Different Subcellular Localization and Phosphoinositides Binding of Insulin Receptor Substrate Protein Pleckstrin Homology Domains

Abstract

Insulin evokes diverse biological effects through receptor-mediated tyrosine phosphorylation of the insulin receptor substrate (IRS) proteins. Here, we show that, in vitro, the IRS-1, -2 and -3 pleckstrin homology (PH) domains bind with different specificities to the 3-phosphorylated phosphoinositides. In fact, the IRS-1 PH domain binds preferentially to phosphatidylinositol 3,4,5-trisphosphate (PtdIns-3,4,5-P3), the IRS-2 PH domain to phosphatidylinositol 3,4-bisphosphate (PtdIns-3,4-P2), and the IRS-3 PH domain to phosphatidylinositol 3-phosphate. When expressed in NIH-IR fibroblasts and L6 myocytes, the IRS-1 and -2 PH domains tagged with green fluorescent protein (GFP) are localized exclusively in the cytoplasm. Stimulation with insulin causes a translocation of the GFP-IRS-1 and -2 PH domains to the plasma membrane within 3-5 min. This translocation is blocked by the phosphatidylinositol 3-kinase (PI 3-K) inhibitors, wortmannin and LY294002, suggesting that this event is PI 3-K dependent. Interestingly, platelet-derived growth factor (PDGF) did not induce translocation of the IRS-1 and -2 PH domains to the plasma membrane, indicating the existence of specificity for insulin. In contrast, the GFP-IRS-3 PH domain is constitutively localized to the plasma membrane. These results reveal a differential regulation of the IRS PH domains and a novel positive feedback loop in which PI 3-K functions as both an upstream regulator and a downstream effector of IRS-1 and -2 signaling.