SNAP23 promotes insulin-dependent glucose uptake in 3T3-L1 adipocytes: possible interaction with cytoskeleton

The acute stimulation of glucose uptake by insulin in fat and muscle cells is primarily the result of translocation of facilitative glucose transporte r 4 (GLUT-4) from an internal compartment to the plasma membrane. Here, we investigate the role of SNAP23 (a 23-kDa molecule resembling the 25-kDa syn aptosome associated protein) in GLUT-4 translocation and glucose uptake in 3T3-L1 adipocytes. Microinjection of a polyclonal antibody directed to the carboxy terminus of SNAP23 inhibited GLUT-4 incorporation into the membrane in response to insulin, whereas microinjection of full-length recombinant SNAP23 enhanced the insulin effect. Introduction of recombinant SNAP23 into chemically permeabilized cells also enhanced insulin-stimulated glucose tr ansport. These results indicate that SNAP23 is required for insulin-depende nt, functional incorporation of GLUT-4 into the plasma membrane and that th e carboxy terminus of the protein is essential for this process. SNAP23 is therefore likely to be a fusion catalyst along with syntaxin-4 and vesicle- associated membrane protein (VAMP)-2. Furthermore, the endogenous content o f SNAP23 appears to be limiting for insulin-dependent GLUT-4 exposure at th e cell surface. A measurable fraction of SNAP23 was sedimented with cytoske letal elements when extracted with Triton X-100, unlike VAMP-2 and syntaxin -4, which were exclusively soluble in detergent. We hypothesize that SNAP23 and its interaction with the cytoskeleton may be targets for regulation of GLUT-4 traffic.