Modulation of membrane fusion by ionotropic and thermotropic phase transitions.

We have studied the relationship of ionotropic and thermotropic phase transitions to divalent cation induced fusion of large unilamellar phospholipid vesicles. Fusion was monitored by the Tb/dipicolinic acid fluorescence assay for the intermixing of internal aqueous contents of vesicles. The phase behavior of the membranes was followed by differential scanning calorimetry. (1) Sr2+ and Ba2+ shifted the phase transition temperature (Tc) of bovine brain phosphatidylserine vesicles from 6 to 27 and 31.5 degrees C, respectively. These cations induced vesicle fusion at temperatures above or below the Tc of that cation/phospholipid complex, indicating that an isothermal phase change from the liquid-crystalline to the gel phase is not a requirement for membrane fusion. (2) The temperature dependence of the initial rate of fusion of phosphatidylserine/dipalmitoylphosphatidylcholine (1:1) vesicles in the presence of Ca2+ exhibited a pronounced maximum at 17 degrees C, at the lower part of the broad phase transition endotherm whose Tc was about 25 degrees C; fusion was inhibited completely at 30 degrees C when the membrane was in the liquid-crystalline state. These observations suggest that molecular clusters rich in phosphatidylserine, formed when the membrane is in the phase transition region, allow the vesicles to fuse. (3) The fusion of phosphatidylserine/dimyristoylphosphatidylethanolamine (1:1) vesicles, whose Tc was also around 25 degrees C, had a different temperature dependence in that the initial rate increased sharply above the Tc, with a local maximum within the transition region. Phase separation of dimyristoylphosphatidylethanolamine was induced by Ca2+ but not by Mg2+, although both ions induced fusion.(ABSTRACT TRUNCATED AT 250 WORDS)