The 90-kDa heat shock protein is essential for Ah receptor signaling in a yeast expression system

In an effort to provide a more powerful system to study the Ah receptor (AHR) signaling pathway, we expressed the AHR, its dimerization partner ARNT, and a beta-galactosidase (lacZ) reporter gene, driven by two dioxin-responsive enhancers, in the yeast Saccharomyces cerevisiae. In this system, the agonists beta-naphthoflavone and alpha-naphthoflavone induced transcription of the lacZ gene, with EC50 values of 7.9 x 10(-8) and 3.0 x 10(-7) M, respectively, while the nonagonist dexamethasone was without effect. As a first application of this system, we examined the relationship between the 90-kDa heat shock protein (hsp90) and AHR function. To accomplish this in a manner that was independent of the ARNT protein, we constructed a chimeric receptor in which the DNA binding and primary dimerization domains of the AHR were swapped with analogous domains from the LexA protein. Coexpression of this AHR-LexA chimera and a lacZ reporter gene driven by eight LexA operator sites in a yeast strain with regulatable levels of hsp90, yielded pharmacology that closely mirrored that of the AHR/ARNT/dioxin-responsive enhancer system described above, but only when hsp90 levels were held near their wild type levels. When hsp90 levels were reduced to approximately 5% of normal, AHR signaling in response to agonist was completely blocked despite normal cell growth. These results provide the first genetic evidence for the role of hsp90 in AHR signaling and provide the basis for a powerful new system in which to study this pathway.