MECHANISM OF ACTIVATION OF GUANINE-NUCLEOTIDE EXCHANGE FACTOR BY INSULIN

Insulin increases activity of the guanine nucleotide exchange factor ( CEF) in Rat-1 fibroblasts transfected with human insulin receptors (HI Rc cells), thereby promoting formation of the active form of p21Ras (p 21Ras . GTP). In order to identify the upstream molecules mediating th is aspect of insulin action, we selectively removed some of these mole cules by immunoprecipitation and examined GEF activity in the post-imm unoprecipitation lysates of the insulin-treated HIRc cells. The remova l of Shc or Grb-2 depleted CEF activity from the cell lysates, whereas immuno-precipitation of the insulin receptors, IRS-1, PLC gamma and G AP, were without effect. In summary, the current data demonstrate that a majority of cellular Pas CEF activity after insulin stimulation is associated with She and involves interactions among She, Grb-2 and Sos .