is a relatively weak interaction (and was not detected in membrane-free cell-based assays or conventional pull-down assays; data not shown), it is nevertheless a relatively tight interaction compared with other FERM domain interactions and therefore significant (Anthis et al. 2009)

Structural homology models of the IDOL FERM domain were generated using PHYRE (Kelley LA and Sternberg. 2009). The inventors generated two models with different F3 sub-domain assignments: 1-344 with the deletion of residues 215-272 and 1-276 lacking residues 277-344. These two regions are indicated F3a:F3c and F3a:F3b, respectively, in Figure 13A. The inventors used these alternative models to generate predictions of residues that might be important for the recognition of the LDLR cytoplasmic tail, based on the known mode of interaction between the talin FERM domain and the beta-integrin cytoplasmic tail (see Figure 13B).

To test which of the two models was correct, the inventors introduced designed mutations into the surfaces that could potentially be involved in the interaction with the LDLR. Since there are no antibodies currently available that are capable of efficiently detecting native IDOL protein, and since epitope tags have the potential to affect protein function and interaction, they performed their initial analyses using native IDOL constructs. Construct expression was monitored in these studies by RNA expression (not shown). Mutation of the key amino acids denoted in Figure 13A demonstrated that Y265A and T269R, both of which reside in the IDOL F3b subdomain, appear to be especially important for IDOL-induced degradation of the LDLR (see Figure 13B, left panel). Indeed, the lack of ability of Y265A and T269R to degrade the LDLR was comparable to that of the RING domain mutant IDOL, (C387A) which lacks ubiquitination activity (Zelcer et al. 2009). When lower levels of IDOL were used in the degradation assay, it became apparent that Q232 was also involved in IDOL action, as the Q232A mutant also led to a substantial reduction in activity on the LDLR (see Figure 13B, right panel). Mutations of M285 and Y323, which lie in the F3C subdomain, had a more modest effect on LDLR degradation and an R327 mutant was comparable to WT in activity. These data suggest that the FERM 3b subdomain of IDOL is particularly important for interaction with other proteins and that the model of IDOL FERM domain structure shown on the left in Figure 13A is the correct one.

Given that the LDLR internalizes LDL particles and reduces plasma LDL cholesterol levels when expressed at the cell surface, the inventors assessed the effects of IDOL FERM domain mutants on the degradation of surface LDLR protein levels. Using a biotin-labeling approach they found that the T269R and Y265A mutants were defective in their ability to clear LDLR protein from the plasma membrane, consistent with their observations with total cellular LDLR (see Figure 14A). To test the functional consequence of these mutations, they assayed the uptake of fluorescently labeled LDL particles by transfected 293T cells. As expected, the inhibitory activity of IDOL T269R on LDL uptake was dramatically reduced compared to WT IDOL (see Figure 14B). Q232A IDOL exhibited a partial defect, consistent with its effects on LDLR protein.

In order to further validate the effects of the IDOL mutants on endogenous LDLR degradation, the inventors stably expressed the WT IDOL, RING mutants or F3b domain mutants in IDOL null (IDOL-/-) mouse embryonic fibroblasts (MEFs).

Figure 14C demonstrates that in the presence of lipoprotein-deficient serum (LPDS), stable expression of WT IDOL or IDOL A273E (a mutant with an intact ability to degrade LDLR) was associated with lower LDLR expression than control IDOL-/- MEFs. In contrast, cells stably expressing RING mutant, Q232A, Y265A or T269R IDOL all exhibited greater LDLR abundance. Furthermore, they observed reduced uptake of LDL particles in IDOL-/- MEFs expressing WT and A273E IDOL compared to those expressing RING mutant, Q232A, Y265A or T269R (see Figure 14D). Example 9 - The FERM F3b subdomain of IDOL is required for LDLR ubiquitination but does not affect intrinsic E3 ligase activity Given that IDOL is an E3 ligase, the lack of degradation and subsequent increased uptake of LDL particles associated with mutants in the IDOL F3b subdomain would be consistent with a reduced ability to ubiquitinate the LDLR. To test this hypothesis, the inventors performed immunoprecipitation assays. The data in Figure 14E confirmed enhanced ubiquitination of the LDLR in the presence of WT and

A273E IDOL. However, mutants Y265A and T269R showed markedly reduced ability to ubiquitinate LDLR. Note, background levels of ubiquitinated LDLR in the absence of transfected IDOL constructs (lane 1) reflect endogenous IDOL activity in 293 cells. In order to address potential differences in the expression or stability of the various mutants employed above, the inventors repeated their analysis using tagged IDOL constructs. None of the F3b mutants substantially altered the abundance of FLAG- IDOL protein, but Y265A and T269R again showed markedly reduced ability to degrade the LDLR (Figure 14F and data not shown). Furthermore, mutations in the IDOL protein that affect intrinsic E3 ligase activity (such as the RING mutant) would be expected to lead to stabilized IDOL protein due to loss of autoubiquitination (Zelcer et al. 2009). The observation that all of the FERM domain mutants in Figure 14F showed comparable stability to WT IDOL indicated that the intrinsic E3 ligase activity of these mutants was intact. Thus, these mutants appear to decouple LDLR recognition from intrinsic E3 ligase activity, and strongly suggest that the FERM domain is involved in LDLR recognition (see below).

