ATP induces a conformational change of the 90-kDa heat shock protein (hsp90)

Extracellular Hsp90 Binds to and Aligns Collagen-1 to Enhance Breast Cancer Cell Invasiveness

Cancer cell-secreted eHsp90 binds and activates proteins in the tumor microenvironment crucial in cancer invasion. Therefore, targeting eHsp90 could inhibit invasion, preventing metastasis-the leading cause of cancer-related mortality. Previous eHsp90 studies have solely focused on its role in cancer invasion through the 2D basement membrane (BM), a form of extracellular matrix (ECM) that lines the epithelial compartment. However, its role in cancer invasion through the 3D Interstitial Matrix (IM), an ECM beyond the BM, remains unexplored. Using a Collagen-1 binding assay and second harmonic generation (SHG) imaging, we demonstrate that eHsp90 directly binds and aligns Collagen-1 fibers, the primary component of IM. Furthermore, we show that eHsp90 enhances Collagen-1 invasion of breast cancer cells in the Transwell assay. Using Hsp90 conformation mutants and inhibitors, we established that the Hsp90 dimer binds to Collagen-1 via its N-domain. We also demonstrated that while Collagen-1 binding and alignment are not influenced by Hsp90's ATPase activity attributed to the N-domain, its open conformation is crucial for increasing Collagen-1 alignment and promoting breast cancer cell invasion. These findings unveil a novel role for eHsp90 in invasion through the IM and offer valuable mechanistic insights into potential therapeutic approaches for inhibiting Hsp90 to suppress invasion and metastasis.

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.

Structure of Hsp90-p23-GR reveals the Hsp90 client-remodelling mechanism

Hsp90 is a conserved and essential molecular chaperone responsible for the folding and activation of hundreds of 'client' proteins^1-3. The glucocorticoid receptor (GR) is a model client that constantly depends on Hsp90 for activity^4-9. GR ligand binding was previously shown to nr inhibited by Hsp70 and restored by Hsp90, aided by the co-chaperone p23¹⁰. However, a molecular understanding of the chaperone-mediated remodelling that occurs between the inactive Hsp70-Hsp90 'client-loading complex' and an activated Hsp90-p23 'client-maturation complex' is lacking for any client, including GR. Here we present a cryo-electron microscopy (cryo-EM) structure of the human GR-maturation complex (GR-Hsp90-p23), revealing that the GR ligand-binding domain is restored to a folded, ligand-bound conformation, while being simultaneously threaded through the Hsp90 lumen. In addition, p23 directly stabilizes native GR using a C-terminal helix, resulting in enhanced ligand binding. This structure of a client bound to Hsp90 in a native conformation contrasts sharply with the unfolded kinase-Hsp90 structure¹¹. Thus, aided by direct co-chaperone-client interactions, Hsp90 can directly dictate client-specific folding outcomes. Together with the GR-loading complex structure¹², we present the molecular mechanism of chaperone-mediated GR remodelling, establishing the first, to our knowledge, complete chaperone cycle for any Hsp90 client.

Heat shock protein 90 kDa (Hsp90) from Aedes aegypti has an open conformation and is expressed under heat stress

Cellular proteostasis is maintained by a system consisting of molecular chaperones, heat shock proteins (Hsps) and proteins involved with degradation. Among the proteins that play important roles in the function of this system is Hsp90, which acts as a node of this network, interacting with at least 10% of the proteome. Hsp90 is ATP-dependent, participates in critical cell events and protein maturation and interacts with large numbers of co-chaperones. The study of Hsp90 orthologs is justified by their differences in ATPase activity levels and conformational changes caused by Hsp90 interaction with nucleotides. This study reports the characterization of Hsp90 from Aedes aegypti, a vector of several diseases in many regions of the planet. Aedes aegypti Hsp90, AaHsp90, was cloned, purified and characterized for its ATPase and chaperone activities and structural conformation. These parameters indicate that it has the characteristics of eukaryotic Hsp90s and resembles orthologs from yeast rather than from human. Finally, constitutive and increased stress expression in Aedes cells was confirmed. Taken together, the results presented here help to understand the relationship between structure and function in the Hsp90 family and have strong potential to form the basis for studies on the network of chaperone and Hsps in Aedes.

Interaction of the middle domains stabilizes Hsp90α dimer in a closed conformation with high affinity for p23

The human genome encodes two highly similar cytosolic Hsp90 proteins called isoforms Hsp90α and Hsp90β. Of the 300 client proteins for Hsp90 identified so far only a handful interact specifically with one Hsp90 isoform. Here we report for the first time that Hsp90 cochaperone p23 binds preferentially to Hsp90α and that this interaction is mediated by the middle domain of Hsp90α. Based on the homology modeling, we infer that the middle domains in the Hsp90α dimer bind stronger with each other than in the Hsp90β dimer. Therefore, compared to Hsp90β, Hsp90α may adopt closed conformation more easily. Hsp90 interacts with p23 in the closed conformation. Hsp90α binds human recombinant p23 about three times stronger than Hsp90β but with significantly smaller exothermic enthalpy as determined by isothermal titration calorimetry of direct binding between the purified proteins. As p23 binds to Hsp90 in a closed conformation, stabilization of the Hsp90α dimer in the closed conformation by its middle domains explains preference of p23 to this Hsp90 isoform.

Hsp90 Stabilizes SIRT1 Orthologs in Mammalian Cells and C. elegans

Sirtuin 1 (SIRT1) othologs are ubiquitous NAD⁺-dependent deacetylases that act as nutrient sensors and modulate metabolism and stress responses in diverse organisms. Both mammalian SIRT1 and Caenorhabditis elegans SIR-2.1 have been implicated in dietary restriction, longevity, and healthspan. Hsp90 is an evolutionarily conserved molecular chaperone that stabilizes a plethora of signaling ’client’ proteins and regulates fundamental biological processes. Here we report that Hsp90 is required for conformational stabilization of SIRT1 and SIR-2.1. We find that inhibition of Hsp90 by geldanamycin (GA) induces the depletion of mammalian SIRT1 protein in a concentration and time dependent manner in COS-7 and HepG2 cells. In contrast to SIRT1, SIRT2 level remains unchanged by GA treatment, reflecting a specific Hsp90 SIRT1 interaction. Hsp90 inhibition leads to the destabilization and proteasomal degradation of SIRT1. Moreover, we observe a GA-sensitive physical interaction between SIRT1 and Hsp90 by immunoprecipitation. We also demonstrate that hsp-90 gene silencing also induces SIR-2.1 protein depletion and proteasomal degradation in C. elegans. Our findings identify metazoan SIRT1 orthologs as Hsp90 clients and reveal a novel crosstalk between the proteostasis and nutrient signaling networks, which may have implications in various age related diseases.

