WO1998024915A1 - Protein phosphatase 2a subunit polypeptide - Google Patents

Abstract

The invention provides a novel B subunit of the protein phosphatase 2A complex, shown in SEQ ID NO:2, which has affinity for the pocket protein p107. Nucleic acid encoding the protein is provided. Over-expression of this protein in replicating mammalian cells leads to an increase in the proportion of cells in the G1 phase.

Description

PROTEIN PHOSPHATASE 2A SUBUNIT POLYPEPTIDE

The present invention relates to a novel polypeptide which is a subunit of the protein phosphatase 2A protein complex.

Background to the invention.

Orderly progression through the cell division cycle requires the coordinate expression of groups of genes that are responsible for the various biochemical processes that take place during cell division. The E2F transcription factor family plays an important part in cell cycle-regulated gene expression as it activates a number of genes that are required for DNA synthesis (3, 6) . The E2F transcription factor, in turn, is regulated by the retinoblastoma family of growth-inhibitory proteins. This family consists of three members: the retinoblastoma protein (pRb) and the related pl07 and pl30 (20, 36, 55, 59) . Collectively, these proteins are known as the "pocket" proteins, as they have a domain, named the pocket, with which they can bind and inactivate several cellular proteins. The ability of the pocket proteins to bind proteins such as E2F is abolished through cell cycle-dependent phosphorylation by cyclin-dependent kinases (cdks) . In the Gl stage of the cell cycle the retinoblastoma protein is hypophosphorylated. At the Gl- to S phase transition, pRb receives additional phosphates and phosphorylation increases even further as cells progress through S and G2. Not only pRb, but also pl07 can be phosphorylated and thereby inactivated as a growth suppressor by the cyclin Dl/cdk4 kinase complex. In addition, pRb, but not pl07, is thought to be a substrate for cyclin A/cdk2 and cyclin E/cdk2 (4, 13, 16, 26) . It is somewhat enigmatic that even though pl07 and pl30 do not appear to be phosphorylated by cyclin A/cdk2 and cyclin E/cdk2, both pocket proteins can form stable complexes with cyclins A/cdk2 and E/cdk2 through a domain within the pocket called the spacer (34, 35, 47, 57) .

In contrast, little is known about the way in which phosphates are removed from pocket proteins during the cell cycle. The retinoblastoma protein has been shown to interact directly with the PP1 catalytic subunit during the phase in the cell cycle, when pRb is known to be dephosphorylated (14, 42) . Moreover, a high molecular weight form of PP1 found in mitotic cell extracts was shown to be able to dephosphorylate pRb in vitro (41) . Also, pRb immunoprecipitates from cells which have accumulated DNA damage as a result of exposure to anti cancer drugs contain a phosphatase activity that can dephosphorylate pRb in vi tro and it is likely that PP1 is involved in this dephosphorylation of pRb in response to DNA damage (12) .

Besides PP1, other major cellular phosphatases exist that could potentially contribute to dephosphorylation of cell cycle control proteins. For example, PP2A is a major intracellular Ser/Thr phosphatase that, apart from being involved in cell cycle regulation, plays a role in diverse cellular processes such as DNA-replication, intermediary metabolism, transcription and signal transduction (28, 38, 40, 54) . PP2A consists of a catalytic subunit (PP2Ac) (11) and a structural 65kDa subunit (PR65) (22) . These two proteins form a core dimer (24) to which various other regulatory "B" subunits associate. To date, three families of genes encoding regulatory B subunits have been cloned. The most prevalent isoform, called PR55, B55, or simply B subunit, is encoded by at least three related genes (37, 44) . Five genes have been cloned that fall into one closely related gene family of PR61 subunits (also called B56, or B' subunit (39, 50, 60) . One gene that gives rise to two differentially spliced mRNAs encodes two B-subunits, PR72 (also called B' ' ) and PR130 (23) . It is widely believed that the multitude of B subunits plays a role in regulation of phosphatase activity, control of substrate specificity and targeting to certain cellular compartments of the PP2A core dimer. That substrate specificity is indeed conferred by the subunit composition of PP2A is demonstrated by the finding that PR72 -containing PP2A holo-enzyme purified from rabbit skeletal muscle in vi tro preferentially dephosphorylates SV40 large T antigen phosphorylated by Casein Kinase I, whereas the PP2A core dimer, or PR55 containing holo-enzyme, preferentially dephosphorylates SV40 large T antigen on a site phosphorylated by recombinant cdc2 kinase (8) . The diversity of activities of the PP2A holo-enzyme is thus reflected in a diversity of possible trimeric PP2A complexes .

The notion that PP2A plays a role in cell cycle regulation is supported by several observations. First, okadaic acid, a strong inhibitor of protein phosphatase 2A (PP2A) , is a tumor promoting agent (40) . Furthermore, PP2A is a target of transforming proteins of several DNA tumor viruses (48) . SV40 small t antigen and polyoma virus small t and middle T antigens, which have a helper function in transformation, can replace certain B subunits in PP2A complexes. As a result, SV40 small t antigen inhibits the PP2A phosphatase activity towards signal transduction kinases MEK and ERK (48) . That there is specificity in this interference with the activity of PP2A holo-enzymes is illustrated by the finding that the variable subunit in PP2A trimeric complexes purified from bovine heart is not replaced by SV40 small t (30) . Finally, a complex of HIV-I encoded proteins, NCp7 and vpr, is implicated in direct activation of PP2A and subsequent cell cycle arrest at the G2 to M transition (51) .

Although it is generally believed that the B subunits control the activity of the core PP2A dimer (29, 38), very few substrates are known with which the regulatory B-subunits associate directly and that are subject to dephosphorylation by PP2A. An isoform of the B' (B56, PR56) PP2A regulatory subunits (B'al) has been shown to form stable complexes in vivo with a cellular protein, cyclin G, but it remains unclear wether this B subunit-bound cyclin actually serves as a substrate for PP2A (43) .

Description of the Drawings.

Figure 1 shows PR59 immunoprecipitates contain PP2A activity. Figure 2 shows inhibition of cell proliferation by PR59 as determined by FACS analysis, of U2-0S cells transfected with (a) lOμg empty vector, (b) lOμg PR59 expression vector, (c) 20μg PR59 expression vector and (d) 20μg HA-tagged PR59 expression vector.

Disclosure of the invention.

We have now surprisingly found a novel B subunit which associates with the PP2A core dimer. This subunit, PR59, appears to direct PP2A to the pl07 pocket protein. The present invention is believed to be the first time a B subunit has been found to have a defined target.

Accordingly, the present invention provides a polypeptide in substantially isolated form which comprises:

(a) SEQ ID NO: 2;

(b) a fragment of said sequence which retains the ability to associate with the pocket protein pl07; (c) a variant of (a) or (b) which is at least 90% homologous thereto; or (d) a mammalian homologue of (a) or (b) .

A particular variant of SEQ ID NO : 2 is the polypeptide of SEQ ID NO: 4, which is over 90% homologous (identical) thereto.

The present invention also provides a variant of SEQ ID NO: 4 which is encoded by a nucleic acid which is obtainable by: providing a nucleic acid probe comprising SEQ ID NO: 3; probing a murine cDNA library with said probe under conditions of medium to high stringency; and isolating a positive clone comprising an open reading frame encoding a homologue of SEQ ID NO: 3.

Conditions of medium to high strigency are for example 0.03M sodium chloride and 0.03M sodium citrate at from about 50 to 60°C. The invention further provides a nucleic acid encoding a polypeptide of the invention, including a variant obtainable by the method described above, and vectors, particularly expression vectors comprising such nucleic acid operably linked to a promoter.

The vectors may be introduced into a host cell to replicate the vector and to express a polypeptide of the invention by a method comprising: i) providing the host cell carrying a vector of the invention said vector also comprising a promoter compatible with said host cell operably linked to the nucleic acid of the invention; ii) growing said host cell under conditions to bring about expression of the polypeptide; and iii) recovering the polypeptide.

The present invention also provides antibodies against the polypeptides of the invention, particularly antibodies which recognise epitopes of the polypeptides of the invention but not other B subunit polypeptides.

Polypeptides of the invention may be used for a variety of purposes in the study of cell cycle regulation, the development of therapeutic compounds, and in methods of treatment of the human or animal body by therapy.

Detailed description of the invention.

A. Polypeptides.

Isolated polypeptides of the invention will be those as defined above in isolated form, free or substantially free of material with which it is naturally associated such as other polypeptides with which it is found in the cell. The polypeptides may of course be formulated with diluents or adjuvants and still for practical purposes be isolated - for example the polypeptides will normally be mixed with gelatin or other carriers if used to coat microtitre plates for use in immunoassays . The polypeptides may be glycosylated, either naturally or by systems of heterologous eukaryotic cells, or they may be (for example if produced by expression in a prokaryotic cell) unglycosylated. Polypeptides may phosphorylated and/or acetylated.

A polypeptide of the invention may also be in a substantially purified form, in which case it will generally comprise the polypeptide in a preparation in which more than 90%, e.g. 95%, 98% or 99% of the polypeptide in the preparation is a polypeptide of the invention.

Polypeptides of the invention may be modified for example by the addition of histidine residues to assist their purification or by the addition of a signal sequence to promote their secretion from a cell.

Polypeptides of the invention include polypeptides in substantially isolated form which comprise the sequence set out in SEQ ID NO : 2 , for example as shown in SEQ ID NO: 4.

A fragment of said sequence which retains the ability to associate with the pocket protein pl07 is also part of the invention. Such fragments may be determined by experiment.

For example, we have found that the C-terminal 127 amino acids of the polypeptide of SEQ ID NO: 4 (identical to the corresponding portion of SEQ ID NO: 2) retains the ability to retain a differential affinity for the pl07 pocket protein compared to the affinity for pRb. The assay set out in the accompanying examples may be used to screen other fragments of the polypeptides of SEQ ID NO: 2 or SEQ ID NO: 4 for this ability.

In general, such polypeptides will be fragments of at least 10, preferably at least 20 and most preferably at least 50 amino acids in size. Fragments from the C-terminal region (either C- terminal fragments or internal fragments of the C-terminal region) may be used. The C-terminal region includes the 127 amino acid region described herein and more generally includes the about 250, for example about 200 such as 150 amino acids of SEQ ID NO:2.

Preferably the fragments also will be able to form a complex with the catalytic domain of PP2A, in addition to being able to associate with pl07. This may also be tested by routine experimentation such as by the methods shown in the accompanying examples .

Variants of SEQ ID NO: 2 or SEQ ID NO: 4 and fragments of SEQ ID NO: 2 or SEQ ID NO : 4 and fragments of said polypeptides and variants form a further aspect to the present invention. The variant will be at least 90% homologous to the polypeptide of SEQ ID NO: 2 or SEQ ID NO : 4 or to a fragment thereof. Preferably, the variants have 95% amino acid homology and most preferably 98% homology. Thus a variant may be one which differs from SEQ ID NO: 2 or SEQ ID NO: 4 by for example from 1 to 40 amino acids, for example from 1 to 35, such as from 1 to 20, for example from 1 to 10 such as 2 , 3, 4, 5 or 8 amino acids .

