PROTEIN PHOSPHATASE-1 CATALYTIC SUBUNIT INTERACTIONS

Abstract: A method of identifying a compound which modulates the interaction between a PP1c and a regulatory subunit thereof, the method comprising determining whether a compound enhances or disrupts the interaction between (a) a PP1c or a fragment, variant, derivative or fusion thereof or a fusion of a fragment, variant or derivative and (b) a regulatory subunit which is able to bind to PP1c or a PP1c-binding fragment, variant, derivative or fusion of said subunit or a fusion of said fragment, variant or derivative. A method of affecting cellular metabolism or function, the method comprising administering to a cell (a) a compound which modulates the interaction between a PP1c and a regulatory subunit thereof or (b) a compound which mimics the effect of a regulatory subunit of PP1c or (c) a peptide capable of binding a PP1c and which affects the ability of PP1c to bind to a particular target and/or affects the regulation of PP1c activity, or a functional equivalent thereof.

Description: PROTEIN PHOSPHATASE-1 CATALYTIC SUBUNIT INTERACTIONS The present invention relates to peptides and protein-protein interactions and to the use of peptides, peptide analogues and compounds which modulate protein- protein interactions in the control of cellular metabolism and function. Cellular metabolism or function is controlled by a number of regulatory agents, which are affected by extracellular factors, for example the physical condition of the cell or the binding of a messenger molecule to a receptor located on the cell surface. The extracellular factor may then initiate a cascade of secondary messenger reactions within the cell itself, leading ultimately to changes in some asρects(s) of metabolism or cell function. It is well recognised by those skilled in the art that phosphorylation or dephosphorylation reactions often play a key role in regulating the activity of the proteins affected. Dephosphorylation reactions are catalysed by phosphatase enzymes, the activity of which may themselves be controlled by phosphorylation and/or dephosphorylation events. Whilst a substantial amount of knowledge has been accumulated regarding protein phosphatases as a group, the number and variety of these enzymes is such that detailed information concerning the mode of action of a specific phosphatase is not always available. There remains a need to further elucidate and characterise particular key enzymes. The reversible phosphorylation of proteins regulates most aspects of cell life. About a third of all mammalian proteins are now thought to contain covalently bound phosphate and, since protein kinases and phosphatases probably account for approximately 2-3 % of all human gene products (Hunter, 1995), many of these enzymes must typically phosphorylate/dephosphorylate numerous proteins in vivo. However, it is becoming increasingly clear that some protein kinases and phosphatases do not find their physiological substrates by simple diffusion within cells and that they are frequently directed to particular loci in the vicinity of their substrates by interaction with targeting subunits. In this way, the actions of protein kinases and phosphatases with inherently broad specificities are restricted and their properties tailored to the needs of a particular subcellular location, organelle or process (reviewed in Hubbard and Cohen, 1993; Faux and Scott, 1996). Protein phosphatase- 1 (PPl), one of the major protein serine/threonine phosphatases of eukaryotic cells, participates in the control of a variety of cellular functions that include glycogen metabolism, muscle contraction, the exit from mitosis (reviewed in [1,2]) and the splicing of mRNA [3]. However, evidence has been accumulating that different processes are regulated by distinct forms of PPl in which the phosphatase catalytic subunit (PPlc) is complexed to specific "targeting subunits". These proteins not only direct PPlc to particular subcellular locations, but modify its specificity in unique ways and confer regulation by extracellular agonists (reviewed in [2,3]). Several targeting subunits have been isolated and characterised, including the G M -subunit that targets PPlc to both the glycogen particles and sarcoplasmic reticulum of striated muscles [4,5], the G L subunit that targets PPlc to liver glycogen [6,7], the M-complexes responsible for the association of PPlc with the myofibrils of skeletal muscle [8,9] and smooth muscle [9-12], the p53 binding protein p53BP2 [13] and nuclear proteins such as sds22 [14] and NIPP1 [15,16] . PPlc is also reported to interact with other mammalian proteins such as the retinoblastoma gene product [17], ribosomal protein L5 [18], a 110 kDa nuclear protein that has yet to be identified [15] and two cytosolic proteins, termed inhibitor- 1 and inhibitor^. Inhibitor- 1, and its homologue termed dopamine and cyclic AMP-regulated phosphoprotein (DARPP), become potent PPl inhibitors after phosphorylation by cyclic AMP-dependent protein kinase (PKA). Inhibitor-1 is thought to inactivate PPlc released from glycogen particles when G M is phosphory lated by PKA [19]. Inhibitor-2 is present as a complex with PPl in the cytosol, and there is evidence that one of its roles is to act like a molecular chaperone to ensure that the PPl catalytic centre is folded correctly prior to its delivery to a specific targeting subunit [20]. It seems likely that many other PPl -targeting subunits will be identified over the next few years as a result of the introduction of powerful new techniques such as microcystin Sepharose affinity chromatography [8] and the yeast "two hybrid system" [13]. The forms of PPlc isolated so far each contain a single PPlc-binding subunit, implying that the interaction of different targeting subunits with PPlc may be mutually exclusive. This, in turn, suggests that the binding sites for targeting subunits may overlap, and that the proportion of PPl directed to any particular location may be determined by the amounts of each targeting subunit synthesised and their relative affinities for PPl. However, the different targeting subunits show surprisingly little similarity to one another. G M and G L are structurally related, yet display only 23% amino acid sequence identity over the first 286 residues of G M , while G L lacks the C-terminal 750 residues of G M [7]. p53BP2 [13] and the M 110 subunits from smoom muscle [10,11] and skeletal muscle [8] contain ankyrin repeats, but no other similarities have so far been detected between other PPl targeting subunits. The paradigm for the targeting subunit concept is protein phosphatase- 1 (PPl), one of the major serine/threonine specific protein phosphatases of eukaryotic cells (Stralfors et al, 1985). This enzyme is involved in controlling diverse cellular functions including glycogen metabolism, muscle contraction, the exit from mitosis and the splicing of RNA (Cohen, 1989; Shenolikar, 1994; Wera and Hemmings, 1995). These different processes appear to be regulated by distinct PPl holo-enzymes in which the same catalytic subunit (PPlc) is complexed to different targeting or regulatory subunits. The latter class of subunits act to confer in vivo substrate specificity not only by directing PPlc to the subcellular loci of its substrates, but also by enhancing or suppressing its activity towards different substrates. In addition, the regulatory subunits allow the activity of PPl to be modulated by reversible protein phosphorylation and second messengers in response to extracellular stimuli. Many regulatory subunits modulate the activity of PPl towards its substrates. In the instance of the regulatory M uo subunit that targets PPlc to myosin, the region on the M 110 subunit that enhances the dephosphorylation of myosin by PPl has now been shown to be distinct from the region involved in targetting the PP1-M holoenzyme to myosin. These observations indicate that alterations in the substrate specificity of PPlc are likely to result from conformational changes induced by interactions with the targetting subunit and not simply as a direct result of targetting PPlc to its substrate. However, in the case of the glycogen binding subunit G M , the dephosphorylation of glycogen phosphorylase and glycogen synthase was enhanced only under conditions when both the PP1-G M complex and its substrates were bound to glycogen (Hubbard and Cohen, 1989) suggesting that targetting alone may be sufficient to enhance specificity. Whilst the identity of the PPl-binding site(s) on any targeting subunit is unknown, it has now been realised that the control of the substrate specificity and activity of this key regulatory enzyme and its interactions are of therapeutic importance. Disruption of PPl -targeting subunit interactions provide a way of altering selectively the state of phosphorylation, and hence the activities, of particular PPl substrates. We have now identified relatively small peptides from the G M and M 110 -subunits that interact with PPl, and which either disrupt or mimic the distinctive properties of myofibrillar and glycogen-associated forms of PPl . The binding of the G-subunit and the M-subunit of PPl has also been found to be mutually exclusive. A first aspect of the invention provides a method of identifying a compound which modulates the interaction between a PPlc and a regulatory subunit thereof, the method comprising determining whether a compound enhances or disrupts the interaction between (a) a PPlc or a fragment, variant, derivative or fusion thereof or a fusion of a fragment, variant or derivative and (b) a regulatory subunit which is able to bind to PPlc or a PPlc-binding fragment, variant, derivative or fusion of said subunit or a fusion of said fragment, variant or derivative. Conveniently, the PPlc or a fragment, variant or derivative or fusion thereof or a fusion of a fragment, variant or derivative is one that is produced using recombinant DNA technology. By "fragment, variant, derivative or fusion of PPlc" we mean any such fragment, variant, derivative or fusion that retains the ability to interact with a regulatory subunit or a suitable PPlc-binding fragment, variant, derivative or fusion of said subunit or a fusion of said fragment, variant or derivative. By "regulatory subunit" we mean any such regulatory subunit. Further subunits are being identified all of the time. It is preferred if the regulatory subunit contains the consensus peptide sequence Arg/Lys-Val/Ile-Xaa-Phe as described below. By "PPlc-binding fragment, variant, derivative or fusion of said subunit or a fusion of said fragment, variant or derivative" we include any such fragments, variants, derivatives and fusions which are able to bind to PPlc. Conveniently, the fragments, variants, derivatives are made using recombinant DNA technology or, in the case of peptides and peptide derivatives and analogues they may be made using peptide synthetic methods. The enhancement or disruption of the interaction between the said PPlc or a fragment, variant, derivative or fusion thereof or a fusion of a fragment, variant or derivative and the said regulatory subunit or a fragment, variant, derivative or fusion thereof or a fusion of a fragment, variant or derivative can be measured in vitro using methods well known in the art of biochemistry and including any methods which can be used to assess protein-protein, protein- peptide and protein-ligand interactions. The said interaction can also be measured within a cell, for example using the yeast two-hybrid system as is well known in the art. It should be appreciated that before the present invention the dissociation of a PPlc-regulatory subunit has not been achieved using a small molecule such as a peptide or a peptide analogue or derivative. Thus, it is preferred if the compounds screened in the method of the first aspect of the invention are small molecules and in particular that they are not intact regulatory subunits of PPlc. By "small molecule" we include any compounds which have a molecular weight of less than 5000, preferably less than 2000 and more preferably less than 1000. Conveniently, the compounds screened are compounds which are able to enter a cell either passively via the cell membrane or via an active uptake system. A second aspect of the invention provides a method of identifying a compound which mimics the effect of a regulatory subunit of PPlc, the method comprising contacting said compound with PPlc and determining whether, in the presence of the compound, PPlc adopts the function of properties of a PPlc in the presence of a given regulatory subunit. By "mimics the effect of a regulatory subunit of PPlc" we include the meaning that the compound modifies a property of PPlc in such a way that PPlc acts, in at least one respect, like PPlc that is interacting with a regulatory subunit. Examples of the properties of PPlc that may be modified, and examples of compounds which modify the properties of PPlc which are therefore identifiable in this method are given below. Preferably, in the methods of the first and second aspects the said regulatory subunit of PPlc is any one of M 110 , G L , G M , M-complexes, p53 BP2, sds22, NIPPI, L5, Inhibitor-1, Inhibitor-2, or DARPP. More preferably, the regulatory subunit of PPlc is any one of M n0 , G L , G M , M-complexes or p53BP2, and still more preferably the regulatory subunit of PPlc is M„o or G M . In relation to the method of the first aspect of the invention the fragment of a regulatory subunit which is able to bind to PPlc is any of the peptides [G63- T93], [G63-N75], [E2-R575], [E2-P243], [E2-D118], [H100-P350] and peptide 63-80 of G M or functional equivalents thereof or peptides comprising said peptide sequences provided that they are not the complete G M regulatory subunit. Preferably the peptides are not [E2-R575] or [H100-P350]. As is described in more detail in the Examples, these peptides have been shown to bind to PPlc and it is convenient, in some circumstances, for the method to be carried out such that one of these peptide is displaced from, or the binding is enhanced to, PPlc. Suitably, the peptide may be labelled in a detectable manner to facilitate the detection of the interaction with PPlc. Conveniently, the peptide is labelled radioactively or fluorescently using methods well known in the art. Also in relation to the method of the first aspect of the invention the fragment of a regulatory subunit which is able to bind to PPlc is any of the peptides [M1-E309], [M1-F38], [M1-A150] or [L24-Y496] of M u0 or functional equivalents thereof or peptides comprising said peptide sequences provided that they are not the complete M 110 regulatory subunit. As is shown in more detail in the Examples these peptides have been shown to bind to PPlc. Also in relation to the first aspect of the invention the PPlc-binding fragment, variant or derivative of said regulatory subunit or a fusion of said fragment, variant or derivative comprises the consensus peptide sequence Arg/Lys- Val/Ile-Xaa-Phe wherein Xaa is any amino acid. We have found that, surprisingly, many regulatory subunits that bind to PPlc contain the consensus peptide sequence Arg/Lys-Val/Ile-Xaa-Phe wherein Xaa is any amino acid, preferably a naturally occurring amino acid. Typically, the PPlc-binding fragment, variant or derivative of said regulatory subunit or a fusion of said fragment, variant or derivative is a peptide (typically 8-400 amino acid residues, preferably 8-200, more preferably 8-10 and still more preferably 8-20 amino acid residues in length which comprises the given consensus peptide sequence). It is preferred if the PPlc-binding fragment, variant or derivative comprises, in addition to the said consensus peptide sequence, at least one basic residue in the four residues N-terminal of the consensus peptide sequence. Preferably, there are at least two basic residues in this position, more preferably at least three such residues. It is also preferred wherein in the consensus peptide sequence Xaa is not Asp or Glu because the negative charge is believed to interfere with binding to PPlc. Similarly, it is preferred if Xaa is not a large hydrophobic residue such as Phe, Tyr, Trp, He or Leu. It is particularly preferred if the PPlc-binding fragment is a fragment of a regulatory subunit comprising the said consensus peptide sequence and therefore the peptide sequences which flank the consensus peptide sequence are the same as in the native regulatory subunit. Preferably the PPlc-binding fragment is a fragment of any of the M no , G L , G M , M-complexes, p53BP2, sds22, NIPPI, L5, Inhibitor- 1, Inhibitor-2 or DARPP regulatory subunits comprising said consensus sequence. Although the methods of the first and second aspects of the invention do not rely on any particular mechanism whereby the modulation or mimicking occurs , it is preferred if the compound binds to a PPlc. Alternatively, but still preferably, the compound binds to a regulatory subumt of PPlc. A further aspect of the invention provides a compound identifiable by the method of the first or second aspects of the invention. A further aspect of the invention provides a compound which modulates the interaction between a PPlc and a regulatory subunit thereof said compound comprising any of the peptides [G63-T93], [G63-N75], [E2-R575], [E2-P243], [E2-D118] , [H100-P350] and peptide 63 to 80 of G M or functional equivalents or said compound comprising any of the peptides [M1-E309], [M1-F38], [Ml- A150] or [L24-Y496] of M 110 or functional equivalents thereof or said compound comprising the consensus peptide sequence Arg/Lys- Val/Ile-Xaa-Phe wherein Xaa is any naturally occurring amino acid or functional equivalents thereof, provided that the said compound is not a complete regulatory subunit of PPlc. Preferably, the peptides are not [E2-R575] or [H100-P350]. By "functional equivalent" we include the meaning that the compound, although having a different structure to the said peptides, modulates the interaction between a PPlc and a regulatory subunit thereof in substantially the same way. For example, a functional equivalent may be a peptide in which conservative substitutions have been made. By "conservative substitution" is intended combinations such as Gly, Ala; Val, He, Leu; Asp, Glu; Asn, Gin; Ser, Thr; Lys, Arg; and Phe, Tyr. A functional equivalent may also be a peptide with the given sequence which has been adapted to be more likely to enter a cell. For example, fatty acids or other hydrophobic moieties may be attached to the peptide. By the term "peptide" we mean derivatives of peptides which are resistant to proteolysis, for example those in which the N or C termini are blocked, or both are blocked, and it includes molecules in which one or more of the peptide linkages are modified so that the molecule retains substantially the same molecular configuration in the linkage but the linkage is more resistant to hydrolysis than a peptide linkage. It is particularly preferred if the compound consists of the peptides [G63-T93], [G63-N75], [E2-R575], [E2-P243], [E2-D118], [H100-P350] or peptide 63 to 80 of G M or functional equivalents thereof or if the compound consists of the peptides [M1-E309], [M1-F38], [M1-A150] or [L24-Y496] of M 110 or functional equivalents thereof. Preferably, the peptide is not [E2-R575] or [H100-P350]. A still further aspect of the invention provides a method of identifying a compound which modulates the interaction between a PPlc and a regulatory subunit thereof, or binds PPlc or mimics the effect of a regulatory subunit, the method comprising selecting a compound which is capable of adopting die same or substantially the same conformation as a peptide bound to the regulatory subunit-binding site of PPlc or the same or substantially the same conformation as the portion of PPlc which binds to said peptide. Suitably, the peptide comprises the consensus peptide sequence Arg/Lys-Val/Ile-Xaa-Phe wherein Xaa is any amino acid, preferably a naturally occurring amino acid. Conveniently, the said peptide consists of residues 63 to 75 of G M . It is particularly preferred if the conformation of the said peptide and the conformation of the said portion of PPlc is as defined by reference to the atomic coordinates given in Table A (see also Example 2). Example 2 provides further details of the peptide - PPlc interactions. Table A provides the atomic coordinates for the given PPlc-peptide crystal structure. A further aspect of the invention provides a compound identifiable by the aforementioned method of the invention. It will be appreciated that the aforementioned compounds and peptides will be useful in medicine and, accordingly, the invention includes pharmaceutical compositions of the said compounds in combination with a pharmaceutically acceptable carrier. The formulations may conveniently be presented in unit dosage form and may be prepared by any of the methods well known in the art of pharmacy. Such methods include the step of bringing into association the active ingredient (compound of the invention) with the carrier which constitutes one or more accessory ingredients. In general the formulations are prepared by uniformly and intimately bringing into association the active ingredient with liquid carriers or finely divided solid carriers or both, and then, if necessary, shaping the product. Formulations in accordance with the present invention suitable for oral administration may be presented as discrete units such as capsules, cachets or tablets, each containing a predetermined amount of the active ingredient; as a powder or granules; as a solution or a suspension in an aqueous liquid or a non-aqueous liquid; or as an oil-in-water liquid emulsion or a water-in-oil liquid emulsion. The active ingredient may also be presented as a bolus, electuary or paste. A tablet may be made by compression or moulding, optionally with one or more accessory ingredients. Compressed tablets may be prepared by compressing in a suitable machine the active ingredient in a free-flowing form such as a powder or granules, optionally mixed with a binder (eg povidone, gelatin, hydroxypropylmethyl cellulose), lubricant, inert diluent, preservative, disintegrant (eg sodium starch glycolate, cross-linked povidone, cross-linked sodium carboxymethyl cellulose), surface-active or dispersing agent. Moulded tablets may be made by moulding in a suitable machine a mixture of the powdered compound moistened with an inert liquid diluent. The tablets may optionally be coated or scored and may be formulated so as to provide slow or controlled release of the active ingredient therein using, for example, hydroxypropylmethylcellulose in varying proportions to provide desired release profile. Formulations suitable for topical administration in the mouth include lozenges comprising the active ingredient in a flavoured basis, usually sucrose and acacia or tragacanth; pastilles comprising the active ingredient in an inert basis such as gelatin and glycerin, or sucrose and acacia; and mouth-washes comprising the active ingredient in a suitable liquid carrier. Formulations suitable for parenteral administration include aqueous and non- aqueous sterile injection solutions which may contain anti-oxidants, buffers, bacteriostats and solutes which render the formulation isotonic with the blood of die intended recipient; and aqueous and non-aqueous sterile suspensions which may include suspending agents and thickening agents. The formulations may be presented in unit-dose or multi-dose containers, for example sealed ampoules and vials, and may be stored in a freeze-dried (lyophilised) condition requiring only the addition of die sterile liquid carrier, for example water for injections, immediately prior to use. Extemporaneous injection solutions and suspensions may be prepared from sterile powders, granules and tablets of the kind previously described. Preferred unit dosage formulations are those containing a daily dose or unit, daily sub-dose or an appropriate fraction thereof, of an active ingredient. It should be understood that in addition to die ingredients particularly mentioned above the formulations of this invention may include other agents conventional in the art having regard to the type of formulation in question, for example those suitable for oral administration may include flavouring agents. A further aspect of the invention provides a method of affecting cellular metabolism or function, the method comprising administering to a cell (a) a compound which modulates me interaction between a PPlc and a regulatory subumt thereof or (b) a compound which mimics the effect of a regulatory subunit of PPlc or (c) a peptide capable of binding a PPlc and which affects the ability of PPlc to bind to a