Chaperome heterogeneity and its implications for cancer study and treatment

The chaperome is the collection of proteins in the cell that carry out molecular chaperoning functions. Changes in the interaction strength between chaperome proteins lead to an assembly that is functionally and structurally distinct from each constituent member. In this review, we discuss the epichaperome, the cellular network that forms when the chaperome components of distinct chaperome machineries come together as stable, functionally integrated, multimeric complexes. In tumors, maintenance of the epichaperome network is vital for tumor survival, rendering them vulnerable to therapeutic interventions that target critical epichaperome network components. We discuss how the epichaperome empowers an approach for precision medicine cancer trials where a new target, biomarker, and relevant drug candidates can be correlated and integrated. We introduce chemical biology methods to investigate the heterogeneity of the chaperome in a given cellular context. Lastly, we discuss how ligand-protein binding kinetics are more appropriate than equilibrium binding parameters to characterize and unravel chaperome targeting in cancer and to gauge the selectivity of ligands for specific tumor-associated chaperome pools.

Adapting to stress - chaperome networks in cancer

In this Opinion article, we aim to address how cells adapt to stress and the repercussions chronic stress has on cellular function. We consider acute and chronic stress-induced changes at the cellular level, with a focus on a regulator of cellular stress, the chaperome, which is a protein assembly that encompasses molecular chaperones, co-chaperones and other co-factors. We discuss how the chaperome takes on distinct functions under conditions of stress that are executed in ways that differ from the one-on-one cyclic, dynamic functions exhibited by distinct molecular chaperones. We argue that through the formation of multimeric stable chaperome complexes, a state of chaperome hyperconnectivity, or networking, is gained. The role of these chaperome networks is to act as multimolecular scaffolds, a particularly important function in cancer, where they increase the efficacy and functional diversity of several cellular processes. We predict that these concepts will change how we develop and implement drugs targeting the chaperome to treat cancer.

Stimulation of the ATPase activity of Hsp90 by zerumbone modification of its cysteine residues destabilizes its clients and causes cytotoxicity

Hsp90 is an ATP-dependent molecular chaperone that assists folding and conformational maturation/maintenance of many proteins. It is a potential cancer drug target because it chaperones oncoproteins. A prokaryotic homolog of Hsp90 (HtpG) is essential for thermo-tolerance in some bacteria and virulence of zoonotic pathogens. To identify a new class of small molecules which target prokaryotic and eukaryotic Hsp90s, we studied the effects of a naturally occurring cyclic sesquiterpene, zerumbone, which inhibits proliferation of a wide variety of tumor cells, on the activity of Hsp90. Zerumbone enhanced the ATPase activity of cyanobacterial Hsp90 (Hsp90SE), yeast Hsp90, and human Hsp90α. It also enhanced the catalytic efficiency of Hsp90SE by greatly increasing kcat. Mass analysis showed that zerumbone binds to cysteine side chains of Hsp90SE covalently. Mutational studies identified 3 cysteine residues (one per each domain of Hsp90SE) that are involved in the enhancement, suggesting the presence of allosteric sites in the middle and C-terminal domains of Hsp90SE. Treatment of cyanobacterial cells with zerumbone caused them to become very temperature-sensitive, a phenotype reminiscent of cyanobacterial Hsp90 mutants, and also decreased the cellular level of linker polypeptides that are clients for Hsp90SE. Zerumbone showed cellular toxicity on cancer-derived mammalian cells by inducing apoptosis. In addition, zerumbone inhibited the binding of Hsp90/Cdc37 to client kinases. Altogether, we conclude that modification of cysteine residues of Hsp90 by zerumbone enhances its ATPase activity and inhibits physiological Hsp90 function. The activation of Hsp90 may provide new strategies to inhibit its chaperone function in cells.

Geranylgeranylacetone induction of HSP90α exerts cryoprotective effect on Acipenser sinensis sperm

Heat Shock Protein 90 (HSP90) is a fertility-associated protein, the expression of which positively correlates with sperm quality in many species. Geranylgeranylacetone (GGA) is reported to induce expression of HSP90. The present study aimed to investigate whether GGA induced expression of HSP90 in Acipenser sinensis sperm to exert a cryoprotective effect. Sperm from five male A. sinensis was combined with extender containing 20 mmol/L tris pH = 8.1, 10% v/v methanol, 2-5 mmol/L KCl, 15 mmol/L lactose, and 15 mmol/L trehalose, with GGA at 0, 14, 67, 135, 673, 1346, or 6731 μmol/L. After cryopreservation and thawing, the percentage of motile spermatozoa, spermatozoon curvilinear velocity (VCL), straight-line velocity (VSL), average path velocity (VAP), acrosome integrity, and membrane integrity, as well as fertility were evaluated. Sperm quality increased with the increase of GGA to 673 μmol/L, but decreased at higher concentrations. Expression levels of HSP90α were detected by Western blot in sperm frozen with GGA at 673 μmol/L (highest obtained sperm quality), 6731 μmol/L (highest GGA concentration), and a control without GGA. The expression of HSP90α increased with the increase in GGA, with lowest expression observed in the control. GGA was found to induce increase of HSP90α, and this increase was associated with higher quality cryopreserved sperm at concentrations ≤673 μmol/L. This research suggests a viable technique to increase the quality of cryopreserved A. sinensis sperm by adding GGA to induce expression of HSP90α.

Hsp90 N- and C-terminal double inhibition synergistically suppresses Bcr-Abl-positive human leukemia cells

Heat shock protein 90 (Hsp90) contains amino (N)–terminal domain, carboxyl(C)-terminal domain, and middle domains, which activate Hsp90 chaperone function cooperatively in tumor cells. One terminal occupancy might influence another terminal binding with inhibitor. The Bcr-Abl kinase is one of the Hsp90 clients implicated in the pathogenesis of chronic myeloid leukemia (CML). Present studies demonstrate that double inhibition of the N- and C-terminal termini can disrupt Hsp90 chaperone function synergistically, but not antagonistically, in Bcr-Abl-positive human leukemia cells. Furthermore, both the N-terminal inhibitor 17-AAG and the C-terminal inhibitor cisplatin (CP) have the capacity to suppress progenitor cells; however, only CP is able to inhibit leukemia stem cells (LSCs) significantly, which implies that the combinational treatment is able to suppress human leukemia in different mature states.

Characterization of CNL like protein fragment (CNL-LPF) from mature Lageneria siceraria seeds

Coiled coil domain-nucleotide binding site-leucine rich repeat (CC-NBS-LRR; CNL) proteins are highly conserved family of plant disease resistance proteins, remarkably comprise of coiled-coil domain, which plays significant role in plant innate immunity. The present study reports that moderately elicited oligomerization of plant CNL like protein fragment (CNL-LPF) in presence of ATP/Mg using various biophysical methods Circular dichroism (CD) results depicted a substantial increase in β-sheet structure content of CNL-LPF. ATP/Mg induced conformational change in protein was observed by increase in blue shift with extrinsic fluorescence measurement, which indicates the exposure of hydrophobic regions of CNL-LPF and leads to self-association i.e. oligomerization. Likewise, cluster of protein oligomer and alteration in protein surface morphology were observed in presence of ATP/Mg by Transmission electron microscopy (TEM) and Atomic force microscopy (AFM), respectively. Also, augmented antiproliferation of HT1376 cells (urinary bladder cancer cell lines) was observed by CNL-LPF in presence of ATP/Mg. In conclusion, the current study illustrates that extent of CNL-LPF oligomerization was enhanced in presence of ATP/Mg (as compared to its absence). Utilization of enhanced oligomerization property of CNL-LPF as an anti-proliferative agent needs more assessment.

