CoRNR Box Research Articles

Structural proteomics defines a sequential priming mechanism for the progesterone receptor.

The progesterone receptor (PR) is a steroid-responsive nuclear receptor with two isoforms: PR-A and PR-B. Disruption of PR-A:PR-B signaling is associated with breast cancer through interactions with oncogenic co-regulatory proteins (CoRs). However, molecular details of isoform-specific PR-CoR interactions remain poorly understood. Using structural mass spectrometry, we investigate the sequential binding mechanism of purified full-length PR and intact CoRs, steroid receptor coactivator 3 (SRC3) and p300, as complexes on target DNA. Our findings reveal selective CoR NR-box binding by PR and unique interaction surfaces between PR and CoRs during complex assembly, providing a structural basis for CoR sequential binding on PR. Antagonist-bound PR showed persistent CoR interactions, challenging the classical model of nuclear receptor activation and repression. Collectively, we offer a peptide-level perspective on the organization of the PR transcriptional complex and infer the mechanisms behind the interactions of these proteins, both in active and inactive conformations.

Characterization of the Free State Ensemble of the CoRNR Box Motif by Molecular Dynamics Simulations

Intrinsically disordered proteins (IDPs) and regions are highly prevalent in eukaryotic proteomes, and like folded proteins, they perform essential biological functions. Interaction sites in folded proteins are generally formed by tertiary structures, whereas IDPs use short segments called linear motifs (LMs). Despite their short length and lack of stable structure, LMs may have considerable structural propensities, which often resemble bound-state conformations with targets. Structural data is crucial for understanding the molecular basis of protein interactions and development of targeted pharmaceuticals, but IDPs present considerable challenges to experimental techniques. As a result, IDPs are largely underrepresented in the Protein Data Bank. In the face of experimental challenges, molecular dynamics (MD) simulations have proven to be a useful tool for structural characterization of IDPs. Here, the free state ensemble of the nuclear receptor corepressor 1 (NCOR1) CoRNR box 3 motif, which is important for binding to nuclear receptors to control gene expression, is studied using MD simulations of a total of 8 μs. Transitions between disordered and α-helical conformations resembling a bound-state structure were observed throughout the trajectory, indicating that the motif may have a natural conformational bias toward bound-state structures. The data shows that the disordered and folded populations are separated by a low energy (4-6 kJ/mol) barrier, and the presence of off-pathway intermediates, leading to a C-terminally folded species that cannot efficiently transition into a completely folded conformation. Structural transitions and folding pathways within the free state ensemble were well-described by principal component analysis (PCA) of the peptide backbone dihedral angles, with the analysis providing insight for increasing structural homogeneity of the ensemble.

The adenovirus 55 residue E1A protein is a transcriptional activator and binds the unliganded thyroid hormone receptor

The early region 1A (E1A) of human adenovirus types 2 and 5 is differentially spliced to yield five distinct mRNAs that encode different proteins. The smallest E1A RNA transcript encodes a 55 residue (55R) protein that shares only 28 amino acid residues with the other E1A proteins. Even though it is the most abundant E1A transcript at late times post-infection, little is known about the functions of this E1A isoform. In this study, we show that the E1A 55R protein interacts with, and modulates the activity of the unliganded thyroid hormone receptor (TR). We demonstrate that E1A 55R contains a signature motif known as the CoRNR box that confers interaction with the unliganded TR; this motif was originally identified in cellular corepressors. Using a system reconstituted in the yeast Saccharomyces cerevisiae, which lack endogenous TR and TR coregulators, we show that E1A 55R nonetheless differs from cellular corepressors as it functions as a strong co-activator of TR-dependent transcription and that it possesses an intrinsic transcriptional activation domain. These data indicate that the E1A 55R protein functions as a transcriptional regulator.