particular target and/or affects the regulation of PPlc activity, or a functional equivalent thereof. It will be appreciated that the said compounds are disclosed above with respect to specific compounds or with respect to methods of obtaining such compounds. In particular, it is preferred if the compound administered to me cell is any one or more of the peptides [G63-T93], [G63-N75], [E2-R575], [E2-P243], [E2- D118], [H100-P350] and peptide 63-80 of G M or functional equivalents thereof or peptides comprising said peptide sequences or any one or more of the peptides [M1-E309], [M1-F38], [M1-A150] or [L24-Y496] of M u0 or functional equivalents thereof or peptides comprising said peptide sequences. Preferably, the peptide is not [E2-R575] or [H100-P350]. In this embodiment it will be appreciated that functional equivalents include those compounds defined above as being functional equivalents, in particular, derivatives of peptides which are more readily able to enter a cell. The compound may be administered to the cell in any suitable way, in particular in such a way that the compound will enter the cell in a suitable form to have its desired effect. Method of facilitating the entry of a compound into the cell are known in the art, for example, in relation to peptides the importins and penetrations may be used, or the peptides may be micro-injected or they may enter me cell in a suitable vehicle such as in a liposome. Preferably, the cell is a cell in a mammalian body. The aforementioned compounds of me invention or a formulation thereof may be administered by any conventional method including oral and parenteral (eg subcutaneous or intramuscular) injection. The treatment may consist of a single dose or a plurality of doses over a period of time. Whilst it is possible for a compound of the invention to be administered alone, it is preferable to present it as a pharmaceutical formulation, together with one or more acceptable carriers. The carrier(s) must be "acceptable" in the sense of being compatible with the compound of the invention and not deleterious to the recipients thereof. Typically, die carriers will be water or saline which will be sterile and pyrogen free. A further aspect of the invention provides a method of treating a patient in need of modulation of PPlc activity or function the method comprising administering to me patient an effective amount of a compound which modulates the interaction between a PPlc and a regulatory subunit thereof or (b) a compound which mimics the effect of a regulatory subunit of PPlc or (c) a peptide capable of binding a PPlc and which affects the ability of PPlc to bind to a particular target and/or affects the regulation of PPlc activity, or a functional equivalent thereof. As will be apparent from what is described herein, protein phosphatase- 1 (PPl) is one of the principal serine/threonine-specific protein phosphatases in human cells where it plays key roles in regulating a variety of physiological roles, including me metabolism of glycogen, the splicing of mRNA, the exit from mitosis and the contraction of smooth muscle. The different functions of PPl are carried out by distinct species of this enzyme in which the same catalytic unit is complexed to different "targeting" subunits. The latter class of proteins direct PPl to specific subcellular loci, tailor its properties to the needs of a particular locus and confer die ability to be regulated by extracellular signals (hormones, growth factors, neurotransmitters). Compounds as herein described that disrupt specific PPl-"targeting" subunits interactions or mimic the effect of a targeting subunit are likely to have a number of therapeutic uses as outlined below. PPl interacts widi the MllO-subunit which targets it to myosin in smooth muscle and enhances me rate at which PPl dephosphorylates myosin. The dephosphorylation of myosin underlies e relaxation of smooth muscle. Thus compounds such as those disclosed herein which disrupt the interaction of PPl with MHO in arterial muscle are expected to increase the phosphorylation of arterial myosin and elevate blood pressure. The interaction of PPl with MHO enhances the rate at which PPl dephosphorylates myosin, but suppresses me rate at which it dephosphorylates glycogen phosphorylase. The disruption of the PPl -Ml 10 interaction is therefore measured in a screen by looking for compounds which enhance the dephosphorylation of phosphorylase and/or suppress die dephosphorylation of die myosin P-light chain (see the Examples). Compounds, such as those disclosed herein, that mimic the effect of the MHO subunit in stimulating myosin dephosphorylation are expected to be useful in lowering blood pressure. Such compounds are identified by their ability to stimulate the dephosphorylation of the myosin P-light chain by the catalytic subunit of PPl . An example of such an assay, which shows that the N-terminal 38 residues of me MHO subunit stimulate the dephosphorylation of the myosin P-light chain by PPl, is shown in die Examples. The interaction of PPl with G L targets the phosphatase to liver glycogen. This interaction enhances die dephosphorylation glycogen synthase which stimulates the conversion of glucose to glycogen. A compounds, such as those disclosed herein, disrupts die interaction between PPl and G L is expected to be useful in treating hypoglycaemia. The interaction of G L with PPl strongly suppresses the rate at which PPl dephosphorylates glycogen phosphorylase. A compound, such as mose disclosed herein, which disrupts die interaction of PPl with G L can be screened for very simply by its ability to increase the phosphorylase phosphatase activity of PPl G L . This can be carried out, for example, using rat liver glycogen pellet as described in the Examples. There is no need to use me purified enzyme. PPl interacts with p53 BP2 (Helps et al, 1995) a protein which is known to interact widi the tumour suppressor p53. The phosphorylation of p53 is known to enhance its ability to bind to DNA and hence its ability to function as a tumour suppressor. p53BP2 may be a protein which targets PPl to p53 stimulating me dephosphorylation and inactivation of p53. A compound, such as mose disclosed herein, which disrupts the interaction of PPl with p53BP2 may enhance the phosphorylation of p53 and its ability to function as a tumour suppressor. Since p53BP2 suppresses the dephosphorylation of glycogen phosphorylase (Helps et al, 1995), compounds mat disrupt the p53BP2-PPl complex can be screened by measuring the increase in rate of dephosphorylation of glycogen phosphorylase. The present invention provides peptides able to bind to die catalytic sub-unit of protein phosphatase- 1 (hereinafter referred to as PPlc). Generally die peptides affect die ability of PPlc to bind to particular target(s) and/or die regulation of PPlc activity. Peptides can be designed based on die sequences of regulatory subunits, especially in relation to the peptide consensus sequence found tiierein and its flanking sequences. Peptides can be syndiesised by methods well known in the art. For example, peptides may be synmesised by the Fmoc-polyamide mode of solid-phase peptide synmesis as disclosed by Lu et al (1981) J. Org. Chem. 46, 3433 and references therein. Temporary N-amino group protection is afforded by die 9-fluorenylmethyloxycarbonyl (Fmoc) group. Repetitive cleavage of mis highly base-labile protecting group is effected using 20% piperidine in N,N-dimethylformamide. Side-chain functionalities may be protected as their butyl ethers (in the case of serine threonine and tyrosine), butyl esters (in the case of glutamic acid and aspartic acid), butyloxycarbonyl derivative (in the case of lysine and histidine), trityl derivative (in me case of cysteine) and 4-methoxy-2 ,3 ,6-trimethylbenzenesulphonyl derivative (in the case of arginine). Where glutamine or asparagine are C-terminal residues, use is made of me 4,4'-dimethoxybenzhydryl group for protection of the side chain amido functionalities. The solid-phase support is based on a polydime yl- acrylamide polymer constituted from the three monomers dimethylacrylamide (backbone-monomer), bisacryloylethylene diamine (cross linker) and acryloylsarcosine methyl ester (functionalising agent). The peptide-to-resin cleavable linked agent used is the acid-labile 4-hydroxymethyl-phenoxyacetic acid derivative. All amino acid derivatives are added as their preformed symmetrical anhydride derivatives with the exception of asparagine and glutamine, which are added using a reversed N,N-dicyclohexyl-carbodiimide/l - hydroxybenzotriazole mediated coupling procedure. All coupling and deprotection reactions are monitored using ninhydrin, trinitrobenzene sulphonic acid or isotin test procedures. Upon completion of synmesis, peptides are cleaved from the resin support with concomitant removal of side-chain protecting groups by treatment with 95% trifluoroacetic acid containing a 50% scavenger mix. Scavengers commonly used are ethanedithiol, phenol, anisole and water, the exact choice depending on die constituent amino acids of die peptide being synthesised. Trifluoroacetic acid is removed by evaporation in vacuo, with subsequent trituration with diethyl ether affording die crude peptide. Any scavengers present are removed by a simple extraction procedure which on lyophilisation of the aqueous phase affords die crude peptide free of scavengers. Reagents for peptide syndiesis are generaUy available from Calbiochem-Novabiochem (UK) Ltd, Nottingham NG7 2QJ, UK. Purification may be effected by any one, or a combination of, techniques such as size exclusion chromatography, ion-exchange chromatography and (principally) reverse-phase high performance liquid chromatography. Analysis of peptides may be carried out using thin layer chromatography, reverse-phase high performance liquid chromatography, amino-acid analysis after acid hydrolysis and by fast atom bombardment (FAB) mass spectrometric analysis. The peptides may be derived from the targeting subunit(s) of PPlc, in particular from the subunits G L , G M , M uo and/or M 21 . Additionally die peptides may be derived from other subunits such as different M-complexes, p53BP2, sds22, NIPP1, L5, Inhibitor-1, Inhibitor-2, DARPP or the like. Functional equivalents or portions of these peptides may also be used. In a further aspect the present invention provides die use of peptides derived from targeting subunit(s) of PPlc, functional equivalents or portions thereof to affect cellular metabolism or function. In a further aspect the present invention provides a method of treatment of me human or non-human (preferably mammalian) animal body, said method comprising altering the levels of peptides derived from targeting subunit(s) of PPlc, functional equivalents or portions thereof to an extent that cellular metabolism or function is affected. Aspects of cellular metabolism that may be affected include (but are not limited to) glycogen metabolism, muscle metabolism, physiology and function. Generally the levels of peptides or dieir activity will be enhanced in cells and diis control may be achieved by causing higher levels of expression of nucleotides sequences encoding for such peptides (optionally linked to molecules which allow diem to cross a cell membrane) or dirough the administration of such peptides or precursors diereof. Alternatively, in some circumstances, it may be more desirable to depress die levels of certain peptides or at least to depress die level of peptides in active form. Preferred peptides according to die present invention are derivatives of G M , especially [G63-T93], [G63-N75], [E2-R575], [E2-P243], [E2-D118], [H100-P350] and peptide 63 to 80, and derivatives of M πo , especially [M1-E309], [M1-F38], [M1-A150], [L24-Y496]. Preferably, the peptide is not [E2-R575] or [H100-P350]. Particularly preferred peptides are tiiose derived from amino acid nos. 63 to 93 (including 63-80 and 63-75) of G M ; or from amino acids 1 to 309 (including from 1-150 and 1-38) of M 110 . The sequence of G M is given in Chen et al (1994) Diabetes 43, 1234-1241. In yet further aspect the present invention provides chimeric proteins containing portions of other proteins or peptides or containing additional amino acids. Additionally the present invention provides nucleotide sequences (optionally in die form of plasmids) encoding die peptides or chimeric proteins of interest. DNA which encodes die polypeptides or peptides of die invention or chimeric proteins can be made based on a knowledge of the peptide sequences disclosed herein. The DNA is then expressed in a suitable host to produce a polypeptide comprising the compound of me invention. Thus, die DNA encoding die polypeptide constituting the compound of die invention may be used in accordance widi known techniques, appropriately modified in view of me teachings contained herein, to construct an expression vector, which is then used to transform an appropriate host cell for the expression and production of die polypeptide of die invention. Such techniques include mose disclosed in US Patent Nos. 4,440,859 issued 3 April 1984 to Rutter et al, 4,530,901 issued 23 July 1985 to Weissman, 4,582,800 issued 15 April 1986 to Crowl, 4,677,063 issued 30 June 1987 to Mark et al, 4,678,751 issued 7 July 1987 to Goeddel, 4,704,362 issued 3 November 1987 to Itakura et al, 4,710,463 issued 1 December 1987 to Murray, 4,757,006 issued 12 July 1988 to Toole, Jr. et al, 4,766,075 issued 23 August 1988 to Goeddel et al and 4,810,648 issued 7 March 1989 to Stalker, all of which are incorporated herein by reference. The DNA encoding the polypeptide constituting the compound of me invention may be joined to a wide variety of other DNA sequences for introduction into an appropriate host. The companion DNA will depend upon die nature of the host, the manner of me introduction of die DNA into me host, and whedier episomal maintenance or integration is desired. Generally, die DNA is inserted into an expression vector, such as a plasmid, in proper orientation and correct reading frame for expression. If necessary, the DNA may be linked to die appropriate transcriptional and translational regulatory control nucleotide sequences recognised by die desired host, aldiough such controls are generally available in the expression vector. The vector is then introduced into me host through standard techniques. Generally, not all of the hosts will be transformed by die vector. Therefore, it will be necessary to select for transformed host cells. One selection technique involves incorporating into the expression vector a DNA sequence, widi any necessary control elements, that codes for a selectable trait in the transformed cell, such as antibiotic resistance. Alternatively, die gene for such selectable trait can be on another vector, which is used to co-transform the desired host cell. Host cells diat have been transformed by die recombinant DNA of me invention are then cultured for a sufficient time and under appropriate conditions known to diose skilled in the art in view of die teachings disclosed herein to permit the expression of die polypeptide, which can dien be recovered. Many expression systems are known, including bacteria (for example E. coli and Bacillus subtilis), yeasts (for example Saccharomyces cerevisiae), filamentous fungi (for example Aspergillus), plant cells, animal cells and insect cells. The vectors include a prokaryotic replicon, such as the ColEl ori, for propagation in a prokaryote, even if me vector is to be used for expression in other, non-prokaryotic, cell types. The vectors can also include an appropriate promoter such as a prokaryotic promoter capable of directing die expression (transcription and translation) of the genes in a bacterial host cell, such as E. coli, transformed dierewidi. A promoter is an expression control element formed by a DNA sequence mat permits binding of RNA polymerase and transcription to occur. Promoter sequences compatible with exemplary bacterial hosts are typically provided in plasmid vectors containing convenient restriction sites for insertion of a DNA segment of the present invention. Typical prokaryotic vector plasmids are pUClδ, pUC19, pBR322 and pBR329 available from Biorad Laboratories, (Richmond, CA, USA) and p7Vc99A and pKK223-3 available from Pharmacia, Piscataway, NJ, USA. A typical mammalian cell vector plasmid is pSVL available from Pharmacia, Piscataway, NJ, USA. This vector uses the SV40 late promoter to drive expression of cloned genes, die highest level of expression being found in T antigen-producing cells, such as COS-1 cells. An example of an inducible mammalian expression vector is pMSG, also available from Pharmacia. This vector uses die glucocorticoid-inducible promoter of the mouse mammary tumour virus long terminal repeat to drive expression of die cloned gene. Useful yeast plasmid vectors are pRS403-406 and pRS413-416 and are generally available from Stratagene Cloning Systems, La Jolla, CA 92037, USA. Plasmids pRS403, pRS404, pRS405 and pRS406 are Yeast Integrating plasmids (YIps) and incorporate the yeast selectable markers HIS3, TRPI, LEU2 and URA3. Plasmids pRS413-416 are Yeast Centromere plasmids (YCps) A variety of methods have been developed to operably link DNA to vectors via complementary cohesive termini. For instance, complementary homopolymer tracts can be added to die DNA segment to be inserted to die vector DNA. The vector and DNA segment are men joined by hydrogen bonding between die complementary homopolymeric tails to form recombinant DNA molecules. Synthetic linkers containing one or more restriction sites provide an alternative method of joining die DNA segment to vectors. The DNA segment, generated by endonuclease restriction digestion as described earlier, is treated with bacteriophage T4 DNA polymerase or E. coli DNA polymerase I, enzymes that remove protruding, 3 '-single-stranded termini with their 3'-5'-exonucleolytic activities, and fill in recessed 3 '-ends with dieir polymerizing activities. The combination of these activities therefore generates blunt-ended DNA segments. The blunt-ended segments are men incubated with a large molar excess of linker molecules in me presence of an enzyme mat is able to catalyze the ligation of blunt-ended DNA molecules, such as bacteriophage T4 DNA ligase. Thus, the products of the reaction are DNA segments carrying polymeric linker sequences at eir ends. These DNA segments are then cleaved with die appropriate restriction enzyme and ligated to an expression vector that has been cleaved witii an enzyme mat produces termini compatible with diose of the DNA segment. Syn ietic linkers containing a variety of restriction endonuclease sites are commercially available from a number of sources including International Biotechnologies Inc, New Haven, CN, USA. A desirable way to modify the DNA encoding die polypeptide of die invention is to use me polymerase chain reaction as disclosed by Saiki et al (1988) Science 239, 487-491. In this method me DNA to be enzymatically amplified is flanked by two specific oligonucleotide primers which themselves become incorporated into the amplified DNA. The said specific primers may contain restriction endonuclease recognition sites which can be used for cloning into expression vectors using methods known in die art. In relation to the above section on DNA expression the term "polypeptide" includes peptides and chimeric proteins. Further the present invention provides host cells transformed witii suitable expression vectors and able to express die peptides. The host cells may be prokaryotic (e.g. E. coli) or eukaryotic (e.g. yeast, mammalian cell cultures). Bacterial cells are preferred prokaryotic host cells and typically are a strain of E. coli such as, for example, the E. coli strains DH5 available from Bethesda Research Laboratories Inc., Bethesda, MD, USA, and RR1 available from the American Type Culture Collection (ATCC) of Rockville, MD, USA (No ATCC 31343). Preferred eukaryotic host cells include yeast and mammalian cells, preferably vertebrate cells such as diose from a mouse, rat, monkey or human fibroblastic cell line. Yeast host cells include YPH499, YPH500 and YPH501 which are generally available from Stratagene Cloning Systems, La Jolla, CA 92037, USA. Preferred mammalian host cells include Chinese hamster ovary (CHO) cells available from the ATCC as CCL61, NIH Swiss mouse embryo cells NIH/3T3 available from the ATCC as CRL 1658, and monkey kidney-derived COS-1 cells available from the ATCC as CRL 1650. Transformation of appropriate cell hosts wi a DNA construct of the present invention is accomplished by well known methods mat typically depend on die type of vector used. Widi regard to transformation of prokaryotic host cells, see, for example, Cohen et al (1972) Proc. Natl. Acad. Sci. USA 69, 2110 and Sambrook et al (1989) Molecular Cloning, A Laboratory Manual, Cold Spring Harbor Laboratory, Cold Spring Harbor, NY. Transformation of yeast cells is described in Sherman et al (1986) Methods In Yeast Genetics, A Laboratory Manual, Cold Spring Harbor, NY. The memod of Beggs (1978) Nature 275, 104-109 is also useful. With regard to vertebrate cells, reagents useful in transfecting such cells, for example calcium phosphate and DEAE-dextran or liposome formulations, are available from Stratagene Cloning Systems, or Life Technologies Inc., Gaithersburg, MD 20877, USA. Electroporation is also useful for transforming cells and is well known in die art for fransforming yeast cell, bacterial cells and vertebrate cells. For example, many bacterial species may be transformed by die methods described in Luchansky et al (1988) Mol. Microbiol. 2, 637-646 incorporated herein by reference. The greatest number of transformants is consistently recovered following electroporation of the DNA-cell mixture suspended in 2.5X PEB using 6250V per cm at 25/tFD. Methods for transformation of yeast by electroporation are disclosed in Becker & Guarente (1990) Methods Enz mol. 194, 182. Successfully transformed cells, ie cells that contain a DNA construct of the present invention, can be identified by well known techniques. For example, cells resulting from the introduction of an expression construct of die present invention can be grown to produce die polypeptide of die invention. Cells can be harvested and lysed and their DNA content examined for the presence of the DNA using a method such as that described by Soudiern (1975) J. Mol. Biol. 98, 503 or Berent et al (1985) Biotech. 