Structural and functional reconstitution of the glucocorticoid receptor-hsp90 complex

Hsp90β positively regulates μ-opioid receptor function

AIMS

Many μ-opioid receptor (MOR)-associated proteins can regulate the MOR signaling pathway. Using a bacterial two-hybrid screen, we found that the C-terminal of the MOR associated with heat shock protein 90 isoform β (Hsp90β). Here, we explored the effect of Hsp90β on MOR signaling transduction and function.

MAIN METHODS

The interaction of Hsp90β with MOR was detected by co-immunoprecipitation and immunofluorescence. The effects of Hsp90β on MOR signaling induced by opioids were studied in vitro and in vivo. The effects of the Hsp90β inhibitor 17-N-allylamino-17-demethoxygeldanamycin (17-AAG) on morphine tolerance and dependence were studied via a hot plate test and CPP test.

KEY FINDINGS

Hsp90β, instead of Hsp90α, interacted with the MOR in HEK293 cells and SH-SY5Y cells, and the interaction was augmented after morphine pretreatment. The interaction of Hsp90β and MOR increased the inhibition of cAMP and decreased PKA activity under opioid treatment. The functional Hsp90β-MOR complex also promoted the phosphorylation and internalization of the MOR induced by DAMGO in MOR-CHO cells. 17-AAG blocked Hsp90β-MOR interactions and decreased the effect of Hsp90β on the MOR signal transduction. In C57BL/6 mice, 17-AAG decreased morphine-induced acute anti-nociception in the hot plate test, with an increase in phosphorylated PKA and phosphorylated JNK and a decrease in phosphorylated CREB and phosphorylated ERK in murine brains. Chronic morphine treatment induced tolerance, and dependence was inhibited by 17-AAG co-administration.

SIGNIFICANCE

Hsp90β is a positive co-regulator of the MOR via the activation of a G-protein-dependent and β-arrestin-dependent pathway. Hsp90β has the potential to improve the pharmacologic profile of existing opiates. It is conceivable that in future clinical treatments, the Hsp90β inhibitor, 17-AAG, could decrease the tolerance and dependence in cancer patients induced by opioids.

Quantification of Hsp90 availability reveals differential coupling to the heat shock response

The heat shock response (HSR) is a protective gene expression program that is activated by conditions that cause proteotoxic stress. While it has been suggested that the availability of free chaperones regulates the HSR, chaperone availability and the HSR have never been precisely quantified in tandem under stress conditions. Thus, how the availability of chaperones changes in stress conditions and the extent to which these changes drive the HSR are unknown. In this study, we quantified Hsp90 chaperone availability and the HSR under multiple stressors. We show that Hsp90-dependent and -independent pathways both regulate the HSR, and the contribution of each pathway varies greatly depending on the stressor. Moreover, stressors that regulate the HSR independently of Hsp90 availability do so through the Hsp70 chaperone. Thus, the HSR responds to diverse defects in protein quality by monitoring the state of multiple chaperone systems independently.

In vitro and in silico assessment of the structure-dependent binding of bisphenol analogues to glucocorticoid receptor

Widespread use of bisphenol A (BPA) and other bisphenol analogues has attracted increasing attention for their potential adverse effects. As environmental endocrine-disrupting compounds (EDCs), bisphenols (BPs) may activate a variety of nuclear receptors, including glucocorticoid receptor (GR). In this work, the binding of 11 BPs to GR was investigated by fluorescence polarization (FP) assay in combination with molecular dynamics simulations. The human glucocorticoid receptor was prepared as a soluble recombinant protein. A fluorescein-labeled dexamethasone derivative (Dex-fl) was employed as tracer. Competitive displacement of Dex-fl from GR by BPs showed that the binding affinities of bisphenol analogues were largely dependent on their characteristic functional groups. In order to further understand the relationship between BPs structures and their GR-mediated activities, molecular docking was utilized to explore the binding modes at the atomic level. The results confirmed that structural variations of bisphenol analogues contributed to different interactions of BPs with GR, potentially causing distinct toxic effects. Comparison of the calculated binding energies vs. experimental binding affinities yielded a good correlation (R ² = 0.8266), which might be helpful for the design of environmentally benign materials with reduced toxicities. In addition, the established FP assay based on GR exhibited the potential to offer an alternative to traditional methods for the detection of bisphenols.

Review: The HSP90 molecular chaperone-an enigmatic ATPase

The HSP90 molecular chaperone is involved in the activation and cellular stabilization of a range of 'client' proteins, of which oncogenic protein kinases and nuclear steroid hormone receptors are of particular biomedical significance. Work over the last two decades has revealed a conformational cycle critical to the biological function of HSP90, coupled to an inherent ATPase activity that is regulated and manipulated by many of the co-chaperones proteins with which it collaborates. Pharmacological inhibition of HSP90 ATPase activity results in degradation of client proteins in vivo, and is a promising target for development of new cancer therapeutics. Despite this, the actual function that HSP90s conformationally-coupled ATPase activity provides in its biological role as a molecular chaperone remains obscure. © 2016 Wiley Periodicals, Inc. Biopolymers 105: 594-607, 2016.

The interchange of immunophilins leads to parallel pathways and different intermediates in the assembly of Hsp90 glucocorticoid receptor complexes

Hormone receptors require participation of the chaperones Hsp40/Hsp70 to form client-transfer complexes with Hsp90/Hop. Interaction with the co-chaperone p23 releases Hop and Hsp70, and the immunophilin FKBP52 mediates transfer of the Hsp90-receptor complex to the nucleus. Inhibition of glucocorticoid receptor (GR) transport by FKBP51, but not by FKBP52, has been observed at the cellular level, but the subunit composition of the intermediates involved has not been deduced. Here we use mass spectrometry to show that FKBP51/52 form analogous complexes with GR/Hsp90/Hop/Hsp70/ATP, but differences emerge upon addition of p23 to client-transfer complexes. When FKBP51 is present, a stable intermediate is formed (FKBP51)₁(GR)₁(Hsp90)₂(p23)₂ by expulsion of Hsp70 and Hop. By contrast, in the presence of FKBP52, ejection of p23 also takes place to form the nuclear transfer complex (FKBP52)₁(GR)₁(Hsp90)₂. Our results are therefore consistent with pathways in which FKBP51/52 are interchangeable during the early assembly reactions. Following interaction with p23, however, the pathways diverge with FKBP51 sequestering GR in a stable intermediate complex with p23. By contrast, binding of FKBP52 occurs almost concomitantly with release of p23 to form a highly dynamic transfer complex, primed for interaction with the dynactin transport machinery.

Hypothalamic-pituitary-adrenal (HPA) axis and aging

Human aging is associated with increasing frailty and morbidity which can result in significant disability. Dysfunction of the hypothalamic-pituitary-adrenal (HPA) axis may contribute to aging-related diseases like depression, cognitive deficits, and Alzheimer's disease in some older individuals. In addition to neuro-cognitive dysfunction, it has also been associated with declining physical performance possibly due to sarcopenia. This article reviews the pathophysiology of HPA dysfunction with respect to increased basal adrenocorticotropic hormone (ACTH) and cortisol secretion, decreased glucocorticoid (GC) negative feedback at the level of the paraventricular nucleus (PVN) of the hypothalamus, hippocampus (HC), and prefrontal cortex (PFC), and flattening of diurnal pattern of cortisol release. It is possible that the increased cortisol secretion is secondary to peripheral conversion from cortisone. There is a decline in pregnolone secretion and C-19 steroids (DHEA) with aging. There is a small decrease in aldosterone with aging, but a subset of the older population have a genetic predisposition to develop hyperaldosteronism due to the increased ACTH stimulation. The understanding of the HPA axis and aging remains a complex area with conflicting studies leading to controversial interpretations.

A model in which heat shock protein 90 targets protein-folding clefts: rationale for a new approach to neuroprotective treatment of protein folding diseases

In an EBM Minireview published in 2010, we proposed that the heat shock protein (Hsp)90/Hsp70-based chaperone machinery played a major role in determining the selection of proteins that have undergone oxidative or other toxic damage for ubiquitination and proteasomal degradation. The proposal was based on a model in which the Hsp90 chaperone machinery regulates signaling by modulating ligand-binding clefts. The model provides a framework for thinking about the development of neuroprotective therapies for protein-folding diseases like Alzheimer's disease (AD), Parkinson's disease (PD), and the polyglutamine expansion disorders, such as Huntington's disease (HD) and spinal and bulbar muscular atrophy (SBMA). Major aberrant proteins that misfold and accumulate in these diseases are "client" proteins of the abundant and ubiquitous stress chaperone Hsp90. These Hsp90 client proteins include tau (AD), α-synuclein (PD), huntingtin (HD), and the expanded glutamine androgen receptor (polyQ AR) (SBMA). In this Minireview, we update our model in which Hsp90 acts on protein-folding clefts and show how it forms a rational basis for developing drugs that promote the targeted elimination of these aberrant proteins.

Conjugation of dexamethasone to C60 for the design of an anti-inflammatory nanomedicine with reduced cellular apoptosis

Dexamethasone (DEX) is a well-known anti-inflammatory drug, whose widespread clinical use is nevertheless restricted by its serious side effects. By conjugation of DEX with C60, we found that this nanomedicine retained the anti-inflammatory activity of DEX while reducing side effects in the animal model. In mouse thymocytes, the CCK-8 assay showed that the cytotoxicity of DEX-C60 was significantly lower than that of free DEX. Flow cytometric studies revealed that incubation with DEX-C60 induced much less apoptotic thymocytes. Interestingly, such reduced cytotoxicity and apoptosis were not observed when equal moles of free C60 and free DEX were coincubated with thymocytes, suggesting that the conjugation alters the signal pathway of DEX. Indeed, we found that the binding of DEX-C60 and a glucocorticoid receptor (GR) was partially blocked in the thymocytes, which resulted in down-regulation of several apoptosis-related genes. These findings help understand the mechanism of beneficial effects of this new nanomedicine, DEX-C60, and promote its clinical applications.

Chaperoning steroidal physiology: lessons from mouse genetic models of Hsp90 and its cochaperones

The molecular chaperone Hsp90 is abundant, ubiquitous, and catholic to biological processes in eukaryotes, controlling phosphorylation cascades, protein stability and turnover, client localization and trafficking, and ligand-receptor interactions. Not surprisingly, Hsp90 does not accomplish these activities alone. Instead, an ever-growing number of cochaperones have been identified, leading to an explosion of reports on their molecular and cellular effects on Hsp90 chaperoning of client substrates. Notable among these clients are many members of the steroid receptor family, such as glucocorticoid, androgen, estrogen and progesterone receptors. Cochaperones typically associated with the mature, hormone-competent states of these receptors include p23, the FK506-binding protein 52 (FKBP52), FKBP51, protein phosphatase 5 (PP5) and cyclophilin 40 (Cyp40). The ultimate relevance of these cochaperones to steroid receptor action depends on their physiological effects. In recent years, the first mouse genetic models of these cochaperones have been developed. This work will review the complex and intriguing phenotypes so far obtained in genetically-altered mice and compare them to the known molecular and cellular impacts of cochaperones on steroid receptors. This article is part of a Special Issue entitled: Heat Shock Protein 90 (HSP90).

Role of glucocorticoids and the glucocorticoid receptor in metabolism: insights from genetic manipulations

Since the discovery of the beneficial effects of adrenocortical extracts for treating adrenal insufficiency more than 80 years ago, glucocorticoids and their cognate, intracellular receptor, the glucocorticoid receptor have been characterized as critical checkpoints in the delicate hormonal control of energy homeostasis in mammals. Whereas physiological levels of glucocorticoids are required for proper metabolic control, aberrant glucocorticoid action has been linked to a variety of pandemic metabolic diseases, such as type II diabetes and obesity. Based on its importance for human health, studies of the molecular mechanisms of within the glucocorticoid signaling axis have become a major focus in biomedical research. In particular, the understanding of tissue-specific functions of the glucocorticoid receptor pathway has been proven to be of substantial value for the development of novel therapies in the treatment of chronic metabolic disorders. Therefore, this review focuses on the consequences of endogenous and experimental modulation of glucocorticoid receptor expression for metabolic homeostasis and dysregulation, particularly emphasizing tissue-specific contributions of the glucocorticoid pathway to the control of energy metabolism.

Alterations in mRNA expression of steroid receptors and heat shock proteins in the liver of rare minnow (Grobiocypris rarus) exposed to atrazine and p,p'-DDE

The chaperon role of heat shock proteins (HSPs) throughout the life cycle of steroid receptors have been demonstrated in vitro. However, the actions of HSPs in steroid pathways in animals especially in fish were unclear. In this study, sexually mature rare minnows (Gobiocypris rarus) were exposed to typical endocrine disruptors (atrazine and p,p'-DDE). Hypertrophy of hepatocytes at the 333 microg/l atrazine treatment and atrophy of hepatocytes in all p,p'-DDE treatments were observed. The expression of liver hsp70 and hsp90 in atrazine treatments were significantly up-regulated. Moreover, remarkable increases in the expression of androgen receptor (ar) and estrogen receptor (er) were observed, while alterations of the glucoticorcoid receptor (gr) expression was not significant in atrazine exposed fish. The expression of ar, er, gr, hsp70 and hsp90 were significantly suppressed following p,p'-DDE exposure. These results demonstrate that the expression of hsp70 and hsp90 is altered along with changes of steroid receptors in vivo. Therefore, the results are consistent with the possibility that in fish heat shock proteins (HSPs) play chaperon roles for the steroid receptors in vivo, which also concurs with previous in vitro mammalian studies.

Aging attenuates glucocorticoid negative feedback in rat brain

Aging is thought to be a risk factor to develop vulnerability of the neuroendocrine system, including the hypothalamic-pituitary-adrenal (HPA) axis, and dysregulation of this axis characterized by dexamethasone (DEX)-mediated negative feedback resistance is sometimes observed in elderly humans and animals. However, the influence of aging on the feedback system including an involvement of the brain is not fully understood. In the present study, we examined the suppressive effects of DEX by the systemic injection or the intracranial infusion into the prefrontal cortex (PFC), hippocampus, and hypothalamus on circulating corticosterone levels, and compared between young (3-month-old) and aged (24-month-old) rats. Moreover, we examined expression levels of glucocorticoid receptors (GRs) and their translocation from the cytoplasm to the nucleus using immunohistochemical and Western immunoblot techniques in the pituitary in addition to three brain regions. When DEX was injected systemically, the suppressive response was significantly enhanced in aged rats, compared with young rats. When DEX was infused into three brain regions, the suppressive response to DEX was abolished in aged rats. The immunohistochemical analysis revealed that the number of GR positive cells in the PFC, hippocampus, and hypothalamus was decreased, but that in the pituitary was increased, in aged rats, compared with young rats. The Western immunoblot analysis confirmed these results. Thus, basal expression levels of GRs in three brain regions were decreased, but those in the pituitary were increased, in aged rats. After the injection or infusion of DEX, the translocation of GRs in three brain regions was reduced, but that in the pituitary was enhanced, in aged rats. These results suggest that aging in rats enhances the feedback ability at the systemic level, which mainly involves the pituitary, but it attenuates the ability in the brain. These mechanisms may underlie the vulnerable neuroendocrine systems associated with aging.

Minireview: the intersection of steroid receptors with molecular chaperones: observations and questions

An involvement of molecular chaperones in the action and well-being of steroid receptors was recognized early in the molecular era of hormone research. However, this has continued to be a topic of much enquiry and some confusion. All steroid receptors associate with heat shock protein 90, the main character of a series of multiprotein chaperone complexes generally referred to as the "heat shock protein 90 chaperoning machine." Receptor association with chaperones occurs in an ordered, step-wise fashion and is necessary for the maintenance of unliganded receptor in a state ready to bind and respond to hormone. Chaperones additionally modulate how receptors respond to hormone and activate target genes. Although much is known about the participants in this chaperoning process and the consequences of chaperoning, many key questions remain unanswered, particularly those concerning molecular mechanisms, cellular dynamics, and the functions of an array of cochaperone proteins. Here, we point out several areas in need of investigation to encourage new ideas and participants in this burgeoning field.

The glucocorticoid responses are shaped by molecular chaperones

The glucocorticoid receptor is a known regulator of a variety of physiological processes. Its mode of action is well defined: upon hormone binding, it undergoes a conformational change, translocates to the nucleus and modulates the transcription of target genes. Molecular chaperones have a widely recognized role in the folding of newly made proteins, but their participation in further maturation of folded proteins to their active states and beyond tends to be underestimated. This review presents the current knowledge on how the Hsp70 and Hsp90 chaperone machines help to shape the responses to glucocorticoids. We discuss the contributions of these molecular chaperones to folding, activation, intracellular transport, transcriptional regulation, and decay of the glucocorticoid receptor.

Saikokaryukotsuboreito, a herbal medicine, prevents chronic stress-induced dysfunction of glucocorticoid negative feedback system in rat brain

Disruption of the hypothalamo-pituitary-adrenal (HPA) axis characterized by dysfunction of the glucocorticoid negative feedback system is frequently observed in human depressives and is thought to involve a reduction in glucocorticoid receptor (GR) function in the feedback sites including the brain. Recently, we found that chronic stress in rats induces similar HPA disruption that is caused by abolishment of feedback ability in the prefrontal cortex (PFC) and hippocampus, which involves decreased cytosolic GRs or increased nuclear GRs, respectively. Also, we found that saikokaryukotsuboreito (SRBT), a herbal medicine, prevents the chronic stress-induced HPA disruption. We therefore examined here the effects of this drug on the chronic stress-induced changes in GRs in the PFC and hippocampus. Chronic stress was induced in rats by water immersion and restraint (2 h/day) for 4 weeks. SRBT significantly prevented decreased cytosolic GRs in the PFC and increased nuclear GRs in the hippocampus in the chronically stressed rats. Moreover, SRBT significantly prevented the abolishment of feedback ability in both regions. These results suggest that the beneficial effects of SRBT on the GR level are involved in its ameliorating actions on the HPA disruption. This finding provides information important for the prevention and treatment of depression.

Chaperoning of glucocorticoid receptors

A multiprotein hsp90/hsp70-based chaperone machinery functions as a 'cradle-to-grave' system for regulating the steroid binding, trafficking and turnover of the glucocorticoid receptor (GR). In an ATP-dependent process where hsp70 and hsp90 act as essential chaperones and Hop, hsp40, and p23 act as nonessential co-chaperones, the machinery assembles complexes between the ligand binding domain of the GR and hsp90. During GR-hsp90 heterocomplex assembly, the hydrophobic ligand-binding cleft is opened to access by steroid, and subsequent binding of steroid within the cleft triggers a transformation of the receptor such that it engages in more dynamic cycles of assembly/disassembly with hsp90 that are required for rapid dynein-dependent translocation to the nucleus. Within the nucleus, the hsp90 chaperone machinery plays a critical role both in GR movement to transcription regulatory sites and in the disassembly of regulatory complexes as the hormone level declines. The chaperone machinery also plays a critical role in stabilization of the GR to ubiquitylation and proteasomal degradation. The initial GR interaction with hsp70 appears to be critical for the triage between hsp90 heterocomplex assembly and preservation of receptor function vs CHIP-dependent ubiquitylation and proteasomal degradation. The hsp90 chaperone machinery is ubiquitous and functionally conserved among eukaryotes, and it is possible that all physiologically significant actions of hsp90 require the hsp70-dependent assembly of client protein-hsp90 heterocomplexes.

Glucocorticoid receptor and heat shock factor 1: novel mechanism of reciprocal regulation

Glucocorticoids control a host of bodily responses, ranging from carbohydrate metabolism in the liver to immunity and inflammation in the lymph system. In response to stress, glucocorticoid levels are known to rise-a response thought to provide a protective function against the stress event. It is now understood that the major function of glucocorticoids under stress is to protect not against the stress event itself but against overstimulation by host defenses (e.g., inflammation). Control of these responses is achieved by the glucocorticoid receptor, a member of the steroid receptor transcription factor family. The oldest, most conserved, and most ubiquitous of the stress responses is induced expression of heat shock proteins that act as chaperones against stress-induced denaturation of protein. Expression of heat shock protein genes is controlled by heat shock transcription factor 1. In this work, we review our observations and those of other laboratories demonstrating a relationship between the glucocorticoid and heat shock responses. We show that complex but reciprocal mechanisms of regulation occur between glucocorticoid receptor and heat shock transcription factor 1 and present a model of coordinated action that likely serves to fully reestablish homeostasis following stress.

Physiological Role for the Cochaperone FKBP52 in Androgen Receptor Signaling

Molecular chaperones mediate multiple aspects of steroid receptor function, but the physiological importance of most receptor-associated cochaperones has not been determined. To help fill this gap, we targeted for disruption the mouse gene for the 52-kDa FK506 binding protein, FKBP52, a 90-kDa heat shock protein (Hsp90)-binding immunophilin found in steroid receptor complexes. A mouse line lacking FKBP52 (52KO) was generated and characterized. Male 52KO mice have several defects in reproductive tissues consistent with androgen insensitivity; among these defects are ambiguous external genitalia and dysgenic prostate. FKBP52 and androgen receptor (AR) are coexpressed in prostate epithelial cells of wild-type mice. However, FKBP52 and AR are similarly coexpressed in testis even though testis morphology and spermatogenesis in 52KO males are usually normal. Molecular studies confirm that FKBP52 is a component of AR complexes, and cellular studies in yeast and human cell models demonstrate that FKBP52 can enhance AR-meditated transactivation. AR enhancement requires FKBP52 peptidylprolyl isomerase activity as well as Hsp90-binding ability, and enhancement probably relates to an affect of FKBP52 on AR-folding pathways. In the presence of FKBP52, but not other cochaperones, the function of a minimally active AR point mutant can be dramatically restored. We conclude that FKBP52 is an AR folding factor that has critically important physiological roles in some male reproductive tissues.

Functional specificity of co-chaperone interactions with Hsp90 client proteins

A wide array of proteins in signal transduction pathways depend on Hsp90 and other chaperone components for functional maturation, regulation, and stability. Among these Hsp90 client proteins are steroid receptors, members from other classes of transcription factors, and representatives of both serine/threonine and tyrosine kinase families. Typically, dynamic complexes form on the client protein, and these consist of Hsp90- plus bound co-chaperones that often have enzymatic activities. In addition to its direct influence on client folding, Hsp90 locally concentrates co-chaperone activity within the client complex, and dynamic exchange of co-chaperones on Hsp90 facilitates sampling of co-chaperone activities that may, or may not, act on the client protein. We are just beginning to understand the nature of biochemical and molecular interactions between co-chaperone and Hsp90-bound client. This review focuses on the differential effects of Hsp90 co-chaperones toward client protein function and on the specificity that allows co-chaperones to discriminate between even closely related clients.

Brain glucocorticoid receptor and heat shock protein 70 levels in rats exposed to acute, chronic or combined stress

The pattern and intensity of glucocorticoid receptor (GR) and heat shock 70 protein (Hsp 70) changes in the hippocampus and brain cortex of adult Wistar rat males exposed to acute (immobilization, cold) and chronic (social isolation, crowding, daily swimming) stress or their combinations were followed by Western immunoblotting. Plasma ACTH and CORT were measured by chemiluminescent method and RIA. A significant decrease in cytosol GR and Hsp 70 was observed after acute stress, while chronic stresses led to negligible changes in both these proteins and caused a reduced responsiveness to a novel acute stress. This was valid irrespective of the type of chronic or acute stress combinations for both hippocampal and cortical GR and Hsp 70. The results support the hypothesis that chronic stress-induced deregulation of the LHPA axis may be caused, at least in part, by partial disruption of intracelullar negative feedback control in the higher centers of the brain.

Functional comparison of human and Drosophila Hop reveals novel role in steroid receptor maturation

Hsp70/Hsp90 organizing protein (Hop) coordinates Hsp70 and Hsp90 interactions during assembly of steroid receptor complexes. Hop is composed of three tetratricopeptide repeat (TPR) domains (TPR1, TPR2a, and TPR2b) and two DP repeat domains (DP1 and DP2); Hsp70 interacts directly with TPR1 and Hsp90 with TPR2a, but the function of other domains is less clear. Human Hop and the Saccharomyces cerevisiae ortholog Sti1p, which share a common domain arrangement, are functionally interchangeable in a yeast growth assay and in supporting the efficient maturation of glucocorticoid receptor (GR) function. To gain a better understanding of Hop structure/function relationships, we have extended comparisons to the Hop ortholog from Drosophila melanogaster (dHop), which lacks DP1. Although dHop binds Hsp70 and Hsp90 and can rescue the growth defect in yeast lacking Sti1p, dHop failed to support GR function in yeast, which suggests a novel role for Hop in GR maturation that goes beyond Hsp binding. Chimeric Hop constructs combining human and Drosophila domains demonstrate that the C-terminal domain DP2 is critical for this previously unrecognized role in steroid receptor function.

Hsp70 and Hsp90--a relay team for protein folding

Molecular chaperones are a functionally defined set of proteins which assist the structure formation of proteins in vivo. Without certain protective mechanisms, such as binding nascent polypeptide chains by molecular chaperones, cellular protein concentrations would lead to misfolding and aggregation. In the mammalian system, the molecular chaperones Hsp70 and Hsp90 are involved in the folding and maturation of key regulatory proteins, like steroid hormone receptors, transcription factors, and kinases, some of which are involved in cancer progression. Hsp70 and Hsp90 form a multichaperone complex, in which both are connected by a third protein called Hop. The connection of and the interplay between the two chaperone machineries is of crucial importance for cell viability. This review provides a detailed view of the Hsp70 and Hsp90 machineries, their cofactors and their mode of regulation. It summarizes the current knowledge in the field, including the ATP-dependent regulation of the Hsp70/Hsp90 multichaperone cycle and elucidates the complex interplay and their synergistic interaction.

The Hsp90 Cochaperone p23 Is the Limiting Component of the Multiprotein Hsp90/Hsp70-based Chaperone System in Vivo Where It Acts to Stabilize the Client Protein·Hsp90 Complex

A variety of signaling proteins form heterocomplexes with and are regulated by the heat shock protein chaperone hsp90. These complexes are formed by a multiprotein machinery, including hsp90 and hsp70 as essential and abundant components and Hop, hsp40, and p23 as non-essential cochaperones that are present in much lower abundance in cells. Overexpression of signaling proteins can overwhelm the capacity of this machinery to properly assemble heterocomplexes with hsp90. Here, we show that the limiting component of this assembly machinery in vitro in reticulocyte lysate and in vivo in Sf9 cells is p23. Only a fraction of glucocorticoid receptors (GR) overexpressed in Sf9 cells are in heterocomplex with hsp90 and have steroid binding activity, with the majority of the receptors present as both insoluble and cytosolic GR aggregates. Coexpression of p23 with the GR increases the proportion of cytosolic receptors that are in stable GR.hsp90 heterocomplexes with steroid binding activity, a strictly hsp90-dependent activity for the GR. Coexpression of p23 eliminates the insoluble GR aggregates and shifts the cytosolic receptor from very large aggregates without steroid binding activity to approximately 600-kDa heterocomplexes with steroid binding activity. These data lead us to conclude that p23 acts in vivo to stabilize hsp90 binding to client protein.