Such variants or fragments thereof will also retain the functional properties of PR59 or its fragments as defined above. Fragments of SEQ ID NO: 2 or SEQ ID NO: 4 or their variants and homologoues may be for example from 50 to 400 amino acids in size, for example 75, 100, 150, 200 or 300 amino acids in size.

Unless specified to the contrary, amino acid homology throughout this specification is defined as "hard homology" i.e. the degree of amino acid identity.

The invention also extends to a mammalian homologue of the PR59 polypeptide or its fragments. Mammalian homologues include homologues from other species including animals such as rats or rabbits, and most preferably from primates, particularly humans .

Mammalian homologues may be isolated by routine recombinant DNA techniques using the knowledge of the murine sequence disclosed herein. For example, PCR primers may be made based on two portions of the nucleotide sequence of SEQ ID N0:1 or SEQ ID NO: 3. Typically such primers will be in the opposite orientation to each other and have 3' ends which are separated on the SEQ ID NO : 1 or SEQ ID NO : 3 sequence by around 50 to 500, e.g. 100 to 200 nucleotides. The primers may be used under standard PCR conditions to make a cDNA library from RNA isolated from cells of a mammal in which the PR59 homologue is expressed, for example lung, heart, spleen, kidney, liver or brain cells. The cDNA library is probed with the primers to identify homologous sequences representing PR59. Because PP2A subunits are highly conserved in evolution, a species homologue may readily be identified and distinguished from other PP2A subunits within a particular species. Section C below provides further details of this and other cloning methods.

Variants of PR59 include those which contain conserved substitutions which may be made according to the following table, where amino acids on the same block in the second column and preferably in the same line in the third column may be substituted for each other:

A polypeptide of the invention may be labelled with a revealing label. The revealing label may be any suitable label which allows the polypeptide to be detected. Suitable labels include radioisotopes, e.g. ¹²⁵I, enzymes, antibodies, polynucleotides and linkers such as biotin. Labelled polypeptides of the invention may be used in diagnostic procedures such as immunoassays in order to determine the amount of a polypeptide of the invention in a sample. This is of use in the mechanisms of cell proliferation which involve expression of PR59 and its function in directing the PP2A complex to pocket proteins such as pl07. Polypeptides or labelled polypeptides of the invention may also be used in serological or cell mediated immune assays for the detection of immune reactivity to said polypeptides in animals and humans using standard protocols.

A polypeptide or labelled polypeptide of the invention or fragment thereof may also be fixed to a solid phase, for example the surface of an immunoassay well or dipstick.

Such labelled and/or immobilized polypeptides may be packaged into kits in a suitable container along with suitable reagents, controls, instructions and the like.

Such polypeptides and kits may be used in methods of detection of antibodies to PR59 or fragments thereof or variants or homologues thereof by immunoassay.

Immunoassay methods are well known in the art and will generally comprise:

(a) providing a polypeptide comprising an epitope bindable by an antibody against said protein;

(b) incubating a biological sample with said polypeptide under conditions which allow for the formation of an antibody-antigen complex; and (c) determining whether antibody-antigen complex comprising said polypeptide is formed.

Polypeptides of the invention may be may by synthetic means known in the art as such or, more usually, by recombinant means. Such means are well known in the art and examples of such are described below and in the accompanying examples .

The polypeptides of the invention may be introduced into the cell by in si tu expression of the polypeptide from a recombinant expression vector (see below) . The expression vector optionally carries an inducible promoter to control the expression of the polypeptide.

The invention also includes fusion proteins in which a polypeptide of the invention is fused, at the N- and/or C- terminus, to another polypeptide sequence. Such sequences may be tags for detection. Examples of such tags are given in section B below.

The above described polypeptides (including variants, homologues, allelic forms and fragments) are collectively referred to as polypeptides of the invention. In addition, reference to "PR59" should, unless the context is clearly to the contrary, be interpreted as including polypeptides of the invention.

Polypeptides of the invention may also be used to generate antibodies and such antibodies form a further aspect of the invention. An antibody of the invention may be monoclonal or polyclonal. Preferred antibodies according to the invention are isolated, in the sense of being free from contaminants such as antibodies able to bind other polypeptides and/or free of serum components. Monoclonal antibodies are preferred for some purposes, though polyclonal antibodies are within the scope of the present invention. Antibodies may be obtained using techniques which are standard in the art. Methods of producing antibodies include immunising a mammal (e.g. mouse, rat, rabbit) with a polypeptide of the invention. Antibodies may be obtained from immunised animals using any of a variety of techniques known in the art, and screened, preferably using binding of antibody to antigen of interest. For instance, Western blotting techniques or immunoprecipitation may be used.

As an alternative or supplement to immunising a mammal with a peptide, an antibody specific for a protein may be obtained from a recombinantly produced library of expressed immunoglobulin variable domains, e.g. using lambda bacteriophage or filamentous bacteriophage which display functional immunoglobulin binding domains on their surfaces; for instance see WO92/01047.

Antibodies according to the present invention may be modified in a number of ways. Indeed the term "antibody" should be construed as covering any binding substance having a binding domain with the required specificity. Thus the invention covers antibody fragments, derivatives, functional equivalents and homologues of antibodies, including synthetic molecules and molecules whose shape mimics that of an antibody enabling it to bind an antigen or epitope.

Example antibody fragments, capable of binding an antigen or other binding partner are the Fab fragment consisting of the VL, VH, Cl and CHI domains; the Fd fragment consisting of the VH and CHI domains; the Fv fragment consisting of the VL and VH domains of a single arm of an antibody; the dAb fragment which consists of a VH domain; isolated CDR regions and F(ab')2 fragments, a bivalent fragment including two Fab fragments linked by a disulphide bridge at the hinge region. Single chain Fv fragments are also included. Humanized antibodies in which CDRs from a non-human source are grafted onto human framework regions, typically with the alteration of some of the framework amino acid residues, to provide antibodies which are less immunogenic than the parent non-human antibodies, are also included within the present invention.

Hybridomas capable of producing antibody with desired binding characteristics are within the scope of the present invention, as are host cells, eukaryotic or prokaryotic, containing nucleic acid encoding antibodies (including antibody fragments) and capable of their expression. The invention also provides methods of production of the antibodies including growing a cell capable of producing the antibody under conditions in which the antibody is produced, and preferably secreted.

The reactivities of antibodies on a sample may be determined by any appropriate means. Tagging with individual reporter molecules is one possibility. The reporter molecules may directly or indirectly generate detectable, and preferably measurable, signals. The linkage of reporter molecules may be directly or indirectly, covalently, e.g. via a peptide bond or non-covalently . Linkage via a peptide bond may be as a result of recombinant expression of a gene fusion encoding antibody and reporter molecule. Reporter molecules include fluorochromes such as fluorescein, rhodamine, phycoerythrin and Texas Red; chromogenic dyes such as diaminobenzidine; macromolecular colloidal particles or particulate material such as latex beads that are coloured, magnetic or paramagnetic, and biologically or chemically active agents that can directly or indirectly cause detectable signals to be visually observed, electronically detected or otherwise recorded. Reporters may also be enzymes which catalyse reactions that develop or change colours or cause changes in electrical properties. Biotin/avidin or biotin/streptavidin and alkaline phosphatase detection systems may be employed. The mode of determining binding is not a feature of the present invention and those skilled in the art are able to choose a suitable mode according to their preference and general knowledge .

Antibodies according to the present invention may be used in screening for the presence of a polypeptide, for example in a test sample containing cells or cell lysate as discussed, and may be used in purifying and/or isolating a polypeptide according to the present invention, for instance following production of the polypeptide by expression from encoding nucleic acid therefor. Antibodies may modulate the activity of the polypeptide to which they bind and so, if that polypeptide has a deleterious effect in an individual, may be useful in a therapeutic context (which may include prophylaxis) .

An antibody may be provided in a kit, which may include instructions for use of the antibody, e.g. in determining the presence of a particular substance in a test sample. One or more other reagents may be included, such as labelling molecules, buffer solutions, elutants and so on. Reagents may be provided within containers which protect them from the external environment, such as a sealed vial.

Antibodies of the invention will be specific for proteins such as SEQ ID NO: 2 or SEQ ID NO : 4 , their variants, fragments and homologues, that is they will bind to such polypeptides witti an affinity substantially greater (e.g at least 1000-fold greater) _^ than they do to other regulatory B subunits including PR72.

B. Assays,

Polypeptides of the invention are useful in studying the mechanisms of cell proliferation and for screening compounds which have the potential to enhance the binding of a PP2A complex comprising PR59 to pocket proteins such as pl07, or enhance the phosphatase activity of the complex. An assay for screening binding enhancing compounds will typically comprise bringing a PR59 polypeptide of the invention into contact with a pocket protein in the presence of a candidate enhancing agent and measuring the degree of binding between the two. The PR59 and pocket protein may be brought into contact in any suitable environment.

A preferred embodiment of this aspect of the invention is a two hybrid assay. In such an assay, the PR59 polypeptide and a pocket protein (or fragment thereof capable of binding SEQ ID NO : 2 or SEQ ID NO : 4 ) are expressed as fusion proteins, one being a fusion protein comprising a DNA binding domain (DBD) , such as the yeast GAL4 binding domain, and the other being a fusion protein comprising an activation domain, such as that from GAL4 or VP16. In such a case the host cell (which again may be bacterial, yeast, insect or mammalian, particularly yeast or mammalian) will carry a reporter gene construct with a promoter comprising a DNA binding elements compatible with the DBD. Constructs expression the fusion proteins may be introduced into the cell and expressed transiently or stably.

For example the PR59 polypeptide and the pocket protein may be co-expressed in a yeast host cell as fusion proteins. The PR59 polypeptide is expressed fused to one of a GAL4 DNA binding domain or transactivation domain, and the pocket protein is expressed fused to the other. The yeast host cell additionally contains a reporter gene driven by a GAL4 promoter so that when both fusion proteins are expressed and associate with one another the reporter gene (e.g. LacZ or HIS3) is expressed. The amount of expression may be measured by any means, either at the RNA level (e.g. northern blotting, SI analysis) or the protein level, (e.g by SDS-PAGE, Western blotting or other immunochemical analysis) .

This assay format is described by Fields and Song, 1989, Nature 340 ; 245-246. This type of assay format is described in further detail in the Examples which follow. A putative enhancing agent may be added to the environment of the yeast (or may be a polypeptide compound expressed recombinantly within the yeast) to observe what effect if any this has on the transcription of the reporter gene.

An assay for screening for compounds which enhance the phosphatase activity of the complex will typically comprise bringing a polypeptide of the invention into contact with the catalytic subunit of PP2A. The phosphatase activity of the complex may be determined by adding a phosphorylated peptide substrate and measuring the phosphatase activity of the complex in the presence and absence of a candidate enhancing agent .

Desirably the assay will be conducted in the presence of the PR65 subunit of the PP2A complex. In either case, complex may be prepared by recombinant expression of the polypeptide of the invention, catalytic domain and optionally the PR65 subunit in mammalian (e.g. human) cells and the complex recovered and tested in vi tro . The complex may be recovered by immunological means. Conveniently, one or more components of the complex may be expressed as a fusion protein with an HA epitope such as the epitope recognized by the 12CA5 antibody.