Negative regulation of ERα by a novel protein CAC1 through association with histone demethylase LSD1

ERα, a critical transcriptional factor for breast cancer proliferation, is regulated by a complex binding repertoire that includes coactivators and corepressors. Here, we identified a novel class of ERα coregulator called CAC1. The CoRNR box of CAC1 was required for the binding to and inactivation of ERα. CAC1 also associated with histone demethylase LSD1 and suppressed LSD1-enhanced ERα activity. CAC1 impaired recruitment of ERα and LSD1 to the ERα-responsive promoter, leading to greater H3K9me3 accumulation. This effect was reversed by CAC1 depletion. Finally, CAC1 increased paclitaxel-induced cell death in ERα-positive MCF7 cells, which are paclitaxel-resistant. Overall, our study provides the first evidence that CAC1, associated with LSD1, functions as an ERα corepressor, implicating a potential antitumor target in ERα-positive breast cancer. Structured summary of protein interactionsER-alphaphysically interacts with CAC1 by anti tag coimmunoprecipitation (View Interaction: 1, 2, 3)LSD1physically interacts with CAC1 by anti tag coimmunoprecipitation (View interaction)CAC1binds to ER-alpha by pull down (View interaction)CAC1 and ER-alphacolocalizebyfluorescence microscopy (View interaction)

CAC1 negatively regulates RARα activity through cooperation with HDAC

Retinoic acid (RA) plays pleiotropic roles in cellular differentiation and animal development. RA responses are mediated by transcriptional activation by the retinoic acid receptor (RAR) and retinoid X receptor (RXR) in cooperation with various types of coregulators at RA-responsive gene promoters. Here, we identified CDK2-associated cullin (CAC1) as a novel type of RARα coregulator that interacts with RARα and inhibits its transcriptional activity. The CoRNR box of CAC1 is required for the binding to and inactivation of RARα. In addition, CAC1 cooperates with histone deacetylases (HDACs) to suppress RARα, probably by associating with HDAC. Finally, depletion of CAC1 increases RA-induced neuronal differentiation of P19 cells, a response accompanied by significant upregulation of the neuronal marker nestin. From these results, we suggest that CAC1 is a novel corepressor of RARα that cooperates with HDACs and is involved in the regulation of RA-induced cellular differentiation.

Stearoyl CoA desaturase (SCD) facilitates proliferation of prostate cancer cells through enhancement of androgen receptor transactivation.

Stearoyl-CoA desaturase (SCD), the rate-limiting enzyme in the biosynthesis of monounsaturated fatty acids, is highly expressed in prostate cancer although the SCD protein has been known to be rapidly turned over by proteolytic cleavage. The present data demonstrate that SCD can promote proliferation of androgen receptor (AR)-positive LNCaP prostate cancer cells and enhance dihydrotestosterone (DHT)-induced AR transcriptional activity, resulting in increased expression of prostate-specific antigen (PSA) and kallikrein-related peptidase 2 (KLK2). Interestingly, among the previously reported SCD-derived peptides produced by proteolytic cleavage of SCD, a peptide spanning amino acids 130-162 of SCD (SCD-CoRNR) contained the CoRNR box motif (LFLII) and enhanced AR transcriptional activity. In contrast, a mutant SCD-CoRNR in which Leu136 was replaced by Ala had no effect on AR transcriptional activity. Moreover, SCD-CoRNR directly interacted with AR and inhibited RIP140 suppression of AR transactivation. Knockdown of the SCD gene by SCD microRNA suppressed AR transactivation with decreased cell proliferation, suggesting that SCD may regulate the proliferation of LNCaP cells via modulation of AR transcriptional activity. Moreover, ectopic expression of SCD in LNCaP cells facilitated LNCaP tumor formation and growth in nude mice. Together, the data indicate that SCD plays a key role in the regulation of AR transcriptional activity in prostate cancer cells.

A Novel Cytoplasmic Adaptor for Retinoic Acid Receptor (RAR) and Thyroid Receptor Functions as a Derepressor of RAR in the Absence of Retinoic Acid*

In most mammalian cells, the retinoic acid receptor (RAR) is nuclear rather than cytoplasmic, regardless of its cognate ligand, retinoic acid (RA). In testis Sertoli cells, however, RAR is retained in the cytoplasm and moves to the nucleus only when RA is supplied. This led us to identify a protein that regulates the translocation of RAR. From yeast two-hybrid screening, we identified a novel RAR-interacting protein called CART1 (cytoplasmic adaptor for RAR and TR). Systematic interaction assays using deletion mutants showed that the C-terminal CoRNR box of CART1 was responsible for the interaction with the NCoR binding region of RAR and TR. Such interaction was impaired in the presence of ligand RA, as further determined by GST pulldown assays in vitro and immunoprecipitation assays in vivo. Fluorescence microscopy showed that unliganded RAR was captured by CART1 in the cytoplasm, whereas liganded RAR was liberated and moved to the nucleus. Overexpression of CART1 blocked the transcriptional repressing activity of unliganded apoRAR, mediated by corepressor NCoR in the nucleus. CART1 siRNA treatment in a mouse Sertoli cell line, TM4, allowed RAR to move to the nucleus and blocked the derepressing function of CART1, suggesting that CART1 might be a cytoplasmic, testis-specific derepressor of RAR.

LIM protein Ajuba functions as a nuclear receptor corepressor and negatively regulates retinoic acid signaling

Corepressors play an essential role in nuclear receptor-mediated transcriptional repression. In general, corepressors directly bind to nuclear receptors via CoRNR boxes (L/I-X-X-I/V-I) in the absence of ligand and appear to act as scaffolds to further recruit chromatin remodeling complexes to specific target genes. Here, we describe the identification of the multiple LIM domain protein Ajuba as a unique corepressor for a subset of nuclear hormone receptors. Ajuba contains functional nuclear-receptor interacting motifs and selectively interacts with retinoic acid receptors (RARs) and rexinoid receptor (RXRs) subtypes in a ligand-dependent manner. Simultaneous mutation of these motifs abolishes RAR binding and concomitantly leads to loss of repression on RARE reporter genes. P19 cells depleted of Ajuba are highly sensitized to all-trans retinoic acid (atRA)-induced transcription and differentiation. In the absence of atRA, Ajuba can be readily found at the RARE control elements of RAR endogenous target genes. Stimulation of cells with atRA results in the dissociation of Ajuba from these regions. Moreover, we observed that coexpression of the known Ajuba binding partner Prmt5 (protein arginine methyltransferase-5) inhibited the Ajuba/RAR interaction. The high-affinity Ajuba-RAR/RXR interaction site overlaps the region responsible for Ajuba/Prmt5 binding, and thus binding appears to be mutually exclusive, providing a potential mechanism for these observations. Identification of Ajuba as a unique corepressor for nuclear receptors sheds new light on mechanisms for nuclear receptor-mediated repression and provides a unique target for developing more effective therapeutics to modulate this important pathway.

Development of Androgen Receptor Antagonists with Promising Activity in Castration-Resistant Prostate Cancer

Androgen receptor (AR) continues to play a central role in prostate cancers that relapse after androgen deprivation therapy, but these tumors are refractory to available AR antagonists. In a recent issue of Science, Tran et al. describe an antagonist that prevents AR recruitment to chromatin and shows efficacy in relapsed prostate cancer.

Structural basis for nuclear receptor corepressor recruitment by antagonist-liganded androgen receptor

Androgen receptor (AR) recruitment of transcriptional corepressors NCoR and SMRT can be enhanced by antagonists such as mifepristone. This study shows that enhanced NCoR binding to the mifepristone-liganded AR is mediated by the NCoR COOH-terminal N1 CoRNR box and that this selectivity is due to charged residues unique to the COOH-terminal CoRNR boxes of NCoR and SMRT. Significantly, these residues are on a helical face adjacent to oppositely charged residues in helix 4 of the AR ligand-binding domain. Mutagenesis of these AR residues in helix 4, as well as mutation of lysine 720 in helix 3 (predicted to interact with the CoRNR box), markedly impaired AR recruitment of NCoR, indicating that N1 CoRNR box binding is being stabilized by these ionic interactions in the AR ligand-binding domain coactivator/corepressor binding site. Finally, results using a helix 12-deleted AR indicate that mifepristone induces allosteric changes in addition to helix 12 displacement that are critical for NCoR binding. These findings show that AR antagonists can enhance corepressor recruitment by stabilizing a distinct antagonist conformation of the AR coactivator/corepressor binding site and support the development of additional antagonists that may be able to further enhance AR recruitment of corepressors.

