Genetic dissection of the signaling domain of a mammalian steroid receptor in yeast

Saccharomyces cerevisiae as a tool for deciphering Hsp90 molecular chaperone function

Yeast is a valuable model organism for their ease of genetic manipulation, rapid growth rate, and relative similarity to higher eukaryotes. Historically, Saccharomyces cerevisiae has played a major role in discovering the function of complex proteins and pathways that are important for human health and disease. Heat shock protein 90 (Hsp90) is a molecular chaperone responsible for the stabilization and activation of hundreds of integral members of the cellular signaling network. Much important structural and functional work, including many seminal discoveries in Hsp90 biology are the direct result of work carried out in S. cerevisiae. Here, we have provided a brief overview of the S. cerevisiae model system and described how this eukaryotic model organism has been successfully applied to the study of Hsp90 chaperone function.

Activation of autophagy depends on Atg1/Ulk1-mediated phosphorylation and inhibition of the Hsp90 chaperone machinery

Cellular homeostasis relies on both the chaperoning of proteins and the intracellular degradation system that delivers cytoplasmic constituents to the lysosome, a process known as autophagy. The crosstalk between these processes and their underlying regulatory mechanisms is poorly understood. Here, we show that the molecular chaperone heat shock protein 90 (Hsp90) forms a complex with the autophagy-initiating kinase Atg1 (yeast)/Ulk1 (mammalian), which suppresses its kinase activity. Conversely, environmental cues lead to Atg1/Ulk1-mediated phosphorylation of a conserved serine in the amino domain of Hsp90, inhibiting its ATPase activity and altering the chaperone dynamics. These events impact a conformotypic peptide adjacent to the activation and catalytic loop of Atg1/Ulk1. Finally, Atg1/Ulk1-mediated phosphorylation of Hsp90 leads to dissociation of the Hsp90:Atg1/Ulk1 complex and activation of Atg1/Ulk1, which is essential for initiation of autophagy. Our work indicates a reciprocal regulatory mechanism between the chaperone Hsp90 and the autophagy kinase Atg1/Ulk1 and consequent maintenance of cellular proteostasis.

Steroid hormone signaling: What we can learn from insect models

Ecdysteroids are a group of steroid hormones in arthropods with pleiotropic functions throughout their life history. Ecdysteroid research in insects has made a significant contribution to our current understanding of steroid hormone signaling in metazoans, but how far can we extrapolate our findings in insects to other systems, such as mammals? In this chapter, we compare steroid hormone signaling in insects and mammals from multiple perspectives and discuss similarities and differences between the two lineages. We also highlight a few understudied areas and remaining questions of steroid hormone biology in metazoans and propose potential future research directions.

A specialized Hsp90 co-chaperone network regulates steroid hormone receptor response to ligand

Heat shock protein-90 (Hsp90) chaperone machinery is involved in the stability and activity of its client proteins. The chaperone function of Hsp90 is regulated by co-chaperones and post-translational modifications. Although structural evidence exists for Hsp90 interaction with clients, our understanding of the impact of Hsp90 chaperone function toward client activity in cells remains elusive. Here, we dissect the impact of recently identified higher eukaryotic co-chaperones, FNIP1/2 (FNIPs) and Tsc1, toward Hsp90 client activity. Our data show that Tsc1 and FNIP2 form mutually exclusive complexes with FNIP1, and that unlike Tsc1, FNIP1/2 interact with the catalytic residue of Hsp90. Functionally, these co-chaperone complexes increase the affinity of the steroid hormone receptors glucocorticoid receptor and estrogen receptor to their ligands in vivo. We provide a model for the responsiveness of the steroid hormone receptor activation upon ligand binding as a consequence of their association with specific Hsp90:co-chaperone subpopulations.

Dynamic allosteric communication pathway directing differential activation of the glucocorticoid receptor

Allosteric communication within proteins is a hallmark of biochemical signaling, but the dynamic transmission pathways remain poorly characterized. We combined NMR spectroscopy and surface plasmon resonance to reveal these pathways and quantify their energetics in the glucocorticoid receptor, a transcriptional regulator controlling development, metabolism, and immune response. Our results delineate a dynamic communication network of residues linking the ligand-binding pocket to the activation function-2 interface, where helix 12, a switch for transcriptional activation, exhibits ligand- and coregulator-dependent dynamics coupled to graded activation. The allosteric free energy responds to variations in ligand structure: subtle changes gradually tune allostery while preserving the transmission pathway, whereas substitution of the entire pharmacophore leads to divergent allosteric control by apparently rewiring the communication network. Our results provide key insights that should aid in the design of mechanistically differentiated ligands.

Construction of reporter gene assays using CWP and PDR mutant yeasts for enhanced detection of various sex steroids

Background

Sex steroid hormone receptors are classified into three classes of receptors: estrogen receptors (ER) α and β, androgen receptor (AR), and progesterone receptor (PR). They belong to the nuclear receptor superfamily and activate their downstream genes in a ligand-dependent manner. Since sex steroid hormones are involved in a wide variety of physiological processes and cancer development, synthetic chemical substances that exhibit sex steroid hormone activities have been applied as pharmaceuticals and consumed in large amounts worldwide. They are potentially hazardous contaminants as endocrine disruptors in the environment because they may induce inappropriate gene expression mediated by sex steroid hormone receptors in vivo.

Results

To develop simple reporter gene assays with enhanced sensitivity for the detection of sex steroid hormones, we newly established mutant yeast strains lacking the CWP and PDR genes encoding cell wall mannoproteins and plasma membrane drug efflux pumps, respectively, and expressing human ERα, ERβ, AR, and PR. Reporter gene assays with mutant yeast strains responded to endogenous and synthetic ligands more strongly than those with wild-type strains. Sex steroid hormone activities in some pharmaceutical oral tablets and human urine were also detectable in these yeast assays.

Conclusions

Yeast reporter gene assay systems for all six steroid hormone receptors, including previously established glucocorticoid receptor (GR) and mineralocorticoid receptor (MR) assay yeasts, are now available. Environmental endocrine disrupters with steroid hormone activity will be qualitatively detectable by simple and easy procedures. The yeast-based reporter gene assay will be valuable as a primary screening tool to detect and evaluate steroid hormone activities in various test samples. Our assay system will strongly support the detection of agonists, antagonists, and inverse agonists of steroid hormone receptors in the field of novel drug discovery and assessments of environmental pollutants.

