Binding of the complexin N terminus to the SNARE complex potentiates synaptic-vesicle fusogenicity.

Complexins facilitate and inhibit neurotransmitter release through distinct domains, and their function was proposed to be coupled to the Ca(2+) sensor synaptotagmin-1 (Syt1). However, the mechanisms underlying complexin function remain unclear. We now uncover an interaction between the complexin N terminus and the SNARE complex C terminus, and we show that disrupting this interaction abolishes the facilitatory function of complexins in mouse neurons. Analyses of hypertonically induced exocytosis show that complexins enhance synaptic-vesicle fusogenicity. Genetic experiments crossing complexin- and Syt1-null mice indicate a functional interaction between these proteins but also show that complexins can promote Ca(2+)-triggered release in the absence of Syt1. We propose that the complexin N terminus stabilizes the SNARE complex C terminus and/or helps release the inhibitory function of complexins, thereby activating the fusion machinery in a manner that may cooperate with Syt1 but does not require Syt1.