Complexin gains effective access to the assembling SNAREs via its membrane-binding C-terminal domain

The conserved presynaptic SNARE-binding protein complexin (Cpx) promotes Ca²⁺-triggered synaptic vesicle (SV) fusion and inhibits spontaneous fusion at some synapses. A membrane-binding motif in the C-terminal domain (CTD) of Cpx plays a critical role in Cpx function, but it remains unclear whether the CTD participates in Cpx regulation of synaptic transmission beyond targeting Cpx to membranes. We examined the impact of the Caenorhabditis elegans CPX-1 CTD in vivo and found that this domain profoundly boosted the efficiency of CPX-1-mediated inhibition of spontaneous SV fusion as a function of protein abundance at the synapse. Removing the C-terminal half of CPX-1 and substituting it with the SV protein RAB-3 was able to fully restore both the fusogenic and inhibitory functions of CPX-1 whereas other SV proteins failed to restore CPX-1 function with the same efficiency regardless of abundance. These results indicate that regulation of spontaneous SV fusion requires a specific interaction of CPX-1 with the SV membrane. We propose that Cpx cannot efficiently access assembling SNAREs from the cytoplasm and that interactions of its CTD with the SV membrane guide Cpx to these sites of SNARE assembly. KEY POINTS: Complexin (Cpx) regulates presynaptic SNARE assembly to control synaptic transmission. A membrane curvature-sensing motif within the Cpx C-terminal domain (CTD) recruits Cpx to vesicles. Replacement of the CTD with the synaptic vesicle protein Rab3 can restore full Cpx function whereas other vesicle proteins fail to substitute regardless of abundance. The efficiency of Cpx-mediated inhibition of synaptic vesicle fusion is profoundly enhanced by the specific localization supplied by its CTD. These results suggest that Cpx reaches the assembling SNARE complexes via its specific CTD-membrane interactions and these SNAREs are inaccessible from the cytoplasmic compartment.