Post-translational Regulation of FNIP1 Creates a Rheostat for the Molecular Chaperone Hsp90

The molecular chaperone Hsp90 stabilizes and activates client proteins. Co-chaperones and post-translational modifications tightly regulate Hsp90 function and consequently lead to activation of clients. However, it is unclear whether this process occurs abruptly or gradually in the cellular context. We show that casein kinase-2 phosphorylation of the co-chaperone folliculin-interacting protein 1 (FNIP1) on priming serine-938 and subsequent relay phosphorylation on serine-939, 941, 946, and 948 promotes its gradual interaction with Hsp90. This leads to incremental inhibition of Hsp90 ATPase activity and gradual activation of both kinase and non-kinase clients. We further demonstrate that serine/threonine protein phosphatase 5 (PP5) dephosphorylates FNIP1, allowing the addition of O-GlcNAc (O-linked N-acetylglucosamine) to the priming serine-938. This process antagonizes phosphorylation of FNIP1, preventing its interaction with Hsp90, and consequently promotes FNIP1 lysine-1119 ubiquitination and proteasomal degradation. These findings provide a mechanism for gradual activation of the client proteins through intricate crosstalk of post-translational modifications of the co-chaperone FNIP1.

The natural anticancer agent cantharidin alters GPI-anchored protein sorting by targeting Cdc1-mediated remodeling in endoplasmic reticulum

Cantharidin (CTD) is a potent anticancer small molecule produced by several species of blister beetle. It has been a traditional medicine for the management of warts and tumors for many decades. CTD suppresses tumor growth by inducing apoptosis, cell cycle arrest, and DNA damage and inhibits protein phosphatase 2 phosphatase activator (PP2A) and protein phosphatase 1 (PP1). CTD also alters lipid homeostasis, cell wall integrity, endocytosis, adhesion, and invasion in yeast cells. In this study, we identified additional molecular targets of CTD using a Saccharomyces cerevisiae strain that expresses a cantharidin resistance gene (CRG1), encoding a SAM-dependent methyltransferase that methylates and inactivates CTD. We found that CTD specifically affects phosphatidylethanolamine (PE)-associated functions that can be rescued by supplementing the growth media with ethanolamine (ETA). CTD also perturbed endoplasmic reticulum (ER) homeostasis and cell wall integrity by altering the sorting of glycosylphosphatidylinositol (GPI)-anchored proteins. A CTD-dependent genetic interaction profile of CRG1 revealed that the activity of the lipid phosphatase cell division control protein 1 (Cdc1) in GPI-anchor remodeling is the key target of CTD, independently of PP2A and PP1 activities. Moreover, experiments with human cells further suggested that CTD functions through a conserved mechanism in higher eukaryotes. Altogether, we conclude that CTD induces cytotoxicity by targeting Cdc1 activity in GPI-anchor remodeling in the ER.

Evaluation of tetrabromobisphenol A effects on human glucocorticoid and androgen receptors: A comparison of results from human- with yeast-based in vitro assays

The incidence of immune-related diseases increased over the last years in industrialized countries, suggesting a contribution of environmental factors. Impaired glucocorticoid action has been associated with immune disorders. Thus, there is an increasing interest to identify chemicals disrupting glucocorticoid action. The widely used flame retardant tetrabromobisphenol A (TBBPA) was reported earlier to potently inhibit glucocorticoid receptor (GR) and moderately androgen receptor (AR) activity in yeast-based reporter gene assays. To further characterize possible GR disrupting effects of TBBPA, transactivation experiments using a human HEK-293 cell-based reporter gene assay and cell-free receptor binding experiments were performed in the present study. Both, transactivation and GR binding experiments failed to detect any activity of TBBPA on GR function. Molecular docking calculations supported this observation. Additionally, the current study could confirm the antiandrogenic activity of TBBPA seen in the yeast assay, although the effect was an order of magnitude less pronounced in the HEK-293 cell-based system. In conclusion, TBBPA does not directly affect GR function and, considering its rapid metabolism and low concentrations found in humans, it is unlikely to cause adverse effects by acting through AR. This study emphasizes the use of cell-free assays in combination with cell-based assays for the in vitro evaluation of endocrine disrupting chemicals.

Chemical Dimerizers in Three-Hybrid Systems for Small Molecule-Target Protein Profiling

The identification of the molecular targets and mechanisms underpinning the beneficial or detrimental effects of small-molecule leads and drugs constitutes a crucial aspect of current drug discovery. Over the last two decades, three-hybrid (3H) systems have progressively taken an important position in the armamentarium of small molecule-target protein profiling technologies. Yet, a prerequisite for successful 3H analysis is the availability of appropriate chemical inducers of dimerization. Herein, we present a comprehensive and critical overview of the chemical dimerizers specifically applied in both yeast and mammalian three-hybrid systems for small molecule-target protein profiling within the broader scope of target deconvolution and drug discovery. Furthermore, examples and alternative suggestions for typical components of chemical dimerizers for 3H systems are discussed. As illustrated, more tools have become available that increase the sensitivity and efficiency of 3H-based screening platforms. Hence, it is anticipated that the great potential of 3H systems will further materialize in important contributions to drug discovery.

Yeast-based assays for screening 11β-HSD1 inhibitors

BACKGROUND

Intracellular metabolism of glucocorticoid hormones plays an important role in the pathogenesis of metabolic syndrome and regulates, among many physiological processes, collagen metabolism in skin. At the peripheral level the concentration of active glucocorticoids is mainly regulated by the 11β-hydroxysteroid dehydrogenase 1 (11β-HSD1) enzyme, involved in the conversion of cortisone into the biologically active hormone cortisol. Cortisol interacts with the glucocorticoid receptor and regulates the expression of different classes of genes within the nucleus. Due to its implication in glucocorticoid metabolism, the inhibition of 11β-HSD1 activity has become a dominant strategy for the treatment of metabolic syndrome. Moreover, inhibitors of this target enzyme can be used for development of formulations to counteract skin ageing. Here we present the construction of two yeast cell based assays that can be used for the screening of novel 11β-HSD1 inhibitors.

RESULTS

The yeast Saccharomyces cerevisiae is used as a host organism for the expression of human 11β-HSD1 as well as a genetically encoded assay system that allows intracellular screening of molecules with 11β-HSD1 inhibitory activity. As proof of concept the correlation between 11β-HSD1 inhibition and fluorescent output signals was successfully tested with increasing concentrations of carbenoxolone and tanshinone IIA, two known 11β-HSD1 inhibitors. The first assay detects a decrease in fluorescence upon 11β-HSD1 inhibition, whereas the second assay relies on stabilization of yEGFP upon inhibition of 11β-HSD1, resulting in a positive read-out and thus minimizing the rate of false positives sometimes associated with read-outs based on loss of signals. Specific inhibition of the ABC transporter Pdr5p improves the sensitivity of the assay strains to cortisone concentrations by up to 60 times.

CONCLUSIONS

Our yeast assay strains provide a cost-efficient and easy to handle alternative to other currently available assays for the screening of 11β-HSD1 inhibitors. These assays are designed for an initial fast screening of large numbers of compounds and enable the selection of cell permeable molecules with target inhibitory activity, before proceeding to more advanced selection processes. Moreover, they can be employed in yeast synthetic biology platforms to reconstitute heterologous biosynthetic pathways of drug-relevant scaffolds for simultaneous synthesis and screening of 11β-HSD1 inhibitors at intracellular level.

