Vesicular restriction of synaptobrevin suggests a role for calcium in membrane fusion

Abstract

Release of neurotransmitter occurs when synaptic vesicles fuse with the plasma membrane. This neuronal exocytosis is triggered by calcium and requires three SNARE (soluble-N-ethylmaleimide-sensitive factor attachment protein receptors) proteins: synaptobrevin (also known as VAMP) on the synaptic vesicle, and syntaxin and SNAP-25 on the plasma membrane1,2,3,4. Neuronal SNARE proteins form a parallel four-helix bundle that is thought to drive the fusion of opposing membranes5,6. As formation of this SNARE complex in solution does not require calcium, it is not clear what function calcium has in triggering SNARE-mediated membrane fusion. We now demonstrate that whereas syntaxin and SNAP-25 in target membranes are freely available for SNARE complex formation, availability of synaptobrevin on synaptic vesicles is very limited. Calcium at micromolar concentrations triggers SNARE complex formation and fusion between synaptic vesicles and reconstituted target membranes. Although calcium does promote interaction of SNARE proteins between opposing membranes, it does not act by releasing synaptobrevin from synaptic vesicle restriction. Rather, our data suggest a mechanism in which calcium-triggered membrane apposition enables syntaxin and SNAP-25 to engage synaptobrevin, leading to membrane fusion.

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Figure 1: SNARE complex formation in solution is not enhanced by calcium.
Figure 2: Synaptic vesicles fuse with target membranes in a calcium- and SNARE-dependent manner.
Figure 3: Availability of SNAREs in biological membranes for complex formation.
Figure 4: Apposition of vesicular and target membranes drives SNARE complex formation and fusion.

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Acknowledgements

We thank H. McMahon for syntaxin and synaptobrevin plasmids and H. Hirling for advice on anti-SNAP-25 immunochromatography. K.H. was supported in part by the University of Cambridge MB/PhD Programme. A.S. and S.F. were supported by postdoctoral fellowships from the Royal Society and Wellcome Trust, respectively.

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Correspondence to Bazbek Davletov.

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Hu, K., Carroll, J., Fedorovich, S. et al. Vesicular restriction of synaptobrevin suggests a role for calcium in membrane fusion. Nature 415, 646–650 (2002). https://doi.org/10.1038/415646a

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