Ligands specify coactivator nuclear receptor (NR) box affinity for estrogen receptor subtypes

Nuclear receptors (NRs) require coactivators to efficiently activate transc ription of their target genes. Many coactivators including the p160 protein s utilize a short NR box motif to recognize the ligand-binding domain of th e NR when it is activated by ligand. To investigate the ability of various ligands to specify the affinity of NR boxes for a ligand-bound NR, we compa red the capacity of p160 NR boxes to be recruited to estrogen receptor (ER alpha) and ER beta in the presence of 17 beta -estradiol, diethylstilbestro l, and genestein. A time-resolved fluorescence-based binding assay was used to determine the dissociation constants for the 10 NR boxes derived from t he three p160 coactivators for both ER subtypes in the presence of the each of the agonists. While the affinity of some NR boxes for ER was independen t of the agonist, we identified several NR boxes that had significantly dif ferent affinities for ER depending on which agonist was bound to the recept or. Therefore, an agonist may specify the affinity of an NR for various NR boxes and thus regulate the coactivator selectivity of the receptor.