Nuclear import of the thyroid hormone receptor alpha 1 is mediated by importin 7, importin beta 1, and adaptor importin alpha 1

Vincent R. Roggero, Coll William & Mary, Dept Biol, 540 Landrum Dr,ISC 2117, Williamsburg, VA 23185 USA
Jibo Zhang, Coll William & Mary, Dept Biol, 540 Landrum Dr,ISC 2117, Williamsburg, VA 23185 USA
Laura E. Parente, Coll William & Mary, Dept Biol, 540 Landrum Dr,ISC 2117, Williamsburg, VA 23185 USA
Yazdi Doshi, Coll William & Mary, Dept Biol, 540 Landrum Dr,ISC 2117, Williamsburg, VA 23185 USA
Rose C. Dziedzic, Coll William & Mary, Dept Biol, 540 Landrum Dr,ISC 2117, Williamsburg, VA 23185 USA
Emma L. McGregor, Coll William & Mary, Dept Biol, 540 Landrum Dr,ISC 2117, Williamsburg, VA 23185 USA
Arev D. Varjabedian, Coll William & Mary, Dept Biol, 540 Landrum Dr,ISC 2117, Williamsburg, VA 23185 USA
Sara E. Schad, Coll William & Mary, Dept Biol, 540 Landrum Dr,ISC 2117, Williamsburg, VA 23185 USA
Lizabeth A. Allison, Coll William & Mary, Dept Biol, 540 Landrum Dr,ISC 2117, Williamsburg, VA 23185 USA
Cornelius Bondzi, Hampton Univ, Dept Biol Sci, Hampton, VA 23668 USA

Abstract

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.