Characterization of the Ah receptor-associated protein, ARA9

The unliganded aryl hydrocarbon receptor (AHR) is found in a complex with o ther proteins including the 90-kDa heat shock protein (Hsp90) and a 37-kDa protein we refer to as ARA9. We found that the three tetratricopeptide repe ats found in the COOH terminus of ARA9 are necessary and sufficient for int eraction with the AHR complex. Conversely, the AHR's "repressor"/Hsp90 bind ing domain is required for interaction with ARA9. Because ARA9 closely rese mbles the 52-kDa FK506-binding protein (FKBP52), found in the unliganded gl ucocorticoid receptor (GR) complex, we compared the binding specificities o f ARA9 and FKBP52 for AHR and GR. In co-immunoprecipitation experiments, AR A9 specifically associated with AHR-Hsp90 complex but not with GR-Hsp90 com plexes. In addition, ARA9 showed a greater capacity than FKBP52 to associat e with AHR-Hsp90 complexes. The biological importance of this interaction w as suggested by the observation that in a yeast expression system ARA9 expr ession enhanced the response of AHR to the agonist beta-napthoflavone, decr easing the EC,, by greater than 5-fold and increasing the maximal response 2.5-fold. In contrast, co-expression of FKBP52 had no effect on AHR signali ng. In addition, although ARA9 contains a domain similar to that found in o ther FK506-binding proteins, ARA9 binding to H-3-FK506 could not be detecte d. Finally, we have characterized the developmental and expression pattern of ARA9 in the developing mouse embryo and mapped the ARA9 locus to human c hromosome 11q13.3.