Calcium- and magnesium-induced fusion of mixed phosphatidylserine/phosphatidylcholine vesicles: effect of ion binding.
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.
D001665:Binding Sites; D002118:Calcium / Q000494:pharmacology*; D002413:Cations, Divalent / Q000378:metabolism; D002459:Cell Fusion / Q000187:drug effects; D008081:Liposomes; D008274:Magnesium / Q000494:pharmacology*; D008563:Membrane Lipids; D008954:Models, Biological; D010713:Phosphatidylcholines; D010718:Phosphatidylserines; D013010:Sonication
D002413:Cations, Divalent; D008081:Liposomes; D008563:Membrane Lipids; D010713:Phosphatidylcholines; D010718:Phosphatidylserines; D008274:Magnesium; D002118:Calcium