Aberrant Association of Promyelocytic Leukemia Protein-Retinoic Acid Receptor-α with Coactivators Contributes to Its Ability to Regulate Gene Expression

The aberrant association of promyelocytic leukemia protein-retinoic acid receptor-alpha (PML-RARalpha) with corepressor complexes is generally thought to contribute to the ability of PML-RARalpha to regulate transcription. We report here that PML-RARalpha acquires aberrant association with coactivators. We show that endogenous PML-RARalpha interacts with the histone acetyltransferases CBP, p300, and SRC-1 in a hormoneindependent manner, an association not seen for RARalpha. This hormone-independent coactivator binding activity requires an intact ligand-binding domain and the NR box of the coactivators. Confocal microscopy studies demonstrate that exogenous PML-RARalpha sequesters and colocalizes with coactivators. These observations correlate with the ability of PML-RARalpha to attenuate the transcription activation of the Notch signaling downstream effector, CBF1, and of the glucocorticoid receptor. This includes attenuation of the glucocorticoid-induced leucine zipper (GILZ) and FLJ25390 target genes of the endogenous glucocorticoid receptor. Furthermore, treatment of NB4 cells with all-trans-retinoic acid, which promotes PML-RARalpha degradation, resulted in increased activation of GILZ. On the basis of these findings, we propose a model in which the hormone-independent association between PML-RARalpha and coactivators contributes to its ability to regulate gene expression.

Xenopus embryos lacking specific isoforms of the corepressor SMRT develop abnormal heads

The corepressor SMRT acts on a range of transcription factors, including the retinoid and thyroid hormone nuclear receptors. The carboxy-terminal region of SMRT contains CoRNR box motifs that mediate these interactions. We have shown, in Xenopus, that SMRT can exist as isoforms containing either two or three CoRNR boxes depending on the alternative splicing of exon 37b. The number of SMRT transcript isoforms expressed increases during development until all sixteen possible isoforms are identified in the swimming tadpole. To eliminate specific SMRT isoforms, we have developed a process that uses an antisense morpholino oligonucleotide in Xenopus to dictate the outcome of alternative splicing at a defined exon and used this to inhibit the formation of transcripts containing exon 37b. These embryos are therefore limited to the expression of SMRT isoforms that contain two rather than three CoRNR boxes. Analysis of responsive genes in these embryos shows that targets of thyroid hormone, but not retinoid signaling are affected by the elimination of exon 37b. Morpholino-injected embryos have swimming abnormalities and develop altered head morphology, an expanded olfactory epithelium and disorganized peripheral axons. These experiments indicate a critical role for the alternative splicing of SMRT in development.

Corepressor/coactivator paradox: potential constitutive coactivation by corepressor splice variants.

The functional consequences of the interaction of transcriptional coregulators with the human thyroid hormone receptor (TR) in mammalian cells are complex. We have used the yeast, Saccharomyces cerevisiae, which lack endogenous nuclear receptors (NRs) and NR coregulators, as a model to decipher mechanisms regulating transcriptional activation by TR. In effect, this system allows the reconstitution of TR mediated transcription complexes by the expression of specific combinations of mammalian proteins in yeast. In this yeast system, human adenovirus 5 early region 1A (E1A), a natural N-CoR splice variant (N-CoRI) or an artificial N-CoR truncation (N-CoRC) coactivate unliganded TRs and these effects are inhibited by thyroid hormone (TH). E1A contains a short peptide sequence that resembles known corepressor-NR interaction motifs (CoRNR box motif, CBM), and this motif is required for TR binding and coactivation. N-CoRI and N-CoRC contain three CBMs, but only the C-terminal CBM1 is critical for coactivation. These observations in a yeast model system suggest that E1A and N-CoRI are naturally occurring TR coactivators that bind in the typical corepressor mode. These findings also raise the possibility that alternative splicing events which form corepressor proteins containing only C-terminal CBM motifs could represent a novel mechanism in mammalian cells for regulating constitutive transcriptional activation by TRs.

SMRT has tissue-specific isoform profiles that include a form containing one CoRNR box

SMRT acts as a corepressor for a range of transcription factors. The amino-terminal part of the protein includes domains that mainly mediate transcriptional repression whilst the carboxy-terminal part includes domains that interact with nuclear receptors using up to three motifs called CoRNR boxes. The region of the SMRT primary transcript encoding the interaction domains is subject to alternative splicing that varies the inclusion of the third CoRNR box. The profile in mice includes an abundant, novel SMRT isoform that possesses just one CoRNR box. Mouse tissues therefore express SMRT isoforms containing one, two or three CoRNR boxes. In frogs, the SMRT isoform profile is tissue-specific. The mouse also shows distinct profiles generated by differential expression levels of the SMRT transcript isoforms. The formation of multiple SMRT isoforms and their tissue-specific regulation indicates a mechanism, whereby cells can define the repertoire of transcription factors regulated by SMRT.

