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Science 313 (5787): 676-680

Copyright © 2006 by the American Association for the Advancement of Science

A Clamping Mechanism Involved in SNARE-Dependent Exocytosis

Claudio G. Giraudo, William S. Eng, Thomas J. Melia, James E. Rothman*

Abstract: During neurotransmitter release at the synapse, influx of calcium ions stimulates the release of neurotransmitter. However, the mechanism by which synaptic vesicle fusion is coupled to calcium has been unclear, despite the identification of both the core fusion machinery [soluble N-ethylmaleimide–sensitive factor attachment protein receptor (SNARE)] and the principal calcium sensor (synaptotagmin). Here, we describe what may represent a basic principle of the coupling mechanism: a reversible clamping protein (complexin) that can freeze the SNAREpin, an assembled fusion-competent intermediate en route to fusion. When calcium binds to the calcium sensor synaptotagmin, the clamp would then be released. SNARE proteins, and key regulators like synaptotagmin and complexin, can be ectopically expressed on the cell surface. Cells expressing such "flipped" synaptic SNAREs fuse constitutively, but when we coexpressed complexin, fusion was blocked. Adding back calcium triggered fusion from this intermediate in the presence of synaptotagmin.

Department of Physiology and Cellular Biophysics, Columbia University, New York, NY 10032, USA.

* To whom correspondence should be addressed. E-mail: jr2269{at}

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