Multiple Pathways Mediate Thyroid Hormone Receptor Nuclear Import

Abstract

The thyroid hormone receptor α1 (TRα1) is a nuclear receptor for thyroid hormone. While TRα1 carries out its function as a transcription factor in the nucleus, the receptor also shuttles rapidly between the nucleus and cytoplasm. Our prior studies show that nuclear import of TRα1 is directed by two nuclear localization signal (NLS) motifs, one in the N‐terminal A/B domain and the other in the hinge domain. Here, we investigated which importins mediate nuclear import of TRα1, using a combined approach of in vitro nuclear import assays and small RNA interference. Nuclear localization of TRα1 in digitonin‐permeabilized HeLa cells was temperature and energy‐dependent and could be reconstituted by the addition of cytosol or recombinant importins α1 and β1. In transient transfection assays, knockdown of importin 7, importin β1, and importin α1 markedly reduced nuclear localization of TRα1, while importin α3 had a lesser effect. Taken together, data suggest that TRα1 utilizes multiple import pathways, with nuclear entry facilitated primarily by importin 7, as well as by importin β1 in association with importin α1.