3, 208. Alternatively, the presence of the protein in the supernatant can be detected using antibodies as described below. In addition to direcdy assaying for the presence of recombinant DNA, successful transformation can be confirmed by well known immunological methods when die recombinant DNA is capable of directing d e expression of die protein. For example, cells successfully transformed with an expression vector produce proteins displaying appropriate antigenicity. Samples of cells suspected of being transformed are harvested and assayed for the protein using suitable antibodies. Thus, in addition to die transformed host cells diemselves, the present invention also contemplates a culture of those cells, preferably a monoclonal (clonally homogeneous) culture, or a culture derived from a monoclonal culmre, in a nutrient medium. In anomer aspect the present invention provides antibodies to PPlc which act in an analogous manner to the peptides of interest. Antibodies to die peptides diemselves are also provided and diese may themselves be used to affect cell metabolism or function. Peptides in which one or more of the amino acid residues are chemically modified, before or after the peptide is syndiesised, may be used providing diat die function of the peptide, namely the production of specific antibodies in vivo, remains substantially unchanged. Such modifications include forming salts with acids or bases, especially physiologically acceptable organic or inorganic acids and bases, forming an ester or amide of a terminal carboxyl group, and attaching amino acid protecting groups such as N-t-butoxycarbonyl. Such modifications may protect the peptide from in vivo metabolism. The peptides may be present as single copies or as multiples, for example tandem repeats. Such tandem or multiple repeats may be sufficiently antigenic diemselves to obviate me use of a carrier. It may be advantageous for the peptide to be formed as a loop, with the N-terminal and C-terminal ends joined together, or to add one or more Cys residues to an end to increase antigenicity and/or to allow disulphide bonds to be formed. If the peptide is covalently linked to a carrier, preferably a polypeptide, then die arrangement is preferably such that the peptide of the invention forms a loop. According to current immunological ieories, a carrier function should be present in any immunogenic formulation in order to stimulate, or enhance stimulation of, the immune system. It is thought tiiat die best carriers embody (or, together with die antigen, create) a T-cell epitope. The peptides may be associated, for example by cross-linking, with a separate carrier, such as serum albumins, myoglobins, bacterial toxoids and keyhole limpet haemocyanin. More recently developed carriers which induce T-cell help in die immune response include die hepatitis-B core antigen (also called the nucleocapsid protein), presumed T-cell epitopes such as Thr-Ala-Ser-Gly-Val-Ala-Glu-Thr- Thr-Asn-Cys (SEQ ID No 1), beta-galactosidase and die 163-171 peptide of interleukin-1. The latter compound may variously be regarded as a carrier or as an adjuvant or as both. Alternatively, several copies of die same or different peptides of die invention may be cross-linked to one anoώer; in mis situation there is no separate carrier as such, but a carrier function may be provided by such cross-linking. Suitable cross-linking agents include diose listed as such in e Sigma and Pierce catalogues, for example glutaraldehyde, carbodiimide and succinimidyl 4-(N-maleimidomethyl)cyclohexane-l -carboxy late, the latter agent exploiting the -SH group on die C-terminal cysteine residue (if present). If die peptide is prepared by expression of a suitable nucleotide sequence in a suitable host, dien it may be advantageous to express die peptide as a fusion product with a peptide sequence which acts as a carrier. Kabigen's "Ecosec" system is an example of such an arrangement. The peptide of die invention may be linked to odier antigens to provide a dual effect. In a yet further aspect the present invention provides a method of diagnosis of abnormalities of cellular metabolism, said mediod comprising analysing die naturally occurring peptide(s) or die nucleotide sequences encoding dierefore and comparing the results to die peptides described herein. The peptides of the present invention may also be used in diagnosis and this aspect is also covered by die present invention. The specificity of the catalytic subunit of protein phosphatase- 1 (PPlc) is modified by regulatory subunits diat target it to particular subcellular locations. For the first time we have identified PPlc-binding domains on G L and G M , the subunits that target PPlc to hepatic and muscle glycogen, respectively, and on M H0 , die subunit diat targets PPlc to smooth muscle myosin. The peptide G M -(G63-T93) interacted widi PPlc and prevented G L from suppressing the dephosphorylation of glycogen phosphorylase, but it did not dissociate G L from PPlc or affect other characteristic properties of the PP1 G _. complex. These results indicate diat G L contains two PPlc-binding sites, die region which suppresses the dephosphorylation of glycogen phosphorylase being distinct from that which enhances the dephosphorylation of glycogen syndiase. At higher concentrations, G M -(G63-N75) had die same effect as G M -(G63-T93), but not if Ser67 was phosphory lated by cyclic AMP-dependent protein kinase. Thus phosphorylation of Ser67 dissociates G M from PPlc because phosphate is inserted into e PPlc-binding domain of G M . The fragments M π0 -(M1-E309) and M 110 -(M1-F38), but not M 110 -(D39-E309), mimicked die M n0 subunit in stimulating dephosphorylation of die smooth muscle myosin P-light chain and heavy meromyosin in vitro. However, in contrast to the M n0 subunit and M 110 -(M1-E309), neither M πo -(Ml-F38) nor M I10 -(D39-E309) suppressed the PPlc-catalysed dephosphorylation of glycogen phosphorylase. These observations suggest that die region which stimulates me dephosphorylation of myosin is situated widiin the N-terminal 38 residues of die M uo subunit, while the region which suppresses the dephosphorylation of glycogen phosphorylase requires the presence of at least part of he region 39-296 which contains seven ankyrin repeats. M U0 -(M1- F38) displaced G L from PPlc, while G M -(G63-T93) displaced M 1I0 from PPlc in vitro. These observations indicate mat the region(s) of PPlc that interact with G M /G L and M 110 overlap, explaining why different forms of PPlc contain just a single targeting subunit. We also disclose die structure of PPlc in complex with a portion of a targeting subunit, and show diat changing key amino acid residues in die subunit disrupts its interaction widi PPlc. These studies identify a critical structural motif in targeting subunits involved in die interaction with PPlc as well as the recognition site on PPlc itself. These findings will facilitate the rational design of agents such as peptides or odier forms of small cell-permeant molecules that act by disrupting PPl -targeting subunit interactions. Given die structural motif and me coordinates of die atoms in the crystal structure, it is within die scope of die abilities of a skilled molecular modeller to produce small cell-permeant molecules, which can enter cells naturally, and possess eidier the same motif, or an analogous structure to give die same functional properties to die molecule. Thus die small cell-permeant molecule can have a precise copy of me motif, or one which is functionally equivalent. The molecule can be a peptide, but otihier types of molecules, which are transferred across die plasma membrane of cells, may be preferred. Several mammalian PPlc targeting subunits have been isolated and characterised, including die G M subunit diat targets PPlc to both die glycogen particles and sarcoplasmic reticulum of striated muscles (Tang et al, 1991; Chen et al, 1994), the G L subunit diat targets PPlc to liver glycogen (Moorhead et al, 1995; Doherty et al, 1995), the M no subunits responsible for the association of PPlc widi the myofibrils of skeletal muscle (Moorhead et al , 1994; Alessi et al , 1992) and smooth muscle (Alessi et al , 1992; Chen et al, 1994), the p53 binding protein p53BP2 (Helps et al, 1995) and the nuclear protein NIPP-1 (Jagiello et al, 1995; Van Eynde et al, 1995). PPlc is also reported to interact wi odier mammalian proteins such as me retinoblastoma gene product (Durphee et al, 1993), an RNA splicing factor (Hirano et al, 1996), ribosomal proteins L5 (Hirano et al, 1995) and RIPP-1 (Beullens et al, 1996), a 110 kDa nuclear protein yet to be identified (Jagiello et al, 1995), kinesin-like proteins and small cytosolic proteins, inhibitor- 1, DARPP-32 and inhibitor^ (Cohen, 1989; Cohen, 1992, Hubbard and Cohen, 1993). Moreover, a number of distinct PPl -regulatory subunits have been identified in yeast (reviewed by Stark, 1996). We attempted to identify which regions of die G M and M u0 subunits were involved in binding to PPlc. These studies led to die identification of relatively small peptides from each targeting subunit that were capable of interacting wim PPlc. Peptides comprising residues 63-93, 63-80 and 63-75 of G M bound to PPlc, dissociating it from G L , while the N-terminal 38 residues of die M π0 subunit (M U0 [l-38]) mimicked die intact M n0 subunit in enhancing e rate at which PPlc dephosphorylated the 20 kDa myosin light chain (MLC 20 ) subunit of smooth muscle myosin (Johnson et al, 1996). The present invention thus provides peptides comprising me N-terminal 38 residues of die M 110 subunit, and diose comprising residues 63-93, 63-80 and 63-75 of G M . Phosphorylation of Ser 67 of G M by protein kinase A (PKA) disrupts the interaction of G M with PPlc (Dent et al , 1990) and a similar disruption is also observed following me phosphorylation of Ser 67 of the G M [63-75] peptide (Johnson et al, 1996). The finding that G M [63-93] disrupted the interaction between PPlc and die M no subunit, and prevented M no from enhancing the MLC 20 phosphatase activity of PPlc implies mat the binding of M n0 and G M to PPlc are mutually exclusive. Thus me invention contemplates the substitution or modification of an amino acid in any such peptide. To understand die basis for the recognition by PPlc of regulatory subunits, and peptides derived from these subunits, we co-crystallised a complex of PPlc with die G M [63-75] peptide and determined die structure at 3.0 A resolution. These experiments have demonstrated diat residues 64 to 69 of the peptide are bound in an extended conformation to a hydrophobic channel widiin die C-teπninal region of PPlc. The residues in G M [63-75] that interact with PPlc lie in an Arg/Lys-Val/Ile~Xaa-Phe motif common to M no [l-38] and almost all known mammalian PPl -binding proteins. Substituting Val or Phe by Ala in die G M [63-75] peptide, and deleting the VxF motif from die M 110 [l-38] peptide, abolished die ability of both peptides to interact wim PPlc. These findings identify a recognition site on PPlc for a critical structural motif involved in the interaction of targeting subunits widi PPl. Particularly preferred peptides are derived from residues 63 to 69 of G M and comprise the motif Arg/Lys-Val/Ile-Xaa-Phe. Peptides derived from M uo (or any odier source) and also including die motif are also included in the scope of the invention. Preferred peptides may also be substantially or wholly made up of hydrophobic residues. The identification of ώis area of PPlc necessary for binding to die various subunits allows die design of agents to specifically disrupt die interaction at this area. Such disruption may, for example, increase the phosphorylation of the protein phospholamban in cardiac muscle and dius increase die force and rate of contraction of the muscle. This provides a possible treatment for congestive heart failure. Also, die specific disruption of the complex of PPl and p53BP2 may prevent PPl from dephosphorylating the tumour suppressor protein p53, thus enhancing phosphorylation of p53, its ability to bind to DNA, and dius its ability to act as a tumour suppressor. The identification of die key motif in targetting subunits mat bind to PPl also provides die means to produce targetting subunits that can no longer interact with PPl . Over-expression of these mutant targetting subunits provides a powerful new way to determine me functions of different targetting subunits in vivo. Abbreviations PPl, protein phosphatase- 1 PPlc, catalytic subunit of PPl PPl, -isofoπn of PPlc PP1 G , glycogen-associated form of PPl PP1 M , myosin-associated form of PPl G M , glycogen-binding subunit of PPl from striated muscle G L , glycogen-binding subunit of PPl from liver NIPP1, nuclear inhibitor of PPl DARPP, dopamine and cyclic AMP-regulated phosphoprotein M 2 ι and M π0 , myofibrillar-binding subunits of PPl with molecular masses of 21 kDa and 110 kDa respectively. PKA, cyclic AMP-dependent protein kinase PhMeS0 2 F, phenylmethylsuphonyl fluoride GST, glutathione-S-transferase MLC 20 , myosin light chain of molecular mass 20 kDa. The invention is now described in more detail by reference to die following Examples and Figures wherein: Figure 1 shows that die N-terminal 118 residues of human G M interact with PPlc. GST-G M fusion proteins were electrophoresed on 10% SDS/polyacrylamide gels and stained widi Coomassie blue (lanes 1-3) or, after transfer to nitrocellulose, probed widi digoxygenin-labelled PPl (lanes 4-6) as in [9]. Lanes 1 and 4, GST-G M -(E2-D118); Lanes 2 and 5, GST-G M -(H100-P350); Lanes 3 and 6, GST. The positions of d e marker proteins glycogen phosphorylase (97 kDa), bovine serum albumin (66 kDa), ovalbumin (43 kDa), carbonic anhydrase (29 kDa) and soybean trypsin inhibitor (20 kDa) are indicated. Figure 2 shows diat syndietic peptides between residues 63 and 93 of rabbit G M stimulate die phosphorylase phosphatase activity of PP1 GL . Hepatic glycogen particles were diluted in assay buffer to 0.6 phosphorylase phosphatase (PhP) mU per ml, incubated for 15 minutes at 30 °C with G M -(G63-T93) (closed circles), G M -(G63-K80) (open circles) or G M -(G63-N75) (closed triangles) and assayed as described in Example 1. The open triangles show the effect of G M -(G63-N75) which had been phosphorylated at Ser67 by PKA (pG M -(G63-N75)). Similar results were obtained in four experiments. Figure 3 shows that removal of die M 21 subunit from smooth muscle PP1 M does not affect its MLC 20 phosphatase.phosphorylase phosphatase activity ratio. (A) Purified smooth muscle PP1 M was electrophoresed on a 12% SDS/polyacrylamide gel, and eidier stained widi Coomassie blue (lane 1) or immunoblotted [32] with antibodies specific for die M 21 subunit (lane 2) or die M 110 subunit (lane 3). The positions of the M 110 subunit, me M 21 subunit and PPlc are marked. (B) Purified PP1 M (lane 1) or PP1 M lacking die M 21 subunit (lane 2) were electrophoresed on a 12 % SDS polyacrylamide gel, transferred to nitrocellulose and immunoblotted with mixed, affinity-purified antibodies to die M n0 and M 21 subunits. The M no and M 2 , subunits are marked. The activity ratio, MLC 20 phosphatase (MP):phosphorylase phosphatase (PhP) of the two preparations is also shown. Similar results were obtained in diree different experiments. The activity ratio MP:PhP of PPlc is 0.07. Figure 4 shows expressed fragments of the M 110 subunit before and after cleavage of the GST-fusion proteins widi tiirombin . Purified GST-fusion proteins were electrophoresed on a 15% SDS/polyacrylamide gel and stained widi Coomassie blue. Lane 1, GST-M H0 -(M1-A150); Lane 2, GST-M 110 -(D39- E309); Lane 3, GST-M 110 -(M1-E309); Lane 4, GST-M 110 -(L24- Y496). Lanes 5-8 are the same as Lanes 1-4 except that the fusion proteins were cleaved widi dirombin. The positions of the marker proteins glycogen phosphorylase (97 kDa), bovine serum albumin (66 kDa), ovalbumin (43 kDa), carbonic anhydrase (29 kDa), GST (26 kDa) and soybean trypsin inhibitor (20 kDa) are marked. Figure 5 shows the effect of M u0 subunit fragments on PPlc-catalysed dephosphorylation of MLC 20 and glycogen phosphorylase A,B; Effects of M no -(Ml-E309) (closed circles), M 110 -(M1-F38) (open circles) and M 110 -(D39-E309) (open triangles) on the MLC 20 phosphatase (B) and phosphorylase phosphatase (B) activities of PPlc were measured after incubating PPlc for 15 minutes at 30°C with each fragment. The results are presented as a percentage of those obtained in experiments where the M 110 fragments were omitted. C,D; The effect of M 110 -(M1-A150) (open circles) and M 110 -(L24-Y496) (closed circles) on the MLC 20 phosphatase (C) and phosphorylase phosphatase (D) activities of PPlc were measured as in A, B. Figure 6 shows the effect of M U0 -(M1-F38) and M U0 -(M1-E309) on the dephosphorylation of glycogen syntiiase by PPlc. The glycogen syndiase phosphatase activity of PPlc was measured after a 15 minute incubation at 30°C wim die indicated concentrations of M n0 -(M1-F38) and M no -(Ml-E309). Similar results were obtained in three different experiments. Figure 7 shows that G M -(G63-T93) dissociates PP1 M . (A) The phosphorylase phosphatase (PhP) activity of PPl M (closed circles) and its MLC 20 phosphatase (MLCP) activity (open circles) were assayed after preincubation for 15 minutes at 30 °C with die indicated concentrations of G M -(G63-T93). Activities are shown relative to control incubations in which G M -(G63-T93) was omitted. Similar results were obtained in diree experiments. (B,C) PPIM was incubated for 15 minutes at 30°C in the absence (B) and presence (C) of 10 M G M -(G63-T93), then passed through a 30 x 1 cm column of Superose 12 equilibrated at ambient temperature in 50 mM Tris/HCl pH 7.5, 0.2M NaCl, 0.1 mM EGTA, 0.1 % (by vol) 2-mercaρtoethanol, 0.03% (by mass) Brij 35 in die absence (B) or presence (C) of 1 μM G M -(G63-T93). Fractions (0.25 ml) were assayed for MLC 20 phosphatase (MLCP) in B and for phosphorylase phosphatase (PhP) activity in C. The arrows denote die position of ferritin (450 kDa) and ovalbumin (43 kDa). Figure 8 shows that G M -(G63-T93) prevents M no -(Ml-F38) or M no -(Ml- E309) from modulating die substrate specificity of PPlc. (A) The MLC 20 phosphatase activity of PPlc was assayed after incubation for 15 minutes at 30 °C in me presence or absence of 1 μM G M -(G63-T93) and either 0.1 μM M 110 -(M1-F38) or 0.1 nM M 110 -(M1-E309). (B) The phosphorylase phosphatase activity of PPlc was assayed as in A in the presence or absence of 1 μM G M -(G63-T93) and 1.0 nM M 110 -(M1-E309). The results are presented (SEM for three experiments) as a percentage of the PPlc activity measured in me absence of G M -(63-T93), M U0 -(M1-F38) or M 110 -(M1-E309). Figure 9 shows die location of PPlc-binding domains on me G M and M no targeting subunits and their effects on PPl activity. The hatched boxes in me M 110 subunit denote the positions of die ankyrin repeats. Figure 10 shows a stereo view of die electron density corresponding to the peptide. A: Initial 2-fold averaged electron map. B: map calculated using 3Fo-2Fc coefficients and phases calculated from the final refined model. Displayed using TURBO-FRODO. Figure 11 shows die structure of PPl-G M [63-75] peptide complex. A. Stereo view of a ribbons diagram of PPlc to indicate die position of the peptide binding channel at die interface of die two β-sheets of the 3-sandwich. The peptide atoms are represented as ball-and-stick (MOLSCRIPT, Kraulis, 1991). B. View of the surface of PPlc to show die hydrophobic peptide binding channel. Residues 63' to 69' (GRRVSFA) (SEQ ID No 2) of the G M [63 75] peptide are shown as sticks. Drawn widi TURBO-FRODO. C. Stereo view of the G M [63-75] peptide at die recognition site of PPl to indicate polar interactions between peptide and protein and die formation of me 3-sheet between Ser 67' - Ala 69' and 14 of PPl. Drawn with TURBO-FRODO. D. Solvent accessible surface and surface electrostatic potential of PP1-G M [63 75] peptide complex calculated widi PPl coordinates alone and showing the peptide as a stick representation in die vicinity of me peptide binding site. The figure was created widi GRASP (Nicholls and Honig, 1991). The protein surface is coloured according to electrostatic potential from red (most negative) to blue (most positive). The figure shows pronounced negative electrostatic potential in me region surrounding die N-terminus of me peptide binding site diat results from seven conserved acidic residues. E. Details of the structure of die peptide binding site to show hydrophobic interactions between PPlc and Val 66', Phe 68' and Ala 69' of the G M [68-75] peptide (MOLSCRIPT, Kraulis, 1991). Figure 12 shows a sequence alignment of PPl -regulatory subunits in me vicinity of die (R/K)(V/I) x F motif. (A) mammalian PPl -binding subunits. G M , Tang et al, 1991; GL, Docherty et al, 1995; G L -related protein, Doherty et al, 1996; p53BP2, Helps et al, 1995; NIPP-1, Bollen et al, 1995; splicing factor PSF, Hirano et al , 1996; M 110 subunit, Chen et al, 1994; inhibitor-1, Aitken et al, 1982; DARPP-32, Williams et al, 1986. (B) PPl-binding proteins in S. cerevisiae. GAC1 (Francois et al, 1992); PIG2 GIP1, GIP2, YIL045W (Tu et al, 1996); REG1, REG2 (Tu and Carlson, 1995; Frederick and Tatchell, 1996); SCD5 (Nelson et al 1996; Tu et al 1996). The region homologous to the RRVSFA (SEQ ID No 3) motif in G M which intersects with PPlc is boxed. Figure 13 shows die disruption of die interactions between PPlc and die G L and M no subunits by a syndietic peptide from p53BP2. (A) PP1 M from chicken gizzard smooth muscle (Alessi et al, 1992) was diluted and incubated for 15 m i n a t 3 0 ° C w i t h t h e p e p t i d e GKRTNLRKTGSERIAHGMRVKFNPLALLLDSC (SEQ ID No 4), corresponding to die sequence in p53BP2 that contains die RVxF motif. Reactions were started widi eidier 32 P-labelled MLC 20 or glycogen phosphorylase and die MLC 20 phosphatase (open circles) and phosphorylase phosphatase (PhP, closed circles) activities were determined. The results are expressed as a percentage of die activity determined in control incubations where the p53BP2 peptide was omitted (100%). Similar results were obtained in three separate experiments. (B) same as (A) except that die peptide was incubated widi diluted hepatic glycogen particles containing PP1-G L before measuring the PhP activity. Similar results were obtained in three separate experiments. Figure 14 shows the effect of M 110 [M1-F38] and M 110 [M1-K35] on the PPlc-catalysed dephosphorylation of MLC 20 M π0 [Ml-F38] (1-38, open circles) or M U0 [M1-K35] (1-35, closed circles) were incubated with PPlc for 15 min at 30°C and reactions started widi die 32 P-labelled MLC 20 substrate. The results are expressed as a % of the activity determined in control incubations where the M 110 peptides were omitted (100%). Similar results were obtained in diree separate experiments. Figure 15 shows die effect of syndietic peptides derived from the M uo and G M subunits on the phosphorylase phosphatase activity of PP1-G L . (A) Hepatic glycogen protein particles containing PP1-G L were diluted and incubated for 15 min at 30°C wim the indicated concentrations of eitiier M 110 [M1-F38] (open circles) or M I10 [M1-K35] (closed circles) and die phosphatase reactions were initiated by addition of 32 P-labelled glycogen phosphorylase. The results are expressed as a percentage of the activity determined in control incubations where the M n0 peptides were omitted. Similar results were obtained in three separate experiments. (B) The experiment was carried out as in (A), except mat the peptide G G63-N75] ("wild type", WT) and variants in which either Val 66 (V66A) (closed triangles) or Phe 68 (F68A) (closed circles) were changed to Ala, were used instead of die M, 10 peptides. Similar results were obtained in three separate experiments. Figure 16 shows a stereo view of a ribbons diagram of a model of PPl -phospho-inhibitor-1 complex. The side chains of He 10, Phe 12 and pThr 35 of phospho-inhibitor-1 are shown with the main-chain atoms of residues 8 to 36 of die inhibitor indicated as a shaded ribbon. Drawn widi MOLSCRIPT (Kraulis 1991). Figure 17 shows a comparison of rat and chicken gizzard M u0 and M 2I subunits. Vertical lines indicate identical residues, colons denote similar residues in die rat and chicken M 1I0 sequences and deletions are shown by dots. (A) Comparison of M 110 subunits. Underlmed residues in die rat M no subunit (Ratl) are deleted in some rat aorta forms and underlined residues in die chicken M H0 subunit (Chi) are deleted in some chicken gizzard forms [5, 8]. Dashed lines above residues indicate amino acids deleted in die rat kidney M no subunit [9] . The alternative C-terminal sequences of rat uterus M 110 subunit are shown as Ratl and Rat2. Leucine residues in die C-terminal leucine zipper motif are double underlined . (B) . The C-terminal sequence of the M , 10 subunit is structurally related to the M 21 subunit. The sequence of the chicken M 21 subunit [5] is compared widi die C-terminal sequences of Rat2 and Chi from A. Identities between Chi and Rat2 are shown in boldface type. Figure 18 shows immunoprecipitation and immunoblotting of PPl M in extracts from chicken gizzard myofibrils. A. Antibodies specific for the M I10 and/or M 21 subunits immunoprecipitate most of the myosin P-light chain phosphatase activity in myofibrillar extracts. PP1 M was immunoprecipitated with either control IgG, antibody raised against me PP1 M holoenzyme, antibody specific for the M uo subunit or antibody specific for die M 21 subunit, as described under Meώods in Example 3. The figure shows activity present in die supernatant (S, open bars) or pellet (P, filled bars) as a percentage of diat measured before centrifugation. The results shown are the average (± S.E.M.) for tiiree separate experiments each assayed in duplicate. B, The M 110 and M 21 subunits are present in similar molar proportions in myofibrillar extracts and in purified PP1 M - 10 ng (track 1) or 3 ng (track 3) of purified PP1 M or 12 μg (track 2) or 3.6 μg (track 4) of myofibrillar extract was electrophoresed on a 12% SDS/polyacrylamide gel, transferred to nitrocellulose and immunoblotted widi mixed affinity-purified antibodies to the M H0 and M 2 , subunits as in [22]. The positions of die two subunits are marked. The results indicate that PP1 M comprises about 0.1 % of me myofibrillar protein. Figure 19 shows the identification of die region on die M 110 subunit diat interacts widi die M 2I subunit. A) PP1 M 5 μg (track 1), 10 μg bacterial extract containing M U0 -(R714-11004) (track 2), MBP-M U0 -(R714-I1004) 1 μg (track 3), MBP-M 110 -(R714-L934) 1 μg (track 4), MBP-M U0 -(K933-I1004) 1 μg (track 5), MBP lμg (track 6), M I 10 - (M1-E309) 2 μg (lane 7) and M 110 -(M1-S477) 2 μg (track 8) were run on a 12% SDS/polyacrylamide gel and stained wim Coomassie Blue. B) same as A) except that 10-fold less protein was electrophoresed