Hsp90 chaperone activity requires the full-length protein and interaction among its multiple domains

Hsp90, a team player in protein quality control and the stress response in bacteria

SUMMARYHeat shock protein 90 (Hsp90) participates in proteostasis by facilitating protein folding, activation, disaggregation, prevention of aggregation, degradation, and protection against degradation of various cellular proteins. It is highly conserved from bacteria to humans. In bacteria, protein remodeling by Hsp90 involves collaboration with the Hsp70 molecular chaperone and Hsp70 cochaperones. In eukaryotes, protein folding by Hsp90 is more complex and involves collaboration with many Hsp90 cochaperones as well as Hsp70 and Hsp70 cochaperones. This review focuses primarily on bacterial Hsp90 and highlights similarities and differences between bacterial and eukaryotic Hsp90. Seminal research findings that elucidate the structure and the mechanisms of protein folding, disaggregation, and reactivation promoted by Hsp90 are discussed. Understanding the mechanisms of bacterial Hsp90 will provide fundamental insight into the more complex eukaryotic chaperone systems.

Heat Shock Response and Heat Shock Proteins: Current Understanding and Future Opportunities in Human Diseases

The heat shock response is an evolutionarily conserved mechanism that protects cells or organisms from the harmful effects of various stressors such as heat, chemicals toxins, UV radiation, and oxidizing agents. The heat shock response triggers the expression of a specific set of genes and proteins known as heat shock genes/proteins or molecular chaperones, including HSP100, HSP90, HSP70, HSP60, and small HSPs. Heat shock proteins (HSPs) play a crucial role in thermotolerance and aiding in protecting cells from harmful insults of stressors. HSPs are involved in essential cellular functions such as protein folding, eliminating misfolded proteins, apoptosis, and modulating cell signaling. The stress response to various environmental insults has been extensively studied in organisms from prokaryotes to higher organisms. The responses of organisms to various environmental stressors rely on the intensity and threshold of the stress stimuli, which vary among organisms and cellular contexts. Studies on heat shock proteins have primarily focused on HSP70, HSP90, HSP60, small HSPs, and ubiquitin, along with their applications in human biology. The current review highlighted a comprehensive mechanism of heat shock response and explores the function of heat shock proteins in stress management, as well as their potential as therapeutic agents and diagnostic markers for various diseases.

K64 acetylation of heat shock protein 90 suppresses nucleopolyhedrovirus replication in Bombyx mori

HSP90 is a highly conserved chaperone that facilitates the proliferation of many viruses, including silkworm (bombyx mori) nucleopolyhedrovirus (BmNPV), but the underlying regulatory mechanism was unclear. We found that suppression of HSP90 by 17-AAG, a HSP90-specific inhibitor, significantly reduced the expression of BmNPV capsid protein gp64 and viral genome replication, whereas overexpression of B. mori HSP90(BmHSP90) promoted BmNPV replication. Furthermore, in a recent study of the lysine acetylome of B. mori infected with BmNPV, we focused on the reduced viral proliferation due to changes of BmHSP90 lysine acetylation. Site-directed introduction of acetylated (K/Q) or deacetylated (K/R) mimic mutations into BmHSP90 revealed that lysine 64 (K64) acetylation activated the JAK/STAT pathway and reduced BmHSP90 ATPase activity, leading to diminished chaperone activity and ultimately inhibiting BmNPV proliferation. In this study, a single lysine 64 acetylation change of BmHSP90 was elucidated as a model of posttranslational modifications occurring in the wake of host-virus interactions, providing novel insights into potential antiviral strategies.

Functional and physical interaction between yeast Hsp90 and Hsp70

Heat shock protein 90 (Hsp90) is a highly conserved ATP-dependent molecular chaperone that is essential in eukaryotes. It is required for the activation and stabilization of more than 200 client proteins, including many kinases and steroid hormone receptors involved in cell-signaling pathways. Hsp90 chaperone activity requires collaboration with a subset of the many Hsp90 cochaperones, including the Hsp70 chaperone. In higher eukaryotes, the collaboration between Hsp90 and Hsp70 is indirect and involves Hop, a cochaperone that interacts with both Hsp90 and Hsp70. Here we show that yeast Hsp90 (Hsp82) and yeast Hsp70 (Ssa1), directly interact in vitro in the absence of the yeast Hop homolog (Sti1), and identify a region in the middle domain of yeast Hsp90 that is required for the interaction. In vivo results using Hsp90 substitution mutants showed that several residues in this region were important or essential for growth at high temperature. Moreover, mutants in this region were defective in interaction with Hsp70 in cell lysates. In vitro, the purified Hsp82 mutant proteins were defective in direct physical interaction with Ssa1 and in protein remodeling in collaboration with Ssa1 and cochaperones. This region of Hsp90 is also important for interactions with several Hsp90 cochaperones and client proteins, suggesting that collaboration between Hsp70 and Hsp90 in protein remodeling may be modulated through competition between Hsp70 and Hsp90 cochaperones for the interaction surface.

