The hemifused state on the pathway to membrane fusion.

Fusion of compartments enclosed by membrane bilayers enables secretion and other vital cellular processes and is widely studied in model synthetic membrane systems. Experiments suggest the fusion pathway passes through a hemifused intermediate where only outer monolayers are fused. Here we show membrane tension and divalent cations drive vesicles to hemifused equilibrium with expanded hemifusion diaphragms (HDs) where inner monolayers engage. Predicted HD sizes agree with recent measurements of Nikolaus et al. [Biophys. J. 98, 1192 (2010).]. The fusion pathway is completed by HD lysis provided HD tension is sufficiently high.