Neurotransmitter release depends critically on Munc18-1, Munc13, the Ca2+ sensor synaptotagmin-1 and the SNAREs syntaxin-1, synaptobrevin and SNAP-25. In-vitro reconstitutions have shown that syntaxin-1-SNAP-25-liposomes fuse efficiently with synaptobrevin-liposomes in the presence of synaptotagmin-1-Ca2+, but neurotransmitter release also requires Munc18-1 and Munc13 in vivo. Here we found that Munc18-1 could displace SNAP-25 from syntaxin-1 and that fusion of syntaxin-1-Munc18-1-liposomes with synaptobrevin-liposomes required Munc13, in addition to SNAP-25 and synaptotagmin-1-Ca2+. Moreover, when starting with syntaxin-1-SNAP-25 liposomes, NSF-α-SNAP disassembled the syntaxin-1-SNAP-25 heterodimers and abrogated fusion, which then required Munc18-1 and Munc13. We propose that fusion does not proceed through syntaxin-1-SNAP-25 heterodimers, but starts with the syntaxin-1-Munc18-1 complex; Munc18-1 and Munc13 then orchestrate membrane fusion together with the SNAREs and synaptotagmin-1-Ca2+ in an NSF- and SNAP-resistant manner.

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Neurotransmitter release requires SNARE complexes to bring membranes together, NSF-SNAPs to recycle the SNAREs, Munc18-1 and Munc13s to orchestrate SNARE complex assembly, and Synaptotagmin-1 to trigger fast Ca2+-dependent membrane fusion. However, it is unclear whether Munc13s function upstream and/or downstream of SNARE complex assembly, and how the actions of their multiple domains are integrated. Reconstitution, liposome-clustering and electrophysiological experiments now reveal a functional synergy between the C1, C2B and C2C domains of Munc13-1, indicating that these domains help bridging the vesicle and plasma membranes to facilitate stimulation of SNARE complex assembly by the Munc13-1 MUN domain. Our reconstitution data also suggest that Munc18-1, Munc13-1, NSF, αSNAP and the SNAREs are critical to form a ‘primed’ state that does not fuse but is ready for fast fusion upon Ca2+ influx. Overall, our results support a model whereby the multiple domains of Munc13s cooperate to coordinate synaptic vesicle docking, priming and fusion.DOI: http://dx.doi.org/10.7554/eLife.13696.001

SUMMARY Rapid neurotransmitter release depends on the Ca2+-sensor Synaptotagmin-1 and the SNARE complex formed by synaptobrevin, syntaxin-1 and SNAP-25. How Synaptotagmin-1 triggers release remains unclear, in part because elucidating high-resolution structures of Synaptotagmin-1-SNARE complexes has been challenging. An NMR approach based on lanthanide-induced pseudocontact shifts now reveals a dynamic binding mode where basic residues in the concave side of the Synaptotagmin-1 C2B domain β-sandwich interact with a polyacidic region of the SNARE complex formed by syntaxin-1 and SNAP-25. The physiological relevance of this dynamic structural model is supported by mutations in basic residues of Synaptotagmin-1 that markedly impair SNARE-complex binding in vitro and Synaptotagmin-1 function in neurons. Mutations with milder effects on binding have correspondingly milder effects on Synaptotagmin-1 function. Our results support a model whereby their dynamic interaction facilitates cooperation between synaptotagmin-1 and the SNAREs in inducing membrane fusion.

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GABA (γ-aminobutyric acid) stimulation of the metabotropic GABA B receptor results in prolonged inhibition of neurotransmission that is central to brain physiology 1 . GABA B belongs to the Family C of G protein-coupled receptors (GPCRs), which operate as dimers to relay synaptic neurotransmitter signals into a cellular response through the binding and activation of heterotrimeric G proteins 2 , 3 . GABA B , however, is unique in its function as an obligate heterodimer in which agonist binding and G protein activation take place on distinct subunits 4 , 5 . Here we show structures of heterodimeric and homodimeric full-length GABA B receptors. Complemented by cellular signaling assays and atomistic simulations, the structures reveal an essential role for the GABA B extracellular loop 2 (ECL2) in relaying structural transitions by ordering the linker connecting the extracellular ligand-binding domain to the transmembrane region. Furthermore, the ECL2 of both GABA B subunits caps and interacts with the hydrophilic head of a phospholipid occupying the extracellular half of the transmembrane domain, thereby providing a potentially crucial link between ligand binding and the receptor core that engages G protein. These results provide a starting framework to decipher mechanistic modes of signal transduction mediated by GABA B dimers and have important implications for rational drug design targeting these receptors.