We demonstrate that there are two experimentally distinguishable steps in the fusion of phospholipid vesicles with planar bilayer membranes. In the first step, the vesicles form a stable, tightly bound pre-fusion state with the planar membrane; divalent cations (Ca++) are required for the formation of this state if the vesicular and/or planar membrane contain negatively charged lipids. In the second step, the actual fusion of vesicular and planar membranes occurs. The driving force for this step is the osmotic swelling of vesicles attached (in the pre-fusion state) to the planar membrane. We suggest that osmotic swelling of vesicles may also be crucial for biological fusion and exocytosis.

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