Protein-mediated intermembrane contact specifically enhances Ca2+-induced fusion of phosphatidate-containing membranes.

Ca2+-induced fusion of glycolipid-phospholipid vesicles containing several different anionic phospholipids was investigated, with and without lectin-mediated intervesicle contact. In vesicles containing phosphatidylserine, phosphatidylinositol or its mono- or diphosphate as the anionic phospholipid fusion was induced only at 1-10 mM Ca2+ both in the absence and presence of lectin. In contrast, the Ca2+-threshold for fusion of phosphatidate-containing vesicles was reduced to less than or equal to 0.1 mM Ca2+ by lectin-mediated intermembrane contact.