Chronic stress attenuates glucocorticoid negative feedback: involvement of the prefrontal cortex and hippocampus

Disruption of the glucocorticoid negative feedback system is observed in approximate one half of human depressives, and a similar condition is induced in animals by chronic stress. This disruption is thought to involve down-regulation of glucocorticoid receptors (GRs) in the feedback sites of the brain. However, the responsible site of the brain has not been well elucidated. Here we examined the effects of chronic stress induced by water immersion and restraint (2 h/day) for 4 weeks followed by recovery for 10 days on the GR levels in the prefrontal cortex (PFC), hippocampus, and hypothalamus of rats using a Western immunoblot technique. In the PFC, the cytosolic GR levels were decreased, but the nuclear GR levels were not changed. In the hippocampus, the levels of cytosolic and nuclear GRs were increased. However, there were no marked changes in the GR levels in the hypothalamus. The changes in the cytosolic GR levels were confirmed at the mRNA level by an in situ hybridization technique. We next examined the suppressive effects of dexamethasone (DEX) infusions into these regions on the circulating corticosterone levels. When DEX was infused into the PFC or hippocampus of the chronically stressed rats, the suppressive response to DEX was abolished, but the response was normal in the hypothalamus. In addition, when DEX was injected systemically to the chronically stressed rats, the suppressive response to DEX was significantly attenuated. These results suggest that the abnormal changes in GRs in the higher centers of the hypothalamo-pituitary-adrenal axis are involved in the chronic stress-induced attenuation of the feedback. Since dysfunction of the PFC or hippocampus is implicated in the pathogenesis of depression, the present findings would help to understand the mechanisms underlying the disrupted feedback system and its relation to brain dysfunction in depression.

Visualization and mechanism of assembly of a glucocorticoid receptor.Hsp70 complex that is primed for subsequent Hsp90-dependent opening of the steroid binding cleft

A minimal system of five proteins, hsp90, hsp70, Hop, hsp40, and p23, assembles glucocorticoid receptor (GR).hsp90 heterocomplexes and causes the simultaneous opening of the steroid binding cleft to access by steroid. The first step in assembly is the ATP-dependent and hsp40 (YDJ-1)-dependent formation of a GR.hsp70 complex that primes the receptor for subsequent ATP-dependent activation by hsp90, Hop, and p23. This study focuses on three aspects of the GR priming reaction with hsp70. First, we have visualized the primed GR.hsp70 complexes by atomic force microscopy, and we find the most common stoichiometry to be 1:1, with some complexes of a size approximately 1:2 and a few complexes of larger size. Second, in a recent study of progesterone receptor priming, it was shown that hsp40 binds first, leading to the notion that it targets hsp70 to the receptor. We show here that hsp40 does not perform such a targeting function in priming the GR. Third, we focus on a short amino-terminal segment of the ligand binding domain that is required for GR.hsp90 heterocomplex assembly. By using two glutathione S-transferase (GST)/ligand binding domain fusions with (GST/520C) and without (GST/554C) hsp90 binding and steroid binding activity, we show that the priming step with hsp70 occurs with GST/554C, and it is the subsequent assembly step with hsp90 that is defective.

Sti1 is a novel activator of the Ssa proteins

The molecular chaperones Hsp70 and Hsp90 are involved in the folding and maturation of key regulatory proteins in eukaryotes. Of specific importance in this context is a ternary multichaperone complex in which Hsp70 and Hsp90 are connected by Hop. In Saccharomyces cerevisiae two components of the complex, yeast Hsp90 (yHsp90) and Sti1, the yeast homologue of Hop, had already been identified, but it remained to be shown which of the 14 different yeast Hsp70s are part of the Sti1 complex and what were the functional consequences resulting from this interaction. With a two-hybrid approach and co-immunoprecipitations, we show here that Sti1 specifically interacts with the Ssa group of the cytosolic yeast Hsp70 proteins. Using purified components, we reconstituted the dimeric Ssa1-Sti1 complex and the ternary Ssa1-Sti1-yHsp90 complex in vitro. The dissociation constant between Sti1 and Ssa1 was determined to be 2 orders of magnitude weaker than the affinity of Sti1 for yHsp90. Surprisingly, binding of Sti1 activates the ATPase of Ssa1 by a factor of about 200, which is in contrast to the behavior of Hop in the mammalian Hsp70 system. Analysis of the underlying activation mechanism revealed that ATP hydrolysis is rate-limiting in the Ssa1 ATPase cycle and that this step is accelerated by Sti1. Thus, Sti1 is a potent novel effector for the Hsp70 ATPase.

She4p/Dim1p interacts with the motor domain of unconventional myosins in the budding yeast, Saccharomyces cerevisiae

She4p/Dim1p, a member of the UNC-45/CRO1/She4p (UCS) domain-containing protein family, is required for endocytosis, polarization of actin cytoskeleton, and polarization of ASH1 mRNA in Saccharomyces cerevisiae. We show herein that She4p/Dim1p is involved in endocytosis and actin polarization through interactions with the type I myosins Myo3p and Myo5p. Two-hybrid and biochemical experiments showed that She4p/Dim1p interacts with the motor domain of Myo3/5p through its UCS domain. She4p/Dim1p was required for Myo5p localization to cortical patch-like structures. Using random mutagenesis of the motor region of MYO5, we identified four independent dominant point mutations that suppress the temperature-sensitive growth phenotype of the she4/dim1 null mutant. All of the amino acid substitutions caused by these mutations, V164I, N168I, N209S, and K377M, could suppress the defects of endocytosis and actin polarization of the she4/dim1 mutant as well. She4p/Dim1p also showed two-hybrid interactions with the motor domain of a type II myosin Myo1p and type V myosins Myo2p and Myo4p, and was required for proper localization of Myo4p, which regulates polarization of ASH1 mRNA. Our results suggest that She4p/Dim1p is required for structural integrity or regulation of the motor domain of unconventional myosins.

Essential role of the unusual DNA-binding motif of BAG-1 for inhibition of the glucocorticoid receptor

The co-chaperone BAG-1 is involved in the regulation of steroid hormone receptors, including the glucocorticoid receptor (GR). More recently, BAG-1 was found in the nucleus where it decreases GR transactivation. Moreover, nonspecific DNA binding of BAG-1 has been reported. We discovered that of the N-terminal part of BAG-1M, the first 8 amino acids are sufficient for DNA binding, containing a stretch of three lysines and a stretch of three arginines. Changing the spacing between these stretches had no effect on DNA binding. Surprisingly, this small, nonsequence-specific DNA binding domain was nonetheless necessary for the inhibitory function of BAG-1 for GR-dependent transcription, whereas the following serine- and threonine-rich E(2)X(4) repeat domain was not. Mutational analysis of these two domains revealed that only mutants retaining DNA binding capability were able to down-regulate GR-mediated transactivation. Intriguingly, lack of DNA binding could not be functionally rescued by BAG-1M harboring a point mutation abolishing interaction with hsp70. Thus, DNA binding and hsp70 interaction are required in cis. We propose that the nonsequence-specific DNA-binding protein BAG-1 acts at specific chromosomal loci by interacting with other proteins.

