II. Mechanism

After ligand-bound AHR dissociates its chaperone proteins that keep it dormant within the cytoplasm, it translocates into the nucleus to form a heterodimer with ARNT. Although, if the AHR repressor (AHRR) is present within a cell, it will bind ARNT instead and repress AHR from forming its heterodimer. This repression is done not through competition, but rather a process known as transrepression, which functions through protein-protein interactions that restrict AHR heterodimerization.

If AHRR is not present in a cell, TCDD will bind the PAS B domain of AHR and the AHR-ARNT heterodimer will form. Following the formation of the AHR-ARNT-TCDD complex, the ligand-bound heterodimer will attach to the target DNA known as the dioxin response element (DRE) and the heterodimer will act as an activator that will induce increased expression of target genes within the DRE, forming proteins such as CYP1A1. CYP1A1, which belongs to the Cytochrome p450 family of enzyme , are found in most animal cells, functioning in the metabolism of endogenous substances and the detoxification of pollutants within the organism. Although, when expression is largely upregulated with the activation of the ligand-bound transcriptional factor AHR by TCDD, these proteins catalyze harmful carcinogenic effects which include malignant tumor initiation and promotion.

III. General Structure

AHR and its activator protein ARNT both belong to the PER-ARNT-SIM (PAS)/basic helix-loop-helix (bHLH) family of transcriptional factors. The PAS domains, which are comprised of two imperfect 50 amino acid repeats known as PAS A and PAS B, are a highly conserved binding motif in which protein-protein interactions or protein signaling can occur. The PAS domain consists of a five-stranded antiparallel beta sheet and multiple alpha helices that make up the hydrophobic core, allowing both AHR and ARNT to interact with each other and ligands. The basic helix-loop-helix (bHLH) binding motif, which is found near the N-terminal region of AHR and ARNT, are binding arrangements that consist of two connected alpha helices and aid in the interactions of protein-DNA and protein-protein hetero/homodimerization. PAS-A domains aid in AHR-ARNT heterodimer formation, and the bHLH of both AHR and ARNT allow for DNA binding with the heterodimer.

The PAS B domain of AHR, also known as the ligand binding domain (LBD), specifically binds to various ligands, allowing for a conformational change in AHR. This conformational change removes the allosterically inhibiting chaperone proteins, Hsp90, P23, and ARA9 that keep AHR dormant in the cytoplasm. Once these proteins dissociate and the ligand binds, the AHR-ligand complex is able to translocate into the nucleus.

After entering the nucleus, ARNT wraps around AHR in a largely intertwined asymmetric construction. AHR-ARNT dimerization is partially governed by the largely hydrophobic points of contact to the dimerization interfaces between AHR and ARNT . The heterodimer contains up to 35 hydrophobic residues that help dimerization occur, as well as increase stability of the heterodimer and its interaction with the DRE. The bHLH domains of AHR and ARNT orient to form two similar protein extensions, one from AHR and one from ARNT, that interact with the major groove of the DRE. Sequence-specific interactions occur here as well, recognizing specifically the DRE consensus sequence T T G C G T G and differentiating it from extremely similar response elements through interactions with the bHLH protein extensions found on both the AHR and ARNT DNA binding sites. AHR-ARNT Structure Formation and It's Interaction with the DRE

IV. Recognition of and Interaction with the Dioxin Response Element