| Upon ligand binding, the insulin receptor tyrosine kinase is activated and phosphorylates intracellular substrates, like IRS-proteins, which then act as docking proteins to propagate signals. We investigated the characteristics of insulin receptor/IRS-1 signaling that were both necessary and sufficient to promote cell growth and survival. Analysis of a naturally occurring chimeric insulin-like receptor, the Drosophila insulin receptor (DIR), demonstrated that multiple IRS-dependent pathways were stimulated through the IRS-like tail of the DIR; however, IRS-1 was still required to mediate insulin-stimulated cell growth. In addition, studies of deletion mutations showed that the PH and PTB domains of IRS-1 were required for substrate recognition and cell growth. We hypothesized that IRS-dependent signals, specifically PI-3 kinase, must be dissociated from the receptor for proper signaling to occur. We further postulated that the PH and PTB domains of IRS-proteins would be required for the recognition and perhaps targeting of the IRS-dependent signals. These hypotheses were proved through analysis of chimeric molecules that fused a PI-3 kinase interacting region of IRS-1 to either the COOH-terminus of the insulin receptor or to the PH and PTB domains of IRS-1. Further analysis revealed that the PH and PTB domains of IRS-1 were sufficient for maintenance of insulin-stimulated cell growth at high insulin doses. Our results demonstrated that the NH... |
