The role of membrane lateral tension in calcium-induced membrane fusion

Calcium-induced fusion of liposomes was studied with a view to understand t he role of membrane tension in this process. Lipid mixing due to fusion was monitored by following fluorescence of rhodamine-phosphatidyl-ethanolamine incorporated into liposomal membrane at a self-quenching concentration. Th e extent of lipid mixing was found to depend on the rate of calcium additio n: at slow rates it was significantly lower than when calcium was injected instantly. The vesicle inner volume was then made accessible to external ca lcium by adding calcium ionophore A23187. No effect on fusion was observed at high rates of calcium addition while at slow rates lipid mixing was elim inated. Fusion of labeled vesicles with a planar phospholipid membrane (BLM ) was studied using fluorescence microscopy. Above a threshold concentratio n specific for each ion, Ca2+, Mg2+, Cd2+ and La3+ induce fusion of both ch arged and neutral membranes. The threshold calcium concentration required f or fusion was found to be dependent on the vesicle charge, but not on the B LM charge. Pretreatment of vesicles with ionophore and calcium inhibited ve sicle fusion with BLM. This effect was reversible: chelation of calcium pri or to the application of vesicle to BLM completely restored their ability t o fuse. These results support the hypothesis that tension in the outer mono layer of lipid vesicle is a primary reason for membrane destabilization pro moting membrane fusion. How this may be a common mechanism for both purely lipidic and protein-mediated membrane fusion is discussed.