Phosphorylation of Human Thyroid Hormone Receptor beta-1 by Casein Kinase II

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Bacterially-expressed human thyroid hormone receptor beta-1 (hTRbeta1) recently has been shown to be phosphorylated in vitro by HeLa cytoso1ic extract. In the present study, we first demonstrated that hTRbeta1 also could be phosphorylated in vitro b purified casein kinase II (CKII). Phosphoamino acid analysis revealed hTrbeta1 was phosphorylated on both serine and threonine residues. In vitro CKII phosphorylation of glutathione S-transferase (GST)-hTRbeta1 fusion proteins whose predicted CKII phosphorylation sites were mutated demonstrated that a threonine residue located in the hinge region(Thr-210) could be phosphorylated by CKII. In order to elucidate the functional significance of CKfl phosphorylation of Thr-210 in hTRbeta1, we next performed transient transfection studies using either wild type or Thr-210 mutated hTRbeta1.In terestingly, the basal repression eve in the absence of ligand was attenuated significantly when Thr-210 mutated hTRbeta1 was used. Since Thr-210 is located within the interacting domain with nudear receptor co-repressor (N-CoR), we next performed electrophoretic mobility shifi assay (EMSA)to examine the interaction between amino terminal-truncated N-CoR (NCoRI) and wild type or Thr-210 mutated hTRbeta1. Interestingly, in contrast to retinoid OMEGA receptor beta(RXRbeta) which equally formed heterodimers with both types of hTR beta1, N CoRIp referentially formed heterodimers with wild type hTRbeta1 than Thr-210mutated hTRbeta1. Taken together, we speculate that CKII phosphorylation ofThr-210 m hTRbeta1 might modulate interaction with N-Co R, and contribute tomediate basal repression in the absence of ligand.