Helical extension of the neuronal SNARE complex into the membrane

Abstract

Neurotransmission critically relies on synaptic vesicles fusing with the membrane of nerve cells to release their neurotransmitter content into the synaptic cleft, a process requiring the assembly of several members of the SNARE protein family. Stein et al. have now solved the X-ray crystallographic structure of an extended neuronal SNARE complex, which suggest that these proteins operate like nanomachines whose zippering all the way into the membranes triggers their fusion. Other SNAREs are likely to function as such robust and simple membrane fusion catalysts during most secretory or endocytosis events in eukaryotes. In neurotransmission, synaptic vesicles fuse with the membrane of nerve cells to release neurotransmitter content into the synaptic cleft. This process requires the assembly of several members of the SNARE protein family. Here, the X-ray structure of a neuronal SNARE complex is solved, providing insight into how these proteins assemble. Neurotransmission relies on synaptic vesicles fusing with the membrane of nerve cells to release their neurotransmitter content into the synaptic cleft, a process requiring the assembly of several members of the SNARE (soluble N-ethylmaleimide-sensitive factor attachment protein receptor) family. SNAREs represent an evolutionarily conserved protein family that mediates membrane fusion in the secretory and endocytic pathways of eukaryotic cells1,2,3. On membrane contact, these proteins assemble in trans between the membranes as a bundle of four α-helices, with the energy released during assembly being thought to drive fusion4,5,6. However, it is unclear how the energy is transferred to the membranes and whether assembly is conformationally linked to fusion. Here, we report the X-ray structure of the neuronal SNARE complex, consisting of rat syntaxin 1A, SNAP-25 and synaptobrevin 2, with the carboxy-terminal linkers and transmembrane regions at 3.4 Å resolution. The structure shows that assembly proceeds beyond the already known core SNARE complex7, resulting in a continuous helical bundle that is further stabilized by side-chain interactions in the linker region. Our results suggest that the final phase of SNARE assembly is directly coupled to membrane merger.