Interaction of a Novel Chaperone PhLP2A With the Heat Shock Protein Hsp90

PhLP2 is a small cytosolic protein that belongs to the highly conserved phosducin-like family of proteins. In amniote genomes there are two PhLP2 homologs, PhLP2A and PhLP2B. It has been shown that mammalian PhLP2A modulates the CCT/TRiC chaperonin activity during folding of cytoskeletal proteins. In order to better understand the function of PhLP2A in cellular protein quality control system, in the present study we have searched for its protein targets. Applying immunoprecipitation followed by mass spectrometry analysis we have identified Hsp90 as a partner of PhLP2A. With pull down experiments, we have confirmed this interaction in protein lysate and using purified proteins we have shown that PhLP2A interacts directly with Hsp90. Furthermore, the proximity ligation assay (PLA) performed on mIMCD-3 cells has shown that PhLP2A forms complexes with Hsp90 which are mainly localized in the cytoplasm of these cells. Further analysis has indicated that the level of PhLP2A increases after heat shock or radicicol treatment, similarly as the level of Hsp90, and that expression of PhLP2A after heat shock is regulated at the transcriptional level. Moreover, using recombinant luciferase we have shown that PhLP2A stabilizes this enzyme in a folding competent state and prevents its denaturation and aggregation. In addition, overexpression of PhLP2A in HEK-293 cells leads to increased heat stress resistance. Altogether, our results have shown that PhLP2A interacts with Hsp90 and exhibits molecular chaperone activity toward denatured proteins. J. Cell. Biochem. 118: 420-429, 2017. © 2016 Wiley Periodicals, Inc.

Semi-automated microplate monitoring of protein polymerization and aggregation

Static light scattering (SLS) is a commonly used technique for monitoring dynamics of high molecular weight protein complexes such as protein oligomers or aggregates. However, traditional methods are limited to testing a single condition and typically require large amounts of protein and specialized equipment. We show that a standard microplate reader can be used to characterize the molecular dynamics of different types of protein complexes, with the multiple advantages of microscale experimental volumes, semi-automated protocols and highly parallel processing.

Mitochondrial Hsp90 is a ligand-activated molecular chaperone coupling ATP binding to dimer closure through a coiled-coil intermediate

Heat-shock protein of 90 kDa (Hsp90) is an essential molecular chaperone that adopts different 3D structures associated with distinct nucleotide states: a wide-open, V-shaped dimer in the apo state and a twisted, N-terminally closed dimer with ATP. Although the N domain is known to mediate ATP binding, how Hsp90 senses the bound nucleotide and facilitates dimer closure remains unclear. Here we present atomic structures of human mitochondrial Hsp90N (TRAP1N) and a composite model of intact TRAP1 revealing a previously unobserved coiled-coil dimer conformation that may precede dimer closure and is conserved in intact TRAP1 in solution. Our structure suggests that TRAP1 normally exists in an autoinhibited state with the ATP lid bound to the nucleotide-binding pocket. ATP binding displaces the ATP lid that signals the cis-bound ATP status to the neighboring subunit in a highly cooperative manner compatible with the coiled-coil intermediate state. We propose that TRAP1 is a ligand-activated molecular chaperone, which couples ATP binding to dramatic changes in local structure required for protein folding.

Overexpression of Hsp90 from grass carp (Ctenopharyngodon idella) increases thermal protection against heat stress

With homologous DNA probes, we had screened a grass carp heat shock protein 90 gene (CiHsp90). The full sequence of CiHsp90 cDNA was 2793 bp, which could code a 798 amino acids peptide. The phylogenetic analysis demonstrated that CiHsp90 shared the high homology with Zebrafish Grp94. Quantitative RT-PCR analysis showed that CiHsp90 was ubiquitously expressed at lower levels in all detected tissues and up-regulated after heat shock at 34 °C or cold stress at 4 °C. To understand the function of CiHsp90 involving in thermal protection, an expression vector containing coding region cDNA was expressed in E. coli BL21 (DE3) plysS. Upon transfer from 37 °C to 42 °C, these cells that accumulated CiHsp90 peptides displayed greater thermoresistance than the control cells. While incubated at 4°C for different periods, it could also improve the cell viability. After transient transfected recombinant plasmid pcDNA3.1/CiHsp90 into mouse myeloma cell line SP2/0, we found that CiHsp90 could contribute to protecting cells against both thermal and cold extremes. On the contrary, the mutant construct ΔN-CiHsp90 (256-798aa) could abolish the protection activity both in prokaryotic cells and eukaryotic cells. Additionally, both CiHsp90 and ΔN-CiHsp90 peptides could reduce the level of citrate synthase aggregation at the high temperature.

