G protein betagamma directly regulates SNARE protein fusion machinery for secretory granule exocytosis.

The activation of G protein-coupled receptors (GPCRs) can result in an inhibition of Ca(2+)-dependent hormone and neurotransmitter secretion. This has been attributed in part to G protein inhibition of Ca(2+) influx. However, a frequently dominant inhibitory effect, of unknown mechanism, also occurs distal to Ca(2+) entry. Here we characterize direct inhibitory actions of G protein betagamma (Gbetagamma) on Ca(2+)-triggered vesicle exocytosis in permeable PC12 cells. Gbetagamma inhibition was rapid (<1 s) and was attenuated by cleavage of synaptosome-associated protein of 25 kD (SNAP25). Gbetagamma bound soluble N-ethylmaleimide-sensitive factor attachment protein receptor (SNARE) complexes, and binding was reduced to SNARE complexes containing cleaved SNAP25 or by Ca(2+)-dependent synaptotagmin binding. Here we show inhibitory coupling between GPCRs and vesicle exocytosis mediated directly by Gbetagamma interactions with the Ca(2+)-dependent fusion machinery.