GAP-43 CONTROLS THE AVAILABILITY OF SECRETORY CHROMAFFIN GRANULES FORREGULATED EXOCYTOSIS BY STIMULATING A GRANULE-ASSOCIATED G(O)

Besides having a role in signal transduction, heterotrimeric G protein s may also be involved in membrane trafficking events as suggested by their presence in specific intracellular compartments. In chromaffin c ells, G(alpha o) is associated with secretory organelles, and its acti vation inhibits exocytosis. Although plasma membrane-bound G proteins are activated by cell surface receptors, the intracellular proteins co ntrolling organelle-associated G proteins are currently unknown. GAP-4 3, a neuronal protein enriched in axonal growth cones and presynaptic terminals, is one possible candidate since it can directly stimulate p urified G(o). We have investigated the interaction of adrenal medullar y GAP-43 with chromaffin granule-associated G(o) and its effect on cat echolamine secretion. Cytosolic and depalmitoylated membrane-extracted GAP-43 were found to stimulate guanine nucleotide binding and exchang e activity in chromaffin granule membranes. In permeabilized chromaffi n cells, both forms of GAP-43 blocked calcium dependent exocytosis, an d this effect was inhibited by specific antibodies against G(alpha o). A synthetic peptide corresponding to the GAP-43 domain that interacts with G(o) inhibited catecholamine secretion. This effect could be sel ectively reversed by the COOH-terminal peptide of G(alpha o). These re sults indicate that GAP-43 may be an endogenous pseudoreceptor for the secretory granule-bound form of G(o) and can thereby control calcium- regulated exocytosis in chromaffin cells.