Hsp90: structure and function

Production of Recombinant Pharmaceutical Proteins

The proteins produced in the body control and mediate the metabolic processes and help in its routine functioning. Any kind of impairment in protein production, such as production of mutated protein, or misfolded protein, leads to disruption of the pathway controlled by that protein. This may manifest in the form of the disease. However, these diseases can be treated, by supplying the protein from outside or exogenously. The supply of active exogenous protein requires its production on large scale to fulfill the growing demand. The process is complex, requiring higher protein expression, purification, and processing. Each product needs unique settings or standardizations for large-scale production and purification. As only large-scale production can fulfill the growing demand, thus it needs to be cost-effective. The tools of genetic engineering are utilized to produce the proteins of human origin in bacteria, fungi, insect, or mammalian host. Usage of recombinant DNA technology for large-scale production of proteins requires ample amount of time, labor, and resources, but it also offers many opportunities for economic growth. After reading this chapter, readers would be able to understand the basics about production of recombinant proteins in various hosts along with the advantages and limitations of each host system and properties and production of some of the important pharmaceutical compounds and growth factors.

Telomerase Regulation from Beginning to the End

The vast body of literature regarding human telomere maintenance is a true testament to the importance of understanding telomere regulation in both normal and diseased states. In this review, our goal was simple: tell the telomerase story from the biogenesis of its parts to its maturity as a complex and function at its site of action, emphasizing new developments and how they contribute to the foundational knowledge of telomerase and telomere biology.

Heat Shock Protein 90 kDa (Hsp90) Has a Second Functional Interaction Site with the Mitochondrial Import Receptor Tom70

To accomplish its crucial role, mitochondria require proteins that are produced in the cytosol, delivered by cytosolic Hsp90, and translocated to its interior by the translocase outer membrane (TOM) complex. Hsp90 is a dimeric molecular chaperone and its function is modulated by its interaction with a large variety of co-chaperones expressed within the cell. An important family of co-chaperones is characterized by the presence of one TPR (tetratricopeptide repeat) domain, which binds to the C-terminal MEEVD motif of Hsp90. These include Tom70, an important component of the TOM complex. Despite a wealth of studies conducted on the relevance of Tom70·Hsp90 complex formation, there is a dearth of information regarding the exact molecular mode of interaction. To help fill this void, we have employed a combined experimental strategy consisting of cross-linking/mass spectrometry to investigate binding of the C-terminal Hsp90 domain to the cytosolic domain of Tom70. This approach has identified a novel region of contact between C-Hsp90 and Tom70, a finding that is confirmed by probing the corresponding peptides derived from cross-linking experiments via isothermal titration calorimetry and mitochondrial import assays. The data generated in this study are combined to input constraints for a molecular model of the Hsp90/Tom70 interaction, which has been validated by small angle x-ray scattering, hydrogen/deuterium exchange, and mass spectrometry. The resultant model suggests that only one of the MEEVD motifs within dimeric Hsp90 contacts Tom70. Collectively, our findings provide significant insight on the mechanisms by which preproteins interact with Hsp90 and are translocated via Tom70 to the mitochondria.

Size dependent classification of heat shock proteins: a mini-review

Molecular chaperones are ubiquitous and abundant within cellular environments, functioning as a defense mechanism against outer environment. The range of molecular chaperones varies from 10 to over 100 kDa. Depending on the size, the specific locations and physiological roles of molecular chaperones vary within the cell. Multifunctionality of heat shock proteins (HSPs) expressed via various cyto-stress including heat shock have been spotlighted as a reliable prognostic target biomarker for therapeutic purpose in neuromuscular disease or cancer related studies. HSP also plays a critical role in the maintenance of proteins and cellular homeostasis in exercise-induced adaptation. Such various functions of HSPs give scientists insights into intracellular protective mechanisms in the living body thus HSPs can be target molecules to know the defense mechanism in cellular environment. Based on experimental results regarding small to large scaled HSPs, this review aims to provide updated important information regarding the modality of responses of intracellular HSPs towards extracellular stimulations. Further, the expressive mechanisms of HSPs data from tremendous in vivo and in vitro studies underlying the enhancement of the functionality of living body will be discussed.

