Modulation of membrane fusion by ionotropic and thermotropic phase transitions

Publisher Website
Google Scholar
Abstract
The relationship of ionotropic and thermotropic phase transitions to divalent cation induced fusion of large unilamellar phospholipid vesicles was studied. 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. Sr2+ and Ba2+ shifted the phase transition temperature (Tc) of bovine brain phosphatidylserine [PS] vesicles from 6 to 27 and 31.5.degree. 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. The temperature dependence of the initial rate of fusion of PS/dipalmitoylphosphatidylcholine (1:1) vesicles in the presence of Ca2+ exhibited a pronounced maximum at 17.degree. C, at the lower part of the broad phase transition endotherm whose Tc was about 25.degree. C; fusion was inhibited completely at 30.degree. C when the membrane was in the liquid-crystalline state. Molecular clusters rich in the phosphatidylserine probably formed when the membrane is in the phase transition region, probably allow the vesicles to fuse. The fusion of PS/dimyristoylphosphatidylethanolamine (1:1) vesicles, whose Tc was also around 25.degree. 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. The observation that PS/egg phosphatidylethanolamine (1:1) vesicles fused in the presence of Ca2+ or Mg2+ at temperatures below or above the lamellar to hexagonal (HII) transition temperature of the phosphatidylethanolamine and that Mg2+ could induce fusion without causing a transition into the HII phase suggests that this transition is not essential for membrane fusion. On the basis of all 3 systems, it is proposed that fusion occurs via defects in molecular packing and dehydration of the polar groups of phospholipids at the region of interbilayer contact.

This publication has 32 references indexed in Scilit: