G protein βγ directly regulates SNARE protein fusion machinery for secretory granule exocytosis

The activation of G protein - coupled receptors (GPCRs) can result in an inhibition of Ca2+-dependent hormone and neurotransmitter secretion. This has been attributed in part to G protein inhibition of Ca2+ influx. However, a frequently dominant inhibitory effect, of unknown mechanism, also occurs distal to Ca2+ entry. Here we characterize direct inhibitory actions of G protein beta gamma (G beta gamma) on Ca2+-triggered vesicle exocytosis in permeable PC12 cells. G beta gamma inhibition was rapid (<1 s) and was attenuated by cleavage of synaptosome-associated protein of 25 kD (SNAP25). G beta gamma bound soluble N-ethylmaleimide-sensitive factor attachment protein receptor ( SNARE) complexes, and binding was reduced to SNARE complexes containing cleaved SNAP25 or by Ca2+-dependent synaptotagmin binding. Here we show inhibitory coupling between GPCRs and vesicle exocytosis mediated directly by G beta gamma interactions with the Ca2+-dependent fusion machinery.