Thyroid hormone receptor functions as monomeric ligand-induced transcription factor on octameric half-sites. Consequences also for dimerization.
The thyroid hormone (3,5,3'-triiodothyronine) receptor (T3R) belongs to the nuclear receptor superfamily of ligand-inducible transcription factors. T3Rs are known to bind as homodimers and heterodimers with retinoid X receptors (RXRs) to two hexameric half-sites in directly repeated, palindromic, and inverted palindromic orientations. The binding of T3R monomers to individual half-sites was often reported, but no clear ligand-induced transactivational activity has been shown. Here, we analyzed interactions of T3R monomers with individual half-sites of the sequence NNAGGTCA. We found that the two nucleotides 5' of the AGGTCA core half-site strongly influence T3R binding and transcriptional activity: octameric half-sites of the consensus sequence (T/C)(A/G)AGGTCA were bound by T3Rs with the highest affinity. This suggests T3R functioning also as a monomeric transcription factor like the orphan nuclear receptors NGFI-B and FTZ-F1. Moreover, we observed that the function of T3R-RXR heterodimers on response elements composed of two half-sites in a directly repeated orientation spaced by 4 nucleotides is determined in major parts by the 5'-flanking sequence of the upstream half-site. Consequently, we noted that the affinity of T3R homodimers is influenced by both 5'-flanking sequences. Our findings suggest that the binding of dimerizing receptors like T3R and other nuclear receptors to their cognate response elements is determined not only by the half-site core sequence, orientation, and number of spacing nucleotides, but also by the nucleotide sequence preceding the half-sites.
Schräder
M
M
Clinique de Dermatologie, Hôpital Cantonal Universitaire, Genève, Switzerland.
Becker-André
M
M
Carlberg
C
C
D000818:Animals; D001483:Base Sequence; D001665:Binding Sites; D002460:Cell Line; D015500:Chloramphenicol O-Acetyltransferase / Q000096:biosynthesis / Q000378:metabolism; D004247:DNA / Q000378:metabolism*; D004330:Drosophila; D007700:Kinetics; D008024:Ligands; D046911:Macromolecular Substances; D008969:Molecular Sequence Data; D009838:Oligodeoxyribonucleotides; D014176:Protein Biosynthesis; D018160:Receptors, Cytoplasmic and Nuclear / Q000096:biosynthesis / Q000302:isolation & purification / Q000378:metabolism*; D018168:Receptors, Retinoic Acid; D011988:Receptors, Thyroid Hormone / Q000096:biosynthesis / Q000302:isolation & purification / Q000378:metabolism*; D047488:Retinoid X Receptors; D014157:Transcription Factors / Q000096:biosynthesis / Q000302:isolation & purification / Q000378:metabolism*; D014162:Transfection; D014284:Triiodothyronine / Q000494:pharmacology*
D008024:Ligands; D046911:Macromolecular Substances; D009838:Oligodeoxyribonucleotides; D018160:Receptors, Cytoplasmic and Nuclear; D018168:Receptors, Retinoic Acid; D011988:Receptors, Thyroid Hormone; D047488:Retinoid X Receptors; D014157:Transcription Factors; D014284:Triiodothyronine; D004247:DNA; D015500:Chloramphenicol O-Acetyltransferase