Post-translational modification of the thyroid hormone receptor interferes with its transcriptional activity
The thyroid hormone receptor (TR) belongs to a family of transcription factors which activity is modulated by the presence or absence of their particular ligand. Additionally, the transcriptional activity of TR is modulated by post-translational modifications such as phosphorylation, acetylation or ubiquitination. Here, I investigate whether TR might also be regulated by sumoylation. Sumoylation of transcription factors at defined lysine residues often leads to an alteration of their transcriptional activity.
Various fusion proteins of the Gal4 DNA binding domain (Gal4) fused to different fragments of TRα were investigated in reporter gene assays. These fusion constructs show a strong activation of reporter gene activity in the mammalian one-hybrid assay upon stimulation with T3 (up to 100-fold induction rates). Several point mutations at suggested sumoylation sites were introduced into the Gal4-TR construct. These mutated constructs show (i) an increased basal reporter activity in the absence of T3 and (ii) reduced fold induction rates upon stimulation with T3. These data indicate that sumoylation defective point mutants of TR have defective repressor function. TR wild type and sumoylation deficient mutants were investigated on natural promoter sequences of both positively and negatively regulated T3 target genes. These investigations confirmed an alteration of ligand-depending activation by some of the mutants. Chromatin immunoprecipitation assays revealed an increased concentration of the transcriptional corepressor NCoR in the presence of TR wild type compared to its sumoylation deficient mutant in the absence of the ligand T3.
All these data suggest that post-translational modifications of TR are essential for appropriate transcriptional silencing by unliganded TR.
Declaration of interest: The authors declare that there is no conflict of interest that could be perceived as prejudicing the impartiality of the research project.
Funding: This research did not receive any specific grant from any funding agency in the public, commercial or not-for-profit sector.