RESEARCH HIGHLIGHTS MEMBRANE DYNAMICS rescue the phenotype of spontane- ous fusion events (that is, clamp spontaneous release), but not the Clamping complexin impaired evoked response. These results suggest that the 27 amino complexin competes with VAMP for acids of the N terminus are impor- the same binding site in the SNARE tant for mediating membrane fusion complex and thereby acts as a fusion events, and that different complexin clamp. domains mediate spontaneous and Using an in vitro fusion assay, evoked release. they showed that membrane fusion Furthermore, when a VAMP was triggered when complexin and mutant that does not bind complexin a recombinant VAMP peptide that but still forms SNARE complexes was homologous to the complexin was introduced into cortical cultures accessory helix were added at the from VAMP-knockout mice that same time in the absence of Ca2+ and lack spontaneous or evoked release, synaptotagmin. When the VAMP spontaneous fusion events were peptide was added after complexin, rescued and even elevated above CORBIS fusion did not occur, supporting the those of cells expressing wild-type The protein complexin interacts with hypothesis. Furthermore, mutations VAMP, whereas evoked events the SNARE (soluble NSF-attachment in the accessory helix that enhanced were decreased. This indicates that protein receptor) complex and regu- clamping rendered complexin more complexin executes its clamping lates neurotransmitter release. Two resistant to VAMP competition function by binding to VAMP. groups have analysed the molecular in the fusion assays. By contrast, Interestingly, a point mutation where structure of complexin and conclude mutations in this helix that reduced VAMP inserts into the membrane that its accessory helix acts as a the clamping ability of complexin (outside the SNARE complex) had fusion clamp (preventing fusion), were dominant-negative and did not the same phenotype, indicating that whereas its amino-terminal domain prevent fusion, confirming that it is complexin acts on the force translation acts as a fusion enhancer. the accessory helix of complexin that of SNAREs onto the membrane. SNARE proteins are localized to exerts the clamping function. These results suggest that the synaptic vesicles and cell membranes, Maximov et al. studied the role accessory helix of complexin acts and their interaction attaches of complexin in cultured mouse as a fusion clamp, by competing vesicles to the membrane. Cytosolic cortical neurons. Knocking down with VAMP to bind to the SNARE complexin binds through its central complexin with RNA interference complex, and that the 27 amino domain to the aligned α-helices of increased spontaneous vesicle-fusion acids of the N terminus of complexin the SNARE complex and is displaced events but impaired evoked synaptic are important for the activation of on Ca2+-activated synaptotagmin release. These effects were rescued membrane-fusion events, revealing a binding prior to membrane fusion. by the addition of wild-type previously unknown task division for Giraudo et al. found that the complexin but not complexin the domains of complexin. sequence of the accessory helix of that lacked 40 amino acids at the Claudia Wiedemann, Chief Editor, complexin is homologous to that N terminus (and that therefore Nature Reviews Neuroscience of the membrane-proximal region of lacked most of the accessory helix VAMP (also called synaptobrevin), but not the central domain). A ORIGINAL RESEARCH PAPERS Giraudo, C. G. et al. Alternative zippering as an on-off switch for one of the SNAREs, when the two deletion mutant that lacked the SNARE-mediated fusion. Science 323, 512–516 sequences are aligned in an anti- N-terminal 27 amino acids but had (2009) | Maximov, A. et al. Complexin controls the force transfer from SNARE complexes to parallel manner. They proposed that an intact accessory helix was able to membranes in fusion. Science 323, 516–521 (2009) NATURE REVIEWS | MOLECULAR CELL BIOLOGY VOLUME 10 | MARCH 2009 © 2009 Macmillan Publishers Limited. All rights reserved.