Structural Basis for an Unexpected Mode of SERM-Mediated ER Antagonism

Tamoxifen is effective for the prevention and treatment of estrogen-dependent breast cancers, but is associated with an increased incidence of endometrial tumors. We report the crystal structure of the estrogen receptor alpha (ERalpha) ligand binding domain (LBD) bound to the structurally similar compound GW5638, which has therapeutic potential and does not stimulate the uterus. Like tamoxifen, GW5638 relocates the carboxy-terminal helix (H12) to the known coactivator-docking site in the ERalpha LBD. However, GW5638 repositions residues in H12 through specific contacts with the N terminus of this helix. In contrast to tamoxifen, the resulting increase in exposed hydrophobic surface of ERalpha LBD correlates with a significant destabilization of ERalpha in MCF-7 cells. Thus, the GW5638-ERalpha LBD structure reveals an unexpected mode of SERM-mediated ER antagonism, in which the stability of ERalpha is decreased through an altered position of H12. This dual mechanism of antagonism may explain why GW5638 can inhibit tamoxifen-resistant breast tumors.

E1A and a nuclear receptor corepressor splice variant (N-CoR I ) are thyroid hormone receptor coactivators that bind in the corepressor mode

Unliganded thyroid hormone (TH) receptors (TRs) and other nuclear receptors (NRs) repress transcription of hormone-activated genes by recruiting corepressors (CoRs), such as NR CoR (N-CoR) and SMRT. Unliganded TRs also activate transcription of TH-repressed genes. Some evidence suggests that these effects also involve TR/CoR contacts; however, the precise reasons that CoRs activate transcription in these contexts are obscure. Unraveling these mechanisms is complicated by the fact that it is difficult to decipher direct vs. indirect effects of TR-coregulator contacts in mammalian cells. In this study, we used yeast, Saccharomyces cerevisiae, which lack endogenous NRs and NR coregulators, to determine how unliganded TRs can activate transcription. We previously showed that adenovirus 5 early-region 1A coactivates unliganded TRs in yeast, and that these effects are blocked by TH. We show here that human adenovirus type 5 early region 1A (E1A) contains a short peptide (LDQLIEEVL amino acids 20-28) that resembles CoR-NR interaction motifs (CoRNR boxes), and that this motif is required for TR binding and coactivation. Although full-length N-CoR does not coactivate TR in yeast, a naturally occurring N-CoR variant (N-CoR(I)) and an artificial N-CoR truncation (N-CoR(C)) that retain CoRNR boxes but lack N-terminal repressor domains behave as potent and direct TH-repressed coactivators for unliganded TRs. We conclude that E1A and N-CoR(I) are naturally occurring TR coactivators that bind in the typical CoR mode and suggest that similar factors could mediate transcriptional activation by unliganded TRs in mammals.

Alternative mRNA Splicing of SMRT Creates Functional Diversity by Generating Corepressor Isoforms with Different Affinities for Different Nuclear Receptors

Many eukaryotic transcription factors are bimodal in their regulatory properties and can both repress and activate expression of their target genes. These divergent transcriptional properties are conferred through recruitment of auxiliary proteins, denoted coactivators and corepressors. Repression plays a particularly critical role in the functions of the nuclear receptors, a large family of ligand-regulated transcription factors involved in metazoan development, differentiation, reproduction, and homeostasis. The SMRT corepressor interacts directly with nuclear receptors and serves, in turn, as a platform for the assembly of a larger corepressor complex. We report here that SMRT is expressed in cells by alternative mRNA splicing to yield two distinct variants or isoforms. We designate these isoforms SMRTalpha and SMRTtau and demonstrate that these isoforms have significantly different affinities for different nuclear receptors. These isoforms are evolutionarily conserved and are expressed in a tissue-specific manner. Our results suggest that differential mRNA splicing serves to customize corepressor function in different cells, allowing the transcriptional properties of nuclear receptors to be adapted to different contexts.