Probing molecular mechanisms of the Hsp90 chaperone: biophysical modeling identifies key regulators of functional dynamics

Deciphering functional mechanisms of the Hsp90 chaperone machinery is an important objective in cancer biology aiming to facilitate discovery of targeted anti-cancer therapies. Despite significant advances in understanding structure and function of molecular chaperones, organizing molecular principles that control the relationship between conformational diversity and functional mechanisms of the Hsp90 activity lack a sufficient quantitative characterization. We combined molecular dynamics simulations, principal component analysis, the energy landscape model and structure-functional analysis of Hsp90 regulatory interactions to systematically investigate functional dynamics of the molecular chaperone. This approach has identified a network of conserved regions common to the Hsp90 chaperones that could play a universal role in coordinating functional dynamics, principal collective motions and allosteric signaling of Hsp90. We have found that these functional motifs may be utilized by the molecular chaperone machinery to act collectively as central regulators of Hsp90 dynamics and activity, including the inter-domain communications, control of ATP hydrolysis, and protein client binding. These findings have provided support to a long-standing assertion that allosteric regulation and catalysis may have emerged via common evolutionary routes. The interaction networks regulating functional motions of Hsp90 may be determined by the inherent structural architecture of the molecular chaperone. At the same time, the thermodynamics-based "conformational selection" of functional states is likely to be activated based on the nature of the binding partner. This mechanistic model of Hsp90 dynamics and function is consistent with the notion that allosteric networks orchestrating cooperative protein motions can be formed by evolutionary conserved and sparsely connected residue clusters. Hence, allosteric signaling through a small network of distantly connected residue clusters may be a rather general functional requirement encoded across molecular chaperones. The obtained insights may be useful in guiding discovery of allosteric Hsp90 inhibitors targeting protein interfaces with co-chaperones and protein binding clients.

Destabilization of Bcr-Abl/Jak2 Network by a Jak2/Abl Kinase Inhibitor ON044580 Overcomes Drug Resistance in Blast Crisis Chronic Myelogenous Leukemia (CML)

Bcr-Abl is the predominant therapeutic target in chronic myeloid leukemia (CML), and tyrosine kinase inhibitors (TKIs) that inhibit Bcr-Abl have been successful in treating CML. With progression of CML disease especially in blast crisis stage, cells from CML patients become resistant to imatinib mesylate (IM) and other TKIs, resulting in relapse. Because Bcr-Abl is known to drive multiple signaling pathways, the study of the regulation of stability of Bcr-Abl in IM-resistant CML cells is a critical issue as a possible therapeutic strategy. Here, we report that a new dual-kinase chemical inhibitor, ON044580, induced apoptosis of Bcr-Abl+ IM-sensitive, IM-resistant cells, including the gatekeeper Bcr-Abl mutant, T315I, and also cells from blast crisis patients. In addition, IM-resistant K562-R cells, cells from blast crisis CML patients, and all IM-resistant cell lines tested had reduced ability to form colonies in soft agar in the presence of 0.5 µM ON044580. In in vitro kinase assays, ON044580 inhibited the recombinant Jak2 and Abl kinase activities when the respective Jak2 and Abl peptides were used as substrates. Incubation of the Bcr-Abl+ cells with ON044580 rapidly reduced the levels of the Bcr-Abl protein and also reduced the expression of HSP90 and its client protein levels. Lysates of Bcr-Abl+ cell lines were found to contain a large signaling network complex composed of Bcr-Abl, Jak2, HSP90, and its client proteins as detected by a gel filtration column chromatography, which was rapidly disrupted by ON044580. Therefore, targeting Jak2 and Bcr-Abl kinases is an effective way to destabilize Bcr-Abl and its network complex, which leads to the onset of apoptosis in IM-sensitive and IM-resistant Bcr-Abl+ cells. This inhibitory strategy has potential to manage all types of drug-resistant CML cells, especially at the terminal blast crisis stage of CML, where TKIs are not clinically useful.

A systematic protocol for the characterization of Hsp90 modulators

Several Hsp90 modulators have been identified including the N-terminal ligand geldanamycin (GDA), the C-terminal ligand novobiocin (NB), and the co-chaperone disruptor celastrol. Other Hsp90 modulators elicit a mechanism of action that remains unknown. For example, the natural product gedunin and the synthetic anti-spermatogenic agent H2-gamendazole, recently identified Hsp90 modulators, manifest biological activity through undefined mechanisms. Herein, we report a series of biochemical techniques used to classify such modulators into identifiable categories. Such studies provided evidence that gedunin and H2-gamendazole both modulate Hsp90 via a mechanism similar to celastrol, and unlike NB or GDA.

