Membrane fusion: what may transpire at the atomic level

Target membrane proteins, SNAP-25 and syntaxin (t-SNARE), and secretory vesicleassociated membrane protein (v-SNARE), are part of the conserved protein complex involved in fusion of opposing bilayers in biological systems in the presence of calcium. It is known that SNARE interaction allows opposing bilayers to come close within a distance of approximately 3 Å, enabling calcium to drive membrane fusion. To understand the possible mechanism of membrane fusion at the atomic level, X-ray diffraction studies and light scattering measurements were performed in SNARE-reconstituted liposomes. Our study demonstrates that if t-SNARE vesicles and v-SNARE vesicles are allowed to interact prior to calcium addition, membrane fusion fails to occur. These studies suggest that hydrated calcium ions are too large (∼ 6 Å) to fit between the space in SNARE-apposed bilayers, and therefore unable to induce membrane fusion. However in the presence of calcium, t-SNARE vesicles interact with v-SNARE vesicles, allowing the formation of calcium-phosphate bridges between the opposing bilayers, resulting in the expulsion of water due to disruption of the water shell around the calcium ion, enabling lipid mixing and membrane fusion.