ISOLATION AND USE OF SH3 BINDING PEPTIDES

Abstract: Peptides having general and specific binding affinities for the Src homology region 3 (SH3) domains of proteins are disclosed in the present invention. In particular, SH3 binding peptides have been isolated from phage-displayed random peptide libraries which had been screened for isolates that bind to bacterial fusion proteins comprising SH3 and glutathione S-transferase (GST). Preferred peptides are disclosed which comprise a core 7-mer sequence (preferably, a consensus motif) and two or more, preferably at least six, additional amino acid residues flanking the core sequence, for a total length of 9, preferably at least 13, amino acid residues and no more than about 45 amino acid residues. Such peptides manifest preferential binding affinities for certain SH3 domains. The preferred peptides exhibit specific binding affinities for the Src-family of proteins. In vitro and in vivo results are presented which demonstrate the biochemical activity of such peptides.

Description: ISOLATION AND USE OF SH3 BINDING PEPTIDES 1. Field of the Invention The present invention relates to SH3 binding peptides having a broad range of binding specificities. That is, certain members of the SH3 binding peptides disclosed bind with approximately the same facility with SH3 domains derived from different SH3 domain-containing proteins. Other members, in contrast, bind with a much greater degree of affinity for specific SH3 domains. The SH3 binding peptides are obtained from random peptide libraries that are also phage-displayed. Methods are described of obtaining the phage clones that bind to the SH3 domain targets and of determining their relevant nucleotide sequences and consequent primary amino acid sequence of the binding peptides. The resulting SH3 binding proteins are useful in a number of ways, including, but not limited to, providing a method of modulating signal transduction pathways at the cellular level, of modulating oncogenic protein activity or of providing lead compounds for development of drugs with the ability to modulate broad classes, as well as specific classes, of proteins involved in signal transduction. 2. Background of the Invention 2.1. Src and the SH3 Domain Among a number of proteins involved in eukaryotic cell signaling, there is a common sequence motif called the SH3 domain. It is 50-70 amino acids in length, moderately conserved in primary structure, and can be present from one to several times in a large number of proteins involved in signal transduction and in cytoskeletal proteins. The protein pp60c-src represents a family of at least nine non-receptor protein tyrosine kinases (NR-PTKs) . Members of this family share an overall structural organization comprising a series of catalytic and non- catalytic domains. In Src, a 14-amino-acid myristylation signal resides at the extreme amino-terminus, and is followed by a unique region that is not highly conserved among family members. Following this region are two highly conserved 60- and 100-amino-acid regions, the Src homology (SH) domains 3 and 2, respectively. SH2 and SH3 domains have been shown to play an important role in mediating protein-protein interactions in a variety of signaling pathways. Koch, C.A., et al., in Science (1991) 252:668-74. The carboxy-terminal half of Src contains the PTK catalytic domain, as well as a negative regulatory tyrosine (Y527) near the carboxy terminus. Phosphorylation of this residue (e.g., by Csk) results in the inhibition of PTK activity. Cooper, J.A. , et al., in Science (1986) 231:1431-1434. Mutation of Y527->F generates forms of Src with increased PTK and oncogenic activity. Cartwright, CA. , et al., in Cell (1987) 49:83-91; Kmiecik, T.E., et al., in Cell (1987) 49:65-73; and Piwicna- Worms, H., et al., in Cell (1987) 75-82. The fact that some mutations which result in increased Src PTK and transforming activity map to the Src SH2 (Seidel- Dugan, C. , et al., in Mol. Cell. Biol. (1992) 12:1835-45; and Hirai, H. and Varmus, H.E. in Mol. Cell. Biol. (1990) 10:1307-1318) and SH3 domains (Seidel-Dugan, C, et al., supra; Hirai, H. and Varmus, H.E., supra ; Superti-Furga, G. , et al., in Embo■ J. (1993) 12:2625-34; and Potts, .M. , et al., in Oncoqene Res. (1988) 3:343-355) suggests a negative regulatory role for these domains. That phosphotyrosine residues within specific sequence contexts represent high affinity ligands for SH2 domains suggests a model in which the SH2 domain participates in Y527-mediated inhibition of PTK activity by binding phosphorylated Y527, thereby locking the kinase domain in an inactive configuration. Matsuda, M. , Mayer, B.J., et al. , in Science (1990) 248:1537-1539. This model is supported by the observation that phosphopeptides corresponding to the carboxy-terminal tail of Src bind active, but not inactive, variants of Src. Roussel, R.R. , et al., in Proc. Natl. Acad. Sci. U S A (1991) 88:10696-700; and Liu, X., et al., in Oncoqene (1993) 8:1119-1126. The mechanism of SH3-mediated inhibition of Src PTK activity remains unclear. There is evidence that pY527- mediated inhibition of Src PTK activity involves the SH3 domain as well as the SH2 domain. Okada, M. , Howell, et al., in J. Biol. Chem. (1993) 268:18070-5; Murphy, S.M. , et al., in Mol. Cell. Biol. (1993) 13:5290-300; and Superti-Furga, G., et al., supra . Although these effects are thought to be a consequence of SH3-mediated protein-protein interactions, precisely how the Src SH3 domain exerts its negative regulatory effect is unclear. Identification of high affinity ligands for the Src SH3 domain could help resolve these issues. 2.2. Protein Tyrosine Kinases and The immune Response Src-related tyrosine kinases are expressed in a variety of cell types including those of the immune system (lymphocytes, T cells, B cells, and natural killer cells) and the central nervous system (neural cells, neurons, oligodendrocytes, parts of the cerebellum, and the like) . Umemori, H. et al., in Brain Res. Mol. Brain Res. (1992) Dec. 16(3-4) :303-310. Their presence in these cells and tissues and their interaction with specific cell surface receptors and immunomodulatory proteins (such as T cell antigen receptor, CD14, CD2, CD4, CD40 or CD45) suggest that these kinases serve an important role in the signalling pathways of not only the central nervous system but of the immune system, as well. See, e.g., Ren, CL. et al. , in J. Exp. Med. (1994) 179(2) :673-680 (signal transduction via CD40 involves activation of Lyn kinase) ; Donovan, J.A. and Koretzky, G.A. , in J. Am. Soc. Nephrol. (1993) 4(4):976-985 (CD45, the immune response, and regulation of Lck and Fyn kinases) ; and Carmo, A.M. et al., in Eur. J. Immunol. (1993) 23 (9) :2196-2201 (physical association of the cytoplasmic domain of CD2 with p561ck and p59fyn) . For instance, mice with disruptions in their Src-like genes, Hck and Fgr, possess macrophages with impaired phagocytic activity or exhibit a novel immunodeficiency characterized by an increased susceptibility to infection with Listeria monocytogenes. Lowell, CA. et al., in Genes Dev. (1994) 8(4) :387-398. Also, it has been shown that bacterial lipopolysaccharide (LPS) activates CD14-associated p561yn, p68hck, and p59c-fgr, while inducing the production of lymphokines, such as TNF-alpha, IL-1, IL-6, and IL-8. Inhibition of the protein tyrosine kinases blocks production of TNF-alpha and IL-l. 2.3. SH3 Binding Peptides As mentioned above, it has long been suspected that SH3 domains are sites of protein-protein interaction, but it has been unclear what SH3 domains actually bind. Efforts to identify ligands for SH3 domains have led to the characterization of a number of SH3-binding proteins, including 3BP1 and 2 (Ren, R. , Mayer, et al., in Science (1993) 259:1157-61), SOS (Olivier, J.P., et al., in Cell (1993) 73:179-91; and Rozakis-Adcock, M. , et al., in Nature (1993) 363:83-5), p85 PI-3' Kinase (Xingquan, L. , et al., in Mol. Cell. Biol. (1993) 13:5225-5232), dynamin (Gout, 1., et al., in Cell (1993) 75:25-36), AFAP-110 (Flynn, D.C, et al., in Mol. Cell. Biol. (1993) 13:7892-7900), and CD42 (Barfod, E.T., et al., in J. Biol. Chem. (1993) 268:26059-26062). These proteins tend to possess short, proline-rich stretches of amino acids, some of which have been directly implicated in SH3 binding. A variety of consensus sequences have been proposed, although the similarity among proline-rich regions of different SH3-binding proteins tends to be fairly low. Also, attempts to build consensus sequences are likely complicated by the incorporation of data from proteins that bind different SH3 domains. Thus, Cicchetti, P., et al., in Science (1992) 257:803- 806, published their work relating to the isolation and sequencing of two naturally-occurring proteins that could be bound in vitro by the SH3 domain of the abl oncogene product. These workers found that SH3 domains bind short, proline-rich regions of such proteins. Subsequently, this same group disclosed further results (Ren, R. et al., supra) in which the SH3 binding sites of the SH3 binding proteins were localized to "a nine- or ten-amino acid stretch rich in proline residues." A consensus sequence incorporating the features of the SH3 binding sites of four SH3 binding proteins was proposed: XPXXPPP¥XP (SEQ ID NO:l), wherein X indicates a position in the amino acid sequence which is not conserved among the four SH3 binding proteins, P represents proline, and ¥ indicates a hydrophobic amino acid residue, such as P or L. The screening of complex random peptide libraries has been used to identify peptide epitopes for monoclonal (Scott, J.K. and Smith, G.P. in Science (1990) 249:386-390) and polyclonal (Kay, B.K. , et al., in Gene (1993) 128:59-65) antibodies, as well as peptide ligands for a variety of proteins, including streptavidin (Devlin, J.J., et al., in Science (1990) 249:404-406; and Lam, K. , et al., in Nature (1991) 354:82-84), the endoplasmic reticulum chaperone BiP (Blond-Elguindi, S., et al., in Cell (1993) 75:717-728) , and CaM (Dedman, J.R., et al., in J. Biol. Chem. (1993) 268:23025-23030) . Recently, Chen, J.K. et al., in J. Am. Chem. Soc. (1993) 115:12591-12592, described ligands for the SH3 domain of phosphatidylinositol 3-kinase (PI-3' Kinase) which were isolated from a biased combinatorial library. A "biased" library is to be distinguished from a "random" library in that the amino acid residue at certain positions of the synthetic peptide are fixed, i.e., not allowed to vary in a random fashion. Indeed, as stated by these research workers, screening of a "random" combinatorial library failed to yield suitable ligands for a PI-3' Kinase SH3 domain probe. The binding affinities of these unsuitable ligands was described as weak, >100 μM, based on dissociation constants measured by the Biosensor System (BIAcore) . More recently, Yu, et al. (Yu, H., et al., in Cell (1994) 76:933-945) used a "biased" synthetic peptide library of the form XXXPPXPXX (SEQ ID NO:2), wherein X represents any amino acid other than cysteine, to identify a series of peptides which bind the Src and PI-3' Kinase SH3 domains. The bias was accomplished by fixing the proline residues at the specific amino acid positions indicated for the "random" peptide. As stated previously, without this bias, the technique disclosed fails to identify any SH3 domain-binding peptides. A consensus sequence, based on 13 binding peptides was suggested: RXLPPRPXX (SEQ ID NO:3), where X tends to be a basic residue (like R, K or H) . The binding affinities of several SH3 binding peptides were disclosed as ranging from 8.7 to 30 μM. A "composite" peptide, RKLPPRPRR (SEQ ID NO:4), was reported to have a binding affinity of 7.6 μM. This value compares favorably to the binding affinity of the peptide, VPPPVPPRRR (SEQ ID NO:5), to the N-terminal SH3 domain of Grb2. See, Kraulis, P.J. J. Appl. Crvstalloqr. (1991) 24:946. Recognizing the limitations of their technique, Chen and co-workers, supra , stated that their results "illustrate the utility of biased combinatorial libraries for ligand discovery in systems where there is some general knowledge of the ligand-binding characteristics of the receptor" (emphasis added) . Yu and co-workers, supra , further described an SH3 binding site consensus sequence, XpøPpXP (SEQ ID NO:6), wherein X represents non-conserved residues, 0 represents hydrophobic residues, P is proline, and p represents residues that tend to be proline. A consensus motif of RXLPPRPXX (SEQ ID NO:7), where X represents any amino acid other than cysteine, was proposed for ligands of PI-3' Kinase SH3 domain. A consensus motif of RXLPPLPRø (SEQ ID NO:8), where φ represents hydrophobic residues, was proposed for ligands of Src SH3 domain. Still, the dissociation constants reported for the 9-mer peptides ranged only from about 8-70 μM and selectivity between one type of SH3 domain and another was relatively poor, the K D s differing by only about a factor of four. Hence, there remains a need to develop techniques for the identification of Src SH3 binding peptides which do not rely on such "biased" combinatorial peptide libraries that are limited to a partially predetermined set of amino acid sequences. Indeed, the isolation of SH3 binding peptides from a "random" peptide library has not been achieved successfully before now. Furthermore, particular peptides having much greater binding affinities, whether general or more selective binding for specific SH3 domains, remain to be identified. Binding peptides specific for particular SH3 domains are useful, for example, in modulating the activity of a particular SH3 domain-containing protein, while leaving others bearing an SH3 domain unaffected. Still, the more promiscuous general binding peptides are useful for the modulation of a broad spectrum of SH3 domain-containing proteins. The present invention relates to such SH3 binding peptides, methods for their identification, and compositions comprising same. In particular, peptides comprising particular sequences of amino acid residues are disclosed which were isolated from random peptide libraries. In the present invention, clones were isolated from a phage- displayed random peptide library which exhibited strong binding affinities for SH3 domain-containing protein targets. Some of these protein targets, include Abl, Src, Grb2, PLC-5, PLC-γ, Ras GAP, Nek, and p85 PI-3' Kinase. From the nucleotide sequence of the binding phage, the amino acid sequence of the peptide inserts has been deduced. Synthetic peptides having the desired amino acid sequences are shown to bind the SH3 domain of the target proteins. In particular, synthetic peptides combining a core consensus sequence and additional amino acid residues flanking the core sequence are especially effective at binding to particular target protein SH3 domains. The SH3 binding peptides disclosed herein can be utilized in a number of ways, including the potential modulation of oncogenic protein activity in vivo . These peptides also serve as useful leads in the production of peptidomimetic drugs that modulate a large class of proteins involved in signal transduction pathways and oncogenesis. 3. Summary of the Invention Accordingly, three phage-displayed random peptide libraries were screened for isolates that bind to bacterial fusion proteins consisting of the Src homology region 3 (SH3) and glutathione S-transferase (GST) . DNA sequencing of the isolates showed that they contained sequences that resemble the consensus motif, RPLPPLP (SEQ ID NO:9), within their 8, 22, or 36 amino acid long random regions. When peptides were synthesized corresponding to the pill inserts of the SH3- binding phage, they bound to the GST fusions of the SH3 domains of Src and the Src-related proteins, such as Yes, but not of Grb2, Crk, Abl, or PLCγl. The synthesized peptides bind quite well to the Src SH3 domain and act as potent competitors of natural Src SH3 interactions in cell lysates. For instance, these peptides can compete with radiolabelled proteins from cell lysates in binding to immobilized Src-GST, with an apparent IC 50 of 1-10 μM. When a peptide, bearing the consensus sequence RPLPPLP (SEQ ID NO:9) was injected into Xenopus laevis oocytes, it accelerated the rate of progesterone-induced maturation. These results demonstrate the utility of phage-displayed random peptide libraries in identifying SH3-binding peptide sequences and that such identified peptides exhibit both in vivo and in vitro biological activity. Thus, it is an object of the present invention to provide peptides having at least nine and up to forty-five amino acid residues, including an amino acid sequence of the formula, R-2-L-P-5-6-P-8-9 (SEQ ID NO:10), positioned anywhere along the peptide, in which each number represents an amino acid residue, such that 2 represents any amino acid residue except cysteine, 5 and 6 each represents a hydrophobic amino acid residue, 8 represents any amino acid residue except cysteine, and 9 represents a hydrophilic amino acid residue except cysteine, each letter being the standard one-letter symbol for the corresponding amino acid, said peptide exhibiting a binding affinity for the SH3 domain of Src, provided that said peptide is not R-P-L-P-P-L-P-T-S (SEQ ID NO:11). In a particular embodiment of the present invention, the peptides also exhibit a binding affinity for the SH3 domain of Src-related proteins, including Yes, Fyn, Lyn, Lck, Hck and Fgr. The present invention also contemplates SH3 domain- binding peptides that further comprise a C-terminal-flanking amino acid sequence of the formula 10, 10-11, 10-11-12, 10- 11-12-13 (SEQ ID NO:12) or 10-11-12-13-14 (SEQ ID NO:13), in which each number represents any amino acid residue except cysteine, such that 10 is bound to 9 by a peptide bond. Furthermore, peptides are also provided which further comprise an N-terminal-flanking amino acid sequence of the formula 1', 2 ' -! ' , 3'-2'-l' or 4'-3'-2'-l' (SEQ ID NO:14) in which each number represents any amino acid residue except cysteine, such that 1' is bound to R by a peptide bond. Thus, in a particular embodiment, a peptide is disclosed having at least thirteen and up tc forty-five amino acid residues, including an amino acid sequence of the formula, 3'-2'-l'-R-2-L-P-5-6-P-8-9-10 (SEQ ID NO:15) , positioned anywhere along the peptide, in which each number represents an amino acid residue, such that 3', 2 ' , 1 ' , 2 , 8, and 10 each represents any amino acid residue except cysteine, 5 and 6 each represents a hydrophobic amino acid residue, and 9 represents a hydrophilic amino acid residue except cysteine, each letter being the standard one-letter symbol for the corresponding amino acid, said peptide exhibiting a binding affinity for the SH3 domain of Src. The present invention also seeks to provide new consensus sequences or motifs that reflect variations in SH3 domain binding selectivities or specificities. The present invention also contemplates conjugates of the SH3 binding peptides and a second molecule or chemical moiety. This second molecule may be any desired substance whose delivery to the region of the SH3 domain of a particular protein (or cell containing the protein) is sought. Possible target cells include, but are not limited to, neural cells, immune cells (e.g., T cells, B cells, natural killer cells, and the like), osteoclasts, platelets, epidermal cells, and the like, which cells express Src, Src-related proteins, and potentially, other SH3 domain-containing proteins. In this manner, the modulation of the biological activity of proteins bearing an SH3 domain can be accomplished. Other methods and compositions consistent with the objectives of the present invention are likewise disclosed. In particular, a method is disclosed of modulating the activity of Src or Src-related proteins comprising administering a composition comprising an effective amount of a peptide of the present invention and a carrier, preferably a pharmaceutically acceptable carrier. In a specific embodiment, the contemplated method results in the inhibition of the activity of Src or Src-related proteins. Alternatively, the method is effective to activate Src or Src-related proteins. In yet another embodiment, a method is disclosed of identifying a peptide having a region that binds to an SH3 domain comprising: (a) providing an immobilized target protein comprising an SH3 domain; (b) incubating the immobilized target protein with an aliquot taken from a random peptide library; (c) washing unbound library peptides from the immobilized target protein; (d) recovering the peptide bound to the immobilized target protein; and (e) determining the primary sequence of the SH3 domain-binding peptide. Moreover, a method is disclosed of imaging cells, tissues, and organs in which Src or Src-related proteins are expressed, which comprises administering an effective amount of a composition comprising an SH3 domain-binding peptide conjugated to detectable label or an imaging agent. Other objectives of the present invention will become apparent to one of ordinary skill in the art after consideration of the above disclosure and the following detailed description of the preferred embodiments. The invention also provides assays for identifying a compound that affects the binding between a first molecule comprising an SH3 domain and a second molecule that binds to the SH3 domain comprising incubating one or more candidate compounds from which it is desired to select such a compound with the first molecule and the second molecule under conditions conducive to binding and detecting the one or more compounds that affect binding of the first molecule to the second molecule. Also provided are kits for performing such assays comprising a first molecule comprising an SH3 domain and a second molecule that binds to the SH3 domain. 