Ca2+ and pH induced fusion of small unilamellar vesicles consisting of phosphatidylethanolamine and negatively charged phospholipids: a freeze fracture study.

Fusion processes in small (sonicated) unilamellar vesicle (SUV) systems composed of 80 mol % unsaturated phosphatidylethanolamine and 20 mol % of phosphatidylserine, phosphatidylinositol, phosphatidic acid or cardiolipin have been examined by freeze-fracture techniques. All these mixtures have the property that Ca2+ and, in some cases low pH, can trigger bilayer to hexagonal (HII) phase transitions in large multilamellar dispersions of these lipid mixtures. It is shown that when the SUV systems are subjected to similar protocols, the vesicles first fuse to form larger systems prior to HII phase formation and this fusion is accompanied by the appearance of lipidic particle structures, often localized to the fusion interface. We conclude that factors promoting HII phase structure initially result in fusion of unilamellar systems and that this fusion proceeds via intermediary formation of non-lamellar, possibly inverted micellar, structure.