Mechanisms of Hsp90 regulation

Heat shock protein 90 (Hsp90) is a molecular chaperone that is involved in the activation of disparate client proteins. This implicates Hsp90 in diverse biological processes that require a variety of co-ordinated regulatory mechanisms to control its activity. Perhaps the most important regulator is heat shock factor 1 (HSF1), which is primarily responsible for upregulating Hsp90 by binding heat shock elements (HSEs) within Hsp90 promoters. HSF1 is itself subject to a variety of regulatory processes and can directly respond to stress. HSF1 also interacts with a variety of transcriptional factors that help integrate biological signals, which in turn regulate Hsp90 appropriately. Because of the diverse clientele of Hsp90 a whole variety of co-chaperones also regulate its activity and some are directly responsible for delivery of client protein. Consequently, co-chaperones themselves, like Hsp90, are also subject to regulatory mechanisms such as post translational modification. This review, looks at the many different levels by which Hsp90 activity is ultimately regulated.

The Hsp70/Hsp90 Chaperone Machinery in Neurodegenerative Diseases

The accumulation of misfolded proteins in the human brain is one of the critical features of many neurodegenerative diseases, including Alzheimer's disease (AD). Assembles of beta-amyloid (Aβ) peptide—either soluble (oligomers) or insoluble (plaques) and of tau protein, which form neurofibrillary tangles, are the major hallmarks of AD. Chaperones and co-chaperones regulate protein folding and client maturation, but they also target misfolded or aggregated proteins for refolding or for degradation, mostly by the proteasome. They form an important line of defense against misfolded proteins and are part of the cellular quality control system. The heat shock protein (Hsp) family, particularly Hsp70 and Hsp90, plays a major part in this process and it is well-known to regulate protein misfolding in a variety of diseases, including tau levels and toxicity in AD. However, the role of Hsp90 in regulating protein misfolding is not yet fully understood. For example, knockdown of Hsp90 and its co-chaperones in a Caenorhabditis elegans model of Aβ misfolding leads to increased toxicity. On the other hand, the use of Hsp90 inhibitors in AD mouse models reduces Aβ toxicity, and normalizes synaptic function. Stress-inducible phosphoprotein 1 (STI1), an intracellular co-chaperone, mediates the transfer of clients from Hsp70 to Hsp90. Importantly, STI1 has been shown to regulate aggregation of amyloid-like proteins in yeast. In addition to its intracellular function, STI1 can be secreted by diverse cell types, including astrocytes and microglia and function as a neurotrophic ligand by triggering signaling via the cellular prion protein (PrP^C). Extracellular STI1 can prevent Aβ toxic signaling by (i) interfering with Aβ binding to PrP^C and (ii) triggering pro-survival signaling cascades. Interestingly, decreased levels of STI1 in C. elegans can also increase toxicity in an amyloid model. In this review, we will discuss the role of intracellular and extracellular STI1 and the Hsp70/Hsp90 chaperone network in mechanisms underlying protein misfolding in neurodegenerative diseases, with particular focus on AD.

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.

AHSA1 regulates proliferation, apoptosis, migration, and invasion of osteosarcoma

Activator of 90kDa heat shock protein ATPase homolog 1 (AHSA1) is a chaperone of heat shock 90kDa (HSP90) and stimulates ATPase activity of HSP90. The function of AHSA1 in osteosarcoma (OS) has not been reported yet. A previous study showed AHSA1 was overexpressed in OS cells. In this study, we investigated the role of AHSA1 in OS cells by silencing AHSA1. We report that silencing AHSA1 inhibited cell growth, migration, and invasion, and increased apoptosis of MG-63 and Saos2 cells. We also found that silencing AHSA1 decreased the ATPase activity of HSP90 in OS cells. In addition, silencing AHSA1 increased the levels of negative regulators of Wnt/β-catenin signalling pathway, Axin-2 and GSK3β, and decreased the levels of two key members of Wnt/β-catenin signalling pathway, namely, Wnt-5a and β-catenin. In conclusion, silencing AHSA1 regulates cell growth, apoptosis, migration, and invasion by regulating Wnt/β-catenin signalling pathway and their negative regulators.

The Chemical Biology of Molecular Chaperones--Implications for Modulation of Proteostasis

Protein homeostasis (proteostasis) is inextricably tied to cellular health and organismal lifespan. Aging, exposure to physiological and environmental stress, and expression of mutant and metastable proteins can cause an imbalance in the protein-folding landscape, which results in the formation of non-native protein aggregates that challenge the capacity of the proteostasis network (PN), increasing the risk for diseases associated with misfolding, aggregation, and aberrant regulation of cell stress responses. Molecular chaperones have central roles in each of the arms of the PN (protein synthesis, folding, disaggregation, and degradation), leading to the proposal that modulation of chaperone function could have therapeutic benefits for the large and growing family of diseases of protein conformation including neurodegeneration, metabolic diseases, and cancer. In this review, we will discuss the current strategies used to tune the PN through targeting molecular chaperones and assess the potential of the chemical biology of proteostasis.

A dynamic view of ATP-coupled functioning cycle of Hsp90 N-terminal domain

Heat-shock protein 90 (Hsp90) is one of the most important chaperones involved in multiple cellular processes. The chaperoning function of Hsp90 is intimately coupled to the ATPase activity presented by its N-terminal domain. However, the molecular mechanism for the ATP-dependent working cycle of Hsp90 is still not fully understood. In this study, we use NMR techniques to investigate the structural characteristics and dynamic behaviors of Hsp90 N-terminal domain in its free and AMPPCP (ATP analogue) or ADP-bound states. We demonstrated that although AMPPCP and ADP bind to almost the same region of Hsp90, significantly different effects on the dynamics behaviors of the key structural elements were observed. AMPPCP binding favors the formation of the active homodimer of Hsp90 by enhancing the slow-motion featured conformational exchanges of those residues (A117–A141) within the lid segment (A111–G135) and around region, while ADP binding keeps Hsp90 staying at the inactive state by increasing the conformational rigidity of the lid segment and around region. Based on our findings, a dynamic working model for the ATP-dependent functioning cycle of Hsp90 was proposed.

Heat shock proteins in multiple myeloma

Heat shock proteins are molecular chaperones with a central role in protein folding and cellular protein homeostasis. They also play major roles in the development of cancer and in recent years have emerged as promising therapeutic targets. In this review, we discuss the known molecular mechanisms of various heat shock protein families and their involvement in cancer and in particular, multiple myeloma. In addition, we address the current progress and challenges in pharmacologically targeting these proteins as anti-cancer therapeutic strategies.