4. Brief Description of the Figures FIG. 1 illustrates a scheme for the generation of a random 36 amino acid peptide library (TSAR-9; e.g., SEQ ID NO:16) . Oligonucleotides were synthesized (SEQ ID NOS:17- 18) , converted into double-stranded DNA, cleaved with restriction enzymes (SEQ ID NOS: 19-20), and cloned into the M13 vector, m663. The random peptide region encoded by the oligonucleotides is shown in the box (SEQ ID NO: 16) and is situated at the N-terminus of mature protein III (SEQ ID NO:21) . SEQ ID NO:22 includes the three amino acids preceding the signal peptidase cleavage site. FIG. 2 illustrates a scheme for the generation of a random 22 amino acid peptide library (TSAR-12; e.g., SEQ ID NO:23). Oligonucleotides were synthesized (SEQ ID NOS:24- 25) , converted into double-stranded DNA, cleaved with restriction enzymes (SEQ ID NOS:26-27) , and cloned into the M13 vector, m663. The random peptide region encoded by the oligonucleotides is shown in the box (SEQ ID NO:23) and is situated at the N-terminus of mature protein III (SEQ ID N0:28). SEQ ID NO:29 includes the three amino acids preceding the signal peptidase cleavage site. FIG. 3 illustrates a scheme for the generation of a random 8 amino acid peptide library (R8C; SEQ ID NO: 30). Oligonucleotides were synthesized (SEQ ID NOS: 31-32), converted into double-stranded DNA, cleaved with restriction enzymes (SEQ ID NOS:33-34) , and cloned into the M13 vector, m663. The random peptide region (SEQ ID NO:30) is flanked by cysteine residues and is situated at the N-terminus of mature protein III (SEQ ID NO:35). FIG. 4 illustrates the possible origin of one class of double-insert R8C reco binants (e.g., encoding SEQ ID NO:36). Double-stranded oligonucleotides (e.g., SEQ ID NO:37) may have ligated in a head-to-head fashion at the Xba I site prior to cloning in the Xho I- Xba I cleaved M13 vector. FIG. 5 shows a list of random peptide recombinants (SEQ ID NOS:38-61 and 106) isolated by the method of the present invention and the displayed peptide sequence. The amino acid sequences are aligned to highlight the core sequences. The flanking sequences are shown to the N-terminal and C-terminal ends of the core sequence. SEQ ID NOS: 38-61 are shown in order from top to bottom except that SSCDHTLGLGWCGSRSTRQLPIPP TTTRPSR is SEQ ID NO:106 and RPLPPLP is SEQ ID NO:9. T12.Src3.1 is a Class II ligand (See Section 6.14.5) . FIG. 6 graphically illustrates the relative binding affinities of selected phage clones for various SH3 domains. The results indicate that certain amino acid sequences provide generic SH3 domain binding, while others can provide greater selectivity for the SH3 domain of Src. Still other clones exhibit Src SH3 domain preferential binding. FIG. 7 shows the binding of synthetic peptides (SEQ ID NOS:9 and 62-70) representing Src SH3-selected phage inserts to Src SH3-GST fusion target (shaded columns) over background GST binding (unshaded columns) relative to the core peptide RPLPPLP (SEQ ID NO:9) and proline-rich peptide segments derived from naturally occurring proteins. Bound biotinylated peptide was detected with streptavidin-alkaline phosphatase ELISA. Each point was performed in triplicate; average absorbance at 405 nm is presented. Error bars represent SD. SEQ ID NOS: 62-70 are shown in order from top to bottom except that RPLPPLP is SEQ ID NO:9. FIG. 8 illustrates the relative specificity of selected peptides (SEQ ID NOS:9 and 62-70) for SH3 domains derived from different proteins. In particular, the binding affinities of the peptides for the SH3 domains of the following protein fusion targets were tested: Src SH3-GST, Yes SH3-GST, Grb2-GST, Crk SH3-GST, Abl SH3-GST, PLCγl SH2SH3-GST. Bound biotinylated peptide was detected with streptavidin-alkaline phosphatase. Each point was performed in triplicate; values are average signal (absorbance at 405 nm) above GST background, with error bars representing standard deviation. Hatched bars indicate saturation of the ELISA signal. SEQ ID NOS:62-70 are shown in order from top to bottom except that RPLPPLP is SEQ ID NO:9. FIG. 9 presents the results of competition experiments in which selected peptides were found to inhibit the binding of proteins from cell lysates to immobilized Src SH3-GST or Abl SH3-GST protein fusion targets. FIG. 10 presents a graph illustrating the increased rate of progesterone-induced maturation of oocytes injected with an SH3 domain-binding peptide, VLKRPLPIPPVTR (SEQ ID NO:64) , of the present invention. Briefly, Stage VI oocyted were prepared and injected as previously described (see, Kay, B.K., in Methods in Cell Biol. (1991) 36:663-669) . Oocytes were injected with 40 nL of 100 μM test peptide or water. After injection, the oocytes were placed in 2 μg/mL progesterone (Sigma, St. Louis, MO) and scored hourly for germinal vesicle breakdown (GVBD) . LAPPKPPLPEGEV is SEQ ID N0:70. FIG. 11 shows the results of fluorescence experiments in which certain peptides, Panel A = VLKRPLPIPPVTR (SEQ ID N0:64), Panel B = GILAPPVPPRNTR (SEQ ID NO:63), Panel C = RSTPRPLPPLPTTR (SEQ ID NO:67), of the invention were shown to localize within cellular compartments thought to contain Src or Src-related proteins. FIG. 12 illustrates a scheme for the generation of a biased peptide library. Oligonucleotides were synthesized (SEQ ID NOS: 162-163) , converted into double-stranded DNA (SEQ ID NO:454), cleaved with restriction enzymes Xhol and Xbal (SEQ ID NOs:455-456) , and cloned into the mBAX vector (SEQ ID NOs:457-458) , described further below in the Examples section. The biased peptide region (SEQ ID NO:459) is situated at the N-terminus of mature pill protein. CTAGACGTGTCAGT is a portion of SEQ ID NO: 162. ACTGACACGT is a portion of SEQ ID NO:454. TCGAGGCACAG is a portion of SEQ ID NO:454. FIG. 13 illustrates the peptide sequence encoded in the mBAX vector situated at the N-terminus of mature piII protein. TCCTCGAGTATCGACATGCCTTAGACTGCTAGCACTATGTACAACATGCTT CATCGCAACGAGCCA is SEQ ID NO:460. SSIDMP*TASTMYNM LHRNEP is SEQ ID NO:461. GGTGGGAGGAAGTTGAGCCCGCCCGCCAACGA CATGCCGCCCGCCCTCCTGAAGAGGTCTAGA is SEQ ID NO:462. GGRKLSPPANDMPPALLKRSR is SEQ ID NO:463. FIG. 14 illustrates the relative binding of SH3-selected phage clones to various SH3 domains. Two clones (A and B) representing each consensus motif were assayed for binding to 1 μg of each immobilized GST-SH3 fusion protein. Bound phage were detected by anti-phage ELISA. Sequences of peptides displayed by each clone are aligned with their respective consensus motifs. Invariant proline residues are underlined. Solid bars, specific binding; open bars, cross-reactive binding. Values are average OD 405 ± SD (N =3) . 5. Detailed Description of the Invention 5.1. General considerations The present invention relates to peptides that exhibit a binding affinity for an SH3 domain, which domain has been found to be present in a number of physiologically significant proteins. In particular, peptides are disclosed which exhibit general binding characteristics to the SH3 domains found in a group of proteins, including but not limited to Abl, Src, Grb2, PLC-ό", PLC-γ, Ras GAP, Nek, and p85 PI-3' Kinase. Preferred peptides exhibit selective, if not specific, binding affinity for the SH3 domain of Src. As described herein, the peptides of the present invention include a core sequence, preferably a consensus seqeunce, and additional amino acid residues that flank the core sequence. These peptides, including the methods for their identification, are described in greater detail, below. Thus, in a specific embodiment of the invention, peptides are provided which have at least nine and up to about forty-five amino acid residues, including an amino acid sequence resembling the formula, R-2-L-P-5-6-P-8-9 (SEQ ID NO:10), positioned anywhere along the peptide. In the above- mentioned formula, each number represents an amino acid residue, such that 2 represents any amino acid residue except cysteine, 5 and 6 each represents a hydrophobic amino acid residue, 8 represents any amino acid residue except cysteine, and 9 represents a hydrophilic amino acid residue except cysteine. Each letter used in the formulas herein represent the standard one-letter symbol for the corresponding amino acid. When the peptide is a 9-mer, the peptide R-P-L-P-P-L-P-T-S (SEQ ID NO: 11) is excluded. The peptides of particular interest are those that exhibit a binding affinity for the SH3 domain of Src and Src-related proteins, including Yes, Fyn, Lyn, Lck, Hck and Fgr. Preferably, the peptides of the invention exhibit a binding affinity for the SH3 domain of Src, which is at least three-fold, more preferably at least four-fold, most preferably at least about five-fold greater than that exhibited by the peptide RPLPPLP (SEQ ID N0:9). In still other embodiments, the peptides exhibit a binding affinity for the SH3 domain of Src which is at least ten-fold greater than that exhibited by the peptide RPLPPLP (SEQ ID NO:9) . In specific embodiments, peptides are disclosed in which the various amino acid residues at the indicated positions may independently have the following preferred identities: 2 is a P, R, A, L, Q, E or S, more preferably P or R; 5 represents a P, M, I or L, more preferably P or M; 6 is a P, L, I or V, more preferably P or L; 8 is a T, R, P, I, N, E, V, S, A, G or L, more preferably T or R; and 9 is a T, R, S, H or D, more preferably T or R. Despite the preference for hydrophobic amino acid residues at 5 and 6, in some cases it may be desirable to have hydrophilic amino acid residues at these positions. Specifically, amino acid residue 5 may be a T, R or S, and amino acid residue 6 may be a T or R. Likewise, while a hydrophilic amino acid residue is preferred at position 9, in some instances a hydrophobic residue, such as a P or A, may be desirable. The present invention also contemplates SH3 domain- binding peptides with a minimum length of 10, 11, 12, 13, 14, 15 or more amino acids. Such peptides contain additional amino acid residues flanking the core sequence of R-2-L-P-5-6-P (SEQ ID NO:71) either at the C-terminal end, the N-terminal end or both. Thus, for example, such peptides include those that further comprise a C-terminal-flanking amino acid sequence of the formula 10, 10-11, 10-11-12, 10- 11-12-13 (SEQ ID NO:12) or 10-11-12-13-14 (SEQ ID NO:13), in which each number represents any amino acid residue except cysteine, such that the amino acid residue 10 is bound to the amino acid residue 9 by a peptide bond. In that case, specific embodiments include an amino acid residue 10 which is T, R, L, S, D, P, A or N, preferably T or R, an amino acid residue 11 which is R, P, A, Q, S or T, preferably R or P, an amino acid residue 12 which is P, S, R or T, preferably P or S, an amino acid residue 13 which is P, S, R, F, H or T, preferably P or S, and an amino acid residue 14 which is S, R, G or T, preferably, S or R. Furthermore, peptides are also provided which further comprise an N-terminal-flanking amino acid sequence of the formula 1', 2 -l , 3'-2'-l' or 4'-3 -2'-l' (SEQ ID NO:14) in which each number represents any amino acid residue except cysteine, such that 1' is bound to R by a peptide bond. In such a case, specific embodiments are provided in which the amino acid residue 1' is T, P, S, N, F, W, K, H, Q or G, preferably T or P, wherein the amino acid residue 2' is S, T, G, P, R, Q, L, A or H, preferably S or T, wherein the amino acid residue 3' is R, S, P, G, A, V, Y or L, preferably S or T, and wherein the amino acid residue 4' is R, S, V, T, G, L or F, preferably R or S. In a particular embodiment, a peptide is disclosed having at least thirteen and up to forty-five amino acid residues, including an amino acid sequence of the formula, 3'-2'-l'-R-2-L-P-5-6-P-8-9-10 (SEQ ID NO:15), positioned anywhere along the peptide, in which each number represents an amino acid residue, such that 3', 2 ' , 1', 2 , 8, and 10 each represents any amino acid residue except cysteine, 5 and 6 each represents a hydrophobic amino acid residue, and 9 represents a hydrophilic amino acid residue except cysteine, each letter being the standard one-letter symbol for the corresponding amino acid, said peptide exhibiting a binding affinity for the SH3 domain of Src. Preferred 13-mers include, but are not limited to, those having an amino acid residue 5 which is a P or M, an amino acid residue 1' which is T, P, S or N, an amino acid residue 2' which is S or T, an amino acid residue 3' which is R or S, and an amino acid residue 10 which is T or R. In all the SH3 domain-binding peptides described herein, the prohibition against the use of the hydrophilic amino acid residue cysteine (C) does not extend beyond the 7-mer "core" sequence and the additional amino acid residues ϊlanking the core up to a total (core + flanking) of about 20 amino acids. That is, the occasional use of a cysteine is not absolutely prohibited. What should be kept in mind is that the potential for the formation of intramolecular disulfide bonds, to form a cyclic structure, be minimized as much as possible. Applicants have found that cyclized structures appear to be disfavored, at least with potential binding peptides of less than about 15 amino acid residues in length. The concern for the formation of cyclized structures comprising the core sequence diminishes with increasing size of the peptide. Presumably, a large enough structure, though cyclic, may allow the critical core sequence to adopt a more or less linear conformation. In particular, specific peptides are disclosed which exhibit binding affinities to SH3 domains. These include the peptides, RSTPRPLPMLPTTR (SEQ ID NO. 62), RSTPRPLPPLPTTR (SEQ ID NO. 67), GILAPPVPPRNTR (SEQ ID NO. 63), VLKRPLPIPPVTR (SEQ ID NO. 64), GPHRRLPPTPATR (SEQ ID NO. 65), and ANPSPATRPLPTR (SEQ ID NO. 66) . Phage clones are also disclosed, along with the amino acid sequences that are responsible for SH3 domain binding. These phage clones are identified in Figure 5. In other embodiments of the present invention, SH3 domain-binding peptides are contemplated which have a total of 11, 13, 14, 18, 20, 22, 23, 25, 30, 36, 38 or 45 amino acid residues. The peptides of the present invention, having been disclosed herein, may be prepared by any number of practicable methods, including but not limited to solution- phase synthesis, solid-phase synthesis, protein expression by a transformed host, cleavage from a naturally-derived, synthetic or semi-synthetic polypeptide, or a combination of these techniques. The SH3 binding peptides exhibit a wide range of biological activity which includes the enhancement (or inhibition, depending on the particular peptide or the nature of the peptide's target molecule, in this case a protein bearing an SH3 domain) of the natural function or biological activity of the peptide's target molecule. For example, the interaction of the binding peptide of the present invention could result in the modulation of the oncogenic activity of the target molecule bearing the SH3 domain. If the target molecule has, in turn, a natural binding partner or ligand, the peptides of the present invention may also exhibit antagonistic or agonistic activity in relation to the biological activity of the natural binding partner. Thus, it is an object of the present invention to provide a method of activating Src or Src-related protein tyrosine kinases by administering an effective amount of the SH3 domain-binding peptides generally described herein. The intensity of the immune response can thus be stimulated, for example, by the increased production of certain lymphokines, such as TNF-alpha and interleukin-1. As is generally known to those of ordinary skill in the art, a more intense immune response may be in order in certain conditions, such as in combating a particularly tenacious infection, viral or otherwise, or a malignancy. Furthermore, in a specific embodiment of the present invention, a conjugate compound is contemplated which comprises the peptide of the present invention and a second chemical moiety. The second chemical moiety can be selected from a wide variety of chemical compounds including the peptide itself. Typically, however, the second chemical moiety is selected to be other than the peptide of the present invention, including but not limited to an amino acid, a peptide other than an SH3 binding peptide of the present invention, a polypeptide or protein (i.e., the conjugate is a fusion protein), a nucleic acid, a nucleoside, a glycosidic residue (i.e., any sugar or carbohydrate), a label or image-enhancing agent (including metals, isotopes, radioisotopes, chromophores, fluorophores (such as FITC, TRITC, and the like) , and enzyme substrates) , a drug (including synthetic, semisynthetic, and naturally-occurring compounds), small molecules (e.g., biotin, hormones, factors) and the like. The peptide of the present invention can be conjugated to the second chemical moiety either directly (e.g., through appropriate functional groups, such as an amine or carboxylic acid group to form, for example, an amine, imine, amide, ester, acyl or other carbon-carbon bond) or indirectly through the intermediacy of a linker group (e.g., an aliphatic or aromatic polyhydroxy, poiyamine, polycarbox lic acid, polyolefin or appropriate combinations thereof) . Moreover, the term "conjugate," as used herein, is also meant to encompass non-covalent interactions, including but not limited to ionic, affinity or other complexation interactions. Preferably, such other non-covalent interactions provide definable, most preferably, isolatable chemical conjugate species. As described further herein, the peptides of the present invention have been shown to localize within certain cellular compartments which contain Src or Src-related proteins. Consequently, the above-described conjugate can be utilized as a delivery system for introduction of a drug to cells, tissues or organs that include SH3 domain-containing proteins. It should also be pointed out that the present invention seeks to provide a recombinant construct comprising a nucleic acid or its complement that includes codons or nucleotide sequences encoding a peptide having a region that binds to an SH3 domain, preferably the Src SH3 domain. The recombinant nucleic acid may be a DNA or RNA polynucleotide. In a specific embodiment, the present invention contemplates a recombinant construct which is a transforming vector. Such vectors include those well known to those of ordinary skill in the art, which effect the transfer or expression of the nucleotide sequence after introduction to a host, such as recombinant plasmid, phage or yeast artificial chromosome. These vectors may be closed circular loops or they may be linearized. The vectors contemplated include those that exist extrachromosomally after host transformation or transfection, as well as those that integrate within or even displace portions of the host chromosome. The vectors may be introduced to the cell with the help of transfection aids or techniques well-known in the art. For example, these aids or techniques may take the form of electroporation, use of calcium chloride, calcium phosphate, DEAE dextran, liposomes or polar lipid reagents known as LIPOFECTIN or LIPOFECTAMINE. In addition, the present invention contemplates the direct introduction of the desired nucleic acid to the host cell, for instance, by injection. Transformed host cells are also obtained by the methods of the present invention which are capable of reproducing the polynucleotide sequences of interest and/or expressing the corresponding peptide products. A variety of hosts are contemplated, including prokaryotic and eukaryotic hosts. In particular, bacterial, viral, yeast, animal, and plant cells are potentially transformable hosts. Thus, a method is disclosed to obtain a transformed host cell that can produce, preferably secrete, a peptide having a region that binds to an SH3 domain comprising (a) providing an expression vector, preferably a secretory expression vector, comprising a nucleotide sequence encoding at least one copy of a peptide having a region that binds to an SH3 domain; and (b) introducing the vector to a competent host cell. The peptides, thus produced, may then be introduced to cells, tissues, organs, or administered to the subject for the purpose of modulating the biochemical activity of the SH3 domain-containing proteins present therein. Accordingly, in specific embodiments of the present invention, compositions are provided which comprise an SH3 domain-binding peptide, including a core sequence and flanking sequences, and a suitable carrier. The compositions contemplated by the present invention may also include other components, from those that facilitate the introduction or administration of the compositions to those that have their own innate activity, such as a prophylactic, a diagnostic or a therapeutic action. Such innate activity may be distinct from that of the peptides of the present invention or be complementary thereto. In any event, the compositions of the present invention include those that are suitable for administration into mammals, including humans. Preferably, the compositions (including necessarily the carrier) of the present invention are sterile, though others may need only be cosmetically, agriculturally or pharmaceutically acceptable. Still other compositions may be adapted for veterinary use. The compositions, including the drug delivery systems described herein, are contemplated to be administered in a variety of ways, such as parenterally, orally, enterally, topically or by inhalation. The compositions may also be adminstered intranasally, opthal ically or intravaginally. Furthermore, the compositions of the invention can take several forms, such as solids, gels, liquids, aerosols or patches. In another embodiment of the present invention a method is provided of identifying a peptide having a region that binds to an SH3 domain comprising: (a) providing an immobilized target protein comprising an SH3 domain; (b) incubating the immobilized target protein with an aliquot taken from a phage-displayed random peptide library, which library includes peptides having a random sequence of >8 amino acid residues; (c) washing unbound phage from the immobilized target protein; (d) recovering the phage bound to the immobilized target protein; and (e) determining the relevant nucleotide sequence of said binding phage nucleic acid and deducing the primary sequence corresponding to the SH3 domain-binding peptide. Preferably, the method further comprises amplifying the titer of the recovered phage and repeating the steps of incubation, washing and recovery to provide SH3 domain-binding peptide-enriched phage. Any other mode by which the peptide library, random or otherwise, can be "displayed" can be utilized in the present invention, however. Moreover, the present applicants believe that longer random peptide sequences (e.g., >6 amino acid residues, preferably >10, and most preferably, >12) provide not only much greater diversity but also a richer degree of secondary structure conducive to binding activity. If the random region of the peptide is less than or equal to an 8- mer, it should preferably not be cyclized. 