In the assay the phosphatase inhibitor okadaic acid may be added at concentrations which result in a partial or total loss of PP2A phosphatase activity. The candidate enhancer compound may then be added to establish whether the okadaic acid inhibition can be fully or partially negated.

The invention also provides an enhancer compound obtainable from such an assay. The agent may be used in a method of controlling uncontrolled cell proliferation. Such a method may comprise administering to an individual with cells undergoing uncontrolled cell proliferation an effective amount of the compound. Assays of the formats described above may also be used for screening potential modulator compounds which disrupt or modify the interaction between the PR59 subunit and a pocket protein, particularly pl07, or between PR59 and PR65 or core dimer of PP2A. Thus two hybrid and other assay formats may be used in the presence and absence of putative modulator compounds (such as peptides of from 5 to 50 amino acids derived from SEQ ID NO : 2 or SEQ ID NO : 4 to determine if the presence of a putative modulator disrupts the various PR59 interactions described herein.

C. Nucleic acids.

Nucleic acids of the invention may comprise DNA or RNA. They may also be polynucleotides which include within them synthetic or modified nucleotides. A number of different types of modification to oligonucleotides are known in the art. These include methylphosphonate and phosphorothioate backbones, addition of acridine or polylysine chains at the 3' and/or 5' ends of the molecule. For the purposes of the present invention, it is to be understood that the polynucleotides described herein may be modified by any method available in the art. Such modifications may be carried out in order to enhance the in vivo activity or lifespan of polynucleotides of the invention.

Polynucleotides of the invention encoding the polypeptide of SEQ ID NO: 2 or SEQ ID NO: 4 will be preferably at least 70%, preferably at least 80 or 90% and more preferably at least 95% homologous to the open reading frame of the sequences of SEQ ID NO:l or SEQ ID NO : 3 respectively. However the redundancy of the genetic code will allow sequence variation to occur which will not result in a change of the encoded polypeptide sequences. However, where variants of the said polypeptides of the invention are to be obtained, this is conveniently achieved by altering the sequence of the DNA sequence of SEQ ID NO:l or SEQ ID NO: 3 to bring about a change in the translation of the open reading frames thereof .

Polynucleotides of the invention may be used to produce a primer, e.g. a PCR primer, a primer for an alternative amplification reaction, a probe e.g. labelled with a revealing label by conventional means using radioactive or non- radioactive labels, or the polynucleotides may be cloned into vectors. Such primers, probes and other fragments will be at least 15, preferably at least 20, for example at least 25, 30 or 40 nucleotides in length.

In a further aspect, the invention provides nucleic acids which consist essentially of from 15 to 50, for example from 15 to 35, 18 to 35, 15 to 24, 18 to 30, 18 to 21 or 21 to 24 nucleotides of a sequence having at least 70% homology to the nucleic acid sequence of SEQ ID NO:l or SEQ ID NO: 3 or their complements .

The term "consist essentially of" refers to nucleic acids which do not include any additional 5' or 3 ' nucleic acid sequences. In a further aspect of the invention, nucleic acids of the invention which consist essentially of from 15 to 50 nucleotides as defined above may however be linked at the 3' but preferably 5' end to short (e.g from 4 to 15, such as from 4 to 10 nucleotides) additional sequences to which they are not naturally linked. Such additional sequences are preferably linkers which comprise a restriction enzyme recognition site to facilitate cloning when the nucleic acid of the invention is used for example as a PCR primer.

Polynucleotides such as a DNA polynucleotide and primers according to the invention may be produced recombinantly, synthetically, or by any means available to those of skill in the art. They may also be cloned by standard techniques. In general, primers will be produced by synthetic means, involving a step wise manufacture of the desired nucleic acid sequence one nucleotide at a time. Techniques for accomplishing this using automated techniques are readily available in the art .

Longer polynucleotides will generally be produced using recombinant means, for example using a PCR (polymerase chain reaction) cloning techniques. This will involve making a pair of primers (e.g. of about 15-50, for example about 15-30 nucleotides) to a region of the PR59 mRNA or genomic sequence encoding the mRNA which it is desired to clone, bringing the primers into contact with mRNA or cDNA obtained from a human cell (e.g. a spleen, heart, liver, thymus or brain cell), performing a polymerase chain reaction under conditions which bring about amplification of the desired region, isolating the amplified fragment (e.g. by purifying the reaction mixture on an agarose gel) and recovering the amplified DNA. The primers may be designed to contain suitable restriction enzyme recognition sites so that the amplified DNA can be cloned into a suitable cloning vector.

Such techniques may be used to obtain all or part of SEQ ID NO:l or SEQ ID NO : 3 described herein. Genomic clones containing the PR59 gene and its introns and promoter regions may also be obtained in an analogous manner, starting with genomic DNA from a murine cell, e.g. a liver cell.

Although in general the techniques mentioned herein are well known in the art, reference may be made in particular to

Sambrook et al (Molecular Cloning: A Laboratory Manual, 1989) .

Polynucleotides which are not 100% homologous to the sequences of the present invention but fall within the scope of the invention can be obtained in a number of ways. Other murine variants (for example allelic forms) of the PR59 sequence described herein may be obtained for example by probing cDNA or genomic DNA libraries made from murine tissue.

In addition, other animal, particularly mammalian (e.g. rats or rabbits) , more particularly primate including human, homologues of PR59 may be obtained. Such sequences may be obtained by making or obtaining cDNA libraries made from dividing cells or tissues or genomic DNA libraries from other animal species, and probing such libraries with probes comprising all or part of SEQ ID NO: 4 or SEQ ID NO : 2 under conditions of medium to high stringency (for example 0.03M sodium chloride and 0.03M sodium citrate at from about 50oc to about 60oC) .

Alternatively, such polynucleotides may be obtained by site directed mutagenesis of the PR59 sequence of SEQ ID NO : 1 or SEQ ID NO: 3 or allelic variants thereof. This may be useful where for example silent codon changes are required to sequences to optimise codon preferences for a particular host cell in which the polynucleotide sequences are being expressed. Other sequence changes may be desired in order to introduce restriction enzyme recognition sites, or to alter the property or function of the polypeptides encoded by the polynucleotides. Further changes may be desirable to represent particular coding changes found in PR59 which are required to provide, for example, conservative substitutions.

Nucleic acid of the invention may be single or double stranded polynucleotides .

Polynucleotides or primers of the invention may carry a revealing label. Suitable labels include radioisotopes such as ³²p _or ³⁵ _s^ enzyme labels, or other protein labels such as biotin. Such labels may be added to polynucleotides or primers of the invention and may be detected using by techniques known per se . Polynucleotides or primers of the invention or fragments thereof labelled or unlabelled may be used by a person skilled in the art in nucleic acid-based tests for detecting or sequencing the gene encoding PR59 in the human or animal body. In the case of detecting, this may be qualitative and/or quantitative .

Such tests for detecting generally comprise bringing a human or animal body sample containing DNA or RNA into contact with a probe comprising a polynucleotide or primer of the invention under hybridizing conditions and detecting any duplex formed between the probe and nucleic acid in the sample. Such detection may be achieved using techniques such as PCR or by immobilizing the probe on a solid support, removing nucleic acid in the sample which is not hybridized to the probe, and then detecting nucleic acid which has hybridized to the probe. Alternatively, the sample nucleic acid may be immobilized on a solid support, and the amount of probe bound to such a support can be detected. Suitable assay methods of this any other formats can be found in for example WO89/03891 and WO90/13667.

The primers of the invention may conveniently be packaged in the form of a test kit in a suitable container. In such kits the probe may be bound to a solid support where the assay format for which the kit is designed requires such binding. The kit may also contain suitable reagents for treating the sample to be probed, hybridizing the probe to nucleic acid in the sample, control reagents, instructions, and the like.

P. Vectors.

Nucleic acid polynucleotides of the invention can be incorporated into a recombinant replicable vector. The vector may be used to replicate the nucleic acid in a compatible host cell. Thus in a further embodiment, the invention provides a method of making polynucleotides of the invention by introducing a polynucleotide of the invention into a replicable vector, introducing the vector into a compatible host cell, and growing the host cell under conditions which bring about replication of the vector. The vector may be recovered from the host cell. Suitable host cells are described below in connection with expression vectors.

E. Expression Vectors.

Preferably, a polynucleotide of the invention in a vector is operably linked to a control sequence which is capable of providing for the expression of the coding sequence by the host cell, i.e. the vector is an expression vector.

The term "operably linked" refers to a juxtaposition wherein the components described are in a relationship permitting them to function in their intended manner. A control sequence "operably linked" to a coding sequence is ligated in such a way that expression of the coding sequence is achieved under condition compatible with the control sequences.

Such vectors may be transformed into a suitable host cell as described above to provide for expression of a polypeptide of the invention. Thus, in a further aspect the invention provides a process for preparing polypeptides according to the invention which comprises cultivating a host cell transformed or transfected with an expression vector as described above under conditions to provide for expression by the vector of a coding sequence encoding the polypeptides, and recovering the expressed polypeptides.

The vectors may be for example, plasmid, virus or phage vectors provided with an origin of replication, optionally a promoter for the expression of the said polynucleotide and optionally a regulator of the promoter. The vectors may contain one or more selectable marker genes, for example an ampicillin resistance gene in the case of a bacterial plasmid or a neomycin resistance gene for a mammalian vector. Vectors may be used in vi tro, for example for the production of RNA or used to transfect or transform a host cell. We have also found that over-expression of PR59 in a host cell results in inhibition of the cell cycle. Thus vectors of the invention may also be adapted to be used in vivo, for example in methods of gene therapy, particularly of conditions associated with uncontrolled proliferation.

Thus a construct capable of expressing a nucleic acid of the invention may be introduced into cells of a recipient by any suitable means, such that a polypeptide of the invention, preferably a full length or active portion polypeptide, is expressed in the cells.

The construct may be introduced in the form of naked DNA, which is taken up by some cells of animal subjects, including muscle cells of mammalians. In this aspect of the invention the construct will generally be carried by a pharmaceutically acceptable carrier alone. The construct may also formulated in a liposome particle, as described above.

Such methods of gene therapy further include the use of recombinant viral vectors such as adenoviral or retroviral vectors which comprise a construct capable of expressing a polypeptide of the invention. Such viral vectors may be delivered to the body in the form of packaged viral particles.

The constructs may be introduced into target cells of a human or non-human mammalian recipient either in si tu or ex-vivo and reimplanted into the body. Where delivered in si tu, this may be by for example injection into target tissue (s) or in the case of liposomes, inhalation.

Gene therapy methods are widely documented in the art and may be adapted for use in the expression of a polypeptide of the invention. See for example WO95/14091 and Walther, Molecular Biotechnology, 6(3): 267-286, (1996) and Blomer, Human Molecular Genetics, Vol .5 : 1397-1404, (1996), the disclosures of which are incorporated herein by reference.

