PHOSPHATIDYLINOSITOL (3,4,5)P-3 INTERACTS WITH SH2 DOMAINS AND MODULATES PI-3-KINASE ASSOCIATION WITH TYROSINE-PHOSPHORYLATED PROTEINS

Src homology 2 (SH2) domains on the regulatory subunit of phosphoinosi tide 3-kinase (PI 3-kinase) mediate its binding to specific tyrosine-p hosphorylated proteins in stimulated cells. Using a pharmacological an d genetic approach, we show that the amount of PI 3-kinase associated with tyrosine-phosphorylated proteins inversely correlates with the am ount of PI 3-kinase lipid products present in the cell. An explanation for this observation is provided by our finding that phosphatidylinos itol (3,4,5)trisphosphate (PtdIns [3,4,5]P-3) binds directly and selec tively to the SH2 domains of the 85 kDa subunit of PI 3-kinase and the reby blocks binding to tyrosine-phosphorylated proteins. The SH2 domai n of pp60(c-src) also specifically bound PtdIns (3,4,5)P-3, and the bi nding was competed by a phosphopeptide specific for the Src SH2 domain . These results indicate that production of PtdIns (3,4,5)P-3 at the m embrane disrupts the binding of PI 3-kinase to phosphoproteins. This l ipid may also recruit other SH2-containing proteins to the membrane to initiate downstream signaling.