Transitional States in Ligand-Dependent Transformation of the Aryl Hydrocarbon Receptor into Its DNA-Binding Form.

Anatoly A Soshilov,Stefano Motta,Laura Bonati,Michael S Denison

doi:10.3390/ijms21072474

Abstract

The aryl hydrocarbon receptor (AhR) is a ligand-activated transcription factor that mediates the biological and toxicological effects of an AhR lacking the entire PASB structurally diverse chemicals, including halogenated aromatic hydrocarbons. Ligand-dependent transformation of the AhR into its DNA binding form involves a ligand-dependent conformational change, heat shock protein 90 (hsp90), dissociation from the AhR complex and AhR dimerization with the AhR nuclear translocator (ARNT) protein. The mechanism of AhR transformation was examined using mutational approaches and stabilization of the AhR:hsp90 complex with sodium molybdate. Insertion of a single mutation (F281A) in the hsp90-binding region of the AhR resulted in its constitutive (ligand-independent) transformation/DNA binding in vitro. Mutations of AhR residues within the Arg-Cys-rich region (R212A, R217A, R219A) and Asp371 (D371A) impaired AhR transformation without a significant effect on ligand binding. Stabilization of AhR:hsp90 binding with sodium molybdate decreased transformation/DNA binding of the wild type AhR but had no effect on constitutively active AhR mutants. Interestingly, transformation of the AhR in the presence of molybdate allowed detection of an intermediate transformation ternary complex containing hsp90, AhR, and ARNT. These results are consistent with a stepwise transformation mechanism in which binding of ARNT to the liganded AhR:hsp90 complex results in a progressive displacement of hsp90 and conversion of the AhR into its high affinity DNA binding form. The available molecular insights into the signaling mechanism of other Per-ARNT-Sim (PAS) domains and structural information on hsp90 association with other client proteins are consistent with the proposed transformation mechanism of the AhR.

Highlights

The aryl hydrocarbon receptor (AhR) is a ligand-dependent transcription factor that mediates the biological and toxic effects of 2,3,7,8-tetrachlorodibenzo-p-dioxin (TCDD, dioxin) and a structurally diverse range of chemicals [1,2]
Ligand binding to the AhR has been extensively studied, and the overall fingerprint amino acid residues, as well as the amino acid residues implicated in differences in AhR affinity among many species, have been characterized [30,39,40,43,44]
While no structural information on the hsp90:AhR cytosolic complex is available to date, a recent Cryo-EM structure of hsp90 bound to another protein (CDK4) [52] provides interesting clues

Summary

Introduction

The aryl hydrocarbon receptor (AhR) is a ligand-dependent transcription factor that mediates the biological and toxic effects of 2,3,7,8-tetrachlorodibenzo-p-dioxin (TCDD, dioxin) and a structurally diverse range of chemicals [1,2]. Numerous endogenous ligands of the AhR have been proposed, and while none have been established with certainty, tryptophan and indole metabolites are likely candidates [1,2,7,8]. Exposure to dioxin-like compounds results in adverse effects that are mediated by AhR-dependent transcriptional activation [1,9,10]. While AhR-dependent toxic effects are highly species-specific, differences in ligand binding affinity among species do not fully account for the observed variations [11]. It has been previously hypothesized that the species differences in biochemical properties of the AhR complex may somehow contribute to the differences in AhR-dependent toxic outcomes, emphasizing the need for better understanding of the process of ligand-dependent activation of the AhR

Methods

Results

Conclusion