Regulation of signaling protein function and trafficking by the hsp90/hsp70-based chaperone machinery

Nearly 100 proteins are known to be regulated by hsp90. Most of these substrates or "client proteins" are involved in signal transduction, and they are brought into complex with hsp90 by a multiprotein hsp90/hsp70-based chaperone machinery. In addition to binding substrate proteins at the chaperone site(s), hsp90 binds cofactors at other sites that are part of the heterocomplex assembly machinery as well as immunophilins that connect assembled substrate*hsp90 complexes to protein-trafficking systems. In the 5 years since we last reviewed this subject, much has been learned about hsp90 structure, nucleotide-binding, and cochaperone interactions; the most important concept is that ATP hydrolysis by an intrinsic ATPase activity results in a conformational change in hsp90 that is required to induce conformational change in a substrate protein. The conformational change induced in steroid receptors is an opening of the steroid-binding cleft so that it can be accessed by steroid. We have now developed a minimal system of five purified proteins-hsp90, hsp70, Hop, hsp40, and p23- that assembles stable receptor*hsp90 heterocomplexes. An hsp90*Hop*hsp70*hsp40 complex opens the cleft in an ATP-dependent process to produce a receptor*hsp90 heterocomplex with hsp90 in its ATP-bound conformation, and p23 then interacts with the hsp90 to stabilize the complex. Stepwise assembly experiments have shown that hsp70 and hsp40 first interact with the receptor in an ATP-dependent reaction to produce a receptor*hsp70*hsp40 complex that is "primed" to be activated to the steroid-binding state in a second ATP-dependent step with hsp90, Hop, and p23. Successful use of the five-protein system with other substrates indicates that it can assemble signal protein*hsp90 heterocomplexes whether the substrate is a receptor, a protein kinase, or a transcription factor. This purified system should facilitate understanding of how eukaryotic hsp70 and hsp90 work together as essential components of a process that alters the conformations of substrate proteins to states that respond in signal transduction.

Nucleotide binding states of hsp70 and hsp90 during sequential steps in the process of glucocorticoid receptor.hsp90 heterocomplex assembly

A minimal system of five purified proteins, hsp90, hsp70, Hop, hsp40, and p23, assembles glucocorticoid receptor (GR).hsp90 heterocomplexes and causes the simultaneous opening of the steroid binding cleft to access by steroid. The first step in assembly is the ATP-dependent and hsp40 (YDJ-1)-dependent binding of hsp70 to the GR, which primes the receptor for subsequent ATP-dependent activation by hsp90, Hop, and p23 (Morishima, Y., Murphy, P. J. M., Li, D. P., Sanchez, E. R., and Pratt, W. B. (2000) J. Biol. Chem. 275, 18054-18060). Here we have examined the nucleotide-bound states of the two essential chaperones in each step. We show that it is the ATP-bound state of hsp70 that interacts initially with the GR. After rapid priming and washing, the primed GR.hsp70 complex rapidly binds hsp90 in the second step reaction in a nucleotide-independent manner. The rate-limiting step is the ATP-dependent opening of the steroid binding cleft after hsp90 binding. This activating step requires the N-terminal ATP-binding site of hsp90, but we cannot establish any role for a C-terminal ATP-binding site in steroid binding cleft opening. The reported specific inhibitors of the C-terminal ATP site on hsp90 inhibit the generation of steroid binding, but they have other effects in this multiprotein system that could explain the inhibition.

Allosteric effects of dexamethasone and RU486 on glucocorticoid receptor-DNA interactions

The glucocorticoid receptor (GR) is a DNA-binding protein that can regulate the transcription of a large number of genes in a ligand-dependent fashion. Although much progress has been made on the mechanism of transcriptional regulation by GR, a potential allosteric effect of GR-binding ligands on specific GR-DNA interactions is controversial. In this study, gel-shift methods are used to measure the effects of a classical agonist dexamethasone and a prototypical antagonist RU486 on the in vitro interactions of GR with DNA substrates, which contain glucocorticoid response elements (GREs) from promoters of GR-regulated genes. These studies show that cell extracts containing human GR bind specifically and with high affinity to GREs in the absence of ligand. An agonist dexamethasone and antagonist RU486 do not affect the affinity of GR for DNA but subtly alter the electrophoretic mobility of the GR-DNA complex. Importantly, the dissociation rate of GR from DNA increases as a function of the concentration of GRE-containing DNA. At a fixed DNA concentration, dexamethasone-bound GR dissociates from DNA significantly faster than does ligand-free GR or RU486-bound GR. These results are consistent with a model for transcriptional activation in which a dynamic complex is formed between agonist-bound GR and DNA.

A high-throughput fluorescent polarization assay for nuclear receptor binding utilizing crude receptor extract

A homogenous high-throughput assay has been developed to measure the binding between nuclear receptors and test compounds. This assay applies a fluorescence polarization (FP) detection method using human glucocorticoid receptor (GR) as a model system. Crude receptor extract, which requires no additional purification, is used in the assay. The binding conditions (i.e., DMSO tolerance, temperature, stability, and variability) have been investigated and validated. At the optimized conditions, a signal-to-background ratio of 2:1 and a Z'-factor of 0.7 was achieved in a 384-well format. Several known strong and weak GR ligands have been evaluated in this system. Possible interference of fluorescent compounds and methods to identify false positives are also discussed. This FP-based assay system can potentially be used for many soluble nuclear receptors in high-throughput binding assays.

Antidepressants enhance glucocorticoid receptor function in vitro by modulating the membrane steroid transporters

1. Previous data demonstrate that the tricyclic antidepressant, desipramine, induces glucocorticoid receptor (GR) translocation from the cytoplasm to the nucleus in L929 cells and increases dexamethasone-induced GR-mediated gene transcription in L929 cells stably transfected with the mouse mammary tumour virus-chloramphenicol acetyltransferase (MMTV-CAT) reporter gene (LMCAT cells) (Pariante et al., 1997). 2. To extend these findings, the present study has investigated the effects of 24 h coincubation of LMCAT cells with dexamethasone and amitriptyline, clomipramine, paroxetine, citalopram or fluoxetine. 3. All antidepressants, except fluoxetine, enhanced GR-mediated gene transcription, with clomipramine having the greatest effect (10 fold increase). Twenty-four hours coincubation of cells with desipramine, clomipramine or paroxetine, also enhanced GR function in the presence of cortisol, but not of corticosterone. 4. It is proposed that these effects are due to the antidepressants inhibiting the L929 membrane steroid transporter, which actively extrudes dexamethasone and cortisol from the cell, but not corticosterone. This is further confirmed by the fact that clomipramine failed to enhance GR-mediated gene transcription in the presence of dexamethasone when the membrane steroid transporter was blocked by verapamil. 5. The membrane steroid transporters that regulate access of glucocorticoids to the brain in vivo, like the multiple drug resistance p-glycoprotein, could be a fundamental target for antidepressant action.