5.2. Preparation of Random Peptide Libraries The preparation and characterization of the preferred phage-displayed random peptide libraries have been described elsewhere. See, for example, Kay, B.K. et al. in Gene (1992) 128:59-65, for a description of the preparation of the phage- displayed random peptide library known as TSAR-9, more below. In particular, by cloning degenerate oligonucleotides of fixed length into bacteriophage vectors, recombinant libraries of random peptides can be generated which are expressed at the amino-terminus of the pill protein on the surface of M13 viral particles. (There are 3-5 copies of the pill-fusion on the surface of each particle.) Phage display offers several conveniences: first, the expressed peptides are on the surface of the viral particles and accessible for interactions; second, the recombinant viral particles are stable (i.e., can be frozen, exposed to pH extremes) ; third, the viruses can be amplified; and fourth, each viral particle contains the DNA encoding the recombinant genome. Consequently, these libraries can be screened by isolating viral particles that bind to targets. The isolates can be grown up overnight, and the displayed peptide sequence responsible for binding can be deduced by DNA sequencing. These libraries have approximately >10 8 different recombinants, and nucleotide sequencing of the inserts suggests that the expressed peptides are indeed random in amino acid sequence. These libraries are referred to herein as TSAR libraries, where TSAR stands for Totally Synthetic Affinity Reagents. The preparation of the TSAR libraries are described further below. 5.3. SH3 Binding Clones And Their Characteristics Accordingly, peptides have been isolated from an unconstrained random peptide library which exhibit a binding affinity for SH3 domains. Furthermore, the binding affinities exhibited by the disclosed peptides differ in their selectivities with certain peptides showing comparable binding affinities for SH3 domains derived from different proteins, while others manifest greater affinities for specific SH3 domains. The amino acid sequence of various peptides isolated by the present method are listed in Figure 5. As can be seen from this list, certain groups of SH3 domain binding peptides are isolated from three separate random peptide libraries, each based on a different type of random peptide insert, all displayed at the amino-terminus of the pill protein on the surface of M13 viral particles. Ten clones were isolated from the R8C library, seven from the TSAR-12 library, and seven from the TSAR-9 library. The sequences are presented to highlight the particular amino acid residues believed to bind directly to the SH3 domain, as well as to point out the remaining amino acid resiudes of the random insert and the viral flanking sequences and complementary site amino acid residues common to each group of clones. The frequency with which each particular clone is found in each library is also indicated in Figure 5. Thus, clones T12.SRC3.1 and T12.SRC3.2 are by far the most abundant clones found among the three libraries. Interestingly, all the binding peptides are found to have the proline-rich amino acid residue motif, which is apparently responsible for binding, the motif being located predominantly at the C-terminal end of the insert, although each clone also contains an insert at the N-terminal end. The significance of this observation is not presently understood, although this finding may indicate the possible importance of the C-terminal viral flanking sequences in SH3 domain binding. Indeed, a synthetic peptide bearing only the core consensus sequence RPLPPLP (SEQ ID NO:9) was less effective in binding to target SH3 domains than synthetic peptides that also included additional amino acid residues flanking the core sequences. Thus, 13-mers and 14-mers having the sequences RSTPRPLPMLPTTR (SEQ ID NO:62), RSTPRPLPPLPTTR (SEQ ID NO:67), GILAPPVPPRNTR (SEQ ID NO:63), GPHRRLPPTPATR (SEQ ID NO:65), and VLKRPLPIPPVTR (SEQ ID NO:64) have been prepared and shown to bind to SH3 domains, such as those of Src and Yes, much more avidly than the 7-mer, RPLPPLP (SEQ ID NO:9). The 13-mer ANPSPATRPLPTR (SEQ ID NO:66) has been shown to have binding affinities comparable to the core consensus sequence. In each case, the 13-mers comprise a 7- mer "core" sequence plus additional amino acid residues flanking same, some of which additional amino acid residues are contributed by the viral flanking sequences. Thus, in one embodiment of the present invention, a 7- mer core includes a consensus motif of the formula RXLPφφP (SEQ ID NO:71), wherein R is arginine, L is leucine, P is proline, X represents any amino acid except cysteine and φ represents a hydrophobic amino acid residue. By "hydrophobic amino acid residue," the applicants mean to include F, Y, W, V, A, I, L, P or M, each letter representing the standard one-letter designation for the corresponding amino acid residue. Furthermore, a preferred 9-mer peptide comprising two additional amino acids on the C-terminal end of the core sequence is envisioned having a consensus motif of the formula RXLPφφPXψ (SEQ ID NO:10). In this preferred 9-mer consensus motif, the symbol represents a hydrophilic amino acid residue, except cysteine. By "hydrophilic amino acid residue," the applicants mean to include K, R, H, D, E, N, Q, T, S or C, and the other symbols are as defined above. For the purposes of the present invention, a glycine residue (G) may be considered either a hydrophobic or a hydrophilic amino acid residue. The one-letter symbols B and Z, which stand for N or D and Q or E, respectively, are considered hydrophilic amino acid residues. Particular 13-mer peptides of the present invention include those listed, below. It is noted, however, that not all the following 13-mer peptides correlate strictly to or comply with the preferred 9-mer consensus motif, described above. Those peptides that do not comply (indicated in italics, with the non-complying amino acid residues underscored) can, thus, be described as "resembling" those that do comply (indicated in normal type) with the preferred 9-mer consensus motif: PGFRELPPLPPSR (SEQ ID NO:72), AQSRPLPIPPETR (SEQ ID NO:73), VLKRPLPIPPVTR (SEQ ID NO: 64) PPNSPLPPLPTHL (SEQ ID NO: 74) TGRGPLPPLPNDS (SEQ ID NO: 75) YSTRPVPPITRPS (SEQ ID NO:76) SHKSRLPPLPTRP (SEQ ID NO:77) YRFRALPSPPSAS (SEQ ID NO: 78) GPHRRLPPTPATR (SEQ ID NO: 65) LAQRQLPP PGRD (SEQ ID NO:79) ALQRRLPR PPPA (SEQ ID NO: 80) PATRPLP RPSRT (SEQ ID NO:81) YSTRPLPSRPSRT (SEQ ID NO: 82) XPGRILLLPSEPR (SEQ ID NO: 83) SGGILAPPVPPRN (SEQ ID NO: 84) RSTRPLPILPRTT (SEQ ID NO:85) STPRPLPMLPTTR (SEQ ID NO:86) STNRPLPMIPTTR (SEQ ID NO:87) RSTRPLPSLPITT (SEQ ID NO: 88) STSRPLPSLPTTR (SEQ ID NO: 89) RSTRSLPPLPPTT (SEQ ID NO:90) RSTRQLPIPPTTT (SEQ ID NO:91) STPRPLPLIPTTP (SEQ ID NO:92) RSTRPLPPTPLTT (SEQ ID NO:93) and RSTRPQPPPPITT (SEQ ID NO:94). Accordingly, other peptides not specifically disclosed, which either comply with or "resemble" the preferred 9-mer consensus motif, can be readily envisioned by those of ordinary skill in the art and are considered to be equivalent to those that are specifically disclosed above. In particular, non-compliance at positions 1 (S, G, and I, in place of R, are tolerated) , 3 (V, A, and Q, in place of L, are tolerated) , 4 (L, in place of P, is tolerated) , 5 (hydrophilic amino acid residues, S, R, and T, are tolerated in place of a hydrophobic amino acid residue) , 6 (hydrophilic amino acid residues, R and T, are tolerated in place of a hydrophobic amino acid residue) , 7 (T, and S, in place of P, are tolerated) , and 9 (P and A are tolerated in place of a hydrophilic amino acid residue) have been observed. 5.3.1. Binding Specificities It has been discovered that certain of the binding peptides disclosed have a greater relative binding affinity for one SH3 domain over another. Referring now to Figure 8, the relative binding affinities of the various peptides described above toward different SH3 domain targets are graphically presented. As one can see, the relative binding affinities of the respective peptides can differ by orders of magnitude. Thus, as shown in Figure 8, the peptide GPHRRLPPTPATR (SEQ ID NO:65), having the relevant sequence of the phage clone identified as T12.SRC3.3, is specific to Src family SH3 domains, including, but not limited to, Src, Yes, Lck, Hck, Fgr, Fyn, and Lyn. This SH3 binding peptide has little affinity for SH3 domains derived from PLCγ or Grb2. On the other hand, the peptide GILAPPVPPRNTR (SEQ ID NO:63), corresponding to the relevant sequence of the phage clone T12.SRC3.1, which is one of the most abundant binding clones found by the present method, binds generically to a broad range of SH3 domains, including Src, PLCγ, and Grb2. On an intermediate level, the present invention has also uncovered a peptide, VLKRPLPIPPVTR (SEQ ID NO:64), corresponding to the relevant sequence of the phage clone T12.SRC3.6, which is Src preferential; that is, this peptide exhibits strong binding affinities for members of the Src family, some binding affinities for Grb2 proteins, but little binding affinities for PLCγ domains. The peptide ANPSPATRPLPTR (SEQ ID NO:66), corresponding to the relevant sequence of the phage clone T12.SRC3.2, also exhibits Src family specificity similar to GPHRRLPPTPATR (SEQ ID NO:65). The peptides RSTPRPLPMLPTTR (R8CYES3.5; SEQ ID NO:62) and RSTPRPLPPLPTTR (representative consensus motif; SEQ ID NO:67) are highly specific for SH3 domain of Src, Yes, and other Src-related proteins. 5.4. Further Discussion of Binding Experiments At the outset it is apparent that the binding affinity of certain peptides to the SH3 domain of Src and Src-related proteins is governed by more than just the presence of the preferred core consensus sequences, RPLPPLP (SEQ ID NO:9) or RPLPMLP (SEQ ID NO:95; i.e., RPLP(P/M)LP, SEQ ID NO:96). Thus, while the synthetic peptides RSTPi?PLPML.PTTR (R8CYES3.5; SEQ ID NO:62) and RSTPi?PiPPI,PTTR (consensus; (SEQ ID NO:67) exhibit a strong specific binding affinity for Src SH3, the other synthetic peptides tested also exhibited an avid binding affinity to SH3 domains relative to the 7- mer, RPLPPLP (SEQ ID NO:9). These other peptides, GILAPPVPPRNTR (SEQ ID NO:63), VLKRPLPIPPVTR (SEQ ID NO:64) , GPHPPIPPTPATR (SEQ ID NO:65), and ANPSPA.T.RPLPTR (SEQ ID NO: 66) , sport core sequences and flanking sequences that do not closely adhere to the preferred core consensus sequences. Thus, these results suggest that binding affinity tc SH3 domains is governed to a large extent by the nature of the amino acid residues flanking the core 7-mer sequence. The binding characteristics of Src SH3-selected peptides was determined using synthetic biotinylated peptides corresponding to the sequences displayed by Src SH3-selected phage. These biotinylated peptides were assayed for direct binding to immobilized Src SH3-GST. Each of the five library-derived peptides tested were found to bind to Src SH3-GST and Yes SH3-GST over background (Figure 8) . Furthermore, a strong correlation was observed between the similarity of a given peptide to the preferred core consensus sequence RPLP(P/M)LP (SEQ ID NO:96) and the peptide's affinity for Src SH3-GST. The core sequence of the clone T12.SRC3.1 (GILAPPVPPRNTR; SEQ ID NO: 63) appears to provide more generic SH3 domain-binding characteristics. Experiments comparing the relative binding of various phage clones to SH3 domains taken from a variety of proteins demonstrated the preference of these clones for Src and Src- related SH3 domains over SH3 domains taken from other proteins. It was further found that while the 7-mer having the consensus sequence RPLPPLP (SEQ ID NO:9) bound to Src SH3-GST only weakly, peptides comprising the consensus sequence flanked by residues encoded by one of the Src SH3-selected clones (R8CYES3.5), RSTP (SEQ ID NO:97) at the N-terminal end and TTR at the C-terminal end, bound significantly better than any of the peptides tested (Figure 7) . Thus, as stated previously, sequences that flank the RPLP(P/M)LP (SEQ ID NO:96) core appear to be important contributors to SH3 binding. It is further surmised that a peptide having or resembling the sequence RSTPAPPVPPRTTR (SEQ ID NO:98) should exhibit strong but generic binding to a variety of SH3 domains. Similarly, it is observed that most of the Src SH3- binding motifs are located near the carboxy-terminus of the random peptides, adjacent to sequences which are fixed in every clone (Figure 5) . The exceptional clones tend to possess sequences that resemble motifs that include fixed flanking sequences. This clustering contrasts with previous results, in which binding motifs are distributed throughout the random peptide. Kay, B.K., et al. , in Gene (1993) 128:59-65. The binding of the library-derived Src SH3-binding peptides was compared to that of peptides corresponding to proline-rich regions of natural proteins. Peptides corresponding to SH3-binding regions in human PI-3' Kinase (KISPPTPKPRPPRPLPV; SEQ ID NO:69) and human SOSl.20 (GTVEPVPPPVPPRRRPESA; SEQ ID NO:68), as well as a proline- rich region of the cytoskeletal protein vinculin (LAPPKPPLPEGEV; SEQ ID NO:70), bound Src SH3 much less well than the library-derived peptides (Figure 7) . As mentioned above, the relative specificity of binding was explored. Thus, the relative binding of Src SH3-selected peptides to equal amounts of GST fusions to SH3 domains from different proteins was determined (Figure 8) . While all of the library-derived peptides bound the Src and Yes SH3 domains almost equally well, none of the peptides (with the exception of peptide T12.SRC3.1, the most divergent peptide tested) bound the SH3 domains of Grb2, Crk, Abl or PLCγl appreciably. Thus, the library-derived peptides, in contrast with a peptide derived from SOSl, exhibit SH3 binding that is relatively specific for Src-family members. Next, it was determined whether the binding to the Src SH3 domain was qualitatively like the interactions of the SH3 domain and natural proteins found in cell lysates. Thus, radiolabeled proteins were prepared from NIH 3T3 cell lysates and chromatographed over Src SH3-GST immobilized on glutathione linked Sepharose. SDS-PAGE shows that a number of proteins can be affinity purified in this manner. The synthesized peptides bind quite well to the Src SH3 domain, as they can compete the binding of radiolabeled proteins from cell lysates to immobilized Src-GST, with an IC 50 of 1-10 mM (Figure 9) . In conclusion, the peptides can efficiently block the interaction of cellular proteins with Src SH3 in vi tro . Moreover, Xenopus laevis oocytes injected with mRNA encoding constitutively active Src undergo progesterone- induced maturation at an accelerated rate relative to oocytes injected with water or c-Src mRNA. Unger, T.F. and Steele, R.E. in Mol. Cell.Biol. (1992) 12:5485-5498. To explore the ability of the library-derived Src SH3-binding peptides to exert a biochemical effect in vivo, the influence of the peptides on the maturation of Xenopus laevis oocytes was examined. Hence, stage VI oocytes were injected with peptide, exposed to progesterone, and scored for germinal vesicle breakdown. Figure 10 shows that the rate of maturation was accelerated by approximately one hour when oocytes were injected with the SH3-binding peptide consisting of RPLPPLP (SEQ ID NO:9) flanked by residues from clone T12.SRC3.6 (VLKRPLPIPPVTR; SEQ ID NO:64), but not with water or a peptide corresponding to a proline-rich segment of vinculin (LAPPKPPLPEGEV; SEQ ID NO:70) as controls. The magnitude of this effect is roughly equivalent to that seen with injection of mRNA encoding constituitively active Src. See, e.g., Figure 3B in Unger, T.F. and Steele, R.E., supra . This result suggests that the library-derived Src SH3-binding peptide is effectively relieving an inhibitory effect of the Src SH3 domain upon Src PTK activity. This model is consistent with a number of studies which have demonstrated an inhibitory effect of the Src SH3 domain upon Src kinase and transforming activity. See, e.g., Okada, M. , et al., supra ; Murphy, S.M. , et al., supra ; and Superti-Furga, G. , et al. , supra . 5.5. Diagnostic And Therapeutic Agents Based On SH3 Binding Peptides and Additional Methods of Their Use As already indicated above, the present invention also seeks to provide diagnostic, prophylactic, and therapeutic agents based on the SH3 binding peptides described herein. In one embodiment, diagnostic agents are provided, preferably in the form of kits, comprising an SH3 domain- binding peptide and a detectable label conjugated to said peptide directly, indirectly or by complexation, said peptide comprising: (i) a core sequence motif of the formula RXLPøøP (SEQ ID NO:71), wherein X represents any amino acid except cysteine and φ represents a hydrophobic amino acid residue, including F, Y, W, V, A, I, L, P, M or G, each letter representing the standard one-letter designation for the corresponding amino acid residue; and (ii) two or more additional amino acid residues flanking said core sequence at its C-terminal end, N-terminal end or both. The diagnostic agents of the present invention can be used to detect the presence of SH3 domains of a generic or specific type in cells, tissues or organs either in vi tro or in vivo . For in vivo applications, the diagnostic agent is preferably mixed with a pharmaceutically acceptable carrier for administration, either enterally, parenterally or by some other route dictated by the needs of the particular application. In a particular embodiment, for example, an assay based on a fusion product is contemplated which comprises a Src SH3 domain-binding peptide of the invention and a substrate for deregulated or "activated" Src. For instance, a muscle biopsy, taken from a subject suspected of being infected by the Rous sarcoma virus, can be treated with an effective amount of the fusion product. By subsequent analysis of the degree of conversion of the substrate, one can potentially detect infection by the Rous sarcoma virus in the subject, particularly mammals, especially chickens. The presence of the retrovirus, which causes the expression of deregulated or "activated" Src, may thus be indicated by unusually high levels of Src as revealed by large amounts of the converted substrate. See, for example, Paxton, W.G. et al., in Biochem. Biophys. Res. Commun. (1994) 200(1) :260-267 (detection of phosphorylated tyrosine and serine residues of angiotensin II ATI receptor, a substrate of Src family tyrosine kinases) ; another suitable substrate may be the protein p68 (Fu agalli, S. et al., in Nature (1994) 368(6474) :871-874; Taylor, S.J. and Shalloway, D., in Ibid. at 867-871. Alternatively, the enzyme can be isolated by selective binding to a form of the SH3 domain-binding peptides of the present invention (e.g., biotin-peptide conjugate). After isolation of the protein-peptide conjugate complex (e.g., on a column comprising streptavidin) , the activity of the enzyme can then be assayed by conventional methods to determine its level of protein kinase activity which can be taken as an indication of the presence of the deregulated or "activated" form of the enzyme. An assay for Src kinase has been described by Klinz and Maness, in Neuroprotocols (a companion to Neuroscience) (1992) 1(3) :224-231. Moreover, the diagnostic agents of the invention can also serve as imaging agents of cells, tissues or organs, especially those that contain proteins with an SH3 domain. For example, neural cells (e.g., neurons, other areas of the brain) , osteoclasts, osteoblasts, platelets, immune cells, and other dividing cells are known to express or contain proteins with SH3 domains. Thus, an image can be taken of portions of the body to serve as a baseline for subsequent images to detect physiologic or biochemical changes in the subject's body. For instance, changes in the condition of cellular levels of Src or a transformation of the cellular Src to an "activated" form may be detected using the diagnostic or imaging agents of the present invention. Accordingly, it has been demonstrated that an SH3- binding peptide tagged with a fluorescence emitter can provide an image of the cytoskeleton. The images are presented in Figure 11. As can be seen from Figure 11, panels A, B, and C show the fluorescence image that is obtained on treating NIH 3T3 fibroblasts with SH3 domain- binding peptides modified to include a fluorescent tag. In sharp contrast, panel D shows only a