Mps1 Mediated Phosphorylation of Hsp90 Confers Renal Cell Carcinoma Sensitivity and Selectivity to Hsp90 Inhibitors

The molecular chaperone Hsp90 protects deregulated signaling proteins that are vital for tumor growth and survival. Tumors generally display sensitivity and selectivity toward Hsp90 inhibitors; however, the molecular mechanism underlying this phenotype remains undefined. We report that the mitotic checkpoint kinase Mps1 phosphorylates a conserved threonine residue in the amino-domain of Hsp90. This, in turn, regulates chaperone function by reducing Hsp90 ATPase activity while fostering Hsp90 association with kinase clients, including Mps1. Phosphorylation of Hsp90 is also essential for the mitotic checkpoint because it confers Mps1 stability and activity. We identified Cdc14 as the phosphatase that dephosphorylates Hsp90 and disrupts its interaction with Mps1. This causes Mps1 degradation, thus providing a mechanism for its inactivation. Finally, Hsp90 phosphorylation sensitizes cells to its inhibitors, and elevated Mps1 levels confer renal cell carcinoma selectivity to Hsp90 drugs. Mps1 expression level can potentially serve as a predictive indicator of tumor response to Hsp90 inhibitors.

c-Abl Mediated Tyrosine Phosphorylation of Aha1 Activates Its Co-chaperone Function in Cancer Cells

The ability of Heat Shock Protein 90 (Hsp90) to hydrolyze ATP is essential for its chaperone function. The co-chaperone Aha1 stimulates Hsp90 ATPase activity, tailoring the chaperone function to specific "client" proteins. The intracellular signaling mechanisms directly regulating Aha1 association with Hsp90 remain unknown. Here, we show that c-Abl kinase phosphorylates Y223 in human Aha1 (hAha1), promoting its interaction with Hsp90. This, consequently, results in an increased Hsp90 ATPase activity, enhances Hsp90 interaction with kinase clients, and compromises the chaperoning of non-kinase clients such as glucocorticoid receptor and CFTR. Suggesting a regulatory paradigm, we also find that Y223 phosphorylation leads to ubiquitination and degradation of hAha1 in the proteasome. Finally, pharmacologic inhibition of c-Abl prevents hAha1 interaction with Hsp90, thereby hypersensitizing cancer cells to Hsp90 inhibitors both in vitro and ex vivo.

Helix 8 of the ligand binding domain of the glucocorticoid receptor (GR) is essential for ligand binding

Membrane association of estrogen receptors (ER) depends on cysteine palmitoylation and two leucines in the ligand binding domain (LBD), conserved in most steroid receptors. The role of this region, corresponding to helix 8 of the glucocorticoid receptor (GR) LBD, on membrane association of GR was studied in 4B cells, expressing endogenous GR, and Cos-7 cells transfected EGFP-GR constructs. 4B cells preloaded with radiolabeled palmitic acid showed no radioactivity incorporation into immunoprecipitated GR. Moreover, mutation C683A (corresponding to ER palmitoylation site) did not affect corticosterone-induced membrane association of GR. Mutations L687-690A, L682A, E680G and K685G prevented membrane and also nuclear localization through reduced ligand binding. L687-690A mutation decreased association of GR with heat shock protein 90 and transcriptional activity, without overt effects on receptor protein stability. The data demonstrate that palmitoylation does not mediate membrane association of GR, but that the region 680-690 (helix 8) is critical for ligand binding and receptor function.

Asymmetric Hsp90 N domain SUMOylation recruits Aha1 and ATP-competitive inhibitors

The stability and activity of numerous signaling proteins in both normal and cancer cells depends on the dimeric molecular chaperone heat shock protein 90 (Hsp90). Hsp90's function is coupled to ATP binding and hydrolysis and requires a series of conformational changes that are regulated by cochaperones and numerous posttranslational modifications (PTMs). SUMOylation is one of the least-understood Hsp90 PTMs. Here, we show that asymmetric SUMOylation of a conserved lysine residue in the N domain of both yeast (K178) and human (K191) Hsp90 facilitates both recruitment of the adenosine triphosphatase (ATPase)-activating cochaperone Aha1 and, unexpectedly, the binding of Hsp90 inhibitors, suggesting that these drugs associate preferentially with Hsp90 proteins that are actively engaged in the chaperone cycle. Importantly, cellular transformation is accompanied by elevated steady-state N domain SUMOylation, and increased Hsp90 SUMOylation sensitizes yeast and mammalian cells to Hsp90 inhibitors, providing a mechanism to explain the sensitivity of cancer cells to these drugs.

The helix 1-3 loop in the glucocorticoid receptor LBD is a regulatory element for FKBP cochaperones

The heat-shock protein 90 (Hsp90) cochaperone FK506-binding protein 52 (FKBP52) upregulates, whereas FKBP51 inhibits, hormone binding and nuclear targeting of the glucocorticoid receptor (GR). Decreased cortisol sensitivity in the guinea pig is attributed to changes within the helix 1 to helix 3 (H1-H3) loop of the guinea pig GR (gpGR) ligand-binding domain. It has been proposed that this loop serves as a contact point for FKBP52 and/or FKBP51 with receptor. We examined the role of the H1-H3 loop in GR activation by FKBP52 using a Saccharomyces cerevisiae model. The activity of rat GR (rGR) containing the gpGR H1-H3 loop substitutions was still potentiated by FKBP52, confirming the loop is not involved in primary FKBP52 interactions. Additional assays also excluded a role for other intervening loops between ligand-binding domain helices in direct interactions with FKBP52 associated with enhanced receptor activity. Complementary studies in FKBP51-deficient mouse embryo fibroblasts and HEK293 cells demonstrated that substitution of the gpGR H1-H3 loop residues into rGR dramatically increased receptor repression by FKBP51 without enhancing receptor-FKBP51 interaction and did not alter recruitment of endogenous Hsp90 and the p23 cochaperone to receptor complexes. FKBP51 suppression of the mutated rGR did not require FKBP51 peptidylprolyl cis-trans isomerase activity and was not disrupted by mutation of the FK1 proline-rich loop thought to mediate reciprocal FKBP influences on receptor activity. We conclude that the gpGR-specific mutations within the H1-H3 loop confer global changes within the GR-Hsp90 complex that favor FKBP51 repression over FKBP52 potentiation, thus identifying the loop as an important target for GR regulation by the FKBP cochaperones.

