Effect of Ca²⁺ on vesicle fusion on solid surface: An in vitro model of protein-accelerated vesicle fusion

Lipid vesicle fusion is an important reaction in the cell. Calcium ions (Ca²⁺) participate in various important biological events including the fusion of vesicles with cell membranes in cells. We studied the effect of Ca²⁺ on the fusion of egg yolk phosphatidylcholine/brain phosphatidylserine (eggPC/brainPS) lipid vesicles on a mica substrate with fast scanning atomic force microscopy (AFM). When unattached and unfused lipid vesicles on mica were rinsed away, discrete patches of fused vesicles were observed under high Ca²⁺ concentrations. At 0 mM Ca²⁺, lipid vesicles were fused on mica and formed continuous supported lipid bilayers (SLBs) covering almost the entire mica surface. The effect of Ca²⁺ on SLB formation was offset by a Ca²⁺ chelating agent. When lipid vesicles were added during AFM observation, vesicles fused on mica and covered almost all areas even under high Ca²⁺ concentrations. These results indicate that force between AFM tip and vesicles overcomes the Ca ²⁺-reduced fusion of lipid vesicles.