dark image that is produced when the cells are treated with a proline-rich segment of vinculin as a control. In another embodiment, an SH3 domain-binding peptide- horseradish immunoperoxidase complex or related immunohistochemical agent could be used to detect and quantitate specific receptor molecules in tissues, serum or body fluids. In particular, the present invention provides useful diagnostic reagents for use in immunoassays, Southern or Northern hybridization, and in situ assays. Accordingly, the diagnostic agents described herein may be suitable for use in vitro or in vivo . In addition, the diagnostic or imaging agent of the present invention is not limited by the nature of the detectable, label. Hence, the diagnostic agent may contain one or more such labels including, but not limited to, radioisotope, fluorescent tags, paramagnetic substances, heavy metals, or other image-enhancing agents. Those of ordinary skill in the art would be familiar with the range of label and methods to incorporate or conjugate them into the SH3 domain-binding peptide to form diagnostic agents. In yet a further embodiment, pharmaceutical compositions are provided comprising an SH3 domain-binding peptide and a pharmaceutically acceptable carrier. In a specific embodiment of the invention, the pharmaceutical composition is useful for the modulation of the activity of SH3 domain- containing proteins. By "modulation" is meant either inhibition or enhancement of the activity of the protein target. Accordingly, a pharmaceutical composition is disclosed comprising an SH3 domain-binding peptide and a pharmaceutically acceptable carrier, said peptide comprising: (i) a 9-mer sequence motif of the formula RXLPφφPX^ (SEQ ID NO:10), wherein X represents any amino acid except cysteine, ø represents a hydrophobic amino acid residue, and wherein is a hydrophilic amino acid residue except cysteine, each letter representing the standard one-letter designation for the corresponding amino acid residue; and, optionally, (ii) additional amino acid residues flanking the 9-mer sequence at its C-terminal end, N-terminal end or both, up to a total of 45 amino acid residues, including said 9-mer sequence. Preferably, the peptide comprises at least one, more preferably at least two, and most preferably at least three additional amino acids flanking the 9-mer sequence. As stated above, the therapeutic or diagnostic agents of the invention may also contain appropriate pharmaceutically acceptable carriers, diluents and adjuvants. Such pharmaceutical carriers can be sterile liquids, such as water and oils including those of petroleum, animal, vegetable or synthetic origin, such as peanut oil, soybean oil, mineral oil, sesame oil and the like. Water is a preferred carrier when the pharmaceutical composition is administered intravenously. Saline solutions and aqueous dextrose and glycerol solutions can also be employed as liquid carriers, particularly for injectable solutions. Suitable pharmaceutical excipients include starch, glucose, lactose, sucrose, gelatin, malt, rice, flour, chalk, silica gel, magnesium carbonate, magnesium stearate, sodium stearate, glycerol monostearate, talc, sodium chloride, dried skim milk, glycerol, propylene, glycol, water, ethanol and the like. These compositions can take the form of solutions, suspensions, tablets, pills, capsules, powders, sustained- release formulations and the like. Suitable pharmaceutical carriers are described in "Remington's Pharmaceutical Sciences" by E.W. Martin. Such compositions will contain an effective therapeutic amount of the active compound together with a suitable amount of carrier so as to provide the form for proper administration to the subject. While intravenous injection is a very effective form of administration, other modes can be employed, including but not limited to intramuscular, intraperitoneal, and subcutaneous injection, and oral, nasal, enteral, and parenteral administration. The therapeutic agents and diagnostic agents of the instant invention are used for the treatment and/or diagnosis of animals, and more preferably, mammals including humans, as well as dogs, cats, horses, cows, pigs, guinea pigs, mice and rats. Accordingly, other methods contemplated in the present invention, include, but are not limited to, a method of modulating, i.e., inhibiting or enhancing, bone resorption in a mammal (see, e.g., Hall, T.J., in Biochem. Biophys. Res. Commun. (1994) 199(3) :1237-44) , a method of disrupting protein tyrosine kinase-mediated signal transduction pathways or a method of regulating the processing, trafficking or translation of RNA in a cell by introducing or administering an effective amount of an SH3 domain-binding peptide of the present invention (see, e.g., Taylor, S.J. and Shalloway, D. , supra) . The diagnostic or therapeutic agents of the present invention can be modified by attachment to soluble acromolecules such as proteins, polysaccharides, or synthetic polymers. For example, the peptide could be coupled to styrene-maleic acid copolymers (see, e.g. , Matsumura and Maeda, Cancer Res. (1986) 46:6387), methacrylamide copolymers (Kopececk and Duncan, J. Controlled Release (1987) 6:315), or polyethylene glycol (PEG) (e.g., Hershfield and Buckley, N. Enαl. J. Med. (1987) 316:589; Ho et al., Drug Metab. Dispos. (1986) 14:349; Chua et al., Ann. Intern. Med. (1988) 109:114). The agents, if desired, are further targeted by attachment to an antibody, especially a monoclonal antibody. Such antibodies include but are not limited to chimeric, single chain, Fab fragments, and Fab expression libraries. In one embodiment the agent is coupled to the macromolecule via a degradable linkage so that it will be released in vivo in its active form. In another embodiment, the therapeutic or diagnostic agent may be delivered in a vesicle, in particular a liposome. See, Langer, Science (1990) 249:1527-1533; Treat et al., in Liposomes in the Therapy of Infectious Disease and Cancer, Lopez-Berestein and Fidler (eds.) , Liss, New York (1989) pp. 353-365; Lopez-Berestein, ibid. , pp. 317-327. In yet another embodiment, the therapeutic or in vivo diagnostic agent can be delivered in a controlled release system. In one embodiment, a pump may be used (see Langer, supra ; Sefton, CRC Crit. Ref. Biomed. Eng. (1987) 14:201; Buchwald et al.. Surgery (1980) 88:507; Saudek et al., N. Engl. J. Med. (1989) 321:574) . In another embodiment, polymeric materials may be used (see Medical Applications of Controlled Release, Langer and Wise (eds.), CRC Pres., Boca Raton, Florida, 1974; Controlled Drug Bioavailability, Drug Product Design and Performance, Smolen and Ball (eds.) Wiley, New York 1984; Raner and Peppas, J. Macro ol. Sci. Rev. Macromol. Chem. (1983) 23:61; see, also, Levy et al., Science (1985) 228:190; During et al., Ann. Neurol. (1989) 25:351; Howard et al., J. Neurosurq. (1989) ~>1:105) . In a preferred embodiment, a controlled release system may be placed next to the therapeutic target, thus requiring only a fraction of the systemic dose (see, e.g. , Goodson, in Medical Applications of Controlled Release, supra , (1984) 2:115-138) . It will be recognized by one of ordinary skill in the art that a particular advantage of the invention is that a peptide will not be subject to the problems of denaturation and aggregation associated with proteins held in the warm, most environment of a body in a controlled release system. Other controlled release systems are discussed in the review by Langer, in Science (1990) 249:1527-1533. 5.6. Identification of Compounds that Affect Binding of SH3 Domain-containing Proteins and their Ligands A common problem in the development of new drugs is that of identifying a single, or a small number, of compounds that possess a desirable characteristic from among a background of a large number of compounds that lack that desired characteristic. This problem arises both in the testing of compounds that are natural products from plant, animal, or microbial sources and in the testing of man-made compounds. Typically, hundreds, or even thousands, of compounds are randomly screened by the use of in vitro assays such as those that monitor the compound's effect on some enzymatic activity or its ability to bind to a reference substance such as a receptor or other protein. The compounds which pass this original screening test are known as "lead" compounds. These lead compounds are then put through further testing, including, eventually, in vivo testing in animals and humans, from which the promise shown by the lead compounds in the original in vitro tests is either confirmed or refuted. See Remington's Pharmaceutical Sciences. 1990, A.R. Gennaro, ed. , Chapter 8, pages 60-62, Mack Publishing Co., Easton, PA; Ecker and Crooke, 1995, Bio/Technology 13:351-360. There is, of course, a continual need for new compounds to be tested in the in vitro assays that make up the first testing step described above. There is also a continual need for new assays by which the pharmacological activities cf these compounds may be tested. It is an object of the present invention to provide such new assays to determine whether a candidate compound is capable of affecting the binding between a protein or polypeptide containing an SH3 domain and a ligand of the SH3 domain. A compound capable of affecting this binding would be useful as a means of modulating the pharmacological activity of proteins or polypeptides containing the SH3 domain. The present invention provides suitable ligands for SH3 domains for use in such assays. Such assays can be performed where the SH3 domains include, but are not limited to, SH3 domains from Cortactin, Nek, Abl, PLCγ, Src, p53bp2, Crk, Yes, and Grb2. The present invention provides methods of identifying a compound that affects the binding of a molecule comprising an SH3 domain and a ligand of the SH3 domain. The effect on binding can be an increase or decrease in total amount of binding or in affinity of bidning. Preferably, the effect is an inhibition (reduction in or loss of binding) . Accordingly, the invention provides a method of identifying an inhibitor of the binding between a first molecule comprising an SH3 domain and a second molecule that binds to the SH3 domain comprising incubating one or more compounds from which it is desired to select such an inhibitor with the first molecule and the second molecule under conditions conducive to binding and detecting the one or more compounds that inhibit binding of the first molecule to the second molecule. In a particular embodiment of the above-described metnod, the second molecule is obtained by: (i) screening a peptide library with the SH3 domain to obtain peptides that bind the SH3 domain; (ii) determining a consensus sequence for the peptides obtained in step (i) ; (iii) producing a peptide comprising the consensus sequence; wherein the second molecule comprises the peptide comprising the consensus sequence. In another embodiment, the second molecule is obtained by: (i) screening a peptide library with the SH3 domain to obtain peptides that bind the SH3 domain; (ii) determining a consensus sequence for the peptides obtained in step (i) ; (iii) searching a database to identify amino acid sequences that resemble the consensus sequence of step (ii) ; (iv) producing a peptide comprising an amino acid sequence identified in step (iii) ; wherein the second molecule comprises the peptide comprising an amino acid sequence identified in step (iii) - Second molecules that bind SH3 domains can be obtained by, e . g. , the use of diversity libraries, such as random or combinatorial peptide or nonpeptide libraries which can be screened for molecules that specifically bind to SH3 domains. Many libraries are known in the art that can be used, e . g. , chemically synthesized libraries, recombinant (e.g., phage display libraries) , and in vitro translation-based libraries. Examples of chemically synthesized libraries are described in Fodor et al., 1991, Science 251:767-773; Houghten et al., 1991, Nature 354:84-86; Lam et al., 1991, Nature 354:82-84; Medynski, 1994, Bio/Technology 12:709-710; Gallop et al., 1994, J. Medicinal Chemistry 37(9) :1233-1251; Ohlmeyer et al., 1993, Proc. Natl. Acad. Sci. USA 90:10922-10926; Erb et al. , 1994, Proc. Natl. Acad. Sci. USA 91:11422-11426; Houghten et al., 1992, Biotechniques 13:412; Jayawickreme et al., 1994, Proc. Natl. Acad. Sci. USA 91:1614-1618; Salmon et al., 1993, Proc. Natl. Acad. Sci. USA 90:11708-11712; PCT Publication No. WO 93/20242; and Brenner and Lerner, 1992, Proc. Natl. Acad. Sci. USA 89:5381-5383. Examples of phage display libraries are described in Scott and Smith, 1990, Science 249:386-390; Devlin et al., 1990, Science, 249:404-406; Christian, R.B., et al. , 1992, J. Mol. Biol. 227:711-718); Lenstra, 1992, J. Immunol. Meth. 152:149-157; Kay et al., 1993, Gene 128:59-65; and PCT Publication No. WO 94/18318 dated August 18, 19S4. In vitro translation-based libraries include but are not limited to those described in PCT Publication No. WO 91/05058 dated April 18, 1991; and Mattheakis et al., 1994, Proc, Natl. Acad. Sci. USA 91:9022-9026. By way of examples of nonpeptide libraries, a benzodiazepine library (see e .g. , Bunin et al., 1994, Proc. Natl. Acad. Sci. USA 91:4708-4712) can be adapted for use. Peptoid libraries (Simon et al. , 1992, Proc. Natl. Acad. Sci. USA 89:9367-9371) can also be used. Another example of a library that can be used, in which the amide functionalities in peptides have been permethylated to generate a chemically transformed combinatorial library, is described by Ostresh et al. (1994, Proc. Natl. Acad. Sci. USA 91:11138-11142). Screening the libraries can be accomplished by any of a variety of commonly known methods. See, e.g., the following references, which disclose screening of peptide libraries: Parmley and Smith, 1989, Adv. Exp. Med. Biol. 251:215-218; Scott and Smith, 1990, Science 249:386-390; Fowlkes et al., 1992; BioTechniques 13:422-427; Oldenburg et al., 1992, Proc. Natl. Acad. Sci. USA 89:5393-5397; Yu et al. , 1994, Cell 76:933-945; Staudt et al. , 1988, Science 241:577-580; Bock et al., 1992, Nature 355:564-566; Tuerk et al., 1992, Proc. Natl. Acad. Sci. USA 89:6988-6992; Ellington et al., 1992, Nature 355:850-852; U.S. Patent No. 5,096,815, U.S. Patent No. 5,223,409, and U.S. Patent No. 5,198,346, all to Ladner et al.; Rebar and Pabo, 1993, Science 263:671-673; and PCT Publication No. WO 94/18318. In a specific embodiment, screening can be carried out by contacting the library members with an SH3 domain immobilized on a solid phase and harvesting those library members that bind to the SH3 domain. Examples of such screening methods, termed "panning" techniques are described by way of example in Parmley and Smith, 1988, Gene 73:305-318; Fowlkes et al. , 1992, BioTechniques 13:422-427; PCT Publication No. WO 94/18318; and in references cited hereinabove. In another embodiment, the two-hybrid system for selecting interacting proteins in yeast (Fields and Song, 1989, Nature 340:245-246; Chien et al., 1991, Proc. Natl. Acad. Sci. USA 88:9578-9582) can be used to identify molecules that specifically bind to SH3 domains. A typical assay of the present invention consists of at least the following components: (1) a molecule (e.g., protein or polypeptide) comprising an SH3 domain; (2) a ligand of the SH3 domain; (3) a candidate compound, suspected of having the capacity to affect the binding between the protein containing the SH3 domain and the ligand. The assay components may further comprise (4) a means of detecting the binding of the protein comprising the SH3 domain and the ligand. Such means can be e.g., a detectable label affixed to the protein, the ligand, or the candidate compound. In another specific embodiment, the invention provides a method of identifying a compound that affects the binding of a molecule comprising an SH3 domain and a ligand of the SH3 domain comprising: (a) contacting the SH3 domain and the ligand under conditions conducive to binding in the presence of a candidate compound and measuring the amount of binding between the SH3 domain and the ligand; (b) comparing the amount of binding in step (a) with the amount of binding known or determined to occur between the molecule and the ligand in the absence of the candidate compound, where a difference in the amount of binding between step (a) and the amount of binding known or determined to occur between the molecule and the ligand in the absence of the candidate compound indicates that the candidate compound is a compound that affects the binding of the molecule comprising an SH3 domain and the ligand. A kit is provided that comprises, in one or more containers, one or more components cf the assay cf the invention, e.g., a first molecule comprising an SH3 domain and a second molecule that binds to the SH3 domain. In one embodiment, the assay comprises allowing the protein or polypeptide containing an SH3 domain to contact the ligand of the SH3 domain in the presence and in the absence of the candidate compound under conditions such that binding of the ligand to the protein containing an SH3 domain will occur unless that binding is disrupted or prevented by the candidate compound. By detecting the amount of binding of the ligand to the protein containing an SH3 domain in the presence of the candidate compound and comparing that amount of binding to the amount of binding of the ligand to the protein or polypeptide containing an SH3 domain in the absence of the candidate compound, it is possible to determine whether the candidate compound affects the binding and thus is a useful lead compound for the modulation of the activity of proteins containing the SH3 domain. The effect of the candidate compound may be to either increase or decrease the binding. One version of an assay suitable for use in the present invention comprises binding the protein containing an SH3 domain to a solid support such as the wells of a microtiter plate. The wells contain a suitable buffer and other substances to ensure that conditions in the wells permit the binding of the protein or polypeptide containing an SH3 domain to its ligand. The ligand and a candidate compound are then added to the wells. The ligand is preferably labeled, e . g . , it might be biotinylated or labeled with a radioactive moiety, or it might be linked to an enzyme, e.g., alkaline phosphatase. After a suitable period of incubation, the wells are washed to remove any unbound ligand and compound. Tf the candidate compound does not interfere with the binding of the protein or polypeptide containing an SH3 domain to the labeled ligand, the labeled ligand will bind to the protein or polypeptide containing an SH3 domain in the well. This binding can then be detected. If the candidate compound interferes with the binding of the protein or polypeptide containing an SH3 domain and the labeled ligand, label will not be present in the wells, or will be present to a lesser degree than is the case when compared to control wells that contain the protein or polypeptide containing an SH3 domain and the labeled ligand but to which no candidate compound is added. Of course, it is possible that the presence of the candidate compound will increase the binding between the protein or polypeptide containing an SH3 domain and the labeled ligand. Alternatively, the ligand can be affixed to solid substrate during the assay. The present invention provides ligands capable of binding SH3 domains that are suitable for incorporation into assays such as those described above. Ligands provided by the present invention include those SH3 domain-binding amino acid sequences disclosed in Tables 1-13 below and proteins or polypeptides containing those amino acid sequences. Also provided are nucleic acids encoding the SH3 domain-binding amino acid sequences disclosed in Tables 1-13 below. 6. EXAMPLES 6.1. Preparation of the TSAR-9 Library 6.1.1. Synthesis and Assembly of Oligonucleotides Figure 1 shows the formula of the oligonucleotides and the assembly scheme used in construction of the TSAR-9 library. The oligonucleotides were synthesized with an applied Biosystems 380a synthesizer (Foster City, CA) , and the full-length oligonucleotides were purified by HPLC Five micrograms of each of the pair of oligonucleotides were mixed together in buffer (10 mM Tris-HCl, pH 8.3, 15 mM KCl, 0.001% gelatin, 1.5 mM magnesium chloride) , with 0.1 % Triton X-100, 2 mM dNTP's, and 20 units of Tag DNA polymerase. The assembly reaction mixtures were incubated at 72 °C for 30 seconds and then 30 °C for 30 seconds; this cycle was repeated 60 times. It should be noted that the assembly reaction is not PCR, since a denaturation step was not used. Fill-in reactions were carried out in a thermal cycling, device (Ericomp, LaJolla, CA) with the following protocol: 30 seconds at 72 °C, 30 seconds at 30 °C, repeated for 60 cycles. The lower temperature allows for annealing of the six base complementary region between the two sets of the oligonucleotide pairs. The reaction products were phenol/chloroform extracted and ethanol precipitated. Greater than 90% of the nucleotides were found to have been converted to double stranded synthetic oligonucleotides. After resuspension in 300 μL of buffer containing 10 mM Tris-HCl, pH 7.5, 1 mM EDTA (TE buffer) , the ends of the oligonucleotide fragments were cleaved with Xba I and Xho I (New England BioLabs, Beverly, MA) according to the supplier's recommendations. The fragments were purified by 4% agarose gel electrophoresis. The band of correct size was removed and electroeluted, concentrated by ethanol precipitation and resuspended in 100 μL TE buffer. Approximately 5% of the assembled oligonucleotides can be expected to have internal Xho I or Xba I sites; however, only the full-length molecules were used in the ligation step of the assembly scheme. The concentration of the synthetic oligonucleotide fragments was estimated by comparing the intensity on an ethidium bromide stained gel run along with appropriate quantitated markers. All DNA manipulations not described in detail were performed according to Maniatis, supra . To demonstrate that the assembled enzyme digested oligonucleotides could be ligated, the synthesized DNA fragments were examined for their ability to self-ligate. The digested fragments were incubated overnight at 18 °C in ligation buffer with T4 DNA ligase. When the ligation products were examined by agarose gel electrophoresis, a concatamer of bands was visible upon ethidium bromide staining. As many as five different unit length concatamer bands (i.e., dimer, trimer, tetramer, pentamer, hexamer) were evident, suggesting that the synthesized DNA fragments were efficient substrates for ligation. 