Distinct phosphotyrosines on a growth factor receptor bind to specific molecules that mediate different signaling pathways

The receptor for platelet-derived growth factor (PDGF) binds two proteins containing SH2 domains, GTPase activating protein (GAP) and phosphatidylinositol 3-kinase (Pl3-kinase). The sites on the receptor that mediate this interaction were identified by using phosphotyrosine-containing peptides representing receptor sequences to block specifically binding of either Pl3-kinase or GAP. These results suggested that Pl3-kinase binds two phosphotyrosine residues, each located in a 5 aa motif with an essential methionine at the fourth position C-terminal to the tyrosine. Point mutations at these sites caused a selective elimination of Pl3-kinase binding and loss of PDGF-stimulated DNA synthesis. Mutation of the binding site for GAP prevented the receptor from associating with or phosphorylating GAP, but had no effect on Pl3-kinase binding and little effect on DNA synthesis. Therefore, GAP and Pl3-kinase interact with the receptor by binding to different phosphotyrosine-containing sequence motifs.