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

The cDNAs for the four murine aryl hydrocarbon (Ah) receptor alleles w ere cloned and sequenced, and the deduced amino acid sequences were co mpared. The Ah(b-1) allele encodes a protein of 805 amino acids, the A h(d) and Ah(b-2) alleles encode proteins of 848 amino acids, and the A h(b-3) allele encodes a protein of 883 amino acids. The alleles differ by eight point mutations in the common open reading frame (the initia l 805 amino acids) and by additional sequences at the carboxyl end. Th e 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 af finity, differs from the Ah(b-2) receptor by only two amino acids. Mut agenesis experiments with these cloned cDNAs, using in vitro transcrip tion and translation and 2-[I-125]iodo-7,8-dibro-modibenzo-p-dioxin bi nding, indicate that the low ligand binding affinity of the Ah(d) alle le is attributable to a valine at residue 375; changing this amino aci d to an alanine, as in the Ah(b-2) protein, enhances the affinity 4-fo ld. 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 trunc ation sites, we mapped the ligand binding region for the Ah(b-1) allel e.