Example 10 - A conserved SI/MxF sequence as an IDOL recognition motif

The inventors have previously shown that the 50 amino acid cytoplasmic tail of the LDLR is required for IDOL-induced degradation (Zelcer et al. 2009; Hong et al.

2010). Since only three proteins (LDLR, VLDLR and apoER2) appear to be targeted by IDOL for degradation, the inventors hypothesized that these proteins must harbor a specific recognition sequence. To identify amino acids within the tail that make up the IDOL degradation motif, they combined sequence analysis and structural modeling (see Figure 15A, B and C). They hypothesized that the IDOL FERM domain might bind directly to lipoprotein receptor tails and generated a homology model of the IDOL FERM domain (1-276) with PHYRE using the structure of the Protein 4.1R core domain (PDB ID: igg3). The LDLR cytoplasmic tail was docked with reference to the structures of talin in complex with layillin, ΡΙΡΚΙ-γ and integrin- iD (PDB IDs: 2koo, 2G35 and 3G9W respectively). The resulting model suggests that F823 and I821 in the LDLR cytoplasmic tail should be key residues mediating the interaction with IDOL F3b subdomain (see Figures 15B and 15C). Interestingly, the model of IDOL reveals a pocket adjacent to residues Y265 and T269 that is not present in other PTB domains (see Figure 15C). The phenylalanine, F823 at the -5 position relative to the NPVY motif (where Y is position o) on the LDLR tail is positioned optimally to fit into this pocket. Another key determinant suggested from the complex model was the interaction with I821 of the LDLR tail interacting with a non-polar surface on the surface of the FERM domain (see Figure 15B).

Extensive site-directed mutagenesis of the LDLR tail led to the identification of 3 amino acids that conferred resistance to IDOL-mediated degradation when mutated (see Figure 15D and Figure 20). These data served to strongly validate the structural modeling. Point mutants in the phenylalanine at position 823 (F823A) and the isoleucine at position 821 (I821E) appeared to be completely resistant to degradation, whereas mutation of the serine at position 820 (S820D) was partially resistant (see Figure 15C). It is noteworthy that basal LDLR protein levels observed following transfection of constructs containing the F823A mutation were consistently higher compared to WT LDLR constructs (see Figure 15D and Figure 20A). This

observation is consistent with enhanced LDLR stability due to lack of degradation by endogenous IDOL. Mutation of other conserved residues in the LDLR tail, including the key tyrosine in the NPVY internalization motif (Y828C; see Figure 20A), did not inhibit LDLR degradation. A chimeric protein in which LDLR was fused to GFP just after Y823 retained its ability to be degraded by IDOL, but a fusion after I821 (lacking F823) was resistant, indicating that the sequences upstream of Y823 are sufficient for IDOL targeting (see Figure 20B). Transfection of these mutated LDLR constructs into 293T cells revealed that resistance to IDOL-dependent degradation also translated to resistance to IDOL-dependent inhibition of LDL uptake (see Figure 15E).

To establish a link between the LDLR SI/MxF motif and IDOL-dependent

ubiquitination, the inventors analyzed the ability of WT and mutant LDLR proteins to be ubiquitinated in 293T cells. Mutations at positions F823, 1821 and S820 each led to reduced ubiquitination by IDOL (see Figure 15F), consistent with the results of the degradation assays.

To verify the importance of the SI/MxF motif for endogenous LDLR degradation, the inventors stably expressed WT or mutant lipoprotein receptors in LDLR-/- MEFs. Treatment with the LXR agonist GW3965, known to induce IDOL expression (Zelcer et al. 2009), effectively reduced the expression of WT LDLR and inhibited LDL uptake, but had little effect on any of the LDLR mutants (see Figure 20C and data not shown). Example 11 - A conserved IDOL recognition site in ApoER2 and VLDLR

Surprisingly, both F823 and S820 are conserved across all three IDOL targets (see Figure 15A). In place of the isoleucine, the VLDLR and ApoER2 have a conservative substitution of the neutral and non-polar methionine. The inventors investigated the importance of the amino acids corresponding to F823 and S820, as well as the methionine aligning with I821, for degradation of VLDLR and ApoER2. For both receptors, mutation of the phenylalanine and the methionine rendered the receptor resistant to degradation, further suggesting that these amino acids are part of the IDOL recognition motif (see Figure 15G and 15H). In contrast, mutation of the equivalent serine of S820 in the LDLR had a minor effect on VLDLR degradation (S829D) and a small effect on ApoER2 degradation (S858D). Again, these results are good agreement with the structural modeling pointing to the particular importance of the -5 position (relative to the NPxY motif) for IDOL recognition of LDLR, VLDLR and apoER2. Together, the results of Figures 13-15 define structural determinants for IDOL target recognition that explain the stringent specificity of this unusual E3 ligase.

