Multiple novel signals mediate thyroid hormone receptor nuclear import and export

J Biol Chem. 2012 Sep 7;287(37):31280-97. doi: 10.1074/jbc.M112.397745. Epub 2012 Jul 19.

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.

Publication types

  • Research Support, N.I.H., Extramural
  • Research Support, Non-U.S. Gov't
  • Research Support, U.S. Gov't, Non-P.H.S.

MeSH terms

  • Active Transport, Cell Nucleus / physiology
  • Amino Acid Motifs
  • Animals
  • HeLa Cells
  • Humans
  • Mutation
  • Nuclear Localization Signals / genetics
  • Nuclear Localization Signals / metabolism*
  • Oncogene Proteins v-erbA / genetics
  • Oncogene Proteins v-erbA / metabolism
  • Protein Structure, Tertiary
  • Rats
  • Thyroid Hormone Receptors alpha / genetics
  • Thyroid Hormone Receptors alpha / metabolism*
  • Thyroid Hormone Receptors beta / genetics
  • Thyroid Hormone Receptors beta / metabolism*

Substances

  • Nuclear Localization Signals
  • Oncogene Proteins v-erbA
  • Thyroid Hormone Receptors alpha
  • Thyroid Hormone Receptors beta