Separation of the osmotically driven fusion event from vesicle-planar membrane attachment in a model system for exocytosis.

TLDR

The fusion process involves an initial Ca²⁺‑dependent pre‑fusion binding of vesicles to the planar membrane, followed by osmotic‑swelling‑driven membrane merger. We identify two distinct fusion steps and propose that osmotic swelling is essential for biological exocytosis.

Abstract

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.