Two HSP90 genes in mandarin fish Siniperca chuatsi: identification, characterization and their specific expression profiles during embryogenesis and under stresses

HSP90 plays important roles in multiple cellular stress responses. Here, two cytoplasmic HSP90 isoforms, ScHSP90α and ScHSP90β, were identified from Siniperca chuatsi. Their cDNA and gDNA structures, amino acid sequence features, and sequence identities and phylogenetic analysis with other species were described. Their expression profiles during embryonic development in different tissues and under stressful conditions were analyzed using real-time quantitative PCR. During embryogenesis, transcripts of both genes were detected at low levels during the early developmental stages and were up-regulated from appearance of myomere for ScHSP90a and closure of blastopore for ScHSP90β. ScHSP90α showed a tissue-specific variation with high expression in ovary and brain under non-stressed conditions, while ScHSP90β was ubiquitously highly expressed in different tissues. Acute heat shock resulted in a strong up-regulation of ScHSP90α in heart, liver, and head kidney, while it only weakly induced ScHSP90β in these tissues. ScHSP90α was also markedly induced in liver in a time-dependent manner under hypoxia, while the expression of ScHSP90β was not affected by hypoxia. Additionally, Aeromonas hydrophila infection markedly augmented ScHSP90α in head kidney and spleen and mildly up-regulated ScHSP90β in spleen, while suppressing ScHSP90β in head kidney. These results suggest that ScHSP90α and ScHSP90β are differently involved in embryogenesis and under different environmental conditions including high temperature, hypoxia, and bacterial infection. This study will benefit to further clarify the roles of fish HSP90 isoforms in embryogenesis and under stressful conditions and contribute to further study on enhancing stress tolerance and disease resistance of mandarin fish.

Heat shock protein 90 is required for sexual and asexual development, virulence, and heat shock response in Fusarium graminearum

Eukaryotic cells repress global translation and selectively upregulate stress response proteins by altering multiple steps in gene expression. In this study, genome-wide transcriptome analysis of cellular adaptation to thermal stress was performed on the plant pathogenic fungus Fusarium graminearum. The results revealed that profound alterations in gene expression were required for heat shock responses in F. graminearum. Among these proteins, heat shock protein 90 (FgHsp90) was revealed to play a central role in heat shock stress responses in this fungus. FgHsp90 was highly expressed and exclusively localised to nuclei in response to heat stress. Moreover, our comprehensive functional characterisation of FgHsp90 provides clear genetic evidence supporting its crucial roles in the vegetative growth, reproduction, and virulence of F. graminearum. In particular, FgHsp90 performs multiple functions as a transcriptional regulator of conidiation. Our findings provide new insight into the mechanisms underlying adaptation to heat shock and the roles of Hsp90 in fungal development.

The role of the dynein light intermediate chain in retrograde IFT and flagellar function in Chlamydomonas

The assembly of cilia and flagella depends on the activity of two microtubule motor complexes, kinesin-2 and dynein-2/1b, but the specific functions of the different subunits are poorly defined. Here we analyze Chlamydomonas strains expressing different amounts of the dynein 1b light intermediate chain (D1bLIC). Disruption of D1bLIC alters the stability of the dynein 1b complex and reduces both the frequency and velocity of retrograde intraflagellar transport (IFT), but it does not eliminate retrograde IFT. Flagellar assembly, motility, gliding, and mating are altered in a dose-dependent manner. iTRAQ-based proteomics identifies a small subset of proteins that are significantly reduced or elevated in d1blic flagella. Transformation with D1bLIC-GFP rescues the mutant phenotypes, and D1bLIC-GFP assembles into the dynein 1b complex at wild-type levels. D1bLIC-GFP is transported with anterograde IFT particles to the flagellar tip, dissociates into smaller particles, and begins processive retrograde IFT in <2 s. These studies demonstrate the role of D1bLIC in facilitating the recycling of IFT subunits and other proteins, identify new components potentially involved in the regulation of IFT, flagellar assembly, and flagellar signaling, and provide insight into the role of D1bLIC and retrograde IFT in other organisms.