The Androgen Receptor Recruits Nuclear Receptor CoRepressor (N-CoR) in the Presence of Mifepristone via Its N and C Termini Revealing a Novel Molecular Mechanism for Androgen Receptor Antagonists

The androgen receptor (AR) activates target gene expression in the presence of agonist ligands via the recruitment of transcriptional coactivators, but recent work shows that overexpression of the nuclear corepressors NCoR and SMRT attenuates this agonist-mediated AR activation. Here we demonstrate using NCoR siRNA and chromatin immunoprecipitation that endogenous NCoR is recruited to and represses the dihydrotestosterone (DHT)-liganded AR. Furthermore this study shows that NCoR and coactivators compete for AR in the presence of DHT. AR antagonists such as bicalutamide that are currently in use for prostate cancer treatment can also mediate NCoR recruitment, but mifepristone (RU486) at nanomolar concentrations is unique in its ability to markedly enhance the AR-NCoR interaction. The RU486-liganded AR interacted with a C-terminal fragment of NCoR, and this interaction was mediated by the two most C-terminal nuclear receptor interacting domains (RIDs) present in NCoR. Significantly, in addition to the AR ligand binding domain, the AR N terminus was also required for this interaction. Mutagenesis studies demonstrate that the N-terminal surface of the AR-mediating NCoR recruitment was distinct from tau5 and from the FXXLF motif that mediates agonist-induced N-C-terminal interaction. Taken together these data demonstrate that NCoR is a physiological regulator of the AR and reveal a new mechanism for AR antagonism that may be exploited for the development of more potent AR antagonists.

Alternative splicing generates multiple SMRT transcripts encoding conserved repressor domains linked to variable transcription factor interaction domains.

Silencing mediator for retinoid and thyroid hormone receptor (SMRT) and nuclear receptor corepressor protein (NCoR) are corepressors that interact with a range of transcription factors. They both consist of N-terminal repressor domains that associate with histone deacetylases and C-terminal interaction domains (IDs) that contain CoRNR box motifs. These motifs mediate the interaction between corepressors and nuclear receptors (NRs), such as the retinoid and thyroid hormone receptors. However, whilst NCoR produces a single transcript during Xenopus development, xSMRT is subject to alternative splicing at four sites in the 3' part of the transcript, the region encoding the C-terminal IDs. Although this provides the potential to produce 16 different transcripts, only five isoforms are found in early embryos. The sites of alternative splicing predict that the resultant isoforms will differ in their ability to interact with NRs, as one site varies the number of CoRNR boxes, the second site changes the sequence flanking CoRNR box-1 and the other sites delete amino acid residues between CoRNR boxes 1 and 2 and so alter the critical spacing between these motifs. SMRT and NCoR therefore represent paralogues in which one form, SMRT, has evolved the ability to generate multiple isoforms whereas the other, NCoR, is invariant in Xenopus development.

Equilibrium interactions of corepressors and coactivators with agonist and antagonist complexes of glucocorticoid receptors.

Corepressors and coactivators can modulate the dose-response curve and partial agonist activity of glucocorticoid receptors (GRs) complexed with agonist and antagonist steroids, respectively, in intact cells. We recently reported that GR-antagonist complexes bind to the coactivator TIF2, (transcriptional intermediary factor 2), which is consistent with the whole-cell effects of coactivators being mediated by direct interactions with GR complexes. We now ask whether the whole-cell modulatory activity of corepressors also entails binding to both GR-agonist and -antagonist complexes and whether the association of corepressors and coactivators with GR complexes involves competitive equilibrium reactions. In mammalian two-hybrid assays with two different cell lines and in cell-free pull-down and whole-cell immunoprecipitation assays, the corepressors NCoR (nuclear receptor corepressor) and SMRT (silencing mediator of retinoid and thyroid hormone receptor) associate with agonist and antagonist complexes of GRs. Both N- and C-terminal regions of GR are needed for corepressor binding, which requires the CoRNR box motifs that mediate corepressor binding to other nuclear/steroid receptors. Importantly, whole-cell GR interactions with corepressors are competitively inhibited by excess coactivator and vice versa. However, the regions of the coactivator TIF2 that compete for GR binding to corepressor and coactivator are not the same, implying a molecular difference in GR association with coactivators and corepressors. Finally, when the whole-cell ratio of coactivators to corepressors is altered by selective cofactor binding to exogenous thyroid receptor beta +/- thyroid hormone, the GR dose-response-curve and partial agonist activity are appropriately modified. Such modifications are independent of histone acetylation. We conclude that mutually antagonistic equilibrium interactions of corepressors and coactivators modulate the dose-response curve and partial agonist activity of GR complexes in a manner that is responsive to the intracellular ratio of these two classes of cofactors. This modulation provides an attractive mechanism for differential control of gene expression during development, differentiation, homeostasis, and endocrine therapies.