Example 12 - Key residues in the IDOL FERM domain and LpR are functionally conserved

Given that integral physiological processes tend to be conserved through evolution, the inventors utilized protein sequence alignment to examine whether the key IDOL FERM residues were conserved across species. Figure 16A demonstrates that the most important residues for LDLR recognition in the IDOL F3b subdomain are conserved in vertebrates as well as in the insect IDOL homolog, DNRi. To determine whether the function of these residues was also conserved, the inventors examined the effect of mutation in these residues in drosophila DNRi. Consistent with previous observations, DNRi was capable of degrading human LDLR when expressed in 293T cells (Hong et al. 2010). Remarkably, however, DNRi point mutations in the tyrosine and threonine residues corresponding to human Y265 and T269 (drosophila Y405 and T409) were associated with reduced ability to degrade the LDLR compared to WT DNRi (see Figure 16B). The reduced inhibitory activity towards LDL uptake of DNRi Y405A and T409R compared to WT DNRi further confirmed the functional importance of these residues for regulation of cholesterol uptake (see Figure 16C). Sequence alignment also revealed conservation of the SI/MxF motif in LDLRs across vertebrate species (see Figure 16D). Remarkably, this sequence was even present in the lipophorin receptor (LpR), the major lipoprotein carrying receptor in insects. As shown in Figure 15E, IDOL promoted the degradation of LpR in 293T cells, indicating that this receptor can indeed be recognized by the FERM domain.

Furthermore, consistent with the mutational analysis of VLDLR and ApoER2, the phenylalanine residue corresponding to LDLR F823 (F992) was critical for IDOL- dependent degradation of LpR. Thus, key aspects of the IDOL mechanism of action on the LDLR appear to be highly conserved through evolution.

Example 13 - IDOL ubiquitinates itself on the FERM 3c subdomain

As is common for E3 ligases, IDOL controls its own by stability through

autoubiquitination, presumably of one or more lysine residues. By introducing a series of point mutations at lysine residues throughout the IDOL protein, the inventors identified specific sites in the F3C subdomain that appear to influence the abundance of IDOL protein (see Figure 17A). Interestingly, K293R and K309R had the greatest influence on IDOL abundance (see Figure 17B) and are also the most highly conserved of the lysines in the F3C subdomain (see Figure 17A). Subsequent compound mutations, each including K293R and K309R, were all associated with increased IDOL abundance compared to WT, as well as increased degradation of the LDLR (see Figure 17C). The abundance of each of the 4X lysine mutants was similar to that of RING mutant IDOL (which does not undergo autoubiquitination), suggesting that mutation of these lysines is sufficient to inhibit IDOL

autodegradation. Studies in the absence or presence of the proteasomal inhibitor, MG-132 revealed little difference in the protein levels of each of the 4X mutants, further confirming that they are no longer undergoing proteasomal degradation (see Figure 21). Analysis of 293T cells treated with MG132 confirmed that mutation of lysine residues in the F3C subdomain strongly reduced IDOL autoubiquitination as predicted (see Figure 17D).

Interestingly, drosophila DNRi also appears to undergo RING domain-dependent autodegradation in both drosophila S2 cells and mammalian 293 cells (see Figure 17E and data not shown). Alignment of the DNRi and IDOL F3C domains revealed two conserved lysine residues (see Figure 17A). Mutation of the highly conserved lysine corresponding to IDOL K293 (K433R) increased DNRi protein stability and increased LDLR degradation, consistent with reduced capacity for autoubiquitination and degradation (see Figure 17E). Thus, the mechanism for IDOL protein turnover also appears to be largely conserved from insects to humans.

Example 14 - Membrane context is important for IDOL-dependent LDLR

degradation

Since the LDLR is a transmembrane protein, if IDOL interacts directly with the LDLR, it must do so in the context of the plasma membrane or membrane vesicles such as endosomes. Interestingly, IDOL was unable to promote the degradation of a fusion protein consisting of the LDLR cytoplasmic domain fused to GFP (see Figure 22). This observation suggested that a simultaneous IDOL-membrane interaction might be required for efficient LDLR recognition and ubiquitination. Consistent with this hypothesis, the inventors were unable to demonstrate a stable interaction between the soluble LDLR cytoplasmic tail and IDOL in immunoprecipitation assays (data not shown). To investigate the ability of IDOL to interact with membrane- bound LDLR, they established a membrane interaction assay. They isolated membrane fractions from 293T cells transfected with LDLR and WT or FERM domain mutant IDOL constructs. They then analyzed the ability of IDOL to associate with these membranes by immunoblotting. Figure 18A demonstrates that the abundance of WT and mutant (Q232A, Y265R) IDOL proteins in total 293T cell lysates from transfected cells was similar. However, in isolated membrane fractions, the inventors readily detected the presence of WT IDOL protein in cells transfected with LDLR, but not those transfected with vector alone, suggesting that IDOL associates with membrane fractions in an LDLR-dependent manner. Furthermore, the Q232A and Y265R IDOL mutants, which were defective in LDLR degradation, showed reduced ability to associate with the membrane fraction (see Figure 18A). Note, even in the presence of much higher levels of LDLR in the membrane (due to lack of IDOL degradation), very little mutant IDOL was recovered in the membrane fraction. The cell-based assays suggested that IDOL requires membrane-inserted LDLR tail for a tight and measurable interaction. To explore this further, the inventors employed in vitro assays that are able to detect weak but relevant protein-lipid and protein- peptide interactions. Their finding that IDOL localizes to membranes in an LDLR- dependent manner raised the question as to whether the FERM domain interacts directly with membranes. Structural modeling suggested that, similar to other FERM proteins such as Talin and Radixin, the IDOL FERM domain has a high proportion of positively charged residues, predominantly on one face of the protein (see Figure i8B). This face is predicted to be proximal to the plasma membrane. To determine whether there was a direct interaction between the IDOL FERM domain and the membrane, the inventors performed vesicle cosedimentation assays in which a solution containing protein and vesicles was separated by centrifugation into a pellet consisting of vesicles plus bound protein and a supernatant containing unbound material (see Figure i8C).

