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Binding of the complexin N terminus to the SNARE complex potentiates synaptic-vesicle fusogenicity

Abstract

Complexins facilitate and inhibit neurotransmitter release through distinct domains, and their function was proposed to be coupled to the Ca2+ sensor synaptotagmin-1 (Syt1). However, the mechanisms underlying complexin function remain unclear. We now uncover an interaction between the complexin N terminus and the SNARE complex C terminus, and we show that disrupting this interaction abolishes the facilitatory function of complexins in mouse neurons. Analyses of hypertonically induced exocytosis show that complexins enhance synaptic-vesicle fusogenicity. Genetic experiments crossing complexin- and Syt1-null mice indicate a functional interaction between these proteins but also show that complexins can promote Ca2+-triggered release in the absence of Syt1. We propose that the complexin N terminus stabilizes the SNARE complex C terminus and/or helps release the inhibitory function of complexins, thereby activating the fusion machinery in a manner that may cooperate with Syt1 but does not require Syt1.

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Figure 1: The N terminus of CplxI facilitates Ca2+-triggered neurotransmitter release.
Figure 2: Identification of Met5 and Lys6 as crucial residues for CplxI N-terminal function.
Figure 3: The CplxI N terminus facilitates spontaneous release.
Figure 4: The CplxI N terminus binds the SNARE complex, and the M5E K6E mutation disrupts this interaction.
Figure 5: The CplxI N terminus binds the C terminus of the SNARE complex.
Figure 6: Complexins regulate synaptic vesicle fusogenicity.
Figure 7: Complexins facilitate Ca2+-evoked neurotransmitter release independently and cooperatively with Syt1.
Figure 8: Proposed model for the key facilitatory function of the complexin N terminus.

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Acknowledgements

We thank H. Deng, H. Chen and X. Zhu for technical assistance, K. Reim and N. Brose (Max Planck Institute of Experimental Medicine, Germany) for providing CplxI KO, CplxII KO and CplxIII KO mice and T. Sudhof (Stanford University) for providing Syt1 KO mice. This work was supported by the US National Institutes of Health (F31MH078678 to H.T.C., NS037200 to J.R. and NS050655 to C.R.), the Welch Foundation (grant I-1304 to J.R.), Baylor Research Advocates for Student Scientists and a McNair Fellowship (both to H.T.C.) and Baylor College of Medicine Mental Retardation and Developmental Disabilities Research Center.

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Contributions

M.X. performed the physiological studies of Cplx and the Cplx-Syt1 interaction; T.K.C. and J.X. performed the biophysical studies of CplxI–SNARE complex interaction; H.-T.C. contributed to the immunocytochemistry and molecular cloning; M.X., J.R. and C.R. wrote the paper.

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Correspondence to Josep Rizo or Christian Rosenmund.

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The authors declare no competing financial interests.

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Xue, M., Craig, T., Xu, J. et al. Binding of the complexin N terminus to the SNARE complex potentiates synaptic-vesicle fusogenicity. Nat Struct Mol Biol 17, 568–575 (2010). https://doi.org/10.1038/nsmb.1791

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