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The Journal of Lipid Research, Vol. 40, 1254-1262, July 1999
Copyright © 1999 by Lipid Research, Inc.

Original Article

Ca2+-induced fusion of sulfatide-containing phosphatidylethanolamine small unilamellar vesicles

Xiaofeng Wua and Qiu-Tian Lia
a Department of Biochemistry, Faculty of Medicine, National University of Singapore, 10 Kent Ridge Crescent, Singapore 119260

Correspondence to: Qiu-Tian Li

The fusogenic properties of sulfatide-containing 1,2-dioleoyl-3-sn -phosphatidylethanolamine (DOPE) small unilamellar vesicles (SUVs) in the presence of CaCl2 were studied by mixing membrane lipids based on an assay of fluorescence resonance energy transfer (FRET). Fusion of the vesicles was also confirmed by mixing aqueous contents with the Tb/dipicolinate (DPA) assay. The half-times of lipid mixing revealed that the fusion rate decreased with increasing 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 was no longer pH-sensitive in the range of pH 6.0 – 9.0. Similar inhibitory effect was also observed in Ca2+-induced fusion of DOPE/ganglioside GM1 vesicles but at a lower concentration of the glycosphingolipid (20 mol%). In contrast, increasing the concentration of phosphatidylserine (PS) in DOPE/PS SUVs resulted in an increase in the rate of Ca2+-induced lipid mixing and the pH sensitivity of this system was not affected.

These results are consistent with an increasing 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 were allowed to fuse with synaptosomes in the absence of Ca2+ by a mechanism involving protein mediation.—Wu, X., and Q-T. Li. Ca2+- induced fusion of sulfatide-containing phosphatidylethanolamine small unilamellar vesicles. J. Lipid Res. 1999. 40: 1254;–1262.

Supplementary key words: pH-sensitive liposomes, fluorescence resonance energy transfer, lipid mixing, headgroup steric hindrance, membrane fusion


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