Priming in exocytosis: Attaining fusion-competence after vesicle docking

Membrane contact established by tethering or docking mechanisms is not a su fficient 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 t o Ca2+ elevations. Additional biochemical events termed 'priming' are essen tial to render vesicles competent for Ca2+-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 NS F-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 ve sicles. An emphasis in this review is on recent work indicating that primin g events identified in the pathways of regulated exocytosis share many feat ures with pre-fusion processes characterized in constitutive fusion pathway s. (C) 2000 societe francaise de biochimie et biologie moleculaire / Editio ns scientifiques et medicales Elsevier SAS.