Identification of PDE6D as a molecular target of anecortave acetate via a methotrexate-anchored yeast three-hybrid screen

Glaucoma and age-related macular degeneration are ocular diseases targeted clinically by anecortave acetate (AA). AA and its deacetylated metabolite, anecortave desacetate (AdesA), are intraocular pressure (IOP)-lowering and angiostatic cortisenes devoid of glucocorticoid activity but with an unknown mechanism of action. We used a methotrexate-anchored yeast three-hybrid (Y3H) technology to search for binding targets for AA in human trabecular meshwork (TM) cells, the target cell type that controls IOP, a major risk factor in glaucoma. Y3H hits were filtered by competitive Y3H screens and coimmunoprecipitation experiments and verified by surface plasmon resonance analysis to yield a single target, phosphodiesterase 6-delta (PDE6D). PDE6D is a prenyl-binding protein with additional function outside the PDE6 phototransduction system. Overexpression of PDE6D in mouse eyes caused elevated IOP, and this elevation was reversed by topical ocular application of either AA or AdesA. The identification of PDE6D as the molecular binding partner of AA provides insight into the role of this drug candidate in treating glaucoma.

Glucocorticoid receptor mutants: man-made tools for functional research

The glucocorticoid receptor (GR) is a ligand-dependent transcription factor that can bind to glucocorticoids (GCs). Upon ligand binding, GR sheds its cytoplasmic chaperoning complex and translocates to the nucleus, where it can act as a ligand-dependent transcription factor, transactivating or transrepressing specific gene promoters. Often, GR interacts with specific cofactors to implement a variety of gene promoter effects. GR activity and function is further modulated by post-translational modifications. To assess the diverse aspects of GR mechanisms of activation and gene regulation, researchers continue to use a range of artificial GR mutants. In this review we analyze the characteristics of GR mutants with the aim of assisting the design and interpretation of GR mutant-based experiments.

Mechanisms for the evolution of a derived function in the ancestral glucocorticoid receptor

Understanding the genetic, structural, and biophysical mechanisms that caused protein functions to evolve is a central goal of molecular evolutionary studies. Ancestral sequence reconstruction (ASR) offers an experimental approach to these questions. Here we use ASR to shed light on the earliest functions and evolution of the glucocorticoid receptor (GR), a steroid-activated transcription factor that plays a key role in the regulation of vertebrate physiology. Prior work showed that GR and its paralog, the mineralocorticoid receptor (MR), duplicated from a common ancestor roughly 450 million years ago; the ancestral functions were largely conserved in the MR lineage, but the functions of GRs-reduced sensitivity to all hormones and increased selectivity for glucocorticoids-are derived. Although the mechanisms for the evolution of glucocorticoid specificity have been identified, how reduced sensitivity evolved has not yet been studied. Here we report on the reconstruction of the deepest ancestor in the GR lineage (AncGR1) and demonstrate that GR's reduced sensitivity evolved before the acquisition of restricted hormone specificity, shortly after the GR-MR split. Using site-directed mutagenesis, X-ray crystallography, and computational analyses of protein stability to recapitulate and determine the effects of historical mutations, we show that AncGR1's reduced ligand sensitivity evolved primarily due to three key substitutions. Two large-effect mutations weakened hydrogen bonds and van der Waals interactions within the ancestral protein, reducing its stability. The degenerative effect of these two mutations is extremely strong, but a third permissive substitution, which has no apparent effect on function in the ancestral background and is likely to have occurred first, buffered the effects of the destabilizing mutations. Taken together, our results highlight the potentially creative role of substitutions that partially degrade protein structure and function and reinforce the importance of permissive mutations in protein evolution.

Threonine 22 phosphorylation attenuates Hsp90 interaction with cochaperones and affects its chaperone activity

Heat shock protein 90 (Hsp90) is an essential molecular chaperone whose activity is regulated not only by cochaperones but also by distinct posttranslational modifications. We report here that casein kinase 2 phosphorylates a conserved threonine residue (T22) in α helix-1 of the yeast Hsp90 N-domain both in vitro and in vivo. This α helix participates in a hydrophobic interaction with the catalytic loop in Hsp90's middle domain, helping to stabilize the chaperone's ATPase-competent state. Phosphomimetic mutation of this residue alters Hsp90 ATPase activity and chaperone function and impacts interaction with the cochaperones Aha1 and Cdc37. Overexpression of Aha1 stimulates the ATPase activity, restores cochaperone interactions, and compensates for the functional defects of these Hsp90 mutants.

Swe1Wee1-dependent tyrosine phosphorylation of Hsp90 regulates distinct facets of chaperone function

Saccharomyces WEE1 (Swe1), the only "true" tyrosine kinase in budding yeast, is an Hsp90 client protein. Here we show that Swe1(Wee1) phosphorylates a conserved tyrosine residue (Y24 in yeast Hsp90 and Y38 in human Hsp90alpha) in the N domain of Hsp90. Phosphorylation is cell-cycle associated and modulates the ability of Hsp90 to chaperone a selected clientele, including v-Src and several other kinases. Nonphosphorylatable mutants have normal ATPase activity, support yeast viability, and productively chaperone the Hsp90 client glucocorticoid receptor. Deletion of SWE1 in yeast increases Hsp90 binding to its inhibitor geldanamycin, and pharmacologic inhibition/silencing of Wee1 sensitizes cancer cells to Hsp90 inhibitor-induced apoptosis. These findings demonstrate that Hsp90 chaperoning of distinct client proteins is differentially regulated by specific posttranslational modification of a unique subcellular pool of the chaperone, and they provide a strategy to increase the cellular potency of Hsp90 inhibitors.

Detecting ligands and dissecting nuclear receptor-signaling pathways using recombinant strains of the yeast Saccharomyces cerevisiae

This is a general protocol for the identification of natural and xenobiotic ligands of metazoan nuclear receptors (NRs) expressed in yeast. Yeast engineered to express an NR and a response element-driven reporter gene provide a system to detect and quantify ligand-dependent transcriptional activity. Such assays allow researchers to measure different types of ligands and determine dose-dependent activation of NRs. This methodology can also be used to examine the components of signal transduction pathways when conducted with mutant or engineered yeast strains expressing additional proteins or having alternate DNA response elements. This assay typically takes 2-3 d to complete, but most of this time entails cell growth rather than 'hands on' time.

A conformational switch in the ligand-binding domain regulates the dependence of the glucocorticoid receptor on Hsp90

Steroid hormone receptors (SRs) are transcription factors that act as regulatory switches by altering gene expression in response to ligands. The highly conserved ligand-binding domain of SRs is a precise but versatile molecular switch that can adopt distinct conformations. Differential stabilization of these conformations by ligands, DNA response elements and transcriptional coregulators controls the activity of SRs in a gene-specific and cell-specific manner. In the case of the glucocorticoid receptor (GR), high-affinity ligand binding requires the interaction of the LBD with the heat shock protein 90 (Hsp90). Here, we show that the dependence of the ligand binding ability of GR on Hsp90 can be modified by the replacement of single amino acids within an allosteric network that connects the buried ligand-binding pocket and a solvent-exposed coregulator interaction surface. Each of the identified mutations altered the equilibrium between alternative GR conformations distinctively, indicating that the Hsp90 dependence of SRs may correlate with differences in the conformational dynamics of these receptors. Our results suggest that Hsp90 stabilizes the GR ligand-binding pocket indirectly by utilizing the allosteric network, while allowing the receptor to remain structurally uncommitted. Thus, in addition to ensuring the accessibility of the GR ligand-binding pocket to ligands, Hsp90 seems to enable hormones and coregulators to act as allosteric effectors, which forms the basis for gene-specific and cell-specific responses of GR to ligands.

