Characterization of GAR-2, a novel G protein-linked acetylcholine receptorfrom Caenorhabditis elegans

We have previously identified two G protein-linked acetylcholine receptors (GARs), GAR-1 and GAR-3, in the nematode Caenorhabditis elegans. Whereas GA R-3 is a homologue of muscarinic acetylcholine receptors (mAChRs), GAR-1 is similar to but pharmacologically distinct from mAChRs. In the current work we isolated a new type of GAR using C. elegans genome sequence information . This receptor, named GAR-2, consists of 614 amino acid residues and has s even putative transmembrane domains. Database searches indicate that GAR-2 is most similar to GAR-1 and closely related to GAR-3/mAChRs. The overall a mino acid sequence identities to GAR-1 and GAR-3 are similar to 32 and simi lar to 23%, respectively. When GAR-2 was coexpressed with the G protein-act ivated inwardly rectifying K+ (GIRK1) channel in Xenopus oocytes, acetylcho line was able to evoke the GIRK current in a dose-dependent fashion. Oxotre morine, a classical muscarinic agonist, had little effect on the receptor, indicating that GAR-2 is pharmacologically different from mAChRs but rather similar to GAR-1. GAR-2 differs from GAR-1, however, in that it showed vir tually no response to muscarinic antagonists such as atropine, scopolamine, and pirenzepine. Expression studies using green fluorescent protein report er gene fusion revealed that GAR-2 is expressed in a subset of C. elegans n eurons, distinct from those expressing GAR-1. Together with our previous re ports, this study demonstrates that diverse types of GARs are present in C. elegans.