Kinetics of Divalent Cation Induced Fusion of Phosphatidylserine Vesicles: Correlation between Fusogenic Capacities and Binding Affinities

Fusion and aggregation of sonicated phosphatidylserine small unilamellar vesicles (PS SUV) induced by the divalent cations Ba²⁺, Ca²⁺, Sr²⁺, and Mg²⁺ are studied and correlated with cation binding. Fusion is monitored with the terbium/dipicolinic acid (Tb/DPA) assay, which is supplemented by measuring the dissociation of preencapsulated Tb/DPA complex due to leakage of contents and entry of medium into the vesicles. The separate contributions of aggregation and fusion rates to the overall kinetics of membrane fusion are analyzed. In the presence of Na⁺ or Li⁺ concentrations of 300 mM or less, the rate of the overall fusion reaction induced by the divalent cations decreases in the sequence Ba²⁺ > Ca²⁺ > Sr²⁺ > Mg²⁺ with respect to bulk concentrations. Under these conditions, both aggregation kinetics and bilayer destabilization are shown to affect the overall rate of fusion. In the presence of 500 mM Na⁺ or Li⁺ and subfusogenic concentrations of each of the divalent cations, the PS SUV reversibly aggregate; thus, the effect of the divalent cations (at larger, fusing concentrations) on the rate of bilayer destabilization can be examined directly. Here, Ba²⁺ and Ca²⁺ appear to be equally effective at inducing fusion. However, when the rate constant of fusion (which can be easily obtained in these cases) is compared with the amount of divalent cation bound per PS (calculated from binding constants which apply to the vesicles before aggregation and fusion), the fusogenic capacities of these divalent cations follow the sequence Ca²⁺ > Ba²⁺ > Sr²⁺ > Mg²⁺.