The distinct agonistic properties of the phenylpyrazolosteroid cortivazol reveal interdomain communication within the glucocorticoid receptor

Recent structural analyses of the nuclear receptors establish a paradigm of receptor activation, in which agonist binding induces the ligand binding domain (LBD)/activation function-2 helix to form a charge clamp for coactivator recruitment. However, these analyses have not sufficiently addressed the mechanisms for differential actions of various synthetic steroids in terms of fine tuning of multiple functions of whole receptor molecules. In the present study, we used the glucocorticoid receptor (GR)-specific agonist cortivazol (CVZ) to probe the plasticity and functional modularity of the GR. Structural docking analysis revealed that although CVZ is more bulky than other agonists, it can be accommodated in the ligand binding pocket of the GR by reorientation of several amino acid side chains but without major alterations in the active conformation of the LBD. In this induced fit model, the phenylpyrazole A-ring of CVZ establishes additional contacts with helices 3 and 5 of the LBD that may contribute to a more stable LBD configuration. Structural and functional analysis revealed that CVZ is able to compensate for the deleterious effects of a C-terminal deletion of the LBD in a manner that mimics the stabilizing influence of the F602S point mutation. CVZ-mediated productive recruitment of transcriptional intermediary factor 2 to the C-terminally deleted LBD requires the receptor's own DNA binding domain and is positively influenced by the N-terminal regions of GR or progesterone receptor. These results support a model where ligand-dependent conformational changes in the LBD play a role in GR-mediated gene regulation via modular interaction with the DBD and activation function-1.

Stabilization of the unliganded glucocorticoid receptor by TSG101

The glucocorticoid receptor (GR) has been shown to undergo hormone-dependent down-regulation via transcriptional, post-transcriptional, and posttranslational mechanisms. However, the mechanisms involved in modulating GR levels in the absence of hormone remain enigmatic. Here we demonstrate that TSG101, a previously identified GR-interacting protein, stabilizes the hypophosphorylated form of GR in the absence of ligand. We found that a non-phosphorylated version of GR (S203A/S211A) showed enhanced interaction with TSG101 as compared with the wild type GR, suggesting that TSG101 interacts more favorably with GR when it is not phosphorylated. A significant accumulation of GR S203A/S211A protein is detected in the absence of ligand when TSG101 is overexpressed, whereas no increase in the wild type phosphorylated GR or phosphomimetic GR S203E/S211E was observed in mammalian cells. In contrast, down-regulation of TSG101 expression by siRNA renders the hypophosphorylated form of GR unstable. We further show that TSG101 stabilizes GR by impeding its degradation by the proteasome and extending receptor half-life. Thus, in absence of a ligand, TSG101 binds GR and protects the non-phosphorylated receptor from degradation.

Signature of the oligomeric behaviour of nuclear receptors at the sequence and structural level

Nuclear receptors (NRs) are ligand-dependent transcription factors that control a large number of physiological events through the regulation of gene transcription. NRs function either as homodimers or as heterodimers with retinoid X receptor/ultraspiracle protein (RXR/USP). A structure-based sequence analysis aimed at discovering the molecular mechanism that controls the dimeric association of the ligand-binding domain reveals two sets of differentially conserved residues, which partition the entire NR superfamily into two classes related to their oligomeric behaviour. Site-directed mutagenesis confirms the functional importance of these residues for the dimerization process and/or transcriptional activity. All homodimers belong to class I, in which the related residues contribute a communication pathway of two salt bridges linking helix 1 on the cofactor-binding site to the dimer interface. A salt bridge involving a differentially conserved arginine residue in loop H8-H9 defines the signature motif of heterodimers. RXR/USP and all Caenorhabditis elegans NRs belong to class I, supporting the hypothesis of an earlier emergence of this class.

Modulation of transcriptional sensitivity of mineralocorticoid and estrogen receptors

Recent reports describe the ability of factors to modulate the position of the dose-response curve of receptor-agonist complexes, and the amount of partial agonist activity of receptor-antagonist complexes, of androgen, glucocorticoid (GRs), and progesterone receptors (PRs). We now ask whether this modulation extends to the two remaining steroid receptors: mineralocorticoid (MRs) and estrogen receptors (ERs). These studies of MR were facilitated by our discovery that the antiglucocorticoid dexamethasone 21-mesylate (Dex-Mes) is a new antimineralocorticoid with significant amounts of partial agonist activity. Elevated levels of MR, the co-activators TIF2 and SRC-1, and the co-repressor SMRT do modulate the dose-response curve and partial agonist activity of MR complexes. Interestingly, the precise responses are indistinguishable from those seen with GRs in the same cells. Thus, the unequal transactivation of common genes by MRs versus GRs probably cannot be explained by differential responses to changing cellular concentrations of homologous receptor, co-activators, or co-repressors. We also find that the dose-response curve of ER-estradiol complexes is left-shifted to lower steroid concentrations by higher amounts of exogenous ER. Therefore, the modulation of either the dose-response curve of agonists or the partial agonist activity of antisteroid, and in many cases the modulation of both properties, is a common phenomenon for all of the classical steroid receptors.

The heat shock protein 70 cochaperone hip enhances functional maturation of glucocorticoid receptor

Multiple molecular chaperones interact with steroid receptors to promote functional maturation and stability of receptor complexes. The heat shock protein (Hsp)70 cochaperone Hip has been identified in conjunction with Hsp70, Hsp90, and the Hsp70/Hsp90 cochaperone Hop/Sti1p in receptor complexes during an intermediate stage of receptor assembly, but a functional requirement for Hip in the receptor assembly process has not been established. Because the budding yeast Saccharomyces cerevisiae contains orthologs for most of the receptor-associated chaperones yet lacks an orthologous Hip gene, we exploited the well-established yeast model for steroid receptor function to ask whether Hip can alter steroid receptor function in vivo. Introducing human Hip into yeast enhances hormone-dependent activation of a reporter gene by glucocorticoid receptor (GR). Because Hip does not similarly enhance signaling by mineralocorticoid, progesterone, or estrogen receptors, a general effect on transcription can be excluded. Instead, Hip promotes functional maturation of GR without increasing steady-state levels of GR protein. Unexpectedly, Hip binding to Hsp70 is not critical for boosting GR responsiveness to hormone. In conclusion, Hip functions by a previously unrecognized mechanism to promote the efficiency of GR maturation in cells.

