Membrane fusion and molecular segregation in phospholipid vesicles

Fusion between vesicles prepared from individual or mixed phospholipid species was demonstrated by ultracentrifugation and gel-filtration techniques, electron microscopy and differential scanning calorimetry. Variation of the chemical composition of the vesicles permitted evaluation of the effect of surface charge, Ca²⁺, fluidity and the presence of cholesterol on the fusion reaction and the segregation of lipid species within fused vesicles. Extensive fusion occurred between negatively charged phosphatidylserine vesicles incubated in the presence of CaCl₂ (> 1 mM) and in vesicles prepared from greater than 50% phosphatidylserine in phosphatidylcholine in the presence of CaCl₂ (> 4 mM) and albumin (0.1 mg/ml). Neutral phosphatidylcholine vesicles showed only a limited capacity to fuse. Vesicles containing lipids that were in a liquid-crystalline state were more susceptible to fusion than vesicles composed of lipids that were in the solid phase at experimental temperatures. Incorporation of equimolar amounts of cholesterol into vesicles composed of lipids in a liquid-crystalline state suppressed their ability to fuse. Calorimetric measuremens revealed Ca²⁺ induced segregation of individual lipids to form separate domains within the vesicle membrane (phase separation). The relationship of fusion between vesicles and fusion occurring in natural membranes was discussed.