A conformational switch in syntaxin during exocytosis: role of munc18

Syntaxin 1, an essential protein in synaptic membrane fusion, contains a he lical autonomously folded N-terminal domain, a C-terminal SNARE moth and a transmembrane region. The SNARE moth binds to synaptobrevin and SNAP-25 to assemble the core complex, whereas almost the entire cytoplasmic sequence p articipates in a complex with munc18-1, a neuronal Sec1 homolog, We now dem onstrate by NMR spectroscopy that, in isolation, syntaxin adopts a 'closed' conformation. This default conformation of syntaxin is incompatible with c ore complex assembly which requires an open' syntaxin conformation. Using s ite-directed mutagenesis, we find that disruption of the closed conformatio n abolishes the ability of syntaxin to bind to munc18-1 and to inhibit secr etion in PC12 cells. These results indicate that syntaxin binds to munc18-1 in a closed conformation and suggest that this conformation represents an essential intermediate in exocytosis, Our data suggest a model whereby, dur ing exocytosis, syntaxin undergoes a large conformational switch that media tes the transition between the syntaxin-munc18-1 complex and the core compl ex.