The D domain of the thyroid hormone receptor alpha 1 specifies positive and negative transcriptional regulation functions

Four structural domains are characteristic of the members of the thyroid/steroid receptor superfamily. Of these, the A/B and D domains are the least conserved. We have investigated the role of two clusters of positively charged amino acids within the D domain of the thyroid hormone receptor alpha 1 (TR alpha 1). The sequences Lys134-Arg-Lys and Arg188-Arg-Lys, individually or together, were substituted to the neutral residues TIT in three mutants named alpha 1-1, alpha 1-2, and alpha 1-3, respectively. Subcellular localization of transiently transfected wild-type and mutated TRs was monitored by immunostaining, using a TR alpha 1-specific antibody. The wild-type and the alpha 1-2 TRs were detected exclusively in the nucleus, in the presence or absence of thyroid hormone. In contrast, the alpha 1-1 and alpha 1-3 mutants accumulated in both cytoplasm and nucleus, underscoring the importance of the Lys134-Arg-Lys residues for correct nuclear targeting. More importantly, although the mutants had unimpaired DNA- and hormone-binding activities, all three had lost positive and negative transcriptional regulatory functions. Thus, transactivation and repression functions can be entirely dissociated from the other properties of the receptor. In addition, substitution of either one of the positively charged amino acid clusters was sufficient to convert the native TR alpha 1 into a dominant, thyroid hormone-dependent receptor antagonist. These observations, which underline the functional relevance of the D domain for TR alpha 1 function, may also have implications for the autosomal dominant syndrome of generalized resistance to thyroid hormone.