Interaction networks in yeast define and enumerate the signaling steps of the vertebrate aryl hydrocarbon receptor

The aryl hydrocarbon receptor (AHR) is a vertebrate protein that mediates the toxic and adaptive responses to dioxins and related environmental pollutants. In an effort to better understand the details of this signal transduction pathway, we employed the yeast S. cerevisiae as a model system. Through the use of arrayed yeast strains harboring ordered deletions of open reading frames, we determined that 54 out of the 4,507 yeast genes examined significantly influence AHR signal transduction. In an effort to describe the relationship between these modifying genes, we constructed a network map based upon their known protein and genetic interactions. Monte Carlo simulations demonstrated that this network represented a description of AHR signaling that was distinct from those generated by random chance. The network map was then explored with a number of computational and experimental annotations. These analyses revealed that the AHR signaling pathway is defined by at least five distinct signaling steps that are regulated by functional modules of interacting modifiers. These modules can be described as mediating receptor folding, nuclear translocation, transcriptional activation, receptor level, and a previously undescribed nuclear step related to the receptor's Per-Arnt-Sim domain.

Structure and function of the glucocorticoid receptor ligand binding domain

After binding to an activating ligand, such as corticosteroid, the glucocorticoid receptor (GR) performs an impressive array of functions ranging from nuclear translocation, oligomerization, cofactor/kinase/transcription factor association, and DNA binding. One of the central functions of the receptor is to regulate gene expression, an activity triggered by ligand binding. In this role, GR acts as an adapter molecule by encoding the ligand's message within the structural flexibility of the ligand binding domain (LBD). The purpose of this review is to discuss the many structural and functional features of the GR LBD in light of recent successful biochemical and crystallographic studies. Progress in this area of research promises to reveal new strategies and insights allowing for the design of novel drugs to treat inflammatory diseases, diabetic conditions, steroid resistance, and cancers.

Fluorescent cellular sensors of steroid receptor ligands

Steroid hormone receptors comprise a major class of therapeutic drug targets that control gene expression by binding steroid hormone ligands. These small molecule-protein interactions are typically characterized in living cells by quantification of ligand-mediated reporter gene expression. As an alternative, non-transcriptional approach, we constructed fluorescent cellular sensors by expressing yellow fluorescent protein (YFP) fused to the ligand binding domains (LBDs) of estrogen receptor-alpha (ERalpha), estrogen receptor-beta (ERbeta), androgen receptor (AR), and the glucocorticoid receptor (GR). These proteins were tethered through a short two amino acid linker and expressed in S. cerevisiae yeast. Recombinant yeast treated with cognate steroid receptor ligands exhibited dose-dependent fluorescence enhancements that were correlated with known relative receptor binding affinity values. These effects generally paralleled ligand-mediated receptor dimerization quantified with analogous yeast two-hybrid transcriptional assays, suggesting that the majority of the observed fluorescence enhancements were conferred by conformational changes coupled with receptor dimerization, such as ligand-mediated stabilization of protein folding. Remarkably, certain interactions such as the binding of cortisol, progesterone, and dexamethasone to the GR were undetectable with yeast two-hybrid assays. However, these interactions were detected with the fluorescent cellular sensors, indicating the sensitivity of this system to subtle ligand-induced conformational effects. These sensors provide a novel, non-transcriptional, and high-throughput method to identify and analyze ligands of nuclear hormone receptors.

The three-dimensional structures of antagonistic and agonistic forms of the glucocorticoid receptor ligand-binding domain: RU-486 induces a transconformation that leads to active antagonism

Here we describe the three-dimensional crystal structures of human glucocorticoid receptor ligand-binding domain (GR-LBD) in complex with the antagonist RU-486 at 2.3 A resolution and with the agonist dexamethasone ligand together with a coactivator peptide at 2.8 A. The RU-486 structure was solved in several different crystal forms, two with helix 12 intact (GR1 and GR3) and one with a protease-digested C terminus (GR2). In GR1, part of helix 12 is in a position that covers the co-activator pocket, whereas in the GR3, domain swapping is seen between the crystallographically identical subunits in the GR dimer. An arm consisting of the end of helix 11 and beyond stretches out from one molecule, and helix 12 binds to the other LBD, partly blocking the coactivator pocket of that molecule. This type of GR-LBD dimer has not been described before but might be an artifact from crystallization. Furthermore, the subunits of the GR3 dimers are covalently connected via a disulfide bond between the Cys-736 residues in the two molecules. All three RU-486 GR-LBD structures show that GR has a very flexible region between the end of helix 11 and the end of helix 12.

Synthesis of a beta-estradiol-biotin chimera that potently heterodimerizes estrogen receptor and streptavidin proteins in a yeast three-hybrid system

Small molecules that dimerize proteins in living cells provide powerful probes of biological processes and have potential as tools for the identification of protein targets of natural products. We synthesized 7-alpha-substituted derivatives of beta-estradiol tethered to the natural product biotin to regulate heterodimerization of estrogen receptor (ER) and streptavidin (SA) proteins expressed as components of a yeast three-hybrid system. Addition of an estradiol-biotin chimera bearing a 19-atom linker to yeast expressing DNA-bound ER-alpha or ER-beta LexA fusion proteins and wild-type SA protein fused to the B42 activation domain activated reporter gene expression by as much as 450-fold in vivo (10 muM ligand). Comparative analysis of lower affinity Y43A (biotin Kd approximately 100 pM) and W120A (biotin Kd approximately 100 nM) mutants of SA indicated that moderate affinity interactions can be readily detected with this system. Comparison of a 7-alpha-substituted estradiol-biotin chimera with a structurally similar dexamethasone-biotin chimera revealed that yeast expressing ER proteins can detect cognate ligands with up to 5-fold greater potency and 70-fold higher activity than yeast expressing analogous glucocorticoid receptor (GR) proteins. This approach may facilitate the identification of protein targets of biologically active small molecules screened against genetically encoded libraries of proteins expressed in yeast three-hybrid systems.

The Hsp90-binding peptidylprolyl isomerase FKBP52 potentiates glucocorticoid signaling in vivo

Hsp90 is required for the normal activity of steroid receptors, and in steroid receptor complexes it is typically bound to one of the immunophilin-related co-chaperones: the peptidylprolyl isomerases FKBP51, FKBP52 or CyP40, or the protein phosphatase PP5. The physiological roles of the immunophilins in regulating steroid receptor function have not been well defined, and so we examined in vivo the influences of immunophilins on hormone-dependent gene activation in the Saccharomyces cerevisiae model for glucocorticoid receptor (GR) function. FKBP52 selectively potentiates hormone-dependent reporter gene activation by as much as 20-fold at limiting hormone concentrations, and this potentiation is readily blocked by co-expression of the closely related FKBP51. The mechanism for potentiation is an increase in GR hormone-binding affinity that requires both the Hsp90-binding ability and the prolyl isomerase activity of FKBP52.

