In Vivo Analysis of Conserved C. elegans Tomosyn Domains

Neurosecretion is critically dependent on the assembly of a macromolecular complex between the SNARE proteins syntaxin, SNAP-25 and synaptobrevin. Evidence indicates that the binding of tomosyn to syntaxin and SNAP-25 interferes with this assembly, thereby negatively regulating both synaptic transmission and peptide release. Tomosyn has two conserved domains: an N-terminal encompassing multiple WD40 repeats predicted to form two b-propeller structures and a C-terminal SNARE-binding motif. To assess the function of each domain, we performed an in vivo analysis of the N- and C- terminal domains of C. elegans tomosyn (TOM-1) in a tom-1 mutant background. We verified that both truncated TOM-1 constructs were transcribed at levels comparable to rescuing full-length TOM-1, were of the predicted size, and localized to synapses. Unlike full-length TOM-1, expression of the N- or C-terminal domains alone was unable to restore inhibitory control of synaptic transmission in tom-1 mutants. Similarly, co-expression of both domains failed to restore TOM-1 function. In addition, neither the N- nor C-terminal domain inhibited release when expressed in a wild-type background. Based on these results, we conclude that the ability of tomosyn to regulate neurotransmitter release in vivo depends on the physical integrity of the protein, indicating that both N- and C-terminal domains are necessary but not sufficient for effective inhibition of release in vivo.