Localization of the Insulin-like Growth Factor I Receptor Binding Sites for the SH2 Domain Proteins p85, Syp, and GTPase Activating Protein

Abstract Potential signaling substrates for the insulin-like growth factor I (IGF-I) receptor are SH2 domain proteins including the p85 subunit of phosphatidylinositol 3-kinase, the tyrosine phosphatase Syp, GTPase activating protein (GAP), and phospholipase C- (PLC-). In this study, we demonstrate an association between the IGF-I receptor and p85, Syp, and GAP, but not with PLC- in lysates of cells overexpressing the human IGF-I receptor. We further investigated these interactions using glutathione S-transferase (GST) fusion proteins containing the amino-terminal SH2 domains of p85 or GAP, or both SH2 domains of Syp or PLC- to precipitate the IGF-I receptor from purified receptor preparations and from whole cell lysates. p85-, Syp-, and GAP-GSTs precipitated the IGF-I receptor, whereas the PLC--GST did not. Using phosphopeptides corresponding to IGF-I receptor phosphorylation sites, we determined that the p85- and Syp-GST association with the IGF-I receptor could be inhibited by a carboxyl-terminal peptide containing pY1316 and that the GAP-GST association could be inhibited by a NPXY domain peptide. The GAP-GST binding site was confirmed by showing that a mutant IGF-I receptor with a deletion of the NPXY domain including tyrosine 950 was poorly precipitated by the GAP-GST. We conclude that p85 and Syp may bind directly to the IGF-I receptor at tyrosine 1316, and that GAP may bind to the IGF-I receptor at tyrosine 950. An association between the IGF-I receptor and PLC- was not evident. p85, Syp, and GAP are potential modulators of IGF-I receptor signal transduction.