and after transfer to nitrocellulose die proteins were probed widi digoxigenin-labelled M 2) subunit (0.2 /ig/ml). C) same as B) except that, after electrophoresis, the proteins were transferred to nitrocellulose and probed widi digoxigenin-labelled M 21 - (M1-L146) (0.2 μg/ml). Figure 20 shows me identification of die region of die M 21 subunit involved in interaction with die M no subunit and in dimerization. A) GST-M 2] 5 μg (track 1), M 21 5 μg (track 2), M 21 -(M1-L146) 5 μg (track 3), M 21 -(M1-E110) 20 μg (track 4) and M 21 -(E110-K186) 5 μg (track 5) were run on 16.5% polyacrylamide gels and stained widi Coomassie Blue. The marker proteins ovalbumin (43 kDa) and carbonic anhydrase (29 kDa) are indicated. B) GST-M 21 0.5 μg (track 1), M 2] 0.5 μg (track 2), M 21 -(M1-L146) 0.5 μg (track 3), M 21 -(M1-E110) 5 μg (track 4) and M 21 -(E110-K186) 5 μg (track 5) were electrophoresed as in A) and after transfer to nitrocellulose me blots were probed widi digoxigenin-labelled MBP-M 110 -(K933-I1004) (0.2 μg/ml). C) same as B) except that, after electrophoresis, the proteins were transferred to nitrocellulose and probed widi digoxigenin-labelled M 2! subunit (0.2 μg/ml). Figure 21 shows diat die M 21 subunit and M 21 -(M1-L146) interact wim me M 110 subunit and diemselves, but not widi PPl. PP1 M (0.5 μg) was electrophoresed on a 12% SDS/polyacrylamide gel, transferred to nitrocellulose and probed witii digoxigenin-labelled M 21 subunit (0.2 μg/ml) (track 1) or digoxigenin-labelled M 2] -(M1-L146) (0.2 μg/ml) (track 2). The positions of die M, 10 subunit, the M 21 subunit and PPlc are marked. Figure 22 shows diat removal of me M 21 subunit from smooth muscle PP1 M does not prevent it from being pelleted widi myosin. The PPl catalytic subunit (PPlc), PP1 M , or PP1 M lacking the M 21 subunit, PP1 M (ΔM 21 ), each at 30 nM, were incubated for 15 min at 0°C with 1 μM myosin and centrifuged (see Metiiods of Example 3). The figure shows the myosin P-light chain phosphatase activity present in die supernatant (S, open bars) or pellet (P, filled bars) as a percentage of that measured before centrifugation. The results shown are die average (± S.E.M.) for three separate experiments each assayed in duplicate. Figure 23 shows the identification of a region of die M 110 subunit which binds to myosin. (A); PP1 M , M U0 -(M1-S477) and GST-M U0 -(M377-K976), each at 30 nM were incubated for 15 min at 0°C widi 1 μM myosin and centrifuged. The supernatants (S), resuspended pellets (P) and die suspension before centrifugation (T, total) were electrophoresed on 12% SDS/polyacrylamide gels, transferred to nitrocellulose and immunoblotted widi antibodies raised against the PP1 M holoenzyme. No protein was pelleted in me absence of myosin (not shown). The positions of me marker proteins myosin heavy chain (200 kDa), glycogen phosphorylase (97 kDa), bovine serum albumin (66 kDa), ovalbumin (43 kDa), carbonic anhydrase (29 kDa) and soybean trypsin inhibitor (20 kDa) are indicated. (B) The experiments were carried out as in (A), except mat me M 110 fragments and M 21 subunit were used at 100 nM, the 8.5 kDa M 110 -(K933-I1004) fragment was electrophoresed on a 16.5% polyacrylamide gel and immunoblotting was carried out widi affinity purified antibodies (see Metiiods). A small amount of M 110 -(R714-11004) pelleted in die absence of myosin. This was probably due to aggregation in the bacterial extract since mis did not happen when it was complexed to die M 21 subunit (data not shown). No other protein was pelleted in me absence of myosin. Figure 24 shows mat the isolated M 21 subunit binds to myosin. (A); Myosin (1 μM) was mixed widi 50 μM, 20 μM or 10 μM M 2 , subunit to give die molar ratios M 21 :myosin dimer indicated. After 15 min at 0°C, the solutions were centrifuged and die supernatants (S), resuspended pellets (P) and die suspension before centrifugation (T, total) were electrophoresed on 12% SDS/polyacrylamide gels and stained widi Coomassie blue. The positions of me myosin heavy chain (MHC) and die M 2[ subunit are indicated. The myosin light chains migrate faster than die M 21 subunit and are not visible at tiiese loadings. (B); Myosin (track A) was purified from chicken gizzard, and die myosin "rod" domain (track B) and light meromyosin (track C) produced by digestion of myosin wim papain and chymotrypsin, respectively. These three proteins, all at 1 μM, were then mixed widi M 21 subunit (track D) to give a molar ratio M 2 ,:myosin dimer of 10:1 and, after 15 min at 0°C, the solutions were centrifuged and die supernatants (S), resuspended pellets (P) and die suspension before centrifugation (T, total) were electrophoresed on 12% SDS/polyacrylamide gels and stained widi Coomassie blue. The slightiy faster migrating band in the M 21 subunit preparation was shown by amino acid sequencing to be N-terminally truncated commencing at residue 16. (C); same as (B), except that M 21 -(M1-L146) (track D) replaced die M 21 subunit. Figure 25 gives a schematic representation of the regions on the M n0 subunit from chicken gizzard diat interact widi PPlc, myosin and the M 2] subunit. PPlc binds to the KVKF (SEQ ID No 5) motif between residue 35 and 38, just N-terminal to the seven ankyrin repeats (hatched vertical lines) mat suppress the dephosphorylation of substrates otiier than myosin. Residues 1-38 of die M 110 subunit enhance me dephosphorylation of myosin. The M 21 subunit binds to die C-terminal 72 residues of the M M0 subunit which are 43% identical in amino acid sequence to residues 87-161 of die M 21 subunit. The dephosphorylated form of myosin binds to M no -(R714-11004) but not to M 110 -(K933-I1004), suggesting mat myosin binds N-terminal to the M 21 subunit. Example 1: Identification of protein phosphatase 1 -binding domains on the glycogen and myofibrillar targeting subunits MATERIALS AND METHODS Materials. The myosin-associated form of PPl (PP1 M ) w s from chicken gizzard [9] and die glycogen-associated foπn of PPl (PP1 G ) from rabbit skeletal muscle [21]. The β isoform of PPlc was released from PP1 G by incubation for 2 hours in 2M LiBr, then purified by gel-filtration on a 30 x 1 cm column of Superose 12 (Pharmacia, Milton Keynes, U.K.) in die presence of 0.5M LiBr. Glycogen protein particles from rat liver [22] were used as die source of hepatic PP1 G . Digoxygenin-labeUed PPlc (γ,-isofoπn, hereafter termed PPl) was prepared as in [9]. G L was expressed in E. coli as a glutathione-S-transferase (GST) fusion protein [7], termed GST-G L . The catalytic subunit of PP2A from bovine heart (PP2AC) was provided by Dr R. MacKintosh in mis Unit. The phosphorylatable myosin light chain (MLC 20 ) and heavy meromyosin from chicken gizzard were a gift from Dr M. Ikebe (Case Western Reserve University, Cleveland, USA). Thrombin and benzamidine- Agarose were purchased from Sigma (Poole, UK). Peptide synthesis. Peptides were syntiiesised on an Applied Biosystems 430A peptide syntiiesiser and tiieir purity and concentration established by high performance liquid chromatography, mass spectrometry and amino acid analysis. The sequence of rabbit G M -(G63-T93) is GRRVSFADNFGFNLVSVKEFDTWELPSVSTT (SEQ ID No 6) and the sequence of M U0 -(M1-F38) is MKMADAKQKRNEQLKRWIGSETDLEPPVVKRQKTKVKF (SEQ ID No 7). The peptide G M -(G63-T93) was cleaved with Lys-C endoproteinase (Boehringer) and die peptide G M -(E81-T93) thus generated was purified on a C 18 column. The peptides G M -(G63-K80) and G M -(G63-N75), were synthesised, and die latter phosphorylated at Ser67 with die catalytic subunit of cyclic AMP-dependent protein kinase (PKA), then bound to a 1 ml C 18 column equilibrated in 0.1 % (v/v) trifluoroacetic acid, washed widi 0.1 % trifluoroacetic acid to remove excess ATP, eluted widi 0.1 % trifluoroacetic acid containing 70% acetonitrile, dried and dissolved in water. The peptide G M -(S40-Y55) was a gift from Dr Bruce Kemp (St Vincent's Institute, Australia). Preparation of phosphorylated proteins and phosphatase assays. 32 P-labelled rabbit skeletal muscle phosphorylase a (containing 1.0 mol phosphate per mol subunit) was prepared by phosphorylation widi phosphorylase kinase [23] , 32 P-labeIled rabbit skeletal muscle glycogen syntiiase (containing 1.5 mol/mol subunit in the sites 3 region) was prepared by phosphorylation with glycogen syntiiase kinase-3 [24]), 32 P-labelled chicken gizzard MLC 20 and 32 P-labelled chicken gizzard heavy meromyosin (containing 1.0 mol phosphate per mol subunit) were prepared by phosphorylation with smooth muscle myosin light chain kinase [9]. The dephosphorylation of phosphorylase a (10 μM), glycogen synthase (1 μM) and MLC 20 (1 μM) and heavy meromyosin (1 μM) was carried out as in [24]. One unit of activity (U) was that amount which released 1 mole of phosphate in one minute. Construction of vectors for the expression of N-terminal fragments of the G M subunit as glutathione-S-transferase (GST) fusion proteins in E. coli. G M -(E2-R575) was produced by inserting a Smal-Smal restriction fragment, encoding amino acids 2-575 of human G M , from clone HIGH [5] into me Smal site of pGEX-KG (Pharmacia, Milton Keynes, U.K.). This resulted in the addition after residues 2-575 of amino acids EFPVVVVEF (SEQ ID No 8) before the stop codon. G M -(E2-P243) was made by deleting an Ncol-Hindlll fragment of me G M -(E2-R575) construct, resulting in termination after residue 243. G M -(E2-D118), encoding amino acids 2-118, widi a C-terminal addition of QLNSS was produced by deleting a BgHI-HindTII fragment of the G M -(E2-R575) construct. G M -(H100-P350) encoding amino acids 100-350 was made by inserting an EcoRI-Hindlll digested PCR fragment prepared using primers 5 ' GCCGAATTC AC AC AGAAGAAT ATGTTTT AGCC 3 ' (SEQ ID No 9) and 5 ' GCCGAAGCTTATGGAAAATTGACTGGATCTGTTG 3 ' (SEQ ID No 10) into me same sites of pGEX-KG. Restriction sites in me primers are underlined. Construction of vectors for the expression of the chicken gizzard M 2 , subunit in E. coli. The entire coding region (M1-K186) of die M 21 subunit [10] was amplified by PCR using primers 5' CGCGCATATGTCGTCGCTGTTCACCAGG 3' (SEQ ID No 11) and 5' GGCGGATCCCTACTTGGAGAGTTTGC 3' (SEQ ID No 12), containing restriction sites Ndel and BamHI (underlined). After cleavage with die restriction enzymes, the PCR fragment was cloned into die same sites of the bacterial expression vector pT7-7. Production of fragments of the chicken gizzard and rat aorta M 0 subunits. The C-terminal 291 residues M, 10 -(R714-11004) of the chicken gizzard M 1I0 subunit were amplified by PCR using a primer 5' AGGAAGAATTCGTTCCACACGAAC 3' (SEQ ID No 13) containing an EcoRI restriction site (underlined) and a KS primer in the Bluescript vector of die cDNA clone [10]. The EcoRI digested PCR fragment was subcloned into die same site of pT7-7. Rat aorta M no fragments were produced as GST-fusion proteins. M U0 -(M1-A150) was amplified by PCR using primers A (5' CCTAGCCCGGGGATGAAGATGGCGGAC 3') (SEQ ID No 14) and B (5' GCGGAAGCTTATGCTTCCTCCTCTGCAATATC 3') (SEQ ID No 15), containing Smal and Hindlll restriction sites (underlined) and die Smal-Hindlll digested PCR fragment subcloned into the same sites of pGEX-KG. M 110 -(M1-E309) was produced by subcloning a Smal-Hindlll digested PCR fr a g m e n t amp l i fi e d u s i n g p r im e r s A a nd C ( 5 ' CTAGAAGCTTCCATATTTGCTGTTGATTCAATC 3 ') (SEQ ID No 16) into the same sites of pGEX-KG. This resulted in one amino acid (A) being added after E309. M U0 -(D39-E309) was produced by subcloning a Smal-Hindlll digested PCR fragment amplified using primers D (5' CCTAGCCCGGGGGACGATGGCGCCGTCTTCC 3') (SEQ ID No 17) and C into the same sites of pGEX-KG. An M 110 -(L24-K976) was prepared by inserting a Xhol-Xhol restriction fragment of the entire M 110 cDNA in Bluescript into Xhol site of pGEX-KG, and M 1I0 -(L24-Y496) expressed by deleting a Ndel-Ndel fragment of the L24-K976 construct and filling the overhanging ends before ligating them. This resulted in e addition after Y496 of amino acids MVAD (SEQ ID No 18) before the stop codon. The sequence of all subclones produced after PCR amplification were verified using an Applied Biosystems 373A automated DNA sequencer and Taq dye terminator cycle sequencing according to die manufacturer's instructions. Expression of proteins in E.coli. All constructs were expressed in E. coli strain BL21(DE3)plysS. Cultures were grown at 37 °C in Luria-Bertani medium in the presence of 100 μg/ml ampicillin and 30 μg/ml chloramphenicol to an A600 of 0.4-0.6, and induced widi 50 μg/ml isopropylthiogalactoside for 8 hours at 25 °C or overnight at ambient temperature. After centrifugation for 10 minutes at 7000 x g (4°C), cells from one litre of culture were resuspended in 20 ml of 50 mM Tris-HCl pH 8.0, 0.1 M NaCl, 1 mM EDTA, 0.1 % (by vol) 2-mercaptoethanol, 0.2 mM phenylmethylsulphonylfluoride (PhMeS0 2 F), 1 mM benzamidine (buffer A) and frozen at -80°C. After thawing, sodium deoxycholate (1 mg/ml ), 8 mM MgS04 and 10 g/ml DNAase I were added, die extract incubated until it was no longer viscous, then made 6 mM in EDTA, 1 mM in benzamidine and 0.2 mM in PhMeS0 2 F and centrifuged for 10 minutes at 10,000 x g. The soluble GST-fusion proteins were then purified from the supernatant by affinity chromatography on glutathione-Sepharose (Pharmacia). The M 2! subunit and M I10 -(R714-11004) C-terminal fragment from chicken gizzard M 110 subunit, which were used for affinity purification of the anti-M 2ι and anti-M n0 antibodies (see below) were obtained in inclusion bodies and dierefore recovered in me pellets after centrifuging E. coli extracts at 10,000 x g. M no -(R714-11004) was solubilised by resuspension in Buffer A containing 0.5% (by mass) Triton X-100 and was >95% pure. The M 21 subunit was not solubilised by this procedure but, after washing die pellets in 0.5% Triton X-100, was dissolved by sonication in 0.5% trifluoroacetic acid; its purity was about 20% . M uo GST-fusion proteins (1-9 mg/ml in 50 mM Tris/HCl, 2.5 mM CaC12, 150 mM NaCl and 0.1 % (by vol) 2-mercaptoethanol) were cleaved by incubation for 20 minutes at 30°C wim 20 μg/ml thrombin. Benzamidine-Agarose (0.2 ml) was added and, after incubation (widi rotation) for 30 minutes at ambient temperature, die benzamidine- Agarose containing die attached dirombin was removed, and die supernatant dialysed against 50 mM Tris-HCl pH 7.5, 0.1 mM EGTA, 0.1% (by vol) 2-mercaptoethanol, 10% glycerol and stored in aliquots at -80°C. After cleavage with dirombin, all fragments of the M 110 subunit, except M 110 -(L24-Y496), commenced widi die sequence GSPG (SEQ ID No 19) before the initiating residue of die GST-fusion proteins. The M no -(24-Y496) was preceded by the sequence GSPGISGGGGGILDSMGR (SEQ ID No 20). Production of antibodies that recognise the M m and M 21 subunits of chicken gizzard PP1 M . Polyclonal sheep antibodies to the PPl M holoenzyme were raised in die Scottish Antibody Production Unit (Carluke, Ayrshire, U.K.). Antibodies which recognise me M uo subunit specifically were obtained by passing me antiserum down a 4 ml affinity column comprising 40 mg of M UO -(R714-I1004) coupled covalently to lg of dried CNBr-activated Sepharose 4B (Sigma). After washing widi 10 column volumes of 50 mM Tris/HCl pH 7.5, 1 % (by mass) Triton X-100, 0.1 mM EGTA, 0.1 % (by vol) 2-mercaptoethanol (Buffer B) plus 0.5 M NaCl, followed by 10 volumes of Buffer B plus 1 M LiBr, the anti-M no antibody was eluted widi 50 mM glycine pH 2.0, neutralised immediately widi 1 M Tris/HCl pH 8.0 and stored in aliquots at -80°C. Antibodies which recognise me M 2] subumt specifically were obtained in an identical manner, except mat the affinity column comprised about 40 mg of the expressed chicken gizzard M 21 subunit coupled to 6 g (dry weight) of CNBr-activated Sepharose. Removal of the M 21 subunit from PP1 M . PP1 M (0.01 ml, 0.4U/ml) was dissociated by incubation for 30 minutes with 500 μM arachidonic acid [25] and then for 30 minutes with 0.08 ml of packed Protein G-Sepharose coupled to 0.08 mg of affinity purified anti-M 21 antibody. The Protein G-Sepharose was pelleted, and die supernatant diluted at least 15-fold to allow die M π0 subunit and PPlc to recombine. The M no -PPlc complex was further purified by gel filtration on Superose 12 (30 x 1 cm) to ensure complete removal of any free PPlc. RESULTS. Identification of a PPlc-inter action domain on the G M -subunit of PP1 GM . The amino acid sequence of rat hepatic G L is 23% identical (39% similar) to residues 1-286 of G M from human skeletal muscle [7]. There is no homology over the first 63 residues but identity is > 40% over die regions 63-86, 144-166 and 186-227 of G M suggesting diat one or more of these sequences comprise a PPl -binding domain. Fusion proteins in which GST was linked to fragments of G M were therefore tested for their ability to bind to PPlc. GST-G M -(E2-D118) (Fig 1) and GST-G M -(E2-P243) (data not shown), but not GST-G M - (H100-P350) or GST itself (Fig 1) interacted with PPl in Far Western experiments, indicating diat die first 118 residues of G M contain a PPlc-binding domain. Moreover, a proteolytic fragment derived from GST-G M -(E2-D118) whose molecular mass was 5 kDa less than GST-G M -(E2-D118), but not a proteolytic fragment mat was 6 kDa smaller, also interacted widi PPlc (Fig 1). Taken together, die observations suggested that die region comprising residues 63-86 was likely to bind to PPlc. We therefore synthesised G M -(G63-T93) and examined its effect on the enzymatic properties of PP1 GL , me form of PPl associated widi rat hepatic protein-glycogen particles. The interaction of PPlc with G L suppresses die dephosphorylation of muscle glycogen phosphorylase by 80% and enhances die dephosphorylation of muscle glycogen syntiiase by 2-3 fold [21, 26]. Disruption of the characteristic properties of hepatic PP1 GL can dierefore be monitored very simply by changes in its specificity. G M -(G63-T93) induced a sixfold increase in the phosphorylase phosphatase activity of PP1 GL , the concentrations required for 50% activation being 30 nM (Fig 2). G M -(G63-T93) also prevented bacterially expressed GST-G L from suppressing die phosphorylase phosphatase activity of PPlc (data not shown). However, G M -(G63-T93) had no effect on the glycogen syntiiase phosphatase activity of PP1 GL , nor was there any alteration of die otiier characteristic properties of PP1 GL , namely allosteric inhibition of die glycogen syntiiase phosphatase activity by phosphorylase a and binding to glycogen (data not shown). Thus die interaction of G M -(G63-T93) with PP1 GL does not displace G L from PPlc. G M -(G63-T93) also increased die phosphorylase phosphatase activity of PPlc, indicating diat it binds to PPlc, rather than to G L . However, the maximal stimulation was only 37 + 1.4% (SEM for three experiments), establishing mat far greater activation of PP1 GL is explained by die ability of G M -(G63-T93) to overcome the suppressive effect of G L on the phosphorylase phosphatase activity of PPlc. Several other peptides, including a 32 residue peptide related to the C-terminus of ribosomal protein S6([G245,G246]S6[218-249]), G M -(S40-Y55) and G M -(E81-T93) (data not shown), had no effect on the phosphorylase phosphatase activity of PP1 GL or PPlc at concentrations up to 10 μM. The peptides G M -(G63-K80) and G M -(G63-N75) also increased the phosphorylase phosphatase activity of PP1 GL , but were less effective than G M -(G63-T93) and higher concentrations were needed (Fig 2). G M -(G63-K80) and G M -(G63-N75) did not increase the phosphorylase phosphatase activity of PPlc significantiy at concentrations up to 10 μM (data not shown). The phosphorylation of G M at Ser67 by cyclic AMP-dependent protein kinase (PKA) triggers the dissociation of PPl from G M in vitro and in vivo [18] and phosphorylation of the peptide G M -(G63-N75) at Ser67 prevented it from increasing die phosphorylase phosphatase activity of PP1 GL (Fig 2A). The increase in phosphorylase phosphatase activity observed at die highest phosphopeptide concentrations (10 μM) may be explained by trace contamination ( < 10%) wim dephosphopeptide, resulting either from incomplete phosphorylation of Ser67 or slight dephosphorylation during the assay. Identification of a PPl -interaction domain on the M m subunit. Antibodies were prepared diat recognised eitiier the M 110 or M 21 subunits of die myosin-associated form of PPl (PP1 M ) from chicken gizzard (Fig 3A). Removal of the M 21 subunit using die M 2] -specific antibody (Fig 3B and see Metiiods) did not affect the activity of PPl M towards MLC 20 or phosphorylase, the MLC 20 phosphatase :phosphorylase phosphatase activity ratio (0.95 ± 0.03) remaining 15-fold higher than PPlc (Fig 3B). The M 2i subunit bound to M 110 , but had no effect on the MLC 20 phosphatase or phosphorylase phosphatase activity of PPlc and did not bind to PPlc (D. Johnson unpublished). Thus M 110 is solely responsible for enhancing the dephosphorylation of MLC 20 and suppressing die dephosphorylation of glycogen phosphorylase by PPlc [9]. In order to identify which region(s) of M, 10 modulates die specificity of PPlc, fusion proteins were constructed consisting of glutathione S-transferase (GST) followed by fragments of the M 110 subunit. After expression in E. coli and purification by affinity chromatography on glutathione-Sepharose, the fusion proteins were cleaved widi dirombin to release GST from fragments of die M 110 subunit (Fig 4 and see Metiiods). M 110 -(M1-E309), which contains seven 33 residue ankyrin repeats located between residues 39-296, modified die specificity of PPlc in a similar manner to M no itself, increasing activity towards MLC 20 about 3-fold (Fig 5A) and suppressing activity towards glycogen phosphorylase by about 80% (Fig 5B). The concentration of M 110 -(M1- E309) required to activate die MLC 20 phosphatase activity maximally (0.1 nM) was similar to the PPlc concentration in the assay, indicating an extremely high affinity for PPlc. M M0 -(M1-A150) modified die specificity of PPl similarly, but 10-fold higher concentrations were needed compared to M π0 -(M1-E309) (Figs 5C and 5D). If die GST tags were not cleaved widi dirombin, a 10-fold higher concentration of M no -(Ml-E309) was needed to modulate die substrate specificity of PPlc, while M 110 -(M1-A150) was unable to stimulate me MLC 20 phosphatase activity of PPlc at all (data not shown). GST itself did not interact with PPlc (Fig 1), had no effect on eitiier die MLC 20 phosphatase or phosphorylase phosphatase activity of PPlc (data not shown), and dierefore was not removed from the solution after cleavage of die fusion proteins widi dirombin. In contrast to M U0 -(M1-E309), M 110 -(D39-E309) failed to stimulate the MLC 20 phosphatase activity of PPlc, or to inhibit its phosphorylase phosphatase activity (Figs 5 A and 5B), suggesting that die extreme N-terminus of die M 110 subunit (i.e. before the start of the ankyrin repeats) might be important in modulating die specificity of PPlc. The peptide M n0 -(M1-F38) was dierefore synthesized and found to stimulate die MLC 20 phosphatase activity of PPlc to the same extent as M no -(Ml-E309), altiiough die concentration required for half maximal activation (10 nM) was at least 100-fold higher (Fig 5A). M 110 -(M1-F38) stimulated die dephosphorylation of heavy meromyosin in a similar manner to me dephosphorylation of MLC 20 (data not shown). However, like M 110 -(D39-E309), M π0 -(M1-F38) did not inhibit die phosphorylase phosphatase activity of PPlc (Fig 5B). These observations suggested that residues beyond 38 were needed to suppress phosphorylase phosphatase activity. Consistent with this, M π0 -(L24-Y496) was less effective than M 110 -(M1-A150) or M 110 -(M1-E309) in stimulating the MLC 20 phosphatase activity of PPlc, but inhibited die phosphorylase phosphatase activity of PPlc in a similar manner to M H0 -(M1-A150) (Figs 5C and 5D). Although M no -(D39-E309) and M 110 -(M1-F38) had no effect on the phosphorylase phosphatase activity of PPlc when each peptide was included individually in the assays at concentrations up to 1 μM (Fig 5), a 39 ± 2% inhibition (SEM n=4) was observed when botii peptides were botii present at 1 μM. Surprisingly, M 110 -(D39-E309) prevented (IC50 = 0.1 M) M 110 -(M1-F38) from stimulating the MLC 20 phosphatase activity of PPlc (data not shown). Thus M 110 -(D39-E309) plus M 110 -(M1-F38) do not faithfully mimic the effect of M U0 -(M1-E309). We have reported previously that die M 110 /M 21 complex suppresses die dephosphorylation of glycogen syndiase by PPlc [9] and, consistent with this finding, die dephosphorylation of glycogen syndiase was also inhibited by M 110 -(M1-E309) (Fig 6B). However, die dephosphorylation of glycogen