The crystal structure of complexin bound to a prefusion SNAREpin mimetic shows that the accessory helix extends away from the SNAREpin in an 'open' conformation, binding another SNAREpin and inhibiting its assembly, to clamp fusion. In contrast, the accessory helix in the postfusion complex parallels the SNARE complex in a 'closed' conformation. Here we use targeted mutations, FRET spectroscopy and a functional assay that reconstitutes Ca2+-triggered exocytosis to show that the conformational switch from open to closed in complexin is needed for synaptotagmin-Ca2+ to trigger fusion. Triggering fusion requires the zippering of three crucial aspartate residues in the switch region (residues 64–68) of v-SNARE. Conformational switching in complexin is integral to clamp release and is probably triggered when its accessory helix is released from its trans-binding to the neighboring SNAREpin, allowing the v-SNARE to complete zippering and open a fusion pore.
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We wish to thank T. Melia (Yale University) for critical reading of the manuscript. This work was supported by US National Institutes of Health grants to J.E.R. and K.M.R., an Agence Nationale de la Recherche (ANR) Physique et Chimie du Vivant (PCV) grant to F.P., and a grant from the Deutsche Forschungsgemeinschaft to D.K.
The authors declare no competing financial interests.
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Krishnakumar, S., Radoff, D., Kümmel, D. et al. A conformational switch in complexin is required for synaptotagmin to trigger synaptic fusion. Nat Struct Mol Biol 18, 934–940 (2011). https://doi.org/10.1038/nsmb.2103
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