6.1.2. Construction of Vectors The construction of the M13 derived phage vectors useful for expressing a TSAR library has been recently described (Fowlkes, D. et al. BioTech. (1992) 13:422-427). To express the TSAR-9 library, an M13 derived vector, m663, was constructed as described in Fowlkes. The m663 vector contains the pill gene having a c-myc-epitope, i.e., as a stuffer fragment, introduced at the mature N-terminal end, flanked by Xho I and Xba I restriction sites (see also, Figure I of Fowlkes) . 6.1.3. Expression of the TSA -9 Library The synthesized oligonucleotides were then ligated to Xho I and Xba I double-digested m663 RF DNA containing, the pill gene (Fowlkes) by incubation with ligase overnight at 12 °C More particularly, 50 ng of vector DNA and 5 ng of the digested synthesized DNA and was mixed together in 50 μL ligation buffer (50 mM Tris, pH 8.0, 10 mM MgCl 2 , 20 mM DTT, 0.1 mM ATP) with T4 DNA ligase. After overnight ligation at 12°C, the DNA was concentrated by ethanol precipitation and washed with 70% ethanol. The ligated DNA was then introduced into E. coli (DH5αF'; GIBCO BRL, Gaithersburg, MD) by electroporation. A small aliquot of the electroporated cells was plated and the number of plaques counted to determine that 10 8 recombinants were generated. The library of E . coli cells containing recombinant vectors was plated at a high density (~400,000 per 150 mM petri plate) for a single amplification of the recombinant phage. After 8 hr, the recombinant bacteriophage were recovered by washing each plate for 18 hr with SMG buffer (100 mM NaCl, 10 mM Tris-HCl, pH 7.5, 10 mM MgCl 2 , 0.05% gelatin) and after the addition of glycerol to 50% were frozen at -80 °C The TSAR-9 library thus formed had a working titer of ~2 x 10 11 pfu/ml. 6.2. Preparation of the TSAR-12 Library Figure 2 shows the formula for the synthetic oligonucleotides and the assembly scheme used in the construction of the TSAR-12 library. As shown in Figure 2, the TSAR-12 library was prepared substantially the same as the TSAR-9 library described in Section 6.1 above with the following exceptions: (1) each of the variant non-predicted oligonucleotide sequences, i.e., NNB, was 30 nucleotides in length, rather than 54 nucleotides; (2) the restriction sites included at the 5' termini of the variant, non-predicted sequences were Sal I and Spe 1, rather than Xho I and Xba I; and (3) the invariant sequence at the 3' termini to aid annealing of the two strands was GCGGTG and CGCCAC rather than CCAGGT and GGTCCA (5' to 3'). After synthesis including numerous rounds of annealing and chain extension in the presence of dNTP's and Taq DNA polymerase, and purification as described above in Section 6.1.1, the synthetic double stranded, oligonucleotide fragments were digested with Sal I and Spe I restriction enzymes and ligated with T4 DNA ligase to the nucleotide sequence encoding the M13 pill gene contained in the m663 vector to yield a library of TSAR-expression vectors as described in Sections 6.1.2 and 6.1.3. The ligated DNA was then introduced into E . coli (DH5αF' ; GIBCO BRL, Gaithersburg, MD by electroporation. The library of E. coli cells were plated at high density (-400,000 per 150 mm petri plate) for amplification of the recombinant phage. After about 8 hr, the recombinant bacteriophage were recovered by washing, for 18 hr with SMG buffer and after the addition of glycerol to 50% were frozen at -80 °C The TSAR-12 library thus formed had a working titer of ~2 x 10 pfu/mL. 6.3. Characterization of the TSAR-9 and -12 Libraries The inserted synthetic oligonucleotides for each of the TS.AR libraries, described in Sections 6.1 and 6.2 above, had a potential coding complexity of 20 ?6 (~10 4? ) and 20 20 , respectively, and since -10 14 molecules were used in each transformation experiment, each member of these TSAR libraries should be unique. After plate amplification the library solution or stock has 10" copies of each member/mL. It was observed that very few (<10%) of the inserted oligonucleotide sequences characterized so far in both of the libraries have exhibited deletions or insertions. This is likely a reflection of the accuracy assembling the oligonucleotides under the conditions used and the fact that certain types of mutations (i.e., frame-shifts) would not be tolerated as pill an essential protein for phage propagation. In order to determine whether any coding bias existed in the variant non-predicted peptides expressed by these libraries, perhaps due to biases imposed in vitro during synthesis of the oligonucleotides or in vivo during expression by the reproducing phage, inserts were sequenced as set forth below. 6.3.1. Characterization of TSA -9 Library Inserted synthetic oligonucleotide fragments of 23 randomly chosen isolates were examined from the TSAR-9 library. Individual plaques were used to inoculate I ml of 2XYT broth containing E. coli (DH5αF') cells and the cultures were allowed to grow overnight at 37°C with aeration. DNA was isolated from the culture supematants according to Maniatis, supra . Twenty-three individual isolates were sequenced according to the method of Sanger (Proc. Natl. Acad. Sci. USA (1979) 74:5463-5467) using as a primer the oligonucleotide 5'-AGCGTAACGATCTCCCG (SEQ ID NO. 99), which is 89 nucleotides downstream of the pill gene cloning site of the m663 vector used to express the TSARS. Nucleotide sequences and their encoded amino acid sequences were analyzed with the MacVector computer program (IBI, New Haven, CT) . The Microsoft EXCEL program was used to evaluate amino acid frequencies. Such analyses showed that the nucleotide codons coding for and hence most amino acids, occurred at the expected frequency in the TSAR-9 library of expressed proteins. The notable exceptions were glutamine and tryptophan, which were over- and under- represented, respectively. It is of interest to note the paucity of TAG stop codons in the inserts, i.e., only 2 of -200 isolates characterized contained a TAG stop codon. About half [1-(47/48) 36 ] of the phage inserts were expected to have at least one TAG codon in view of the assembly scheme used. However, most of the TAG- bearing phage appear to have been lost from the library, even though the bacterial host was supE. This may be a consequence of suppression being less than 100% effective. The amino acids encoded by the inserted double stranded synthesized oligonucleotide sequences, excluding the fixed PG-encoding centers, were concatenated into a single sequence and the usage frequency determined for each amino acid using the Microsoft EXCEL program. These frequencies were compared to that expected from the assembly scheme of the oligonucleotides, and the divergence from expected values represented by the size of the bars above and below the baseline. Chi square analysis was used to determine the significance of the deviations. The majority of amino acids were found to occur at the expected frequency, with the notable exceptions that glutamine and tryptophan were somewhat over- and under-represented, respectively. Thus, except for the invariant Pro-Gly, any position could have any amino acid; hence, the sequences are unpredicted or random. 6.3.2. Characterization of TSAR-12 Library Approximately 10 randomly chosen inserted oligonucleotides from the TSAR-12 library were examined by DNA sequencing as described above in Section 6.3.1. The isolates were chosen at random from the TSAR-12 library and prepared for sequencing, as were the TSAR-9 isolates. Analysis showed that except for the invariant Gly any position could have any amino acid; hence, the sequences are unpredicted or random. 6.4. Preparation of R8C Library Referring now to Figure 3, two oligonucleotides were synthesized on an Applied Biosystems Model 380a machine with the sequence 5'- TGACGTCTCGAGTTGTNNKNNKNNKNNKNNKNNKNNKNNKTGTGGATCTAGAAGGATC-3' (SEQ ID NO:31) and 5'-GATCCTTCTAGATCC-3' (SEQ ID NO:32), where N is an equimolar ratio of deoxynucleotides A, C, G, and T, and K is an equimolar ratio of G and T. Fifty pmol of each oligonucleotide was incubated at 42 °C for 5 min, then 37 °C for 15 min, in 50 μL of Sequenase™ buffer (U.S. Biochemicals, Cleveland, OH) with 0.1 μg/μL acetylated BSA, and 10 mM DTT. After annealing, 10 units of Sequenase™ (U.S. Biochemicals) and 0.2 mM of each dNTP were added and incubated at 37 °C for 15 min. The sample was then heated at 65 °C for 2 hr, digested with 100 units of both Xho I and Xba I (New England BioLabs, Beverly, MA) , phenol extracted, ethanol precipitated, and resolved on a 15% non-denaturing polyacrylamide gel. The assembled, digested fragment was gel purified prior to ligation. The vector, m663 (Fowlkes, D. et al. Biotech. (1992) 13:422-427), was prepared by digestion with Xho I and Xba I, calf alkaline phosphatase (Boehringer Mannheim, Indianapolis, IN) treatment, phenol extracted, and purified by agarose gel electrophoresis. To ligate, 20 μg vector was combined with 0.2 μg insert in 3 mL with T4 DNA ligase (Boehringer Mannheim) , according to the manufacturer. After removal of the protein and buffer by phenol extraction and ethanol precipitation, the ligated DNA was electroporated into XLl-Blue E. coli (Stratagene, San Diego, CA) and plated for eight hours at 37 °C To recover the recombinant phage, the top agar was collected with a spatula, mixed with an equal volume of 100 mM NaCl, 10 mM MgCl 2 , and 50 mM Tris-HCl (pH7.5), and disrupted by two passes through an 18-gauge syringe needle. The bacterial cells were removed by centrifugation, and phage particles were collected by polyethylene glycol precipitation and stored at -70 °C in 25% glycerol. The library had 10 6 total recombinants and a working titer of 6 x 10 12 pfu/mL. Members of the library were checked for inserts by the polymerase chain reaction (Saiki, et al. Science (1988) 239:487-491). Individual plaques on a petri plate were touched with a sterile toothpick and the tip was stirred into 2xYT with F* E . coli bacteria and incubated overnight at 37 °C with aeration. Five microliters of the phage supernatant were then transferred to new tubes containing buffer (67 mM Tris-HCl, pH 8.8/10 mM β- mercaptoethanol/16.6 mM ammonium sulfate/6.7 mM EDTA/50 μg bovine serum albumin per mL) , 0.1 mM deoxynucleotide triphosphates, and 1.25 units of Taq DNA polymerase (Boehringer Mannheim, Indianapolis, IN) with 100 p oles of oligonucleotide primers. The primers flanked the cloning site in gene III of m663 (5'-TTCACCTCGAAAGCAAGCTG-3' (SEQ ID NO:100) and 5'-CCTCATAGTTAGCGTAACG-3' (SEQ ID NO:101)). The assembly reactions were incubated at 94 °C for 1 min, 56 °C for 2 min, and 72 °C for 3 min; this cycle was repeated 24 times. The reaction products were then resolved by electrophoresis on a NuSieve 2.0% agarose gel (FMC, Rockland, ME) . Gels revealed that for 20 plaques tested, all were recombinant and had single inserts of the expected size. Based on the sample size of the library, it was anticipated that 100% of the recombinants had single inserts. However, all of the SH3-binding phage isolated from the R8C library had double-inserts. Such phage are presumed rare (i.e., <5%) within the library, yet because the SH3-binding peptide appears to need to be linear they were selected for by our screening methods. Most likely they were formed during the generation of the library; one scenario is that the inserts ligated together to form head-to-head dimers and that they were subsequently cloned into m663 DNA by ligation with the vector's Xho I sticky end and by illegitimate ligation with the vector's Xba I site (see, Figure 4) . 6.5. Preparation Of Target-Coated Microtiter Wells 6.5.1. Preparation Of GST-SH3 Fusion Proteins The preparation of Src-GST fusion protein was first described by Smith and Johnson, in Gene (1988) 67:31, the disclosure of which is incorporated by reference herein. Briefly, pGEX-derived (Pharmacia, Piscataway, NJ) constructs expressing GST fusion proteins containing the SH3 domains of Src, Grb2, Crk, Abl, or PLCγ were obtained from Dr. Channing Der (University of North Carolina at Chapel Hill) ; a construct expressing the SH3 domain of Yes was obtained from Dr. Marius Sudol (Rockefeller University) . The use of the pGEX bacterial expression vector for the production of GST- SH3 fusion proteins is well-known to those in the art. See, e.g., Cicchetti, P. et al., in Science (1992) 257:803-806. Briefly, the coding region for a particular SH3 domain can be fused in-frame at the Bam HI site of pGEX-2T. Thus, fusion proteins were prepared as per the manufacturer's instructions, and quantified by Coomassie Blue staining of SDS-polyacrylamide gels. Microtiter wells were coated with 5-20 μg GST-SH3 fusion protein in 100 mM NaHC0 3 , pH 8.5, blocked with 100 mM NaHC0 3 (pH 8.5) 1% BSA, and washed. All washes consisted of five applications of 1XPBS, 0.1% Tween 20, 0.1% BSA (Buffer A). Where appropriate, the amount of protein bound to each well was quantified with an anti-GST antibody-based ELISA (Pharmacia, Piscataway, NJ) , and with a 5 GST-binding phage, isolated during the course of this work. 6.5.2. Coating of Microtiter Wells Bacterially expressed Src SH3 glutathione-S-transferase (Src-GST) fusion protein was purified from bacterial lysates 0 using glutathione agarose 4B (Pharmacia) , according to the manufacturer's instructions. Bound Src-GST fusion protein was eluted from the glutathione agarose with 10 mM glutathione in PBS. Microtiter wells were then coated with Src-GST fusion protein (1-10 μg/well, in 50 mM NaHC0 3 , pH 8.5) 5 overnight at 4 °C To block non-specific binding of phage, 100 μL 1% BSA in 100 mM NaHC0 3 , pH 8.5, was added to each well and allowed to incubate at room temperature for 1 hour. The wells were then washed five times with 200 μL PBS, 0.1% Tween 20, 0.1% BSA (Buffer A). 0 6.6. Biopanning And Subsequent Characterization Of Phage-Displayed Random Peptide Libraries With Src-GST Fusion Protein As Target Molecule 6.6.1. Isolation of Src SH3-Binding Phage _ Library screens were performed as previously described. Kay, B.K., et al., in Gene (1993) 128:59-65. Briefly, 1 X 10 11 pfu TSJAR 9, TSAR 12, or R8C phage in Buffer A were incubated in a Src SH3-GST-coated well for 2 hours. The wells were washed, and bound phage were eluted with 100 μL 50 mM 0 glycine-HCl (pH 2.2), transferred to a new well, and neutralized with 100 mL 200 mM NaHP0 4 (pH 7.0). Recovered phage were used to infect 1 x lθ 9 DH5Q.F' E. coli cells in 20 L 2xYT; the infected cells were grown overnight, resulting in a 1000- to 10,000-fold amplification of phage titer. Amplified phage were panned twice more, as above, excepting the amplification step. Binding phage recovered after the third round of panning were plated at a low density on a lawn of DH5αF' E . coli cells to yield isolated plaques for clonal analysis. Isolated plaques were used to produce small cultures from which phage stocks and DNA were recovered for phage binding experiments and dideoxy sequencing (Sanger, F. , et al., in Proc. Natl. Acad. Sci. USA (1977) 74:5463-5467), respectively. Clones were confirmed as binding the SH3 domain by applying equal titers of phage to wells containing Src SH3-GST or GST alone, and titering the number of eluted particles from each well, or detecting bound phage with an anti-phage antibody-based ELISA (Pharmacia) . Indeed, the ability of isolated phage clones to bind to several SH3 domains derived from a variety of different proteins can be investigated by the manner described above. GST-SH3 fusion proteins containing SH3 domains from a variety of different proteins are bound to microliter wells. An aliquot of the aforementioned phage stocks (50 μL) is introduced into wells containing the different GST-SH3 fusion proteins. After room temperature incubation for 1-2 hours, the liquid contents of the microtiter plates are removed, and the wells are washed 5 times with 200 μL Buffer A. Bound phage are eluted with 100 μL 50 mM glycine (pH 2.2), transferred to a new well, and neutralized with 100 μl. 200 mM NaHP0 4 (pH 7.0). The phage are diluted 10 3 - to 10- 6 -fold, and aliquots are plated onto lawns of DH5αF' E . coli cells to establish the number of plaque forming units in the output sample. From these experiments, the relative specificity of different Src SH3 binding clones for SH3 domains derived from other proteins is determined. 6.6.2. Phage ELISA and Nucleotide Sequencing To evaluate the binding of isolates to various targets proteins, enzyme-linked-immuno-assays (ELISA) were also performed. Bacterial cultures were infected with phage isolates and cultured overnight in 2XYT at 37 °C The cells were spun down and 25 mL of supernatant was added to microtiter plate wells coated with 50 μL of protein (1 mg/mL in 100 mM NaHC0 3 , pH 8.4; overnight at 4 °C or for a few hours at room temperature) and blocked (1 mg/mL BSA in 100 mM NaHC0 3 , pH 8.4; for about one hour). The phage are incubated in the well with 25 μL of PBS-0.1% Tween 20 at RT for 2 hr. The wells are then washed multiple times over 30 minutes. To each well is added 50 μL of polyclonal anti-phage antibody conjugated to horseradish peroxidase. The antibody is diluted 1:3000 in BSA-PBS-Tween 20; it was obtained from Pharmacia (Piscataway, NJ; catalog number 27-9402-01) . After 30 minutes, the wells are washed again with BSA-PBS-Tween 20 for -20 minutes. Finally, 100 μL of ABTS reagent (Pharmacia, with H 2 0 2 ) are added to each well for the development of color. Plates are read with a plate reader (Molecular Devices, Menlo Park, CA) at 405 nm wavelength. The nucleotide sequence of the relevant segments of the Src SH3 binding clones (or phage clones that bind to SH3 domains of other proteins) were sequenced using standard methods. Sanger, F. , et al. , in Proc. Natl. Acad. Sci, USA (1977) 74:5463-5467. The oligo primer 5'-AGCGTAACGATCTAAA--3' (SEQ ID NO:102) was used, which is 89 nucleotides downstream of the gene III cloning site of M13 m666. The nucleotide sequences were analyzed with the MacVector computer program (IBI, New Haven, CT, USA). From this nucleotide sequence information the primary sequence of each Src SH3 binding peptide was deduced. The corresponding synthetic peptides were then prepared by techniques well known in the art with or without flanking sequences. Indeed, these synthetic peptides have been shown to bind to SH3 domain targets, with those possessing the phage flanking amino acid residues exhibiting greater binding affinity. 6.7.In Vi tro Peptide Binding Assays Peptides were obtained from Research Genetics (Birmingham, AL) , Chiron Mimotopes (Victoria, Australia) , or synthesized by conventional techniques by Dr. J. Mark Carter of Cytogen Corporation (Princeton, NJ) . Peptide purity was assessed by HPLC and/or mass spectrometry. Biotinylated peptides were synthesized with either a KSGSG (SEQ ID NO:103) or a GSGS (SEQ ID NO:104) peptide linker (a spacer) between the biotin and the N-terminus of the peptide. Binding experiments were performed as above, excepting the use of 10 μM peptide instead of phage. Bound biotinylated peptide was detected with streptavidin conjugated to alkaline phosphatase (Sigma Chemical Co., St. Louis, MO). After one hour incubation period at room temperature, the wells were washed, and a solution of 3 mM p-nitrophenyl-phosphate (US Biochemicals, Cleveland, OH) in 50 mM NaC0 3 (pH 9.8) , and 50 mM MgCl 2 was added and color allowed to develop . Signals were read with an ELISA plate reader (Molecular Devices, Menlo Park, CA) at 405 nm wavelength. Binding experiments were performed in triplicate. The results are presented in Figures 7 and 8. 6.8> Peptide Competition of GST-SH3 Affinity Precipitations of Cell Lysates Labeled proteins are prepared by incubating a culture of HeLa cells overnight with 100 μCi/mL J5 S-methionine. The cells are then washed and lysed with mild detergent. This mixture of radioactive proteins is incubated with Src-GST fusion protein that has been immobilized cn glutathione- linked Sepharose beads (Pharmacia, Piscataway, NJ) . After several hours of tumbling, the beads are pelleted gently by low-speed centrifugation, and the supernatant is discarded. The beads are then resuspended into a slurry in PBS-0.1% Tween 20, pelleted, and washed several additional times. Finally, a 2% SDS solution is added to the sample, which is then boiled at 100 °C for 3 minutes. Afterward, the sample is centrifuged, and the supernatant loaded on a 10% polyacrylamide SDS gel for electrophoresis. After the proteins have been resolved, the gel is fixed, dried down, and exposed to X-ray film for autoradiography or phosphor plates for scanning by a Molecular Dynamics PhosphorImager. The ability of Src SH3 to bind certain 35 S-labeled proteins is examined for competability with exogenous peptides. Synthetic peptides corresponding to phage- displayed inserts and motifs are added at the time that the lysate is incubated with the Src-GST fusion protein immobilized on glutathione-linked sepharose beads. The SH3 binding peptides block binding of all or some of the labeled proteins while negative control peptides (unrelated peptide sequences) do not. The amount of competition is quantified and correlated with the amount of added SH3-domain binding peptides. Alternatively, NIH 3T3 cells were grown in Dulbecco's Modified Eagle Medium (DME) + 10% fetal calf serum (FCS) + 80 μCi/mL Tran 35 Slabel (ICN) , washed with PBS, lysed in RIPA buffer, and pelleted. Supernatant from 1.5 x 10 6 cells was precleared with 100 μg glutathione-agarose-immobilized GST. The supernatant was then incubated with 10 μg glutathione- agarose-immobilized GST-SH3 fusion protein with or without added test peptide in a final volume of 250 μL. Pelleted beads were washed with 1 mL each of RIPA, RIPA + 1% deoxycholate + 0.1% SDS, and PBS. resuspended in 50 μL SDS-PAGE sample buffer, boiled, and subjected to SDS-PAGE (7.5%). Labeled proteins were detected by phosphorimaging (Molecular Dynamics). The results are presented in Figure 9. 