MLK3 Signaling in Cancer Invasion

Mixed-lineage kinase 3 (MLK3) was first cloned in 1994; however, only in the past decade has MLK3 become recognized as a player in oncogenic signaling. MLK3 is a mitogen-activated protein kinase kinase kinase (MAP3K) that mediates signals from several cell surface receptors including receptor tyrosine kinases (RTKs), chemokine receptors, and cytokine receptors. Once activated, MLK3 transduces signals to multiple downstream pathways, primarily to c-Jun terminal kinase (JNK) MAPK, as well as to extracellular-signal-regulated kinase (ERK) MAPK, P38 MAPK, and NF-κB, resulting in both transcriptional and post-translational regulation of multiple effector proteins. In several types of cancer, MLK3 signaling is implicated in promoting cell proliferation, as well as driving cell migration, invasion and metastasis.

A Remodeled Hsp90 Molecular Chaperone Ensemble with the Novel Cochaperone Aarsd1 Is Required for Muscle Differentiation

Hsp90 is the ATP-consuming core component of a very abundant molecular chaperone machine that handles a substantial portion of the cytosolic proteome. Rather than one machine, it is in fact an ensemble of molecular machines, since most mammalian cells express two cytosolic isoforms of Hsp90 and a subset of up to 40 to 50 cochaperones and regulate their interactions and functions by a variety of posttranslational modifications. We demonstrate that the Hsp90 ensemble is fundamentally remodeled during muscle differentiation and that this remodeling is not just a consequence of muscle differentiation but possibly one of the drivers to accompany and to match the vast proteomic changes associated with this process. As myoblasts differentiate into myotubes, Hsp90α disappears and only Hsp90β remains, which is the only isoform capable of interacting with the novel muscle-specific Hsp90 cochaperone Aarsd1L. Artificially maintaining Hsp90α or knocking down Aarsd1L expression interferes with the differentiation of C2C12 myotubes. During muscle differentiation, Aarsd1L replaces the more ubiquitous cochaperone p23 and in doing so dampens the activity of the glucocorticoid receptor, one of the Hsp90 clients relevant to muscle functions. This cochaperone switch protects muscle cells against the inhibitory effects of glucocorticoids and may contribute to preventing muscle wasting induced by excess glucocorticoids.

The association of Hsp90 expression induced by aspirin with anti-stress damage in chicken myocardial cells

The protective effect of aspirin during exposure to heat stress in broiler chickens was investigated. We assayed pathological damage, expression and distribution of Hsp90 protein and hsp90 mRNA expression in chicken heart tissues after oral administration of aspirin following exposure to high temperature for varying times. Heat stress induced increases in plasma aspartate aminotransferase, creatine kinase and lactate dehydrogenase activities while causing severe heart damage, which was characterized by granular and vacuolar degeneration, nuclear shrinkage and even myocardium fragmentation in cardiac muscle fibers. After aspirin administration, myocardial cells showed fewer pathological lesions than broilers treated with heat alone. A high positive Hsp90 signal was always detected in the nuclei of myocardial cells from broilers treated with aspirin, while in myocardial cells treated with heat alone, Hsp90 in the nuclei decreased, as did that in the cytoplasm. Aspirin induced rapid and significant synthesis of Hsp90 before and at the initial phase of heat stress, and significant expression of hsp90 mRNA was stimulated throughout the experiment when compared with cells exposed to heat stress alone. Thus, specific pre-induction of Hsp90 in cardiovascular tissue was useful for resisting heat stress damage because it produced stable damage-related enzymes and fewer pathologic changes.

