Testing the SNARE/SM protein model of membrane fusion.

I n eukaryotic cells, the budding and fusion of membranes mediates diverse but essential processes, ranging from cell division to organelle biogenesis to neurotransmitter secretion. All intracellular membrane fusion except for mitochondrial fusion is driven by SNARE and SM (Sec1/Munc18-like) proteins (1–3). Membrane fusion has been particularly intensely studied for neurotransmitter secretion. In neurotransmitter secretion, fusion is mediated by the plasma membrane SNARE proteins syntaxin and synaptosomal-associated protein 25 (SNAP25), the vesicle SNARE protein synaptobrevin/vesicle-associated membrane protein (VAMP), and the SM protein Munc18-1 (Fig. 1 A and B). Mechanistically, SNARE proteins are thought to fuel fusion by forming a transcomplex between the vesicle and target membranes; in this complex, progressive zippering of a fourhelical bundle formed by the SNARE motifs of SNARE proteins forces the fusing phospholipid membranes into close proximity, thereby destabilizing their surfaces (1–3). SM proteins are essential coagonists of SNARE proteins in fusion in that all intracellular SNARE-dependent fusion reactions require an SM protein. At least for fusion during neurotransmitter secretion, syntaxin constitutes the central organizer that is composed of multiple domains: a conserved N-terminal unstructured peptide, an N-terminal Habc domain, a SNARE motif, and a C-terminal transmembrane region (Fig. 1A). Syntaxin assumes two conformations: a closed conformation outside of the SNARE complex in which the Habc domain folds back onto the SNARE motif, and an open conformation in the SNARE complex with a mobile Habc domain (4, 5) (Fig. 1B). Both conformations bind to Munc18, but only Munc18 binding to open syntaxin requires the N-terminal syntaxin peptide (6–8). Although both Munc18/syntaxin binding modes are known to be essential for fusion in vivo (9–12), how syntaxin orchestrates fusion remains unclear. Using elegant in vitro liposome fusion and in vivo Caenorhabditis elegans experiments, Rathore et al. (13) now address the critical question of whether the N-terminal peptide of syntaxins acts autonomously in fusion, independent of its anchorage to syntaxin, or whether it is required to be coupled to syntaxin. In a first set of experiments, Rathore et al. (13) use an in vitro liposome fusion assay that monitors lipid mixing to confirm previous data (14) showing that Munc18 is essential for efficient liposome fusion and that the syntaxin N-peptide is required, whereas the Habc domain is dispensable. Ha Hb Hc TMR N-terminal peptide