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Mol Cell Biol, February 1998, p. 978-988, Vol. 18, No. 2
0270-7306/98/$04.00+0
Copyright © 1998, American Society for Microbiology. All rights reserved.

Hepatitis B Virus X-Associated Protein 2 Is a Subunit of the Unliganded Aryl Hydrocarbon Receptor Core Complex and Exhibits Transcriptional Enhancer Activity

Brian K. Meyer,1 Marilyn G. Pray-Grant,2 John P. Vanden Heuvel,2 and Gary H. Perdew1,2,*

Graduate Program in Biochemistry and Molecular Biology1 and Center for Molecular Toxicology and Department of Veterinary Science,2 The Pennsylvania State University, University Park, Pennsylvania 16802

Received 25 August 1997/Returned for modification 12 October 1997/Accepted 12 November 1997

Prior to ligand activation, the unactivated aryl hydrocarbon receptor (AhR) exists in a heterotetrameric 9S core complex consisting of the AhR ligand-binding subunit, a dimer of hsp90, and an unknown subunit. Here we report the purification of an ~38-kDa protein (p38) from COS-1 cell cytosol that is a member of this complex by coprecipitation with a FLAG-tagged AhR. Internal amino acid sequence information was obtained, and p38 was identified as the hepatitis B virus X-associated protein 2 (XAP2). The simian ortholog of XAP2 was cloned from a COS-1 cDNA library; it codes for a 330-amino-acid protein containing regions of homology to the immunophilins FKBP12 and FKBP52. A tetratricopeptide repeat (TPR) domain in the carboxy-terminal region of XAP2 was similar to the third and fourth TPR domains of human FKBP52 and the Saccharomyces cerevisiae transcriptional modulator SSN6, respectively. Polyclonal antibodies raised against XAP2 recognized p38 in the unliganded AhR complex in COS-1 and Hepa 1c1c7 cells. It was ubiquitously expressed in murine tissues at the protein and mRNA levels. It was not required for the assembly of an AhR-hsp90 complex in vitro. Additionally, XAP2 did not directly associate with hsp90 upon in vitro translation, but was present in a 9S form when cotranslated in vitro with murine AhR. XAP2 enhanced the ability of endogenous murine and human AhR complexes to activate a dioxin-responsive element-luciferase reporter twofold, following transient expression of XAP2 in Hepa 1c1c7 and HeLa cells.

* Corresponding author. Mailing address: Department of Veterinary Science, 114 Henning Building, University Park, PA 16802. Phone: (814) 863-0400. Fax: (814) 863-6140. E-mail: ghp2{at}psu.edu.




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