title: Aryl hydrocarbon (Ah) receptor levels are selectively modulated by hsp90-associated immunophilin homolog XAP2.
abstract: The aryl hydrocarbon receptor (AhR) is a ligand-inducible transcription factor that mediates biological responses to halogenated aromatic hydrocarbons. The unliganded AhR is a cytoplasmic, tetrameric complex consisting of the AhR ligand-binding subunit, a dimer of hsp90, and the hepatitis B virus X-associated protein 2 (XAP2). The role of XAP2 as a member of the AhR core complex is poorly understood. XAP2 shares significant homology with the immunophilins FKBP12 and FKBP52, including a highly conserved, C-terminal, tetratricopeptide repeat (TPR) domain. XAP2 forms a complex with hsp90 and the AhR but can also bind to both independently. This binding is mediated by the conserved TPR domain. Single-point mutations in this region are sufficient to disrupt the association of XAP2 with both the AhR and hsp90 in cells. Cotransfection of the AhR and XAP2 in COS-1 cells results in increased AhR levels compared with cells transfected with the AhR alone. In contrast, coexpression of the AhR with the TPR containing proteins FKBP52, protein phosphatase 5 (PP5), or XAP2 TPR-mutants deficient in binding to the AhR and hsp90 does not affect AhR levels and coexpression of the AhR with the TPR domain of PP5 results in AhR down-regulation. These results demonstrate that XAP2 is apparently unique among hsp90-binding proteins in its ability to enhance AhR levels. A yellow fluorescent protein (YFP)-XAP2-FLAG was constructed and biochemically characterized, and no loss of function was detected. YFP-XAP2-FLAG was transiently transfected into NIH 3T3 and was found to localize in both the nucleus and the cytoplasm when visualized by fluorescence microscopy. Treatment of Hepa-1 cells with the hsp90-binding benzoquinone ansamycin, geldanamycin, and the macrocyclic antifungal compound radicicol resulted in AhR but not XAP2 or FKBP52 turnover. Taken together, these results suggest that XAP2/hsp90 and FKBP52/hsp90 complexes are similar yet exhibit unique functional specificity.
authors.affiliation: Graduate Program in Biochemistry and Molecular Biology, The Pennsylvania State University, University Park 16802,USA.
authors.forename: B K
authors.initials: BK
authors.lastname: Meyer
authors.forename: J R
authors.initials: JR
authors.lastname: Petrulis
authors.forename: G H
authors.initials: GH
authors.lastname: Perdew
mesh_terms: D020816:Amino Acid Motifs; D000595:Amino Acid Sequence; D000818:Animals; D000900:Anti-Bacterial Agents / Q000494:pharmacology; D016227:Benzoquinones; D019556:COS Cells; D002460:Cell Line; D002467:Cell Nucleus / Q000378:metabolism; D002474:Cell-Free System; D003593:Cytoplasm / Q000378:metabolism; D017930:Genes, Reporter; D018841:HSP90 Heat-Shock Proteins / Q000737:chemistry / Q000235:genetics / Q000378:metabolism*; D047908:Intracellular Signaling Peptides and Proteins; D047029:Lactams, Macrocyclic; D007783:Lactones / Q000494:pharmacology; D018942:Macrolides; D018613:Microscopy, Confocal; D008969:Molecular Sequence Data; D009154:Mutation; D009687:Nuclear Proteins / Q000378:metabolism; D010749:Phosphoprotein Phosphatases / Q000378:metabolism; D020836:Protein Structure, Quaternary; D011506:Proteins / Q000737:chemistry / Q000235:genetics / Q000378:metabolism*; D011809:Quinones / Q000494:pharmacology; D018336:Receptors, Aryl Hydrocarbon / Q000235:genetics / Q000378:metabolism*; D011993:Recombinant Fusion Proteins / Q000378:metabolism; D016415:Sequence Alignment; D022021:Tacrolimus Binding Proteins / Q000235:genetics / Q000378:metabolism; D014162:Transfection
chemical_list: D000900:Anti-Bacterial Agents; D016227:Benzoquinones; D018841:HSP90 Heat-Shock Proteins; D047908:Intracellular Signaling Peptides and Proteins; D047029:Lactams, Macrocyclic; D007783:Lactones; D018942:Macrolides; D009687:Nuclear Proteins; D011506:Proteins; D011809:Quinones; D018336:Receptors, Aryl Hydrocarbon; D011993:Recombinant Fusion Proteins; C103265:aryl hydrocarbon receptor-interacting protein; D010749:Phosphoprotein Phosphatases; C089641:protein phosphatase 5; D022021:Tacrolimus Binding Proteins; C403485:tacrolimus binding protein 4; C035359:monorden; C001277:geldanamycin