Identifying a C-terminal ATP binding sites-based novel Hsp90-Inhibitor in silico: a plausible therapeutic approach in Alzheimer's disease

BACKGROUND

Alzheimer's disease (AD) is a progressive brain disorder, which gradually and irreversibly destroys the intellectual and cognitive abilities of the brain. Heat shock protein 90 (Hsp90α) is a molecular chaperone which was found to regulate the function of number of client proteins including tau that is involved in the cause of the AD. Inhibition of Hsp90α by C-Terminal domain (CTD) ATP binding-site blockage might be used as an effective treatment strategy against the disease via degradation of tau proteins that are involved in the progression of the disease. Till date, a variety of drugs have been identified as Hsp90α inhibitors, which include Novobiocin, Clorobiocin, Epigallocatechingallate (EGCG) and Derrubone. However, which drug among the four binds to the CTD ATP binding site strongly and what are the specific residue responsible for such binding, have not been reported so far.

HYPOTHESIS

We hypothesize that binding site for ATP of Hsp90α CTD contains multiple ATP binding sites. We also hypothesize that a drug which can bind to the ATP binding site of CTD strongly can inhibit Hsp90α function which is in turn redirects towards the proteasomal degradation of diseased client protein like tau in AD. Such inhibition will find a novel therapeutic approach in the treatment of AD.

EXPERIMENTAL DESIGN

The identification of ATP binding site of Hsp90α CTD was done using various software tools like Hex 6.3, CastP, protein Hydrophobicity plots, ATPint and LigPlot+ v.1.4.5. Docking experiments were conducted between Hsp90αCTD and its inhibitors at these ATP binding site using the Autodock 4.0. The docking energies were further compared to obtain the most effective Hsp90α inhibitor of CTD.

RESULTS

From our experiments, Leucine (Leu) 665, Leu 666 and Leu 694 were predicted to be located in CTD ATP binding site. Furthermore, docking studies were performed of various Hsp90α inhibitors like Novobiocin, Clorobiocin, Epigallocatechingallate (EGCG) and Derrubone with the previously recognized ATP binding residues of CTD i.e. Leu 665, Leu 666 and Leu 694. The docking results of Derrubone showed the highest binding energy at all the three sites of ATP interaction. Additionally, Derrubone showed the best binding energy at Leu 666 (-7.53kcal/mol) compared to Leu 665 (-7.20kcal/mol) and Leu 694 (-6.67kcal/mol).

CONCLUSION

Based on our findings, we propose that the recognized sites i.e. Leu665, Leu 666 and Leu694 could possibly be the binding sites of Hsp90α CTD for ATP and the Hsp90 inhibitors. It was predicted that Derrubone could bind with CTD of Hsp90α strongly and resulted tau protein degradation which might be considered to be a therapeutic approach in AD.

Proteomic data from human cell cultures refine mechanisms of chaperone-mediated protein homeostasis

In the crowded environment of human cells, folding of nascent polypeptides and refolding of stress-unfolded proteins is error prone. Accumulation of cytotoxic misfolded and aggregated species may cause cell death, tissue loss, degenerative conformational diseases, and aging. Nevertheless, young cells effectively express a network of molecular chaperones and folding enzymes, termed here "the chaperome," which can prevent formation of potentially harmful misfolded protein conformers and use the energy of adenosine triphosphate (ATP) to rehabilitate already formed toxic aggregates into native functional proteins. In an attempt to extend knowledge of chaperome mechanisms in cellular proteostasis, we performed a meta-analysis of human chaperome using high-throughput proteomic data from 11 immortalized human cell lines. Chaperome polypeptides were about 10% of total protein mass of human cells, half of which were Hsp90s and Hsp70s. Knowledge of cellular concentrations and ratios among chaperome polypeptides provided a novel basis to understand mechanisms by which the Hsp60, Hsp70, Hsp90, and small heat shock proteins (HSPs), in collaboration with cochaperones and folding enzymes, assist de novo protein folding, import polypeptides into organelles, unfold stress-destabilized toxic conformers, and control the conformal activity of native proteins in the crowded environment of the cell. Proteomic data also provided means to distinguish between stable components of chaperone core machineries and dynamic regulatory cochaperones.

Nuclear transport of Epstein-Barr virus DNA polymerase is dependent on the BMRF1 polymerase processivity factor and molecular chaperone Hsp90

Epstein-Barr virus (EBV) replication proteins are transported into the nucleus to synthesize viral genomes. We here report molecular mechanisms for nuclear transport of EBV DNA polymerase. The EBV DNA polymerase catalytic subunit BALF5 was found to accumulate in the cytoplasm when expressed alone, while the EBV DNA polymerase processivity factor BMRF1 moved into the nucleus by itself. Coexpression of both proteins, however, resulted in efficient nuclear transport of BALF5. Deletion of the nuclear localization signal of BMRF1 diminished the proteins' nuclear transport, although both proteins can still interact. These results suggest that BALF5 interacts with BMRF1 to effect transport into the nucleus. Interestingly, we found that Hsp90 inhibitors or knockdown of Hsp90β with short hairpin RNA prevented the BALF5 nuclear transport, even in the presence of BMRF1, both in transfection assays and in the context of lytic replication. Immunoprecipitation analyses suggested that the molecular chaperone Hsp90 interacts with BALF5. Treatment with Hsp90 inhibitors blocked viral DNA replication almost completely during lytic infection, and knockdown of Hsp90β reduced viral genome synthesis. Collectively, we speculate that Hsp90 interacts with BALF5 in the cytoplasm to assist complex formation with BMRF1, leading to nuclear transport. Hsp90 inhibitors may be useful for therapy for EBV-associated diseases in the future.

Molecular pathways: targeting hsp90--who benefits and who does not

Many kinases and hormone receptors, important for cancer cell proliferation and survival, bind to and are dependent on the Hsp90 cycle for their folding and maturation. This provides the rationale for the development of small-molecule ATP competitors that, inhibiting Hsp90 function, lead to degradation of the "client" proteins. After continual efforts to improve the pharmacologic properties and the tolerability of these molecules, several Hsp90 inhibitors have exhibited activity in both preclinical models and in the clinical setting. As is the case with many other targeted agents, patient selection seems to be the major limitation to the success of these compounds. ERBB2-positive patients with breast cancer are exquisitely sensitive to Hsp90 inhibition. This is because ERBB2 is indispensable for growth and survival of this subtype of cancer, and at the same time ERBB2 is a client protein strictly dependent on Hsp90 for its maturation and stability. Extensive preclinical work identifying other ERBB-like client proteins will likely lead to the ability to enhance selection of appropriate patients for enrollment in more rational clinical trials. Hsp90 inhibition has also been reported to synergize with other therapeutic agents. Several ongoing studies testing different combinations of Hsp90 inhibitors with other targeted agents will confirm whether Hsp90 inhibition can potentiate the efficacy of targeted therapy and/or prevent the emergence of drug resistance.