Selective expression in target cells may be achieved by the use of delivery systems which target the vector to those cells (e.g direct injection of the vector (or viral particles containing the vector) into the tumour and/or the use of tissue specific promoters which are only expressed (or selectively expressed) in tumour cells . Such promoters include the ras or cErbB2 promoter in some tumour cells such as breast or colon cancer cells. Gene therapy vectors and methods for their use are widely documented in the art and may be adapted for use in the expression of a polypeptide of the invention. See for example WO95/14091 the disclosure of which is incorporated herein by reference.

A further embodiment of the invention provides host cells transformed or transfected with the vectors for the replication and expression of polynucleotides of the invention. The cells will be chosen to be compatible with the said vector and may for example be bacterial, yeast, insect or mammalian.

Polynucleotides according to the invention may also be inserted into the vectors described above in an antisense orientation in order to provide for the production of antisense RNA.

Antisense RNA or other antisense polynucleotides may also be produced by synthetic means.

Promoters and other expression regulation signals may be selected to be compatible with the host cell for which the expression vector is designed. For example, yeast promoters include S. cerevisiae GAL4 and ADH promoters, S. pombe nmtl and adh promoter. Mammalian promoters include the metallothionein promoter which is can be included in response to heavy metals such as cadmium. Viral promoters such as the SV40 large T antigen promoter or adenovirus promoters may also be used. All these promoters are readily available in the art . The following examples are provided to illustrate the invention in a non-limiting manner.

Example 1 : Identification of clones that interact with pl07. To identify proteins that interact with the retinoblastoma protein-related pl07, we performed a yeast two-hybrid screen with the pocket domain of human pl07 fused to the DNA binding domain of the yeast transcription factor GAL4 as previously described (25) . Briefly, yeast strain Y190 (21) , containing "bait" plasmid pPC97-pl07, encoding the GAL4 DNA binding domain (DBD) fused to the pocket region of pl07, was transformed with a day 14.5 CD1 mouse embryo library (9) by the lithium acetate method (46) . Transformants were selected for growth on plates lacking histidine and supplemented with 25 mM 3-aminotriazole . His+ colonies were subsequently analyzed for β-galactosidase activity as previously described (9, 25) . cDNA library plasmids derived from double positive yeast colonies were tested for bait specificity by re-transformation with different GAL4 DBD fusion plasmids: pPC97-pl07, pPC97-bmi, and pPC97 without an insert. pPC97-pl07 was generated by cloning a cDNA fragment that specifies the pocket region of pl07 (amino acids 240 to 816) in frame with the GAL4 DBD (amino acids 1 to 147) of pPC97 (9) .

Partial sequence analysis of two positive clones indicated that they had significant sequence homology to human PR72, a regulatory subunit of protein phosphatase 2A (PP2A) (27) . Upon further sequence analysis, both clones (pp44 and pp31) were found to be derived from the same gene and contained an open reading frame of 603 nt and 381 nt, respectively.

The clone pp44 was used to screen additional mouse cDNA libraries by DNA-DNA hybridization. The full-length PR59 cDNA described here was obtained from a day 16 mouse embryo cDNA library in λ-ex/lox (Novagen) . This yielded seven independent positive phages, the largest of which contained an insert of 1988 bp. The largest cDNA contains an ORF of 1782 bp (SEQ ID NO : 3 ) . Two potential ATG initiator codons are present at the 5' end of the ORF, one at nt 93, the other at nt 219 of the cDNA sequence. Only the second ATG matches well the consensus start of translation as defined by Kozak (44) . There are no in frame stop codons in the cDNA upstream of these putative ATG initiator codons. To ask which of the two possible ATG codons is used as a start of translation, we transcribed both the full length cDNA and a 5' truncated cDNA that lacks the first 189 bp (including the first ATG) in vi tro into RNA and translated the RNAs in a rabbit reticulocyte lysate . Both the full length RNA and the 5' truncated RNA generated a protein of the same apparent molecular weight (approximately 65 kDa) on SDS-PAGE. This indicates that most likely the initiator codon at nt 219 is used as a start of translation. The predicted molecular weight from this methionine to the stop codon at 1783 of this putative PP2A regulatory subunit is 59 kDa. Following the nomenclature used for its homologue PR72 (27) , we named this novel protein PR59.

The cDNA of SEQ ID NO : 3 was used to screen a mouse PCC4 teratocarcinoma cDNA library, which yielded a 1.9 kilo base pair cDNA with an open reading frame of 1692 base pairs (SEQ ID N0:1) . The only difference between the SEQ ID NO: 3 and SEQ ID N0:1 is an insert of 90 base pairs in the inactive cDNA which specifies a 30 amino acid insert between amino acid 209 and 210 of SEQ ID NO: 2. The insert maps to a domain that is completely conserved with PR72 and encodes the sequence:

NH₂-SRYVPNKLPALGHAHLSKTPPTALCHNHRV-COOH ( SEQ ID NO: 5 ) that has no significant homology in the databases. We presume that the 90 base pair insert resulted from a mutation introduced by reverse transcriptase in an incompletely processed primary transcript, causing a mutation in a splice acceptor site.

Although the polypeptide encoded by SEQ ID NO: 3 was found to be inactive, it was used to raise antiserum which was able to co- precipitate PP2A activity, and its C-terminal region, which is identical to that of SEQ ID NO: 2, was used to demonstrate interaction with pocket proteins in a two-hybrid assay in yeast .

The amino acid comparison between PR59 and PR72 was determined. PR59 shares 56% identity and 65 % similarity with PR72. PR72 and PR130 are splice variants from the same gene, that differ in their N-terminal sequences. The homology between PR59 and PR72 extends into the diverged N-terminus of PR72, which suggests that PR59 is a homologue of PR72 rather than of PR130. The region of highest homology between PR59 and PR72 is evolutionary very conserved, as it is also found in D. melanogaster (DM25H1T) , T. Brucii (W99281) , C. elegans (U41014), A . thaliana (AB005239) , and in O. sa tiva (RICC10055A) , as determined by a search of various databases (accession number of clone with highest homology given in brackets) .

Sequence alignments were performed with the University of Wisconsin gcg package (19) or using the BLAST algorithm (2) to search Genbank.

Example 2 : Northern Blot Analysis .

To assess the expression of PR59 mRNA, a panel of murine tissue RNAs and RNA from different cell lines was probed with the coding region of clone pp44 at high stringency. Equal loading and transfer was verified by staining the RNA with ethidium bromide .

Briefly, total cytoplasmic RNA was prepared from murine tissues and from a panel of cell lines. Fifteen or twenty micrograms of total mRNA were electrophoresed through a 1% formaldehyde agarose gel containing 1 μg/ml ethidium bromide as previously described (7) , transferred to nitrocellulose, crosslinked by UV- irradiation, and the filter was photographed under UV light to check for equal loading and transfer of RNA. Subsequently, the filter was probed with a ³²P-labeled partial mouse PR59 cDNA (nucleotides 1179 to 1733 of SEQ ID NO : 3 ) .

It was found that the PR59 mRNA transcript is about 2 kb, indicating that the largest PR59 cDNA clone is full-length or very close to being full length. We did not detect any larger PR59 transcripts on the Northern blot, suggesting that our cDNA is indeed full length and that there are no PR130-like splice variants of PR59.

It was also found that PR59 mRNA is ubiquitously expressed at approximately equal levels in all tissues tested, and no splice variants were detected. RNA from the following murine tissues was found to be postive for PR59: lung, heart, spleen, kidney, liver and brain; together with the following cell lines: U2-OS (human osteosarcoma) ; COS (SV40 transformed monkey kidney cell line) ; SAOS-2 (human osteosarcoma) ; 293 (human adenovirus transformed embryonic kidney cell line) ; RET; (human adenovirus transformed retinal cell line) ; and DD3 (mouse hybridoma cell line) . That the mouse cDNA also detected a transcript in human and monkey cell lines indicates that the human homologue of PR59 is closely related to the mouse sequence, and can be obtained by a process analogous to that used to obtain SEQ ID NO:l using part of SEQ ID NO: 3 as a probe, under conditions of medium to high stringency, thus allowing a cDNA encoding the human PR59 to be isolated and used for expression of human PR59, thus providing this protein in isolated form.

Example 3: PR59 associates with PP2A activity.

To determine whether PR59 is a bona fide PP2A regulatory subunit that can associate with a functional PP2A catalytic subunit, we first generated several reagents.

A GST-PR59 fusion protein was prepared in E. coli by expression of the plasmid GST-PR59. This plasmid was made by cloning the ORF of the PR59 cDNA from the Apa I site at nucleotide -26 relative to the initiator ATG codon in frame in the Sma I site of pGEX-2TK. PR59 was and purified using glutathione Sepharose beads (Pharmacia) . The fusion protein (200 μg) in complete Freund's adjuvant was injected in a rabbit. Subsequent booster injections (200 μg of protein in incomplete Freund's adjuvant) were given at three-weekly intervals. After five rounds of immunization, polyclonal serum was obtained. Before the first immunization, pre-immune serum was obtained from the rabbit. By repeated immunization of a rabbit with a GST-PR59 fusion protein, a polyclonal antiserum was generated that specifically reacted with PR59 in both immunoprecipitation and Western blotting experiments (see below) .

Expression vectors in which the coding sequence of PR59, PR65, PR72 or the PP2A catalytic subunit (PP2Ac) was fused to a 10 amino acid epitope (HA) that is recognized by the monoclonal antibody 12CA5 were also made. pMV-HAtag was created by cloning an oligonucleotide that specifies a consensus AUG start codon, followed by the 10 amino acid influenza virus haemagglutinin epitope tag from the Xho I to Sal I sites of pBSK+ . The PR59 cDNA was then cloned from the Apa I site 26 bases upstream of the putative initiator codon in frame in pMV-HAtag, introducing 17 amino acids between the HA tag and the first PR59 methionine . HA-PR59 was then subcloned into a cytomegalo virus promoter-containing expression vector (pCMV) . HA-tagged PP2A catalytic subunit was created by cloning human catalytic subunit (49) from nucleotide 58 to 1022 in pMV-HAtag, introducing 19 additional amino acids between the HA tag and the first methionine. HA-PR65 was created by cloning the human PR65 α cDNA (22), starting from amino acid 3, in pMV-HAtag. HA-PR72 was created by PCR using as 5' primer 5' -TCGCGTCGACGATGATGATCAAGGAAACATC-3' (SEQ ID NO: 6) and T7 as 3' primer on the full length human PR72 cDNA in pBSK (23) . The PCR product was cloned into the Sal I site of pMV-HAtag. HA tagged PP2Ac and PR72 were then subcloned into pCMV. HA tagged PR65 was subcloned into the mammalian expression vector pRC/CMV (Invitrogen) . Using these reagents, we tested whether PR59 associates with PP2A phosphatase activity in vivo . For this purpose, an immunoprecipitation-phosphatase assay was used in which PP2A catalytic subunit activity was measured through the release of ³² [P0₄] from a peptide containing a phosphorylated PP2A consensus site. U2-0S human osteosarcoma cells were transfected with the HA-tagged PR59 (HA-PR59) , PR65 (HA-PR65) , PR72 (HA-PR72) or PP2A catalytic subunit (HA-PP2Ac) expression vectors. U2-0S and B104 cells were cultured in Dulbecco's modified Eagle medium supplemented with 10% fetal calf serum. Transfections were performed by the calcium phosphate precipitation method (53). Total DNA transfected was adjusted to 20 μg with empty vector. Mild detergent 12CA5 immunoprecipitates of transfected cells were tested for phosphatase activity.

Briefly, immunoprecipitations from the transfected U2-OS were performed as described previously (5) . The cells were scraped in ELB (250 mM NaCl, 0.1 % NP40, 50 mM HEPES pH 7.0 , 5 mM EDTA) supplemented with protease inhibitors (Complete, Boehringer

Mannheim and 1 mM phenylmethylsulfonylfluoride) and incubated on ice for 25 minutes. Insoluble material was removed by centrifugation and the supernatant was then rocked with 25 μl of a pre-formed complex of antibody coupled to protein A Sepharose beads. After 1 hour, the beads were washed three times in protease inhibitor-supplemented ELB buffer and once in phosphatase wash buffer (250 mM NaCl , 50 mM HEPES pH 7.0) . The beads were then resuspended in 50 μl phosphatase assay buffer (50 mM Tris pH 7.5, 0.1 mM EDTA, 0.9 mg/ml Bovine Serum Albumin (Sigma), 0.09% β-mercaptoethanol , 1 mM MnCl2) . Fifty μl phosphatase assay buffer with 125 nM ³²P-phosphorylated peptide (see below) and rocked at 30°C for three hours. After this, 350 μl stop mix (80 mM NaPPi, 1.6 mM NaH2P04, 0.9 M HC1, 3.2% w/v active charcoal) was added to the reaction, tubes were vortexed for 10 minutes, spun down, and 300 μl of the supernatant was counted. The amount of released ³²P0₄ never exceeded 25% of the input counts . The peptide Leu-Arg-Arg-Ala-Ser-Val-Ala (Kemptide Val6-Ala7, Bachem, SEQ ID NO: 7) was phosphorylated in the presence of γ32P-ATP by the catalytic subunit of protein kinase A as described previously (17) . Subsequently, the peptide was separated from un- incorporated g32P-ATP on a Dowex 1 (Fluka AG) column with 30% acetic acid, freeze dried twice, and aliquoted at 25 mM in 50 mM Tris-HCl pH 7.5 , 0.1 mM EDTA.

Figure la shows phosphatase activity measure as release of ³²P0₄ from the phosphorylated substrate peptide (vertical axis is released cpm minus released cpm from peptide incubated with assay buffer alone) . The average with standard error bars is shown. All phosphatase assays are representative of at least three independent experiments. As expected, the 12CA5 immunoprecipitate of HA-tagged PP2Ac-transfected cells contained significant amounts of phosphatase activity. Similarly, the 12CA5 immunoprecipitate from HA-PR65 and HA-PR72 transfected cells contained substantial amounts of phosphatase activity, suggesting that the endogenous PP2A catalytic subunit can be immunoprecipitated via a PP2A regulatory subunit in this assay.

To confirm that the phosphatase activity seen in these assays was indeed PP2A activity, okadaic acid was added to several of the immunoprecipitates to a final concentration of 5 nM. This concentration of okadaic acid will specifically inhibit PP2A, but not PP1 (10, 27) . Figure lc shows that the phosphatase activity in immunoprecipitates of the HA-PR65 and the HA-PP2Ac-transfected cells could be inhibited by this concentration of okadaic acid, strongly suggesting that the phosphatase activity seen was indeed PP2A.

Although all HA-tagged proteins were expressed and immunoprecipitated at comparable levels, as determined by 12CA5 Western blot analysis of an aliquot of the immunoprecipitate used for the phosphatase assay, no significant phosphatase activity was found in the HA-PR59 immunoprecipitate. In contrast, when U2-OS cells were transfected with expression vectors encoding non-tagged PR59 in combination with HA-PP2Ac and HA-PR65 subunits, and PR59 was immunoprecipitated with polyclonal PR59 antiserum, a significant amount of phosphatase activity could be detected in complex with PR59 (Fig. lb, lane 7) . No significant phosphatase activity was seen when PR59 was transfected alone (Fig. lb, lane 5) , which suggests that PR59 competes poorly with endogenous B subunits for binding to the PP2Ac/PR65 core dimer.

Next, we asked whether endogenous PR59 can be found associated with endogenous PP2A catalytic subunit. Untransfected rat neuroblastoma B104 cells were cultured as described for U2-0S cells and immunoprecipitated with polyclonal rabbit PR59 serum and tested for phosphatase activity. This polyclonal antiserum specifically immunoprecipitates in vi tro translated PR59, but does not cross react with in vi tro translated PR72. Figure Id shows that the PR59 immunoprecipitate contained significant phosphatase activity as compared to the control immunoprecipitate with pre-immune rabbit serum. Furthermore, the PR59-associated phosphatase activity could be completely inhibited with 5 nM okadaic acid (Fig. Id) . Together, this suggests that endogenous PR59 is associated with PP2A activity.

To further investigate whether PR59 can associate with the PP2A catalytic subunit, cells were transfected with HA-PP2AC and PR59 expression vectors. Lysates of transfected cells were immunoprecipitated with anti PR59 or control serum and analyzed by Western blot for the presence of the catalytic subunit. It was found that PP2Ac is present in anti-PR59 immunoprecipitates from cells expressing PR59 and PP2Ac, but not in control immunoprecipitates, nor in anti-PR59 immunoprecipitates from cells not expressing exogenous PR59. Taken together, these data show that the PR59 cDNA encodes a functional PP2A regulatory subunit. Example 4 : PR59 binds preferentially to p!07 in eukarvotic cells.

To compare the affinity of this novel PP2A subunit for the different members of the retinoblastoma protein family, the pp31 "bait" plasmid which encodes the carboxyl terminal 127 amino acids of PR59 was co-transformed with GAL4-DBD fusions of pRb, pl07 or pl30 in yeast strain Y190. Protein interaction was determined via activation of the LacZ reporter-gene as measured by the -galactosidase activity in the colonies (21) .

The results are summarized in Table 1:

TABLE 1

Table 1 legend: Kinetics of appearance of blue staining of the transformed yeast colonies was scored as a measure of association."-": no blue color detectable after overnight staining; no association, "+" : blue color detectable after overnight staining; association, "++" : blue color detectable after 1 hour of staining; strong association. Mouse E2F-5 (25) was used as a positive control. These data are representative results of 3 independent experiments .

Table 1 shows that in this assay PR59 interacted with pl07 and pl30, but not with pRb. In contrast, mouse E2F-5 interacted equally well with all three members of the retinoblastoma family, indicating that all DBD fusion proteins used in this assay are functional. These results suggest that the newly identified PP2A variable subunit has differential affinity for the various members of the pocket protein family.

Example 5: PR59 binds preferentially to p!07 in mammalian cells. To determine whether PR59 interacted also in mammalian cells with pl07, and whether this interaction was specific for this member of the retinoblastoma family of proteins, we performed a co-immunoprecipitation experiment. U2-0S cells were cultured and co-transfected (conditions as described above) with an expression vector that directs the synthesis of HA-PR59 together with expression vectors for both human pl07 and human pRb. As controls, expression vectors encoding PR59 without HA tag or HA-PR72 were co-transfected. Transfected cells were immunoprecipitated as described previously (5) . The immunoprecipitate or the total cell lysate were separated on a SDS 10% polyacrylamide gel and transferred to nitrocellulose. The membrane was blocked in PBS containing 0.05% Tween-20 and 5% dried non-fat milk for one hour at room temperature, and incubated for 16 hours at 4°C with the relevant antibodies in PBS with Tween and 1% milk.

The presence of pRb in the immunoprecipitates was detected with a polyclonal rabbit antiserum against the C-terminal 15 amino acids of pRb (C15) . After this, the blot was stripped of antibodies, and reprobed with a polyclonal antiserum against the C-terminal 18 amino acids of pl07 (C18) to detect pl07 in the immunoprecipitates .

Anti HAtag (12CA5) culture supernatant was used 1:500 diluted, anti-pl07 (C18) , anti-pRb (C15) and polyclonal anti-PR59 were used 1:4000. The membrane was then washed in PBS with Tween and 1% milk, and the antibody was detected using horseradish peroxidase-linked goat anti mouse IgG (Biorad) or peroxidase-linked goat anti rabbit (Biosource Ltd.) and enhanced chemiluminescence (Amersham) . The polyclonal antibodies against pl07 (C18) and pRb (C15) were obtained from Santa Cruz .

It was found that pl07 is present only in the HA-PR59 immunoprecipitate, but not in the control or HA-PR72 immunoprecipitates. No pRb was detected in the HA-PR59 immunoprecipitate, even though the expression of pRb and pl07 was equivalent in the transfected cells. Consistent with these data, we found that in a yeast two hybrid assay clone pp31, which encodes the carboxyl terminal 127 amino acids of PR59, interacted with pl07 and not with pRb. Furthermore, in an in vi tro GST pull down assay, PR59, but not PR72 , associated specifically with GST-pl07. Taken together, these data indicate that PR59 associates with pl07, but not with pRb, in mammalian cells, whereas PR72 associates with neither of the two pocket proteins under these conditions.

Example 6:PR59 co-localizes with p!07 and the PP2A catalytic subunit in nuclear structures.

Confocal Laser Scanning Microscopy (CLSM) was used to determine whether the association of PR59 with pl07 and with the PP2A catalytic subunit is reflected in their subcellular localization. U2-0S cells were grown and transfected (conditions as described above) with PR59, pl07 and HA-PP2AC expression vectors on glass cover slips, washed twice in PBS, and fixation was done by incubation in 80% acetone for 5 min. Cells were subsequently washed, and blocked with PBS with 0.05% Tween20 and 1% BSA. Incubations with antibodies were also done in PBS/Tween/BSA. Polyclonal antibodies C18, anti-PP2Ac and anti-PR59 were diluted 1:100, 12CA5 supernatant was diluted 1:3.

Co-expression of PR59 (detected by red fluorescence with anti PR59 polyclonal rabbit serum) and pl07 (detected by green fluorescence with SD9 monoclonal anti pl07) resulted in cytoplasmic staining of PR59, nuclear staining of pl07, as well as co-localization (seen as yellow) in dots in the nucleus. These experiments were repeated with similar results using different proteins (HA-tagged PR59 and pl07) in combination with a different set of antibodies (Anti-HA tag and C18) , and also using a different fixation method. As expected (because of the lack of interaction between PR59 and pRb) co-expression of PR59 with pRb did not result in co-localization, and PR59 was not present in the nucleus.

Co-expression of PR59 with HA-PP2AC also resulted in nuclear dots. When a polyclonal anti PP2Ac serum was used (from B.A. Hemmings (F.M.I., Basel)), PP2Ac was also detected in dots in nuclei of untransfected cells, indicating that these dots are not a result of over-expression. To determine wether PR59 indeed recruits pl07 to PP2Ac in these nuclear dots, a triple transfection was performed in which PR59, pl07 and HA-PP2AC were co-expressed. Using a polyclonal anti pl07 serum (C18) and 12CA5 to detect HA-PP2AC, pl07 was found to localize to the same nuclear dots were PP2Ac was found to be present.

