Characterization of the AhR-hsp90-XAP2 core complex and the role of the immunophilin-related protein XAP2 in AhR stabilization

The unliganded aryl hydrocarbon receptor (AhR) exists in the cytoplasm in a tetrameric 9S core complex, consisting of the AhR ligand-binding subunit, a dimer of hsp90, and the hepatitis B virus X-associated protein 2 (XAP2), an immunophilin-related protein sharing homologous regions with FKBP12 and FKBP52. Interactions between the recently identified XAP2 subunit and other members of the unliganded AhR complex and its precise role in the AhR sign al transduction pathway are presently unknown. Mapping studies indicate tha t XAP2 requires the PAS, hsp90, and ligand binding domain(s) of the AhR for binding, and that both proteins directly interact in the absence of hsp90. XAP2 is also able to interact with hsp90 complexes in the absence of the A hR, and C-terminal sequences of XAP2 are required for this interaction. XAP 2 binds to the C-terminal end of hsp90, which contains a tetratricopeptide repeat domain acceptor site, whereas the AhR binds to a domain in the middl e of hsp90. XAP2 was not found to be associated with the AhR-Arnt heterocom plex either in vitro or in nuclear extracts isolated from Hepa 1 cells trea ted with TCDD. Transient expression of XAP2 in COS-1 cells resulted in enha nced cytosolic AhR levels, suggesting a role for XAP2 in regulating the rat e of AhR turnover.