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 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 a 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-a 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 a 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 inter-face 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-a subunits. only found in C. elegans to date, suggest that at least some of these subunits may contribute to heteromers in vivo.