A MUTATION IN THE ARYL-HYDROCARBON RECEPTOR (AHR) IN A CULTURED MAMMALIAN-CELL LINE IDENTIFIES A NOVEL REGION OF AHR THAT AFFECTS DNA-BINDING

Introduction of a retroviral expression vector far the aryl hydrocarbo n receptor (AHR) restores CYPIA1 inducibility to a mutant derivative o f the Hepa-1 cell line that is defective in induction of CYP1A1 by lig ands for the receptor. An AHR protein with normal ligand binding activ ity is expressed in the mutant but ligand treatment of mutant cell ext ract fails to induce binding of the AHR.ARNT (aryl hydrocarbon recepto r nuclear translocator) dimer to the xenobiotic responsive element (XR E). ABR cDNAs derived from the mutant encode a protein that is unimpai red in ligand-dependent dimerization with ARNT, but the AHR.ARNT dimer so formed is severely impaired in XRE binding activity. The mutant cD NAs contain a C to G mutation at base 648, causing a cysteine to trypt ophan alteration at amino acid 216, located between the PER-ARNT-SIM h omology region (PAS) A and PAS B repeats, Introduction of the same mut ation in the wild-type AHR sequence by site-directed mutagenesis simil arity impaired XRE binding activity. Substitution with the conservativ e amino acid, serine, had no effect on XRE binding, The tryptophan mut ation, but not the wild-type allele, was detectable in genomic DNA of the mutant, The implication that an amino acid within the PAS region m ay be involved in DNA binding indicates that the DNA binding behavior of AHR may be more anomalous than previously suspected.