Synaptotagmin-1 Is an Antagonist for Munc18-1 in SNARE Zippering

Background: The molecular mechanisms of the critical necessity of Munc18-1 protein for neurotransmitter release remain unclear. Results: Synaptotagmin-1 competes with Munc18-1 in SNARE zippering and fusion pore opening. Conclusion: Synaptotagmin-1 wins the tug-of-war in gaining control of the SNAREpin at the moment of membrane fusion. Significance: This work clarifies an ambiguity concerning the Munc18-1 function in neuroexocytosis. In neuroexocytosis, SNAREs and Munc18-1 may consist of the minimal membrane fusion machinery. Consistent with this notion, we observed, using single molecule fluorescence assays, that Munc18-1 stimulates SNARE zippering and SNARE-dependent lipid mixing in the absence of a major Ca2+ sensor synaptotagmin-1 (Syt1), providing the structural basis for the conserved function of Sec1/Munc18 proteins in exocytosis. However, when full-length Syt1 is present, no enhancement of SNARE zippering and no acceleration of Ca2+-triggered content mixing by Munc18-1 are observed. Thus, our results show that Syt1 acts as an antagonist for Munc18-1 in SNARE zippering and fusion pore opening. Although the Sec1/Munc18 family may serve as part of the fusion machinery in other exocytotic pathways, Munc18-1 may have evolved to play a different role, such as regulating syntaxin-1a in neuroexocytosis.