STRUCTURAL BASIS FOR THE BINDING OF PROLINE-RICH PEPTIDES TO SH3 DOMAINS

A common RXL motif was found in proline rich ligands that were selecte d from a biased combinatorial peptide library on the basis of their ab ility to bind specifically to the SH3 domains from phosphatidylinosito l 3-kinase (P13K) or c-Src. The solution structure of the P13K SH3 dom ain complexed to one of these ligands, RKLPPRPSK (RLP1), was determine d. Structure-based mutations were introduced into the P13K SH3 domain and the RLP1 ligand, and the influence of these mutations on binding w as evaluated. We conclude that SH3 domains recognize proline-rich moti fs possessing the left-handed type II polyproline (PPII) helix conform ation. Two proline residues directly contact the receptor. Other proli nes in the ligands appear to function as a molecular scaffold, promoti ng the formation of the PPII helix. Three nonproline residues consisti ng of combinations of arginine and leucine interact extensively with t he SH3 domain and appear to confer ligand specificity.