Calcium and calmodulin in membrane fusion

Regulated exocytosis was the first intracellular membrane fusion step that was suggested to involve both Ca2+ and calmodulin. In recent years, it has become clear that calmodulin is not an essential Ca2+ sensor for exocytosis but that it is likely to have a more regulatory role. A requirement for cytosolic Ca2+ in other vesicle fusion events within cells has become apparent and in certain cases, such as homotypic fusion of early endosomes and yeast vacuoles, calmodulin may be the primary Ca2+ sensor. A number of distinct targets for calmodulin have been identified including SNARE proteins and subunits of the vacuolar ATPase. The extent to which calmodulin regulates different intracellular fusion events through conserved SNARE-dependent or other mechanisms remains to be resolved. (C) 2003 Elsevier B.V. All rights reserved.