6.9. Peptide Competition of GST-SH3 Affinity Precipitations of PI-3' Kinase From Cell Lysates It is possible to follow the precipitation of PI-3' Kinase by Src from cell lysates in the presence or absence of SH3-binding peptides. HeLa cells are lysed with detergent and the protein mixtures are incubated for several hours with the Src-GST fusion protein immobilized on glutathione-linked Sepharose beads. After several hours of tumbling, the beads are pelleted gently by low-speed centrifugation and the supernatant is discarded. The beads are then resuspended into a slurry in PBS-0.1% Tween 20, pelleted, and washed several additional times. Finally, an SDS solution is added to the sample, which is then boiled at 100 °C for 3 minutes. Subsequently, the sample is centrifuged, and the supernatant is loaded on a 10% polyacrylamide SDS gel for electrophoresis. After the proteins have been resolved, the gel is blotted to nitrocellulose or nylon (i.e., western blot). The filter is then probed with a PI-3' Kinase antibody (monoclonal and polyclonal antibodies are available from Upstate Biotechnology Incorporated, Lake Placid, NY) and an enzyme-linked secondary antibody. The amount of PI-3' Kinase is then quantitated. The ability of Src SH3 to bind PI-3' Kinase is examined for competability with exogenous peptides. Synthetic peptides corresponding to phage-displayed inserts and motifs are added at the time that the lysate is incubated with the Src-GST fusion protein that has been immobilized on glutathione-linked sepharose beads. Ten-fold and one hundred-fold molar excess of peptides are used relative to SH3 proteins. The SH3 binding peptides block binding of the PI-3' Kinase as detected on western blots while negative control peptides (unrelated peptide sequences) do not. The amount of competition is quantified and correlated with the amount of added SH3-domain binding peptides. 6.10. In Vivo Association Of SH3-Binding Peptides With SH3-Domains Of Proteins To demonstrate association of the SH3-binding peptides with SH3-domains of proteins inside cells, the SH3-binding peptides are tagged and localized in cells. For example, Bar-Sagi et al., in Cell (1993) 74:83-91, have shown that SH3-binding proteins localize to the cytoskeleton when expressed in cells. Thus, the SH3 domain-binding peptides of the present invention can serve as cellular targetting signals (e.g., to the cytoskeleton). Accordingly, the peptides are tagged with biotin and, subsequently, injected into cells. Alternatively, one can transfect into cells a recombinant plasmid that expresses a fusion protein comprising of the SH3-binding peptide and the green fluorescent protein (GFP, Chalfie et al., in Science (1994) 263:802-805). The location of the biotinylated peptide or the GFP fusion protein is then assayed with FITC-labeled streptavidin in paraformaldehyde-fixed cells or by direct fluorescence in living cells, respectively. Localization of the SH3-binding peptides to the cytoskeleton demonstrates that the SH3-binding peptides can bind SH3-domain proteins in vivo . In addition, focal adhesions, which are rich in Src, are also sites of potential subcellular localization of SH3- binding peptides. Thus, NIH 3T3 fibroblasts were cultured in vitro on glass coverslips coated with fibronectin. After two days of growth at 37 °C, the cells were fixed for one hour at room temperature in the presence of 2% paraformaldehyde (pH 7.5). The coverslips were washed with PBS-0.1% Tween 20 several times to remove the fixative. Next, the coverslips were dipped into acetone (chilled at -2C °C) for approximately 20 seconds and allowed to air-dry. The coverslips were washed again with PBS-0.1% Tween 20, containing BSA (1 mg/mL) , and incubated for 2 hours at room temperature with different biotinylated peptides in PBS-0.1% Tween 20. The coverslips were washed and then incubated with 1 mg/mL streptavidin-Cy3 (Jackson Immunoresearch Co., West Grove, PA) for 1 hour at room temperature. Finally, the coverslips were washed in PBS-0.1% Tween 20, mounted in a glycerol solution on a glass slide, and viewed with a Nikon Optiphot epifluorescence microscope and a 60x oil immersion lens. The results are presented in Figure 11, in which panel A displays cells stained with the conjugate biotin-spacer- VLKRPLPIPPVTR (SEQ ID NO:64); panel B exhibits cells stained with the conjugate, biotin-spacer-GILAPPVPPRNTR (SEQ ID NO:63); panel C shows cells stained with the long consensus peptide, biotin-spacer-RSTPRPLPPLPTTR (SEQ ID NO:67); and panel D shows cells stained with the proline-rich vinculin peptide conjugate, biotin-spacer-LAPPKPPLPEGEV (SEQ ID N0:70). The "spacer" sequence is KSGSG (SEQ ID N0:l03). As shown in Figure 11, the panels in which SH3 domain-binding peptides were used present a bright display of fluorescence activity that is in sharp contrast to the relatively "dark" features of panel D (non-SH3 domain binding vinculin segment) . These results demonstrate further the ability of the SH3 domain-binding peptides of the present invention to localize to protein targets (e.g., Src and Src-related proteins) within cells and provide an image thereof. 6.11. In Vivo Modulation Of Src In Oocytes With SH3-Binding Peptides When Xenopus laevis oocytes are injected with mRNA encoding deregulated Src, there are dramatic cytological and biochemical changes in the oocyte (Unger, T.F. and Steele, R.E., in Mol. Cell. Biol. (1992) 12:5485-5498). The applicants have obtained piasmids for generating wild type and deregulated Src mRNA, which are available from Dr. Robert Steele (University of California at Irvine) . Synthetic SH3- binding peptides are injected into oocytes that have been previously injected with Src mRNA. The state of the cytoskeleton i inspected visually by observing the arrangement of cortical pigment granules under a dissecting microscope. The state of phosphorylation of several proteins is examined by western blotting with an anti-phosphotryosine monoclonal antibody (4G10; Upstate Biotechnology Incorporated) , as described in Unger and Steele, above. 6.12. Progesterone-induced X. laevis Oocyte Maturation Segments of adult ovary were removed surgically and incubated in 0.1% collagenase type D (Boehringer .Mannheim, Indianapolis, IN) in Ca 2+ -free 0R2 (82.5 mM NaCl, 2.5 mM KCl, 1.0 mM MgCl 2 , 1.0 mM Na 2 HP0 4 , 5.0 mM HEPES, and 3.8 mM NaOH, pH 7.6). Oocytes were then washed 3-5 times with OR2 containing 1.0 mM CaCl 2 and allowed to recover in 0R2 overnight at 18 °C Stage VI oocytes were injected with 40 nL of 100 mM peptide or water. After injection, the oocytes were placed in 0R2 with 2 mg/mL progesterone (Sigma, St Louis, MO) and incubated at 20 °C Oocytes were scored at hourly time points for germinal vesicle breakdown (GVBD) . Figure 10 presents the results of this experiment. As shown by the graph, oocytes injected with the SH3 domain- binding peptide VLKRPLPIPPVTR (SEQ ID NO:64) exhibit a faster rate of progesterone-induced germinal vesicle breakdown relative to oocytes that had been injected with water or with the proline-rich vinculin peptide, LAPPKPPLPEGEV (SEQ ID NO:70). These results parallel those of Unger and Steele, supra , wherein oocytes injected with deregulated or active Src RNA matured at a faster rate than oocytes injected with water or wild-type Src mRNA (See Figrure 3B of the Unger and Steele article) . The present results obtained with Src SH3 domain-binding peptides suggest that these peptides modulate the biochemical activity of "cellular" Src; in particular, it is proposed that at least some of the Src SH3 domain-binding peptides of the present invention upregulate the biochemical activity of "cellular" Src, which may be downregulated or inhibited in its normal state. Hence, the administration of the SH3 domain-binding peptides of the present invention can constitute a novel method of modulating the activity of Src or Src-related proteins. Specifically, certain of these peptides are able to activate Src-family proteins. 6.13. In Vivo Antagonism Of Src In Src Transformed Cells With SH3-Binding Peptides The coding regions for SH3-binding peptides are cloned into vectors that direct their expression in animal cells. A bipartite gene is constructed, encoding a protein with c-myc epitope and SH3-binding peptide, which is transcribed from a strong constitutive promoter (e.g., SV40, CMV, HSV TK, calmodulin) . The vector is introduced into either normal or Src-transformed cells via transfection (e.g., electroporation, calcium phosphate, liposomes, DEAE dextran) . Transfected cells express the bipartite gene transiently in culture. To create stable transformed cell lines, the vector carries a selectable marker (e.g., neomycin resistance) or transfection is performed in the presence of excess plasmid carrying a selectable marker (e.g., neomycin resistance) and cells selected for the marker. Transfected cells are stained by immunofluorescence to detect expression of the bipartite protein. The hybridoma 9E10 secretes a monoclonal antibody that is highly specific for the c-myc epitope (EQKLISEEDLN [SEQ ID NO:105]; see, Evan, G.A. et al. , in Mol. Cell. Biol. (1985) 5:3610-3616). This antibody is used in immunofluorescence experiments to demonstrate that the bipartite protein is expressed inside the cells, and in some cases, localized to subcellular structures enriched in SH3 domain bearing proteins. There are several controls used in these experiments. First, cells are transfected with vectors that do not have the SH3-binding peptide coding region. Second, normal (non- transformed) cells are transfected. Third, cells transformed by oncogenes other than Src are used in the transfection experiments. Fourth, cells are stained with other monoclonal antibodies that do not recognize the c-myc epitope„ Transfected cells are examined for any changes in cell shape, behavior, and metabolism as a consequence of expressing the SH3 binding peptides. Cell shape is examined by phase contrast microscope at several times after transfection; in particular, the flatness of the cells, their adhesion to the substrate, and the degree of cell ruffling are monitored. Cell division rates, cell migration, and contact inhibition are also observed over time. Finally, the amount of phosphorylated tyrosine in transfected cells is quantitated by phosphoaminoacid analysis and with an anti- phosphotryosine monoclonal antibody (4G10; Upstate Biotechnology Incorporated) in western blotting experiments. 6.14. Distinct Ligand Preferences of Various SH3 Domains 6.14.1. Preparation of PXXP (SEQ ID NO: 161) Biased Peptide Libraries Using procedures similar to those described in Sections 6.1 and 6.4 and also described in Sparks, A. B. , et al., in Methods in Enzymoloqy, (1995) 255:498-509, oligonucleotide inserts were constructed according to the schematic provided in FIG. 12. The two synthetic oligonucleotides (5'- ctgtgcctcgagk(nnk) 6 cca(nnk) 2 cca(nnk) 6 tctagacgtgtcagt-3' (SEQ 10 ID NO:162) and 5'-actgacacgtctaga-3' (SEQ ID NO:163), where k=g+t and n=g+aκt+c) were annealed and filled in with Sequenase (Amersham, Arlington Heights, II) . The inserts were then digested with Xho I and Xba I and were ligated into gene III of the mBAX vector. The mBAX vector was created by generating cloning sites in gene III of the M13mpl8 vector (Messing, J. , 1991, "Cloning in M13 phage or how to use biology at its best," Gene 100, 3-12) in the manner of Fowlkes et al., 1992, Biotechniques 13, 422-427. The mBAX vector displays a peptide sequence at the N-terminus of the mature pill protein that encodes the epitope for the mouse monoclonal antibody 7E11 (see FIG. 13) ; it includes the stop codon TAG in the coding region, which is suppressed in E. coli carrying suppressor tRNA gene mutations known as supE or supF . There are no other stop codons in the mBAX genome. The mBAX vector also carries a segment of the alpha fragment of jβ-galactosidase; bacterial cells expressing the omega fragment of 3-galactosidase that are infected by a bacteriophage that expresses the alpha fragment convert the clear XGal substrate into an insoluble blue precipitate. Thus, plaques of such bacteriophage on such cells appear blue. Recombinant mBAX molecules can be distinguished easily __ from non-recombinant molecules due to the TAG codon in the Xhol - XJ al segment in gene III of mBAX. When recombinants are generated by replacing the Xho I - Xba I fragment with oligonucleotides encoding random peptides, the recombinants can be grown in bacteria with (e.g., DH5 F') or without (e.g., JS5) suppressor tRNA mutant genes. On the other hand, the non-recombinant mBAX molecules fail to produce plaques on bacterial lawns where the bacteria (e.g., JS5) lack such suppressor genes. This is because in JS5, the TAG codon serves as a stop codon to yield a truncated pill molecule during translation; since pill is an essential protein component of viable M13 viral particles, no plaques will form. The ligated DNA was electroporated into JS5 E . coli and recombinant phage were propagated on two hundred 100 mm 2xYT + 0.8% agar plates as described in Sambrook, J. , Frisch, Ξ, F. , & Maniatis, T. (1989) Molecular Cloning: A Laboratory Manual (Cold Spring Harbor Laboratory, Plainview, NY) (Sambrook et al.). To minimize the recovery of sibling clones during affinity purification of binding phage, six distinct library fractions were prepared by dividing the plates into six roughly equal groups. Each fraction was treated separately in all subsequent manipulations. Phage particles were harvested from each fraction by diffusion into 100 ml PBS (137 mM NaCl, 2.7 mM KCl, 4.3 mM Na 2 HP0 4 , 1.4 mM KH 2 P0 4 ) , concentrated by polyethylene glycol precipitation as in Sambrook et al. (1989, supra) , and resuspended in 10 ml PBS + 10% glycerol. Each fraction contained approximately 5xl0 7 unique recombinants, for a total library complexity of approximately 3xl0 8 . The resulting phage-displayed library contained peptides of the form X S PXXPX 6 (SEQ ID NO:164), where X represents any amino acid. 6.14.2. Affinity purification of SH3-binding phage Library screens were performed as described in Sparks, A. B., et al., in Methods in Enzvmoloqy. (1995) 255:498-509. Briefly, wells of an ELISA microtiter plate were coated with 10 μg GST-SH3 fusion protein in 100 mM NaHC0 3 (pH 8.5) for 3 hours and blocked with Superblock (Pierce, Rockford, IL) for 1 hour. Approximately 5 x 10 11 infectious particles from each library fraction were diluted in 200 μl PBS + 0.1% Tween 20 and incubated in a GST-SH3-coated well for 3 hours. The wells were washed five times with PBS + 0.1% Tween 20, and bound phage were eluted with 50 mM glycine-HCl (pH 2.2) . Recovered phage were propagated in 10 ml 2xYT media and 100 μl of a saturated DH5αF' E . coli culture and affinity purified twice more as above. Affinity purified phage were plated onto 2xYT + 0.8% agar plates to yield isolated plaques from which clonal phage stocks and DNA were produced. Phage binding was confirmed by incubating equal amounts of a clonal phage stock in wells coated with 1 μg GST-SH3 or GST. The wells were washed five times with PBS + 0.1% Tween 20, and bound phage were detected by anti-phage ELISA according to the manufacturer's instructions (Pharmacia, Piscataway, NJ) . Clones with strong SH3-binding activity were selected for further analysis. The sequences of peptides displayed by these clones were determined by DNA sequencing of phage inserts. 6.14.3 * Preparation of GST-SH3 fusion proteins Constructs encoding GST fusions to the Grb2 N-terminal (Grb2 N, aa 1-58), Nek N-terminal (Nek N, aa 1-68), Nek middle (Nek M, aa 101-166) , Nek C-terminal (Nek C, aa 191- 257) , p53bp2 (aa 454-530), or Src (aa 87-143) SH3 domains were generated by PCR cloning of the appropriate cDNAs into pGEX-2T (Pharmacia, Piscataway, NJ; a general reference for the pGEX vectors is Smith, D. B. , & Johnson, K. S. (1988) Gene 67, 31-40) . The integrity of the constructs was confirmed by DNA sequencing. pGEX-derived constructs expressing GST fusions to the SH3 domains of Yes, Cortactin, Crk, Abl, and PLCγ were kindly provided by M. Sudol (Rockefeller University) , J. T. Parsons (University of Virginia at Charlottsville) , M. Matsuda (Tokyo, Japan) , A. M. Pendergast (Duke University) , and S. Earp (University of North Carolina at Chapel Hill) , respectively. Alternatively, the GST-SH3 fusion proteins for Yes, Cortactin, Crk, Abl, and PLCγ could have been prepared as above for Grb2 N, Nek N, Nek M, Nek C, p53bp2, and Src, using published sequence information for these proteins. See, e.g., Suen et al., (1993) Mol . Cell . Biol . 13, 5500-5512 (Grb2); Lehmann et al., (1990) Nucleic Acids Res . 18, 1048 (Nek); Iwabuchi et al., (1994) Proc . Natl . Acad . Sci . USA 91, 6098-6102 (p53bp2) ; Takeya et al., (1983) Cell 32, 881-890 (Src); Sudol et al., (1988) Nucleic Acids Res . 16, 9876 (Yes); Wu et al., (1991) Mol . Cell . Biol . 11, 5113-5124 (Cortactin); Matsuda et al. , (1992) Mol . Cell . Biol . 12, 3482-3489 (Crk); Shtivelman et al., (1986) Cell 47, 277-284 (Abl); Burgess et al., (1990) Mol . Cell . Biol . 10, 4770-4777 (PLCγ) . GST-SH3 fusion proteins were prepared as described in Smith, D. B. , & Johnson, K. S. (1988) Gene 67, 31-40. The integrity and purity of the fusion proteins were confirmed by SDS-PAGE. Protein concentrations were determined α inq a the BioRad protein assay (BioRad, Hercules, CA) . 6.14.4. SH3 Domain Binding Peptides and Consensus Sequences The use of second generation or biased peptide libraries, which fix all or part of the PXXP (SEQ ID NO: 161) consensus motif for SH3 domain binding peptides and randomize flanking residues, has defined additional sequence residues exhibiting selective SH3 domain binding. Tables 1-5, below, list some of the relevant amino acid sequences obtained when the biased peptide library described in Section 6.14.1 was screened with GST-SH3 fusion proteins. The underscored amino acid residues in Tables 1-5 indicate the fixed positions. Also, indicated for each set of new binders is a "consensus" sequence, which seeks to include the additional features gleaned from the new binding peptides. The symbol "ø" in the consensus sequences of Tables 1-5 represents a hydrophobic residue. The symbol x in the consensus sequences of Tables 1-5 represents any amino acid. For the Nek SH3 domain binding clones, a GST-SH3 fusion protein containing the middle SH3 domain of Nek was used. TABLE 1 CORTACTIN SH3-BINDING PEPTIDES TABLE 2 NCK SH3-BINDING PEPTIDES TABLE 3 ABL SH3 BINDING PEPTIDES TABLE 5 SRC SH3-BINDING PEPTIDES In addition to the consensus sequence shown in Table 5, the amino acid sequences of the inserts from the Src SH3 domain-binding phage isolated from the PXXP (SEQ ID NO: 161) biased peptide library described in Section 6.14.1 also give rise to the consensus sequence LXXRPLPX^P (SEQ ID NO: 165), as shown in Table 6, below. In the consensus sequence LXXRPLPXi/P (SEQ ID NO:165), represents aliphatic amino acid residues (A, V, L, I, P) ; X represents any amino acid. TABLE 6 Src SH3 Binding Peptides LASRPLPLLPNSAPGQ LTGRPLPALPPPFSDF PAYRPLPRLPDLSVIY RALRVRPLPPVPGTSL DAPGSLPFRPLPPVPT LKWRALPPLPETDTPY ISQRALPPLPLMSDPA LTSRPLPDIPVRPSKS NTNRPLPPTPDGLDVR MKDRVLPPIPTVESAV LQSRPLPLPPQSSYPT FINRRLPALPPDNSLL FRALPLPPTPDNPFAG LYSAIAPDPPPRNSSS♦ LXXRPLPX^P « CONSENSUS In Table 6, represents aliphatic amino acid residues (A, V, L, I, P) ; X represents any amine acid; ♦ putative class II peptide (see Section 6.14.5). Invariant proline residues are underlined. Another consensus sequence that can be derived from the amino acid sequences of the inserts from the Src SH3 domain- binding phage is: LX 1 X 2 RPLPX 3 ιPX 4 X 5 (SEQ ID NO: 454) where ψ represents aliphatic amino acid residues (A, V, L, I, P) and X l t X 2 , X 3 , X 4 , and X 5 represent any amino acid; except that if X 3 = P, ψ = 1,, X 4 = P, and X 5 = P, then: where X λ = F, then X 2 is not H or R; or where X = S, then X 2 is not R, H, A, N, T, G, V, M, or W; or where X α = c, then X 2 is not S or G; or where X 1 = R , then X 2 is not T or F; or where X j = A, then X 2 is not R, Q, N, S, or L; or where X α = Q, then X 2 is not M; or where X 1 = L, then X 2 is not R; or where X λ = I, then X 2 is not A; or where X x = P, then X 2 is not P, W, or R; or where X λ = G, then X 2 is not S or R; or where X λ = T, then X 2 is not T. In addition to the consensus sequence shown in Table 1, the amino acid sequences of the inserts from the cortactin SH3 domain-binding phage isolated from the PXXP (SEQ ID NO: 161) biased peptide library described in Section 6.14.1 also give rise to the consensus sequence +PPι/'PXKPXWL (SEQ ID NO:166) , as shown in Table 7, below. TABLE 7 Cortactin SH3 Binding Peptides LTPQSKPPLPPKPSAV (a portion of SEQ ID NO: 112) SSHNSRPPLPEKPSWL (a portion of SEQ ID NO: 111) PVKPPLPAKPWWLPPL (SEQ ID NO: 