Stoichiometry, abundance, and functional significance of the hsp90/hsp70-based multiprotein chaperone machinery in reticulocyte lysate

Rabbit reticulocyte lysate contains a multiprotein chaperone system that assembles the glucocorticoid receptor (GR) into a complex with hsp90 and converts the hormone binding domain of the receptor to its high affinity steroid binding state. This system has been resolved into five proteins, with hsp90 and hsp70 being essential and Hop, hsp40, and p23 acting as co-chaperones that optimize assembly. Hop binds independently to hsp70 and hsp90 to form an hsp90.Hop.hsp70 complex that acts as a machinery to open up the GR steroid binding site. Because purified hsp90 and hsp70 are sufficient for some activation of GR steroid binding activity, some investigators have rejected any role for Hop in GR.hsp90 heterocomplex assembly. Here, we counter that impression by showing that all of the Hop in reticulocyte lysate is present in an hsp90.Hop.hsp70 complex with a stoichiometry of 2:1:1. The complex accounts for approximately 30% of the hsp90 and approximately 9% of the hsp70 in lysate, and upon Sephacryl S-300 chromatography the GR.hsp90 assembly activity resides in the peak containing Hop-bound hsp90. Consistent with the notion that the two essential chaperones cooperate with each other to open up the steroid binding site, we also show that purified hsp90 and hsp70 interact directly with each other to form weak hsp90.hsp70 complexes with a stoichiometry of 2:1.

Glucocorticoid receptors in major depression: relevance to pathophysiology and treatment

Hyperactivity of the hypothalamic--pituitary--adrenal (HPA) axis has been reliably observed in patients with major depression. One of the primary features of this HPA axis hyperactivity is reduced sensitivity to the inhibitory effects of the glucocorticoid dexamethasone on the production of adrenocorticotropic hormone and cortisol during the dexamethasone suppression test and, more recently, the dexamethasone--corticotropin-releasing hormone test. Because the effects of glucocorticoids are mediated by intracellular receptors including, most notably, the glucocorticoid receptor (GR), a number of studies have considered the possibility that the number and/or function of GRs are reduced in depressed patients. Moreover, whether antidepressants act by reversing these putative GR changes has been examined. The extant literature on GR receptors in major depression was reviewed along with studies examining the impact of antidepressants on the GR. The data support the hypothesis that the function of the GR is reduced in major depression in the absence of clear evidence of decreased GR expression. The data also indicate that some antidepressants have direct effects on the GR, leading to enhanced GR function and increased GR expression. Hypotheses regarding the mechanism of these receptor changes involve relevant second messenger pathways that regulate GR function. The findings indicate that the GR is an important molecular target in major depression. Further elucidation of the biochemical and molecular mechanisms involved in GR changes in major depression is an exciting frontier that will no doubt lead to new insights into the pathophysiology and treatment of affective disorders.

Evidence for iterative ratcheting of receptor-bound hsp70 between its ATP and ADP conformations during assembly of glucocorticoid receptor.hsp90 heterocomplexes

hsp90 and hsp70 are essential components of a five-protein system, including also the nonessential cochaperones Hop, hsp40, and p23, that assembles glucocorticoid receptor (GR).hsp90 heterocomplexes and causes the simultaneous opening of the steroid binding pocket to access by steroid. The first event in assembly is the ATP-dependent and hsp40 (YDJ-1)-dependent binding of hsp70 to the GR, which primes the receptor for subsequent ATP-dependent activation by hsp90 [Morishima, Y., Murphy, P. J. M., Li, D. P., Sanchez, E. R., and Pratt, W. B. (2000) J. Biol. Chem. 275, 18054-18060]. Here, we demonstrate that, during the priming step, ATP-bound hsp70 is converted to GR-bound hsp70 that is approximately 1/3 in the ADP- and approximately 2/3 in the ATP-dependent conformation. In the second step, hsp90, which is provided in the non-nucleotide-bound state, is converted to GR-bound hsp90 in the ATP-dependent conformation. The ATPase activity of hsp70 is K(+)-dependent, and the priming step is K(+)-dependent. Surprisingly, the subsequent hsp90-dependent step, which is rate-limiting for receptor activation, is also potassium-dependent. This suggests that GR-bound hsp70 is also converted from the ATP-dependent to the ADP-dependent conformation while it cooperates with hsp90 to activate steroid binding activity. Because the priming step requires both sustained high levels of ATP and YDJ-1 for optimal activity and because both steps require potassium, we predict that receptor-bound hsp70 undergoes iterative ratcheting between its ATP- and ADP-dependent conformations in opening the hydrophobic steroid binding pocket.

The co-chaperone CHIP regulates protein triage decisions mediated by heat-shock proteins

To maintain quality control in cells, mechanisms distinguish among improperly folded peptides, mature and functional proteins, and proteins to be targeted for degradation. The molecular chaperones, including heat-shock protein Hsp90, have the ability to recognize misfolded proteins and assist in their conversion to a functional conformation. Disruption of Hsp90 heterocomplexes by the Hsp90 inhibitor geldanamycin leads to substrate degradation through the ubiquitin-proteasome pathway, implicating this system in protein triage decisions. We previously identified CHIP (carboxyl terminus of Hsc70-interacting protein) to be an interaction partner of Hsc70 (ref. 4). CHIP also interacts directly with a tetratricopeptide repeat acceptor site of Hsp90, incorporating into Hsp90 heterocomplexes and eliciting release of the regulatory cofactor p23. Here we show that CHIP abolishes the steroid-binding activity and transactivation potential of the glucocorticoid receptor, a well-characterized Hsp90 substrate, even though it has little effect on its synthesis. Instead, CHIP induces ubiquitylation of the glucocorticoid receptor and degradation through the proteasome. By remodelling Hsp90 heterocomplexes to favour substrate degradation, CHIP modulates protein triage decisions that regulate the balance between protein folding and degradation for chaperone substrates.

Role of HSP90 in salt stress tolerance via stabilization and regulation of calcineurin

The role of HSP90 in stress tolerance was investigated in Saccharomyces cerevisiae. Cells showing 20-fold overexpression of Hsc82, an HSP90 homologue in yeast, were hypersensitive to high-NaCl or H-LiCl stresses. Hsc82-overexpressing cells appeared similar to calcineurin-defective cells in salt sensitivity and showed reduced levels of calcineurin-dependent gene expression. Co-overexpression of Cna2, the catalytic subunit of calcineurin, suppressed the hypersensitivity. Cna2 and Hsc82 coimmunoprecipitated from control cells grown under normal conditions but not from stressed cells. In contrast, coimmunoprecipitation was detected with Hsc82-overexpressing cells even after exposure to stresses. Cna2 immune complexes from stressed control cells showed a significant level of calcineurin activity, whereas those from stressed Hsc82-overexpressing cells did not. Treatment of extracts from Hsc82-overexpressing cells with Ca(2+)-calmodulin increased the calcineurin activity associated with Cna2 immune complexes. Geldanamycin, an inhibitor of HSP90 abolished the coimmunoprecipitation but did not activate calcineurin. When the expression level of Hsc82 decreased to below 30% of the normal level, cells also became hypersensitive to salt stress. In these cells, the amount of Cna2 was reduced, likely as a result of degradation. The present results showed that Hsc82 binds to and stabilizes Cna2 and that dissociation of Cna2 from Hsc82 is necessary for its activation.