In the absence of vesicles or in the presence of neutral phosphatidylcholine vesicles (or even vesicles containing 20% negatively charged phosphatidylserine), the IDOL FERM remained in the supernatant fraction. However, increasing the negatively charged content of the vesicles to ioo% phosphatidylserine caused 8o% of WT IDOL to precipitate with the vesicles. Phosphatidylserine was used due to its negatively charged head groups that had previously been shown to be a good model when the mode of binding is due to the general interaction with negatively charged lipid head groups (Goult et al. 2010). Interestingly, the interaction of IDOL with these vesicles was considerably weaker than that of the Talin FERM domain (see Figure 18C). This is suggestive of a more transient IDOL-LDLR-membrane interaction, and is consistent with the requirement for the LDLR tail in the cell-based membrane association assays.

To confirm that membrane-facing residues in the IDOL FERM domain were important for LDLR degradation, the inventors performed cosedimentation assays and LDLR degradation assays with mutant IDOL proteins. A FLAG-IDOL expression construct encoding R73E/K75E (residues in subdomain Fi) showed a partial reduction in LDLR degradation activity and a R193E/K199E/R259E mutant

(subdomain F3) construct showed a prominent deficit (see Figure 18D). A

K137E/K146E (subdomain F2) mutant effectively degraded the LDLR, indicating that the primary membrane contacts are mediated by the Fi and F3 subdomains.

Cosedimentation assays further verified the importance of the F3 domain residues (R193/K199/R259) for association with phospholipid vesicles (see Figure 18C).

These results strongly suggest that IDOL FERM interaction with cellular membranes is important for efficient targeting and therefore subsequent degradation of the LDLR. Taken together, the results suggest that the IDOL FERM domain mediates direct interactions with negatively charged membrane surfaces and with the cytoplasmic peptides of its targets. High affinity interaction, sufficient to be detected in cell-based assays, clearly requires both membrane and the LDLR tail. Discussion

Examples 8 to 14 provide compelling data concerning the FERM-dependent E3 ligase recognition. Induction of the E3 ubiquitin ligase IDOL in response to sterol activation of LXR provides a complementary pathway to SREBPs for feedback inhibition of the LDLR pathway. However, until the present invention, the mechanism by which IDOL specifically targets the LDLR had not been elucidated. The inventors previously showed that increased IDOL levels in cells correlate with LDLR

ubiquitination, but a central unresolved issue has been the question of whether IDOL interacts directly with the LDLR, or whether the primary target for binding or ubiquitination is an intermediate protein. Surprisingly, the inventors have now demonstrated that IDOL binds directly to the cytoplasmic tails of lipoprotein receptors and cellular membranes through its FERM domain. Furthermore, they have defined the structural requirements for these interactions using complementary cell-based, computational and biochemical approaches. The inventors also establish that FERM domain binding to both membrane and the LDLR tail is critical for the ability of IDOL to trigger ubiquitination and degradation of the LDLR receptor.

These studies provide mechanistic insight into sterol-dependent regulation of lipoprotein receptor expression. In the ubiquitin system for protein degradation, the role of the E3 ligase is to confer target specificity. Regulation of E3 targeting can be achieved by changes in abundance or activity via transcriptional or post-translational mechanisms.

Ubiquitin-mediated protein degradation often involves direct binding of the E3 and subsequent ubiquitination the target, but this is not always the case. For example, cbl-mediated degradation of the EGFR requires an intermediate factor. Since IDOL is the only E3 ligase that contains a FERM domain (Zelcer et al. 2010), the inventors postulated that this domain may be responsible for target recognition. Structural homology modeling and mutagenesis studies revealed that FERM subdomains (denoted F3b and F3C) harbor the critical residues for lipoprotein receptor recognition. Interestingly, the F3bc subdomains do not align with other FERM domain sequences. Thus, the structural basis for IDOL target recognition is unique amongst FERM domain proteins.