The 'active life' of Hsp90 complexes

Hsp90 forms a variety of complexes differing both in clientele and co-chaperones. Central to the role of co-chaperones in the formation of Hsp90 complexes is the delivery of client proteins and the regulation of the ATPase activity of Hsp90. Determining the mechanisms by which co-chaperones regulate Hsp90 is essential in understanding the assembly of these complexes and the activation and maturation of Hsp90's clientele. Mechanistically, co-chaperones alter the kinetics of the ATP-coupled conformational changes of Hsp90. The structural changes leading to the formation of a catalytically active unit involve all regions of the Hsp90 dimer. Their complexity has allowed different orthologues of Hsp90 to evolve kinetically in slightly different ways. The interaction of the cytosolic Hsp90 with a variety of co-chaperones lends itself to a complex set of different regulatory mechanisms that modulate Hsp90's conformation and ATPase activity. It also appears that the conformational switches of Hsp90 are not necessarily coupled under all circumstances. Here, I described different co-chaperone complexes and then discuss in detail the mechanisms and role that specific co-chaperones play in this. I will also discuss emerging evidence that post-translational modifications also affect the ATPase activity of Hsp90, and thus complex formation. Finally, I will present evidence showing how Hsp90's active site, although being highly conserved, can be altered to show resistance to drug binding, but still maintain ATP binding and ATPase activity. Such changes are therefore unlikely to significantly alter Hsp90's interactions with client proteins and co-chaperones. This article is part of a Special Issue entitled: Heat Shock Protein 90 (HSP90).

HSP90 at the hub of protein homeostasis: emerging mechanistic insights

Heat shock protein 90 (HSP90) is a highly conserved molecular chaperone that facilitates the maturation of a wide range of proteins (known as clients). Clients are enriched in signal transducers, including kinases and transcription factors. Therefore, HSP90 regulates diverse cellular functions and exerts marked effects on normal biology, disease and evolutionary processes. Recent structural and functional analyses have provided new insights on the transcriptional and biochemical regulation of HSP90 and the structural dynamics it uses to act on a diverse client repertoire. Comprehensive understanding of how HSP90 functions promises not only to provide new avenues for therapeutic intervention, but to shed light on fundamental biological questions.

Specific regulation of noncanonical p38alpha activation by Hsp90-Cdc37 chaperone complex in cardiomyocyte

RATIONALE

p38 is an important stress activated protein kinase involved in gene regulation, proliferation, differentiation, and cell death regulation in heart. p38 kinase activity can be induced through canonical pathway via upstream kinases or by noncanonical autophosphorylation. The intracellular p38 kinase activity is tightly regulated and maintained at low level under basal condition. The underlying regulatory mechanism for canonical p38 kinase activation is well-studied, but the regulation of noncanonical p38 autophosphorylation remains poorly understood.

OBJECTIVE

We investigated the molecular basis for the regulation of noncanonical p38 autophosphorylation and its potential functional impact in cardiomyocytes.

METHODS AND RESULTS

Using both proteomic and biochemical tools, we established that heat shock protein (Hsp)90-Cdc37 chaperones are part of the p38alpha signaling complex in mammalian cells both in vitro and in vivo. The Hsp90-Cdc37 chaperone complex interacts with p38 via direct binding between p38 and Cdc37. Cdc37 expression is both sufficient and necessary to suppress noncanonical p38 activation via autophosphorylation at either basal state or under TAB1 (TAK1 binding protein-1) induction. In contrast, Cdc37 expression has no impact on p38 activation by canonical upstream kinase MKK3 or oxidative stress. Furthermore, Hsp90 inhibition results in p38 activation via autophosphorylation, and p38 activity contribute to apoptotic cell death induced by Hsp90 inhibition.

CONCLUSION

Our study has revealed a so far uncharacterized function of Hsp90-Cdc37 as an endogenous regulator of noncanonical p38 activity.

pH-dependent conformational changes in bacterial Hsp90 reveal a Grp94-like conformation at pH 6 that is highly active in suppression of citrate synthase aggregation

The molecular chaperone Hsp90 depends upon large conformational rearrangements for its function. One driving force for these rearrangements is the intrinsic ATPase activity of Hsp90, as seen with other chaperones. However, unlike other chaperones, structural and kinetic studies have shown that the ATPase cycle of Hsp90 is not conformationally deterministic. That is, rather than dictating the conformational state, ATP binding and hydrolysis shift the equilibrium between a preexisting set of conformational states in an organism-dependent manner. While many conformations of Hsp90 have been described, little is known about how they relate to chaperone function. In this study, we show that the conformational equilibrium of the bacterial Hsp90, HtpG, can be shifted with pH. Using small-angle X-ray scattering, we identify a two-state pH-dependent conformational equilibrium for apo HtpG. Our structural modeling reveals that this equilibrium is observed between the previously observed extended state and a second state that is strikingly similar to the recently solved Grp94 crystal structure. In the presence of nonhydrolyzable 5'-adenylyl-beta,gamma-imidodiphosphate, a third state, which is identical with the solved AMPPNP-bound structure from yeast Hsp90, is populated. Electron microscopy confirmed the observed conformational equilibria. We also identify key histidine residues that control this pH-dependent equilibrium; using mutagenesis, we successfully modulate the conformational equilibrium at neutral pH. Using these mutations, we show that the Grp94-like state provides stronger aggregation protection compared to the extended apo conformation in the context of a citrate synthase aggregation assay. These studies provide a more detailed view of HtpG's conformational dynamics and provide the first linkage between a specific conformation and chaperone function.

A common conformationally coupled ATPase mechanism for yeast and human cytoplasmic HSP90s

The conformationally coupled mechanism by which ATP is utilized by yeast Hsp90 is now well characterized. In contrast, ATP utilization by human Hsp90s is less well studied, and appears to operate differently. To resolve these conflicting models, we have conducted a side-by-side biochemical analysis in a series of mutant yeast and human Hsp90s that have been both mechanistically and structurally characterized with regard to the crystal structure of the yeast Hsp90 protein. We show that each monomer of the human Hsp90 dimer is mutually dependent on the other for ATPase activity. Fluorescence studies confirmed that the N-terminal domains of Hsp90β come into close association with each other. Mutations that directly affect the conformational dynamics of the ATP-lid segment had marked effects, with T31I (yeast T22I) and A116N (yeast A107N) stimulating, and T110I (yeast T101I) inhibiting, human and yeast ATPase activity to similar extents, showing that ATP-dependent lid closure is a key rate-determining step in both systems. Mutation of residues implicated in N-terminal dimerization of yeast Hsp90 (L15R and L18R in yeast, L24R and L27R in humans) significantly reduced the ATPase activity of yeast and human Hsp90s, showing that ATP-dependent association of the N-terminal domains in the Hsp90 dimer is also essential in both systems. Furthermore, cross-linking studies of the hyper-active yeast A107N and human A116N ATP-lid mutants showed enhanced dimerization, suggesting that N-terminal association is a direct consequence of ATP binding and lid closure in both systems.

