GUANOSINE TRIPHOSPHATASE-ACTIVATING PROTEIN-ASSOCIATED PROTEIN, BUT NOT SRC-ASSOCIATED PROTEIN P68 IN MITOSIS, IS A PART OF INSULIN SIGNALING COMPLEXES

The insulin receptor, following insulin stimulation of cells, triggers formation of various signaling complexes. In rat HTC hepatoma cells o verexpressing normal human insulin receptors (HTC-LR), p85 regulatory subunit of phosphatidylinositol-3-kinase (PI3K) forms signaling comple xes containing the insulin receptor, insulin receptor substrate 1 (IRS -1), guanosine triphosphatase-activating protein (GAP) and 60-70 kDa p hosphotyrosine proteins (p60-70). In the present study, Ne demonstrate that p60-70 interacts directly with the p85 subunit via src homology 2 domain of the latter. Employing antibodies specific to two p85 isofo rms, p85 alpha and p85 beta, we demonstrate that HTC-IR cells express both p85 isoforms, and these isoforms induce the formation of similar signaling complexes in response to insulin. p60-70, present in both al pha-p85 alpha and alpha-p85 beta immunoprecipitates, is a GAP-associat ed protein, but is distinct from the p68 src-associated protein in mit osis (Sam68) by several criteria. These data suggest that 1) GAP-assoc iated protein, but not Sam68, is a part of insulin signaling complexes ; and 2) p85 alpha and p85 beta form similar, but distinct, insulin re ceptor signaling complexes.