Fusion in phospholipid spherical membranes. II. Effect of cholesterol, divalent ions and pH

Effect of cholesterol, divalent ions and pH on spherical bilayer membrane fusion was studied as a function of increasing temperature. Spherical bilayer membranes were composed of natural [phosphatidylcholine (PC) and phosphatidylserine (PS)] as well as synthetic (dipalmitoyl-PC, dimyristoyl-PC and dioleoyl-PC) phospholipids. Incorporation of cholesterol into the membrane (33% by weight) suppressed the fusion temperature and also greatly reduced the percentage of membrane fusion. The presence of 1 mM divalent ions (Ca++, Mg++ or Mn++) on both sides or one side of the PC membrane did not affect appreciably its fusion characteristic with temperature, but the PS membrane fusion with temperature was greatly enhanced by the presence of divalent ions. The variation of pH of the environmental solution in the range of 5.5 approximately 7.0 did not affect the membrane fusion characteristic. However, at pH 8.5, the fusion with respect to temperature was shifted toward the lower temperature by approximately 3degreesC for PC and PS membranes, and at pH 3.0 the opposite situation was observed as the fusion temperature was increased by 6degreesC for PS membranes and by 4degreesC for PC membranes The results seem to indicate that membrane fluidity and structural instability in the bilayer are important for membrane fusion to occur.