Abstract
The dioxin receptor together with its partner factor Arnt belongs to the basic helix-loop-helix/PAS family of transcription factors. The receptor is the mediator of the biological effects of a number of environmental pollutants such as dioxin and other related compounds. At the molecular level the dioxin receptor has been shown to, in the presence of agonists to the receptor, activate a battery of genes involved in metabolic activation and clearing of xenobiotic compounds from the cell. In the absence of ligand the dioxin receptor is present in the cytosolic compartment of the cell coupled to a dimer of the molecular chaperone hsp90. In the presence of ligand the dioxin receptor is found in the nuclear compartment of the cell as a heterodimeric form with its partner factor Arnt. This heterodimer interacts with xenobiotic responsive elements (XRE) in the vicinity of dioxin inducible genes to activate transcription. We have studied the activation mechanisms of the dioxin receptor.
We show that the molecular chaperone hsp90 is required to maintain the dioxin receptor in a ligand binding conformation as well to inhibit an otherwise constitutive DNA binding activity. This DNA binding activity is critically dependent on dimerization with Arnt and appears to be regulated by a protein kinase C-dependent mechanism. In addition we have performed a detailed analysis of the determinants of dimerization and DNA binding activity of the dioxin receptor. We describe a critical role of the PAS domain in determining a dimerization specificity: it restricts an otherwise broad dimerization activity of the DNA binding basic helix-loop-helix domain.