The inventors have described lysine residues within LDLR, VLDLR and apoER2 that serve as sites for ubiquitination. However, the recognition sequence for IDOL in its target proteins has remained elusive. E3 ligase recognition signals are commonly short amino acid sequences, such as RXALGIXIXN in the case of the destruction box, the first ubiquitination signal to be identified (Glotzer et al. 1991). The LDLR has previously been shown to interact with proteins that have a PTB-like domain such as sorting nexin-17 (SNX17). This interaction requires residues within and downstream of the NPVY endocytosis signal in the LDLR cytoplasmic tail (Burden et al. 2004). FERM-NPxY interactions are common among membrane proteins, such as that reported for the Talin FERM F3 domain and integrin (Garcia- Alvarez et al. 2003). However, the F3b subdomain of IDOL binds to a distinct recognition sequence (⁸²⁰SI/MXF⁸²3) immediately N-terminal to the NPxY motif in the cytoplasmic tail of the LDLR. In fact, mutation of each of the 4 residues in the NPVY sequence of LDLR, or complete deletion of this motif, had no effect on IDOL-induced degradation, suggesting that the IDOL degradation pathway may be independent of clathrin- mediated endocytosis.

The structural modeling also strongly supported the importance of the -5 position (relative to the NPxY motif) to the specificity of IDOL for LDLR, VLDLR and apoER2. The model predicts the existence of a pocket in the F3b subdomain adjacent to the critical amino acids required for target degradation (Y265 and T269) that

accommodates the phenylalanine residue in -5 position of the LDLR, VLDLR and apoER2 cytoplasmic tails. Interestingly, the sequence motif FxNPxY only occurs in 13 other proteins of which only one, seizure6-like protein, is located in a cytoplasmic tail of a membrane protein. Seizure6-like lacks the key ubiquitinated lysine, however, and is therefore unlikely to be an IDOL target. Disabled-i (DABi) also binds the LDLR cytoplasmic tail and the structure of DABi in complex with the apoER2 cytoplasmic tail has been solved (PDB ID: iNTV). In this structure the phenylalanine equivalent to F823 in the LDLR also seems to play an important role in the interaction.

Fluorescence polarization-based interaction assays confirmed that the IDOL FERM binds to a single site in lipoprotein receptor tails. Moreover, the structural requirements for this interaction are in agreement with the amino acid sequence requirements for IDOL-dependent LDLR degradation.

Despite the fact that the LDLR internalization factor ARH readily associates with the soluble LDLR cytoplasmic tail in biochemical assays, the inventors' attempts to co- immunoprecipitate IDOL with the soluble LDLR tail were unsuccessful. This led them to hypothesize that the cell membrane may be a key component of IDOL- lipoprotein receptor interactions. Indeed, they found that IDOL interacts with negatively charged phospholipid membranes, although this association is weaker than that reported for talin (Anthis et al. 2009). They further defined positively charged residues on the predicted membrane-facing surface of the IDOL FERM domain important for this interaction. The IDOL-membrane interaction may serve several different purposes. Since the affinity between IDOL and the LDLR tail is relatively weak, simultaneous membrane interaction likely provides stability to the complex. In addition, by helping IDOL to localize with its targets in the cells, membrane association of IDOL imparts a spatial constraint on IDOL-dependent protein degradation. Finally, it is likely that the FERM-membrane interaction positions IDOL in the correct orientation to bind lipoprotein receptor tails.

The analysis also revealed a distinct regulatory function for the FERM F3C subdomain: regulation of IDOL protein stability. Autoubiquitination is a strategy employed by many E3 ligases as a means of regulating turnover. The inventors defined a series of lysine residues in the IDOL F3C subdomain domain critical for autoubiquitination and subsequent proteasomal degradation. Furthermore, mutation of these residues allowed them to establish that IDOL autoubiquitination is functionally independent of LDLR degradation. Indeed, an IDOL protein lacking the F3C ubiquitinated lysines is resistant to proteasomal degradation, but shows enhanced ability to degrade the LDLR. Although certain other E3 ligases have been reported to undergo autoubiquitination on a single residue (e.g. cyclin Di), promiscuity is also commonly observed, as E2-E3 ligase complexes often favor lysine accessibility rather than sequence context once they have been recruited to their specific targets (Danielsen et al. 2011). The clustering of lysine ubiquitination targets in the F3C subdomain suggests that this region may be particularly accessible to

RING domain-catalyzed ubiquitin transfer (see the schematic model shown in Figure 23)·

The central role of the IDOL FERM domain in LDLR recognition is in line with recent studies showing that a non-synonymous single nucleotide polymorphism in this domain (N342S) is associated with total cholesterol levels in humans. The presence of a serine at this site results in attenuated ubiquitination of the LDLR and a concomitant increase in LDLR expression and uptake of LDL particles (Weissglss- Volkov, JCI). However, compared with mutation of key residues in the F3b subdomain (Y265 and T269) the N342S change exerts more modest functional effect. These data suggest that residues in the F3C subdomain may make secondary contacts with the LDLR that stabilize the complex or that the N342S polymorphism may lead to conformational changes that affect the F3D-LDLR interaction. The fact that an IDOL protein lacking the F3C domain is inactive also supports the hypothesis that this region is important for the overall conformation of the FERM domain. Although LXR nuclear receptors are not present in organisms lower than vertebrates, the IDOL pathway for lipoprotein receptor degradation is conserved in insects. The inventors have shown here that the same molecular strategy for recognition of the LDLR receptor by IDOL is employed by the drosophila DNRi E3 ligase to bind the insect lipophorin receptor (LpR). Key residues predicted by the structural modeling to be involved in FERM domain-lipoprotein receptor interactions are conserved between IDOL and DNRi. Furthermore, the IDOL SI/MxF recognition sequence is conserved in the mammalian IDOL targets (LDLR, apoER2 and VLDLR) and the insect receptor LpR. Thus, the IDOL FERM-LDLR interaction is an evolutionarily- conserved mechanism for the post-translational control of membrane lipoprotein receptor activity.

