IRS‐1 undergoes rapid tyrosine phosphorylation during insulin stimulation and forms a stable complex containing the 85 kDa subunit (p85) of the phosphatidylinositol (PtdIns) 3′‐kinase, but p85 is not tyrosyl phosphorylated. IRS‐1 contains nine tyrosine phosphorylation sites in YXXM (Tyr‐Xxx‐Xxx‐Met) motifs. Formation of the IRS‐1‐PtdIns 3′‐kinase complex in vitro is inhibited by synthetic peptides containing phosphorylated YXXM motifs, suggesting that the binding of PtdIns 3′‐kinase to IRS‐1 is mediated through the SH2 (src homology‐2) domains of p85. Furthermore, overexpression of IRS‐1 potentiates the activation of PtdIns 3‐kinase in insulin‐stimulated cells, and tyrosyl phosphorylated IRS‐1 or peptides containing phosphorylated YXXM motifs activate PtdIns 3′‐kinase in vitro. We conclude that the binding of tyrosyl phosphorylated IRS‐1 to the SH2 domains of p85 is the critical step that activates PtdIns 3′‐kinase during insulin stimulation.

In response to harsh environmental conditions, C. elegans larvae undergo dauer arrest at the second molt. The past decade has yielded many insights into the signaling pathways and the molecular mechanisms that govern this developmental transition. Dauer pheromone, the major physiologic signal promoting dauer arrest, has been purified, identified, and synthesized. The molecular identities of the vast majority of dauer regulatory genes isolated in initial genetic screens are now known. Physiologic ligands for DAF-12, a nuclear receptor that is the final common target of dauer regulatory pathways, have been identified. The discovery of the Hid (high temperature induction of dauer) phenotype and the results of enhancer screens have greatly expanded the repertoire of dauer regulatory genes. Genomic analysis of dauer arrest has highlighted the role of pathway crosstalk in dauer regulation. Nonetheless, critical questions remain about the mechanistic underpinnings of dauer arrest.

The C. elegans insulin/IGF-1 signaling (IIS) pathway connects nutrient levels to metabolism, growth, development, longevity, and behavior. This fundamental pathway is regulated by insulin-like peptide ligands that bind to the insulin/IGF-1 transmembrane receptor (IGFR) ortholog DAF-2. DAF-2/IGFR controls the activity of a conserved phosphoinositide 3-kinase (PI3K)/Akt kinase cascade, culminating in the regulation of a FoxO transcription factor, DAF-16, that governs most of the functions of this pathway. In light of the evolutionary conservation of the IIS pathway, its study in C. elegans is likely to shed light on its functions and regulation in higher organisms, including humans. Originally identified based on its role in the regulation of larval development and aging, IIS also controls a host of other biological processes. Here we review what is currently known about the biological functions and the molecular components of C. elegans IIS.

IRS-1 is an insulin receptor substrate that undergoes tyrosine phosphorylation and associates with the phosphatidylinositol (PtdIns) 3'-kinase immediately after insulin stimulation. Recombinant IRS-1 protein was tyrosine phosphorylated by the insulin receptor in vitro and associated with the PtdIns 3'-kinase from lysates of quiescent 3T3 fibroblasts. Bacterial fusion proteins containing the src homology 2 domains (SH2 domains) of the 85-kDa subunit (p85) of the PtdIns 3'-kinase bound quantitatively to tyrosine phosphorylated, but not unphosphorylated, IRS-1, and this association was blocked by phosphotyrosine-containing synthetic peptides. Moreover, the phosphorylated peptides and the SH2 domains each inhibited binding of PtdIns 3'-kinase to IRS-1. Phosphorylated IRS-1 activated PtdIns 3'-kinase in anti-p85 immunoprecipitates in vitro, and this activation was blocked by SH2 domain fusion proteins. These data suggest that the interaction between PtdIns 3'-kinase and IRS-1 is mediated by tyrosine phosphorylated motifs on IRS-1 and the SH2 domains of p85, and IRS-1 activates PtdIns 3'-kinase by binding to the SH2 domains of p85. Thus, IRS-1 likely serves to transmit the insulin signal by binding and regulating intracellular enzymes containing SH2 domains.