Chromatin remodeling and tissue-selective responses of nuclear hormone receptors

Chromatin structure of eukaryotic genes regulates gene expression by controlling the accessibility of regulatory factors. To overcome the inhibitory nature of chromatin, protein complexes that modify higher order chromatin organization and histone-DNA contacts are critical players in regulating transcription. For example, nuclear hormone receptors regulate transcription by interacting with ATP-dependent chromatin-remodeling complexes and coactivators, which include histone acetyltransferases and histone methylases that modify the basic residues of histones. A growing number of tissue-specific nuclear hormone receptor ligands, termed "selective modulators", owe their specificity, at least in part, to the differential recruitment of these chromatin-modifying coactivators. The molecular mechanisms by which these compounds modulate the functions of nuclear hormone receptors are discussed here.

Crystal structure of the glucocorticoid receptor ligand binding domain reveals a novel mode of receptor dimerization and coactivator recognition

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.

Comparison of acute toxicities of indolo[3,2-b]carbazole (ICZ) and 2,3,7,8-tetrachlorodibenzo-p-dioxin (TCDD) in TCDD-sensitive rats

2,3,7,8-Tetrachlorodibenzo-p-dioxin (TCDD) and related halogenated aromatic hydrocarbons are environmental toxicants that act via the AH receptor (AHR). In vitro studies have demonstrated that some indole derivatives present in cruciferous vegetables also bind to the AHR. One of the highest AHR binding affinities is exhibited by indolo[3,2-b]carbazole (ICZ). Since exposure to these dietary indoles is quantitatively far larger than that to halogenated aromatic compounds, their potential toxic risks have raised concern. In the present study, we compared the effects of ICZ with those of a single dose of 20 microg/kg TCDD in the most TCDD-sensitive rat strain (Long-Evans [Turku AB]) (L-E). Whereas TCDD elicited the expected toxicity syndrome, ICZ, either as a single subcutaneous dose (63.5, 127 or 508 microg/kg) or with repeated sc dosing (508 microg/kg for 5 days) failed to reproduce any toxic impacts of TCDD. Furthermore, a simultaneous ICZ treatment (63.5 or 127 microg/kg for 10 days) did not interfere with TCDD (20 microg/kg; single exposure) action. A moderate hepatic induction of CYP1A1 could be triggered by repeated intragastric administration of ICZ (127 microg/kg for 4 days, the last treatment 2.5 h prior to termination). In control experiments in a reconstituted yeast system, ICZ potently and dose-dependently activated L-E rat AHR function demonstrating that it represents a bona fide high-affinity ligand for the rat receptor in vivo. Thus, the present study does not support the view that dietary exposure to ICZ would present a hazard of AHR-mediated adverse health effects to humans.

Identification and characterization of ART-27, a novel coactivator for the androgen receptor N terminus

The androgen receptor (AR) is a ligand-regulated transcription factor that stimulates cell growth and differentiation in androgen-responsive tissues. The AR N terminus contains two activation functions (AF-1a and AF-1b) that are necessary for maximal transcriptional enhancement by the receptor; however, the mechanisms and components regulating AR transcriptional activation are not fully understood. We sought to identify novel factors that interact with the AR N terminus from an androgen-stimulated human prostate cancer cell library using a yeast two-hybrid approach designed to identify proteins that interact with transcriptional activation domains. A 157-amino acid protein termed ART-27 was cloned and shown to interact predominantly with the AR(153-336), containing AF-1a and a part of AF-1b, localize to the nucleus and increase the transcriptional activity of AR when overexpressed in cultured mammalian cells. ART-27 also enhanced the transcriptional activation by AR(153-336) fused to the LexA DNA-binding domain but not other AR N-terminal subdomains, suggesting that ART-27 exerts its effect via an interaction with a defined region of the AR N terminus. ART-27 interacts with AR in nuclear extracts from LNCaP cells in a ligand-independent manner. Interestingly, velocity gradient sedimentation of HeLa nuclear extracts suggests that native ART-27 is part of a multiprotein complex. ART-27 is expressed in a variety of human tissues, including sites of androgen action such as prostate and skeletal muscle, and is conserved throughout evolution. Thus, ART-27 is a novel cofactor that interacts with the AR N terminus and plays a role in facilitating receptor-induced transcriptional activation.

Mammalian transcription factors in yeast: strangers in a familiar land

Many transcription factors in human cells have functional orthologues in yeast, and a common experimental theme has been to define the function of the yeast protein and then test whether the mammalian version behaves similarly. Although, at first glance, this approach does not seem feasible for factors that do not have yeast counterparts, mammalian transcriptional activators or repressors can be expressed directly in yeast. Often, the mammalian factor retains function in yeast, and this allows investigators to exploit the experimental tractability of yeast to ask a diverse set of questions.

Bakers yeast rises to the challenge: reconstitution of mammalian steroid receptor signalling in S. cerevisiae

Steroid hormones are an important class of signalling molecule, regulating a diverse range of processes in metazoan eukaryotes. The actions of these hormones are mediated by intracellular receptor proteins that act as ligand-activated transcription factors. The ability to reconstitute steroid receptor signalling in the budding yeast, Saccharomyces cerevisiae, provides a genetically tractable model system in which to investigate steroid receptor structure and function. Through targeted disruption and genetic screening, an increasing number of genes have been identified that are likely to have a role in steroid receptor action.

Glucocorticoid receptor homodimers and glucocorticoid-mineralocorticoid receptor heterodimers form in the cytoplasm through alternative dimerization interfaces

Steroid hormone receptors act to regulate specific gene transcription primarily as steroid-specific dimers bound to palindromic DNA response elements. DNA-dependent dimerization contacts mediated between the receptor DNA binding domains stabilize DNA binding. Additionally, some steroid receptors dimerize prior to their arrival on DNA through interactions mediated through the receptor ligand binding domain. In this report, we describe the steroid-induced homomeric interaction of the rat glucocorticoid receptor (GR) in solution in vivo. Our results demonstrate that GR interacts in solution at least as a dimer, and we have delimited this interaction to a novel interface within the hinge region of GR that appears to be both necessary and sufficient for direct binding. Strikingly, we also demonstrate an interaction between GR and the mineralocorticoid receptor in solution in vivo that is dependent on the ligand binding domain of GR alone and is separable from homodimerization of the glucocorticoid receptor. These results indicate that functional interactions between the glucocorticoid and mineralocorticoid receptors in activating specific gene transcription are probably more complex than has been previously appreciated.

