Structural hierarchy controlling dimerization and target DNA recognition in the AHR transcriptional complex

Environmental pollutants and cellular metabolites influence a wide range of biological functions through the aryl hydrocarbon receptor (AHR). Prior research has shown that AHR is a PAS domain transcription factor that forms a functional complex with ARNT to regulate gene expression. It was already known that this complex binds to specific DNA sequences, such as the dioxin response element (DRE). However, the structural details of how AHR and ARNT interact with DNA remained unclear. No prior work had resolved the three-dimensional organization of the AHR-ARNT-DRE complex. That uncertainty drove the need for structural analysis to understand how AHR recognizes DNA and how its activation is regulated. This gap motivated the investigation into the molecular architecture of the AHR complex. Understanding the structural hierarchy of AHR could clarify how diverse ligands influence its function. The AHR’s role in both toxicity and normal physiology suggests a need for precise structural insights.

The aim of this study was to determine the crystal structure of the AHR-ARNT heterodimer bound to the DRE. This would provide a structural basis for understanding how AHR recognizes DNA and how its activity is modulated. The researchers sought to clarify the molecular architecture of the AHR-ARNT-DRE complex. They hypothesized that structural features of the complex would reveal mechanisms of DNA recognition and ligand-induced activation. The study also aimed to identify how interdomain interactions influence nuclear localization of AHR. By analyzing the structure, the researchers hoped to uncover the allosteric pathways involved in AHR function. This work could help explain how AHR responds to a wide range of ligands. The study focused on the structural dynamics that underlie AHR’s diverse biological effects.

The researchers used X-ray crystallography to determine the structure of the AHR-ARNT-DRE complex. They purified the AHR and ARNT proteins and co-crystallized them with the DRE DNA fragment. The crystal structure was solved at high resolution to capture the heterodimer and DNA interactions. The structure revealed an asymmetric architecture with ARNT wrapping around AHR. The team analyzed the heterodimerization interfaces and interdomain interactions in detail. They mapped the DNA-binding residues involved in DRE recognition. Computational modeling was used to compare the DRE with the hypoxia response element (HRE). The structural data were interpreted in the context of AHR’s known functional properties.

The crystal structure showed that ARNT wraps around AHR in a highly intertwined, asymmetric configuration. Extensive heterodimerization interfaces and interdomain interactions were observed in the complex. Specific DNA-binding residues in AHR were identified as critical for DRE recognition. These residues distinguish DRE from the closely related HRE. The dimerization interfaces and interdomain interactions globally influence DNA recognition. Mutations at these interfaces altered AHR’s nuclear localization behavior. Some changes caused constitutive nuclear localization, while others prevented nuclear translocation. The structure suggests an allosteric pathway linking ligand binding to nuclear localization signal exposure.

The structural findings suggest a dynamic hierarchy in AHR activation driven by interdomain interactions and dimerization. The AHR-ARNT-DRE complex exhibits a highly intertwined architecture with extensive interfaces. Specific DNA-binding residues are responsible for DRE recognition. The dimerization interfaces and interdomain interactions modulate DNA recognition globally. Allosteric changes in these regions affect nuclear localization of AHR. The structure supports a model where ligand binding induces conformational changes. These changes may expose the nuclear localization signal of AHR. The researchers propose that this structural hierarchy allows AHR to respond to diverse ligands.