Western blotting for detection of glucocorticoid receptors in the brain and pituitary gland from adrenal intact pigs

The objectives of the experiments presented here were to develop a method for measuring total glucocorticoid receptors (GRs), in adrenal intact pigs, and to describe the distribution of this receptor in the pituitary gland and selected brain regions. Six male pigs, 14 days of age, were euthanized and the pituitary gland, hypothalamus, prefrontal cortex, and hippocampus were prepared for western blotting using a traditional soluble cytosolic preparation or a whole cell extract preparation. Samples were electrophoresed and immunoblotted using the anti-human GR antibodies #57 and #51. The #57 antibody labeled a doublet at 95 and 87 kDa whereas the #51 GR antibody labeled a single band at 87 kDa. Affinity labeling of GR with [(3)H] dexamethasone mesylate revealed a single band at 87 kDa. Using the soluble cytosolic preparation GR concentrations were highest in the pituitary gland followed by the hypothalamus (P<0. 0001). The amount of GR in the hippocampus and prefrontal cortex was equivalent but lesser (P<0.0001) than that found in the pituitary gland or hypothalamus. With the whole cell extract tissue preparation, GR concentrations were highest (P<0.05) in the prefrontal cortex and hippocampus with lower (P<0.05) levels detected in the hypothalamus. Taken together, these results indicated that the 87-kDa band was the functional GR and that regional distribution of GRs is specific to the type of tissue preparation used. In addition, we have described a procedure (from whole cell extracts) for measuring total GRs in neural and nonneural tissues from adrenal intact pigs.

Stepwise assembly of a glucocorticoid receptor.hsp90 heterocomplex resolves two sequential ATP-dependent events involving first hsp70 and then hsp90 in opening of the steroid binding pocket

A system of five purified proteins that assembles stable glucocorticoid receptor (GR)-hsp90 heterocomplexes has been reconstituted from reticulocyte lysate. Two proteins, hsp90 and hsp70, are required for the activation of steroid binding activity that occurs with heterocomplex assembly, and three proteins, Hop, hsp40, p23, act as co-chaperones that enhance activation and assembly (Morishima, Y., Kanelakis, K. C., Silverstein, A.M., Dittmar, K. D., Estrada, L., and Pratt, W. B. (2000) J. Biol. Chem. 275, 6894-6900). Here we demonstrate that the first step in assembly is the ATP-dependent and hsp40 (YDJ-1)-dependent binding of hsp70 to the GR. After elimination of free hsp70, these preformed GR.hsp70 complexes can be activated to the steroid binding state by the hsp70 free assembly system in a second ATP-dependent step. hsp90 is required for opening of the steroid binding pocket and is converted to its ATP-dependent conformation during this second step. We predict that hsp70 in its ATP-dependent conformation binds initially to the folded receptor and is then converted to the ADP-dependent form with high affinity for hydrophobic substrate. This conversion initiates the opening of the hydrophobic steroid binding pocket such that it can now accept the hydrophobic binding form of hsp90, which in turn must be converted to its ATP-dependent conformation for the pocket to be accessible by steroid.

Role of molecular chaperones in subnuclear trafficking of glucocorticoid receptors

The delivery of activated steroid receptors to high-affinity genomic sites must be efficient enough to account for the rapidity and selectivity of many transcriptional responses to steroid hormones. Thus, the signal transduction capacity of steroid hormone receptors will be influenced by the efficiency of receptor trafficking both between different subcellular compartments (that is, the cytoplasm and nucleus) and within a specific compartment (that is, the nucleus). Molecular chaperones, such as heat shock proteins, have long been recognized to play important roles in the management of protein folding in both stressed and nonstressed cells. In recent years, the participation of these proteins in various signal transduction pathways (for example, steroid hormone responses) has also been recognized. In this review, recent results that implicate a role for distinct heat shock proteins in subnuclear trafficking of glucocorticoid receptors are discussed. These studies also highlight the importance of mobilizing the cellular chaperone machinery for managing steroid receptor folding within the nucleus.

The Hsp organizer protein hop enhances the rate of but is not essential for glucocorticoid receptor folding by the multiprotein Hsp90-based chaperone system

A system consisting of five purified proteins: Hsp90, Hsp70, Hop, Hsp40, and p23, acts as a machinery for assembly of glucocorticoid receptor (GR).Hsp90 heterocomplexes. Hop binds independently to Hsp90 and to Hsp70 to form a Hsp90.Hop.Hsp70.Hsp40 complex that is sufficient to convert the GR to its steroid binding form, and this four-protein complex will form stable GR.Hsp90 heterocomplexes if p23 is added to the system (Dittmar, K. D., Banach, M., Galigniana, M. D., and Pratt, W. B. (1998) J. Biol. Chem. 273, 7358-7366). Hop has been considered essential for the formation of receptor.Hsp90 heterocomplexes and GR folding. Here we use Hsp90 and Hsp70 purified free of all traces of Hop and Hsp40 to show that Hop is not required for GR.Hsp90 heterocomplex assembly and activation of steroid binding activity. Rather, Hop enhances the rate of the process. We also show that Hsp40 is not essential for GR folding by the five-protein system but enhances a process that occurs less effectively when it is not present. By carrying out assembly in the presence of radiolabeled steroid to bind to the GR as soon as it is converted to the steroid binding state, we show that the folding change is brought about by only two essential components, Hsp90 and Hsp70, and that Hop, Hsp40, and p23 act as nonessential co-chaperones.

Chaperones in progesterone receptor complexes

Studying the components, pathways, and dynamics of progesterone receptor (PR) assembly with chaperones has provided a highly valuable model system for understanding the coordinate actions of chaperones. Chaperones are primarily adapted to facilitate protein folding processes, and the actions of chaperones toward PR and other steroid receptors probably remain within this general functional boundary. Unlike a typical misfolded protein substrate, PR's folding is effectively arrested prior to hormone binding, thus extending indefinitely the chaperone-interaction phase that normally would be transitory during progressive protein folding. While one could consider this a limitation in PR's ability to fold properly, perhaps a more accurate view is that PR is specially adapted to remain 'misfolded', and thus extend chaperone interactions that function efficiently in repressing PR's transcriptional activity while the receptor awaits an activating signal.

The seven amino acids (547-553) of rat glucocorticoid receptor required for steroid and hsp90 binding contain a functionally independent LXXLL motif that is critical for steroid binding

Hsp90 association with glucocorticoid receptors (GRs) is required for steroid binding. We recently reported that seven amino acids (547-553) overlapping the amino-terminal end of the rat GR ligand-binding domain are necessary for hsp90 binding, and consequently steroid binding. The role of a LXXLL motif at the COOH terminus of this sequence has now been analyzed by determining the properties of Leu to Ser mutations in full-length GR and glutathione S-transferase chimeras. Surprisingly, these mutations decreased steroid binding capacity without altering receptor levels, steroid binding affinity, or hsp90 binding. Single mutations in the context of the full-length receptor did not affect the transcriptional activity but the double mutant (L550S/L553S) was virtually inactive. This biological inactivity was found to be due to an increased rate of steroid dissociation from the activated mutant complex. These results, coupled with those from trypsin digestion studies, suggest a model in which the GR ligand-binding domain is viewed as having a "hinged pocket," with the hinge being in the region of the trypsin digestion site at Arg(651). The pocket would normally be kept shut via the intramolecular interactions of the LXXLL motif at amino acids 550-554 acting as a hydrophobic clasp.