syndiase was greatiy enhanced by M U0 -(M1-F38) (Fig 6A). The binding of G M and the M m subunit to PPlc is mutually exclusive. In order to investigate whether G M binds to the same region of PPlc as M 110 , we next examined die effect of G M -(G63-T93) on the properties of PP1 M . G M -(G63-T93) at 10 μM increased die phosphorylase phosphatase activity of PP1 M by about 7-fold and suppressed its MLC 20 phosphatase activity by 60-65 % (Fig 7A), indicating that die distinctive properties of PP1 M had been disrupted. Gel-filtration experiments confirmed diat G M -(G63-T93) had displaced die M 110 subunit from PP1 M , dissociating it to PPlc (Figs 7B and 7C). G M -(G63-T93) also prevented M 110 -(M1-F38) or M 110 -(M1-E309) from stimulating the MLC 20 phosphatase activity of PPlc (Fig 8A), and prevented M 110 -(M1-E309) from suppressing die phosphorylase phosphatase activity of PPlc (Fig 8B). Conversely, die presence of 10 μM M 110 -(M1-F38) increased die phosphorylase phosphatase activity of PP1 GL by 3.5- fold. This resulted from the partial dissociation to PPlc, because die enhanced phosphorylase phosphatase activity was not associated with glycogen, but recovered in die supernatant after centrifugation of die glycogen-protein particles (not shown). DISCUSSION. We have identified a region on G M that binds to PPlc (Fig 9). The peptides G M -(G63-T93), G M -(G63-K80) and G M -(G63-N75) all prevented G L from suppressing die dephosphorylation of glycogen phosphorylase by PPlc and two lines of evidence indicate mat ese peptides interact widi PPlc and not widi G L . Firstiy, the PPlc-catalysed dephosphorylation of glycogen phosphorylase is stimulated slightly by G M -(G63-T93). Secondly, PPlc crystallises in the presence of G M -(G63-K80) or G M -(G63-N75) in a different form than is observed in die absence of tiiese peptides. PKA phosphorylates G M at Ser67 and die introduction of a negative charge directly into the PPlc-binding domain explains why phosphorylation of Ser67 triggers the dissociation of G M from PPlc [18]. Phosphorylation of G M -(G63-N75) at Ser67 also prevented tiiis peptide from interacting with PPl in the PP1 GL complex (Fig 2). Although G M -(G63-T93) prevented G L from suppressing die dephosphorylation of glycogen phosphorylase by PPlc, it did not dissociate G L from PPlc, nor did it affect the otiier characteristic properties of PP1 GL . Moreover, unlike G L , G M -(G63-T93) did not itself suppress die phosphorylase phosphatase activity of PPlc, but actually enhanced it slightiy. These observations demonstrate that anodier region(s) on G L must interact with PPlc and that this other region(s) may play an important role in modulating die substrate specificity of PPlc. The presence of a second PPlc binding site in G M /G L would be somewhat analogous to the situation found in inhibitor- 1 and DARPP which also contain two PPl -binding sites, high (nM) affinity binding being generated by die conjugation of two low affinity binding sites that, individually, only interact widi PPl at μM concentrations [28]. The second PPlc-binding site on G M /G L might correspond to one of the other regions where G M and G L show >40% identity (residues 144-166 and 186-227 of human G M ). Although G M -(H100-P350) was not recognised by PPlc in Far Western experiments (Fig 1) this result is not definitive because G M -(H100-P350) may only interact with PPlc weakly. Alternatively, G M -(H100-P350) might not fold correctly or fail to renature after SDS/polyacrylamide gel electrophoresis. However, it is also possible that residues 144-166 and 186-227 of G M do not represent part of die second PPlc-binding domain, but part of die glycogen-binding domain. In this connection it should be recalled diat residues 144-166 and 186-227 are the regions showing greatest similarity (25 % identity) to GAC1, which appears to be a homologue of G M /G L in budding yeast [7, 27, 28] . Curiously, GAC1 does not contain a region homologous to residues 63-93 of G M /G L . It would clearly be of interest to compare the effect of GAC1 on die enzymatic properties of PPlc with tiiose of G M and G L . We have also identified a region on die M ao subunit diat binds to PPlc. An N-terminal fragment, M100-(M1-E309), enhanced die PPlc-catalysed dephosphorylation of MLC 20 and suppressed die dephosphorylation of glycogen phosphorylase in a similar manner to M 110 itself (Fig 5). However, unlike M no , tiiis fragment does not bind to myosin. Thus die region which enhances die dephosphorylation of MLC 20 is distinct from the myosin-binding domain. The fragment M 110 -(M1-E309) contains seven ankyrin repeats lying between residues 39 and 296. However, M 110 -(D39-E309) was ineffective as an activator of the MLC 20 phosphatase activity of PPlc or as an inhibitor of the phosphorylase phosphatase activity, and tiiis led to die finding diat a peptide comprising me N-terminal 38 residues of die M ι , 0 subunit enhances e dephosphorylation of MLC 20 to the same extent as M UO -(M1-E309), aldiough widi lower potency. However, M U0 -(M1-F38) did not inhibit the dephosphorylation of glycogen phosphorylase by PPlc suggesting diat residues beyond 38 are required to suppress this activity. This view was reinforced by die finding diat, aldiough neither M πo -(Ml-F38) nor M 110 -(D39-E309) inhibited die phosphorylase phosphatase activity of PPlc when present individually, inhibition was observed in the presence of botii peptides. Moreover M πo -(D39-E309) actually prevented M 110 -(M1-F38) from stimulating the dephosphorylation of MLC 20 . These observations suggest that M U0 -(D39-E309) can bind to M 110 -(M1-F38) and/or PPlc. An interaction with PPlc seems likely because it has been found diat M U0 -(D39-E309) can enhance die phosphorylase activity of PP1 GL . The presence of a second PPl-binding site in the ankyrin-repeat domain of die M 110 subunit is also supported by die observation tihat higher concentrations of M 110 -(M1-A150) and M 110 -(M1-E309) are needed to inhibit the phosphorylase phosphatase activity of PPlc than are required to stimulate its MLC 20 phosphatase activity (see Fig 5). The presence of at least two PPl-binding sites may explain why die M no subumt and PPlc interact at picomolar concentrations. The ankyrin repeat domain might suppress the dephosphorylation of some substrates (such as glycogen phosphorylase) by a steric mechanism, preventing diem from gaining easy access to die catalytic centre. This scenario could explain why die dephosphorylation of glycogen syndiase is greatly enhanced by M U0 -(M1-F38) yet suppressed by M no -(M1-E309) (Fig 6). G M -(G63-T93) abolished the distinctive properties of PPl M (Fig 7A), prevented M U0 -(M1-F38) or M πo -(M1-E309) from modulating die substrate specificity of PPlc (Fig 8) and displaced die M 110 subunit from PP1 M (Fig 7B). In addition, die peptide M no -(Ml-F38), was capable of displacing G L from PP1 G . These findings indicate diat die binding site(s) on PPlc for G M and die M 110 subunit are likely to overlap, explaining why different forms of PPl contain a single PPl -targeting subunit. The diree-dimensional structure of PPlc isoforms have recently been solved to high resolution [29,30], and PPlc crystallises in different forms in the presence of G M -(G63-N75) or G M -(G63-K80) or M 110 -(M1-F38) than in me absence of these peptides. Consistent with the results presented here, Gailly et al [31] have recendy shown that M 110 -(M1-F38) or M no (Ml-E309) enhance the ability of PPlc to stimulate the relaxation of microcystin-contracted permeabilised portal vein, while G M -(G63-T93) inhibits the ability of PP1 M to induce die relaxation of tiiis smooth muscle. G M -(G63-T93) also slowed the relaxation of permeabilised femoral artery, indicating diat it competes with die endogenous M n0 subunit for PPlc [31]. Thus die PPlc-binding peptides described constitute useful pharmacological agents widi which to explore die role and regulate the activity of PPl in cell regulation. Example 2: Structural basis for the recognition of regulatory subunits by the catalytic subunit of protein phosphatase 1 MATERIALS AND METHODS Crystallisation and Data Collection The catalytic subunit of PPl 1 was overproduced in Escherischia coli and purified as described previously (Alessi et al. , 1993; Barford and Keller, 1994). The G M [G63-N75] peptide, variants of this peptide in which Val 66' or Phe 68' were changed to Phe, and die peptides M uo [l-38] and M uo [l-35] were synthesised on an Applied Biosystems 430A peptide syndiesiser and purified by chromatography on a C18 column (Johnson et al, 1996) by Mr F.B. Caudwell at Dundee. A tiiree-fold molar excess of G M [G63-N75] was added to die protein solution (8 mg/ml), which had been previously dialysed against 10 mM Tris-HCl (pH 7.8), 0.3 M NaCl, 0.4 mM MnCl 2 and 2 mM DTT. The complex was crystallised at 20 °C using the hanging drop vapour diffusion method, by mixing 2 ml of the protein-peptide solution and 2 ml of the precipitant solution containing 2.0 M ammonium sulphate, 2% (w/v) polyethylene glycol 400,100 mM HEPES (pH 7.5) and 2 mM DTT. These conditions are very much in contrast to the relatively low ionic strength conditions from which the monoclinic PPlc crystals grew (Barford and Keller, 1994; Egloff et al, 1995). Crystals appeared after 3 montiis as a cluster. Individual crystals removed from the cluster had dimensions of ~25 μm x 25 μm x 5 μm. Crystals were frozen in a 100 K nitrogen gas stream and stored. Prior to freezing, crystals were incubated in a cryoprotectant solution consisting of an equilibration buffer; 2.0 M ammonium sulphate, 2% (w/v) PEG 400, 100 mM HEPES (pH 7.5) with increasing amounts of glycerol in steps of 7% , 15%, 22% and 30% (v/v). A partial data set to 3.0 A was collected on Beam Line PX 9.6, SRS, Daresbury, using a 30 cm diameter Mar Research image plate system. Data were processed and scaled using DENZO and SCALEPACK (Otwinowski, 1993). The crystal system is tetragonal with point group symmetry P422 and unit cell dimensions a = b = 62.50A, c = 361.30A. Systematic absences indicate a 21 screw axis along b. The Matthews coefficient was 2.38 A 3 per Dalton, assuming 2 molecules per asymmetric unit. A second data-set was collected on BL4 at the ESRF, Grenoble. Substantial radiation damage was observed during data collection requiring mat three crystals were used in total. Data collected from four crystal at Daresbury and die ESRF were merged together in SCALEPACK. Details of the data collection and processing statistics are given in Table 1. Structure determination The structure of the PPl-G M [63-75] complex was solved by molecular replacement using as a model die protein atoms coordinates of die 2.5 A refined structure of the catalytic subunit of PPI I determined by MAD methods (Egloff et al , 1995). Rotation and translation functions searches were performed with AMORE (Navaza, 1992). Using data between 8 and 3 A resolution, die peak in me rotation search was 6.7 standard deviations (SD) above die mean. The translation search was best performed using data between 8 and 3.5 A, giving a maximal peak at 13.8 SD above the mean for the space group P41212. After the first rigid body refinement performed in AMORE, the R-factor was 0.494 and die correlation factor 0.30. Crystallographic Refinement The solution from molecular replacement was optimized by 20 cycles of rigid body refinement performed widi X-PLOR version 3.1 (Brunger, 1992), using data between 8.0 A and 3.0 A resolution. After a round of conjugate gradient positional refinement and simulated annealing molecular dynamics to 2000 K, followed by 25 cycles of grouped B-factor refinement (2 B-factor groups for each residue), die R factor (respectively free-R) was 0.295 (0.367). Fourier difference maps (Fo-Fc) and (3Fo-2Fc) revealed die presence of tiiree strong peaks at (over diree-times the sigma level of the map) at e catalytic site of PPlc. From the previously refined PPlc-structure, we identified two as manganese and iron ions. The tiiird one, occupying die position of die tungstate ion in die PPlc-W04 complex, was identified as sulphate. The initial difference Fourier maps also revealed strong electron density near die N-terminus of 014. The maps were improved by applying non-crystallographic symmetry 2-fold averaging using PHASES (Furey and Swaminathan, 1990). As shown in Fig. 1A, residues Val 66', Ser 67' and Phe 68' of the G M [63-75] peptide were identified in me averaged map. These 3 residues, as well as die 2 metal and sulphate ions were built in each molecule, using die program TURBO-FRODO (Roussel and Cambillau, 1992) . Refinement of this structure was performed by repeated rounds of manual rebuilding followed by conjugate gradient positional refinement and grouped B-factor refinement using X-PLOR. The final model contains protein residues Lys 6 to Ala 299 and peptide residues Arg 65' to Ala 69' in molecule 1, and protein residues Asn 8 to Lys 297 and peptide residues Gly 63' to Ala 69' in molecule 2. A few well defined water molecules were also observed in botii initial (3Fo-2Fc) and (Fo-Fc) electron density maps. Eventually, 14 water molecules that were above 3 sigma in die (Fo-Fc) difference map, widiin hydrogen bond of die PPl -peptide complex or anodier solvent molecule and present in botii molecules, were included in the model. The crystallographic and refinement data are summarized in Table 1. Representative electron density from the peptide before and after refinement is shown in Figure 10 A and 10B, respectively. Solvent accessible surface areas were calculated using the method of Lee and Richards (1974). Purification and assay of PPl. PPlc was isolated from the rabbit skeletal muscle PP1-G M complex as described previously (Johnson et al, 1996). Glycogen particles isolated from rat liver (Schelling et al, 1988) served as die source of PP1-G L . The dephosphorylation of glycogen phosphorylase (10 μM) and die isolated MLC 20 of smooth muscle myosin (1 μM) by PPlc was carried out as described previously (Cohen et al, 1988; Alessi et al, 1992). Table 1. Crystallographic data and refinement statistics The star (*) indicates residues absolutely conserved in all protein phosphatase 1 sequences available so far, the double start (**) the residues mostly conserved (from sequence alignment from Barton et al, 1994). RESULTS AND DISCUSSION Structure Determination. Crystallographic data to 3.0 A were measured at die ESRF beam-line BL4 at Grenoble and at PX9.6, Daresbury (Table 1). The relatively high merging R-factors and low 1/(1 values of die crystallographic data results from the weak diffraction observed from the PPl-G M [63-73] crystals. This is attributable to both the small crystal size ( ~ 25 μm by 25 μm by 5 μm) and long c-axis of die unit cell. In addition, the high x-ray photon dose required to obtain usable diffraction images resulted in x-ray radiation damage to die crystals, despite being maintained at a temperature of 100 K during the course of the experiment. The structure was solve by the molecular replacement metiiod using as a search model the 2.5 A refined coordinates of PPlc (Egloff et al , 1995). Phases obtained from a single cycle of simulated annealing refinement of the protein coordinates alone using X-PLOR Brunger, 1992), and improved by 2-fold non-cry stallographic symmetry averaging and solvent flattening, were used to calculate an electron density map. This map revealed clear density corresponding to residues Val 66', Ser 67' and Phe 68' (where ' denotes residues of die peptide) of the G M peptide and provided a starting point for further refinement of the PP1-G M peptide complex (Fig. 10A). The final model of die complex was refined at 3.0 A resolution widi a crystallographic R-factor of 0.22 and R-free of 0.31 (Fig. 10B). The two molecules of PPlc within the asymmetric unit are similar with a root mean square deviation between main chain atoms of 0.6 A. Residues 6 to 299 and 8 to 297 from molecules 1 and 2 respectively, are visible in the electron density map. Similar to me structures of native PP γl (Egloff et al, 1995) and PPlα in complex with microcystin LR (Goldberg et al, 1995), residues C-terminal to 299 are disordered. Overall Structure of PPl The conformation of PPlc in the PP1-G M complex is virtually identical to diat of native PPlc in complex with tungstate (Egloff et al. , 1995) with a root mean square deviation between equivalent main-chain atoms of 1.0 A. PPlc is folded into a single elliptical domain consisting of a central /3-sandwich of two mixed β-sheets surrounded on one side by 7o helices and on die otiier by a sub-domain consisting of 3α-helices and a 3 stranded mixed o;-sheet (Fig. 2A, B). The interface of the three /3-sheets at the top of the 3-sandwich creates a shallow catalytic site channel. Three loops connecting /."-strands with o;-helices within a β-a-β-a-β motif in sheet 1 (strand order /S4-93 - 32-313-/314) together with loops emanating from the opposite /3-sheet (sheet 2; strand order, β 1-/35- 36-/310-/312-/311) provide die catalytic site residues. The catalytic site of PPl contains a binuclear metal site consisting of Mn 2+ and Fe 2+ (Egloff et al, 1995) and, in the PP1-G M complex, oxygen atoms of a sulphate ion of crystallisation coordinate bo metal ions, similar to that seen in die PPl -tungstate (Egloff et al. , 1995) and PP2B-phosphate complexes (Griffith et al, 1995). PPlc-G M [63-75] Peptide Interactions Six residues of the G M [63-75] peptide (Arg 64' to Ala 69') are clearly visible in the electron density map of die complex of molecule 2, the remaining residues are not visible and assumed to be disordered (Fig. 10B). Density is not visible for Arg 64' of the peptide bound to molecule 1 , otherwise equivalent residues of die peptide are similar within the two complexes. The six residues (RRVSFA) (SEQ ID No 3) of the G M [63-75] peptide in complex 2 adopt an extended conformation and bind to a hydrophobic channel on the protein surface with dimensions 25 A by 10 A diat is formed at die interface of the two /3-sheets of the /3-sandwich opposite to die catalytic site channel and therefore remote from the catalytic site (Fig. HA). The residues diat form this channel occur on three regions of PPlc, namely (i) the N-terminus of 5 and /35//36 loop of sheet 2; (ii) the three edge /3-strands of sheet 2: /310, β\2, βl 1 and (iii) /313, the 013//314 loop and 014 of die edge of sheet 1 (Fig. 11 A). The total solvent accessible surface area buried on formation of the complex is 980 A2. Three residues of die peptide (Ser 67' to Ala 69') form a /3-strand which is incorporated into / 3-sheet 1 of PPlc as a sixth /3-strand parallel to die N-terminus of the edge / 3-strand, /314 (residues Leu 289 to Leu 296) (Fig. 11C). Main-chain atoms of Ser 67' and Ala 69' form H-bonds to die main-chain atoms of residues of /314. In addition, die main-chain nitrogen of Val 66' forms a H-bond with die side-chain of Asp 242. Oti er polar interactions include die guanidinium group of Arg 64' with the mainchain carbonyl of Glu 287 and a salt bridge to Asp 166. Botii Asp 166 and Asp 242 are invariant in mammalian PPl genes. A water molecule bridges the main-chain carbonyl of Arg 65' and side-chain hydroxyl of Ser 67' widi the main-chain carbonyl of Thr 288 of PPlc (Fig. 11C). A notable feature of the peptide binding site is die presence of a negatively charged region created by seven acidic residues (widi one Lys residue) surrounding die hydrophobic channel at the N-terminus of the peptide in the vicinity of Arg 64' and Arg 65' that includes Asp 166 and Asp 242 (Fig. 11D). This would suggest a favourable electrostatic environment for the side chains of Arg 64' and Arg 65'. The predominant interactions between the peptide and PPlc involve hydrophobic contacts between the side chains of Val 66' and Phe 68' and solvent exposed, invariant, hydrophobic residues of PPlc diat form the hydrophobic channel (Fig. 11C, E). In particular, the binding site for the side chain of Val 66' is formed from the side chains of He 169, Leu 243, Leu 289 and Cys 291, whereas that for the side chain of Phe 68' is formed from the side chains of Phe 257, Cys 291 and Phe 293. Details of peptide-PPlc contacts are given in Table 2. The structure of the G M [63-75] peptide binding site is likely to be conserved in other forms of PPl from diverse species. Each hydrophobic residue of PPlc diat interacts with die Val 66' and Phe 68' residues of the G M [63-75] peptide are invariant and die acidic residues that surround die N-terminus of die peptide binding site are highly conserved amongst all isofoπns of PPl from species as diverse as yeast, Drosophila, mammals and higher plants (Barton et al, 1994). However, since these residues are not conserved widiin die PP2A and PP2B sequences, tiiese proteins will not recognise PPl -regulatory subunits. Presence of an (R/K) (V/I) x F Motif in other PPlc Regulatory Proteins Over a dozen regulatory subunits of PPlc are now known which appear to bind to PPlc in a mutually exclusive manner that suggests either an overlapping binding site or sites. Sequence comparisons of these subunits reveals little similarity except for the motif (R/K) (V/I) x F, that is not only present in G M [63-75] but also in G M , G L , M no , NIPP-1, p53BP2, and an RNA splicing factor (Fig. 12A). Moreover, a 38 residue peptide from the HOkDa M, 10 subunit that binds to PPlc contain this motif (Johnson et al, 1996), as do fragments of NIPP-1 (Beullens et al, 1992; Van Eynde et al, 1995), an RNA splicing factor (Hirano et al, 1996) and p53BP2 (Helps et al, 1995). A 32 residue peptide from p53BP2, which contains this motif, disrupted the interaction of the M 110 subumt with PPlc, as shown by a decrease in die rate of dephosphorylation of die MLC 20 subunit of smooth muscle myosin and by an increase in the rate of dephosphorylation of glycogen phosphorylase (Fig. 13A). This peptide also disrupted die interaction of die G L subunit widi PPlc, as shown by an increase in the rate of dephosphorylation of glycogen phosphorylase (Fig. 13B). Peptides comprising me motif (R/K) (V/I) x F are thus encompassed widiin the scope of the invention. In further support of the notion of a common PPlc recognition motif present widiin PPl-binding proteins, previous studies had revealed diat die sequence KIQF (SEQ ID No 22) (similar to the R KVxF motif) at the N-terminus of inhibitor 1 and its homologue DARPP-32 (Fig. 12 A) is necessary for mediating e inhibitory effects of these proteins. Loss of He 10 of the KIQF (SEQ ID No 22) motif of inhibitor 1 disrupts the inhibitory effects on PPlc by phospho-inhibitor-1 (Aitken and Cohen, 1984; Endo et al, 1996) and die binding of eitiier dephospho-inhibitor-1 or phospho-inhibitor-1 to PPlc (Endo et al. , 1996). A similar result was found on disrupting the equivalent residue (He 9) of DARPP-32 (Hemmings et al, 1990; Desdouits et al, 1995). These results were interpreted to indicate that inhibitor- 1 and DARPP-32 bind to PPl through two low affinity binding sites, one that encompasses the sequence KIQF (SEQ ID No 22) and another which includes die phosphorylated Thr residue (35 in I- 1 , 34 in DARPP-32) and which presumably binds at die catalytic site. Analysis of die PPl-G M [63-75] complex structure suggests that an isoleucine residue could be readily accommodated widiin die peptide binding site in place of Val 66' such that the additional metiiyl group on He compared to Val would contribute to favourable van der Waals interactions between the peptide and Leu 243 and Cys 291 of PPl. More bulky hydrophobic residues such as Leu, Met and Phe cannot be accommodated, however. It is interesting to note that as well as die (R K) (V/I) x F motif shared by PPl -regulatory subunits, the four residues N-terminal to this motif contain an abundance of basic residues. These residues may provide further favourable interactions with the negative electrostatic surface potential at the N-terminus of the G M (63-75] peptide binding site of PPlc (Fig. 11D). Mutagenesis of the R/K) (V/I) x F motif The structural studies presented here suggest a dominant role for Val 66' and Phe 68' in stabilising the interaction between G M [63-75] and PPlc and this notion is further reinforced by die finding diat otiier PPl -regulatory subunit sequences contain an (R K)(V/I) x F motif yet share little overall sequence similarity. To test the hypothesis that Val 66' and Phe 68' are required for the interaction of G 63-75] with PPlc and also that the KVKF (SEQ ID No 5) sequence present within the M U0 [M1-F38) peptide is important in mediating its interaction with PPlc, we synthesised variations of the G M and M uo peptides where die R/KVxF motif was disrupted. The two variants of the G M