Background information
The Aryl hydrocarbon receptor (AhR), also designated dioxin receptor (DR), a ligand-activated transcription factor, becomes activated upon binding of dioxins or structurally related forms of xenobiotics. Upon ligand binding, AhR translocates from the cytoplasm to the nucleus where it complexes with Arnt to form a DNA binding heterodimer. This complex activates transcription of target genes involved in xenobiotic metabolism. Until ligand binding occurs, AhR remains latent in the cytoplasm, which is maintained by its association with the molecular chaperones HSP 90, the hepatitis B virus X-associated protein (XAP2, also designated AIP and ARA9) and the heat shock protein p23. XAP2, a ubiquitously expressed protein, binds to HSP 90 and AhR through a highly conserved carboxy-terminal tetraticopeptide repeat domain. XAP2 participates in stabilizing AhR as well as enhancing the cytoplasmic localization of the receptor. It may also be involved in regulating the degradation of AhR.
