Insulin causes the translocation of the glucose transporter GLUT4 and other proteins from an intracellular location to the cell surface in adipocytes. The basis of this effect appears to be the stimulation of the fusion of specialized small secretory vesicles containing these proteins with the plasma membrane. The lon term objective is to elucidate at the molecular level how this stimulated fusion occurs and what other surface functions beside glucose transport are modified as the result of it. The main experimental approach is to characteriz the proteins in the insulin- regulated secretory vesicles (IR vesicles) from rat adipocytes. These will include proteins required for vesicle formation, movement, and fusion, and cargo proteins, like GLUT4, that alter functions at the cell surface. During the coming grant period, the specific aims are as follows: 1. Sortilin and vp22, two novel proteins that have recently been found in the vesicles, will be further characterized. In the case of sortilin, which appear to be a receptor, its degree of colocalization with GLUT4, its extent of translocation to the cell surface in response to insulin, and its rate of recycling will be determined. In the case of vp22, its cDNA will be cloned, an then vp22 will be characterized as described for sortilin. In addition, the function of vp22 will be investigated by a variety of approaches, including assay for any activity suggested by its amino acid sequence, determination of any role in insulin-stimulated vesicle trafficking, and identification of associated proteins. 2. A method for the isolation of more highly purified IR vesicles will be developed. Then vesicle proteins present at low copy number (one or several pe vesicle) will be subjected to peptide analysis, identified in the data base or cloned, and characterized as described under 1. This research is directly relevant to diabetes. Detailed knowledge of insulin-elicited vesicle trafficking and alteration of plasma membrane functions may provide the basis for identifying a cause of NIDDM and may lead to better therapeutic agents or regimes for both NIDDM and IDDM.
