title: An aryl hydrocarbon receptor (AHR) homologue from the soft-shell clam, Mya arenaria: evidence that invertebrate AHR homologues lack 2,3,7,8-tetrachlorodibenzo-p-dioxin and beta-naphthoflavone binding.
abstract: The aryl hydrocarbon receptor (AHR) mediates numerous toxic effects following exposure of vertebrate animals to certain aromatic environmental contaminants, including 2,3,7,8-tetrachlorodibenzo-p-dioxin (TCDD). To investigate possible effects of TCDD on invertebrates, a cDNA encoding an AHR homologue was cloned from the soft-shell clam, Mya arenaria. The predicted amino acid sequence contains regions characteristic of vertebrate AHRs: basic helix-loop-helix (bHLH) and PER-ARNT-SIM (PAS) domains and a glutamine-rich region. Phylogenetic analysis shows that the clam AHR sequence groups within the AHR subfamily of the bHLH-PAS family, in a clade containing AHR homologues from Drosophila melanogaster and Caenorhabditis elegans. AHR mRNA expression was detected in all tissue types tested: adductor muscle, digestive gland, foot, gill, gonad, mantle, and siphon. The in vitro-expressed clam AHR exhibited sequence-specific interactions with a mammalian xenobiotic response element (XRE). Velocity sedimentation analysis using either in vitro-expressed clam AHR or clam cytosolic proteins showed that this AHR homologue binds neither [(3)H]TCDD nor [(3)H]beta-naphthoflavone (BNF). Similarly, in vitro-expressed D. melanogaster and C. elegans AHR homologues lacked specific 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 AHRs. Comparative studies of phylogenetically diverse organisms may help identify an endogenous ligand(s) and the physiological role(s) for this protein.
authors.affiliation: University of Maine, School of Marine Sciences, Orono, ME 04469, USA.
authors.forename: R A
authors.initials: RA
authors.lastname: Butler
authors.forename: M L
authors.initials: ML
authors.lastname: Kelley
authors.forename: W H
authors.initials: WH
authors.lastname: Powell
authors.forename: M E
authors.initials: ME
authors.lastname: Hahn
authors.forename: R J
authors.initials: RJ
authors.lastname: Van Beneden
mesh_terms: D000595:Amino Acid Sequence; D000818:Animals; D001483:Base Sequence; D001665:Binding Sites / Q000235:genetics; D001667:Binding, Competitive; D049872:Bivalvia / Q000235:genetics* / Q000378:metabolism; D003001:Cloning, Molecular; D018076:DNA, Complementary / Q000737:chemistry / Q000235:genetics; D015870:Gene Expression; D008969:Molecular Sequence Data; D010802:Phylogeny; D000072317:Polychlorinated Dibenzodioxins / Q000378:metabolism; D011485:Protein Binding; D012313:RNA / Q000235:genetics / Q000378:metabolism; D011869:Radioligand Assay; D018336:Receptors, Aryl Hydrocarbon / Q000235:genetics* / Q000378:metabolism; D016415:Sequence Alignment; D017422:Sequence Analysis, DNA; D017386:Sequence Homology, Amino Acid; D014018:Tissue Distribution; D014316:Tritium; D019324:beta-Naphthoflavone / Q000378:metabolism
chemical_list: D018076:DNA, Complementary; D000072317:Polychlorinated Dibenzodioxins; D018336:Receptors, Aryl Hydrocarbon; D014316:Tritium; D019324:beta-Naphthoflavone; D012313:RNA