Calcium- and magnesium-induced fusion of mixed phosphatidylserine/phosphatidylcholine vesicles: Effect of ion binding

Abstract

The aggregation, leakage, and fusion of pure PS (phosphatidylserine) and mixed PS/PC (phosphatidylcholine) sonicated vesicles were studied by light scattering, the release of encapsulated carboxyfluorescein, and a new fusion assay which monitors the mixing of the internal compartments of fusing vesicles. On a time scale of 1 min the extent of fusion was considerably greater than leakage. The Ca2+ and Mg2+ concentrations required to induce fusion increased when the PS content of the vesicles was decreased, and/or when the NaCl concentration was increased. Calculations employing a modified Gouy-Chapman equation and experimentally determined intrinsic binding constants of Na+ and Ca2+ to PS were shown to predict correctly the amount of Ca2+ bound in mixed PS/PC vesicles. For vesicles composed of either pure PS or of mixtures with PC in 100 mM NaCl (4:1 and 2:1 PS/PC); the induction of fusion (on a time scale of minutes) occurred when the amount of Ca or Mg bound/PS molecule exceeded 0.35-0.39. The induction of fusion for both pure PS and PS/PC mixed vesicles (with PS exceeding 50%) can be explained by assuming that destabilization of these vesicles requires a critical binding ratio of divalent cations to PS.

High-Generation Polycationic Dendrimers Are Unusually Effective at Disrupting Anionic Vesicles: Membrane Bending Model

“…Surprisingly, the extent of lipid mixing increases with increasing NaCl concentration. This relationship is the reverse to that observed with Ca 2+ -mediated fusion (27).…”

“…Apparently, the polycations are able to overcome the surface hydration barrier provided by the external PEG chains and force the apposed membranes to undergo lipid mixing. Similarly, Ca 2+ -mediated fusion is strongly inhibited by increasing NaCl concentrations (27), but the lipid mixing induced by PAMAM dendrimers and poly(lysine) actually increases with NaCl concentration (Table 4). Na + ions are thought to affect fusion in opposing ways (50).…”

Imaging single membrane fusion events mediated by SNARE proteins

“…Divalent cations are known to bridge negatively charged phospholipid head groups such as phosphatidylserine as well as uncharged phosphatidylcholine head groups and have been reported to induce fusion between lipid vesicles (17)(18)(19). However, when the above experiments were repeated in the absence of SNARE proteins, no fusion events were observed in the absence of or after addition of Ca 2ϩ to a final concentration of 100 M (Fig.…”

Membrane Fusion Induced by Small Molecules and Ions

“…A wide range of studies were done starting from early seventies to late eighties on cations-induced fusion. From almost two decades of work, two cations, namely, Ca 2+ [52–58] and Mg 2+ [16, 59, 60] were found, which can induce membrane fusion effectively. Other cations like Mn 2+ [49], Zn 2+ [61–63], La 3+ [64], Sr 3+ [16], and H + [65] also have some potential to induce membrane fusion, but Ca 2+ is the principal player.…”