A genetic analysis of glucocorticoid receptor signaling. Identification and characterization of ligand-effect modulators in Saccharomyces cerevisiae

To find novel components in the glucocorticoid signal transduction pathway, we performed a yeast genetic screen to identify ligand-effect modulators (LEMs), proteins that modulate the cellular response to hormone. We isolated several mutants that conferred increased glucocorticoid receptor (GR) activity in response to dexamethasone and analyzed two of them in detail. These studies identify two genes, LEM3 and LEM4, which correspond to YNL323w and ERG6, respectively. LEM3 is a putative transmembrane protein of unknown function, and ERG6 is a methyltransferase in the ergosterol biosynthetic pathway. Analysis of null mutants indicates that LEM3 and ERG6 act at different steps in the GR signal transduction pathway.

BRG-1 is recruited to estrogen-responsive promoters and cooperates with factors involved in histone acetylation

Several factors that mediate activation by nuclear receptors also modify the chemical and structural composition of chromatin. Prominent in this diverse group is the steroid receptor coactivator 1 (SRC-1) family, which interact with agonist-bound nuclear receptors, thereby coupling them to multifunctional transcriptional coregulators such as CREB-binding protein (CBP), p300, and PCAF, all of which have potent histone acetyltransferase activity. Additionally factors including the Brahma-related gene 1 (BRG-1) that are involved in the structural remodeling of chromatin also mediate hormone-dependent transcriptional activation by nuclear receptors. Here, we provide evidence that these two distinct mechanisms of coactivation may operate in a collaborative manner. We demonstrate that transcriptional activation by the estrogen receptor (ER) requires functional BRG-1 and that the coactivation of estrogen signaling by either SRC-1 or CBP is BRG-1 dependent. We find that in response to estrogen, ER recruits BRG-1, thereby targeting BRG-1 to the promoters of estrogen-responsive genes in a manner that occurs simultaneous to histone acetylation. Finally, we demonstrate that BRG-1-mediated coactivation of ER signaling is regulated by the state of histone acetylation within a cell. Inhibition of histone deacetylation by trichostatin A dramatically increases BRG-1-mediated coactivation of ER signaling, and this increase is reversed by overexpression of histone deacetylase 1. These studies support a critical role for BRG-1 in ER action in which estrogen stimulates an ER-BRG-1 association coupling BRG-1 to regions of chromatin at the sites of estrogen-responsive promoters and promotes the activity of other recruited factors that alter the acetylation state of chromatin.

A role for the Hsp40 Ydj1 in repression of basal steroid receptor activity in yeast

In addition to its roles in translocation of preproteins across membranes, Ydj1 facilitates the maturation of Hsp90 substrates, including mammalian steroid receptors, which activate transcription in yeast in a hormone-dependent manner. To better understand Ydj1's function, we have constructed and analyzed an array of Ydj1 mutants in vivo. Both the glucocorticoid receptor and the estrogen receptor exhibited elevated activity in the absence of hormone in all ydj1 mutant strains, indicating a strict requirement for Ydj1 activity in hormonal control. Glucocorticoid receptor containing a mutation in the carboxy-terminal transcriptional activation domain, AF-2, retained elevated basal activity, while mutation of the amino-terminal transactivation domain, AF-1, eliminated the elevated basal activity observed in ydj1 mutant strains. This result indicates that the source of activity is AF-1, which is normally repressed by the carboxy-terminal hormone binding domain in the absence of hormone. Chimeric proteins containing the hormone binding domain of the estrogen or glucocorticoid receptor fused to heterologous activation and DNA binding domains also exhibited elevated activity in the absence of hormone. Thus, Ydj1 mutants appear to increase basal receptor activity by altering the ability of the hormone binding domain of the receptor to repress nearby activation domains. We propose that Ydj1 functions to present steroid receptors to the Hsp90 pathway for folding and hormonal control. In the presence of Ydj1 mutants that fail to bind substrate efficiently, some receptor escapes the Hsp90 pathway, resulting in constitutive activity.

The Hsp90 of Candida albicans can confer Hsp90 functions in Saccharomyces cerevisiae: a potential model for the processes that generate immunogenic fragments of this molecular chaperone in C. albicans infections

During infections with a number of important eukaryotic pathogens the Hsp90 molecular chaperone of the pathogen is recognized as an immunodominant antigen by the host immune system. Yeast molecular genetics should allow study of the extent of sequence variation within conserved immunodominant epitopes on pathogen Hsp90s that is compatible with essential Hsp90 functions, as well as the processes that generate antigenic subfragments of these Hsp90s. The Hsp90 of the fungal pathogen Candida albicans was shown in this study to provide both essential and nonessential (pheromone signalling and mammalian steroid receptor activation) Hsp90 functions in Saccharomyces cerevisiae cells. Much of the C. albicans Hsp90 expressed in respiratory S. cerevisiae cells was shown to undergo a partial degradation in vivo, a degradation that closely resembles that of the native Hsp82 (one isoform of the homologous Hsp90) in S. cerevisiae. Allowing for the differences in the length of the charged linker region between the N- and C-terminal domains of C. albicans Hsp90 and S. cerevisiae Hsp82, these two proteins expressed in S. cerevisiae appear to give the same major degradation products. These Hsp90 fragments are similar to the products of incomplete Hsp90 degradation found in C. albicans cultures.

Hormone-independent transcriptional activation and coactivator binding by novel orphan nuclear receptor ERR3

Orphan nuclear receptors share sequence homology with members of the nuclear receptor superfamily, but ligands are unknown or unnecessary. A novel orphan receptor, estrogen receptor-related protein 3 (ERR3), was identified by yeast two-hybrid screening, using the transcriptional coactivator glucocorticoid receptor interacting protein 1 (GRIP1) as bait. The putative full-length mouse ERR3 contains 458 amino acids and is closely related to two known orphan receptors ERR1 and ERR2. All the ERR family members share an almost identical DNA-binding domain, which has 68% amino acid identity with that of estrogen receptor. ERR3 bound specifically to an estrogen response element and activated reporter genes controlled by estrogen response elements, both in yeast and in mammalian cells, in the absence of any added ligand. A conserved AF-2 activation domain located in the hormone-binding domain of ERR3 was primarily responsible for transcriptional activation. The ERR3 AF-2 domain bound GRIP1 in a ligand-independent manner both in vitro and in vivo, through the LXXLL motifs of GRIP1, and GRIP1 functioned as a transcriptional coactivator for ERR3 in both yeast and mammalian cells. Expression of ERR3 in adult mouse was restricted; highest expression was observed in heart, kidney, and brain. In the mouse embryo no expression was observed at day 7, and highest expression occurred around the 11-15 day stages. Although ERR3 is much more closely related to ERR2 than to ERR1, the expression pattern for ERR3 was similar to that of ERR1 and distinct from that for ERR2, suggesting a unique role for ERR3 in development.