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Ca2+–synaptotagmin directly regulates t-SNARE function during reconstituted membrane fusion

Nature Structural & Molecular Biology volume 13, pages 323–330 (2006)Cite this article

Abstract

In nerve terminals, exocytosis is mediated by SNARE proteins and regulated by Ca2+ and synaptotagmin-1 (syt). Ca2+ promotes the interaction of syt with anionic phospholipids and the target membrane SNAREs (t-SNAREs) SNAP-25 and syntaxin. Here, we have used a defined reconstituted fusion assay to determine directly whether syt–t-SNARE interactions couple Ca2+ to membrane fusion by comparing the effects of Ca2+–syt on neuronal (SNAP-25, syntaxin and synaptobrevin) and yeast (Sso1p, Sec9c and Snc2p) SNAREs. Ca2+–syt aggregated neuronal and yeast SNARE liposomes to similar extents via interactions with anionic phospholipids. However, Ca2+–syt was able to bind and stimulate fusion mediated by only neuronal SNAREs and had no effect on yeast SNAREs. Thus, Ca2+–syt regulates fusion through direct interactions with t-SNAREs and not solely through aggregation of vesicles. Ca2+–syt drove assembly of SNAP-25 onto membrane-embedded syntaxin, providing direct evidence that Ca2+–syt alters t-SNARE structure.

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Figure 1: Ca2+–syt binds neuronal, but not yeast, t-SNAREs.
Figure 2: Ca2+–syt–t-SNARE interactions are crucial for Ca2+-stimulated membrane fusion.
Figure 3: Ca2+–syt mediates assembly of SNARE complexes.
Figure 4: Ca2+–syt enhances the rate of assembly of t-SNARE heterodimers in the absence of cd-syb.
Figure 5: SNARE complexes assembled by Ca2+–syt are resistant to cleavage by botulinum neurotoxin E.
Figure 6: Syt stimulates fusion of both the inner and outer membrane leaflets of SNARE-bearing vesicles.
Figure 7: Model depicting the role of Ca2+–syt in SNARE-catalyzed membrane fusion.

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Acknowledgements

We thank members of E.R.C.'s laboratory for discussions and comments. This study was supported by grants from the US National Institutes of Health (NIGMS GM 56827 and NIMH MH61876) and from the American Heart Association (0440168N) to E.R.C. A.B. and M.C.C. are supported by an American Heart Association predoctoral fellowship. W.C.T. was supported by a postdoctoral National Research Service Award from the US National Institutes of Health. E.R.C. is an Investigator of the Howard Hughes Medical Institute.

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  1. Akhil Bhalla and Michael C Chicka: These authors contributed equally to this work.

Authors and Affiliations

  1. Howard Hughes Medical Institute, University of Wisconsin, 1300 University Avenue, Madison, SMI 129, Wisconsin, USA

    Akhil Bhalla, Michael C Chicka & Edwin R Chapman

  2. Department of Physiology, University of Wisconsin, 1300 University Avenue, Madison, SMI 129, Wisconsin, USA

    Akhil Bhalla, Michael C Chicka, Ward C Tucker & Edwin R Chapman

  3. Molecular and Cellular Pharmacology Program, University of Wisconsin, 1300 University Avenue, Madison, SMI 129, Wisconsin, USA

    Akhil Bhalla

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Correspondence to Edwin R Chapman.

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Supplementary information

Supplementary Fig. 1

Syt aggregates vesicles in a Ca2+-dependent manner (PDF 1237 kb)

Supplementary Fig. 2

Rate of inner leaflet mixing is similar to rate of outer plus inner leaflet mixing (PDF 1081 kb)

Supplementary Methods (PDF 28 kb)

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Bhalla, A., Chicka, M., Tucker, W. et al. Ca2+–synaptotagmin directly regulates t-SNARE function during reconstituted membrane fusion. Nat Struct Mol Biol 13, 323–330 (2006). https://doi.org/10.1038/nsmb1076

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