Meyer B K, Petrulis J R, Perdew G H
Graduate Program in Biochemistry and Molecular Biology, The Pennsylvania State University, University Park 16802,USA.
Cell Stress Chaperones. 2000 Jul;5(3):243-54. doi: 10.1379/1466-1268(2000)005<0243:aharla>2.0.co;2.
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.
芳烃受体（AhR）是一种配体诱导型转录因子，介导对卤代芳烃的生物学反应。未结合配体的AhR是一种细胞质四聚体复合物，由AhR配体结合亚基、hsp90二聚体和乙型肝炎病毒X相关蛋白2（XAP2）组成。XAP2作为AhR核心复合物成员的作用尚不清楚。XAP2与免疫亲和蛋白FKBP12和FKBP52具有显著同源性，包括一个高度保守的C末端四肽重复（TPR）结构域。XAP2与hsp90和AhR形成复合物，但也能独立与两者结合。这种结合由保守的TPR结构域介导。该区域的单点突变足以破坏细胞中XAP2与AhR和hsp90的结合。与单独转染AhR的细胞相比，在COS-1细胞中共转染AhR和XAP2会导致AhR水平升高。相反，AhR与含TPR的蛋白FKBP52、蛋白磷酸酶5（PP5）或缺乏与AhR和hsp90结合能力的XAP2 TPR突变体共表达不会影响AhR水平，而AhR与PP5的TPR结构域共表达会导致AhR下调。这些结果表明，XAP2在增强AhR水平的能力方面在hsp90结合蛋白中显然是独特的。构建了黄色荧光蛋白（YFP）-XAP2-FLAG并进行了生化特性分析，未检测到功能丧失。YFP-XAP2-FLAG被瞬时转染到NIH 3T3细胞中，通过荧光显微镜观察发现其定位于细胞核和细胞质中。用hsp90结合的苯醌安莎霉素、格尔德霉素和大环抗真菌化合物雷帕霉素处理Hepa-1细胞会导致AhR周转，但不会导致XAP2或FKBP52周转。综上所述，这些结果表明XAP2/hsp90和FKBP52/hsp90复合物相似但具有独特的功能特异性。
