GENETIC AND BIOCHEMICAL-EVIDENCE FOR A NOVEL AVERMECTIN-SENSITIVE CHLORIDE CHANNEL IN CAENORHABDITIS-ELEGANS - ISOLATION AND CHARACTERIZATION

Avermectins are a class of macrocyclic lactones that is widely used in crop protection and to treat helminth infections in man and animals. Two complementary DNAs (GluCl alpha and GluCl beta) encoding chloride channels that are gated by avermectin and glutamate, respectively, wer e isolated from Caenorhabditis elegans. To study the role of these sub units in conferring avermectin sensitivity we isolated a mutant C. ele gans strain with a Tc1 transposable element insertion that functionall y inactivated the GluCl alpha gene (GluCl alpha::Tc1). GluCl alpha::Tc 1 animals exhibit a normal phenotype including typical avermectin sens itivity, Xenopus oocytes expressing GluCl alpha::Tc1 strain mRNA elici ted reduced amplitude avermectin and glutamate-dependent chloride curr ents. Avermectin binding assays in GluCl alpha::Tc1 strain membranes s howed the presence of high affinity binding sites, with a reduced B-ma x. These experiments suggest that GluCl alpha is a target for avermect in and that additional glutamate-gated and avermectin-sensitive chlori de channel subunits exist in C. elegans. We isolated a cDNA (GluCl alp ha 2) encoding a chloride channel that shares 75% amino acid identity with GluCl alpha. This subunit forms homomeric channels that are gated irreversibly by avermectin and reversibly by glutamate, GluCl alpha 2 coassembles with GluCl beta to form heteromeric channels that are gat ed by both ligands. The presence of subunits related to GluCl alpha ma y explain the low level and rarity of target site involvement in resis tance to the avermectin class of compounds.