Hsp90 cleavage by an oxidative stress leads to its client proteins degradation and cancer cell death

The heat shock protein 90 (Hsp90) plays a crucial role in the stability of several proteins that are essential for malignant transformation. Hsp90 is therefore an interesting therapeutic target for cancer therapy. In this paper, we investigated whether an oxidative stress generated during ascorbate-driven menadione redox cycling (ascorbate/menadione), affects Hsp90 leading to the degradation of some critical proteins and cell death. Unlike 17-AAG, which inhibits Hsp90 but enhances Hsp70 levels, ascorbate/menadione-treated cells present an additional Hsp90 protein band of about 70kDa as shown by Western blot analysis, suggesting Hsp90 cleavage. This Hsp90 cleavage seems to be a selective phenomenon since it was observed in a large panel of cancer cell lines but not in non-transformed cells. Antibodies raised against either the N-terminus or the C-terminus domains of Hsp90 suggest that the site of cleavage should be located at its N-terminal part. Furthermore, antibodies raised against either the alpha- or the beta-Hsp90 isoform show that Hsp90beta is cleaved while the alpha isoform is down-regulated. We have further shown that different Hsp90 client proteins like Bcr-Abl (a chimerical protein expressed in K562 leukemia cells), RIP and Akt, were degraded when K562 cells were exposed to an oxidative stress. Both Hsp90 cleavage and Bcr-Abl degradation were observed by incubating K562 cells with another H(2)O(2)-generating system (glucose/glucose oxidase) and by incubating KU812 cells (another leukemia cell line) with ascorbate/menadione. Due to the major role of Hsp90 in stabilizing oncogenic and mutated proteins, these results may have potential clinical applications.

Apo-Hsp90 coexists in two open conformational states in solution

BACKGROUND INFORMATION

Hsp90 (90 kDa heat-shock protein) plays a key role in the folding and activation of many client proteins involved in signal transduction and cell cycle control. The cycle of Hsp90 has been intimately associated with large conformational rearrangements, which are nucleotide-binding-dependent. However, up to now, our understanding of Hsp90 conformational changes derives from structural information, which refers to the crystal states of either recombinant Hsp90 constructs or the prokaryotic homologue HtpG (Hsp90 prokaryotic homologue).

RESULTS AND DISCUSSION

Here, we present the first nucleotide-free structures of the entire eukaryotic Hsp90 (apo-Hsp90) obtained by small-angle X-ray scattering and single-particle cryo-EM (cryo-electron microscopy). We show that, in solution, apo-Hsp90 is in a conformational equilibrium between two open states that have never been described previously. By comparing our cryo-EM maps with HtpG and known Hsp90 structures, we establish that the structural changes involved in switching between the two Hsp90 apo-forms require large movements of the NTD (N-terminal domain) and MD (middle domain) around two flexible hinge regions.

CONCLUSIONS

The present study shows, for the first time, the structure of the entire eukaryotic apo-Hsp90, along with its intrinsic flexibility. Although large structural rearrangements, leading to partial closure of the Hsp90 dimer, were previously attributed to the binding of nucleotides, our results reveal that they are in fact mainly due to the intrinsic flexibility of Hsp90 dimer. Taking into account the preponderant role of the dynamic nature of the structure of Hsp90, we reconsider the Hsp90 ATPase cycle.

Hsp90 regulates the Fanconi anemia DNA damage response pathway

Heat shock protein 90 (Hsp90) regulates diverse signaling pathways. Emerging evidence suggests that Hsp90 inhibitors, such as 17-allylamino-17-demethoxygeldanamycin (17-AAG), enhance DNA damage-induced cell death, suggesting that Hsp90 may regulate cellular responses to genotoxic stress. However, the underlying mechanisms are poorly understood. Here, we show that the Fanconi anemia (FA) pathway is involved in the Hsp90-mediated regulation of genotoxic stress response. In the FA pathway, assembly of 8 FA proteins including FANCA into a nuclear multiprotein complex, and the complex-dependent activation of FANCD2 are critical events for cellular tolerance against DNA cross-linkers. Hsp90 associates with FANCA, in vivo and in vitro, in a 17-AAG-sensitive manner. Disruption of the FANCA/Hsp90 association by cellular treatment with 17-AAG induces rapid proteasomal degradation and cytoplasmic relocalization of FANCA, leading to impaired activation of FANCD2. Furthermore, 17-AAG promotes DNA cross-linker-induced cytotoxicity, but this effect is much less pronounced in FA pathway-defective cells. Notably, 17-AAG enhances DNA cross-linker-induced chromosome aberrations. In conclusion, our results identify FANCA as a novel client of Hsp90, suggesting that Hsp90 promotes activation of the FA pathway through regulation of intracellular turnover and trafficking of FANCA, which is critical for cellular tolerance against genotoxic stress.

Structure and mechanism of the Hsp90 molecular chaperone machinery

Heat shock protein 90 (Hsp90) is a molecular chaperone essential for activating many signaling proteins in the eukaryotic cell. Biochemical and structural analysis of Hsp90 has revealed a complex mechanism of ATPase-coupled conformational changes and interactions with cochaperone proteins, which facilitate activation of Hsp90's diverse "clientele." Despite recent progress, key aspects of the ATPase-coupled mechanism of Hsp90 remain controversial, and the nature of the changes, engendered by Hsp90 in client proteins, is largely unknown. Here, we discuss present knowledge of Hsp90 structure and function gleaned from crystallographic studies of individual domains and recent progress in obtaining a structure for the ATP-bound conformation of the intact dimeric chaperone. Additionally, we describe the roles of the plethora of cochaperones with which Hsp90 cooperates and growing insights into their biochemical mechanisms, which come from crystal structures of Hsp90 cochaperone complexes.

The beta4-beta8 groove is an ATP-interactive site in the alpha crystallin core domain of the small heat shock protein, human alphaB crystallin

The site for ATP interactions in human alphaB crystallin, the archetype of small heat-shock proteins, was identified and characterized to resolve the controversial role of ATP in the function of small heat-shock proteins. Comparative sequence alignments identified the alphaB crystallin sequence, (82)KHFSPEELKVKVLGD(96) as a Walker-B ATP-binding motif that is found in several ATP-binding proteins, including five molecular chaperones. Fluorescence resonance energy transfer and mass spectrometry using a novel fluorescent ATP analog, 8-azido-ATP-[gamma]-1-naphthalenesulfonic acid-5(2-aminoethylamide) (azido-ATP-EDANS) and a cysteine mutant of human alphaB crystallin (S135C) conjugated with a fluorescent acceptor, eosin-5-maleimide (EMA) identified the beta4-beta8 groove as the ATP interactive site in alphaB crystallin. A 44% decrease in the emitted fluorescence of azido-ATP-EDANS at the absorption maximum of S135C-EMA and a corresponding 50% increase in the fluorescence emission of S135C-EMA indicated a close spatial relationship between azido-ATP-EDANS and the center of the beta8 strand ((131)LTITSSLS(138)). Liquid chromatography, electrospray ionization mass spectrometry identified two peptide fragments of the alphaB crystallin Walker-B motif photo-affinity-labeled with azido-ATP-EDANS confirming the beta4-beta8 groove as an ATP interactive site. The results presented here clearly establish the beta4-beta8 groove as the ATP interactive region in alphaB crystallin, and are in contrast to the existing paradigm that classifies small heat-shock proteins as ATP-independent chaperones.

