Many hormones and neurotransmitters, stored in intracellular vesicles, are released from neurons and endocrine cells by calcium-dependent exocytosis. This process can be blocked by G protein-coupled receptors (GPCRs) whose activation triggers a number of inhibitory events, including blocking the entry of calcium through ion channels at the cell surface. Blackmer et al. report that the βγ subunits of heterotrimeric G proteins (Gβγ) can also block exocytosis by directly binding to secretory vesicles and blocking their fusion with the plasma membrane. The authors examined permeabilized mammalian PC12 cells that release catecholamines in response to an increase in intracellular calcium (an exocytosis assay in which ion channels play no role). Addition of Gβγ subunits to permeabilized cells blocked secretion within one second. Recombinant Gβγ subunits bound to purified soluble N-ethylmaleimide-sensitive factor attachment protein receptor (SNARE) complex, a protein assembly that is involved in the fusion step of vesicle exocytosis. In the presence of calcium, synaptotagmin, a SNARE-associated calcium sensor for exocytosis, bound more efficiently to SNARE complexes than Gβγ bound to SNARE complexes in vitro. This suggests that activated GPCRs may block secretion by both reducing calcium influx and blocking the SNARE fusion machinery that mediates calcium-dependent exocytosis.
T. Blackmer, E. C. Larsen, C. Bartleson, J. A. Kowalchyk, E.-J. Yoon, A. M. Preininger, S. Alford, H. E. Hamm, T. F. J. Martin, G protein directly regulates SNARE protein fusion machinery for secretory granule exocytosis. Nat. Neurosci. 8, 421-425 (2005). [PubMed]