Specificity of the PTB Domain of Shc for β Turn-forming Pentapeptide Motifs Amino-terminal to Phosphotyrosine

Shc phosphorylation in cells following growth factor, insulin, cytokine, and lymphocyte receptor activation leads to its association with Grb2 and activation of Ras. In addition to being a cytoplasmic substrate of tyrosine kinases, Shc contains an SH2 domain and a non-SH2 phosphotyrosine binding (PTB) domain. Here we show that the Shc PTB domain, but not the SH2 domain, binds with high affinity (ID50 approximately equal to 1 microM) to phosphopeptides corresponding to the sequence surrounding Tyr250 of the polyoma virus middle T (mT) antigen (LLSNPTpYSVMRSK). Truncation studies show that five residues amino-terminal to tyrosine are required for high affinity binding, whereas all residues carboxyl-terminal to tyrosine can be deleted without loss of affinity. Substitution studies show that tyrosine phosphorylation is required and residues at -5, -3, -2, and -1 positions relative to pTyr are important for this interaction. 1H NMR studies demonstrate that the phosphorylated mT antigen-derived sequence forms a stable beta turn in solution, and correlations between structure and function indicate that the beta turn is important for PTB domain recognition. These results show that PTB domains are functionally distinct from SH2 domains. Whereas SH2 domain binding specificity derives from peptide sequences carboxyl-terminal to phosphotyrosine, the Shc PTB domain gains specificity by interacting with beta turn-forming sequences amino-terminal to phosphotyrosine.