FUNCTIONAL REGULATION OF THYROID-HORMONE RECEPTOR VARIANT TR-ALPHA-2 BY PHOSPHORYLATION

The thyroid hormone (T3) receptor (TR) variant TR alpha 2 is abundant in brain but does not bind T3 because of its unique C terminus. The on ly known function of TR alpha 2, inhibition of TR-dependent transactiv ation, involves competition for T3 response elements. Paradoxically, i n vitro-translated TR alpha 2 bound poorly to these sites, We report h ere that dephosphorylation of TR alpha 2 restored its DNA binding. Mut ation of C-terminal serine residues to alanine (TR alpha 2-SA) was equ ally effective. The C terminus of TR alpha 2 was phosphorylated in a h uman cell line, whereas that of TR alpha 2-SA was not. Conversely, TR alpha 2-SA was a much better inhibitor of T3 action than was wild-type TR alpha 2. The dominant negative activity of TR alpha 2-SA was less than stoichiometric with TR concentration, possibly because it was una ble to heterodimerize with retinoid X receptor, which enhances the bin ding of other TRs. Purified casein kinase II as well as a reticulocyte casein kinase II-like activity phosphorylated TR alpha 2 on serines 4 74 and 475. Mutation of these two residues to alanine was sufficient t o restore DNA binding, Thus, DNA binding by TR alpha 2 is regulated by phosphorylation at a site distant from the DNA-binding domain. The in creased dominant negative activity of a nonphosphorylatable form of TR alpha 2 suggests that phosphorylation may provide a rapid, T3-indepen dent mechanism for cell-specific modulation of the expression of T3-re sponsive genes.