Thyroid hormone response elements differentially modulate the interactions of thyroid hormone receptors with two receptor binding domains in the steroid receptor coactivator-1

Ligand-dependent transcriptional activation by nuclear receptors is mediated by interactions with coactivators. Recently, a consensus interaction motif (LXXLL) has been identified in a number of coactivators such as steroid receptor coactivator-l (SRC-1). SRC-1 contains three such motifs in the central (nuclear receptor binding domain-1, NBD-1) and a single one in the C-terminal (NBD-S) regions. To define the nature and role of the two NBDs in SRC-1, interaction studies between the two NBDs and thyroid hormone receptor (TR) were performed. Although NBD-1 and NBD-S showed similar ligand- and AF-a dependent interactions with TR, in solution, these two NBDs possessed distinct interaction properties with TR when TR is bound to a thyroid hormone-response element (TRE). Both in vitro and in vivo interaction studies demonstrate that NBD-1, but not NBD-2, exhibits ligand-dependent interaction with TR in the presence of TREs. In addition, a natural isoform of SRC-1, SRC-1E, which lacks NBD-2, preserved TR as well as progesterone receptor-mediated coactivator function on reporter gene expression. Finally, we found that NBD-1 failed to interact with a TR and retinoid X receptor heterodimer complex on a transcriptionally inactive direct repeat +4 TRE in electrophoretic mobility shift assays. These observations indicate that DNA-induced, as well as ligand-induced, conformational change(s) of TR may influence the nature of its binding to SRC-1, and that the two NBDs of SRC-I may play different roles to regulate ligand-dependent transactivation of TRs.