Characterization of Ca2(+)-dependent phospholipid binding, vesicle aggregation and membrane fusion by annexins

The annexins are a family of structurally similar, Ca2(+)-dependent, phospholipid-binding proteins. We compared six members of this family (calpactin I heavy chain, lipocortins I and III, endonexin II, p68 and protein II) to determine their phospholipid-binding specificities, as well as their ability to promote aggregation and fusion of phospholipid vesicles. The Ca2+ requirement for all of the proteins was lowest for binding to vesicles composed of phosphatidic acid, followed by phosphatidylserine and then phosphatidylinositol. Only protein II, p68, lipocortin III and endonexin II bound to vesicles composed of phosphatidylethanolamine, and none bound to phosphatidylcholine. Both calpactin I heavy chain and lipocortin I promoted aggregation of phosphatidylserine- or phosphatidylinositol-containing vesicles in the presence of less than 10 microM-Ca2+. Lipocortin I promoted fusion of liposome membranes by lowering threshold Ca2+ concentrations. Although calpactin I heavy chain did not affect threshold Ca2+ concentrations, it did increase the rate and extent of spontaneous fusion. In contrast, p68 inhibited fusion at threshold Ca2+ concentrations. Whereas previous reports have emphasized properties that the annexins have in common, these findings reveal quantitative and qualitative differences among the annexins which may relate to distinct intracellular functions.