Summary

The E3 ubiquitin ligase IDOL is an important regulator of cholesterol uptake, but its mechanism of action, including the molecular basis for its stringent target specificity, is poorly understood. The inventors have also shown that IDOL employs a unique strategy among E3 ligases for target recognition. The IDOL FERM domain binds directly to a SI/MxF recognition sequence in the cytoplasmic tails of lipoprotein receptors. This interaction is independent of IDOL's RING domain E3 ligase activity and its capacity for autoubiquitination. Surprisingly, the key interacting residues in IDOL and the LDLR are functionally conserved in their insect homologues. The inventors have also demonstrated that target recognition by IDOL involves a tripartite interaction between the FERM domain, membrane phospholipids and the lipoprotein receptor tail. The data identify the IDOL- LDLR interaction as an evolutionarily-conserved mechanism for the regulation of lipid uptake and suggest that this interaction can be exploited for the pharmacologic modulation of lipid metabolism using agents capable of preventing or inhibiting this interaction, i.e. the FERM-LDLR interaction might be tractable target for the pharmacologic

manipulation of lipid metabolism. Example 15 - Preparation of molecules which inhibit degradation of LDLR The inventors set out to develop agents or inhibitors, which are able to block the various protein interactions described herein in order to inhibit degradation of LDLR, VLDLR or apoER2. In summary, these agents are as follows:-

(a) A molecule which inhibits binding of FERM domain of IDOL with the target receptor (i.e. the conserved motifs in LDLR, VLDLR or apoER2);

(b) A molecule which inhibits binding of FERM domain of IDOL with membrane phospholipids;

(c) A molecule which inhibits binding of RING domain of IDOL with UBE2D1-4;

(d) A molecule which inhibits binding of iron ions with the RING domain of IDOL; or

(e) A molecule which inhibits the dimerisation of IDOL.

Peptide design, based on the sequence of a natural protein partner have been successfully used. For example, in the case of BCL6, peptides based on the BCOR protein bind BCL6 and blocks SMRT from interacting at the same site and in doing so blocks BCL6-mediated transcriptional repression and kills lymphoma cells (Ghetu et al 2008). Similarly, the design of a synthetic, cell-permeable, stabilised peptide that targets the protein-protein interface in the NOTCH transactivation complex has also been successfully used in leukaemic cells in culture.

In a similar way, a wildtype peptide corresponding to the LDLR tail sequence, SEQ ID No: 2, would compete for binding to the IDOL FERM and prevent degradation of the LDLR receptor. Modifications/optimisation of the peptide sequence could be made to increase the affinity so that it is tighter than the wildtype which is a relatively weak and short lived interaction by design. A peptide from VLDLR or apoER2 or from other species could possibly have higher affinity, and this would be explored.

The inclusion of a ubiquitination motif, as in the LDLR tail, in the synthetic peptide would serve a secondary function; as well as binding the FERM domain it could be possible for it to be ubiquitinated reducing the available pool of active E2 for ubiquitinating endogenous LDLR.

A peptide similar in nature to the conserved motif of the LDLR tail could dock into the unique surface on the IDOL F3ab domain and prevent LDLR interaction with IDOL and thus block LDLR ubiquitination. Structure-guided optimisation of the contacts would enhance the affinity. The inventors also consider the addition of sequence modules to enhance cell-permeability, and stability of transient structures. Furthermore, a sequence with a LDLR-like motif and also a thiol-reactive moiety that could target the iron binding cysteines could have a dual mechanism. An antibody that targets the LDLR binding pocket on the F3ab domain or the iron-binding cysteines could also be used to block LDLR ubiquitination.

Having identified the critical interaction surfaces on the proteins, the inventors expect that therapeutic antibodies can be developed that target these specific surfaces, block the interaction of IDOL with the relevant partners and hence prevent degradation of the LDL receptor. Examples of such approaches in other systems can be found in Nature Reviews Immunology io, 285, May 2010.

Example 16 - Agents which have been developed showing proof principle

Agent 1: Over expression of catalytically dead UBE2D blocks IDOL function.

The agent is transfected catalytically dead UBE2D which binds to the endogenous WT-IDOL and prevents endogenous UBE2D binding. This shows that disruption of the ID0L:E2 interaction via an agent that targets the E2 binding site abolishes IDOL function (see Example 2).

Agent 2: Disruption ofdimer interface. The agent here is over expression of catalytically dead IDOL.

This agent disrupts the function of WT-IDOL in a dose dependent manner. This effect could also be due to the over-expressed inactive IDOL binding to the LDLR tail and blocking the interaction of WT-IDOL in which case that would make it an agent for disrupting the interaction between the FERM domain and the LDLR tail (see

Example 6).

