Effect of cholesterol on Ca²⁺-induced aggregation and fusion of sonicated phosphatidylserine/cholesterol vesicles

Small unilamellar vesicles composed of phosphatidylserine (PS) and cholesterol at various ratios were employed in studying the effect of cholesterol on Ca²⁺-induced vesicle aggregation and fusion using the Tb/dipicolinic acid assay. The leakage of preencapsulated Tb³⁺ was also measured. The analysis of the data provided estimates for the rate of aggregation, C₁₁, and the rate of fusion per se, f₁₁. An increase in cholesterol contents results in a decrease in C₁₁ values. Similarly, aggregation of PS/cholesterol vesicles is slower than that of PS vesicles in the presence of 650 mM NaCl. With 100 or 200 mM NaCl, the overall fusion rate of PS/cholesterol vesicles is slower than that of PS vesicles; the rate being reduced by an increase in cholesterol contents. With 600 mM NaCl, the overall fusion rate of PS/cholesterol 9:1 vesicles is faster than that of PS vesicles, and results are well-simulated by assuming no delay in vesicle aggregation up to dimers. Emerging f₁₁ values are larger in PS/cholesterol than in PS vesicles. An analysis of fusion kinetics of several lipid concentrations shows that f₁₁ values of PS/cholesterol 3:1 vesicles are 5-times larger than those of PS vesicles, when fusion occurs in a medium containing 200 mM NaCl and 1.5 mM Ca²⁺. The increase in Na⁺ concentration from 100 to 200 mM, or 600 mM results in a 50- or 150-fold reduction in f₁₁ values of PS vesicles. It is suggested that incorporation of cholesterol in PS vesicles results in enhancement of Ca²⁺-induced fusogenic capacity.