Transcriptional regulation by the glucocorticoid receptor (GR) is mediated by hormone binding, receptor dimerization, and coactivator recruitment. Here, we report the crystal structure of the human GR ligand binding domain (LBD) bound to dexamethasone and a coactivator motif derived from the transcriptional intermediary factor 2. Despite structural similarity to other steroid receptors, the GR LBD adopts a surprising dimer configuration involving formation of an intermolecular beta sheet. Functional studies demonstrate that the novel dimer interface is important for GR-mediated activation. The structure also reveals an additional charge clamp that determines the binding selectivity of a coactivator and a distinct ligand binding pocket that explains its selectivity for endogenous steroid hormones. These results establish a framework for understanding the roles of protein-hormone and protein-protein interactions in GR signaling pathways.

The androgen receptor (AR) is required for male sex development and contributes to prostate cancer cell survival. In contrast to other nuclear receptors that bind the LXXLL motifs of coactivators, the AR ligand binding domain is preferentially engaged in an interdomain interaction with the AR FXXLF motif. Reported here are crystal structures of the ligand-activated AR ligand binding domain with and without bound FXXLF and LXXLL peptides. Key residues that establish motif binding specificity are identified through comparative structure-function and mutagenesis studies. A mechanism in prostate cancer is suggested by a functional AR mutation at a specificity-determining residue that recovers coactivator LXXLL motif binding. An activation function transition hypothesis is proposed in which an evolutionary decline in LXXLL motif binding parallels expansion and functional dominance of the NH(2)-terminal transactivation domain in the steroid receptor subfamily.

Ligand binding is the first step in hormone regulation of mineralocorticoid receptor (MR) activity. Here, we report multiple crystal structures of MR (NR3C2) bound to both agonist and antagonists. These structures combined with mutagenesis studies reveal that maximal receptor activation involves an intricate ligand-mediated hydrogen bond network with Asn 770 which serves dual roles: stabilization of the loop preceding the C-terminal activation function-2 helix and direct contact with the hormone ligand. In addition, most activating ligands hydrogen bond to Thr 945 on helix 10. Structural characterization of the naturally occurring S810L mutant explains how stabilization of a helix 3/helix 5 interaction can circumvent the requirement for this hydrogen bond network. Taken together, these results explain the potency of MR activation by aldosterone, the weak activation induced by progesterone and the antihypertensive agent spironolactone, and the binding selectivity of cortisol over cortisone.