Characterization of plant p23-like proteins for their co-chaperone activities

The small acidic protein p23 is best described as a co-chaperone of Hsp90, an essential molecular chaperone in eukaryotes. p23 binds to the ATP-bound form of Hsp90 and stabilizes the Hsp90-client protein complex by slowing down ATP turnover. The stabilizing activity of p23 was first characterized in studies of steroid receptor-Hsp90 complexes. Earlier studies of the Hsp90 chaperone complex in plants suggested that a p23-like stabilizing activity was absent in plant cell lysates. Here, we show that p23-like proteins are present in plants and are capable of binding Hsp90, but unlike human p23 and yeast ortholog Sba1, the plant p23-like proteins do not stabilize the steroid receptor-Hsp90 complexes formed in wheat germ lysate. Furthermore, these proteins do not inhibit the ATPase activity of plant Hsp90. While transcripts of Arabidopsis thaliana p23-1 and Atp23-2 were detected under normal growing conditions, those of the closely related Brassica napus p23-1 were present only after moderate heat stress. These observations suggest that p23-like proteins in plants are conserved in their binding to Hsp90 but have evolved mechanisms of action different from their yeast and animal counterparts.

Functional characterization of orchardgrass endoplasmic reticulum-resident Hsp90 (DgHsp90) as a chaperone and an ATPase

Hsp90 proteins are essential molecular chaperones regulating multiple cellular processes in distinct subcellular organelles. In this study, we report the functional characterization of a cDNA encoding endoplasmic reticulum (ER)-resident Hsp90 from orchardgrass (DgHsp90). DgHsp90 is a 2742bp cDNA with an open reading frame predicted to encode an 808 amino acid protein. DgHsp90 has a well conserved N-terminal ATPase domain and a C-terminal Hsp90 domain and ER-retention motif. Expression of DgHsp90 increased during heat stress at 35 degrees C or H(2)O(2) treatment. DgHsp90 also functions as a chaperone protein by preventing thermal aggregation of malate dehydrogenase (EC 1.1.1.37) and citrate synthase (EC 2.3.3.1). The intrinsic ATPase activity of DgHsp90 was inhibited by geldanamycin, an Hsp90 inhibitor, and the inhibition reduced the chaperone activity of DgHsp90. Yeast cells overexpressing DgHsp90 exhibited enhanced thermotolerance.

Neuroprotective activity and evaluation of Hsp90 inhibitors in an immortalized neuronal cell line

Alzheimer's disease (AD) neuropathology is characterized by loss of synapses and neurons, neuritic plaques consisting of beta-amyloid (Abeta) peptides, and neurofibrillary tangles consisting of intracellular aggregates of hyperphosphorylated tau protein in susceptible brain regions. Abeta oligomers trigger a cascade of pathogenic events including tau hyperphosphorylation and aggregation, inflammatory reactions, and excitotoxicity that contribute to the progression of AD. The molecular chaperone Hsp90 facilitates the folding of newly synthesized and denatured proteins and is believed to play a role in neurodegenerative disorders in which the defining pathology results in misfolded proteins and the accumulation of protein aggregates. Some agents that inhibit Hsp90 protect neurons against Abeta toxicity and tau aggregation, and assays for rapidly screening potential Hsp90 inhibitors are of interest. We used the release of the soluble cytosolic enzyme lactate dehydrogenase (LDH) as an indicator of the loss of cell membrane integrity and cytotoxicity resulting from exposure to Abeta peptides to evaluate the neuroprotective properties of novel novobiocin analogues and established Hsp90 inhibitors. Compounds were assessed for potency in protecting proliferating and differentiated SH-SY5Y neuronal cells against Abeta-induced cell death; the potential toxicity of each agent alone was also determined. The data indicated that several of the compounds decreased Abeta toxicity even at low nanomolar concentrations and, unexpectedly, were more potent in protecting the undifferentiated cells against Abeta. The novobiocin analogues alone were not toxic even up to 10 microM concentrations whereas GDA and the parent compound, novobiocin, were toxic at 1 and 10 microM, respectively. The results suggest that novobiocin analogues may provide novel leads for the development of neuroprotective drugs.

