N- to C-terminal SNARE complex assembly promotes rapid membrane fusion.

Assembly of the soluble N-ethylmaleimide-sensitive factor attachment protein receptors (SNAREs) syntaxin 1, SNAP-25, and synaptobrevin 2 is thought to be the driving force for the exocytosis of synaptic vesicles. However, whereas exocytosis is triggered at a millisecond time scale, the SNARE-mediated fusion of liposomes requires hours for completion, which challenges the idea of a key role for SNAREs in the final steps of exocytosis. We found that liposome fusion was dramatically accelerated when a stabilized syntaxin/SNAP-25 acceptor complex was used. Thus, SNAREs do have the capacity to execute fusion at a speed required for neuronal secretion, demonstrating that the maintenance of acceptor complexes is a critical step in biological fusion reactions.