Fusogenic capacities of divalent cations and effect of liposome size

The initial kinetics of divalent cation (Ca2+, Ba2+, Sr2+) induced fusion of phosphatidylserine (PS) liposomes, LUV, is examined to obtain the fusion rate constant, f11, for two apposed liposomes as a function of bound divalent cation. The aggregation of dimers is rendered very rapid by having Mg2+ in the electrolyte, so that their subsequent fusion is rate limiting to the overall reaction. In this way the fusion kinetics are observed directly. The bound Mg2+, which by itself is unable to induce the PS LUV to fuse, is shown to affect only the aggregation kinetics when the other divalent cations are present. There is a threshold amount of bound divalent cation below which the fusion rate constant f11 is small and above which it rapidly increases with bound divalent cation. These threshold amounts increase in the sequence Ca2+ less than Ba2+ less than Sr2+, which is the same as found previously for sonicated PS liposomes, SUV. While Mg2+ cannot induce fusion of the LUV and much more bound Sr2+ is required to reach the fusion threshold, for Ca2+ and Ba2+ the threshold is the same for PS SUV and LUV. The fusion rate constant for PS liposomes clearly depends upon the amount and identity of bound divalent cation and the size of the liposomes. However, for Ca2+ and Ba2+, this size dependence manifests itself only in the rate of increase of f11 with bound divalent cation, rather than in any greater intrinsic instability of the PS SUV. The destabilization of PS LUV by Mn2+ and Ni2+ is shown to be qualitatively distinct from that induced by the alkaline earth metals.