ANALYSIS OF THE 4 ALLELES OF THE MURINE ARYL-HYDROCARBON RECEPTOR

The cDNAs for the four murine aryl hydrocarbon (Ah) receptor alleles were cloned and sequenced, and the deduced amino acid sequences were compared. The Ah(b-1) allele encodes a protein of 805 amino acids, the Ah(d) and Ah(b-2) alleles encode proteins of 848 amino acids, and the Ah(b-3) allele encodes a protein of 883 amino acids. The alleles differ by eight point mutations in the common open reading frame (the initial 805 amino acids) and by additional sequences at the carboxyl end. The amino halves of the proteins, containing a spliced leader sequence, a basic helix-loop-helix motif, and two 50-amino acid repeats (PAAS), have identical sequences except for a single amino acid change in the second PAAS box. The Ah(d) allele, which has a lower ligand binding affinity, differs from the Ah(b-2) receptor by only two amino acids. Mutagenesis experiments with these cloned cDNAs, using in vitro transcription and translation and 2-[I-125]iodo-7,8-dibro-modibenzo-p-dioxin binding, indicate that the low ligand binding affinity of the Ah(d) allele is attributable to a valine at residue 375; changing this amino acid to an alanine, as in the Ah(b-2) protein, enhances the affinity 4-fold. For in vitro translated Ah(b-1) and Ah(b-2) alleles the K-d values were similar to 6-10 pM and for Ah(d) the K-d value was similar to 37 pM. Using 5' truncation and mutations to produce 3' translation truncation sites, we mapped the ligand binding region for the Ah(b-1) allele.