167) TERPPLPQRPDWLSYS (a portion of SEQ ID NO:109) LGEFSKPPIPQKPTWM (a portion of SEQ ID NO: 108) YPQFRPPVPPKPSLMQ (SEQ ID NO:168) VTRPPLPPKPGHMADF (SEQ ID NO: 169) VSLGLKPPVPPKPMQL (SEQ ID NO:170) LLGPPVPPKPQTLFSF (a portion of SEQ ID NO: 107) YKPEVPARPIWLSEL (SEQ ID NO:171) GAGAARPLVPKKPLFL (SEQ ID NO: 172) +PP^PXKPXWL = CONSENSUS (SEQ ID NO:166) In Table 7, + represents basic amino acid residues (R, K ; φ represents aliphatic amino acid residues (A, V, L, I, P) ; X represents any amino acid. Invariant proline residues are underlined. In addition to the consensus sequence shown in Table 3 , the amino acid sequences of the inserts from the Abl SH3 domain-binding phage isolated from the PXXP (SEQ ID NO: 161) biased peptide library described in Section 6.14.1 also give rise to the consensus sequence PPXøXPPPi/P (SEQ ID NO: 173), as shown in Table 8, below. In Table 8, θ represents aromatic amino acid residues; ψ represents aliphatic amino acid residues (A, V, L, I, P) ; X represents any amino acid. Invariant proline residues are underlined. t This clone contained a three nucleotide deletion in the random peptide coding sequence. The amino acid sequences of the inserts from the PLCγ SH3 domain-binding phage isolated from the PXXP (SEQ ID NO:161) biased peptide library described in Section 6.14.1 give rise to the consensus sequence PPVPPRPXXTL (SEQ ID NO:175), as shown in Table 9, below. TABLE 9 PLCγ SH3 Binding Peptides MPPPVPPRPPGTLQVA (SEQ ID NO: 176) LSYSPPPVPPRPDSTL (SEQ ID NO: 177) VLAPPVPPRPGNTFFT (SEQ ID NO: 178) YRPPVAPRPPSSLSVD (SEQ ID NO:179) LQCPDCPRVPPRPIPI (SEQ ID NO:180) VPPLVAPRPPSTLNSL (a portion of SEQ ID NO: 143) LTPPPFPKRPRWTLPE (SEQ ID NO: 181) YWPHRPPLAPPQTTLG (SEQ ID NO: 182) PPVPPRPXXTL = CONSENSUS (SEQ ID NO: 75) In Table 9, the symbol X represents any amino acid. Invariant proline residues are. underlined. The PXXP (SEQ ID NO: 161) biased peptide library described in Section 6.14.1 was also used to obtain phagti clones that specifically bound the SH3 domain from the p53op2 protein. The amino acid sequences of the peptides expressed by the p53bp2 SH3 domain-binding phage are shown in Table 10 below. TABLE 10 p53bp2 SH3 Binding Peptides YDASSAPORPPLPVRKSRP (SEQ ID NO:183) EYVNASPERPPIPGRKSRP (SEQ ID NO: 184) WNGIAIPGRPEIPPRASRP (SEQ ID NO:185) SMIFIYPERPSPPPRFSRP (SEQ ID NO:186) GVEEWNPERPQIPLRLSRP (SEQ ID NO:187) WWDSRPDIPLRRSLP (SEQ ID NO:188) WPLGRPEIPLRKSLP (SEQ ID NO: 189) GGTVGRPPIPERKSVD (SEQ ID NO:190) YSHAGRPEVPPRQSKP (SEQ TD NO: 191) FS.AAARPDIPSRASTP (SEQ ID NO:192) LYIPKRPEVPPRRHEA (SEQ ID NO:193) NNISARPPLPSRQNPP (SEQ ID NO: 194) MAGTPRPAVPQRMNPP SEQ ID NO:195) RPX^PlR+SXP = CONSENSUS (SEQ ID NO: 196) In Table 10, + represents basic amino acid residues (R, K) ; ψ represents aliphatic amino acid residues (A, V, L, I, P) ; X represents any amino acid. Invariant proline or flanking residues are underlined. The PXXP (SEQ ID NO:161) biased peptide library described in Section 6.14.1 was also used to obtain phage clones that specifically bound the SH3 domain from the N terminal portion of the Crk protein. The amino acid sequences of the peptides expressed by the Crk N terminal SH3 domain-binding phage are shown in Table 11 below. TABLE 11 Crk N SH3 Binding Peptides GQPAGDPDPPPLPAKF FEQTGVPLLPPKSFKY IFGDPPPPIPMKGRSL SNQGSIPVLPIKRVQY NYVNALPPGPPLPAKN SSDPERPVLPPKLWSV HFGPSKPPLPIKTRIT DWKVPEPPVPKLPLKQ ATSEGLPILPSKVGSY NANVSAPRAPAFPVKT EMVLGPPVPPKRGTW AGSRHPPTLPPKESGG SVAADPPRLPAKSRPQ ^ ^LP^K = CONSENSUS In Table 11, represents aliphatic amino acid residues (A, V, L, I, P) . Invariant proline residues are underlined. The present invention provides a purified peptide that binds to the SH3 domain of Crk, the purified peptide comprising the amino acid sequence (SEQ ID NO:210), where ψ represents aliphatic amino acid residues (A, V, L, I, P) , with the proviso that the peptide does not comprise the amino acid sequence WNERQPAPALPPKPPKPT (SEQ ID NO:456) . The PXXP (SEQ ID NO: 161) biased peptide library described in Section 6.14.1 was also used to obtain phage clones that specifically bound the SH3 domain from the Yes protein. The amino acid sequences of the peptides expressed by the Yes SH3 domain-binding phage are shown in Table 12 below. TABLE 12 Yes SH3 Binding Peptides ITMRPLPALPGHGQIH (SEQ ID NO:211) LPRRPLPDLPMAAGKG (SEQ ID NO:212) LGSRPLPPTPRQWPEV (SEQ ID NO:213) STIRPLPAIPRDTLLT (SEQ ID NO:214) RSGRPLPPIPEVGHNV (SEQ ID NO:215) IGSRPLPWTPDDLGSA (SEQ ID NO:216) LAQRELPGLPAGAGVS (SEQ ID NO:217) IPGRALPELPPQRALP (SEQ ID NO:218) FVGRELPPTPRTVIPW (SEQ ID NO:219) DPRSALPALPLTPLQT (SEQ ID NO:220) SPHDVLPALPDSHSKS (SEQ ID NO:221) ^XXRPLPXLP = CONSENSUS (SEQ ID NO:222) In Table 12, φ represents aliphatic amino acid residues (A, V, L, I, P) ; X represents any amino acid. Invariant proline residues are underlined. Another consensus sequence that can be derived from the amino acid sequences of the inserts from the Yes SH3 domain- binding phage is: ^X 1 X 2 RPLPX 3 LPX 4 X 5 (SEQ ID NO:455) where φ represents aliphatic amino acid residues (A, V, L, I, P) and x lf X 2/ X 3 , X 4 , and X 5 represent any amino acid; except that if X 3 = P, X 4 = P, and X b = P, then: when φ = L, where X, = F, then X 2 is not H or R; or where X α = S, then X 2 is not R, H, A, N, T, G, V, M, or W; or where X x = C, then X 2 is not S or G; or where X x = R, then X 2 is not T or F; or where X = A, then X 2 is not R, Q, N, S, or L; or where X 1 = Q, then X 2 is not M; or where X x = L, then X 2 is not R; or then X 2 is not A; or then X 2 is not P, W, or R; or then X 2 is not S or R; or then X 2 is not T; and then X 2 is not R; or then X 2 is not R or Y; or then X 2 is not S; or then X 2 is not G; or then X 2 is not S; or then X 2 is not R; and then X 2 is not K; and then X 2 is not C or Q; or then X 2 is not P; and then X 2 is not H; or then X ? is not S; or then X- is not S. The present invention also provides a purified peptide that binds to the SH3 domain of Yes, the purified peptide comprising the amino acid sequence ι/ ' X 1 X 2 RPLPX 3 LPX 4 X s (SEQ ID NO:455), where φ represents aliphatic amino acid residues (A, V, L, I, P) and X l t X 2 , X 3 , X 4 , and X 5 represent any amino acid, with the proviso that the peptide does not comprise the amino acid sequence AGDRPLPPLPYNPKS (SEQ ID NO:457). The PXXP (SEQ ID NO:161) biased peptide library described in Section 6.14.1 was also used to obtain phage clones that specifically bound the SH3 domain from the N terminal portion of the Grb2 protein. The amino acid sequences of the peptides expressed by the Grb2 N terminal SH3 domain-binding phage are shown in Table 13 below. These sequences can be arranged into three groups of sequences that have different, but related, consensus sequences. An overall consensus sequence, +0DXPLPXLP (SEQ ID NO:223), can be derived for the three groups. TABLE 13 Grb2 N SH3 Binding Peptides KWDSLLPALPPAFTVE (SEQ ID NO:224) RWDQVLPELPTSKGQI (SEQ ID NO:225) RFDFPLPTHPNLQKAH (SEQ ID NO:226) RLDSPLPALPPTVMQN (SEQ ID NO:227) RWGAPLPPLPEYSWST (SEQ ID NO:228) YWDMPLPRLPGEEPSL (SEQ ID NO:229) RFDYNLPDVPLSLGTA (SEQ ID NO:230) TKKPNAPLPPLPAYMG (SEQ ID NO:231) KWDLDLPPEPMSLGNY (SEQ ID NO:232) +0DXPLPXLP = CONSENSUS (SEQ ID NO:223) YYQRPLPPLPLSHFES (SEQ ID NO:234) YYRKPLPNLPRGQTDD (SEQ ID NO:235) YFDKPLPESPGALMSL (SEQ TD NO:236) YFSRALPGLPERQEAH (SEQ ID NO:237) Y0X+PLPXLP = CONSENSUS (SEQ ID NO:238) SLWDPLPPIPQSKTSV (SEQ ID NO:239) SYYDPLPKLPDPGDLG (SEQ ID NO:240) KLYYPLPPVPFKDTKH (SEQ ID NO:241) DPYDALPETPSMKASQ (SEQ ID NO:242) 0DPLPXLP = CONSENSUS (SEQ ID NO:243) +0DXPLPXLP = OVERALL CONSENSUS (SEQ ID NO:223) In Table 13, + represents basic amino acid residues (R, K) ; θ represents aromatic amino acid residues; X represents any amino acid. Invariant proline residues are underlined. 6.14.5. SH3 Ligand Binding Orientation Peptide ligands bound to SH3 domains have been shown to assume a left-handed polyproline type II (PPII) helix conformation (Yu, H. , Chen, J. K. , Feng, S., Dalgarno, D. C, Brauer, A. W. , & Schreiber, S. L. (1994) Cell 76, 933-45). SH3 ligands are pseudo-symmetrical and may therefore bind in one of two opposite orientations (Feng, S., Chen, J. K. , Yu, H. , Simon, J. A. , & Schreiber, S. L. (1994) Science 266, 1241-7) (Feng et al.). Feng et al . , supra , have demonstrated that peptides that bind in one or the other orientation share different consensus motifs. Specifically, ligands that bind in the Class I or Class II orientation conform to the consensus +pYPpYP (SEQ ID NO:244) or YPpYPp+ (SEQ ID NO:245) respectively, where uppercase positions represent conserved residues that contact the SH3 domain and confer specificity, and lowercase positions represent scaffolding residues that tend to be proline. According to this model, we predict that the peptides selected by the Src, Yes, Abl, and Grt N SH3 domains bind in the Class I orientation, whereas the peptides selected by the Cortactin, p53bp2, PLCγ, and Crk N SH3 domains bind in the Class II orientation (see Table 14) . Interestingly, most of the SH3 ligand consensus motifs identified in this work contain additional conserved residues flanking the SH3- binding core defined by Feng et al., supra . Furthermore, these conserved residues are situated N- and C-terminal of the SH3-binding core in Class I and Class II motifs, respectively, and are therefore predicted to interact with equivalent regions of their target SH3 domains (see Table 14) . TABLE 14 Alignment of SH3 ligand consensus motifs SEQ ID NO: Class I + P 244 165 222 173 223 246 243 245 166 196 247 210 In Table 14, each SH3 ligand consensus motif was assigned to class I or II based on its agreement with the class T or II consensus motif. Highly (>90%) conserved positions in each SH3 ligand consensus motif are listed in boldface and were interpreted as SH3 contact residues. + represents basic amino acid residues (K, R) ; φ represents aliphatic amino acid residues (A, V, L, I, P) ; θ represents aromatic amino acid residues; X represents any amino acid; lower case p represents residues that tend to be proline. The Src SH3 domain is capable of binding both Class I and Class II peptides Feng et al., supra . Although Class I peptides predominate in the population of Src SH3 ligands selected from the PXXP (SEQ ID NO: 161) library, one clone conforms well to the Class II consensus (see Table 6) . Previously, Sparks, A. B. , Quilliam, L. A., Thorn, J. M. , Der, C. J., & Kay, B. K. (1994) J. Biol. Chem. 269, 23853-6 and Yu, H. , Chen, J. K. , Feng, S., Dalgarno, D. C, Brauer, A. W., & Schreiber, S. L. (1994) Cell 76, 933-45 had isolated Class II Src SH3 ligands sharing the consensus PPi/'PPR (SEQ ID NO:248). Similarly, whereas the Grb2 N SH3 domain has been shown to bind peptides from SOS with the Class II consensus sequence PPι<PPR (SEQ ID NO:248) (Rozakis-Adcock, M. , Fernley, R. , Wade, J. , Pawson, T. , & Bowtell, D. (1993) Nature 363, 83-5) , we have isolated Grb2 N SH3 ligands that conform to the Class I consensus (see Table 14) . Thus, both the Src and the Grb2 N SH3 domains apparently have the capacity to bind both Class I and Class II peptide ligands. 6.14.6. SH3 Ligand Binding Characteristics To explore further the capacity of SH3 domains to discriminate between different SH3 ligands, we investigated the binding of phage expressing various peptide ligands to a panel of SH3 domains. Equal titers of clonal phage stocks were incubated in microtiter wells coated with different GST- SH3 fusion proteins. The wells were washed several times, and bound phage were detected with an anti-phage antibody (see Fig. 14) . Positive ELISA signals were equivalent to those obtained with previously characterized Src SH3-binding clones (Sparks, A. B., Quilliam, L. A., Thorn, J. M. , Der, C J., & Kay, B. K. (1994) J. Biol. Chem. 269, 23853-6) and are indicative of SH3:peptide affinities in the 5 to 75 μM range (Yu, H. , Chen, J. K. , Feng, S., Dalgarno, D. C , Brauer, A. W. , & Schreiber, S. L. (1994) Cell 76, 933-945; Rickles, R. J. , Botfield, M. C, Weng, Z., Taylor, J. A., Green, 0. M. , Brugge, J. S., & Zoller, M. J. (1994) EMBO J. 13, 5598-604). Whereas the Src, Yes, Crk, and Grb2 N SH3 domains cross- reacted with a few phage clones selected with other SH3 domains, the Abl, Cortactin, p53bp2, and PLCγ SH3 domains displayed considerable specificity. Significantly, only 33 of 220 potential instances of cross-reactivity were observed, suggesting that SH3 selectivity is the rule rather than the exception. Each instance of cross-reactivity may be explained by similarities between the sequences of the peptides and the ligand preferences of the cross-reactive SH3 domains. For example, Crk SH3 cross-reacted with three phage clones selected with other SH3 domains; each of these clones coincidentally expressed peptides conforming to the Crk SH3 preferred ligand consensus motif. Similarly, the cross- reactivity observed between the Src, Yes, and Grb2 SH3 domains and clones selected by other SH3 domains within this group may be a consequence of the fact that these SH3 domains prefer the same proline-rich core. Finally, the Src and Yes SH3 domains cross-reacted with the PLCγ SH3 ligand MPPPVPPRPPGTL (a portion of SEQ ID NO: 176), which contains the Class II Src SH3-binding sequence PPVPPR (SEQ ID N0:249). Taken together, these data demonstrate the capacity of SH3 domains to discern subtle differences in the primary structure of potential ligands. 6.15. Use of consensus Sequences to Determine the Amino Acid Sequences Responsible for Binding in Proteins that are Known to Bind SH3 Domains There are many proteins that are known to bind SH3 domains but for which the specific sequences of those proteins that are responsible for binding to SH3 domains are not known. The consensus sequences shown above in Tables 1- 13 can be used to search databases (e.g., GenBank) containing the amino acid sequences of those proteins in order to determine which sequences are responsible for the binding of those proteins to SH3 domains. This was done for a number of known SH3 domain binding proteins and sequences resembling the consensus sequences of Tables 1-13 were identified. The results are shown in Table 15. For comparison, also shown in Table 15 are the amino acid sequences that had previously been demonstrated to be responsible for SH3 domain binding for a number of proteins. PLCγ SH3 PPVPPRPXXTL 175 Hs Dynamin (812-823) APPVPSRPGASP 270 6 (829-840) PPQVPSRPNRNR 271 6 SEQ ID NO: Reference Hs c-Cbl (493-504) LPPVPPRLDLLP 272 7 Crk N SH3 VφLPφK 210 Hs Abl (524-533) *QAPELPTKTR 273 13 (568-577) *VSPLLPRKER 274 13 (758-767) EKPALPRKRA 275 13 Hs C3G (282-291) *PPPALPPKKR 276 14 (452-461) *TPPALPEKKR 277 14 (539-548) *KPPPLPEKKN 278 14 (607-616) *PPPALPPKQR 279 14 Grb2 N SH3 Class I He c-Cbl (560-571) 8 (589-600) 8 Grb2 N SH3 Class II HS Abl (523-533)* 13 (567-577)* 13 (609-619)* 13 Hs c-Cbl (491-501) 8 Hs Dynamin(810-820) 6 (827-837) 6 (838-848) 6 Hs SOSl (1148-1158)* 15 (1177-1187) 15 (1209-1219)* 15 (1287-1297)* 15 Rn Synapsin 1(592-602) 16 (670-680) 16 In Table 15, + represents basic amino acid residues (R, K) ; φ represents aliphatic amino acid residues (A, V, L, I, P) ; θ represents aromatic amino acid residues; X represents any amino acid. * represents amino acid sequences previously demonstrated to bind their respective SH3 domains. Residues within the sequences that agree with the most highly conserved residues of the consensus motifs are shown in bold. Each entry shows an abbreviation of the name of the SH3 domain binding protein and the species from which it was derived. The amino acid positions in the mature proteins of the sequences shown are indicated in parentheses. For more details, see the reference listed for each protein. Reference 1 is Flynn, D. C, Leu, T. H. , Reynolds, A. B., & Parsons, J. T. (1993) Mol Cell Biol 13, 7892-7900. Reference 2 is Barfod, E. T. , Zheng, Y., Kuang, W, J. , Hart, M. J. , Evans, T. , Cerione, R. A., & Ashkenazi, A. (1993) J Biol Chem 268, 26059-62. Reference 3 is Weng, Z., Thomas, S. M. , Rickles, R. J. , Taylor, J. A., Brauer, A. W. , Seidel-Dugan, C, Michael, W. M., Dreyfuss, G. , & Brugge, J. S. (1994) Mol Cell Biol 14, 4509-21. Reference 4 is Sudol, M. (1994) Oncogene 9, 2145-52. Reference 5 is Gertler, F. B. , Comer, A. R. , Juang, J. L., Ahern, S. M. , Clark, M. J. , Liebl, E. C, & Hoffmann, F. M. (1995) Genes Dev 9, 521-33. Reference 6 is Gout, I., Dhand, R. , Hiles, I. D. , Fry, M. J., Panayotou, G. , Das, P., Truong, 0. , Totty, N. F., Hsuan, J., Booker, G, W. & et al. (1993) Ceil 75, 25-36. Reference 7 is Rivero-Lezcano, 0. M. , Sameshima, J. H. , Marcilla, A., & Robbins, K. C (1994) J Biol Chem 269, 17363- 6. Reference 8 is Odai, H., Sasaki, K. , Iwamatsu, A., Hanazono, Y. , Tanaka, T., Mitani, K. , Yazaki, Y. & Hirai, H. (1995) J Biol Chem 270, 10800-5. Reference 9 is Kapeller, R. , Prasad, K. V., Janssen, 0., Hou, W., Schaffhausen, B. S., Rudd, C. E. , & Cantley, L. C. (1994) J Biol Chem 269, 1927-33. Reference 10 is Weng, Z., Taylor, J. A., Turner, C E. , Brugge, J. S. , & Seidel-Dugan, C (1993) J Biol Chem 268, 14956-63. Reference 11 is Karlsson, T., Songyang, Z. ; Landgren, E., Lavergne, C, Di-Fiore, P. P., Anafi, M. , Pawson, T. , Cantley, L. C, Claesson-Welsh, L. , & Welsh, M. (1995) Oncogene 10, 1475-83. Reference 12 is Ren, R. , Mayer, B. J. , Cicchetti, P., & Baltimore, D. (1993) Science 259, 1157-61. Reference 13 is Ren, R. , Ye, Z. S., & Baltimore, D. (1994) Genes Dev 8, 783-95. Reference 14 is Knudsen, B. S., Feller, S. M., & Hanafusa, H.. (1994) J Biol Chem 269, 32781-7. Reference 15 is Rozakis-Adcock, M. , Fernley, R. , Wade, J., Pawson, T. , & Bowtell, D. (1993) Nature 363, 83-5. Reference 16 is McPherson, P, S., Czernik, A, J. , Chilcore, T, J. , Onofri, F. , Benfenati, F. , Greengard, P., Schlessinger, J. , & De-Camilli, P. (1994) Proc Natl Acad Sci USA 91, 6486-90. The sequences shown in Table 15 are useful in that they can be used as ligands in the assays for the identification of compounds that affect binding cf SH3 domain-containing proteins and their ligands that is described above in Section 5.6. 6.16. Use of Consensus Sequences to Identify Amino Acid Sequences Resembling SH3 Domain-binding Sequences in Proteins that are Mot Known to Bind SH3 Domains The consensus sequences shown above in Tables 1-13 can be used to search databases (e.g., GenBank) containing the amino acid sequences of proteins that are not known to bind to SH3 domains. In this way, a large number of proteins not previously suspected of containing amino acid sequences that bind SH3 domains have been shown to contain such sequences. The portions of the amino acid sequences of these proteins that resemble one or more of the consensus motifs of Tables 1-13 are shown below in Table 16. The SH3 domain-binding sequences of the proteins shown in Table 16 can be used as ligands in the assays for the identification of compounds that affect binding of SH3 domain-containing proteins and their ligands that are described above in Section 5.6. TABLE 16 In Table 16, locus and accession number refer to the entries' names and accession numbers in GenBank or the Swiss- Prot database. The two numbers immediately to the left of the displayed sequences refer to the amino acid positions of the displayed sequences in the mature proteins. The leftmost of these two numbers refers to the starting amino acid number of the displayed sequence in the mature protein. The numbers in parentheses immediately to the right of the displayed sequences refer to the sequences' SEQ ID NOs: . The eight columns to the extreme right of Table 16 show the discrepancies between the displayed sequences and the consensus motifs of Tables 6-15. The leftmost Src column refers to Class I motifs; the rightmost Src column refers to Class II motifs. It should be apparent to one of ordinary skill that many other embodiments of the present invention can be contemplated beyond the preferred embodiments described above but which other embodiments nevertheless fall within the scope and spirit of the present invention. Hence, the present invention should not be construed to be limited to the preferred embodiments described herein, which serve only to illustrate the present invention, but only by the claims that follow. Also, numerous references are cited throughout the specification. The complete disclosures of these references are incorporated by reference herein.