Agent 3: Disruption of iron binding site.

Mutation of the iron binding site cysteines to alanine clearly prevent iron binding and this increases IDOL stability (see Example 7).

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Claims

1. An agent capable of:

(i) a Low-Density Lipoprotein receptor (LDLR),

(ii) a Very Low Density Lipoprotein Receptor (VLDLR) and/or

(iii) a Low density lipoprotein receptor-related protein 8 (apoER2);

(b) inhibiting binding or interaction between IDOL and a

K/RNWXXKNXXSI/MXF motif present in the LDLR, VLDLR and/or apoER2;

(d) inhibiting or preventing binding of iron ions with IDOL; or

(e) inhibiting or preventing the dimerisation of IDOL,

for use in inhibiting LDLR, VLDLR and/or apoER2 degradation and/or promoting lipoprotein uptake.

2. An agent capable of:

(i) a Low-Density Lipoprotein receptor (LDLR),

(ii) a Very Low Density Lipoprotein Receptor (VLDLR) and/or

(iii) a Low density lipoprotein receptor-related protein 8 (apoER2);

(b) inhibiting binding or interaction between IDOL and a

K/RNWXXKNXXSI/MXF motif present in the LDLR, VLDLR and/or apoER2;

(d) inhibiting or preventing binding of iron ions with IDOL; or

(e) inhibiting or preventing the dimerisation of IDOL, for use in the treatment, prevention or amelioration of hypercholesterolemia or cardiovascular disease.

3. An agent according to either claim 1 or claim 2, wherein the agent is used for the treatment, amelioration or prevention of a cardiovascular disease selected from a group consisting of disorders of the heart and vascular system, such as congestive heart failure; myocardial infarction; ischemic diseases of the heart; ischemic cardiomyopathy; myocardial disease; all kinds of atrial and ventricular arrhythmias; hypertensive vascular diseases; peripheral vascular diseases; atherosclerotic coronary artery disease; heart failure; hypertrophic cardiomyopathy; restrictive

cardiomyopathy; congestive heart failure; cardiogenic shock; and hypertension.

4. An agent according to any preceding claim, wherein the agent is capable of inhibiting binding or interaction between the receptor and the F3 sub-domain of the FERM domain of IDOL, wherein the F3 sub-domain is defined by amino acid residues 183-344 of SEQ ID No:i, or a functional fragment or variant thereof.

5. An agent according to any preceding claim, wherein the agent is capable of inhibiting binding or interaction between the receptor and an F3a, F3b or F3C sub- domain of the FERM domain of IDOL, wherein sub-domain F3a is defined by amino acid residues 183-214 of SEQ ID No:i, or a functional fragment or variant thereof, sub-domain F3b is defined by amino acid residues 215-272 of SEQ ID No:i, or a functional fragment or variant thereof, and sub-domain F3C is defined by amino acid residues 272-344 of SEQ ID No:i, or a functional fragment or variant thereof.

6. An agent according to any preceding claim, wherein the agent is capable of inhibiting binding or interaction between an F3b sub-domain of the FERM domain of IDOL and the receptor, the F3b sub-domain being represented by amino acid residues 215-272 of SEQ ID No:i, or a functional fragment or variant thereof.

7. An agent according to claim 6, wherein the amino acid residues in the F3b sub-domain of IDOL, which are targeted by the agent to prevent binding or interaction with the receptor, are selected from a group of residues consisting of residues: 232; 265; and 269 of SEQ ID No:i.

8. An agent according to any preceding claim, wherein the agent is capable of inhibiting binding or interaction between an F3C sub-domain of the FERM domain of IDOL and the receptor, the F3C sub-domain being represented by amino acid residues 273-344 of SEQ ID No:i.

9. An agent according to claim 8, wherein the amino acid residues in the F3C sub-domain of IDOL, which are targeted by the agent to prevent binding or interaction with the receptor, are selected from a group of residues consisting of residues: 285; 323; 327 and 342 of SEQ ID No:i.

10. An agent according to any preceding claim, wherein the agent is capable of inhibiting binding or interaction between a sub-domain of the FERM domain of

IDOL and amino acid residues conserved between (i) the LDLR, (ii) the VLDLR, and (iii) the apoER2 receptors.

11. An agent according to any preceding claim, wherein the agent is capable of inhibiting binding or interaction between IDOL and: (i) a ⁸²⁰SI/MXF⁸²³ motif; (ii) a

8ⁱo_WKNW⁸ⁱ3 motif; (iii) a ^8l6KN⁸¹? motif; and/or (iv) a ^DNPVY⁸²⁸ motif, each motif being present in the LDLR, VLDLR and/or apoER2, as represented in SEQ ID No:2.

12. An agent according to any preceding claim, wherein the agent is capable of inhibiting interaction or binding between the RING domain of IDOL and a member of the ubiquitin-conjugating enzyme (UBE2D) family, which member is UBE2D1, UBE2D2, UBE2D3 or UBE2D4.

