Priming in exocytosis: attaining fusion-competence after vesicle docking

Membrane contact established by tethering or docking mechanisms is not a sufficient condition for membrane fusion. In neural and neuroendocrine cells, only a small fraction of secretory vesicles docked at the plasma membrane are fusion-competent and undergo rapid ATP-independent fusion in response to Ca(2+) elevations. Additional biochemical events termed 'priming' are essential to render vesicles competent for Ca(2+)-triggered fusion. The priming of vesicles is ATP-dependent and a number of ATP-dependent priming reactions have been characterized in permeable neuroendocrine cells. These involve NSF-mediated priming of SNARE protein complexes, the ATP-dependent synthesis of phosphoinositides, and protein kinase-mediated protein phosphorylation. In addition, munc13 is an important protein involved in priming synaptic vesicles. An emphasis in this review is on recent work indicating that priming events identified in the pathways of regulated exocytosis share many features with pre-fusion processes characterized in constitutive fusion pathways.