Structural Studies on the Exocyst Tethering Complex
Final Report Abstract
In neurons and neurosecretory cells, complexin binds to SNARE (Soluble N-ethylmaleimide-sensitive factor Attachment protein REceptor) complexes during regulated exocytosis and inhibits fusion prior to neuronal or other stimulation. The molecular mechanism how this “clamped” state forms and is released is not understood. To capture the half-zippered SNARE complexes that constitute the inhibited state, the portion of the v-SNARE VAMP2 which zippers last to trigger fusion was removed, and I determined the structure of this truncated complex bound to complexin. Complexin forms a continuous helix which is anchored to one SNARE complex at one end. Surprisingly, the other end of complexin binds to a second SNARE complex in the site that the v-SNARE must occupy to trigger fusion, thus blocking zippering. Furthermore, as a consequence of the SNARE crosslinking by complexin, the SNARE complexes are arranged into a continuous zig-zag topology that is incompatible with fusion. Only small perturbations seem necessary to disassemble this cooperative structure, suggesting a simple mechanism for triggering fusion. This crystal structure provides the molecular basis for how neuronal fusion is halted and suggests a model how this inhibition can be efficiently released. It therefore presents a major advancement in our standing of the mechanism of fast, synchronous neurotransmitter release.
