The differential hormone-dependent transcriptional activation of thyroid hormone receptor isoforms is mediated by interplay of their domains.
Human thyroid hormone nuclear receptor isoforms (TRalpha1 and TRbeta1) express differentially in a tissue-specific and development-dependent manner. It is unclear whether these two isoforms have differential functions. We analyzed their interaction with a thyroid hormone response element with half-site binding motifs arranged in an everted repeat separated by six nucleotides (F2). Despite extensive sequence homologies, the two isoforms bound to F2 with different affinities and ratios of homodimer/monomer. Using F2-containing reporter gene, we found that the transcriptional activity of TRbeta1 was approximately 6-fold higher than that of TRalpha1. The lower activity of TRalpha1 was not due to differences in expression of the two isoforms because similar nuclear localization patterns were observed. To understand the structural determinants responsible for these differences, we constructed chimeric receptors in which hinge regions (domain D), hormone binding domains (domain E), and domains (D + E) were sequentially interchanged and their activities were compared. Chimeric TRs containing the domains D, E or (D + E) of TRbeta1 showed increased propensities to form homodimers and mediated higher transactivation activities than TRalpha1. Thus, differential transactivation activities of TR isoforms are mediated by interplay of their domains and could serve as an important regulatory mechanism to achieve diversity and specificity of pleiotropic T3 effect.
Zhu
X G
XG
Laboratory of Molecular Biology, Division of Basic Sciences, NIDDKD, National Institutes of Health, Bethesda, Maryland 20892, USA.
McPhie
P
P
Lin
K H
KH
Cheng
S Y
SY
D001665:Binding Sites; D015153:Blotting, Western; D004268:DNA-Binding Proteins / Q000378:metabolism; D004591:Electrophoresis, Polyacrylamide Gel; D006801:Humans; D007700:Kinetics; D008956:Models, Chemical; D011487:Protein Conformation; D011988:Receptors, Thyroid Hormone / Q000737:chemistry / Q000235:genetics*; D011993:Recombinant Fusion Proteins / Q000737:chemistry; D014158:Transcription, Genetic; D014162:Transfection; D014284:Triiodothyronine / Q000378:metabolism
D004268:DNA-Binding Proteins; D011988:Receptors, Thyroid Hormone; D011993:Recombinant Fusion Proteins; D014284:Triiodothyronine