The hsp90 Co-chaperone XAP2 Alters Importin β Recognition of the Bipartite Nuclear Localization Signal of the Ah Receptor and Represses Transcriptional Activity

Abstract

The mouse aryl hydrocarbon receptor (mAhR) is a ligand-activated transcription factor that exists in a tetrameric, core complex with a dimer of the 90-kDa heat shock protein, and the hepatitis B virus X-associated protein 2 (XAP2). Transiently expressed mAhR-YFP (yellow fluorescent protein fused with the mAhR) localizes throughout cells, with a majority occupying nuclei. Coexpression of XAP2 with mAhR-YFP results in a distinct redistribution to the cytoplasm. We have utilized several approaches to attempt to identify the mechanism by which XAP2 modulates the sub-cellular localization of the mAhR. The nuclear export inhibitor, leptomycin B, was used to demonstrate that XAP2 inhibits ligand-independent nucleocytoplasmic shuttling of the receptor. Results from cytoskeletal disruption and the addition of an alternate nuclear localization sequence (NLS) to mAhR-YFP suggest that XAP2 does not physically tether the complex in the cytoplasm. The use of a rabbit polyclonal antibody raised against a portion of the bipartite NLS of the mAhR revealed that XAP2 does not appear to block access to the NLS. However, XAP2 hinders importin ␤ binding to the mAhR complex, suggesting that XAP2 alters the conformation of the bipartite NLS of mAhR. XAP2 also represses the transactivation potential of the AhR, in contrast to previously published reports, perhaps by stabilizing the receptor complex and/or blocking nucleocytoplasmic shuttling of the AhR complex.

Aryl Hydrocarbon Receptor Interacting Protein Targets IRF7 to Suppress Antiviral Signaling and the Induction of Type I Interferon

“…Interestingly, the inhibition of IRF7 nuclear localization by AIP is strikingly similar to the regulation of AhR by AIP. AIP binds to AhR and inhibits AhR nuclear translocation by triggering a conformational change that prevents binding of its bipartite NLS to importin-␤ (38). Therefore, AIP may antagonize the nuclear localization of AhR and IRF7 transcription factors by similar mechanisms, although further studies are needed to address this possibility.…”

“…Hsp90 and AIP retain AhR in the cytoplasm, and AIP prevents the ubiquitination and degradation of AhR (36). Upon AhR ligand binding, AIP is dissociated from the complex, allowing AhR to enter the nucleus, dimerize with AhR nuclear translocator, and activate genes involved in the xenobiotic response (37,38). AIP was also identified as a protein associated with several viral proteins, such as the X protein of the hepatitis B virus (31) and EBNA-3 of the Epstein-Barr virus (39).…”

Role of the Per/Arnt/Sim Domains in Ligand-dependent Transformation of the Aryl Hydrocarbon Receptor

“…This mechanism was based on predicted exposure of the NLS contained within the bHLH domain following ligand binding due to the observed nuclear import of the AhR complex (12). However, a more recent study suggested that nuclear import of the AhR complex may depend on a change in the NLS conformation rather than its exposure and that XAP2 could be involved in this process (31). Moreover, the prediction of ligand-dependent dissociation of Hsp90 from the bHLH domain (to account for presumed NLS exposure) was also questioned by the findings that the Hsp90 binding stabilizing agent sodium molybdate did not inhibit nuclear translocation of the AhR complex (8), and by the observation that deletion of the C-terminal transactivation domain of the AhR, which does not affect Hsp90 binding, resulted in constitutive ligand-independent AhR nuclear localization (32,33).…”

When evolution is the solution to pollution: Key principles, and lessons from rapid repeated adaptation of killifish (Fundulus heteroclitus) populations

“…For example, the most significant QTL identified, explaining up to 46% of the phenotypic variance within families analyzed (Nacci, Proestou, et al., 2016), was nearby to the gene encoding aryl hydrocarbon receptor interacting protein ( AIP , also known as HBV x‐associated protein 2 , XAP2 , and AH receptor‐activated 9 , ARA9 ; Trivellin & Korbonits, 2011) (Figure 4). While the roles of AIP are not precisely known and may vary by species and life stage, it is known to modulate the function of AHR signaling (Petrulis, Kusnadi, Ramadoss, Hollingshead, & Perdew, 2003), and influences DLC toxicity (Nukaya, Lin, et al., 2010). Another genetic marker near AIP interacted significantly as an additive QTL with HSP90 .…”