Nuclear Import Of The Thyroid Hormone Receptor Alpha 1 Is Mediated By Importin 7, Importin Beta 1, And Adaptor Importin Alpha 1

The thyroid hormone receptor alpha 1 (TR alpha 1) is a nuclear receptor for thyroid hormone that shuttles rapidly between the nucleus and cytoplasm. Our prior studies showed that nuclear import of TR alpha 1 is directed by two nuclear localization signals, one in the N-terminal A/B domain and the other in the hinge domain. Here, we showed using in vitro nuclear import assays that TR alpha 1 nuclear localization is temperature and energy-dependent and can be reconstituted by the addition of cytosol. In HeLa cells expressing green fluorescent protein (GFP)-tagged TR alpha 1, knockdown of importin 7, importin beta 1 and importin alpha 1 by RNA interference, or treatment with an importin beta 1-specific inhibitor, significantly reduced nuclear localization of TR alpha 1, while knockdown of other importins had no effect. Coimmunoprecipitation assays confirmed that TR alpha 1 interacts with importin 7, as well as importin beta 1 and the adapter importin alpha 1, suggesting that TR alpha 1 trafficking into the nucleus is mediated by two distinct pathways. (C) 2015 Elsevier Ireland Ltd. All rights reserved.