We previously identified
aex-3 as a regulator of synaptic transmission 1 .
aex-3 mutants show a variety of behavioral defects including those in defecation, pharyngeal pumping, and male mating.
aex-3 encodes a protein highly homologous to Rat Rab3 GTP/GDP exchange factor (GEF). RAB-3, a small GTP binding protein implicated in synaptic transmission, normally localizes to presynaptic terminals but is mislocalized in
aex-3 mutants to neural cell bodies. This suggests that AEX-3 regulates presynaptic activities through the RAB-3-dependent pathway. Unlike
aex-3 mutants,
rab-3 mutants show no defecation motor defects 2 . This suggests that AEX-3 may have functions in addition to that as a Rab3 GEF. Furthermore AEX-3 was recently identified as a homologue of the human protein MADD [MAP kinase activating protein containing the Death Domain 3 ]. MADD interacts with the Tumor Necrosis Factor receptor through Death Domains (DD). Therefore, we hypothesized that AEX-3 may also interact through its C terminus or DD region with another protein(s) and that this interaction might be important for defecation control. To investigate this possibility, we used the yeast two hybrid system to identify AEX-3 C-terminal binding proteins. From a library of 10 6 clones, 62 positive clones were isolated of which 41 were derived from a single gene, which now we call
cab-1 ( C terminus of A EX-3 b inding). CAB-1 has no homology to other known proteins.
cab-1 is expressed in various neurons, consistent with the notion that AEX-3 and CAB-1 interact in vivo .
cab-1 mutants have defects in the defecation motor program, suggesting that CAB-1 functions in the
aex-3 pathway. However, unlike
aex-3 and
rab-3 mutants,
cab-1 mutants show no defects in other behaviors including pharyngeal pumping and male mating. These observations indicate that unlike other GEFs, AEX-3 is a regulator of two different pathways (the
rab-3 and
cab-1 pathways) for presynaptic activities. 1) Iwasaki et al. Neuron 18:613 2) Nonet et al. J Neurosci. 17:8061 3) Schievella et al. J Biol Chem. 272:12069