Hsp90 protein interacts with phosphorothioate oligonucleotides containing hydrophobic 2'-modifications and enhances antisense activity

RNase H1-dependent antisense oligonucleotides (ASOs) are chemically modified to enhance pharmacological properties. Major modifications include phosphorothioate (PS) backbone and different 2′-modifications in 2–5 nucleotides at each end (wing) of an ASO. Chemical modifications can affect protein binding and understanding ASO-protein interactions is important for better drug design. Recently we identified many intracellular ASO-binding proteins and found that protein binding could affect ASO potency. Here, we analyzed the structure-activity-relationships of ASO-protein interactions and found 2′-modifications significantly affected protein binding, including La, P54nrb and NPM. PS-ASOs containing more hydrophobic 2′-modifications exhibit higher affinity for proteins in general, although certain proteins, e.g. Ku70/Ku80 and TCP1, are less affected by 2′-modifications. We found that Hsp90 protein binds PS-ASOs containing locked-nucleic-acid (LNA) or constrained-ethyl-bicyclic-nucleic-acid ((S)-cEt) modifications much more avidly than 2′-O-methoxyethyl (MOE). ASOs bind the mid-domain of Hsp90 protein. Hsp90 interacts with more hydrophobic 2′ modifications, e.g. (S)-cEt or LNA, in the 5′-wing of the ASO. Reduction of Hsp90 protein decreased activity of PS-ASOs with 5′-LNA or 5′-cEt wings, but not with 5′-MOE wing. Together, our results indicate Hsp90 protein enhances the activity of PS/LNA or PS/(S)-cEt ASOs, and imply that altering protein binding of ASOs using different chemical modifications can improve therapeutic performance of PS-ASOs.

Identification of epipolythiodioxopiperazines HDN-1 and chaetocin as novel inhibitor of heat shock protein 90

The molecular chaperone heat shock protein 90 (Hsp90) has emerged as an important target for cancer treatment. HDN-1, an epipolythiopiperazine-2, 5-diones (ETPs) compound, was here identified as a new Hsp90 inhibitor. HDN-1 bound directly to C-terminus of Hsp90α, resulting in a potential conformational change that interfered with the binding of 17-AAG and novobiocin to Hsp90α. In contrast, association of 17-AAG, novobiocin or ATP with Hsp90α did not prevent the binding HDN-1 to Hsp90α. HDN-1 in combination with 17-AAG exhibited an enhanced inhibitory effect on non-small lung cancer cell proliferation. Molecular docking analyses revealed that HDN-1 bound to Hsp90α at C-terminal 526–570 region. In addition, HDN-1 degraded multiple oncoproteins and promoted EGF-induced wild type and mutated EGFR downregulation. Notably, chaetocin, used as a SUV39H1 inhibitor with similar structure to HDN-1, bound to Hsp90 and degraded Hsp90 client proteins and SUV39H1 as did HDN-1. These results indicate that HDN-1 and chaetocin are inhibitors of Hsp90 and that SUV39H1 is a novel client protein of Hsp90.

Altered Expression of High Molecular Weight Heat Shock Proteins after OCT4B1 Suppression in Human Tumor Cell Lines

Objective

OCT4B1, a novel variant of OCT4, is expressed in cancer cell lines and tis- sues. Based on our previous reports, OCT4B1 appears to have a crucial role in regulating apoptosis as well as stress response [heat shock proteins (HSPs)] pathways. The aim of the present study was to determine the effects of OCT4B1 silencing on the expression of high molecular weight HSPs in three different human tumor cell lines.

Materials and Methods

In this experimental study, OCT4B1 expression was suppressed in AGS (gastric adenocarcinoma), 5637 (bladder tumor) and U-87MG (brain tumor) cell lines using RNAi strategy. Real-time polymerase chain reaction (PCR) array was em- ployed for expression level analysis and the fold changes were calculated using RT2 Pro- filer PCR array data analysis software version 3.5.

Results

Our data revealed up-regulation of HSPD1 (from HSP60 family) as well as HSPA14, HSPA1L, HSPA4, HSPA5 and HSPA8 (from HSP70 family) following OCT4B1 knock-down in all three cell lines. In contrast, the expression of HSP90AA1 and HSP90AB1 (from HSP90 family) as well as HSPA1B and HSPA6 (from HSP70 family) was down-regulated under similar conditions. Other stress-related genes showed varying ex- pression pattern in the examined tumor cell lines.

