Control of membrane fusion by phospholipid head groups II. The role of phosphatidylethanolamine in mixtures with phosphatidate and phosphatidylinositol

Membrane fusion induced by Ca2+ and Mg2+ in large unilamellar vesicles composed of mixtures of phosphatidylethanolamine with phosphatidate and phosphatidylinositol was studied by means of a fluorescence assay for the intermixing of internal aqueous contents of the vesicles. The threshold concentrations of Ca2+ or Mg2+ required for fusion increased only moderately when up to 80 mol% phosphatidylethanolamine was included with phosphatidate at pH 7.4, but no fusion could be detected in vesicles containing 70 mol% phosphatidylcholine even at high concentrations of Ca2+ or Mg2+. Phosphatidate-phosphatidylethanolamine (1 : 4) vesicles could be induced to fuse by 0.1 mM Ca2+ in the presence of a Mg2+ concentration which alone was insufficient for fusion. When equimolar amounts of phosphatidylethanolamine was included with phosphatidylinositol, the vesicles were susceptible to fusion by Ca2+, although pure phosphatidylinositol vesicles themselves merely aggregate and do not fuse (Sundler, R. and Papahadjopoulos, D. (1981) Biochim. Biophys. Acta 649, 743-750, accompanying paper). The role of phosphatidylethanolamine acyl chains, and hence the possible involvement of the bilayer-hexagonal (HII) transition in membrane fusion, was examined by the temperature dependence of Ca2+-induced fusion in phosphatidylinositol-dimyristoylphosphatidylethanolamine (1 : 1) vesicles. Fusion was strictly dependent on the gel-liquid crystalline transition of the mixture and not on the phase behavior of the phosphatidylethanolamines. Comparable fusion rates were obtained for both egg yolk phosphatidylethanolamine and dimyristoylphosphatidylethanolamine at 50°C. As the dimyristoylphosphatidylethanolamine does not convert to a non-bilayer phase in this temperature range, we conclude that the bilayer-hexagonal transition is not necessary for membrane fusion. We propose that the dehydration characteristics of the phospholipids and their metal ion complexes are the critical factors determining fusion suceptibility of phospholipid membranes. © 1981