Control of Synaptic Transmission Through SNARE Complex Regulation

Synaptic transmission requires the assembly of a highly conserved complex composed of the SNARE proteins Syntaxin, SNAP-25, and Synaptobrevin. The Syntaxin binding protein Tomosyn has been implicated in the regulation of this secretory process through interactions with the SNARE complex. Here we used truncated Tomosyn constructs in C. elegans to demonstrate that both the N and C terminal domains contribute to the inhibitory function of this protein. Based on this observation, we isolated a novel Tomosyn N –terminal interacting protein, VPS-39. Our analysis suggests that VPS-39 does regulate Tomosyn levels, but the major synaptic phenotype is consistent with a permissive role in exocytosis. The characterization of the vps-39 mutant phenotype indicates a role upstream of UNC-13 function. Based on known biochemical interactions and our own experimental data we postulate that VPS-39 promotes the transition of Syntaxin from its default closed to open configuration required for priming. We also explored the role of a second SNARE binding protein, Snapin. Our analysis indicates that Snapin is involved in synaptic vesicle docking consistent with its known interaction with SNAP-25. Importantly, our genetic analysis suggests that this function is independent and upstream of the calcium sensor, Synaptotagmin. Together these observations further our understanding of the mechanisms regulating SNARE complex assembly, a critical event underlying normal synaptic transmission.