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Division of Genetics, Department of Medicine Brigham and
Women’s Hospital and Harvard Medical School Boston,
Massachusetts 02115
Department of Endocrinology and
Metabolism (T.N.) Research Institute of Environmental
Medicine Nagoya University Nagoya, Japan
Genetisches
Institut (A.B.) Justus-Liebig-Universitat D-35392, Giessen,
Germany
We have employed a chimeric receptor system in which we cotransfected yeast GAL4 DNA-binding domain/retinoid X receptor ß ligand-binding domain chimeric receptor (GAL4RXR), thyroid hormone receptor-ß (TRß), and upstream activating sequence-reporter plasmids into CV-1 cells to study repression, derepression, and transcriptional activation. In the absence of T3, unliganded TR repressed transcription to 20% of basal level, and in the presence of T3, liganded TRß derepressed transcription to basal level. Using this system and a battery of TRß mutants, we found that TRß/RXR heterodimer formation is necessary and sufficient for basal repression and derepression in this system. Additionally, an AF-2 domain mutant (E457A) mediated basal repression but not derepression, suggesting that interaction with a putative coactivator at this site may be critical for derepression. Interestingly, a mutant containing only the TRß ligand binding domain (LBD) not only mediated derepression, but also stimulated transcriptional activation 10-fold higher than basal level. Studies using deletion and domain swap mutants localized an inhibitory region to the TRß DNA-binding domain. Titration studies further suggested that allosteric changes promoting interaction with coactivators may account for enhanced transcriptional activity by LBD. In summary, our findings suggest that TR heterodimer formation with RXR is important for repression and derepression, and coactivator interaction with the AF-2 domain may be needed for derepression in this chimeric system. Additionally, there may be an inhibitory region in the DNA-binding domain, which reduces TR interaction with coactivators, and prevents full-length wild-type TRß from achieving transcriptional activation above basal level in this chimeric receptor system.
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