Micromolar Ca²⁺ stimulates fusion of lipid vesicles with planar bilayers containing a calcium-binding protein

Fusion of phospholipid vesicles with a planar phospholipid bilayer membrane that contains a calcium-binding protein appears to mimic the essential aspects of cytoplasmic-vesicle fusion with plasma membranes (exocytosis) in that (i) there is a low basal rate of fusion in the absence of Ca²⁺, (ii) this basal rate is enormously increased by micromolar (∼ 10 μM) amounts of Ca²⁺, and (iii) this rate is not increased by millimolar Mg ²⁺. Essential to this process is an osmotic gradient across the planar membrane, with the side containing the vesicles hyperosmotic to the opposite side. Similar osmotic gradients or their equivalent may be crucial for biological fusion events.