Figure 1.

Multistage pathway to fusion through hemifusion. Divalent cations and membrane tension drive membrane bilayers to hemifusion and fusion. Evidence suggests the pathway to vesicle fusion begins with an initial hemifused connection (fused outer monolayers, red) that is a minimally sized hemifusion diaphragm (HD), i.e., the stalk, and tension- and cation-induced leaflet-condensation forces drive HD expansion to a final equilibrium state (dead-end hemifusion) unless HD tension is sufficient to cause rupture (fusion).