Effect of monovalent cations on polyvalent cation-induced fusion of phosphatidylserine small unilamellar vesicles

Fluorescence internal contents mixing assay was used to monitor the fusion of phosphatidylserine (PS) small unilamellar vesicles, initiated by metal ions (Ca²⁺, La³⁺ and Tb³⁺), at various concentrations of monovalent cations (Li⁺, Na⁺ and K⁺). The influence of ionic strength (0.02-1.0 M) on the threshold concentratino of "fusogenic" cations required to induce fusion was measured. The threshold concentrations increased monotonically (1 mM at 0.1 M to 3.1 mM at 1 M) with the increasing ionic strength of the solution for Ca²⁺, but remained unchanged for both La³⁺ and Tb³⁺. Changes in the ionic strength of the encapsulated solution did not alter the threshold concentrations for all the ions studied, in the range 0.02-0.3 M. The results are analyzed in terms of competitive binding between the monovalent ions and the "fusogenic" ions (Ca²⁺, Tb³⁺ and La³⁺). It is shown that there is a critical value for calcium bound-PS, below which no massive fusion occurs. Bound and free fractions of PS are calculated based on the Gouy-Chapman model, taking activities rather than concentrations of metal ions into account. Our experiments also show that monovalent ions alone do not induce fusion even at high concentrations.