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 recepto r (AHR) signaling pathway, we expressed the AHR, its dimerization part ner ARNT, and a beta-galactosidase (lacZ) reporter gene, driven by two dioxin-responsive enhancers, in the yeast Saccharomyces cerevisiae. I n this system, the agonists beta-naphthoflavone and alpha-naphthoflavo ne induced transcription of the lacZ gene, with EC(50) values of 7.9 x 10(-8) and 3.0 x 10(-7) M, respectively, while the nonagonist dexamet hasone was without effect. As a first application of this system, we e xamined 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 w ith analogous domains from the LexA protein. Coexpression of this AHR- LexA chimera and a lacZ reporter gene driven by eight LexA operator si tes in a yeast strain with regulatable levels of hsp90, yielded pharma cology that closely mirrored that of the AHR/ARNT/dioxin-responsive en hancer system described above, but only when hsp90 levels were held ne ar their wild type levels. When hsp90 levels were reduced to approxima tely 5% of normal, AHR signaling in response to agonist was completely blocked despite normal cell growth. These results provide the first g enetic evidence for the role of hsp90 in AHR signaling and provide the basis for a powerful new system in which to study this pathway.