Identification and characterization of Src SH3 ligands from phage-displayed random peptide libraries

We have used the Src homology 3 (SH3) domain to screen two phage-displayed random peptide libraries, each containing 2 x 10(8) unique members, and have identified a series of high affinity peptide ligands. The peptides possess similar proline-rich regions, which yield a consensus Src SH3-binding motif of RPLPPLP. We have confirmed this motif by screening a phage-displayed peptide library biased for SH3 ligands and identifying the same consensus sequence. Binding studies using synthetic peptides suggest that the RPLPPLP motif is important for SH3 binding and confers specificity for the Src SH3 domain, and that residues which flank the motif may also contribute to binding. Peptides that contain the RPLPPLP motif compete Src, but not Abl or phospholipase C gamma, SH3 interactions with SH3-binding proteins from cell lysates (IC50 = 1-5 microM). Furthermore, RPLPPLP-related peptides are able to accelerate progesterone-induced maturation of Xenopus laevis oocytes. A similar acceleration has been observed in oocytes treated with activated, but not normal, Xenopus Src, suggesting the possibility that the peptides are able to antagonize the negative regulation of Src activity by Src SH3 in vivo.