Novel alpha7-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 alpha and one non-alpha) 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 alpha7 subunit. Comparison of the predicted amino acid sequences of the new nAChR subunits with those described previously in C. elegans reveals five subunits (four alpha and one non-alpha) 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-alpha 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 alpha 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 alpha7 subunits and the identification of related non-alpha subunits, only found in C. elegans to date, suggest that at least some of these subunits may contribute to heteromers in vivo.