Ca2+-induced fusion of sulfatide-containing phosphatidylethanolamine smallunilamellar vesicles

The fusogenic properties of sulfatide-containing 1,2-dioleoyl-3-sn-phosphat idylethanolamine (DOPE) small unilamellar vesicles (SUVs) in the presence o f CaCl2 were studied by mixing membrane lipids based on an assay of fluores cence resonance energy transfer (FRET). Fusion of the vesicles was also con firmed by mixing aqueous contents with the Tb/dipicolinate (DPA) assay. The half-times of lipid mixing revealed that the fusion rate decreased with in creasing molar concentration of sulfatide. This inhibitory effect was more obvious at sulfatide concentrations higher than 30 mol%, where hydration at the membrane surface reached its maximum and the fusion tvas no longer pH- sensitive in the range of pH 6.0-9.0. Similar inhibitory effect was also ob served in Ca2+-induced fusion of DOPE/ganglioside GM(1) vesicles but at a l ower concentration of the glycosphingolipid (20 mol%). In contrast, increas ing the concentration of phosphatidylserine (PS) in DOPE/PS SUVs resulted i n an increase in the rate of Ca2+-induced lipid mixing and the pH sensitivi ty of this system was not affected. These results are consistent with an in creasing steric hindrance to membrane fusion at higher molar concentration and larger headgroup size of the glycosphingolipids. Interestingly, the pH sensitivity of the sulfatide-containing liposomes was retained when they we re allowed to fuse with synaptosomes in the absence of Ca2+ by a mechanism involving protein mediation.