Next, we examined the phosphorylation status of pl07 which is present in the nuclear dots with PR59. Because SD9 , the anti-pl07 monoclonal antibody (monoclonal antibodies to pl07 (SD9) and the 12CA5 haemagglutinin tag antibody were as described previously (18, 59)) is specific for the hypophosphorylated form of pl07 in Western blot, we determined whether this specificity also holds true in immunofluorescence experiments. To this end, we manipulated the phosphorylation status of pl07 by co-expression of pl07 with either cyclin Dl and cdk4 to generate hyperphosphorylated pl07, or co-transfected pl07 with a dominant negative mutant of cdk4 (cdk4 d.n.) to generate hypophosphorylated pl07. Transfected cells were then stained with both the phosphorylation- insensitive polyclonal C18 serum and with monoclonal antibody SD9. We found that SD9 does not stain pl07 in its hyperphosphorylated state, whereas C18 does. In contrast, both SD9 and C18 serum are both able to recognize hypophosphorylated pl07. These data indicate that monoclonal antibody SD9 only reacts with hypophosphorylated pl07 in these experiments. Since the nuclear dots that contain pl07 can be stained both with SD9 and with C18 these results suggest strongly that the pl07 in the nuclear dots is in a hypophosphorylated state. Example 7: p!07 is dephosphorylated in vivo by PR59.

Next, we determined whether association of PR59 to pl07 also results in dephosphorylation of pl07 in vivo . For this purpose, we again used U2-0S cells, which have high endogenous cyclin Dl/cdk4 kinase activity towards pocket proteins and consequently hyperphosphorylate both endogenous and ectopically expressed pl07 and pRb. U2-0S cells were transfected with 3μg of expression vectors for the HA-tagged pl07 (to distinguish transfected from endogenous pl07) and increasing amounts (2, 5 14 μg) of PR59 expression vector.As a control, HA-pl07 was co-transfected with lOg μg cdk4 dominant negative to generate hypophosphorylated pl07. It was found that expression of HA-pl07 in U2-0S cells resulted in hyperphosphorylated pl07, whereas co-expression of increasing amounts of PR59 resulted in increasing amounts of hypophosphorylated pl07. HA-pRb is also hyperphosphorylated in U2-OS cells, but no hypophosphorylated pRb appeared when PR59 was co-expressed with pRb (from 6 μg vector) , whereas co-expression of cdk4 dominant negative did result in the appearance of hypo-phosphorylated pRb. To establish that the difference seen here in dephosphorylation of pl07 and pRb was not due to a difference in expression of PR59, the cell lysates were also analyzed by Western blot with antiserum against PR59. These results showed that co-expression of PR59 with pl07 leads to an increase in the amount of hypophosphorylated pl07, but does not influence pRb phosphorylation status.

The results were measured by western blotting whole cell lysates which were corrected for transfection efficiency and separated on a SDS-PAGE gel (7% PAA for HA-pl07 and HA-pRb, 10% PAA for PR59) .

Example 8 : Overexpression of PR59 results in inhibition of cell cycle progression. Expression of hypophosphorylated pl07 inhibits progression through the cell cycle by causing cells to acculumate in Gl (4, 59) . We therefore asked whether expression of PR59 caused an increase in the Gl population of transfected cells. To this end U2-0S cells, which contain pl07 in an inactive, hyperphosphorylated form, were transfected with increasing amounts (10 and 20 μg) of PR59, or lOμg HAtagged PR59 expression vector, together with 2μg of a CD20 vector. 48 hours after transfection, cells were stained for DNA content and CD20 expression and the cell cycle profile of the CD20 positive transfected cells was analyzed by FACS. Expression of PR59 resulted in a marked increase in cells in Gl compared to control cells (Fig. 2a-d, which indicates the percentages of cells in Gl, S or G2/M phase) .

To further investigate the effects of PR59 on cell proliferation, a colony formation assay was performed. U2-OS cells were transfected in triplicate with 10 or 20μg of PR59 expression vector or control empty vector, together with 0.5 μg of a puromycin resistance vector. 48 hours after transfection, the cells were placed under puromycin selection, and after 10 days colonies were counted. The number of colonies and the standard deviation is given as a percentage of colonies with the control vector alone. It was found that transfection of U2-0S cells with a puromycin-selectable marker and increasing amounts PR59 expression vector, causes a marked reduction in the number of reduces the number of puromycin-resistant colonies. Taken together, these data indicate that elevated expression of PR59 causes a reduction of cell proliferation that results from an accumulation of cells in the Gl phase of the cell cycle.

The following documents cited above are incorporated herein by Reference : 1. Altiok, S., M. Xu, and B. M. Spiegelman. 1997. PPAR-gamma induces cell cycle withdrawal : inhibition of E2F/DP DNA-binding activity via downregulation of PP2A. Genes & Dev. 11:1987-1998.

2. Altschul, S. F., W. Gish, W. Miller, E. W. Myers, and D. J. Lipman. 1990. Basic local alignment search tool. J. Mol . Biol. 215:403-410.

3. Beijersbergen, R. L. , and R. Bernards. 1996. Cell cycle regulation by the retinoblastoma family of growth inhibitory proteins. Biochem. Biophys . Acta, Reviews on Cancer. 1287:103-120.

4. Beijersbergen, R. L., L. Carlee, R. M. Kerkhoven, and R. Bernards. 1995. Regulation of the retinoblastoma-related pl07 by Gl cyclin complexes. Genes & Dev. 9:1340-1353.

5. Beijersbergen, R. L., R. M. Kerkhoven, L. Zhu, L. Carlee, P. M. Voorhoeve, and R. Bernards. 1994. E2F-4, a new member of the E2F gene family, has oncogenic activity and associates with pl07 in vivo . Genes & Dev. 8:2680-2690.

6. Bernards, R. 1997. E2F, a nodal point in cell cycle regulation. Biochem. Biophys. Acta. Reviews on Cancer 1333 :M33-M40.

7. Bernards, R. , S. K. Dessain, and R. A. Weinberg. 1986. N-myc amplification causes down-modulation of MHC class I antigen expression in neuroblastoma . Cell 47:667-674.

8. Cegielska, A., S. Shaffer, R. Derua, J. Goris, and D. M. Virshup. 1994. Different oligomeric forms of protein phosphatase 2A activate and inhibit simian virus 40 DNA replication. Mol. Cell. Biol. 14:4616-4623.

9. Chevray, P. M. , and D. Natans . 1992. Protein interaction cloning in yeast: identification of mammalian proteins that react with the leucine zipper of jun. Proc. Nat. Acad. Sci. USA 89:5789-5793.

10. Cohen, P., S. Klumpp, and D. L. Schelling. 1989. An improved procedure for identifying and quantitating protein phosphatases in mammalian tissues. FEBS Lett. 250:596-600.

11. Cohen, P. T., N. D. Brewis, V. Hughes, and D. J. Mann. 1990. Protein serine/threonine phosphatases; an expanding family. FEBS Lett. 268:355-359. 12. Dou, Q. P., B. An, and P. L. Will. 1995. Induction of a retinoblastoma phosphatase activity by anticancer drugs accompanies p53 independent Gl arrest and apoptosis. Proc. Natl. Acad. Sci. USA 92:9019-9023.

13. Dowdy, F. D., P. W. Hinds, K. Louie, S. I. Reed, and R. A. Weinberg. 1993. Physical interaction of the retinoblastoma protein with human D cyclins. Cell 73:499-511.

14. Durfee, T., K. Becherer, P. L. Chen, S. H. Yeh, Y. Yang, A. E. Kilburn, W. H. Lee, and S. J. Elledge. 1993. The retinoblastoma protein associates with the protein phosphatase type 1 catalytic subunit. Genes & Dev. 7:555-569.

15. Dynlacht, B. D., 0. Flores, J. A. Lees, and E. Harlow. 1994. Differential regulation of E2F transactivation by cyclin/cdk2 complexes. Genes & Dev. 8:1772-1786.

16. Ewen, M. E., H. K. Sluss, C. J. Sherr, H. Matsushime, J. Kato, and D. M. Livingston. 1993. Functional interactions of the retinoblastoma protein with mammalian D-type cyclins. Cell 73 :487-497.

17. Favre, B., S. Zolnierowicz, P. Turowski, and B. A. Hemmings. 1994. The catalytic subunit of protein phosphatase 2A is carboxyl-methylated in vivo . J. Biol. Chem. 269:16331-16317.

18. Field, J. , J. I. Nikawa, D. Broek, B. MacDonald, L. Rodgers, I. A. Wilson, R. A. Lerner, and M. Wigler. 1988. Purification of a ras-responsive adenylyl cyclase complex from Saccharomyces cerevisiae by use of an epitope addition method. Mol. Cell. Biol. 8:2159-2165.

19. Genetics Computer Group, I. 1994. Program manual for the Wisconsin Package 8, Madison, WI .

20. Hannon, G. J. , D. Demetrick, and D. Beach. 1993. Isolation of the Rb-related pl30 through its interaction with CDK2 and cyclins. Genes & Dev. 7:2378-2391.

21. Harper, J. W. , G. R. Adami, N. Wei, K. Keyomarsi, and S. J. Elledge. 1993. The p21 Cdk-interacting protein Cipl is a potent inhibitor of Gl cyclin-dependent kinases . Cell 75:805-816.

22. Hemmings, B. A., P. C. Adams, F. Maurer, P. Muller, J. Goris, W. Merlevede, J. Hofsteenge, and S. R. Stone. 1990. Alpha- and beta-forms of the 65-kDa subunit of protein phosphatase 2A have a similar 39 amino acid repeating structure. Biochemistry 29:3166-3173.

23. Hendrix, P., R. E. Mayer-Jackel, P. Cron, J. Goris, J. Hofsteenge, W. Merlevede, and B. A. Hemmings. 1993. Structure and expression of a 72 -kDa regulatory subunit of protein phosphatase 2A. Evidence for different size forms produced by alternative splicing. J. Biol. Chem. 268:15267-15276.

24. Hendrix, P., P. Turowski, J. R. Mayer, J. Goris, J. Hofsteenge, W. Merlevede, and B. A. Hemmings. 1993. Analysis of subunit isoforms in protein phosphatase 2A holoenzymes from rabbit and Xenopus . J. Biol. Chem. 268:7330-7337.

25. Hijmans, E. M., P. M. Voorhoeve, R. L. Beijersbergen, L. J. van 't Veer, and R. Bernards. 1995. E2F-5, a new E2F family member that interacts with pl30 in vivo . Mol. Cell. Biol. 15:3082-3089.

26. Hinds, P. W. , S. Mittnacht, V. Dulic, A. Arnold, S. I. Reed, and R. A. Weinberg. 1992. Regulation of retinoblastoma protein functions by ectopic expression of human cyclins. Cell 70:993-1006.

27. Honkanan, R. E., B. A. Codispoti, K. Tse, and A. L. Boynton. 1994. Characterization of natural toxins with inhibitory activity against serine/threonine protein phosphatases. Toxicon 32:339-350.

28. Hubbard, M. J. , and P. Cohen. 1993. On target with a new mechanism for the regulation of protein phosphorylation. Trends Biochem. Sci. 18:172-177.

