Our lab is interested in elucidating signal transduction mechanisms in the ASE salt receptor neurons (Ortiz et al., 2009). 35 years ago, Lewis and Hodgkin described single alleles of two chemosensory mutants, che-6 and che-7 (Lewis and Hodgkin, 1977). We analyzed these mutants using current standard chemotaxis protocols and found that both mutants have a similar, though not identical, spectrum of chemotaxis defects.
che-6(e1126) animals have intermediate defects in chemotaxis to NaCl, which is sensed by both ASEL and ASER, but are strongly defective in chemotaxis to individual ions that are sensed by either ASEL or ASER, i.e. sodium (ASEL) and chloride (ASER). che-7(e1128) animals are also defective in odortaxis to diacetyl but not benzaldehyd.
Similarly, che-7(e1128) animals have intermediate defects in NaCl chemotaxis, but are strongly defective in chemotaxis to sodium, chloride and lithium. che-7(e1128) animals are defective in odortaxis to both benzaldehyd and diacetyl.
che-6 had previously been linked to LGIV and che-7 to LGVII. We did whole genome sequencing of the che-6 and che-7 mutant strains. We found a missense mutation in the cyclic nucleotide-gated ion channel cng-4/C23H5.7 in che-6(e1126) mutant animals and a missense mutation in the innexin gene inx-4 in che-7(e1128) mutant animals. In both cases, the mutant phenotype can be rescued with genomic DNA pieces that encompass the respective genomic loci. Moreover, independently isolated knockout alleles (ok2373 for inx-4/che-7 and tm5036 for cng-4/che-6) showed a similar spectrum of chemotactic defects.
While we currently have no specific hypothesis in regard to che-7 function, we speculate that che-6 forms a heteromeric CNG channel complex together with tax-2 and tax-4 in the ASE neurons. This is based on the notion that CNG channels are generally thought to be tetramers composed of three different subunits, two beta subunits and two different alpha subunits (Zheng and Zagotta, 2004). In the case of the ASE neurons, we speculate the CNG complex to contain two TAX-2 beta subunits and the alpha subunits TAX-4 and CHE-6.
References
Lewis JA, and Hodgkin JA. (1977). Specific neuroanatomical changes in chemosensory mutants of the nematode Caenorhabditis elegans. J. Comp. Neurol. 172, 489-510. 
Ortiz CO, Faumont S, Takayama J, Ahmed HK, Goldsmith AD, Pocock R, McCormick KE, Kunimoto, H, Iino Y, Lockery S and Hobert, O. (2009). Lateralized gustatory behavior of C.elegans is controlled by specific receptor-type guanylyl cyclases. Curr. Biol. 19, 996-1004.
Zheng J, and Zagotta WN. (2004). Stoichiometry and assembly of olfactory cyclic nucleotide-gated channels. Neuron 42, 411-21.
