ARA9 modifies agonist signaling through an increase in cytosolic aryl hydrocarbon receptor

The aryl hydrocarbon receptor (AHR) is a ligand activated transcription fac tor that mediates the effects of agonists like 2,3,7,8-tetrachlorodibenzo-p -dioxin. In the current model for AHR signaling, the unliganded receptor is found in the cytosol as part of a complex with a dimer of the 90-kDa heat shock protein and an immunophilin-like molecule, ARA9. In yeast, expression of ARA9 results in an increase in the maximal agonist response and a leftw ard shift in the AHR dose-response curve. To better understand the mechanis m by which ARA9 modifies AHR signal transduction, we performed a series of coexpression experiments in yeast and mammalian cells. Our results demonstr ate that ARA9's influence on AHR signaling is not due to inhibition of a me mbrane pump or modification of the receptor's transactivation properties. U sing receptor photoaffinity labeling experiments, we were able to show that ARA9 enhances AHR signal transduction by increasing the available AHR bind ing sites within the cytosolic compartment of the cell. Our evidence sugges ts that ARA9's effects are related to its role as a cellular chaperone; i.e . we observed that expression of ARA9 increases the fraction of AHR in the cytosol and also stabilized the receptor under heat stress.