Functional diversity of vertebrate ARNT proteins: Identification of ARNT2 as the predominant form of ARNT in the marine teleost, Fundulus heteroelitus

The aryl hydrocarbon receptor nuclear translocator (ARNT) is a member of th e bHLH/PAS protein superfamily. ARNT dimerizes with several PAS superfamily members, including the ligand-activated aryl hydrocarbon receptor (AHR), f orming a complex that alters transcription by binding specific elements wit hin the promoters of target genes. Two genes encode different forms of the protein in rodents: ARNT1, which is widely expressed, and ARNT2, which is l imited to the brain and kidneys of adults and specific neural and branchial tissues of embryos. In an effort to characterize aryl hydrocarbon signalin g mechanisms in Fundulus heteroclitus, a marine teleost that can develop he ritable xenobiotic resistance, we have isolated a liver cDNA encoding an AR NT homolog. The protein exhibits AHR-dependent DNA binding capability typic al of other vertebrate ARNTs. Unexpectedly phylogenetic analysis reveals th at the cDNA encodes an ARNT2. This is the only detectable ARNT sequence in Fundulus liver gill, ovary, and brain, suggesting that, ARNT2 is the predom inant form of ARNT in this species. Also surprising is the relative lack of sequence identity with another fish ARNT protein, rainbow trout ARNTb, whi ch we show farms a distinct branch outside the ARNT1 and ARNT2 clades in ph ylogenetic analyses. Functional diversity of ARNT proteins in fish may have important implications for the assessment of aryl hydrocarbon effects on n atural populations, The increasing use of fish models in developmental and toxicological studies underscores the importance of identifying taxon-speci fic roles of ARNT proteins and their potential dimeric partners in the PAS superfamily. (C) 1999 Academic Press.