Calcium sensors in regulated exocytosis

Neurotransmitter release, hormone secretion and a variety of other secretor y process are tightly regulated with exocytotic fusion of secretory vesicle s being triggered by a rise in cytosolic Ca2+ concentration. A series of pr oteins that act as part of a conserved core machinery for vesicle docking a nd fusion throughout the cell have been identified. In regulated exocytosis this core machinery must be controlled by Ca2+-sensor proteins that allow rapid activation of the fusion process following elevation of cytosolic Ca2 + concentration. The properties of such Ca2+ sensors are known from physiol ogical studies but their molecular identity remains to be unequivocally est ablished. The multiple Ca2+-dependent steps in the exocytotic pathway sugge st the likely involvement of several Ca2+-binding proteins with distinct pr operties. Functional evidence for the role of various Ca2+-binding proteins and their possible sites of action is accumulating but a definitive identi fication of the major Ca2+-sensor in the final step of Ca2+-triggered membr ane fusion in different cell types awaits further analysis.