Intrinsic inhibition of the Hsp90 ATPase activity

The molecular chaperone Hsp90 is required for the folding and activation of a large number of substrate proteins. These are involved in essential cellular processes ranging from signal transduction to viral replication. For the activation of its substrates, Hsp90 binds and hydrolyzes ATP, which is the key driving force for conformational conversions within the dimeric chaperone. Dimerization of Hsp90 is mediated by a C-terminal dimerization site. In addition, there is a transient ATP-induced dimerization of the two N-terminal ATP-binding domains. The resulting ring-like structure is thought to be the ATPase-active conformation. Hsp90 is a slow ATPase with a turnover number of 1 ATP/min for the yeast protein. A key question for understanding the molecular mechanism of Hsp90 is how ATP hydrolysis is regulated and linked to conformational changes. In this study, we analyzed the activation process structurally and biochemically with a view to identify the conformational limitations of the ATPase reaction cycle. We showed that the first 24 amino acids stabilize the N-terminal domain in a rigid state. Their removal confers flexibility specifically to the region between amino acids 98 and 120. Most surprisingly, the deletion of this structure results in the complete loss of ATPase activity and in increased N-terminal dimerization. Complementation assays using heterodimeric Hsp90 show that this rigid lid acts as an intrinsic kinetic inhibitor of the Hsp90 ATPase cycle preventing N-terminal dimerization in the ground state. On the other hand, this structure acts, in concert with the 24 N-terminal amino acids of the other N-terminal domain, to form an activated ATPase and thus regulates the turnover number of Hsp90.

Heat-shock protein 90 inhibitors as novel cancer chemotherapeutic agents

Heat-shock protein 90 (Hsp90) is a molecular chaperone whose association is required for the stability and function of multiple mutated, chimeric and overexpressed signalling proteins that promote cancer cell growth and/or survival. Hsp90 client proteins include mutated p53, Bcr-Abl, Raf-1, Akt, HER2/Neu (ErbB2) and hypoxia inducible factor-1alpha (HIF-1alpha). Through specific interaction with a single molecular target, Hsp90 inhibitors cause the destabilisation and eventual degradation of Hsp90 client proteins, and they have shown promising antitumour activity in preclinical model systems. One Hsp90 inhibitor, 17-allylamino-geldanamycin (17-AAG), is currently in Phase I clinical trials. Hsp90 inhibitors are unique in that, although they are directed towards a specific molecular target, they simultaneously inhibit multiple signalling pathways on which cancer cells depend for growth and survival. Further, because of the unique effect that Hsp90 inhibition has on cancer cells, combination of an Hsp90 inhibitor with standard chemotherapeutic agents may dramatically increase the in vivo efficacy of the standard agent.

Dissection of the contribution of individual domains to the ATPase mechanism of Hsp90

Hsp90 is a dimeric, ATP-regulated molecular chaperone. Its ATPase cycle involves the N-terminal ATP binding domain (amino acids (aa) 1-272) and, in addition, to some extent the middle domain (aa 273-528) and the C-terminal dimerization domain (aa 529-709). To analyze the contribution of the different domains and the oligomeric state on the progression of the ATPase cycle of yeast Hsp90, we created deletion constructs lacking either the C-terminal or both the C-terminal and the middle domain. To test the effect of dimerization on the ATPase activity of the different constructs, we introduced a Cys residue at the C-terminal ends of the constructs, which allowed covalent dimerization. We show that all monomeric constructs tested exhibit reduced ATPase activity and a decreased affinity for ATP in comparison with wild type Hsp90. The covalently linked dimers lacking only the C-terminal domain hydrolyze ATP as efficiently as the wild type protein. Furthermore, this construct is able to trap the ATP molecule similar to the full-length protein. This demonstrates that in the ATPase cycle, the C-terminal domain can be replaced by a cystine bridge. In contrast, the ATPase activity of the artificially linked N-terminal domains remains very low and bound ATP is not trapped. Taken together, we show that both the dimerization of the N-terminal domains and the association of the N-terminal with the middle domain are important for the efficiency of the ATPase cycle. These reactions are synergistic and require Hsp90 to be in the dimeric state.

Bimoclomol, a heat shock protein co-inducer, acts by the prolonged activation of heat shock factor-1

The novel hydroxylamine derivative, bimoclomol, has been shown previously to act as a co-inducer of several heat shock proteins (Hsp-s), enhancing the amount of these proteins produced following a heat shock compared to heat shock alone. Here we show that the co-inducing effect of bimoclomol on Hsp expression is mediated via the prolonged activation of the heat shock transcription factor (HSF-1). Bimoclomol effects are abolished in cells from mice lacking HSF-1. Moreover, bimoclomol binds to HSF-1 and induces a prolonged binding of HSF-1 to the respective DNA elements. Since HSF-1 does not bind to DNA in the absence of stress, the bimoclomol-induced extension of HSF-1/DNA interaction may contribute to the chaperone co-induction of bimoclomol observed previously. These findings indicate that bimoclomol may be of value in targeting HSF-1 so as to induce up-regulation of protective Hsp-s in a non-stressful manner and for therapeutic benefit.

Comparative analysis of the ATP-binding sites of Hsp90 by nucleotide affinity cleavage: a distinct nucleotide specificity of the C-terminal ATP-binding site

The 90-kDa heat shock protein (Hsp90) is a molecular chaperone that assists both in ATP-independent sequestration of damaged proteins, and in ATP-dependent folding of numerous targets, such as nuclear hormone receptors and protein kinases. Recent work from our lab and others has established the existence of a second, C-terminal nucleotide binding site besides the well characterized N-terminal, geldanamycin-sensitive ATP-binding site. The cryptic C-terminal site becomes open only after the occupancy of the N-terminal site. Our present work demonstrates the applicability of the oxidative nucleotide affinity cleavage in the site-specific characterization of nucleotide binding proteins. We performed a systematic analysis of the nucleotide binding specificity of the Hsp90 nucleotide binding sites. N-terminal binding is specific to adenosine nucleotides with an intact adenine ring. Nicotinamide adenine dinucleotides and diadenosine polyphosphate alarmones are specific N-terminal nucleotides. The C-terminal binding site is much more unspecific-it interacts with both purine and pirimidine nucleotides. Efficient binding to the C-terminal site requires both charged residues and a larger hydrophobic moiety. GTP and UTP are specific C-terminal nucleotides. 2',3'-O-(2,4,6-trinitrophenyl)-nucleotides (TNP-ATP, TNP-GTP) and pyrophosphate access the C-terminal binding site without the need for an occupied N-terminal site. Our data provide additional evidence for the dynamic domain-domain interactions of Hsp90, give hints for the design of novel types of specific Hsp90 inhibitors, and raise the possibility that besides ATP, other small molecules might also interact with the C-terminal nucleotide binding site in vivo.

