Vesicle exocytosis is an important cellular function that mediates various cell-cell communications, such as synaptic transmission. In C. elegans, many exocytic proteins including Rab family, UNC-13, UNC-18, and SNARE proteins have been identified and characterized by examining their mutant phenotypes. In these genes, a few mutants such as
aex-3 (Rab GEF) and
aex-6 (Rab-27), show a phenotype that specifically lack two muscle contraction steps (aBoc and Exp) in the defecation behavior (aBoc and Exp defective, called Aex phenotype). However, most synaptic mutants such as
snb-1 (VAMP),
unc-64 (Syntaxin 1A),
unc-13 (Munc13-1) etc, do not show severe defects in these steps. We have previously shown that mutations in the
aex-1 gene cause Aex phenotype, and that
aex-1 encodes a Munc13-4-like protein. Although AEX-1 may be a member of Munc13 protein family, it does not have the long N-terminus region seen in UNC-13 protein. Thus, we hypothesized that a distinct mechanism may regulate vesicle exocytosis for aBoc and Exp steps through AEX-1 and Rab-27. Despite our general understanding on the machinery of synaptic transmission, the exact molecular functions of AEX-1 and Rab-27 in the vesicle exocytosis are not known yet. To understand the molecular mechanism controlling vesicle exocytosis for defecation, we performed a genetic screen to isolate mutants that lack aBoc and Exp steps. From this screening, we found that
tg94 mutant animals exhibit strong Aex-mutant phenotype.
tg94 mutants also show a weak defect in egg laying behavior, however, they move well and the response to aldicarb, an acetylecholinesterase inhibitor, looks normal, suggesting that general synaptic transmission is probably not affected. We mapped the
tg94 and found that
tg94 is an allele of the
syn-1 gene. SYN-1 protein is presumably a member of plasma-membrane Syntaxin that functions as a t-SNARE for fusion step in the vesicle trafficking. Although SYN-1 contains a TM region at its C-terminus, SYN-1 is most similar to mammalian Syntaxin 11, which does not contain any TM regions. In C. elegans, another Syntaxin UNC-64, a homologue of mammalian Syntaxin 1A , regulates the exocytosis of synaptic vesicles from presynaptic neurons.
unc-64 is expressed in all the neuronal cells, and
unc-64 mutants exhibit strong resistance to aldicarb but do not show severe defecation defects. On the other hands,
syn-1 is expressed in most muscular organs such as pharyngeal, body-wall, intestinal and vulval muscles, and also expressed in the intestine and a few neurons. These results suggest that SYN-1 is a non-neuronal Syntaxin, and that SYN-1 may regulate the exocytic step for the defecation muscle contractions by interacting with AEX-1.