peptide were Val 66' and Phe 68' to Ala substitutions. In order to disrupt die (R/K)(V/I) x F present within the M 110 peptide, a peptide corresponding to residues Met 1 to Lys 35 was syndiesised which no longer contains the sequence VKF of the VxF motif, which is present at residues 36-38. The results for the M, 10 [l-38] and M 110 [l-35] peptides (Figs. 14, 15) are unequivocal. Whereas M t 10 [l-38] stimulates the myosin light chain phosphatase activity of PPlc with a half-maximal effect at 10 nM reaching maximal (3-fold) activation at a peptide concentration of 1 μM as reported previously (Johnson et al, 1996), the M uo [l-35] peptide was at least 104-fold less effective at activating PPlc (Fig. 14). Unlike M I 10 [l-38], the M u0 [l-35] peptide was also unable to activate die phosphorylase phosphatase activity of liver PP1-G L . This latter result suggests two conclusions. Firstly, that although M U0 [l-38] is able to bind to PPlc and disrupt die interactions between PPlc and die G L -subunit, hence reversing me inhibitory effects of G L on the ability of PPlc to dephosphorylate phosphorylase, loss of the VKF sequence in the M 110 [l-38] peptide abolishes the ability of the peptide to disrupt this interaction. Secondly, die recognition site on PPlc for the VKF sequence of the M 110 [l-38] peptide must overlap with die binding site for the G L subunit, suggesting that die VKF sequence binds to die same site as the VSF sequence of G L diat is identical with that present in the G M [63-75] peptide. Similar conclusions may be reached from the results obtained from disrupting die VxF motif within the G M [63-75] peptide (Fig. 16B). Substitution of Phe 68' for Ala abolishes completely the ability of G M [63-75] to disrupt die PP1-G L complex, whereas replacement of Val 66' widi Ala reduced the effectiveness of the disruption 100-fold. Thus preferred peptides may comprise analogues of G M with substitutions at Val 66' and Phe 68' to some other amino acid such as Ala, so diat binding of die PPlc to G M does not occur and die PPlc is not suitably directed or controlled. Alternatively, suitable peptides could comprise peptides suitable to compete for the binding site(s) of Val 66' and Phe 68' on PPlc. Such peptides can be added in sufficient quantities to compete for the Phe 68' and Val 66' binding site(s) on die PPlc, thereby disrupting die interaction of PPlc and natural G M . Such peptides could comprise structural analogues of G M with Phe 68' and Val 66' in the same positions as G M . Alternatively, other amino acids capable of mimicking the binding of Phe 68' and Val 66' could be used in these locations. Regulation of the PP1-G M Complex by Phosphorylation of Ser 67' Phosphorylation of Ser 67', corresponding to x of die VxF motif, by PKA promotes dissociation of botii G M and G M [63-75] from PPlc. In vivo, this provides a mechanism of inhibiting PPlc during stimulation of skeletal muscle by adrenalin (Dent et al, 1990). The sequence of G M surrounding Ser 67' (RRVSFA) (SEQ ID No 3) conforms to a consensus PKA recognition sequence. Interestingly, the conformation of the peptide is similar to that of residues 18 to 23 corresponding to die pseudo-substrate sequence of PKI bound to the catalytic site of PKA (Knighton et al, 1990). Although the side chain of Ser 67' is exposed within the PPlc-peptide complex, overall the G M peptide is buried, and it is unlikely that Ser 67' would be a substrate for PKA when the peptide is bound to PPlc. This would suggest tihat PKA phosphorylates Ser 67' when G M is not associated widi PPlc and diat tiiis phosphorylation prevents die re-association of PP 1 c with G M . Since phosphorylation of Ser 67 ' promotes the dissociation of die PP1-G M complex both in vivo and in vitro, it is most likely that PKA phosphorylates Ser 67' of G M by competing with PPlc for the RRVSFA (SEQ ID No 3) sequence. This is consistent with a notion that the PP1-G M complex exists in dynamic equilibrium with free PPlc and G M subunits and diat phosphorylation occurs on the regulatory subunit during transient dissociation from PPlc. In the PPlc-peptide complex, the side-chain of Ser 67' adopts die most favourable rotamer conformation. Analysis of me PPlc peptide complex structure suggests diat incorporation of a phosphate group onto the side chain of Ser 67' with die same side-chain rotomer conformation would cause steric hindrance between the peptide and Met 290 of PPl and also introduce a phosphate group into a region of negative charge at the PPlc surface (Fig. 11C, D). This may explain how phosphorylation of Ser 67' prevents peptide association widi PPlc, aldiough it should be noted that rotation of the side-chain of Ser 67' would relieve this steric clash. A similar mechanism of control may also operate for other PPl -regulatory subunits. For example, NIPP-1 a nuclear inhibitor of PPl, inhibits PPl with an inhibitory constant of 1 pM (Beullens et al, 1992). Phosphorylation of NIPP-1 by PKA and/or casein kinase 2 in vitro abolishes this inhibition (Beullens et al, 1993; Van Eynde et al, 1994). Although the sites of phosphorylation on NIPP-1 that mediate these effects are not yet fully characterised it is known that these sites occur within the central ~ 120 residues of NIPP-1 that incorporates the (R K)(V/I) x F motif (Van Eynde et al. , 1995). Interestingly, a consensus phosphorylation site for PKA (RKNS) (SEQ ID No 23) occurs immediately N-terminal to this motif whereas one casein kinase 2 consensus phosphorylation site occurs between the Val and Phe of die motif and another occurs immediately C-terminal to the Phe residue (TFSEDDE) (SEQ ID No 24) (Van Eynde et al, 1995) (Fig. 12A). It is possible that PKA, casein kinase II or other kinases with similar specificity, release PPlc from inhibition by NIPP-1 by phosphorylating NIPP-1 at sites that block its interaction with die (R K)(V/I) x F motif recognition site on PPlc. Model of the PPlc-Phospho-Inhibitor 1 Complex Our model for the interaction of a (R/K)(V/I) x F motif with PPlc, together with previous kinetic data suggesting that the sequence KIQF (SEQ ID No 22) of inhibitor-l (Aitken and Cohen, 1984; Endo et al, 1996) and DARPP-32 (Hemmings et al , 1990; Desdouits et al, 1995) interacts with PPlc, allowed us to construct a plausible model of a complex of PPlc with phospho-inhibitor 1. The major assumptions of this model were (1) die KIQF (SEQ ID No 22) sequence of inhibitor-l binds to the same site as RVSF (SEQ ID No 25) of the G M [63-75] sequence and (2) that the phosphothreonine residue 35 of phospho-inhibitor 1 binds at die phosphate binding site of die PPlc-catalytic site. Secondary structure predictions of inhibitor 1 (Rost and Sander, 1993; Rost, 1996) suggested that residues 9 to 14 and 23 to 31 adopt /3-strand and α-helical conformations, respectively. The prediction of die sequence KIQF (SEQ ID No 22) as a |3-strand is consistent widi our assumption diat this region of inhibitor-l adopts die same conformation as RVSF (SEQ ID No 25) of the G M peptide when bound to die VxF recognition site of PPlc. We have positioned die residues RRPpTP (SEQ ID No 26) encompassing die pThr 35 site widiin die catalytic site channel in an extended conformation, widi the phosphate group of the pThr 35 occupying the phosphate binding site of die catalytic site and die Oy-atom of Thr 35 equivalent to the solvent exposed oxygen of a dianion that forms a H-bond to die side-chain of die putative general acid His 125 (Egloff et al, 1995; Griffith et al, 1995). The four consecutive Arg residues N-terminal to pThr 35 interact with Asp and Glu residues within an acidic groove of PPlc formed from the βl/βS loop on one side and die βlOlβl 1 loop and /311 strand on die other, similar to that proposed by Goldberg et al, (1995) for their model of DARPP-32 bound to PPlc. We propose diat residues 20 to 30 of inhibitor-l form an amphipathic helix which folds around die edge of die /3-sandwich of PPlc. The N-terminus of this helix is disrupted by prolines at residues 19 and 23. Pro 19 and Pro 15 are probably responsible for introducing turns into the polypeptide chain that allows die /3-strand encompassing the KIQF (SEQ ID No 22) sequence (residues 9 to 14) to connect widi the helix. The model of the phospho-inhibitor 1-PPlc complex is shown in Fig. 16. Prediction of PPl Recognition Motifs in Yeast PPl-Binding Proteins The residues in mammalian PPlc mat interact with die sequence RRVSFA (SEQ ID No 3) are conserved in S. cerevisiae PPl suggesting that the proteins in 5. cerevisiae known to interact with PPl (reviewed by Stark, 1996) probably bind to a similar hydrophobic groove on the surface of the enzyme. Examination of their amino acid sequences revealed diat a number of PPl-binding proteins in S. cerevisiae contained putative PPl-binding motifs that were similar to those present in mammalian PPl-binding proteins (Fig. 12A, B). The S. cerevisiae PPl-binding proteins not only contain a V/I x F motif, but also a basic residue equivalent to Arg 64' of G M V the residue that contacts Asp 166, Leu 289 and die main-chain carbonyl of Glu 287 of PPlc. Several of the S. cerevisiae proteins also contain a further basic residue (His or Lys) at die position equivalent to Arg 65' of G M . Another striking feature of the putative PPl-binding sequences in S. cerevisiae is the presence of a basic amino acid between the Val/Ile and Phe residues, as is also found in two mammalian PPl -regulatory subunits, the M, 10 subunit and die p53BP2 (Fig. 12A). The S. cerevisiae proteins GAC1 and PIG2 show some homology to residues 140-230 of mammalian G M V and tiiere is genetic and biochemical evidence diat tiiey may function to regulate glycogen metabolism in budding yeast (Francois et al, 1992). GIP2 also shares sequence similarity with residues 140-230 of mammalian G M , while YIL045W is an open reading frame in the S. cerevisiae genome whose predicted amino acid sequence shows 41 % sequence identity to GIP2. YIL045W contains two putative PPl-binding motifs and site directed mutagenesis will be needed to establish which (if either) of these sequences binds to PPlc. REG1 and REG2 are PPl-binding proteins that play a role in cell growth and, in the case of REG1, glucose repression (Tu and Carlson, 1995; Tu et al, 1996; Frederick and Tatchell, 1996). GIP1, which also contains two putative PPl-binding motifs, is expressed specifically during meiosis, affects the transcription of late meiotic genes and is essential for sporulation (Tu and Carlson, 1996). SCD5 is a PPl -interacting protein (Tu et al, 1996) that was first isolated as a multicopy suppressor of the inviability of clathrin heavy chain-deficient yeast (Nelson et al, 1996). The findings herein demonstrate that die short peptide sequence, die (R/K)(V/I)XF motif, is critical for PPlc to interact with its regulatory subunits. PPlc (when complexed to its targeting subunits) plays key roles in the control of many cellular processed and it is reasonable to predict diat over 100 ppl -binding proteins may exist in mammalian cells. Protein sequence data-base searching has revealed that the (R/K)(V/I)XF motifs are found in 10% of proteins. Thus if ~ 100 PPl-binding proteins occur in mammalian cells, only 1 % of proteins with the (R/K)(V/I)XF motif will be PPl-binding proteins . The reasons why only a few proteins with the (R/K)(V/I)XF motif bind to PPl are numerous. For example, not every residue may be tolerated at position X or immediately N-terminal or C- terminal to this motif. This study has shown diat phosphoserine is not tolerated at position X and it is dierefore likely that Asp or Glu will not be tolerated either. The structure of the PPl-G M [63-75] complex suggests that large hydrophobic residues will also be excluded from position X. Moreover, the Val (or He) and Phe residues in many (R/K)(V/I)XF motifs will be buried in die hydrophobic core of the protein and hence be unable to interact with PPl, since this motif is predicted to form an amphipathic /3-strand conformation. Thirdly, many of the (R/K)(V/I)XF motifs will be in extracellular proteins or extracelluar domains of transmembrane proteins and hence be unable to bind to PPl. Particular feature so the tertiary structure of PPl-binding proteins may allow exposure of this motif on the surface to allow interaction with PPl. Finally, there is evidence diat a second PPl-binding site exists on die G M and M 110 subunits (Johnson et al, 1996) and die high affinity interaction of PPlc with protein inhibitor-l is generated by die binding of PPlc to two low affinity sites (Desdouits et al, 1995), one of which is the KIQF sequence belonging to the (R/K)(V/I)XF motif. The question of how regulatory subunits modulate the substrate specificity of PPlc requires the co-crystallisation of PPlc with a diverse array of regulatory subunits and substrates and is beyond the scope of this paper. However, two models to account for this property of regulatory subunits are that tiiese subunits either alter the conformation of PPlc or simply target PPl to its substrates. Both mechanisms may operate in vivo depending on die regulatory subunits and substrates. For example, evidence for the former model has recently been reported for the enhancement of myosin dephosphorylation by a complex of PPlc and die M πo subunit (Johnson et al, 1996, 1997), whereas the enhancement of the dephosphorylation of glycogen phosphorylase and glycogen synthase by the PP1-G M complex is more consistent with the second model (Hubbard and Cohen, 1989). The identification of the (R/K)(V/I)XF motif also suggests a new approach for determining die physiological roles of PPl -targeting subunits whose functions are unknown. Thus mutation of die (R/K)(I/V)XF motif should disrupt die interaction of many targeting subunits with PPlc without affecting their binding to die target locus. Expression of tiiese mutated proteins under an inducible promoter should lead to displacement of the normal targeting subunit (complexed to PPlc) from its target locus, without disrupting die functions of any otiier PPlc-targeting subunit complex. Finally, die structural information described here will also facilitate the rational design of drugs that act by disrupting PPl -targeting subunit interactions. Example 3: Identification of the regions on the M subunit of protein phosphatase IM that interact with the M 71 subunit and with myosin Abbreviations:- PP1 M , myofibril-associated form of protein phosphatase 1; PPlc, catalytic subunit of protein phosphatase- 1; M ll0 and M 21 , 110 kDa and 21 kDa regulatory subunits of PP1 M ; MBP, maltose-binding protein; GST, glutathione-S-transferase. SUMMARY We have previously isolated a form of protein phosphatase- 1 (PP1 M ) from avian smooth muscle myofibrils which is composed of die catalytic subunit of PPl (PPlc) bound to an M-complex consisting of 110 kDa (M I10 ) and 21 kDa (M 2ι ) subunits. The interaction of PPlc with an N-terminal region of die M πo subumt enhances die dephosphorylation of myosin and suppresses die dephosphorylation of otiier substrates [Alessi, D.R., MacDougall, L.K., Sola, M.M., Ikebe, M. and Cohen, P. (1992) Eur. J. Biochem 210, 1023-1035; Chen, Y.H., Chen, M.X., Alessi, D.R., Campbell, D.G., Shanahan, C, Cohen, P. and Cohen, P.T.W. (1994) FEBS Lett 356, 51-56; Johnson, D.F., Moorhead, G., Caudwell, F.B., Cohen, P., Chen, Y.H., Chen, M.X. and Cohen, P.T.W. (1996) Eur. J. Biochem. 239, 317-325]. In this Example we establish that PP1 M accounts for nearly all the myosin phosphatase activity in myofibrils, that the M no and M 21 subunits are present at similar concentrations in the myofibrillar fraction and that these subunits are entirely bound to PPl . We demonstrate that die M 2ϊ subumt does not interact with PPlc, but with the C-terminal 72 residues of die M uo subunit, a region which is 43% identical to residues 87-161 of die M 21 subunit. A fragment of the M 21 subunit, M 21 -(M1- L146), lacking the C-teπninal leucine zipper, also bound to die M no subunit, but two other fragments M 21 -(M1-E110) and M 21 -(E110-K186) did not. The M no and M 21 subunits were both found to be myosin-binding proteins. The C-terminal 291 residues of die M no subunit, but not the C-terminal 72 residues, bound to myosin, but the N-terminal fragments M no -(Ml-E309) and M 110 -(M1- S477) did not. Thus the region of the M 110 subunit which stimulates the dephosphorylation of myosin by PPlc is distinct from the region which targets PP1 M to myosin. Remarkably, each myosin dimer was capable of binding about 20 moles of M 2l subunit and many of the M 2 ,-binding sites were located in the myosin "rod domain" . The potential significance of this observation is discussed. Introduction Protein phosphatase- 1 (PPl), one of die major serine/threonine-specific protein phosphatases in eukaryotic cells, is regulated by targetting subunits that direct it to particular subcellular loci, modify its substrate specificity and confer the ability to be regulated by extracellular signals (reviewed in [1, 2]). A significant proportion of the PPl in vertebrate muscle extracts is associated with myofibrils and has enhanced activity towards die P-light chain of myosin and reduced activity towards otiier substrates, such as glycogen phosphorylase [3, 4]. When isolated from avian (chicken gizzard) [4, 5] or mammalian (pig bladder) [6] smootii muscle, this form of PPl (PP1 M ) was found to be composed of three subunits, namely the catalytic subunit of PPl (PPlc) and two other proteins with molecular masses of 110 kDa and 21 kDa, termed the M 110 and M 21 subunits, respectively [4, 5]. The M no subunit is complexed to botii PPlc and die M 21 subunit [4], and is die component which modulates die substrate specificity of PPlc because selective removal of the M 21 subunit from PP1 M does not affect the rate at which either myosin or glycogen phosphorylase are dephosphorylated [7]. The M πo subunit has been cloned from rat aorta [5], chicken gizzard [8] and rat kidney [9] cDNA libraries. The N-terminus of the M uo subunit contains seven ankyrin repeats (residues 39-296 of die rat aorta protein), while alternative splicing in rat uterus [5] gives rise to two different C-termini (Fig 17A), termed Ratl and Rat2. The C-terminus of Ratl is virtually identical to die C-terminus of the M uo subunit from chicken gizzard (Fig 17A). The sequence of die M 2j subunit from chicken gizzard is structurally related to the C-terminal region of the M uo subunit, the most striking similarities occurring from residues 76-141 of die M 2[ subunit and residues 921-984 of the chicken gizzard M 110 subunit (54% identity, Fig 17B). However, the C-terminal 53 residues of die M 21 subunit from chicken gizzard are strikingly similar (83% identity) to die C-terminal 53 residues of me rat aorta sequence, both teπninating in a leucine zipper (Fig 17B, [5]). Residues 1-309 of die M 110 subunit from rat aorta, M U0 -(M1-E309), mimic the intact M 110 subunit in stimulating die dephosphorylation of myosin and in suppressing the dephosphorylation of glycogen phosphorylase by PPlc, but a slightly shorter construct M no -(D39-E309) (which still contains die seven ankyrin repeats) is unable to modulate the specificity of PPlc [7]. This observation led to die finding that the N-terminal 38 residues, M 110 -(M1-F38), bind to PPlc and enhance the dephosphorylation of myosin. However, M no - (M1-F38) does not suppress die dephosphorylation of glycogen phosphorylase, suggesting that the ankyrin repeats either contain a second PPlc-binding site or prevent glycogen phosphorylase from binding to the active site of PPlc, perhaps by steric hindrance [7]. A 13 residue peptide G M -(G63-N75) from die subunit (G M ) which targets PPlc to glycogen and die sarcoplasmic reticulum in striated muscle, has been co- crystallised with PPlc and die structure of die complex solved to 3A resolution [2]. These studies showed that a hexapeptide sequence in G M -(G63-N75) (Arg- Arg-Val-Ser-Phe-Ala) (SEQ ID No 3) binds to a small hydrophobic groove on the surface of PPlc forming a /3-sheet which runs parallel to anotiier /3-sheet in PPlc. Moreover, inspection of other mammalian PPlc-binding proteins reveals that almost all contain an Arg/Lys-Val/Ile-Xaa-Phe motif that is likely to be critical for interaction with PPlc [2]. For example, a Lys-Val-Lys-Phe (SEQ ID No 5) motif is present between residues 35 and 38 of the M 110 subunit and die deletion of residues 36-38 from M 110 -(M1-F38) prevents this peptide from stimulating the dephosphorylation of myosin, and from disrupting the interaction of PPlc with other targetting subunits [2]. The finding that a region near the N-terminus of the M no subunit binds to PPlc and modulates its specificity raised die question of which region on die M uo subunit interacted widi die M 21 subunit, and how die PP1 M complex is targeted to the myofibrils. In this Example we identify regions near the C-terminus of the M 110 subunit that interact with die M 21 subunit and widi myosin, and demonstrate that the M 21 subunit is also a myosin-binding protein. These findings indicate that the domain of the M, 10 subunit which enhances the dephosphorylation of the myosin P-light chain is distinct from the region which targets PPlc to the contractile apparatus. MATERIALS AND METHODS Materials PP1 M [4] and the dephosphory lated form of myosin [10] were isolated from chicken gizzard, and die rod-domain and light meromyosin were obtained by subdigestion of chicken gizzard myosin with papain and chymotrypsin, respectively [11]. PP1 G was purified from rabbit skeletal muscle by Dr G. Moorhead in this laboratory [12] and PPlc dissociated from the glycogen- binding subunit by incubation for 2 h in 2 M LiBr and then purified by gel- filtration on a 30 x 1 cm column of Superose 12 (Pharmacia, Milton Keynes, UK) in the presence of LiBr (0.5 M). All other chemicals were from BDH Chemicals (Poole, UK) or Sigma (Poole, UK). Construction of vectors for the expression of fragments of the M m subunit from rat aorta (rat2 sequence in Fig 17 A) as glutathione-S-transferase (GST) fusion proteins in E. coli. A construct pGEX-M 110 -(Ml-E309) for the expression of GST-M 110 -(M1-E309) from rat aorta was produced as described previously [7]. A construct for the expression of GST-M 110 -(M1-S477) was prepared by subcloning a Xhol-Hindlll fragment (encoding L24-S477) of pKS-M u0 -(Ml-S477) described in [5] into die same sites of pGEX-M no -(Ml-E309). The resulting construct expressed a GST-M 110 -(M1-S477) fusion protein with the additional amino acids SAANSISSLIHRD* (SEQ ID No 27) after S477. An expression construct for GST-M 110 -(M377-K976) was produced by deleting a Ncol-Ncol fragment of the construct pGEX-M 110 -(L24-K976) [7]. Construction of vectors for the expression of C-terminal fragments of the M 110 subunit from chicken gizzard (Chi sequence in Fig 17 A, [5]) as maltose binding protein (MBP) fusion proteins in E. coli. A pT7.7 vector for the expression of the C-terminal 291 residues of the M 110 subunit from chicken gizzard, pT7-M U0 -(R714-11004) was described previously [7]. A construct for the expression of MBP-M 1I0 -(R714-11004) was produced by cloning an Ndel-BamHl fragment of pT7-M 110 -(R714-I1004) into the pMAL- HA vector (New England Biolabs). Removal of a HindRl-Hindlll restriction fragment from pMBP-M no -(R714-I1004) allowed expression of MBP-M U0 - (R714-L934) with the sequence GTGRRFTTS (SEQ ID No 28) added to its C- terminus. Removal of a Ndel-Hindlϊl restriction fragment from pMBP-M no - (R714-I1004), followed by filling in the overhanging ends and religating them, allowed expression of MBP-M 110 -(K933-I1004). Construction of vectors for the expression in E. coli. of the M 21 subunit from chicken gizzard [5], with and without the C-terminal leucine zipper domain. A pT7.7 vector for the expression of the entire coding region (M1-K186) of the M 21 subunit was described previously [7]. The leucine zipper motif of the M 21 subunit was deleted by removing a Sacl-BamRl restriction fragment from pT7.7 M 21 , filling in the overhanging ends and religating them. The construct expressed M 21 -(M1-R144) widi an extra I and L after residue 144. The M 21 - (M1-R144) protein was insoluble when expressed and was purified as described for the expressed M 2] subunit [7]. Construction of vectors for the expression of the M 2! subunit from chicken gizzard [5] and fragments of the M 2] subunit as glutathione-S-transferase (GST) fusion proteins in E. coli. A construct expressing GST-M 2I was produced by inserting a Ndel-Hindϊil fragment of pT7.7 M 21 encoding M1-K186 into the same sites of the pGEX vector modified to include an Ndel site. A construct expressing GST-M 21 -(M1- E110) plus an additional Ala residue at the C-terminus was constructed by deleting a Xhol-Hindlϊl fragment of pGEX-M 21 , filling in die overhanging ends and religating them. In order to express GST-M 21 -(E110-K186), a Ndel-Xhol restriction fragment of pGEX-M 2 , was deleted and the overhanging ends filled in and religated. Expression of proteins in E. coli. This was carried out essentially as described in [7] , except diat, after freezing the cells at -80 °C and ώawing, the lysates were not treated with DNAase but sonicated for 4 min on ice (ensuring diat die temperature remained below 4°C) until the suspension was no longer viscous. The soluble GST-fusion proteins and MBP-fusion proteins were purified from the supernatant by affinity chromatography on glutathione-Sepharose (Sigma) and amylose resin (New England Biolabs), respectively, according to die instructions of the manufacturers. After expression in E. coli M U0 -(R714-11004) was the major soluble protein and all experiments with this fragment were performed using the bacterial extracts. The chicken gizzard M 2 , subunit was isolated from E. coli extracts as described [7]. M 2j subunit lacking the leucine zipper domain, M 21 -(M1-L146), like the M 21 subunit itself, was obtained in inclusion bodies and dierefore recovered in the pellet obtained after centrifugation of the bacterial lysates for 30 min at 28 000 x g. The inclusion bodies were washed three times in 50 mM Tris/HCl pH 7.5, 0.1M NaCl, 10 mM EDTA, 0.1 % (by vol) 2-mercaptoethanol, 1 mM benzamidine, 0.2 mM phenylmemylsulphonyl fluoride and 0.5% (by mass) Triton X-100, then resuspended in 50 mM Tris/HCl pH 7.5, 1 mM EDTA, 1 mM EGTA, 0.03% (by mass) Brij-35, 0.1 % (by vol) 2-mercaptoetiιanol. An aliquot (containing 3 mg protein) was made 0.5 % (by vol) in trifluoroacetic acid, sonicated, centrifuged for 2 min at 13,000 x g and die supernatant (containing the solubilised M 21 subunit) loaded on to a Vydac C18 column (Separations Group, Hesperia, CA, USA) equilibrated in 0.1 % (by vol) trifluoroacetic acid. The column was developed widi a linear acetonitrile gradient at a flow rate of 1.0 ml / min with an increase in acetonitrile concentration of 1 % per min. Homogeneous M 21 subunit, which eluted at 42% acetonitrile, and M 2 r(Ml-L146) which eluted at 40% acetonitrile were dried in a vacuum concentrator redissolved in water, redried and then dissolved in 50 mM Tris/HCl pH 7.5, 0.1 mM EGTA, 0.03% (by mass) Brij-35, 0.1 % (by vol) 2-mercaptoethanol. Removal of GST and MBP tags from fusion proteins. GST-M u0 -( 1-477) was cleaved widi dirombin and die proteinase removed using benzamidine agarose [7] . GST-M 21 -(E110-K186) (lmg / ml) was cleaved by incubation for 1 h at 30°C with 10 μg/ml thrombin, while GST-M 21 -(M1- E110) (lmg / ml) was cleaved by incubation for 3 h at 30°C with 1 μg/ml thrombin, because it was more susceptible to degradation by thrombin. MBP- M 110 (K933-I1004) (1 mg / ml) was cleaved by incubation for 8 h at 23 °C with Factor Xa (10 μg/ml). Other conditions and removal of Factor Xa were carried out as described for thrombin [7]. Preparation of phosphorylated myosin P-light chain and phosphatase assays. 