Thyroid hormone receptor (TR) major subtypes and localization signals. The
structural diagram (not to scale) of TRα1, TRβ1, and
TRβ2 shows nuclear localization signal (NLS), nuclear export signal
(NES), and mitochondrial import sequence (MIS) motifs, where known (solid bar)
or predicted (striped bar) based on sequence homology. The positions of
localization signals are indicated in relation to the respective individual
domains of TR: N-terminal A/B domain (A/B), DNA-binding domain (DBD), hinge
domain, and ligand-binding domain (LBD). The TRα1 mRNA encodes several
forms of truncated TR by translation initiation from internal AUG sites encoding
methionine (M); amino acid residue numbers correspond to the position in
full-length TRα1.