DISASSEMBLY OF SON-OF-SEVENLESS PROTEINS FROM GRB2 DURING P21(RAS) DESENSITIZATION BY INSULIN

Insulin receptor signaling acutely stimulates GTP loading of p21(ras), apparently by mobilizing complexes of Grb2 and the guanine nucleotide exchangers Son-of-sevenless (Sos) 1 and 2 to associate with tyrosine-phosphorylated proteins in the plasma membrane. Here we show that in P-32-labeled 3T3-L1 adipocytes the elevated cellular concentrations of [P-32]GTP-bound p21(ras) in response to insulin return to near basal levels after 20-30 min of hormone stimulation, while insulin receptors remain activated. Lysates of such desensitized cells were quantitatively immunoprecipitated with an antiserum recognizing both Sos1 and Sos2 proteins or a specific anti-Sos2 antiserum. Immunoblot analysis of these precipitates revealed that insulin causes a marked hyperphosphorylation of Sos1 and a 50% decrease in Grb2 associated with Sos proteins under these conditions. Similarly, anti-Grb2 immunoprecipitates of such lysates revealed the presence of decreased Sos1 protein due to insulin action. The disassembly of Grb2 from Sos proteins slightly precedes the time course of p21(ras) deactivation in response to insulin, These data are consistent with the hypothesis that the dissociation of Grb2 from Sos proteins caused by insulin in 3T3-L1 cells mediates p21(ras) deactivation and desensitization.