CA2-DEPENDENT INTERACTION OF THE GROWTH-ASSOCIATED PROTEIN GAP-43 WITH THE SYNAPTIC CORE COMPLEX()

The synaptic vesicle exocytosis occurs by a highly regulated mechanism : syntaxin and 25 kDa synaptosome-associated protein (SNAP-25) are ass embled with vesicle-associated membrane protein (VAMP) to form a synap tic core complex and then synaptotagmin participates as a Ca2+ sensor in the final step of membrane fusion. The 43 kDa growth-associated pro tein GAP-43 is a nerve-specific protein that is predominantly localize d in the axonal growth cones and presynaptic terminal membrane. In the present study we have examined a possible interaction of GAP-43 with components involved in the exocytosis. GAP-43 was found to interact wi th syntaxin, SNAP-25 and VAMP in rat brain tissues and nerve growth fa ctor-dependently differentiated PC12 cells, but not in undifferentiate d PC12 cells. GAP-43 also interacted with synaptotagmin and calmodulin . These interactions of GAP-43 could be detected only when chemical cr osslinking of proteins was performed before they were solubilized from the membranes with detergents, in contrast with the interaction of th e synaptic core complex, which was detected without cross-linking. Exp eriments in vitro showed that the interaction of GAP-43 with these pro teins occurred Ca2+-dependently; its maximum binding with the core com plex was observed at 100 mu M Ca2+, whereas that of syntaxin with syna ptotagmin was at 200 mu M Ca2+. These values of Ca2+ concentration are close to that required for the Ca2+-dependent release of neurotransmi tters. Furthermore we observed that the interaction in vitro of GAP-43 with the synaptic core complex was coupled with protein kinase C-medi ated phosphorylation of GAP-43. Taken together, our results suggest a novel function of GAP-43 that is involved in the Ca2+-dependent fusion of synaptic vesicles.