29. Kamibayashi, C, R. Estes, R. L. Lickteig, S. I. Yang, C. Craft, and M. C. Mumby. 1994. Comparison of heterotrimeric protein phosphatase 2A containing different B subunits. J. Biol. Chem. 269:20139-20148.

30. Kamibayashi, C. , and M. C. Mumby. 1995. Control of protein phosphatase 2A by multiple families of regulatory subunits. Adv. Prot. Phosphatases 9:195-210.

31. Kitagawa, M. , H. Higashi, T. I. Suzuki, K. Segawa, S. K. Hanks, Y. Taya, S. Nishimura, and A. Okuyama. 1995. Phosphorylation of E2F-1 by cyclin A-cdk2. Oncogene 10:229-236. 32. Kozak, M. 1992. Regulation of translation in eukaryotic systems. Annu. Rev. Cell. Biol. 8:197-225.

33. Krek, W. , Ewen, M.E., Shirodkar, S., Arany, Z., Kaelin, Jr., W.G. and Livingston, D.M. 1994. Negative regulation of the growth promoting transcription factor E2F-1 by a stably bound cyclin A-dependent protein kinase . Cell 78:161-172.

34. Lees, E., B. Faha, V. Dulic, S. I. Reed, and E. Harlow.

1992. Cyclin E/cdk2 and cyclin A/cdk2 kinases associate with pl07 and E2F in a temporally distinct manner. Genes & Dev. 6:1874-1885.

35. Li, L. J., G. S. Naeve, and A. S. Lee. 1993. Temporal regulation of cyclin A-pl07 and p33cdk2 complexes binding to a human thymidine kinase promoter element important for Gl-S phase transcriptional regulation. Proc. Natl . Acad. Sci. USA 90:3554-3558.

36. Li, Y., C. Graham, S. Lacy, A. M. V. Duncan, and P. Whyte .

1993. The adenovirus ElA-associated 130-kD protein is encoded by a member of the retinoblastoma gene family and physically interacts with cyclins A and E. Genes & Dev. 7:2366-2377.

37. Mayer, R. E., P. Hendrix, P. Cron, R. Matthies, S. R. Stone, J. Goris, W. Merlevede, J. Hofsteenge, and B. A. Hemmings. 1991. Structure of the 55-kDa regulatory subunit of protein phosphatase 2A: evidence for a neuronal-specific isoform. Biochemistry 30:3589-3597.

38. Mayer-Jaekel, R. E., and B. A. Hemmings. 1994. Protein phosphatase 2A- a 'menage a trois'. Trends Cell Biol.

4 :287-291.

39. McCright, B., and D. M. Virshup. 1995. Identification of a new family of protein phosphatase 2A regulatory subunits. J. Biol. Chem. 270:26123-26128.

40. Mumby, M. C. , and G. Walter. 1993. Protein serine/threonine phosphatases: structure, regulation, and functions in cell growth. Physiol. Rev. 73:673-699.

41. Nelson, D. A., N. A. Krucher, and J. W. Ludlow. 1997. High molecular weight protein phosphatase type 1 dephosphorylates the retinoblastoma protein. J. Biol. Chem. 272:4528-4535. 42. Nelson, D. A., and J. W. Ludlow. 1997. Characterization of the mitotic phase pRb directed protein phosphatase activity. Oncogene 14:2407-2415.

43. Okamoto, K. , C. Kamibayashi, M. Serrano, C. Prives, M. C. Mumby, and D. Beach. 1996. p53 -dependent association between cyclin G and the B' subunit of protein phosphatase 2A. Mol. Cell. Biol. 16:6593-6602.

44. Pallas, D. C, W. Weller, S. Jaspers, T. B. Miller, W. S. Lane, and T. M. Roberts. 1992. The third subunit of protein phosphatase 2A (PP2A) , a 55-kilodalton protein which is apparently substituted for by T antigens in complexes with the 36- and 63-kilodalton PP2A subunits, bears little resemblance to T antigens. J. Virol. 66:886-893.

45. Radulescu, R. T. 1995. The 'LXCXE' hydropathic superfamily of ligands for retinoblastoma protein: a proposal. Med. Hypotheses 44:28-31.

46. Schiestl, R. H. , and R. D. Gietz . 1989. High efficiency transformation of intact yeast cells using single stranded nucleic acids as carrier. Curr. Genet. 16:339-346.

47. Shirodkar, S., M. Ewen, J. A. DeCaprio, J. Morgan, D. M. Livingston, and T. Chittenden. 1992. The transcription factor E2F interacts with the retinoblastoma product and a pl07-cyclin A complex in a cell cycle-regulated manner. Cell 68:157-166.

48. Sontag, E., S. Fedorov, C. Kamibayashi, D. Robbins, M. Cobb, and M. C. Mumby. 1993. The interaction of SV40 small tumor antigen with protein phosphatase 2A stimulates the map kinase pathway and induces cell proliferation. Cell 75:887-897.

49. Stone, S. R., R. Mayer, W. Wernet, F. Maurer, J. Hofsteenge, and B. A. Hemmings. 1988. The nucleotide sequence of the cDNA encoding the human lung protein phosphatase 2A alpha catalytic subunit. Nucleic Acids Res. 16:11365.

50. Tehrani, M. A., M. C. Mumby, and C. Kamibayashi. 1996. Identification of a novel protein phosphatase 2A regulatory subunit highly expressed in muscle. J. Biol. Chem. 271:5164-5170.

51. Tung, H. Y., R. H. De, L. J. Zhao, X. Cayla, B. P. Roques, and R. Ozon. 1997. Direct activation of protein phosphatase-2A0 by HIV-1 encoded protein complex NCp7:vpr. FEBS Lett. 401:197-201.

52. van den Heuvel, S., and E. Harlow. 1993. Distinct roles for cyclin-dependent kinases in cell cycle control. Science 262:2050-2054.

53. Van der Eb, A. J. , and F. L. Graham. 1980. Assay of transforming activity of tumor virus DNA. Meth. Enzymol . 65:826-839.

54. Virshup, D. M. , A. Cegielska, A. Russo, T. J. Kelly, and S. Shaffer. 1993. The initiation of SV40 DNA replication is controlled by a phosphorylation-dephosphorylation cycle. Adv. Prot. Phosphatases 7:271-293.

55. Weinberg, R. A. 1995. The retinoblastoma protein and cell cycle control. Cell 81:323-330.

56. Xu, M., K. A. Sheppard, C. Y. Peng, A. S. Yee, and W. H. Piwnica. 1994. Cyclin A/CDK2 binds directly to E2F-1 and inhibits the DNA-binding activity of E2F-1/DP-1 by phosphorylation. Mol. Cell. Biol. 14:8420-8431.

57. Zhu, L., G. Enders, J. A. Lees, R. L. Beijersbergen, R. Bernards, and E. Harlow. 1995. The pRB-related protein pl07 contains two growth suppression domains : independent interactions with E2F and cyclin/cdk complexes. EMBO J.

14 :1904-1913.

58. Zhu, L. , E. Harlow, and B. D. Dynlacht . 1995. pl07 uses a p21CIPl -related domain to bind cyclin/cdk2 and regulate interactions with E2F. Genes & Dev. 9:1740-1752.

59. Zhu, L., S. van den Heuvel, K. Helin, A. Fattaey, M. Ewen, D. Livingston, N. Dyson, and E. Harlow. 1993. Inhibition of cell proliferation by pl07, a relative of the retinoblastoma protein. Genes & Dev. 7:1111-1125.

60. Zolnierowicz, S., H. C. Van, N. Andjelkovic, P. Cron, I. Stevens, W. Merlevede, J. Goris, and B. A. Hemmings. 1996. The variable subunit associated with protein phosphatase 2A0 defines a novel multimember family of regulatory subunits. Biochem. J. 317:187-194. SEQUENCE LISTING

SEQ ID N0:1 AND SEQ ID NO : 2

-218 GAATTCaagcccccgtgggagagaaaaggaaaagcacagacccgagggaggacagaaccgcgggtgaaccccacggtgacccaggtgacgtcatgaccgc

-118 gctgcagggtcgcggatcgcctgagcgcggccctcgcgaatcgcccgggtccccggggtcgcaggccacacccacggattcgcaggccgggccctaccct

-18 gctcgagccacgcccaccATGCCGGAGCGCCCGCCCATACGAGCCCTGCGCCGCGACCCTGACGACCCAGCGGTGGCCCAGGCTCTGGCGTCGCTGGCTC 1 M P E R P P I R A L R R D P D D P A V A Q A L A S L A R

82 GGGGCAGCGATCTGGTGTTTCCGTCGCGCTTCCAGAAGTGGCTGCGGGACTTCCGGCAGGTCCATGCCCACAGGAAGGAGGAGCCCCCGCCCCAGTCCCC 29 G S D L V F P S R F Q K L R D F R Q V H A H R K E E P P P Q S P

182 ACCCCCTGGCCACACGGTCCCCGCCTTCTACTTCCCATGCGGTCGACCCCCGCCCCGTCCCCAGGACACGGAGGATGCCATCGCCCTCGTGGAGTGCGCG 62 P P G H T V P A F Y F P C G R P P P R P Q D T E D A I A L V E C A

282 TTCGAGGGGCTGCCTCGTGGGAGGGCGGGTCTGGGCGACATGGCCGTGGTTGCCAAGGCCTGTGGCTGCCCTCTGTACTGGAAGGCCCCGCTGTTCTACG 95 F E G L P R G R A G L G D M A V V A K A C G C P L Y K A P L F Y A

-P=>

382 CTGCGGGCGGCGAGCGCACAGGCTCTGTATCCGTGCACATGTTCGTAGCTATGTGGCGCAAAGTCCTGCTGACCTGCCATGATGATGCTGCGCGGTTCGT 129 A G G E R T G S V S V H M F V A M W R K V L L T C H D D A A R F V

482 GCGGCTGCTGGGCCACCCCGGGTGTAGTGGCCTCATCCAGGAGGACTTTGTGCCCTTCCTGCAGGATGTGGTGAACTCGCACCCGGGCCTGGCCTTCCTG 162 R L L G H P G C S G L I Q E D F V P F L Q D V V N S H P G L A F L

582 AGGGCAGCGAAGGACTTCCACTCACGTTACATCACCACAGTGATTCAGAGGATCTTCTACACAGTAAACCGGTCATGGTCAGGGATGATCTCGCGTGAGG 195 R A A K D F H S R Y I T T V I Q R I F Y T V N R S W S G M I S R E E

682 AGCTAAGGCGCAGCAGCTTCCTGCAGGCTGTGTCACAGTTGGAGGTAGAGCCTGACATCAACCGCATGACGTCGTTCTTCTCTTATGAGCATTTCTATGT 229 L R R S S F L Q A V S Q L E V E P D I N R M T S F F S Y E H F Y V