Structures of GRP94-nucleotide complexes reveal mechanistic differences between the hsp90 chaperones

GRP94, an essential endoplasmic reticulum chaperone, is required for the conformational maturation of proteins destined for cell-surface display or export. The extent to which GRP94 and its cytosolic paralog, Hsp90, share a common mechanism remains controversial. GRP94 has not been shown conclusively to hydrolyze ATP or bind cochaperones, and both activities, by contrast, result in conformational changes and N-terminal dimerization in Hsp90 that are critical for its function. Here, we report the 2.4 A crystal structure of mammalian GRP94 in complex with AMPPNP and ADP. The chaperone is conformationally insensitive to the identity of the bound nucleotide, adopting a "twisted V" conformation that precludes N-terminal domain dimerization. We also present conclusive evidence that GRP94 possesses ATPase activity. Our observations provide a structural explanation for GRP94's observed rate of ATP hydrolysis and suggest a model for the role of ATP binding and hydrolysis in the GRP94 chaperone cycle.

Humoral immune response upon mild heat-shock conditions in Galleria mellonella larvae

Larvae of Galleria mellonella exposed to mild heat-shock (38 degrees C) showed an enhanced humoral immune response after microbial infection in comparison to infected animals grown at 28 degrees C. This enhanced response was manifested by increased expression of antimicrobial peptide (AMP) genes leading to enhanced antimicrobial activity in the hemolymph. We found an increased level of Hsp90 and changes in the level of a 55kDa protein recognized by anti-Hsp90 antibodies in fat bodies of infected animals reared at 28 degrees C as well as in uninfected animals exposed to elevated temperature. Pre-treatment of animals with an inhibitor of Hsp90, 17-DMAG, prior to immunization resulted in increased expression of AMP genes encoding gallerimycin and cecropin at 38 degrees C. This observation was correlated with the changes in Hsp90 protein and increased level of 55kDa protein. Also G. mellonella larvae pre-treated with 17-DMAG and exposed to mild heat-shock for 30min showed an increased survival rate after infection with entomopathogenic bacteria Pseudomonas aeruginosa. We also show the effect of 17-DMAG on the phosphorylation state of ERK MAP kinase. We postulate that Hsp90 may play a significant role in converging pathways involved in the insect immune response and heat-shock.

Chaperone activation of the hepadnaviral reverse transcriptase for template RNA binding is established by the Hsp70 and stimulated by the Hsp90 system

Hepadnaviruses are DNA viruses that replicate by protein-primed reverse transcription, employing a specialized reverse transcriptase (RT), P protein. DNA synthesis from the pregenomic RNA is initiated by binding of P to the ε signal. Using ε as template and a Tyr-residue for initiation, the RT synthesizes a DNA oligo (priming) as primer for full-length DNA. Priming strictly requires prior RT activation by chaperones. Active P–ε complexes have been reconstituted in vitro, but whether in addition to the heat-shock protein 70 (Hsp70) system the Hsp90 system is essential has been controversial. Here we quantitatively compared Hsp70 versus Hsp70 plus Hsp90 RT activation, and corroborated that the Hsp70 system alone is sufficient; however, Hsp90 as well the Hsp70 nucleotide exchange factor Bag-1 markedly stimulated activation by increasing the steady-state concentration of the activated metastable RT form P*, though by different mechanisms. Hsp90 inhibition in intact cells by geldanamycin analogs blocked hepadnavirus replication, however not completely and only at severely cytotoxic inhibitor concentrations. While compatible with a basal level of Hsp90 independent in vivo replication, unambiguous statements are precluded by the simultaneous massive upregulation of Hsp70 and Hsp90.

Prevention of peroxynitrite-induced apoptosis of motor neurons and PC12 cells by tyrosine-containing peptides

Although peroxynitrite stimulates apoptosis in many cell types, whether peroxynitrite acts directly as an oxidant or the induction of apoptosis is because of the radicals derived from peroxynitrite decomposition remains unknown. Before undergoing apoptosis because of trophic factor deprivation, primary motor neuron cultures become immunoreactive for nitrotyrosine. We show here using tyrosine-containing peptides that free radical processes mediated by peroxynitrite decomposition products were required for triggering apoptosis in primary motor neurons and in PC12 cells cultures. The same concentrations of tyrosine-containing peptides required to prevent the nitration and apoptosis of motor neurons induced by trophic factor deprivation and of PC12 cells induced by peroxynitrite also prevented peroxynitrite-mediated nitration of motor neurons, brain homogenates, and PC12 cells. The heat shock protein 90 chaperone was nitrated in both trophic factor-deprived motor neurons and PC12 cells incubated with peroxynitrite. Tyrosine-containing peptides did not affect the induction of PC12 cell death by hydrogen peroxide. Tyrosine-containing peptides should protect by scavenging peroxynitrite-derived radicals and not by direct reactions with peroxynitrite as they neither increase the rate of peroxynitrite decomposition nor decrease the bimolecular peroxynitrite-mediated oxidation of thiols. These results reveal an important role for free radical-mediated nitration of tyrosine residues, in apoptosis induced by endogenously produced and exogenously added peroxynitrite; moreover, tyrosine-containing peptides may offer a novel strategy to neutralize the toxic effects of peroxynitrite.

