Multiple Novel Signals Mediate Thyroid Hormone Receptor Nuclear Import and Export*

Abstract

Background: Thyroid hormone receptor (TR) shuttles between the cytosol and nucleus and regulates gene expression in response to hormone.

Results: TR shuttling is mediated by multiple, transferable nuclear localization and nuclear export signals.

Conclusion: Functional domains of TR work in concert to modulate shuttling.

Significance: Molecular characterization of signals that regulate shuttling is crucial for understanding nuclear receptor function.

Abstract

Thyroid hormone receptor (TR) is a member of the nuclear receptor superfamily that shuttles between the cytosol and nucleus. The fine balance between nuclear import and export of TR has emerged as a critical control point for modulating thyroid hormone-responsive gene expression; however, sequence motifs of TR that mediate shuttling are not fully defined. Here, we characterized multiple signals that direct TR shuttling. Along with the known nuclear localization signal in the hinge domain, we identified a novel nuclear localization signal in the A/B domain of thyroid hormone receptor α1 that is absent in thyroid hormone receptor β1 and inactive in the oncoprotein v-ErbA. Our prior studies showed that thyroid hormone receptor α1 exits the nucleus through two pathways, one dependent on the export factor CRM1 and the other CRM1-independent. Here, we identified three novel CRM1-independent nuclear export signal (NES) motifs in the ligand-binding domain as follows: a highly conserved NES in helix 12 (NES-H12) and two additional NES sequences spanning helix 3 and helix 6, respectively. Mutations predicted to disrupt the α-helical structure resulted in a significant decrease in NES-H12 activity. The high degree of conservation of helix 12 suggests that this region may function as a key NES in other nuclear receptors. Furthermore, our mutagenesis studies on NES-H12 suggest that altered shuttling of thyroid hormone receptor β1 may be a contributing factor in resistance to thyroid hormone syndrome. Taken together, our findings provide a detailed mechanistic understanding of the multiple signals that work together to regulate TR shuttling and transcriptional activity, and they provide important insights into nuclear receptor function in general.