Novel α7-like nicotinic acetylcholine receptor subunits in the nematode Caenorhabditis elegans

We have used reverse-transcription-polymerase chain reaction (RT-PCR) and DNA sequencing techniques to confirm the transcription of seven (six α and one non-α) novel candidate nicotinic acetylcholine receptor (nAChR) subunit-encoding genes identified in the genome sequence of the nematode Caenorhabditis elegans. Compared to vertebrate nAChR subunits, they most closely resemble the homomer-forming, neuronal α7 subunit. Comparison of the predicted amino acid sequences of the new nAChR subunits with those described previously in C. elegans reveals five subunits (four α and one non-α) which resemble the DEG-3-like group of subunits. To date, this highly divergent nAChR subunit group is unique to C. elegans. ACR-22 is the first non-α member of the DEG-3-like group of subunits to be identified. Two new members of the related ACR-16-like nAChR group of subunits have also been shown to be transcribed, making the ACR-16-like subunit group the largest in C. elegans. Residues in the α subunit second transmembrane region (M2) which contribute to the channel lining show variations with implications for channel function. For example, in ACR-22, the highly conserved 0′ lysine of M2 is replaced by histidine. Restrained molecular dynamics simulations have been used to generate molecular models of homo-pentameric M2 helix bundles for the novel subunits, enabling identification and display of pore-lining and protein interface residues. The number and diversity of genes encoding C. elegans nAChR subunits with similarities to the homomer-forming vertebrate α7 subunits and the identification of related non-α subunits, only found in C. elegans to date, suggest that at least some of these subunits may contribute to heteromers in vivo.