An aryl hydrocarbon receptor (AHR) homologue from the soft-shell clam, Myaarenaria: evidence that invertebrate AHR homologues lack 2,3,7,8-tetrachlorodibenzo-p-dioxin and beta-naphthoflavone binding

The aryl hydrocarbon receptor (AHR) mediates numerous toxic effects followi ng exposure of vertebrate animals to certain aromatic environmental contami nants, including 2,3,7,8-tetrachlorodibenzo-p-dioxin (TCDD). To investigate possible effects of TCDD on invertebrates, a cDNA encoding an AHR homologu e was cloned from the soft-shell clam, Mya arenaria. The predicted amino ac id sequence contains regions characteristic of vertebrate AHRs: basic helix -loop-helix (bHLH) and PER-ARNT-SIM (PAS) domains and a glutamine-rich regi on. Phylogenetic analysis shows that the clam AHR sequence groups within th e AHR subfamily of the bHLH-PAS family, in a clade containing AHR homologue s from Drosophila melanogaster and Caenorhabditis elegans. AHR mRNA express ion was detected in all tissue types tested: adductor muscle, digestive gla nd, foot, gill, gonad, mantle, and siphon. The in vitro-expressed clam AHR exhibited sequence-specific interactions with a mammalian xenobiotic respon se element (XRE). Velocity sedimentation analysis using either in vitro-exp ressed clam AHR or clam cytosolic proteins showed that this AHR homologue b inds neither [H-3]TCDD nor [H-3]beta -naphthoflavone (BNF). Similarly, in v itro-expressed D. melanogaster and C. elegans AHR homologues lacked specifi c binding of these compounds. Thus, the absence of specific, high-affinity binding of the prototypical AHR ligands TCDD and BNF, is a property shared by known invertebrate AHR homologues, distinguishing them from vertebrate A HRs. Comparative studies of phylogenetically diverse organisms may help ide ntify an endogenous ligand(s) and the physiological role(s) for this protei n. (C) 2001 Published by Elsevier Science B.V.