High-throughput assay for the identification of Hsp90 inhibitors based on Hsp90-dependent refolding of firefly luciferase

Previously, we have demonstrated that the renaturation of heat denatured firefly luciferase is dependent upon the activity of Hsp90 in rabbit reticulocyte lysate. Here, we demonstrate that this assay may identify inhibitors that obstruct the chaperone activity of Hsp90 either by direct binding to its N-terminal or C-terminal nucleotide binding sites or by interference with the ability of the chaperone to switch conformations. The assay was adapted and optimized for high-throughput screening. Greater than 20,000 compounds were screened to demonstrate the feasibility of using this assay on a large scale. The assay was reproducible (av Z-factor=0.62) and identified 120 compounds that inhibited luciferase renaturation by greater than 70% at a concentration of 12.5 microg/mL. IC50 values for twenty compounds with varying structures were determined for inhibition of luciferase refolding and in cell-based assays for Hsp90 inhibition. Several compounds had IC50 values <10 microM and represent a number of new lead structures with the potential for further development and optimization as potent Hsp90 inhibitors.

A novel method for detecting intramolecular coevolution: adding a further dimension to selective constraints analyses

Protein evolution depends on intramolecular coevolutionary networks whose complexity is proportional to the underlying functional and structural interactions among sites. Here we present a novel approach that vastly improves the sensitivity of previous methods for detecting coevolution through a weighted comparison of divergence between amino acid sites. The analysis of the HIV-1 Gag protein detected convergent adaptive coevolutionary events responsible for the selective variability emerging between subtypes. Coevolution analysis and functional data for heat-shock proteins, Hsp90 and GroEL, highlight that almost all detected coevolving sites are functionally or structurally important. The results support previous suggestions pinpointing the complex interdomain functional interactions within these proteins and we propose new amino acid sites as important for interdomain functional communication. Three-dimensional information sheds light on the functional and structural constraints governing the coevolution between sites. Our covariation analyses propose two types of coevolving sites in agreement with previous reports: pairs of sites spatially proximal, where compensatory mutations could maintain the local structure stability, and clusters of distant sites located in functional domains, suggesting a functional dependency between them. All sites detected under adaptive evolution in these proteins belong to coevolution groups, further underlining the importance of testing for coevolution in selective constraints analyses.

Substrate transfer from the chaperone Hsp70 to Hsp90

Hsp90 is an essential chaperone protein in the cytosol of eukaryotic cells. It cooperates with the chaperone Hsp70 in defined complexes mediated by the adaptor protein Hop (Sti1 in yeast). These Hsp70/Hsp90 chaperone complexes play a major role in the folding and maturation of key regulatory proteins in eukaryotes. Understanding how non-native client proteins are transferred from one chaperone to the other in these complexes is of central importance. Here, we analyzed the molecular mechanism of this reaction using luciferase as a substrate protein. Our experiments define a pathway for luciferase folding in the Hsp70/Hsp90 chaperone system. They demonstrate that Hsp70 is a potent capture device for unfolded protein while Hsp90 is not very efficient in this reaction. When Hsp90 is absent, in contrast to the in vivo situation, Hsp70 together with the two effector proteins Ydj1 and Sti1 exhibits chaperone activity towards luciferase. In the presence of the complete chaperone system, Hsp90 exhibits a specific positive effect only in the presence of Ydj1. If this factor is absent, the transferred luciferase is trapped on Hsp90 in an inactive conformation. Interestingly, identical results were observed for the yeast and the human chaperone systems although the regulatory function of human Hop is completely different from that of yeast Sti1.

The HSP90 family of genes in the human genome: insights into their divergence and evolution

HSP90 proteins are important molecular chaperones. Transcriptome and genome analyses revealed that the human HSP90 family includes 17 genes that fall into four classes. A standardized nomenclature for each of these genes is presented here. Classes HSP90AA, HSP90AB, HSP90B, and TRAP contain 7, 6, 3, and 1 genes, respectively. HSP90AA genes mapped onto chromosomes 1, 3, 4, and 11; HSP90AB genes mapped onto 3, 4, 6, 13 and 15; HSP90B genes mapped onto 1, 12, and 15; and the TRAP1 gene mapped onto 16. Six genes, HSP90AA1, HSP90AA2, HSP90N, HSP90AB1, HSP90B1 and TRAP1, were recognized as functional, and the remaining 11 genes were considered putative pseudogenes. Amino acid polymorphic variants were detected for genes HSP90AA1, HSP90AA2, HSP90AB1, HSP90B1, and TRAP1. The structures of these genes and the functional motifs and polymorphic variants of their proteins were documented and the features and functions of their proteins were discussed. Phylogenetic analyses based on both nucleotide and protein data demonstrated that HSP90(AA+AB+B) formed a monophyletic clade, whereas TRAP is a relatively distant paralogue of this clade.