782 CATCTACTGCAAGTTCTGGGAGCTCGACCTCGACCGCGACCTCACCATTGACAGGAGCGACCTCGCACGCCATGGGGACGGGGCCATCTCCTCCAGAATG

262 I Y C K F W E L D L D R D L T I D R S D L A R H G D G A I S S R M

882 ATTGACCGCATCTTCTCAGGGGCCGTCACCAGGGCGAGGCTGCCCCGAAAGGTCGGGAAGCTGAGCTACGCAGACTTTGTATGGTTCCTGCTCTCCGAGG

295 I D R I F S G A V T R A R L P R K V G K L S Y A D F V F L L S E E

982 AGGACAAGACTACGCCCACCAGCACCGAGTACTGGTTCCGTTGCATGGACCTGGACGGTGACGGCGCTCTCTCCATGTTTGAGCTCGAGTTCTTCTATGA

329 D K T T P T S T E Y W F R C M D L D G D G A L S M F E L E F F Y E

1082 GGAGCAGGCCCAGCGCATGGCCGCCCGGGGTGTGGAGCCCCTCCCCTTCCACGACCTTGCACGCCAGGTGCTCGACCTCGTCGCCCCGCGCTGCCCCGGC

362 E Q A Q R M A A R G V E P L P F H D L A R Q V L D L V A P R C P G

1182 CGGATCACGCTGCGCGACCTGAAGCAGTGCGGCCTGGCGGGTGAGTTCTTCGACGCCTTCTTCAACGTGGACAAGTACCTGGCACGGGAGCAGCGCGAGC

395 R I T L R D L K Q C G L A G E F F D A F F N V D K Y L A R E Q R E Q

1282 AGGCGGGGACTCCGCAGGACACTGACTCCGACCCCGCGGCATCTGCCTGGGACAGGTATGCCGCCGAGGAGTACGACTTCCTCGTGGCCGAGGAGGCCAT 429 A G T P Q D T D S D P A A S A W D R Y A A E E Y D F L V A E E A M

1382 GGCAGAGGACGACGACGACCATGACGAAGGGTCCGACCCCATAGATCTCTACGGCCTTGCTGACGAGGACTGCGACGACCTGGAGCCTCTGTGAcgccct 462 A E D D D D H D E G S D P I D L Y G L A D E D C D D L E P L *

1482 actgtgcgcccaggaactgcgcacttgtgcctcccagtgatccctcccaaggcaggggtgacatcgggggctcggggtgtgacctcctccccgtggcctg

1582 cagccctgctgtgtgcaggtgcctcacacatgacgtttgtgggtttgttttttttttaataaaaagtttgatttgttttttcaaaaaaaaaaAAGCTT

SEQ ID NO: 3 AND SEQ ID NO : 4

I tn

I

10 30 50 70 90

GAATTCAAGCCCCCGTGGGAGAGAAAAGGAAAAGCACAGACCCGAGGGAGGACAGAACCGCGGGTGAACCCCACGGTGACCCAGGTGACGTCATGACCGC

110 130 150 170 190

GCTGCAGGGTCGCGGATCGCCTGAGCGCGGCCCTCGCGAATCGCCCGGGTCCCCGGGGTCGCAGGCCACACCCACGGATTCGCAGGCCGGGCCCTACCCT

210 230 250 270 290

GCTCGAGCCACGCCCACCATGCCGGAGCGCCCGCCCATACGAGCCCTGCGCCGCGACCCTGACGACCCAGCGGTGGCCCAGGCTCTGGCGTCGCTGGCTC M P E R P P I R A L R R D P D D P A V A Q A L A S L A R

310 330 350 370 390

GGGGCAGCGATCTGGTGTTTCCGTCGCGCTTCCAGAAGTGGCTGCGGGACTTCCGGCAGGTCCATGCCCACAGGAAGGAGGAGCCCCCGCCCCAGTCCCC G S D L V F P S R F Q K L R D F R Q V H A H R K E E P P P Q S P

410 430 450 470 490

ACCCCCTGGCCACCACGGTCCCCGCCTTCTACTTCCTTGCGGTCGACCCCCGCCCCGTCCCCAGGACACGGAGGATGCCATCGCCCTCGTGGAGTGCGCG P P G H H G P R L L L P C G R P P P R P Q D T E D A I A L V E C A

510 530 550 570 590

TTCGAGGGGCTGCCTCGTGGGAGGGCGGGTCTGGGCGACATGGCCGTGGTTGCCAAGGCCTGTGGCTGCCCTCTGTACTGGAAGGCCCCGCTGTTCTACG

F E G L P R G R A G L G D M A V V A K A C G C P L Y K A P L F Y A

610 630 650 670 690

CTGCGGGCGGCGAGCGCACAGGCTCTGTATCCGTGCACATGTTCGTAGCTATGTGGCGCAAAGTCCTGCTGACCTGCCATGATGATGCTGCGCGGTTCGT

A G G E R T G S V S V H M F V A M W R K V L L T C H D D A A R F V

710 730 750 770 790

GCGGCTGCTGGGCCACCCCGGGTGTAGTGGCCTCATCCAGGAGGACTTTGTGCCCTTCCTGCAGGATGTGGTGAACTCGCACCCGGGCCTGGCCTTCCTG

R L L G H P G C S G L I Q E D F V P F L Q D V V N S H P G L A F L

810 830 850 870 890

AGGGCAGCGAAGGACTTCCACTCACGTTACATCACCACAGTGAGCAGATACGTCCCGAACAAGCTCCCGGCCCTGGGACACGCCCACCTCTCCAAGACCC R A A K D F H S R Y I T T V S R Y V P N K L P A L G H A H L S K T P

910 930 950 970 990

CACCCACAGCTCTCTGCCACAACCACAGGGTGATTCAGAGGATCTTCTACACAGTAAACCGGTCATGGTCAGGGATGATCTCGCGTGAGGAGCTAAGGCG P T A L C H N H R V I Q R I F Y T V N R S S G M I S R E E L R R

1010 1030 1050 1070 1090

CAGCAGCTTCCTGCAGGCTGTGTCACAGTTGGAGGTAGAGCCTGACATCAACCGCATGACGTCGTTCTTCTCTTATGAGCATTTCTATGTCATCTACTGC S S F L Q A V S Q L E V E P D I N R M T S F F S Y E H F Y V I Y C

1110 1130 1150 1170 1190 CCGCGACCTCACCATTGACAGGAGCGACCTCGCACGCCATGGGGACGGGGCCATCTCCTCCAGAATGATTGACCGCA -ι

AAGTTCTGGGAGCTCGACCTCGA i CT) K F E L D L D R D L T I D R S D L A R H G D G A I S S R M I D R I '

1210 1230 1250 1270 1290

TCTTCTCAGGGGCCGTCACCAGGGCGAGGCTGCCCCGAAAGGTCGGGAAGCTGAGCTACGCAGACTTTGTATGGTTCCTGCTCTCCGAGGAGGACAAGAC F S G A V T R A R L P R K V G K L S Y A D F V W F L L S E E D K T

1310 1330 1350 1370 1390

TACGCCCACCAGCACCGAGTACTGGTTCCGTTGCATGGACCTGGACGGTGACGGCGCTCTCTCCATGTTTGAGCTCGAGTTCTTCTATGAGGAGCAGGCC T P T S T E Y F R C M D L D G D G A L S M F E L E F F Y E E Q A

1410 1430 1450 1470 1490

CAGCGCATGGCCGCCCGGGGTGTGGAGCCCCTCCCCTTCCACGACCTTGCACGCCAGGTGCTCGACCTCGTCGCCCCGCGCTGCCCCGGCCGGATCACGC Q R M A A R G V E P L P F H D L A R Q V L D L V A P R C P G R I T L

1510 1530 1550 1570 1590

TGCGCGACCTGAAGCAGTGCGGCCTGGCGGGTGAGTTCTTCGACGCCTTCTTCAACGTGGACAAGTACCTGGCACGGGAGCAGCGCGAGCAGGCGGGGAC

R D L K Q C G L A G E F F D A F F N V D K Y L A R E Q R E Q A G T

1610 1630 1650 1670 1690

TCCGCAGGACACTGACTCCGACCCCGCGGCATCTGCCTGGGACAGGTATGCCGCCGAGGAGTACGACTTCCTCGTGGCCGAGGAGGCCATGGCAGAGGAC

P Q D T D S D P A A S A D R Y A A E E Y D F L V A E E A M A E D

1710 1730 1750 1770 1790

GACGACGACCATGACGAAGGGTCCGACCCCATAGATCTCTACGGCCTTGCTGACGAGGACTGCGACGACCTGGAGCCTCTGTGACGCCCTACTGTGCGCC

D D D H D E G S D P I D L Y G L A D E D C D D L E P L *

1810 1830 1850 1870 1890

CAGGAACTGCGCACTTGTGCCTCCCAGTGATCCCTCCCAAGGCAGGGGTGACATCGGGGGCTCGGGGTGTGACCTCCTCCCCGTGGCCTGCAGCCCTGCT

1910 1930 1950 1970

GTGTGCAGGTGCCTCACACATGACGTTTGTGGGTTTGTTTTTTTTTTAATAAAAAGTTTGATTTGTTTTTTCAAAAAAAAAAAAGCTT

SEQ ID NO: 5

NH₂-SRYVPNKLPALGHAHLSKTPPTALCHNHRV-COOH

SEQ ID NO: 6 TCGCGTCGACGATGATGATCAAGGAAACATC

SEQ ID NO: 7 Leu-Arg-Arg-Ala-Ser-Val-Ala

Claims

1. A polypeptide in substantially isolated form which comprises :

(a) the sequence of the polypeptide of SEQ ID NO : 2 ;

(b) a fragment of said sequence which retains the ability to associate with the pocket protein pl07;

(c) a variant of (a) or (b) which is at least 90% homologous thereto; or

(d) a mammalian homologue of (a) or (b) .

2. A polypeptide according to claim 1 encoded by a cDNA obtainable by: providing a nucleic acid probe comprising SEQ ID NO: 3; probing a murine cDNA library with said probe under conditions of medium to high stringency; and isolating a positive clone comprising an open reading frame encoding a homologue of SEQ ID NO: 3.

3. A polypeptide according to claim 1 or 2 which has the sequence of SEQ ID NO: 2.

4. A polypeptide according to claim 1 which comprises the C- terminal 127 amino acids of the polypeptide of SEQ ID NO: 4 or a fragment thereof which retains the ability to retain a differential affinity for the pl07 pocket protein compared to the affinity for pRb.

5. A mammalian homologue of a polypeptide according to claim 1, 2 or 3.

6. A homologue according to claim 5 which is a human homologue .

7. A nucleic acid encoding a polypeptide as defined in any one of claims 1 to 6.

8. An expression vectors comprising the nucleic acid of claim 7 operably linked to a promoter.

9. A host cell capable of expressing a polypeptide of any one of claims 1 to 6 by a method comprising: i) providing the host cell carrying a vector as defined in claim 8 said vector also comprising a promoter compatible with said host cell; ii) growing said host cell under conditions to bring about expression of the polypeptide; and iii) recovering the polypeptide.

10. An antibody capable of binding a polypeptide according to any one of claims 1 to 6.

11. A method of inhibiting proliferation of a mammalian cell which comprises delivering to said cell an expression vector which expresses a polypeptide according to claim 1, 2 or 3 , which polypeptide is capable of binding to and dephosphorylating pl07.