Insulin treatment of fat and muscle cells causes a rapid increase in the rate of glucose transport into these cells. This effect is a major contributor to the lowering of the blood glucose level by insulin. The basis of the effect is the movement to and fusion of specialized intracellular vesicles containing the glucose transporter GLUT4 with the plasma membrane. This process is known as GLUT4 translocation. Our overall goal is to elucidate at the molecular level how GLUT4 translocation occurs. The main focus in the coming grant period is to describe in detail how signaling from the insulin receptor causes GLUT4 translocation in adipocytes. Recently we have discovered and initially characterized a protein that is a likely key connection between insulin signaling and GLUT4 translocation. This protein is a Rab GTPase activating protein (Rab GAP) known as AS160. Our results indicate that insulin-stimulated phosphorylation of AS160, probably by the protein kinase Akt, is a trigger of GLUT4 translocation. They support the hypothesis that this phosphorylation of AS160 inactivates its GAP function, and that as a consequence the GTP form of a critical Rab is elevated. The GTP Rab is expected to be the initiating component in the GLUT4 translocation machinery.
Our specific aims are: 1. To complete the characterization of AS160, We will identify the Rab(s) that is its substrate, determine how phosphorylation affects its GAP activity, define its sub cellular location, and assess its role in other membrane trafficking processes. 2. To characterize the Rab(s) that is required for GLUT4 translocation. We will identify this Rab, and examine its roles at the molecular level in both the movement of GLUT4 vesicles and their fusion with the plasma membrane. This research is directly relevant to diabetes. Detailed knowledge of GLUT4 translocation may lead to better therapeutic regimes or agents for both type 1 and type 2 diabetes. ? ?