Interaction between the N-terminal and middle regions is essential for the in vivo function of HSP90 molecular chaperone

At the primary structure level, the 90-kDa heat shock protein (HSP90) is composed of three regions: the N-terminal (Met(1)-Arg(400)), middle (Glu(401)-Lys(615)), and C-terminal (Asp(621)-Asp(732)) regions. In the present study, we investigated potential subregion structures of these three regions and their roles. Limited proteolysis revealed that the N-terminal region could be split into two fragments carrying residues Met(1) to Lys(281) (or Lys(283)) and Glu(282) (or Tyr(284)) to Arg(400). The former is known to carry the ATP-binding domain. The fragments carrying the N-terminal two-thirds (Glu(401)-Lys(546)) and C-terminal one-third of the middle region were sufficient for the interactions with the N- and C-terminal regions, respectively. Yeast HSC82 that carried point mutations in the middle region causing deficient binding to the N-terminal region could not support the growth of HSP82-depleted cells at an elevated temperature. Taken together, our data show that the N-terminal and middle regions of the HSP90 family protein are structurally divided into two respective subregions. Moreover, the interaction between the N-terminal and middle regions is essential for the in vivo function of HSP90 in yeast.

Binding of ATP to heat shock protein 90: evidence for an ATP-binding site in the C-terminal domain

The presence of a nucleotide binding site on hsp90 was very controversial until x-ray structure of the hsp90 N-terminal domain, showing a nonconventional nucleotide binding site, appeared. A recent study suggested that the hsp90 C-terminal domain also binds ATP (Marcu, M. G., Chadli, A., Bouhouche, I., Catelli, M. G., and Neckers, L. M. (2000) J. Biol. Chem. 275, 37181-37186). In this paper, the interactions of ATP with native hsp90 and its recombinant N-terminal (positions 1-221) and C-terminal (positions 446-728) domains were studied by isothermal titration calorimetry, scanning differential calorimetry, and fluorescence spectroscopy. Results clearly demonstrate that hsp90 possesses a second ATP-binding site located on the C-terminal part of the protein. The association constant between this domain of hsp90 and ATP-Mg and a comparison with the binding constant on the full-length protein are reported for the first time. Secondary structure prediction revealed motifs compatible with a Rossmann fold in the C-terminal part of hsp90. It is proposed that this potential Rossmann fold may constitute the C-terminal ATP-binding site. This work also suggests allosteric interaction between N- and C-terminal domains of hsp90.

A Nucleotide-dependent molecular switch controls ATP binding at the C-terminal domain of Hsp90. N-terminal nucleotide binding unmasks a C-terminal binding pocket

In vivo function of the molecular chaperone Hsp90 is ATP-dependent and requires the full-length protein. Our earlier studies predicted a second C-terminal ATP-binding site in Hsp90. By applying direct biochemical approaches, we mapped two ATP-binding sites and unveiled the C-terminal ATP-binding site as the first example of a cryptic chaperone nucleotide-binding site, which is opened by occupancy of the N-terminal site. We identified an N-terminal gamma-phosphate-binding motif in the middle domain of Hsp90 similar to other GHKL family members. This motif is adjacent to the phosphate-binding region of the C-terminal ATP-binding site. Whereas novobiocin disrupts both C- and N-terminal nucleotide binding, we found a selective C-terminal nucleotide competitor, cisplatin, that strengthens the Hsp90-Hsp70 complex leaving the Hsp90-p23 complex intact. Cisplatin may provide a pharmacological tool to dissect C- and N-terminal nucleotide binding of Hsp90. A model is proposed on the interactions of the two nucleotide-binding domains and the charged region of Hsp90.

Vanadate increases glucocorticoid receptor-mediated gene expression: a novel mechanism for potentiation of a steroid receptor

Transition metal oxyanions, such as molybdate, tungstate and vandadate, have been shown to prevent in vitro hormone-induced activation of the glucocorticoid receptor (GR) by blocking dissociation of the GR/heat shock protein heterocomplex. In this work, we report a novel effect of vanadate: in vivo potentiation of GR-mediated gene expression. In cells stably-transfected with complex (mouse mammary tumor virus (MMTV)) or minimal GR-regulated CAT reporters, treatment with 500muM vanadate caused CAT gene expression to dramatically increase, even at saturating concentrations of dexamethasone; while no such effect was seen in response to RU486 antagonist. Similar treatment with molybdate had no effect on GR activity, suggesting that the response to vanadate was not a general property of transition metal oxyanions. Treatment with vanadate after hormone-induced nuclear translocation of the GR also caused potentiation, demonstrating that vanadate was acting on a post-transformation event, perhaps by affecting the transactivation function of DNA-bound GR. Paradoxically, vanadate caused an apparent but temporary "loss" of GR protein immediately after treatment (as measured by loss of reactivity to BuGR2 antibody and of hormone-binding capacity) that returned to normal at approximately 8h post-treatment, suggesting that potentiation of GR transactivation function (as measured by our CAT assays) was probably occurring during the later stages (8-24h) of this assay. However, gel shift analyses revealed that vanadate could induce binding of the hormone-free GR to glucocorticoid response element (GRE)-containing oligonucleotides immediately after treatment. Thus, the rapid vanadate-induced "loss" of GR was not due to degradation of GR protein. Yet, vanadate in the absence of hormone had no effect on CAT reporter expression, demonstrating that this form of the GR still requires agonist for its enhanced transcriptional activity. As an indication of the potential mechanism of vanadate action, vanadate was found to dramatically stimulate the mitogen-activated protein kinases, ERK-1 and ERK-2. In addition, vanadate potentiation of GR reporter gene expression was completely blocked by the tyrosine kinase inhibitor herbimycin A. Taken as a whole, our results suggest that vanadate can have dramatic and complex effects on GR structure and function, resulting in hormone-free activation of GR DNA-binding function, as well as alterations to the BuGR2 epitope and hormone-binding domains--while at the same time stimulating tyrosine phosphorylation pathways controlling GR-mediated gene transcription.

Coordinated ATP hydrolysis by the Hsp90 dimer

The Hsp90 dimer is a molecular chaperone with an unusual N-terminal ATP binding site. The structure of the ATP binding site makes it a member of a new class of ATP-hydrolyzing enzymes, known as the GHKL family. While for some of the family members structural data on conformational changes occurring after ATP binding are available, these are still lacking for Hsp90. Here we set out to investigate the correlation between dimerization and ATP hydrolysis by Hsp90. The dimerization constant of wild type (WT) Hsp90 was determined to be 60 nm. Heterodimers of WT Hsp90 with fragments lacking the ATP binding domain form readily and exhibit dimerization constants similar to full-length Hsp90. However, the ATPase activity of these heterodimers was significantly lower than that of the wild type protein, indicating cooperative interactions in the N-terminal part of the protein that lead to the activation of the ATPase activity. To further address the contribution of the N-terminal domains to the ATPase activity, we used an Hsp90 point mutant that is unable to bind ATP. Since heterodimers between the WT protein and this mutant showed WT ATPase activity, this mutant, although unable to bind ATP, still has the ability to stimulate the activity in its WT partner domain. Thus, contact formation between the N-terminal domains might not depend on ATP bound to both domains. Together, these results suggest a mechanism for coupling the hydrolysis of ATP to the opening-closing movement of the Hsp90 molecular chaperone.