A partially zipped SNARE complex stabilized by the membrane.

The SNARE complex acts centrally for intracellular membrane fusion, an essential process for vesicular transport in cells. Association between vesicle-associated (v-) SNARE and target membrane (t-) SNARE results in the coiled coil core that bridges two membranes. Here, the structure of the SNARE complex assembled by recombinant t-SNARE Sso1p/Sec9 and v-SNARE Snc2p, which are involved in post-Golgi trafficking in yeast, was investigated using EPR. In detergent solutions, SNAREs formed a fully assembled core. However, when t-SNAREs were reconstituted into the proteoliposome and mixed with the soluble SNARE motif of Snc2p, a partially zipped core in which the N-terminal region is structured, whereas the C-terminal region is frayed, was detected. The partially zipped and fully assembled complexes coexisted with little free energy difference between them. Thus, the core complex formation of yeast SNAREs might not serve as the energy source for the fusion, which is different from what has been known for neuronal SNAREs. On the other hand, the results from the proteoliposome fusion assay, employing cysteine- and nitroxide-scanning mutants of Sso1p, suggested that the formation of the complete core is required for membrane fusion. This implies that core SNARE assembly plays an essential role in setting up the proper geometry of the lipid-protein complex for the successful fusion.