Distinct roles of the molecular chaperone hsp90 in modulating dioxin receptor function via the basic helix-loop-helix and PAS domains.

The intracellular dioxin receptor mediates signal transduction by dioxin and functions as a ligand-activated transcription factor. It contains a basic helix-loop-helix (bHLH) motif contiguous with a Per-Arnt-Sim (PAS) homology region. In extracts from nonstimulated cells the receptor is recovered in an inducible cytoplasmic form associated with the 90-kDa heat shock protein (hsp90), a molecular chaperone. We have reconstituted ligand-dependent activation of the receptor to a DNA-binding form by using the dioxin receptor and its bHLH-PAS partner factor Arnt expressed by in vitro translation in reticulocyte lysate. Deletion of the PAS domain of the receptor resulted in constitutive dimerization with Arnt. In contrast, this receptor mutant showed low levels of xenobiotic response element-binding activity, indicating that the PAS domain may be important for DNA-binding affinity and/or specificity of the receptor. It was not possible to reconstitute dioxin receptor function with proteins expressed in wheat germ lysate. In line with these observations, reticulocyte lysate but not wheat germ lysate promoted the association of de novo synthesized dioxin receptor with hsp90. At least two distinct domains of the receptor mediated interaction with hsp90: the ligand-binding domain located within the PAS region and, surprisingly, the bHLH domain. Whereas ligand-binding activity correlated with association with hsp90, bHLH-hsp90 interaction appeared to be important for DNA-binding activity but not for dimerization of the receptor. Several distinct roles for hsp90 in modulating dioxin receptor function are therefore likely: correct folding of the ligand-binding domain, interference with Arnt heterodimerization, and folding of a DNA-binding conformation of the bHLH domain. Thus, the dioxin receptor system provides a complex and interesting model of the regulation of transcription factors by hsp90.