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.

Hsp90 regulates the activity of wild type p53 under physiological and elevated temperatures

The activity and structural integrity of the tumor suppressor protein p53 is of crucial importance for the prevention of cancer. p53 is a conformational flexible and labile protein, in which structured and unstructured regions function in a synergistic manner. The molecular chaperone Hsp90 is known to bind to mutant and wild type p53 in vivo. Using highly purified proteins we analyzed the interaction and the binding sites between both proteins in detail. Our results demonstrate that Hsp90 binds to a folded, native-like conformation of p53 in vitro with micromolar affinity. Specifically, the DNA-binding domain of p53 and the middle and carboxy-terminal domains of Hsp90 are responsible for this interaction, which is essential to stabilize p53 at physiological temperatures and to prevent it from irreversible thermal inactivation. Our results are in agreement with a model in which Hsp90 is required to maintain the folded, active state of p53 by a reversible interaction, thus introducing an additional level of regulation.

The crystal structure of the carboxy-terminal dimerization domain of htpG, the Escherichia coli Hsp90, reveals a potential substrate binding site

Hsp90 is a ubiquitous, well-conserved molecular chaperone involved in the folding and stabilization of diverse proteins. Beyond its capacity for general protein folding, Hsp90 influences a wide array of cellular signaling pathways that underlie key biological and disease processes. It has been proposed that Hsp90 functions as a molecular clamp, dimerizing through its carboxy-terminal domain and utilizing ATP binding and hydrolysis to drive large conformational changes including transient dimerization of the amino-terminal and middle domains. We have determined the 2.6 A X-ray crystal structure of the carboxy-terminal domain of htpG, the Escherichia coli Hsp90. This structure reveals a novel fold and that dimerization is dependent upon the formation of a four-helix bundle. Remarkably, proximal to the helical dimerization motif, each monomer projects a short helix into solvent. The location, flexibility, and amphipathic character of this helix suggests that it may play a role in substrate binding and hence chaperone activity.

p23, a simple protein with complex activities

p23 is a small but important cochaperone for the Hsp90 chaperoning pathway. It appears to facilitate the adenosine triphosphate-driven cycle of Hsp90 binding to client proteins. It enters at a late stage of the cycle and enhances the maturation of client proteins. Although this role of p23 is fairly well established, recent studies suggest that it may have additional functions in the cell that merit further exploration.

Expansion of Protein Interaction Maps by Phage Peptide Display Using MDM2 as a Prototypical Conformationally Flexible Target Protein

Expanding on the possible protein interaction partners in a biochemical pathway is one key molecular goal in the post-genomic era. Phage peptide display is a versatile in vitro tool for mapping novel protein-protein interfaces and the advantage of this technique in expanding protein interaction maps is that in vitro manipulation of the bait protein conformational integrity can be controlled carefully. Phage peptide display was used to expand on the possible types of binding proteins for the conformationally responsive protein MDM2. Peptides enriched differ depending upon whether MDM2 is ligand-free, zinc-bound, or RNA-bound, suggesting that MDM2 conformational changes alter the type of peptide ligands enriched. Classes of putative/established MDM2-binding proteins identified by this technique included ubiquitin-modifying enzymes (F-box proteins, UB-ligases, UBC-E1) and apoptotic modifiers (HSP90, GAS1, APAF1, p53). Of the many putative MDM2 proteins that could be examined, the impact of HSP90 on MDM2 activity was studied, since HSP90 has been linked with p53 protein unfolding in human cancers. Zinc ions were required to reconstitute a stable MDM2-HSP90 protein complex. Zinc binding converted MDM2 from a monomer to an oligomer, and activated MDM2 binding to its internal RING finger domain, providing evidence for a conformational change in MDM2 protein when it binds zinc. Reconstitution of an HSP90-MDM2 protein complex in vitro stimulated the unfolding of the p53 tetramer. A p53 DNA-binding inhibitor purified from human cells that is capable of unfolding p53 at ambient temperature in vitro contains co-purifying pools of HSP90 and MDM2. These data highlight the utility of phage peptide display as a powerful in vitro method to identify regulatory proteins that bind to a conformationally flexible protein like MDM2.

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.

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.

