SYNERGISTIC EFFECTS OF MICROMOLAR CONCENTRATIONS OF ZN-2+ AND CA-2+ ON MEMBRANE-FUSION

Resonance Energy Transfer between N-(7-nitro-2,1,3 benzoxadiazol-4 yl) phosphatidyl ethanolamine and N-Lissamine-Rhodamine B sulfonyl phosphatidyl ethanolamine embedded in two different populations of small unilamellar vesicles made of phosphatidyl serine has been used to study the fusion process induced by Zn2+ and Ca2+. Lipid intermixing demonstrating fusion of liposome membranes can already be observed at 125 and 250 .mu.mol/l of Zn2+. After short time pre-incubations with micromolar concentrations of Zn2+ as low as 150 .mu.mol/l, Ca2+ induces an instantaneous increase of vesicle fusion. The lipid intermixing induced by micromolar concentrations of Ca2+ (250-500 .mu.mol/l) could be increased up to 4 times when pre-incubated with 150 or 200 .mu.mol/l of Zn2+. The effect of 1 mM of Ca2+ alone on lipid intermixing can be mimicked by 150 .mu.mol/l of Zn2+ followed by 500 .mu.mol/l of Ca2+. Our data demonstrate that Zn2+ and Ca2+ act synergistically to affect cation-induced membrane fusion. We suggest that Zn2+ specifically alters the physical state of phospholipid membranes making them more prone to calcium-triggered fusion.