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.
Molecular and Cellular Endocrinology
Roggero, V.R., Zhang, J., Parente, L.E., Doshi, Y., Dziedzic, R.C., McGregor, E.L., Varjabedian, A.D., Schad, S.E., Bondzi, C., Allison, L.A. (2016) Nuclear import of the thyroid hormone receptor alpha 1 is mediated by importin 7, importin beta 1, and adaptor importin alpha 1. Molecular and Cellular Endocrinology 419, 185-197.