32 P-labelled myosin P-light chains containing 1.0 mol phosphate per mol subunit was prepared by phosphorylation with smooth muscle myosin light chain kinase [4]. The dephosphorylation of myosin P-light chain (1 μM) was carried out as in [4] and one unit of activity (U) was that amount which catalysed die release of 1 μmole of phosphate in one min. When assaying PP1 M in immunoprecipitates from the myofibrillar extracts, the tubes were shaken continuously and 3 nM okadaic acid was included to inhibit any PP2A present. Immunoprecipitation of PPl M from myofibrillar extracts. Antibodies raised against the PPl M holoenzyme (1 μg), which recognise both the M 110 and M 21 subunits, but not PPlc, affinity purified antibodies specific for either die M 110 subunit or M 21 subunit (5 μg) [7], and control IgG (5 μg) were conjugated separately to 10 μl of pelleted protein G-Sepharose. After incubation for 30 min at 4°C , the Protein G-Sepharose-antibody conjugate was washed three times with 50 mM Tris/HCl pH 7.5, 0.1 mM EGTA, 0.03% (by mass) Brij-35, 0.3M NaCl, 0.1 % (by vol) 2-mercaptoethanol before addition of a 100 μl of myofibrillar extract (prepared as in [4]) which had been diluted 10-fold in 50 mM Tris/HCl pH 7.5, 0.1 mM EGTA, 0.1 % (by vol) 2- mercaptoethanol, 0.2 mM phenylmethylsulphonyl fluoride, 1 mM benzamidine, 10 μg/ml leupeptin containing 1 mg/ml bovine serum albumin. After incubation for 1 h at 4°C, with shaking, a 10 μl aliquot of the suspension was removed to measure the total activity. The remaining 90 μl was centrifuged for 1 min at 13,000 x g, the supernatant was removed, and the pellet washed twice in dilution buffer containing 0.2 M NaCl and 0.03% (by mass) Brij-35 (but no bovine serum albumin), once in dilution buffer and then resuspended in 90 μl of dilution buffer. Myosin P-light chain phosphatase activity was then measured in the supernatant and the resuspended pellet at a further 30-fold final dilution. Myosin binding assays. Myosin (0.5 mg / ml, 1 μM in terms of myosin heavy chains) in 10 mM Hepes pH 7.5, 50 mM KC1, 5 mM MgC12, 0.1 % (by vol) 2-mercaptoethanol, was mixed wim PP1 M , M 21 subunit, or fragments of the M n0 and M 2 ι subunits, at the concentrations indicated in figure legends. After incubation for 15 min at 0°C, the solutions were centrifuged for 2 min at 13,000 x g, the supernatants removed, and the pellets washed twice in 10 mM Hepes pH 7.5, 50 mM KC1, 5 mM MgC12, 0.1 % (by vol) 2-mercaptoethanol before resuspension in 50 mM Tris-HCl pH 7.5, 0.1 mM EGTA, 0.03% (by mass) Brij 35, 0.6 M NaCl, 0.1% (by vol) 2-mercaptoethanol. Aliquots of the supernatant, the resuspended pellet and die suspension before centrifugation were either assayed for myosin P-light chain phosphatase activity or denatured in SDS and analysed by SDS/polyacrylamide gel electrophoresis. Preparation of a complex between GST-M 21 and M m -(R714-11004). GST-M 2 , (10 μg) was mixed widi 80 μl of bacterial extract expressing M uo - (R714-I1004). After incubation for 15 min at ambient temperature the solution was added to 20 μl (packed volume) of glutathione-Sepharose equilibrated in 50 mM Tris HCl pH 7.5, 0.1 mM EGTA, 0.03% (by mass) Brij 35, 0.1 % (by vol) 2-mercaptoethanol, 0.2 mM phenylmethylsulphonyl fluoride, 1 mM benzamidine and 0.15 M NaCl. After incubation for 30 min at 4°C widi shaking, the Sepharose was washed three times in the same buffer before eluting the complex with buffer containing 20 mM glutathione pH 8.0. Other procedures. Proteins were labelled witii digoxigenin and Far Western analyses carried out as described [4], except that die digoxigenin-labelled probe was used at a concentration of 0.2 μg/ml instead of 2 μg/ml. SDS/polyacrylamide gel electrophoresis was carried out on 7.5-15% gels according to Laemmli [13] and on 16.5% gels according to Schagger and von Jagow [14]. Protein was estimated according to Bradford [15]. Results PP1 M accounts for nearly all the myosin phosphatase activity in extracts prepared from chicken gizzard myofibrils. 80-90% of the myosin phosphatase activity present in chicken gizzard homogenates is recovered in die myofibrils [4]. In the present study, we used antibodies diat recognise the M 110 and/or die M 21 subunits of chicken gizzard PPI M [7] to immunoprecipitate the myosin P-light chain phosphatase activity from the myofibrillar extracts. About 90% of the activity was immunoprecipitated by antibodies raised against me PP1 M holoenzyme (Fig 18A) which recognise both the M 110 and M 2l subunits in immunoblotting experiments, but not PPlc. Similarly, about 80% of the myosin P-light chain phosphatase activity in the myofibrillar extracts was immunoprecipitated by either the anti-M, 10 antibody or by the anti-M 21 antibody (Fig 18A). Thus, most of the myosin P-light chain phosphatase activity in myofibrillar extracts is catalysed by PPlc present as a complex containing both die M 110 and die M 21 subunits. Immunoblotting experiments demonstrated that the ratio M πo :M 21 in myofibrillar extracts was identical to die ratio of these subunits in purified PP1 M (Fig 18B), which is 1: 1 [4]. These immunoblotting experiments also demonstrated diat PP1 M comprises 0.1 % of the protein in die myofibrillar extract (see legend to Fig 18B), identical to the proportion estimated from the fold-purification needed to obtain pure PP1 M from this fraction (see Table 1 in Ref 4). These experiments imply that PP1 M accounts for virtually all the myosin phosphatase activity associated widi myofibrils, and that neitiier the M u0 nor the M 21 subunit is present in a significant molar excess over PPlc in the myofibrils. Identification of a region on the M m subunit that binds to the M 21 subunit. PP1 M and several fragments of die M 1I0 subunit, were subjected to SDS/polyacrylamide gel electrophoresis (Fig 19A) and, after transfer to nitrocellulose, the blots were probed with digoxigenin-labelled M 21 subunit (Fig 19B). These experiments showed diat die M 21 subunit recognised die full length Muo subunit (Fig 19B, track 1), M n0 -(R714-I1004) (Fig 19B, tracks 2 and 3) and M 110 -(K933-I1004) (Fig 19B, track 5), but not M no -(R714-L934) (Fig 19B, track 4), M 110 -(M1-E309) (Fig 19B, track 7) or M 110 -(M1-S477) (Fig 19B, track 8). Thus, the M 2 , subunit binds to die C-terminal 72 residues of the M m subunit. The specificity of this interaction was indicated by die observation that digoxigenin-labelled M 21 subunit recognised only Mn 0 -(R714- 11004) and no other protein in the E. coli extract (track 2 in Figs 19A and 19B), nor did it recognise the MBP or GST tags, PPlc (Figs 19A and 19B) or any of the molecular mass markers (data not shown). Consistent with the results in Fig 19, digoxigenin-labelled MBP-M 110 -(R714- 11004) (data not shown) and MBP-M 110 -(K933-I1004) (Fig 20B), but not digoxigenin-labelled MBP-Mn 0 -(R714-L934) (data not shown), recognised the full length M 21 subunit and M 21 (M1-L146) in Far Western blotting experiments. The region of the M 21 subunit that interacts with the M n0 subunit. Digoxigenin-labelled M 21 -(M1-L146) recognised die same fragments of die M 110 subunit as the full lengtii M 2 , protein (Fig 19C), demonstrating that the C- terminal leucine zipper of the M 21 subumt is not required for interaction with die M n0 subunit. However, neitiier digoxigenin-labelled GST-M 21 -(M1-E110) nor digoxigenin-labelled GST-M 21 -(E110-K186) recognised M 110 -(K933-I1004) in Far Western blotting experiments (data not shown). Consistent with these findings, digoxigenin-labelled Mι 10 -(K933-I1004) recognised the full length M 21 protein and M 21 -(M1-L146), but not M 21 -(M1-E110) or M 2 ,-(E110-K186) in Far Western blotting experiments (Figs 20 A and B). However, digoxigenin- labelled M 110 -(K933-I1004) did recognise a proteolytic fragment of the M 21 subunit with an apparent molecular mass only slightly larger than M 21 -(M1- El 10) (Fig 20B, track 2 and compare tracks 2 and 4 in Fig 20A). These results are considered further under Discussion. The isolated M 21 subunit dimerizes and the region involved in dimerization is identical to that which interacts with the M no subunit. Although the M no subunit binds to botii PPlc and the M 21 subunit [4], and removal of the M 2I subunit does not alter die specificity of the PPl M complex [7], an interaction between the M 21 subunit and PPlc had not been excluded. In order to examine this point, PPlc and die M 2I subunit were mixed togedier and subjected to gel filtration on Superose 12. The M 21 subunit eluted just before the 37 kDa PPlc protein, demonstrating diat tiiey do not form a high affinity complex and suggesting that me isolated M 21 subunit dimerizes (data not shown). These results were supported by die finding that digoxigenin-labelled full length M 21 subunit recognised the M 21 subunit as well as the M u0 subumt, but not PPlc, in Far Western blotting experiments (Fig 21, track 1). Similar results were obtained widi M 2r (Ml-L146) (Fig 21, track 2). Digoxigenin- labelled M 2j subunit, like digoxigenin-labelled M 110 -(K933-I1004), recognised a fragment of the M 21 subunit that migrated slightly more slowly than M 21 -(M1- E110), but did not recognise M 21 -(M1-E110) or M 21 -(E110-K186) (Tracks 2, 4 and 5 in Figs 20B and 20C). Identification of a region on the M m subunit that binds to myosin. When PP1 M (30 nM) was mixed widi chicken gizzard myosin (1 μM) and centrifuged to pellet die myosin, 85% of the myosin P-light chain phosphatase was recovered in the pellet (Figs 22 and 23 A). In contrast, neither PPlc (Fig 22) nor bovine serum albumin (data not shown) bound to myosin under tiiese conditions. After removal of the M 21 subumt from PP1 M [7], the M 110 -PPlc complex (PP1 M (ΔM 21 ) still pelleted widi myosin in a similar manner to PP1 M itself (Fig 22), indicating diat die M n0 subunit is a myosin-binding protein. In order to identify the myosin-binding domain(s), several fragments of the M uo subunit were expressed and purified from E. coli extracts and their binding to myosin was studied. GST-M U0 -(M1-S477), like GST-M 110 -(M1- E309) [7], stimulated die PPlc-catalysed dephosphorylation of the myosin P- light chain and inhibited the dephosphorylation of glycogen phosphorylase in a similar manner to the full length M u0 subunit (data not shown). However, neither GST-Mi 10 -(M1-S477) nor GST-M 110 -(M1-E309) bound to myosin (data not shown), even after removal of the GST-tag from GST-M U0 -(M1-S477) (Fig 23A). A fragment comprising GST-M 110 -(M377-K976) from rat aorta migrated as multiple bands on SDS/polyacrylamide gels after purification on glutathione- Sepharose (Fig 23 A), indicating cleavage at multiple sites within the M no subunit. Only die largest fragment, with an apparent molecular mass corresponding to undegraded GST-M 110 -(M377-K976) bound to myosin (Fig 23 A), suggesting mat the myosin binding site(s) was located towards the C- terminus of the M 110 subunit. Consistent with this finding, M 110 -(R714-11004) from chicken gizzard also bound to myosin (Fig 23B). However, M 110 -(K933- 11004), which bound to die M 21 subunit (Fig 20B), did not bind to myosin in these experiments (Fig 23B). The M 21 subunit, and a complex between M 21 and M no -(R714-I1004) bind to myosin. After purification on glutathione-Sepharose, GST-M 2 , migrated as four protein staining bands (track 1 in Fig 20 A), the two species of highest apparent molecular mass being recognised by the anti-M 21 antibody (Fig 23B). The apparent molecular mass of the slowest migrating band (47 kDa) corresponds to undegraded GST-M 21 and this species bound to myosin (Fig 23B). The next most slowly migrating band had an apparent molecular mass of 38 kDa, slightly less than that of GST-M 21 -(M1-E110) (data not shown) indicating diat it corresponds to GST fused to about the first 100 residues of die M 2I subunit; this fragment hardly bound to myosin (Fig 23B). Bacterial extracts expressing M 110 -(R714-I1004) were mixed widi GST-M 21 and die resulting complex was purified on glutathione-Sepharose. This complex bound quantitatively to myosin (Fig 23B). In contrast, the GST-M 2I fragment of apparent molecular mass 38 kDa was not complexed to M 110 -(R714-I1004) and did not bind to myosin (Fig 23B). The C-terminal fragment of the M 21 subunit, M 21 -(E110-K186) also did not bind to myosin under these conditions (data not shown). Multiple binding sites for the M 2] subunit on the myosin molecule. The molar ratio myosin:PPl M in chicken gizzard is about 80: 1 in vivo [4] and die myosin binding experiments described above were dierefore carried out using a large (ten fold) molar excess of myosin over either die M 2] or the M 110 subunit. However, further experiments carried out with die M 21 subunit in excess revealed that, remarkably, 20 or more moles of M 2! subunit could be bound to each myosin dimer (Fig 24 A). Many of me binding sites were located in the region of myosin involved in filament formation, because the M 21 subunit was pelleted with the myosin "rod" domain even when the molar ratio M 21 : myosin dimer was 10:1 (Fig 24B). A shorter portion of the rod, termed light meromyosin, also bound the M 21 subunit avidly. However, a fragment of the M 21 subunit lacking the first 15 residues from the N-terminus, which was a contaminant in this preparation, did not bind to light meromyosin (Fig 24B), although it bound to die longer myosin rod (Fig 24B). The M 2! subunit lacking the C-terminal leucine zipper, M 21 -(M1-L146), bound to both myosin and the rod domain, but fewer moles of M 21 -(M1-L146) could be bound and tiiis C- terminally truncated species did not bind to light meromyosin under the conditions studied (Fig 24C). Multiple forms of the M m subunit Comparison of two different clones encoding the M 110 subunit from chicken gizzard revealed a 123 bp (41 amino acid) deletion/insertion after Asn-511 (Fig 17, [8]). Since the rat aorta sequence [5] showed considerable variation from the chicken sequences in tiiis region, compared to the high degree of sequence similarity throughout most of die rest of the molecule (Fig 17), it seemed probable that forms of the rat M π0 subumt also existed tiiat varied in this middle section of die protein. PCR of the "variable region" of several rat aorta clones gave fragments of either 608 bp or 776 bp. Direct sequencing of these fragments showed an in frame insertion of 168bp (56 amino acids) after Ser- 552 (Fig. 1); i.e. a slightly different position from the deletion reported for the chicken gizzard M no subunit (Fig 17). Furthermore, a different 62 amino acid deletion/insertion in this section is apparent by comparison of the rat aorta sequences with that of the M π0 protein from rat kidney (Fig. 1) [9]. While it is likely that most of these variations arise by alternative splicing of the mRNA, Soutiiern blotting of rat genomic DNA revealed die presence of two closely related genes (data not shown). Discussion The contraction of smooth muscle is triggered by phosphorylation of die P-light chain of myosin catalysed by myosin light chain kinase. However, the identity of the myosin P-light chain phosphatase remained unclear for many years. In 1992 we reported diat 80-90% of die myosin phosphatase activity in chicken gizzard homogenates was associated widi myofibrils and purified a myosin phosphatase to homogeneity from this fraction [4]. This enzyme, termed PP1 M , was found to be composed of the /3-isoform of PPlc (termed die δ-isoform in [16]) and an "M-complex" consisting of two other subunits [4] whose molecular masses were 21 kDa (M 21 ) [5] and 110 kDa (M 110 ) [5, 8], respectively. Further evidence diat a form of PPl was the major myosin phosphatase in smooth muscle was indicated by the finding that tautomycin (a much more potent inhibitor of PPl than PP2A [17]) stimulated die contraction of permeabilised mammalian smooth muscle fibres at much lower concentrations than okadaic acid [18] (a much more potent inhibitor of PP2A than PPl [19]). Two further pieces of evidence presented in this Example establish that PP1 M accounts for most, if not all, of the myosin phosphatase activity associated widi chicken gizzard myofibrils, reinforcing the view diat it is likely to be the major myosin P-light chain phosphatase in vivo. Firstly, nearly all the myosin P-light chain phosphatase activity was immunoprecipitated by antibodies tiiat recognise either the M 110 or the M 21 subunit specifically (Fig 18A). Secondly, PP1 M was found to represent 0.1 % of the protein in die myofibrillar extracts whetiier its concentration was calculated from the increase in specific activity needed for purification to homogeneity [4] or from immunoblotting experiments widi the anti-M 110 and anti-M 21 antibodies (Fig 18B). Had anotiier enzyme been die major myosin phosphatase in the myofibrillar extracts the enrichment estimated by immunoblotting with anti-M n0 and anti-M 21 antibodies would have been much higher. The experiments presented in Fig 18 also demonstrate diat die M uo and M 21 subunits are not present in myofibrillar extracts in a significant molar excess over PPlc and tiiat all the M n0 subunits are complexed to M 2 , subunit and vice versa. The M 2 ι subunit was found to bind to the C-terminal 72 residues of the chicken gizzard M ao subunit (Figs 19 and 25), a region whose amino acid sequence is 43% identical to residues 87-161 of the M 21 subunit (Fig 17B). The C-terminal leucine zipper of die M 21 subunit (residues 145-186) is not required for interaction with ti e M 110 subunit, and die site on die M 21 subumt which interacts with the M n0 subunit lies within about the N-terminal 120 residues (Fig 20B). Interestingly, the same region is essential for the dimerisation of the M 2I subunit (compare Figs 20B and 20C), suggesting diat the region(s) involved in interaction is probably located between residues 60 and 120 of the M 21 subunit and 906-965 of die M u0 subunit from chicken gizzard; i.e. die regions with greatest amino acid identity between these two proteins (Fig 17). More digoxigenin-labelled M 21 subunit bound to die M„ 0 subunit tiian to the M 2l subunit in Far Western blotting experiments (Fig 21), consistent with die observation that M 110 /M 21 heterodimers form in vivo, but not M 21 /M 21 homodimers. The finding tiiat the C-terminus of the M uo subunit interacts with the M 21 subunit explains why preparations of PP1 M comprising PPlc complexed to N-terminal fragments of the M no subunit do not contain the M 21 subunit [8, 20]. PP1 M binds to ti e dephosphorylated form of myosin and our data demonstrate that die M no subunit (Fig 22) and die M 21 subunit (Fig 23B and Fig 24) are both myosin-binding proteins. The C-terminal 600 residues of die M uo subunit from rat aorta, M U0 -(M377-K976) (Fig 23A) and the C-terminal 291 residues of the M n0 subunit from chicken gizzard, M,, 0 -(R714-I1004), bound to myosin, but the C-terminal 72 residues of die M„ 0 subunit, M U0 -(K933-1004), did not (Fig 23B), indicating diat a myosin-binding domain is likely to be situated in die Mn o subunit just N-terminal to die M 21 -binding domain (Fig 25). In contrast, two N-terminal fragments of the M π0 subunit Mι, 0 -(M1-S477) (Fig 23A) and M 110 -(M1-E309) (data not shown) did not bind to myosin under the conditions studied. Since M UO -(M1-E309) [7] and M, 10 -(M1-S477) (data not shown) stimulate the dephosphorylation of myosin and inhibit the dephosphorylation of glycogen phosphorylase by PPlc, and in a similar manner to full length M H0 subunit, these results show that the region of the M no subunit which stimulates the dephosphorylation of the myosin P-light chain is distinct from that which binds die dephosphorylated form of myosin and mereby targets PP1 M to the contractile apparatus. Digestion of chicken gizzard PP1 M with chymotrypsin cleaves the M 110 subunit to a fragment with an apparent molecular mass of 58 kDa and a form of PPl , termed here PP1 M *» can then be isolated by gel-filtration which appears to comprise just the 58 kDa fragment and PPlc in a 1 : 1 molar ratio [8] . The 58 kDa fragment, like the M 110 subunit, has a blocked N-terminus and seven tryptic peptides isolated were located between residues 286 and 467, suggesting that the 58 kDa fragment corresponds to die N-terminal portion of the M 110 subunit [8]. PP1 M * was reported to bind to myosin, albeit less effectively than PP1 M [8], suggesting the presence of a myosin-binding domain within the 58 kDa fragment. This result is in apparent conflict with the present s dy, because the fragment M 110 -(M1-S477), which also migrates on SDS/polyacrylamide gels with an apparent molecular mass of 58 kDa, did not bind to dephosphorylated myosin under conditions where 80-90% of the PPl M and M 110 -(R714-11004) was pelleted with myosin (Fig 23 A). One possible explanation for this discrepancy is that PP1 M * also contains small myosin- binding fragments from the C-terminus of the M 1 !0 subunit which still interact with the N-terminal 58 kDa fragment, but are too small to be detected by SDS/polyacrylamide gel electrophoresis. In a separate smdy heavy meromyosin (50 μg) was found to bind partially to 2 mg of M 110 -(l-633) coupled to Affigel 15, at very low ionic strength but not at 150-200 mM NaCl [21]. The significance of this observation is unclear because of the extremely high concentration of the M u0 -( 1-633) used in these experiments. The average intracellular concentration of PPl M in chicken gizzard is about 1 μM, 100-fold lower than the concentration of myosin. In the present study, we analysed the binding of die M uo subunit and its subfragments (30-100 nM) to myosin (1 μM) using