13. An agent according to any preceding claim, wherein the agent is capable of inhibiting interaction of binding between the member of the ubiquitin-conjugating enzyme (UBE2D) family and one or more amino acid residues of the RING domain of IDOL selected from the group of residues consisting of: GIU383; Val389; Leu4i5 and Pro4i9 of SEQ ID No:i.

14. An agent according to any preceding claim, wherein the agent is capable of inhibiting interaction of binding between IDOL and one or more amino acid residues of the member of the ubiquitin-conjugating enzyme (UBE2D) family selected from the group of residues consisting of: Lys8; Argis; Pro6i; Phe62 and Pro95 of SEQ ID No:3.

15. An agent according to any preceding claim, wherein the agent is capable of inhibiting interaction of binding one or more amino acid residues of the RING domain of IDOL selected from the group of residues consisting of: GIU383; Val389; Leu4i5 and Pro4i9 of SEQ ID No:i, and one or more amino acid residues of the member of the ubiquitin-conjugating enzyme (UBE2D) family selected from the group of residues consisting of: Lys8; Argis; Pro6i; Phe62 and Pro95 of SEQ ID No:3.

16. An agent according to any preceding claim, wherein the agent is capable of inhibiting or preventing binding of iron ions with the RING domain of IDOL.

17. An agent according to any preceding claim, wherein the agent is capable of inhibiting or preventing binding of iron ions with amino acid residue C360, C363 and/or C383 of SEQ ID No:i.

18. An agent according to any preceding claim, wherein the agent is capable of inhibiting or preventing binding of membrane phospholipids with IDOL, preferably the FERM domain thereof.

19. An agent according to claim 18, wherein amino acid residues in the FERM domain of IDOL, which are targeted by the agent to prevent binding or interaction with membrane phospholipids, are selected from the group of residues including 73; 75; 193; 199; 259; 137; and 146 of SEQ ID No:i.

20. An agent according to any preceding claim, wherein the agent comprises a competitive polypeptide, or a derivative or analogue thereof, or a peptide-like molecule or a small molecule.

21. An agent according to any one of claims 1-20, wherein the agent is an antibody or a fragment thereof.

22. A hypercholesterolaemia or cardiovascular disease treatment composition comprising a therapeutically effective amount of an agent capable of:

(i) a Low-Density Lipoprotein receptor (LDLR),

(apoER2);

(b) inhibiting binding or interaction between IDOL and a

K/RNWXXKNXXSI/MXF motif present in the LDLR, VLDLR and/or apoER2;

(d) inhibiting or preventing binding of iron ions with IDOL; or

(e) inhibiting or preventing the dimerisation of IDOL,

and optionally a pharmaceutically acceptable vehicle.

23. A process for making the composition according to claim 22, wherein the process comprises contacting a therapeutically effective amount of an agent capable of:

(i) a Low-Density Lipoprotein receptor (LDLR),

(ii) a Very Low Density Lipoprotein Receptor (VLDLR) and/or

(iii) a Low density lipoprotein receptor-related protein 8 (apoER2);

(b) inhibiting binding or interaction between IDOL and a

K/RNWXXKNXXSI/MXF motif present in the LDLR, VLDLR and/or apoER2;

(d) inhibiting or preventing binding of iron ions with IDOL; or

(e) inhibiting or preventing the dimerisation of IDOL,

with a pharmaceutically acceptable vehicle.

24. A composition according to claim 22 or a method according to claim 23, wherein the agent as defined in any one of claims 1-21.

25. A method of inhibiting LDLR, VLDLR and/ or apoER2 degradation and/ or promoting lipoprotein uptake in a subject, the method comprising administering, to a subject in need of such treatment, a therapeutically effective amount of an agent capable of:

(i) a Low-Density Lipoprotein receptor (LDLR),

(ii) a Very Low Density Lipoprotein Receptor (VLDLR) and/or

(iii) a Low density lipoprotein receptor-related protein 8 (apoER2);

(b) inhibiting binding or interaction between IDOL and a

K/RNWXXKNXXSI/MXF motif present in the LDLR, VLDLR and/or apoER2; or

(d) inhibiting or preventing binding of iron ions with IDOL; or

(e) inhibiting or preventing the dimerisation of IDOL,

26. A method of treating, preventing or ameliorating hypercholesterolaemia or cardiovascular disease in a subject, the method comprising administering, to a subject in need of such treatment, a therapeutically effective amount of an agent capable of:

(i) a Low-Density Lipoprotein receptor (LDLR),

(ii) a Very Low Density Lipoprotein Receptor (VLDLR) and/or

(iii) a Low density lipoprotein receptor-related protein 8 (apoER2);

(b) inhibiting binding or interaction between IDOL and a

K/RNWXXKNXXSI/MXF motif present in the LDLR, VLDLR and/or apoER2; or

(c) inhibiting interaction or binding between IDOL and a member of the ubiquitin-conjugating enzyme (UBE2D) family; (d) inhibiting or preventing binding of iron ions with IDOL; or

(e) inhibiting or preventing the dimerisation of IDOL,

to treat, prevent or ameliorate hypercholesterolaemia or cardiovascular disease subject.