FORMATION OF THE INVERTED HEXAGONAL STRUCTURE IN BOVINE BRAIN PHOSPHOLIPID-MEMBRANES INDUCED BY DIOLEOYLGLYCEROL

To evaluate the diacylglycerol (DG) concentration that may induce the membrane fusion, we examined the formation of the inverted hexagonal ( H-II) structure in the multilamellar vesicles comprised of bovine brai n phosphatidylcholine (BBPC), bovine brain phosphatidylethanolamine (B BPE) and cholesterol (1:1:1 molar ratio) in the absence or presence of a small amount of dioleoylglycerol (DOG). These lipid layers are cons idered as the model system for the secreting cell membranes. The forma tion of the H-II structure was detected using P-31-NMR and fluorescenc e techniques. The fluorescent probe used was ne-5-sulfonyl)-dipalmitoy lphosphatidylethanolamine (DNS-PE). Without DOG, the H-II structure sl ightly appeared at 37 degrees C and the extent of the H-II, structure increased with increasing temperature. The onset temperature of the H- II formation decreased to 20 degrees C by the addition of 3 mol % of D OG. Thus, only a few mol % of DG could produce a significant amount of H-II structure, indicating that the physiological amount of DG promot es the fusion of the secreting cell membranes through H-II like interm ediates.