HSP-25 and HSP-90 stabilize Na,K-ATPase in cytoskeletal fractions of ischemic rat renal cortex

BACKGROUND

We recently designed an in vitro system based on differential Triton-extractability of Na,K-ATPase from the cytoskeletal protein fraction isolated from rat renal cortex after renal ischemia. In the present study, we hypothesized that heat shock protein (HSP)-70, HSP-25 and HSP-90 work synergistically to stabilize the cytoskeletal anchorage of Na,K-ATPase.

METHODS

Cellular proteins were fractionated by differential centrifugation into cytoskeletal pellets (I-PEL) obtained early (exhibiting abnormally high Triton extractability of Na,K-ATPase) and non-cytoskeletal supernatants (R-SUP) obtained late (exhibiting high abundance of HSP) after renal ischemia. For assessment of the role of HSP-70, HSP-25 and HSP-90 upon in vitro re-compartmentalization, I-PEL was either incubated in R-SUP with/without HSP antibodies, or in buffer with/without HSPs at different titers and combinations. Effects were evaluated by changes of Triton extractability of Na,K-ATPase after co-incubation.

RESULTS

R-SUP was shown to contain significant amounts of HSP-70, HSP-25 and HSP-90. Incubation of I-PEL in R-SUP reduced Triton extractability of Na,K-ATPase. Addition of antibodies against each HSP significantly abolished these effects of R-SUP. Incubation of I-PEL with purified HSP-70, HSP-25 or HSP-90 each partly reproduced the effects of R-SUP, whereas the combination of all three HSP demonstrated a strong and more than additive effect on the cytoskeletal stabilization of Na,K-ATPase.

CONCLUSIONS

The molecular mechanisms responsible for postischemic re-compartmentalization of Na,K-ATPase in rat renal cortex likely involves interactions between HSP-70, HSP-25 and HSP-90, stress proteins known to be induced in the ischemic kidney.

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.

The action of molecular chaperones in the early secretory pathway

The endoplasmic reticulum (ER) serves as a way-station during the biogenesis of nearly all secreted proteins, and associated with or housed within the ER are factors required to catalyze their import into the ER and facilitate their folding. To ensure that only properly folded proteins are secreted and to temper the effects of cellular stress, the ER can target aberrant proteins for degradation and/or adapt to the accumulation of misfolded proteins. Molecular chaperones play critical roles in each of these phenomena.

The N-terminal adenosine triphosphate binding domain of Hsp90 is necessary and sufficient for interaction with estrogen receptor

To understand how the molecular chaperone Hsp90 participates in conformational maturation of the estrogen receptor (ER), we analyzed the interaction of immobilized purified avian Hsp90 with mammalian cytosolic ER. Hsp90 was either immunoadsorbed to BF4 antibody-Sepharose or GST-Hsp90 fusion protein (GST.90) was adsorbed to glutathione-Sepharose. GST.90 was able to retain specifically ER, similarly to immunoadsorbed Hsp90. When cells were treated with estradiol and the hormone treatment was maintained during cell homogenization, binding, and washing steps, GST.90 still interacted efficiently with ER, suggesting that ER may form complexes with Hsp90 even after its activation by hormone and salt extraction from nuclei. The GST.90-ER interaction was consistently reduced in the presence of increasing concentrations of potassium chloride or when cytosolic ER-Hsp90 complexes were previously stabilized by molybdate, indicating that GST.90-ER complexes behave like cytosolic Hsp90-ER complexes. A purified thioredoxin-ER fusion protein was also able to form complexes with GST.90, suggesting that the presence of other chaperones is not required. ER was retained only by GST.90 deletion mutants bearing an intact Hsp90 N-terminal region (1-224), the interaction being more efficient when the charged region A was present in the mutant (1-334). The N-terminal fragment 1-334, devoid of the dimeric GST moiety, was also able to interact with ER, pointing to the monomeric N-terminal adenosine triphosphate binding region of Hsp90 (1-224) as the region necessary and sufficient for interaction. These results contribute to understand the Hsp90-dependent process responsible for conformational competence of ER.

Hsp90: chaperoning signal transduction

Hsp90 is an ATP dependent molecular chaperone involved in the folding and activation of an unknown number of substrate proteins. These substrate proteins include protein kinases and transcription factors. Consistent with this task, Hsp90 is an essential protein in all eucaryotes. The interaction of Hsp90 with its substrate proteins involves the transient formation of multiprotein complexes with a set of highly conserved partner proteins. The specific function of each component in the processing of substrates is still unknown. Large ATP-dependent conformational changes of Hsp90 occur during the hydrolysis reaction and these changes are thought to drive the chaperone cycle. Natural inhibitors of the ATPase activity, like geldanamycin and radicicol, block the processing of Hsp90 substrate proteins. As many of these substrates are critical elements in signal transduction, Hsp90 seems to introduce an additional level of regulation.