Thyroid hormone receptors (TRs) are ligand-regulated transcription factors that bind to thyroid hormone response elements (TREs) as monomers and homodimers, and as heterodimers with nuclear proteins such as TR auxiliary proteins and retinoid X receptors. Recently, bacterially expressed human TRβ-1 (hTRβ-1) was shown to be phosphorylated in vitro by HeLa cytosolic extract. However, little is known about the consequences of phosphorylation on the nature of TR complexes. Therefore, we studied the effect of phosphorylation on TR binding to TREs. Bacterially expressed hTRβ-1 was phosphorylated in vitro with ATP by HeLa cytosolic extract. The ratio of phosphoserine to phosphothreonine was approximately 5:1. We then analyzed phosphorylated hTRβ-1 binding to several TREs by electrophoretic mobility shift assay. Phosphorylated hTRβ-1 bound better as a homodimer to the TREs than hTRβ-1 incubated with preheated cytosolic extract. Alkaline phosphatase treatment of the phosphorylated hTRβ-1 eliminated the enhanced homodimer binding to DNA. In contrast, phosphorylation did not affect TR/TR auxiliary protein or TR/retinoid X receptor heterodimer binding to DNA. Triiodothyronine decreased both phosphorylated and unphosphorylated hTRβ-1 homodimer binding to several TREs, and the addition of okadaic acid did not alter this triiodothyronine effect. These results indicate that phosphorylation, in addition to ligand binding, modulates TR dimer binding to TREs. As such, it is possible that phosphorylation may also participate in TR-mediated regulation of transcription.
|Number of pages||5|
|Journal||Journal of Biological Chemistry|
|Publication status||Published - 1994|
ASJC Scopus subject areas
- Molecular Biology
- Cell Biology