Insight in the exocytotic process in chromaffin cells: regulation by trimeric and monomeric G proteins.

Catecholamine secretion from chromaffin cells has been used for a long time as a general model to study exocytosis of large dense core secretory granules. Permeabilization and microinjection techniques have brought the possibility to dissect at the molecular level the multi-protein machinery involved in this complex physiological process. Regulated exocytosis comprises distinct and sequential steps including the priming of secretory granules, the formation of a docking complex between granules and the plasma membrane and the subsequent fusion of the granule with the plasma membrane. Key proteins involved in the exocytotic machinery have been identified. For instance, SNAREs which participate in the docking events in most intracellular transport steps along the secretory pathway, play a role in exocytosis in both neuronal and endocrine cells. However, in contrast to intracellular transport processes for which the highest fusion efficiency is required after correct targeting of the vesicles, the number of exocytotic events in activated secretory cells needs to be tightly controlled. We describe here the multistep control exerted by heterotrimeric and monomeric G proteins on the progression of secretory granules from docking to fusion and the molecular nature of some of their downstream effectors in neuroendocrine chromaffin cells.