Claims: WHAT IS CLAIMED IS: 1. A purified peptide that binds to the SH3 domain of Cortactin, said peptide comprising the amino acid sequence ZPPφPxKPxW (SEQ ID NO: 113) , where Z represents K or R; φ represents a hydrophobic amino acid; and x represents any amino acid. 2. A purified peptide that binds to the middle SH3 domain of Nek, said peptide comprising the amino acid sequence øxxxxxPxPPøRZxSL (SEQ ID NO: 127), where Z represents S or T; φ represents a hydrophobic amino acid; and x represents any amino acid. 3. A purified peptide that binds to the SH3 domain of Abl, said peptide comprising the amino acid sequence PPxWxPPPøP (SEQ ID NO: 141), where φ represents a hydrophobic amino acid; and x represents any amino acid. 4. A purified peptide that binds to the SH3 domain of Src, said peptide comprising the amino acid sequence LXXRPLPXi'P (SEQ ID NO:165), where φ represents an aliphatic amino acid; and X represents any amino acid. 5. A purified peptide that binds to the SH3 domain of Cortactin, said peptide comprising the amino acid sequence -i-PPyPXKPXWL (SEQ ID NO: 166) , where + represents a basic amino acid; φ represents an aliphatic amino acid: and X represents any amino acid. 6. A purified peptide that binds to the SH3 domain of Abl, said peptide comprising the amino acid sequence PPX0XPPPι/'P (SEQ ID NO: 173), where θ represents an aromatic amino acid; φ represents an aliphatic amino acid; and X represents any amino acid. 7. A purified peptide that binds to the SH3 domain of PLCγ, said peptide comprising the amino acid sequence PPVPPRPXXTL (SEQ ID NO:175), where X represents any amino acid. 8. A purified peptide that binds to the SH3 domain of p53bp2, said peptide comprising the amino acid sequence RPX^Pi^R+SXP (SEQ ID NO:196), where + represents a basic amino acid; φ represents an aliphatic amino acid; and X represents any amino acid. 9. A purified peptide that binds to the N terminal SH3 domain of Crk, said peptide comprising the amino acid sequence φPφltPφK (SEQ ID NO:210), where φ represents an aliphatic amino acid; and X represents any amino acid. 10. A purified peptide that binds to the SH3 domain of Yes, said peptide comprising the amino acid sequence 1/XXRPLPXLP (SEQ ID NO:222), where φ represents an aliphatic amino acid; and X represents any amino acid. 11. A purified peptide that binds to the N terminal SH3 domain of Grb2, said peptide comprising an amino acid sequence selected from the group consisting of: +0DXPLPXLP (SEQ ID NO:223), Y0X+PLPXLP (SEQ ID NO:238), and 0DPLPXLP (SEQ ID NO:243), where θ represent an aromatic amino acid; + represents a basic amino acid; φ represents an aliphatic amino acid; and X represents any amino acid. 12. A purified peptide that binds to the SH2 domain cf Cortactin, said peptide comprising an amin acid sequence selected from the group consisting of: LTPQSKPPLPPKPSAV (a portion of SEQ ID NO: 112); SSHNSRPPLPEKPSWL (a portion of SEQ ID NO: 111); PVKPPLPAKPWWLPPL (SEQ ID NO:167); TERPPLPQRPDWLSYS (a portion of SEQ ID NO: 109); LGEFSKPPIPQKPTWM (a portion of SEQ ID NO: 108); YPQFRPPVPPKPSLMQ (SEQ ID NO: 168); VTRPPLPPKPGHMADF (SEQ ID NO: 169); VSLGLKPPVPPKPMQL (SEQ ID NO: 170); LLGPPVPPKPQTLFSF (a portion of SEQ ID NO: 107); YKPEVPARPIWLSEL (SEQ ID NO: 171); GAGAARPLVPKKPLFL (SEQ ID NO:172); and SREPDWLCPNCPLLLRSDSR (SEQ ID NO:110). 13. A purified peptide that binds to the middle SH3 domain of Nek, said peptide comprising an amino acid sequence selected from the group consisting of: SSLGVGWKPLPPMRTASLSR (SEQ ID NO: 114) ; SSVGFADRPRPPLRVESLSR (SEQ ID NO:115) ; SSAGILRPPEKPXRSFSLSR (SEQ ID NO: 116) SSPYTGDVPIPPLRGASLSR (SEQ ID NO:117) SSLMGSWPPVPPLRSDSLSR (SEQ ID NO:118) SSIGEDTPPSPPTRRASLSR (SEQ ID NO:119) SRSLSEVSPKPPIRSVSLSR (SEQ ID NO:120) SSVSEGYSPPLPPRSTSLSR (SEQ ID NO:121) SSSFTLAAPTPPTRSLSLSR (SEQ ID NO:122) SSPPYELPPRPPNRTVSLSR (SEQ ID NO:123) SRWDGLAPPPPVRLSSLSR (SEQ ID NO:124) SSLGYSGAPVPPHRxSSLSR (SEQ ID NO: 125); and SSISDYSRPPPPVRTLSLSR (SEQ ID NO: 126). 14. A purified peptide that binds to the SH3 domain of Abl, said peptide comprising an amino acid sequence selected from the group consisting of: PPWWAPPPIPNSPQVL (SEQ ID NO: 174); PPKFSPPPPPYWQLHA (a portion of SEQ ID HO: 132); PPHWAPPAPPAMSPP1 (a portion of SEQ ID NO:130); PPTWTPPKPPGWGWF (a portion of SEQ ID NO: 137); FPSFAPPAAPPRHSFG (a portion of SEQ ID NO: 33) ; PTYPPPPPPDTAKGA (a portion of SEQ ID NO:135); GPRWSPPPVPLPTSLD (a portion of SEQ ID NO:128) APTWSPPALPNVAKYX (a portion of SEQ ID NO: 138) PPDYAAPAIPSSLWVD (a portion of SEQ ID NO: 129) IKGPRFPVPPVPLNGV (a portion of SEQ ID NO:139) PPAWSPPHRPVAFGST (a portion of SEQ ID NO:140) APKKPAPPVPMMAHVM (a portion of SEQ ID NO:134) SSDRCWECPPWPAGGQRGSR (SEQ ID NO:131); and SSPPXXXPPPIPNSPQVLSR (SEQ ID NO:136). 15. A purified peptide that binds to the SH3 domain of PLCγ, said peptide comprising an amino acid sequence selected from the group consisting of: MPPPVPPRPPGTLQVA (SEQ ID NO:176); LSYSPPPVPPRPDSTL (SEQ ID NO:177); VLAPPVPPRPGNTFFT (SEQ ID NO:178); YRPPVAPRPPSSLSVD (SEQ ID NO:179) ; LQCPDCPRVPPRPIPI (SEQ ID NO:180); VPPLVAPRPPSTLNSL (a portion of SEQ ID NO:143); LTPPPFPKRPRWTLPE (SEQ ID NO: 181) ; YWPHRPPLAPPQTTLG (SEQ ID NO: 182); SSMKVHNFPLPPLPSYETSR (SEQ ID NO: 142); SSLYWQHGPDPPVGAPQLSR (SEQ ID NO: 144) ; and SSHPLNSWPGGPFRHNLSSR (SEQ ID NO: 145) . 16. A purified peptide that binds to the SH3 domain of Src, said peptide comprising an amino acid sequence selected from the group consisting of: LASRPLPLLPNSAPGQ a portion of SEQ ID NO 155) LTGRPLPALPPPFSDF a portion of SEQ ID NO 152) PAYRPLPRLPDLSVIY a portion of SEQ ID NO 150) RALRVRPLPPVPGTSL a portion of SEQ ID NO 146) DAPGSLPFRPLPPVPT a portion of SEQ ID NO 148) LKWRALPPLPETDTPY a portion of SEQ ID NO 157) ISQRALPPLPLMSDPA a portion of SEQ ID NO 149) LTSRFLPDIPVRPSKS a portion of SEQ ID NO 156) NTNRPLPPTPDGI,DVR a portion of SEQ ID NO 158) MKDRVLPPIPTVESAV a portion of SEQ ID NO 153) LQSRPLPLPPQSSYPI a portion of SEQ ID NO 159) FINRRLPALPPDNSLL a portion of SEQ ID NO 151) FRALPLPPTPDNPFAG a portion of SEQ ID NO 147) and LYSAIAPDPPPRNSSS a portion of SEQ ID NO: 154) 1 . A purified peptide that binds to the SH3 domain of p53bp2, said peptide comprising an amino acid sequence selected from the group consisting of: YDASSAPQRPPLPVRKSRP SEQ ID NO: 183); EYVNASPERPPIPGRKSRP (SEQ ID NO: 184); WNGIAIPGRPEIPPRASRP SEQ ID NO: 185); SMIFIYPERPSPPPRFSRP (SEQ ID NO:186); GVEEWNPERPQIPLRLSRP (SEQ ID NO:187); WWDSRPDIPLRRSLP (SEQ ID NO: 188) WPLGRPEIPLRKSLP (SEQ ID NO: 189) GGTVGRPPIPERKSVD (SEQ ID NO: 190) YSHAGRPEVPPRQSKP (SEQ ID NO:191) FSAAARPDIPSRASTP (SEQ ID NO:192) LYIPKRPEVPPRRHEA (SEQ ID NO:193) NNISARPPLPSRQNPP (SEQ ID NO: 194); and MAGTPRPAVPQRMNPP (SEQ ID NO: 195) . 18. A purified peptide that binds to the N terminal SH3 domain of Crk, said peptide comprising an amino acid sequence selected from the group consisting of: GQPAGDPDPPPLPAKF FEQTGVPLLPPKSFKY IFGDPPPPIPMKGRSL SNQGSIPVLPIKRVQY NYVNALPPGPPLPAKN SSDPERPVLPPKLWSV HFGPSKPPLPIKTRIT DWKVPEPPVPKLPLKQ ATSEGLPILPSKVGSY NANVSAPRAPAFPVKT EMVLGPPVPPKRGTW AGSRHPPTLPPKESGG and SVAADPPRLPAKSRPQ SEQ ID NO:209) . 19. A purified peptide that binds to the SK domain of Yes, said peptide comprising an amino acid sequence selected from the group consisting of: ITMRPLPALPGHGQIH (SEQ ID NO:211) LPRRPLPDLPMAAGKG (SEQ ID NO:212) LGSRPLPPTPRQWPEV (SEQ ID NO:213) STIRPLPAIPRDTLLT (SEQ ID NO:214) RSGRPLPPIPEVGHNV (SEQ ID NO:215) IGSRPLPWTPDDLGSA (SEQ ID NO:216) LAQRELPGLPAGAGVS (SEQ ID NO:217) IPGRALPELPPQRALP (SEQ ID NO:218) FVGRELPPTPRTVIPW (SEQ ID NO:219) DPRSALPALPLTPLQT (SEQ ID NO:220); and SPHDVLPALPDSHSKS (SEQ ID NO:221). 20. A purified peptide that binds to the N terminal SH3 domain of Grb2, said peptide comprising an amino acid sequence selected from the group consisting of: KWDSLLPALPPAFTVE (SEQ ID NO:224); RWDQVLPELPTSKGQI (SEQ ID NO:225); RFDFPLPTHPNLQKAH (SEQ ID NO:226); RLDSPLPALPPTVMQN RWGAPLPPLPEYSWST YWDMPLPRLPGEEPSL RFDYNLPDVPLSLGTA TKKPNAPLPPLPAYMG KWDLDLPPEPMSLGNY YYQRPLPPLPLSHFES YYRKPLPNLPRGQTDD YFDKPLPESPGALMSL YFSRALPGLPERQEAH SLWDPLPPIPQSKTSV SYYDPLPKLPDPGDLG KLYYPLPPVPFKDTKH and DP DALPETPSMKASQ SEQ ID NO:242) . 21. A purified peptide having an amino acid sequence selected from the group consisting of: SEQ ID NOs: 250-252, 254, 256-259, 261, 262, 264-266, 269-272, 275, 280 , 281, 286- 239, 291, 294, and 295. 22. A purified peptide having an amino acid sequence selected from the group consisting of: SEQ ID NCs: 296-453. 23. method of identifying an inhibitor of the. binding between a first molecule comprising an SH3 domain and a second molecule that binds to the SH3 domain comprising incubating one or more compounds from which it is desired to select such an inhibitor with the first molecule and the second molecule under conditions conducive to binding and detecting the one or more compounds that inhibit binding of the first molecule to the second molecule. 24. The method of claim 23 where the second molecule is obtained by: (i) screening a peptide library with the SH3 domain to obtain peptides that bind the SH3 domain; (ii) determining a consensus sequence for the peptides obtained in step (i) ; ( ϋ) producing a peptide comprising the consensus sequence; wherein the second molecule comprises the peptide comprising the consensus sequence. 25. The method of claim 23 where the second molecule is obtained by: (i) screening a peptide library with the SH3 domain to obtain peptides that bind the SH3 domain; (ii) determining a consensus sequence for the peptides obtained in step (i) ; (iii) searching a database to identify amino acid sequences that resemble the consensus sequence of step (ii) ; (iv) producing a peptide comprising an amino acid sequence identified in step (iii) ; wherein the second molecule comprises the peptide comprising an amino acid sequence identified in step (iii) . 26. The method of claim 23 where the second molecule is a peptide that binds to the SH3 domain of Cortactin, said peptide comprising the amino acid sequence ZPPøPxKPxW (SEQ ID NO: 113), where Z represents K or R; ø represents a hydrophobic amino acid; and x represents any amine ac d. 27. The method of claim 23 where the second molecule is a peptide that binds to the middle SH3 domain of Nek, said peptide comprising the amino acid sequence øxxxxxPxPPøRZxSL (SEQ ID NO: 127) , where Z represents S or T; ø represents a hydrophobic amino acid; and x represents any amino acid. 28. The method of claim 23 where the second molecule is a peptide that binds to the SH3 domain of Abl, said peptide comprising the amino acid sequence PPxWxPPPøP (SEQ ID NO: 141), where ø represents a hydrophobic amino acid; and x represents any amino acid. 29. The method of claim 23 where the second molecule is a peptide that binds to the SH3 domain of Src, said peptide comprising the amino acid sequence LXXRPLPX^P (SEQ ID NO:165), where φ represents an aliphatic amino acid; and X represents any amino acid. 30. The method of claim 23 where the second molecule is a peptide that binds to the SH3 domain of Cortactin, said peptide comprising the amino acid sequence +PPι'PXKPXWL (SEQ ID NO:166), where + represents a basic amino acid; φ represents an aliphatic amino acid; and X represents any amino acid. 31. The method of claim 23 where the second molecule is a peptide that binds to the SH3 domain of Abl, said peptide comprising the amino acid sequence PPXøXPPPi/'P (SEQ ID NO:173), where θ represents an aromatic amino acid; φ represents an aliphatic amino acid; and X represents any amino acid. 32. The method of claim 23 where the second molecule is a peptide that binds to the SH3 domain of PLCγ, said peptide comprising the amino acid sequence PPVPPRPXXTL (SEQ ID NO:175), where X represents any amino acid. 33. The method of claim 23 where the second molecule is a peptide that binds to the SH3 domain of p53bp2, said peptide comprising the amino acid sequence RPXi/ ' Pi ' R+SXP (SEQ ID NO:196), where + represents a basic amino acid; φ represents an aliphatic amino acid; and X represents any amino acid. 34. The method of claim 23 where the second molecule is a peptide that binds to the N terminal SH3 domain of Crk, said peptide comprising the amino acid sequence φPφ PφK (SEQ ID NO:210) , where φ represents an aliphatic amino acid; and X represents any amino acid. 35. The method of claim 23 where the second molecule is a peptide that binds to the SH3 domain of Yes, said peptide comprising the amino acid sequence lXXRPLPXLP (SEQ ID NO:222), where φ represents an aliphatic amino acid; and X represents any amino acid. 36. The method of claim 23 where the second molecule is a peptide that binds to the N terminal SH3 domain of Grb2, said peptide comprising an amino acid sequence selected from the group consisting of: +0DXPLPXLP (SEQ ID NO:223), Y0X+PLPXLP (SEQ ID NO:238), and 0DPLPXLP (SEQ ID NO:243), where θ represent an aromatic amino acid; + represents a basic amino acid; φ represents an aliphatic amino acid; and X represents any amino acid. 37. The method of claim 23 where the second molecule is a peptide that binds to the SH3 domain of Cortactin, said peptide comprising an amino acid sequence selected from the group consisting of: LTPQSKPPLPPKPSAV (a portion of SEQ ID NO: 112); SSHNSRPPLPEKPSWL (a portion of SEQ ID NO: 111); PVKPPLPAKPWWLPPL (SEQ ID NO:167); TERPPLPQRPDWLSYS (a portion of SEQ ID NO: 109); LGEFSKPPIPQKPTWM (a portion of SEQ ID NO: 108); YPQFRPPVPPKPSLMQ (SEQ ID NO:168) ; VTRPPLPPKPGHMADF (SEQ ID NO:169); VSLGLKPPVPPKPMQL (SEQ ID NO: 170); LLGPPVPPKPQTLFSF (a portion of SEQ ID NO:107) ; YKPEVPARPIWLSEL (SEQ ID NO: 171); GAGAARPLVPKKPLFL (SEQ ID NO: 172) ; and SREPDWLCPNCPLLLRSDSR (SEQ ID NO:110). 38. The method of claim 23 where the second molecule is a peptide that binds to the middle SH3 domain of Nek, said peptide comprising an amino acid sequence selected from the group consisting of: SSL VGWKPLPPMRTASLSR (SEQ ID NO:114) SSVGFADRPRPPLRVESLSR (SEQ ID NO: 115) SSAGILRPPEKPXRSFSLSR (SEQ ID NO:116) SSPYTGDVPIPPLRGASLSR (SEQ ID NO:117) SSLMGSWPPVPPLRSDSLSR (SEQ ID NO: 118) SSIGEDTPPSPPTRRASLSR (SEQ ID NO:119) SRSLSEVSPKPPIRSVSLSR (SEQ ID NO:120) SSVSEGYSPPLPPRSTSLSR (SEQ ID NO:121) SSSFTLAAPTPPTRSLSLSR (SEQ ID NO:122) SSPPYELPPRPPNRTVSLSR (SEQ ID NO: 123) SRWDGLAPPPPVRLSSLSR (SEQ ID NO: 124) SSLGYSGAPVPPHRXSSLSR (SEQ ID NO: 125) ; and SSISDYSRPPPPVRTLSLSR (SEQ ID NO:126) . 39. The method of claim 23 where the second molecule is a peptide that binds to the SH3 domain of Abl, said peptide comprising an amino acid sequence selected from the group consisting of: PPWWAPPPIPNSPQVL SEQ ID NO: 174) ; PPKFSPPPPPYWQLHA a portion of SEQ ID NO: 132) PPHWAPPAPPAMSPPI a portion of SEQ ID NO: 130) PPTWTPPKPPGWGWF a portion of SEQ ID NO:137) PPSFAPPAAPPRHSFG a portion of SEQ ID NO: 133) PTYPPPPPPDTAKGA (a portion of SEQ ID NO:135); GPRWSPPPVPLPTSLD a portion of SEQ ID NO:128) APTWSPPALPNVAKYK a portion of SEQ ID NO:138) PPDYAAPAIPSSLWVD a portion of SEQ ID NO: 129) IKGPRFPVPPVPLNGV a portion of SEQ ID NO:139) PPAWSPPHRPVAFGST a portion of SEQ ID NO: 140) APKKPAPPVPMMAHVM a portion of SEQ ID NO: 134) SSDRCWECPPWPAGGQRGSR (SEQ ID NO:131) ; and SSPPXXXPPPIPNSPQVLSR (SEQ ID NO:136). 40. The method of claim 23 where the second molecule is a peptide that binds to the SH3 domain of PLCγ, said peptide comprising an amino acid sequence selected from the group consisting of: MPPPVPFRPPGTLQVA (SEQ ID NO: 176) LSYSPPPVPPRPDSTL (SEQ ID NO:177) VLAPPVPPRPGNTFFT (SEQ ID NO:178) YRPPVAPRPPSSLSVD (SEQ ID NO: 179) LQCPDCPRVPPRPIPI (SEQ ID NO: 180) VPPLVAPRPPSTLNSL (a portion of SEQ ID NO:143); LTPPPFPKRPRWTLPE (SEQ ID NO: 181); YWPHRPPLAPPQTTLG (SEQ ID NO:182); SSMKVHNFPLPPLPSYETSR (SEQ ID NO:142); SSLYWQHGPDPPVGAPQLSR (SEQ ID NO:144); and SSHPLNSWPGGPFRHNLSSR (SEQ ID NO: 145). 41. The method of claim 23 where the second molecule is a peptide that binds to the SH3 domain of Src, said peptide comprising an amino acid sequence selected from the group consisting of: LASRPLPLLPNSAPGQ (a portion of SEQ ID NO:155); LTGRPLPALPPPFSDF (a portion of SEQ ID NO:152) ; PAYRPLPRLPDLSVIY (a portion of SEQ ID NO:150); RALRVRPLPPVPGTSL (a portion of SEQ ID NO: 146); DAPGSLPFRPLPPVPT (a portion of SEQ ID NO: 148) LKWRALPPLPETDTPY (a portion of SEQ ID NO: 157) ISQRALPPLPLMSDPA (a portion of SEQ ID NO: 149) LTSRPLPDIPVRPSKS (a portion of SEQ ID NO:156) NTNRPLPPTPDGLDVR (a portion of SEQ ID NO:158) MKDRVLPPIPTVESAV (a portion of SEQ ID NO: 153) LQSRPLPLPPQSSYPI (a portion of SEQ ID NO: 159) FINRRLPALPPDNSLL (a portion of SEQ ID NO:151) FRALPLPPTPDNPFAG (a portion of SEQ ID NO: 147); and LYSAIAPDPPPRNSSS (a portion of SEQ ID NO:154) . 42. The method of claim 23 where the second molecule is a peptide that binds to the SH3 domain of p53bp2, said peptide comprising an amino acid sequence selected from the group consisting of: YDASSAPQRPPLPVRKSRP SEQ ID NO:183); EYVNΛSPERPPIPGRKSRP (SEQ ID NO:184); WNGIAIPGRPEIPPRASRP SEQ ID NO:185); SMIF.TYPERPSPPPRFSRP (SEQ ID NO:.186); GVEEWNPERPQIPLRLSRP (SEQ ID NO: 187); WVVDSRPDIFLRRSLP (SEQ ID NO: 188) VVPLGRPEIPLRKSLP (SEQ ID NO:189) GGTVGRPPIPERKSVD (SEQ ID NO:190) YSHAGRPEVPPRQSKP (SEQ ID NO: 191) FSAAARPDIPSRASTP (SEQ ID NO: 192) LYIPKRPEVPPRRHEA (SEQ ID NO:193) NNISARPPLPSRQNPP (SEQ ID NO:194) ; and MAGTPRPAVPQRMNPP (SEQ ID NO: 195). 43. The method of claim 23 where the second molecule is a peptide that binds to the N terminal SH3 domain of Crk, said peptide comprising an amino acid sequence selected from the group consisting of: GQPAGDPDPPPLPAKF (SEQ ID NO:197) FEQTGVPLLPPKSFKY (SEQ ID NO:198) IFGDPPPPIPMKGRSL (SEQ ID NO:199) SNQGSIPVLPIKRVQY (SEQ ID NO:200) NYVNALPPGPPLPAKN (SEQ ID NO:201) SSDPERPVLPPKLWSV (SEQ ID NO:202) HFGPSKPPLPIKTRIT (SEQ ID NO:203) DWKVPEPPVPKLPLKQ (SEQ ID NO:204) ATSEGLPILPSKVGSY (SEQ ID NO:205) NJANVSAPRAPAFPVKT (SEQ ID NO:206) EMVLGPPVPPKRGTW (SEQ ID NO:207) AGSRHPPTLPPKESGG (SEQ ID NO:208); and SVAADPPRLPAKSRPQ (SEQ ID NO:209). 44. The method of claim 23 where the second molecule is a peptide that binds to the SH3 domain of Yes, said peptide comprising an amino acid sequence selected from the group consisting of: ITMRPLPALPGHGQIH (SEQ ID NO:211) LPRRPLPDLPMAAGKG (SEQ ID NO:212) LGSRPLPPTPRQWPEV (SEQ ID NO:213) STIRPLPAIPRDTLLT (SEQ ID NO:214) RSGRPLPPIPEVGHNV (SEQ ID NC:215) IGSRPLPWTPDDLGSA (SEQ ID NO:216) LAQRELPGLPAGAGVS (SEQ ID NO:217) IPGRALPELPPQRALP (SEQ ID NO:218) FVGRELPPTPRTVIPW (SEQ ID NO:219) DPRSALPALPLTPLQT (SEQ ID NO:220) ; and SPHDVLPALPDSHSKS (SEQ ID NO:221). 45. The method of claim 23 where the second molecule is a peptide that binds to the N terminal SH3 domain of Grb2, said peptide comprising an amino acid sequence selected from the group consisting of: KWDSLLPALPPAFTVE (SEQ ID NO:224) RWDQVLPELPTSKGQI (SEQ ID NO:225) RFDFPLPTHPNLQKAH (SEQ ID NO:226) RLDSPLPALPPTVMQN (SEQ ID NO:227) RWGAPLPPLPEYSWST (SEQ ID NO:228) YWDMPLPRLPGEEPSL (SEQ ID NO:229) RFDYNLPDVPLSLGTA (SEQ ID NO:230) TKKPNAPLPPLPAYMG (SEQ ID NO:231) KWDLDLPPEPMSLGNY (SEQ ID NO:232) YYQRPLPPLPLSHFES (SEQ ID NO:234) YYRKPLPNLPRGQTDD (SEQ ID NO:235) YFDKPLPESPGALMSL (SEQ ID NO:236) YFSRALPGLPERQEAH (SEQ ID NO:237) SLWDPLPPIPQSKTSV (SEQ ID NO:239) SYYDPLPKLPDPGDLG (SEQ ID NO:240) KLYYPLPPVPFKDTKH (SEQ ID NO:241); and DPYDALPETPSMKASQ (SEQ ID NO:242) . 46. The method of claim 23 where the second molecule is a peptide having an amino acid sequence selected from the group consisting of: SEQ ID NOs: 250-252, 254, 256-259, 261, 262, 264-266, 269-272, 275, 280, 281, 286-289, 291, 294, and 295. 47. The method of claim 23 where the second molecule is a peptide having an amino acid sequence selected from the group consisting of: SEQ ID NOs: 296-453. 48. A method of identifying a compound that affects the binding of a molecule comprising an SH3 domain and a ligand of the SH3 domain, the method comprising: (a) contacting the SH3 domain and the ligand under conditions conducive to binding in the presence of a candidate compound and measuring the amount of binding between the SH3 domain and the ligand; (b) comparing the amount of binding in step (a) with the amount of binding known or determined to occur between the molecule and the ligand in the absence of the candidate compound, where a difference in the amount of binding between step (a) and the amount of binding known or determined to occur between the molecule and the ligand in the absence of the candidate compound indicates that the candidate compound is a compound that affects the binding of the molecule comprising an SH3 domain and the ligand. 49. A kit comprising, in one or more containers: (a) a purified first molecule comprising an SH3 domain; (b) a purified second molecule that binds to the SH3 domain. 50. The kit of claim 49 wherein said second molecule comprises a peptide having an amino acid sequence selected from the group consisting of: SEQ ID NOs: 107- 112, 114-126, 128-140, 142-159, 167, 168-172, 174, 176-195, 197-209, 211- 221, 224-232, 234-237, 239-242, 250-252, 254, 256-259, 261, 262, 264-266, 269-272, 275, 280, 281, 286-289, 291, 294-453. 51. A purified peptide that binds to the SH3 domain of Src, said peptide comprising the amino acid sequence LX^ PLPX j ^PX^ (SEQ ID NO: 54) where φ represents aliphatic amino acid residues and X l t X 2 , X 3 , X 4 , and X 5 represent any amino acid; except that if X j = P, φ = L>, X 4 = P, and X 5 = P, then: where = F, then X 2 is not H or R; or where , = S, then X 2 is not R, H, A, N, T, G, V, M, or W; or where X- = C, then X 2 is not S or G; or where X x = R, then X 2 is not T or F; or where X λ - A, then X 2 is not R, Q, N, S, or L; or where X, = Q, then X ? is not M; or where X L = L, then X 2 is not R; or where X, = I, then X 2 is not Λ; or where j -■ , then X 2 is not P, W, or R; or where X : = G, then X 2 is not S or R; or where X t = T, then X 2 is not T. 52. A purified peptide that binds to the SH3 domain of Yes, said peptide comprising the amino acid sequence where φ represents aliphatic amino acid residues and X l t X 2 , X 3 , X 4 , and X 5 represent any amino acid; except that if X, = P, X 4 = P, and X 5 = P, then: when φ = L, where X x - F , then X is not H or R; or where X x = S, then X 2 is not R, H, A, N, T, G, V, M, or W; or where X x = C, then X 2 is not S or G; or where X x = R, then X 2 is not T or F; or where X x = A, then X 2 is not R, Q, N, S, or L; or where X x = Q, then X 2 is not M; or where X x = L, then X 2 is not R; or where X x = I, then X 2 is not A; or where X x = P, then X 2 is not P, W, or R; or where X x = G, then X 2 is not S or R; or where X x = T, then X 2 is not T; and when φ = P, where X x = A, then X 2 is not R; or where X ± = S, then X 2 is not R or Y; or where X 1 = M, then X 2 is not S; or where X x = V, then X 2 is not G; or where X x = R, then X 2 is not S; or where X L = I, then X 2 is not R; and when φ - A, where X, = A, then X 2 is not K; and when φ = V, where X = A, then X is not C or Q; or where X x = P, then X 2 is not P; and when φ = I, wheχ-e X = G, then X_, is not H; or where X, - T, then X is not S; or where X ^ = R, then X is not S.

Inventors: DER CHANNING J, FOWLKES DANA M, KAY BRIAN K, QUILLIAM LAWRENCE A, RIDER JAMES E, SPARKS ANDREW B, THORN JUDITH M, DER, CHANNING, J., FOWLKES, DANA, M., KAY, BRIAN, K., QUILLIAM, LAWRENCE, A., RIDER, JAMES, E., SPARKS, ANDREW, B., THORN, JUDITH, M.
Applicants: CYTOGEN CORP, UNIV NORTH CAROLINA, CYTOGEN CORPORATION, THE UNIVERSITY OF NORTH CAROLINA AT CHAPEL HILL
Application year: 1997
Main classification: C07K5
IPC codes: C07K5, C07K7, C07K14