Membrane bridging by Munc13-1 is crucial for neurotransmitter release

Munc13-1 plays a crucial role in neurotransmitter release. We recently proposed that the C-terminal region encompassing the C₁, C₂B, MUN and C₂C domains of Munc13-1 (C₁C₂BMUNC₂C) bridges the synaptic vesicle and plasma membranes through interactions involving the C₂C domain and the C₁-C₂B region. However, the physiological relevance of this model has not been demonstrated. Here we show that C₁C₂BMUNC₂C bridges membranes through opposite ends of its elongated structure. Mutations in putative membrane-binding sites of the C₂C domain disrupt the ability of C₁C₂BMUNC₂C to bridge liposomes and to mediate liposome fusion in vitro. These mutations lead to corresponding disruptive effects on synaptic vesicle docking, priming, and Ca²⁺-triggered neurotransmitter release in mouse neurons. Remarkably, these effects include an almost complete abrogation of release by a single residue substitution in this 200 kDa protein. These results show that bridging the synaptic vesicle and plasma membranes is a central function of Munc13-1.