Subtle interplay between synaptotagmin and complexin binding to the SNARE complex

Junjie Xu, Kyle D. Brewer, Raquel Perez-Castillejos, Jose Rizo-Rey

Research output: Contribution to journalArticlepeer-review

36 Scopus citations


Ca2+-triggered neurotransmitter release depends on the formation of SNARE complexes that bring the synaptic vesicle and plasma membranes together, on the Ca2+ sensor synaptotagmin-1 and on complexins, which play active and inhibitory roles. Release of the complexin inhibitory activity by binding of synaptotagmin-1 to the SNARE complex, causing complexin displacement, was proposed to trigger exocytosis. However, the validity of this model was questioned based on the observation of simultaneous binding of complexin-I and a fragment containing the synaptotagmin-1 C2 domains (C2AB) to membrane-anchored SNARE complex. Using diverse biophysical techniques, here we show that C2AB and complexin-I do not bind to each other but can indeed bind simultaneously to the SNARE complex in solution. Hence, the SNARE complex contains separate binding sites for both proteins. However, total internal reflection fluorescence microscopy experiments show that C2AB can displace a complexin-I fragment containing its central SNARE-binding helix and an inhibitory helix (Cpx26-83) from membrane-anchored SNARE complex under equilibrium conditions. Interestingly, full-length complexin-I binds more tightly to membrane-anchored SNARE complex than Cpx26-83, and it is not displaced by C2AB. These results show that interactions of Nand/ or C-terminal sequences of complexin-I with the SNARE complex and/or phospholipids increase the affinity of complexin-I for the SNARE complex, hindering dissociation induced by C2AB. We propose a model whereby binding of synaptotagmin-1 to the SNARE complex directly or indirectly causes a rearrangement of the complexin-I inhibitory helix without inducing complexin-I dissociation, thus relieving the inhibitory activity and enabling cooperation between synaptotagmin-1 and complexin-I in triggering release.

Original languageEnglish (US)
Pages (from-to)3461-3475
Number of pages15
JournalJournal of Molecular Biology
Issue number18
StatePublished - Sep 23 2013


  • Neurotransmitter release
  • Protein-membrane interactions
  • Protein-protein interactions
  • Synaptic vesicle fusion

ASJC Scopus subject areas

  • Structural Biology
  • Molecular Biology


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