low concentrations of these proteins to try and ensure that only high affinity binding sites were identified. The isolated M 21 subunit also bound to myosin and up to 20 moles of M 21 subunit could be bound to each myosin dimer (Fig 24). These observations indicate tiiat each myosin molecule contains multiple binding sites for the M 21 subunit, many of which are located widiin the "rod domain" (Figs 24B and 24C). In vivo, the molar ratio PP1 M : myosin is about 1:80 and yet, during muscle relaxation, all die myosin P-light chains can be dephosphorylated by PP1 M within seconds. This implies that PP1 M must be highly mobile within the myofibrils and move extremely rapidly from one myosin molecule to another. The "off rates" for binding of PP1 M to myosin must therefore be very fast as well as the "on rates". It is tempting to speculate that the presence of multiple binding sites on myosin for the M 21 subunit (and perhaps for the M 110 subunit as well) allows PP1 M to "slide" rapidly from one myosin molecule to another. Example 4: Design of small molecules to modulate the properties of PPl Table A is a print-out of me atomic coordinates of die protein phosphatase- 1 peptide coordinates as deduced in Example 2. The format is Protein Data Bank. The structure of the protein phosphatase- 1 catalytic subunit (PPlc) in complex with a 13-residue peptide (G M peptide) corresponding to a site of interaction between PPlc and die glycogen targeting subunit provides a frame- work for the rational design of small molecules to modulate die functions and properties of PPl in vivo. Knowledge of die structural nature of the interactions between the G M peptide and PPlc allows the design of inhibitors that mimic these interactions. These inhibitors may be designed for increased potency, cell permeability and with improved pharmokinetic properties. Computer graphics systems may be used to design such inhibitors in conjunction with computer graphics modelling software such as SYBIL available from: Tripos Inc, 16995 S Hanley Road, St Louis, Missouri 63144- 2913, USA and LUDI available from: Molecular Simulations Inc, 9685 Scranton Road, San Diego, CA 92121-3752, USA, and in conjunction with the atomic coordinates shown in Table A. Example 5: Effect of peptide derived from p53BP2 binding site to PPl in vivo The function of p53BP2 is ascertained by examining die in vivo effect of peptides based on die sequence of die p53BP2 binding site to PPl . This may be done by reference to the consensus peptide sequence described in the previous Examples and by reference to the crystal structure in Example 2. The peptide is introduced into cultured cells using penetratin available from Appligene. Other importins may also be used. Alternatively cDNA specifying p53BP2 proteins mutant in the p53BP2 binding site to PPl are transfected in cultured cells. The effect of these agents on the cell cycle and apoptosis are assessed by a number of methods, for example WAFl ELISA and Nuclear Matrix Protein ELISA assays (Amersham). The effect of the p53BP2 peptide is to modulate the interaction between PPl and p53BP2 in vivo and affect cell regulation and apoptosis. The p53BP2 peptide may also be micro-injected into the cell. 0.819 54.783 91.585 1.00 79.69 55.525 110.502 1.00 2.00 0 55.818 110 .539 1.00 12.38 0 56.182 112 .881 1.00 8.25 0 57.046 113 .782 1.00 8.25 0 57.949 113 .337 1.00 8.25 0 57.109 115 .133 1.00 8.25 0 58.043 115 .526 1.00 8.25 0 58.516 114 .399 1.00 8.25 0 55.195 109. .385 1 00 11.52 0 55.174 108. .078 1 00 11.52 0 54.266 107. .868 00 11.52 0 54.462 106. .915 00 81.94 0 53.272 108. .730 00 18.96 0 52.364 108. .602 1.00 20.32 0 52.139 109. .957 1.00 6.39 0 53.407 110. .711 1.00 4.06 0 53.239 111. .757 1.00 7.01 0 54.197 112. .111 1.00 8.21 0 52.020 112. .270 1.00 5.84 0 51.039 108. .010 1.00 18.72 0 49.975 108. .634 1.00 10.73 0 51.119 106. 777 1.00 5.70 0 49.970 106. 054 1.00 5.52 0 50.313 104. 587 1.00 2.00 0 49.322 103. 841 1.00 2.00 0 49.099 104. 187 1.00 2.00 0 48.194 103. 477 1.00 2.00 0 48.611 102. 762 1.00 2.00 0 47.701 102. 038 1.00 2.00 0 47.506 102. 408 .00 2.00 0 46.632 101. 715 .00 2.00 0 48.730 106. 188 .00 8.52 0 47.641 106. 393 .00 2.00 0 48.877 106. 093 .00 2.00 0 47.703 106. 186 .00 2.00 0 48.031 105. 741 1.00 64.80 0 47.964 104. 231 1.00 70.91 0 48.993 103. 431 1.00 73.06 0 48.924 102. 041 1.00 68.38 0 46.857 103. 597 1.00 73.86 0 46.780 102. 200 1.00 70.94 0 47.820 101. 433 .00 72.52 0 47.779 100. 062 .00 70.35 0 47.007 107. 538 00 2.00 0 45.771 107. 610 00 64.08 0 47.789 108. 608 00 .87 0 47.203 109. 927 00 .73 0 48.242 110. 998 00 14,.88 0 48.600 111. 054 00 25.87 0 49.778 111. 293 00 24.58 0 47.714 110. 861 1.00 21.52 0 46.622 110. 051 .00 6.73 0 45.697 110. 826 .00 14.14 0 47.148 109. 250 00 2.00 0 46.654 109. 208 00 00 0 47.613 108. 456 00 00 0 46.946 108. 067 00 00 0 46.562 109. 342 00 00 0 47.867 107. 208 00 00 0 45.320 108. 476 00 00 0 44.401 108. 814 00 00 0 45.231 107. 449 00 44.35 0 44.005 106. 693 00 42.59 0 44.218 105. 460 00 2. 00 0 44.759 104. 160 00 2.00 0 44.728 103. 101 00 2 00 0 43.920 103. 725 00 2.00 0 42.978 107.629 1.00 42.42 0 11.059 41.890 107.817 1.00 2.00 0 12.729 43.354 108 .241 1.00 2.00 0 13.462 42.494 109 .179 1.00 2.00 0 14.591 43.285 109 .840 1.00 31.22 0 15.809 43.478 108 .958 1.00 31.18 0 16.944 44.146 109 .710 1.00 37.63 0 17.250 43.459 110 .960 1.00 37.44 0 17.210 44.041 112 .157 1.00 41.10 0 16.888 45.327 112 .265 1.00 39.99 0 17.476 43.339 113 .254 1.00 39.22 0 12.569 41.896 110 .259 1.00 2.00 0 12.621 40.703 110 .544 1.00 29.18 0 11.747 42.742 110 .853 1.00 2.00 0 10.818 42.331 111. .901 1.00 2.00 0 10.069 43.580 112. .407 1.00 3.53 0 8.704 43.654 113. .091 1.00 3.53 0 8.554 45.074 113. .606 1.00 3.53 0 7.561 43.337 112. .138 1.00 3.53 0 9.850 41.279 111. .384 1.00 2.00 0 9.594 40.284 112. .043 1.00 3.53 0 9.325 41.514 110. .190 1.00 2.00 0 8.382 40.604 109. .573 1. 00 2.00 0 7.883 41.173 108. .255 1.00 2.00 0 6.626 41.966 108. .382 1.00 2.00 0 6.447 43.124 107. .642 1, 00 2.00 0 5.616 41.550 109. .240 1.00 2.00 0 5.277 43.852 107. .753 1.00 2.00 0 4.447 42.270 109. .358 1.00 2.00 0 4.272 43.423 108. .615 1.00 2.00 0 9.016 39.271 109. .322 1.00 2.00 0 8.339 38.260 109. .363 1.00 2.00 0 10.316 39.283 109. .043 1.00 2.00 0 11.080 38.065 108. .789 1.00 2.00 0 12.436 38.409 108. .191 1.00 84.85 0 12.331 39.117 106. .861 1.00 89.26 0 13.651 39.690 106. .391 1.00 98.60 0 13.627 40.493 105. .434 1.00 98.04 0 14.709 39.345 106. 973 1.00 .97 0 11.262 37.316 110. 100 00 .00 0 11.469 36.103 110. 099 00 80 .75 0 11.182- 38.045 111. 216 00 15 .29 0 11.325 37.449 112. 538 00 15 .29 0 11.945 38.455 113. 514 00 67 . 13 0 12.459 37.828 114. 796 1.00 69 . 14 0 11.822 36.717 115. 368 1.00 76 . 95 0 12.278 36.142 116. 548 1.00 77 . 96 0 13.577 38.346 115. 446 00 75 .81 0 14.044 37.775 116.b d 00 77 .77 0 13.387 36.673 117. 181 00 77 . 18 0 13.826 36.115 118. 365 00 87 . 14 0 9.967 36.964 113. 053 00 15 . 29 0 9.811 35.796 113. 403 00 62 . 78 0 8.992 37.863 113. 117 00 2. 00 0 7.675 37.485 113. 582 00 2. 00 0 6. .977 36.532 112. 629 00 2.00 0 6 .226 35.649 113. 052 00 34.11 0 7.237 36.699 111. 338 1.00 49.23 0 6.601 35.870 110. 330 00 48.34 0 5.699 36.782 109. 520 00 47.91 0 4.955 37.572 110. 097 00 15.83 0 5.759 36.692 108. 193 00 00 0 4. .948 37.558 107. 343 00 00 0 5 .386 37.403 105. 880 00 00 0 6 .673 38.134 105. 556 00 00 0 7, .899 37.506 105. 685 00 00 0 6. .652 39.467 105. 139 1.00 00 0 9.082 38.193 105. 405 1.00 00 0 7.829 40.154 104. 859 1.00 2.00 0 107 62.313 107.718 1.00 2.00 0 65.914 108 .374 1.00 28.24 0 65.854 107 .951 00 2.00 0 66.971 108 .255 00 2.00 0 68.234 107 .627 00 2.00 0 69.269 108 .006 0 70.673 107 .477 0 70.734 105 .962 0 70.533 105 .434 0 71.479 104. .852 0 72.697 104. .711 0 71.211 104. .424 0 68.731 107. .990 0 69.477 107. .225 0 68.324 109. .127 0 68.801 109. .457 0 69.644 110. .746 0 71.055 110. .441 0 68.999 111. .800 0 69.792 113. .065 0 67.779 109. .553 0 68.073 109. .127 0 66.583 110. .080 0 65.567 110. 270 0 64.766 111. 561 0 65.648 112. 767 0 65.789 113. 486 0 66.651 114. 557 0 66.392 113. 160 0 67.254 114. 228 0 67.384 114. 925 0 68.259 115. 984 0 64.606 109. 139 0 63.404 109. 368 0 65.127 107. 922 0 64.275 106. 787 0 63.772 105. 882 0 64.127 104. 713 0 62.963 106. 393 0 62.412 105. 549 0 61.757 106. 347 1.00 2.00 0 60.831 105. 524 1.00 2.00 0 59.734 104. 870 1.00 2.00 0 61.036 105. 424 1.00 2.00 0 58.846 104. 131 1.00 2.00 0 60.155 104. 688 1.00 2.00 0 59.050 104. 037 1.00 2.00 0 63.358 104. 563 1.00 2.00 0 62.926 103. 445 1.00 2.00 0 64.616 104. 940 1.00 2.00 0 65.530 103. 977 1.00 00 0 66.859 104. 598 1.00 00 0 67.921 103. 564 1.00 00 0 68.203 103. 257 1.00 00 0 69.196 102 ' . 327 1.00 00 0 68.666 102. 901 1 00 00 0 69.644 101. 985 1 00 00 0 69.903 101. 707 1 00 00 0 70.893 100. 823 1 00 00 0 65.785 102. 822 1 00 00 0 65.669 101. 646 1 00 00 0 66.153 103. 155 1 00 00 0 66.439 102. 147 1 00 00 0 66.971 102. 810 1.00 28.59 0 68.173 103. 705 1.00 31.85 0 69.327 103. 255 1.00 34.04 0 67.963 104. 867 1.00 31.26 0 65.203 101. 308 1.00 2.00 0 -7.580 52.409 122.120 1.00 .00 0 -7.567 50.414 120 .637 1 .00 16.67 0 -8.494 50.203 119 .861 1 .00 .00 0 -6.984 49.455 121 .356 1.00 .00 0 -7.379 48.052 121 .267 1.00 .00 0 -6.559 47.374 120 .197 1 .00 14.80 0 -7.232 47.283 122. .604 1 .00 .00 0 -6.373 47.620 123. .425 1.00 .24 0 -8.069 46.263 122. .813 1.00 .09 0 -8.036 45.424 124. .018 1.00 .09 0 -9.323 45.590 124. .860 1 .00 10.56 0 -9.200 44.830 126. .150 1.00 10.56 0 -9.566 47.055 125. .187 1.00 10.56 0 -10.886 47.285 125. .837 1.00 10.56 0 -7.910 43.938 123. .623 1.00 8.09 0 -8.866 43.328 123. .127 1.00 10.56 0 -6.739 43.353 123. .856 1.00 20.53 0 -6.510 41.958 123. .503 1.00 21.83 0 -5.041 41.664 123. .243 1.00 2.00 0 -4.905 40.241 122. .717 1.00 2.00 0 -4.460 42.686 122. .273 1.00 2.00 0 -6.973 40.987 124. .570 1.00 22..04 0 -6.546 41.064 125. .728 1.00 2..00 0 -7.841 40.066 124. .163 1.00 10.50 0 -8.401 39.057 125. 046 1.00 16.73 0 -7.348 37.980 125. 337 1.00 24.26 0 -7.245 36.914 124. 212 1.00 24.02 0 -8.258 36.207 123. 944 1.00 22.19 0 -6.145 36.776 123. 611 1.00 26.91 0 -8.963 39.674 126. 326 1.00 15.15 0 -9.139 39.006 127. 336 1.00 12.89 0 -9.262 40.967 126. 231 1.00 2.00 0 -9.833 41.811 127. 289 1.00 2.00 0 -11.280 41.394 127. 555 1.00 83.32 0 -12.129 41.397 126. 273 1.00 2.00 0 -11.819 42.603 125. 305 1.00 00 0 -11.133 42.398 124. 242 1.00 00 0 -12.268 43.750 125. 622 1.00 00 0 -9.056 41.981 128. 585 1.00 00 0 -9.634 42.138 129. 657 1 .00 74.79 0 -7.733 41.984 128. 460 1.00 22.77 0 -6.829 42.154 129. 589 1.00 14.49 0 -6.098 40.844 129. 910 1.00 21.81 0 -6.956 39.798 130. 600 1.00 17.49 0 -7.677 40.404 131. 802 1.00 18.27 0 -8.654 39.411 132. 466 1.00 26.28 0 -9.658 40.080 133. 373 00 29.04 0 -5.811 43.225 129. 237 00 11 04 0 -5.665 44.214 129. 947 00 17 06 0 -5.109 43.015 128. 131 00 2.00 0 -4..093 43.950 127. 663 00 2.00 0 -3..151 43.267 126. 648 00 2.00 0 -1.989 44.184 126. 302 00 2.00 0 -2.632 41.952 127. 215 00 2.00 0 -1.691 41.218 126. 297 00 2.00 0 -4.731 45.144 126. 964 00 2.00 0 -5.393 44.973 125. 938 00 2.00 0 -4.572 46.337 127. 524 00 17.81 0 -5 .110 47.525 126. 877 00 19.78 0 -5 ,486 48.610 127. 885 00 2.00 0 -5.895 49.920 127. 252 00 2.00 0 -7.095 50.028 126. 538 00 2.00 0 -5.092 51.056 127. 390 00 2.00 0 -7.493 51.248 125. 976 00 2.00 0 -5.473 52.282 126. 836 00 2.00 0 -6.680 52.377 126. 127 00 2.00 0 -3.943 47.992 126. 038 1.00 23.45 0 -2.809 47.983 126. 518 1.00 2.00 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 1.532 64.103 121.420 1.105 64.535 120.148 2.854 64.355 121.813 -0.616 63.541 121.162 -0.212 64.178 120.017 1.950 65.207 119.263 3.697 65.024 120.932 3.238 65.443 119.669 1.246 61.330 124.955 2.419 61.639 125.194 0.418 60.867 125.886 0.830 60.690 127.267 -0.363 60.273 128.121 -0.482 61.120 129.256 1.951 59.699 127.446 2.075 58.743 126.681 2.777 59.943 128.462 3.899 59.056 128.764 5.257 59.714 128.505 5.296 60.532 127.256 5.695 61.706 127.362 4.959 60.008 126.180 3.903 58.705 130.231 3.580 59.537 131.072 4.244 57.457 130.559 4.509 56.316 129.673 4.299 57.040 131.957 4.407 55.522 131.845 5.197 55.356 130.604 5.584 57.675 132.507 6.448 58.078 131.712 5.724 57.774 133.833 6.931 58.360 134.413 6.755 59.863 134.614 8.050 60.570 134.990 9.132 59.956 134.874 7.989 61.756 135.396 7.304 57.734 135.729 6.448 57.320 136.496 8.608. 57.666 135.966 9.170 57.136 137.211 10.647 56.760 137.024 10.941 55.441 136.345 12.451 55.184 136.403 12.852 53.831 135.809 14.336 53.616 135.852 9.100 58.157 138.367 8.746 57.803 139.481 9.456 59.414 138.093 9.468 60.483 139.101 10.322 61.669 138.624 11.483 61.254 137.754 11.823 62.030 136.834 12.051 60.166 137.987 8.074 61.033 139.412 7.943 62.164 139.903 7.040 60.250 139.124 669 60.690 139.340 137 61.376 138.050 652 61.247 137.922 505 62.826 138.076 767 59.526 139.763 5.007 58.356 139.390 3.748 59.844 140.564 815 58.830 141.022 742 58.809 142.543 371 57.438 143.097 507 56.477 142.816 2.109 57.530 144.588 1.00 17.44 52.724 135.376 51.413 136.992 50.787 136.454 51.292 138.275 50.395 139.173 50.487 140.542 50.681 139.245 49.763 139.166 51.954 139.397 52.355 139.477 53.843 139.803 54.160 141.205 53.499 142.257 53.151 143.543 SI.892 143.444 52.036 138.183 51.319 138.206 52.532 137.061 52.319 135.733 52.880 134.640 52.452 133.249 53.199 132.502 51.270 132.706 52.777 131.236 50.834 131.452 51.588 130.710 50.849 135.444 50.471 134.977 50.029 135.690 48.606 135.447 47.900 135.731 48.136 134.710 47.332 135.096 47.737 133.321 47.978 136.278 47.398 135.712 48.118 137.606 47.540 138.504 47.885 139.980 47.380 140.935 47.916 141.329 46.173 141.599 45.989 142.355 47.031 142.210 48.033 138.138 47.232 138.027 49.346 137.947 49.923 137.607 51.424 137.327 52.103 137.104 53.628 136.829 54.444 138.083 55.925 137.845 49.215 136.403 49.205 136.234 48.607 135.584 47.897 134.384 48.603 133.148 50.056 133.091 50.689 132.785 51.046 133.397 52.225 133.284 52.036 132.914 46.417 134.352 45.798 133.295 45.838 135.503 44.433 135.564 43.512 135.737 ATOM 3962 N PHE 618 38.215 72.812 1.00 14.67 0 ATOM 3964 CA PHE 618 39.192 72.736 1.00 14.67 0 ATOM 3965 CB PHE 618 39.146 74.044 1.00 2.00 0 ATOM 3966 CG PHE 618 40.042 74.073 00 2.00 0 ATOM 3967 CDl PHE 618 39.501 74.058 00 2.00 0 ATOM 3968 CD2 PHE 618 41.422 74.147 00 2.00 0 ATOM 3969 CE1 PHE 618 40.322 74.119 00 2.00 0 ATOM 3970 CE2 PHE 618 42.254 74.209 00 2.00 0 ATOM PHE 618 41.696 74.195 00 2.00 0 ATOM PHE 618 40.540 72.570 1.00 14.67 0 ATOM PHE 618 40.893 73.394 1.00 2.00 0 ATOM PHE 619 41.285 71.514 1.00 2.00 0 ATOM PHE 619 42.583 71.271 1.00 2.00 0 ATOM PHE 619 42.468 70.111 1.00 2.00 0 ATOM 619 41.404 70.311 1.00 2.00 0 ATOM 619 40.063 70.159 1.00 2.00 0 ATOM 619 41.738 70.678 1.00 2.00 0 ATOM 619 39.068 70.370 1.00 2.00 0 ATOM 619 40.750 70.892 1.00 2.00 0 ATOM 619 39.408 70.738 1.00 2.00 0 ATOM 619 43.719 71.005 00 2.00 0 ATOM 619 43.501 70.564 00 2.00 0 ATOM 620 44.936 71.282 00 2.00 0 ATOM 620 46.118 71.073 00 2.00 0 ATOM 620 46.647 72.393 00 2.00 0 ATOM 620 45.826 73.321 1.00 2.00 0 ATOM 620 46.741 74.437 1.00 2.00 0 ATOM 620 45.247 72.565 1.00 2.00 0 ATOM 620 47.225 70.438 1.00 2.00 0 ATOM 620 47.548 70.910 1.00 2.00 0 ATOM 621 47.805 69.376 1.00 2.00 0 ATOM 3997 CA LEU 621 48.880 68.669 1.00 2.00 0 ATOM 621 48.577 67.162 1.00 2.00 0 ATOM 621 47.256 66.739 1.00 2.00 0 ATOM 621 47.021 65.253 1.00 .00 0 ATOM 621 47.281 67.062 1.00 .00 0 ATOM 621 50.162 68.943 1.00 .00 0 ATOM 621 50.121 69.285 1.00 .00 0 ATOM 622 51.296 68.774 1.00 .00 0 ATOM 622 52.593 69.013 1.00 2.00 0 ATOM 622 53.620 69.231 1.00 5.18 0 ATOM 622 54.967 69.716 1.00 5.18 0 ATOM 622 55.802 70.196 1.00 5.18 0 ATOM 622 56.969 70.941 1.00 5.18 0 ATOM 622 57.830 71.557 1.00 5.18 0 ATOM 622 57.659 71.525 1.00 6.18 0 ATOM 622 58.870 72.199 .00 5.18 0 ATOM 622 53.070 67.894 ,υ0 2.00 0 ATOM 622 53.032 66.702 00 16.40 0 ATOM 623 53.508 68.286 00 19.04 0 ATOM 4023 CA GLY 623 54.043 67.328 00 20.78 0 ATOM 4024 C GLY 623 55.551 67.390 1.00 18.09 0 ATOM 4025 0 GLY 623 56.069 68.212 0 ATOM 4026 N ASN 624 56.281 66.530 0 ATOM 4028 CA ASN 624 57.728 66.566 0 ATOM 4029 CB ASN 624 58.345 65.220 0 ATOM 4030 CG ASN 624 58.120 64.857 0 ATOM 4031 OD1 ASN 624 57.381 65.538 1. 0 ATOM 4032 ND2 ASN 624 58.721 63.757 1. 0 ATOM 4035 C ASN 624 58.260 67.721 1 0 ATOM 4036 O ASN 624 59.442 68.066 1 0 ATOM 4037 N HIS 625 57.359 68.335 1 0 ATOM 4039 CA HIS 625 57.687 69.477 1.00 8.83 0 ATOM 4040 CB HIS 625 57.030 69.320 1 .00 2.00 0 ATOM 4041 CG HIS 625 57.909 68.656 1.00 2.00 0 ATOM 4042 CD2 HIS 625 59.042 67.933 1.00 2.00 0 ATOM 4043 ND1 HIS 625 57.668 68.721 1.00 2.00 0 ATOM 4045 CE1 HIS 625 58.618 68.069 1 .00 2.00 0 59.394 21.381 62.540 1.00 12.05 References for Example 3 1. 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Claims: CLAIMS 1. A method of identifying a compound which modulates the interaction between a PPlc and a regulatory subunit thereof, the method comprising determining whether a compound enhances or disrupts the interaction between (a) a PPlc or a fragment, variant, derivative or fusion thereof or a fusion of a fragment, variant or derivative and (b) a regulatory subumt which is able to bind to PPlc or a PPlc-binding fragment, variant, derivative or fusion of said subunit or a fusion of said fragment, variant or derivative. 2. A method of identifying a compound which mimics the effect of a regulatory subunit of PPlc, the method comprising contacting said compound with PPlc and deteπnining whether, in the presence of the compound, PPlc adopts the function of properties of a PPlc in the presence of a given regulatory subunit. 3. A method according to Claim 1 or 2 wherein said regulatory subumt of PPlc is any one of M 110 , G L , G M , M-complexes, p53 BP2, sds22, NIPPI, L5, Inhibitor-l , Inhibitor-2, or DARPP. 4. A method according to Claim 3 wherein the regulatory subunit of PPlc is any one of M u0 , G L , G M , M-complexes or p53BP2. 5. A method according to Claim 4 wherein the regulatory subunit of PPlc is M„o or G M . 6. A method according to Claim 1 wherein the fragment of a regulatory subunit which is able to bind to PPlc is any of the peptides [G63-T93], [G63-N75], [E2-P243], [E2-D118], and peptide 63-80 of G M or functional equivalents thereof or peptides comprising said peptide sequences provided that they are not the complete G M regulatory subunit. 7. A method according to Claim 1 wherein the fragment of a regulatory subunit which is able to bind to PPlc is any of the peptides [M1-E309], [M1-F38], [M1-A150] or [L24-Y496] of M 110 or functional equivalents thereof or peptides comprising said peptide sequences provided that they are not the complete M 110 regulatory subunit. 8. A method according to Claim 1 wherein the PPlc-binding fragment, variant or derivative of said regulatory subunit or a fusion of said fragment, variant or derivative comprises the consensus peptide sequence Arg/Lys-Val/Ile-Xaa-Phe wherein Xaa is any amino acid. 9. A method according to Claim 8 wherein the PPlc-binding fragment, variant or derivative comprises, in addition to the said consensus peptide sequence, at least one basic residue in the four residues N-terminal of the consensus peptide sequence. 10. A method according to Claim 8 wherein in the consensus peptide sequence Xaa is not Asp or Glu or a large hydrophobic residue. 11. A method according to Claim 8 wherein the PPlc-binding fragment is a fragment of a regulatory subunit comprising the said consensus peptide sequence. 12. A method according to Claim 10 wherein the PPlc-binding fragment is a fragment of any of the M uo , G L , G M , M-complexes, p53BP2, sds22, NIPPI, L5, Inhibitor-l , Inhibitor-2 or DARPP regulatory subunits comprising said consensus sequence. 13. A method according to any one of the preceding claims wherein the compound binds to a PPlc. 14. A method according to Claim 1 wherein the compound binds to a regulatory subunit of PPlc. 15. A compound identifiable by the method of any one of Claims 1 to 14. 16. A compound which modulates the interaction between a PPlc and a regulatory subunit thereof said compound comprising any of the peptides [G63-T93], [G63-N75], [E2-P243], [E2-D118], and peptide 63 to 80 of G M or functional equivalents or said compound comprising any of the peptides [M1-E309], [M1-F38], [M1-A150] or [L24-Y496] of M 110 or functional equivalents thereof or said compound comprising the consensus peptide sequence Arg/Lys-Val/Ile-Xaa-Phe wherein Xaa is any naturally occurring amino acid or functional equivalents thereof, provided that the said compound is not a complete regulatory subunit of PPlc. 17. A compound according to Claim 16 consisting of the peptides [G63- T93], [G63-N75], [E2-P243], [E2-D118], or peptide 63 to 80 of G M or functional equivalents thereof or consisting of the peptides [M1-E309], [M1-F38], [M1-A150] or [L24-Y496] of M no or functional equivalents thereof. 18. A peptide capable of binding a PPlc and which affects the ability of PPlc to bind to a particular target and/or affects the regulation of PPlc activity, or a functional equivalent thereof. 19. A method of identifying a compound which modulates the interaction between a PPlc and a regulatory subunit thereof, or binds PPlc or mimics the effect of a regulatory subunit, the method comprising selecting a compound which is capable of adopting the same or substantially the same conformation as a peptide bound to the regulatory subunit-binding site of PPlc or the same or substantially the same conformation as the portion of PPlc which binds to said peptide. 20. A method according to Claim 19 wherein said peptide comprises the consensus peptide sequence Arg/Lys-Val/Ile-Xaa-Phe wherein Xaa is any amino acid. 21. A method according to Claim 20 wherein said peptide consists of residues 63 to 75 of G M . 22. A method according to Claim 21 wherein the conformation of the said peptide and the conformation of the said portion of PPlc is as defined by reference to the coordinates in Table A. 23. A compound identifiable by the method of any one of Claims 19 to 22. 24. A compound according to any one of Claims 15 to 18 or 23 for use in medicine. 25. A pharmaceutical composition comprising a compound according to any one of Claims 15 to 18 or 23 and a pharmaceutically acceptable carrier. 26. A method of affecting cellular metabolism or function, the method comprising administering to a cell (a) a compound which modulates the interaction between a PPlc and a regulatory subunit thereof or (b) a compound which mimics the effect of a regulatory subunit of PPlc or (c) a peptide capable of binding a PPlc and which affects the ability of PPlc to bind to a particular target and/or affects the regulation of PPlc activity, or a functional equivalent thereof. 27. A method according to Claim 26 wherein any one or more of the peptides [G63-T93], [G63-N75], [E2-P243], [E2-D118], and peptide 63- 80 of G M or functional equivalents thereof or peptides comprising said peptide sequences are administered. 28. A method according to Claim 26 wherein any one or more of the peptides [M1-E309], [M1-F38], [M1-A150] or [L24-Y496] of M no or functional equivalents thereof or peptides comprising said peptide sequences are administered. 29. A method according to Claim 26 wherein a compound according to any one of Claims 15 to 18 or 23 are administered to the cell. 30. A method according to any one of Claims 26 to 29 wherein the cell is in a mammalian body. 31. A method of treating a patient in need of modulation of PPlc activity or function the method comprising administering to the patient an effective amount of a compound which modulates the interaction between a PPlc and a regulatory subunit thereof or (b) a compound which mimics the effect of a regulatory subunit of PPl c or (c) a peptide capable of binding a PPlc and which affects the ability of PPlc to bind to a particular target and/or affects the regulation of PPlc activity, or a functional equivalent thereof. 32. Use of peptides derived from targeting subunits of PPlc, functional equivalents or portions thereof to affect cellular metabolism. 33. A method of treatment of a mammal said method comprising altering levels of peptides derived from a targeting subunit of PPlc, functional equivalents or portions thereof to an extent that cellular metabolism or function is affected. 34. A PPlc-regulating subunit that is modified so that it cannot interact with PPlc. 35. A PPlc-regulator subumt according to Claim 34 wherein the consensus peptide sequence Arg/Lys-Val/Ile-Xaa-Phe is missing or modified.

Inventors: BARFORD DAVID, COHEN PATRICIA TOWNSEND WADE, COHEN PHILIP, BARFORD, DAVID, COHEN, PATRICIA, TOWNSEND, WADE, COHEN, PHILIP
Applicants: BARFORD DAVID, COHEN PATRICIA TOWNSEND WADE, COHEN PHILIP, MEDICAL RES COUNCIL, BARFORD, DAVID, COHEN, PATRICIA, TOWNSEND, WADE, COHEN, PHILIP, MEDICAL RESEARCH COUNCIL
Application year: 1997
Main classification: G01N33
IPC codes: A61K38, C07K14, C12N9, G01N33