Conclusion

Our data suggest a direct or indirect correlation between the expression of OCT4B1 and HSP90 gene family. However, OCT4B1 expression was not strongly corre- lated with the expression of HSP70 and HSP60 gene families.

CPUY201112, a novel synthetic small-molecule compound and inhibitor of heat shock protein Hsp90, induces p53-mediated apoptosis in MCF-7 cells

Heat-shock protein 90 (Hsp90) is highly expressed in many tumor cells and is associated with the maintenance of malignant phenotypes. Targeting Hsp90 has had therapeutic success in both solid and hematological malignancies, which has inspired more studies to identify new Hsp90 inhibitors with improved clinical efficacy. Using a fragment-based approach and subsequent structural optimization guided by medicinal chemistry principles, we identified the novel compound CPUY201112 as a potent Hsp90 inhibitor. It binds to the ATP-binding pocket of Hsp90 with a kinetic dissociation (K_d) constant of 27 ± 2.3 nM. It also exhibits potent in vitro antiproliferative effects in a range of solid tumor cells. In MCF-7 cells with high Hsp90 expression, CPUY201112 induces the degradation of Hsp90 client proteins including HER-2, Akt, and c-RAF. We prove that treating MCF-7 cells with CPUY201112 results in cell cycle arrest and apoptosis through the wild-type (wt) p53 pathway. CPUY201112 also synergizes with Nutlin-3a to induce cancer cell apoptosis. CPUY201112 significantly inhibited the growth of MCF-7 xenografts in nude mice without apparent body weight loss. These results demonstrate that CPUY201112 is a novel Hsp90 inhibitor with potential use in treating wild-type p53 related cancers.

Novel and less explored chemotypes of natural origin for the inhibition of Hsp90

This review focuses on novel classes of natural products whose structures have not yet been thoroughly explored for medicinal chemistry purposes. These novel chemotypes may be useful starting points to develop compounds that alter Hsp90 functionvianovel mechanisms.

Nuclear-localized AtHSPR links abscisic acid-dependent salt tolerance and antioxidant defense in Arabidopsis

Salt stress from soil or irrigation water limits plant growth. A T-DNA insertion mutant in C24, named athspr (Arabidopsis thaliana heat shock protein-related), showed several phenotypes, including reduced organ size and enhanced sensitivity to environmental cues. The athspr mutant is severely impaired under salinity levels at which wild-type (WT) plants grow normally. AtHSPR encodes a nuclear-localized protein with ATPase activity, and its expression was enhanced by high salinity and abscisic acid (ABA). Overexpression (OE) of AtHSPR significantly enhanced tolerance to salt stress by increasing the activities of the antioxidant system and by maintaining K(+) /Na(+) homeostasis. Quantitative RT-PCR analyses showed that OE of AtHSPR increased the expression of ABA/stress-responsive, salt overly sensitive (SOS)-related and antioxidant-related genes. In addition, ABA content was reduced in athspr plants with or without salt stress, and exogenous ABA restored WT-like salt tolerance to athspr plants. athspr exhibited increased leaf stomatal density and stomatal index, slower ABA-induced stomatal closure and reduced drought tolerance relative to the WT. AtHSPR OE enhanced drought tolerance by reducing leaf water loss and stomatal aperture. Transcript profiling in athspr showed a differential salt-stress response for genes involved in accumulation of reactive oxygen species (ROS), ABA signaling, cell death, stress response and photosynthesis. Taken together, our results suggested that AtHSPR is involved in salt tolerance in Arabidopsis through modulation of ROS levels, ABA-dependent stomatal closure, photosynthesis and K(+) /Na(+) homeostasis.

Chaperone BAG6 is dispensable for MHC class I antigen processing and presentation

Antigen processing for direct presentation on MHC class I molecules is a multistep process requiring the concerted activity of several cellular complexes. The essential steps at the beginning of this pathway, namely protein synthesis at the ribosome and degradation via the proteasome, have been known for years. Nevertheless, there is a considerable lack of factors identified to function between protein synthesis and degradation during antigen processing. Here, we analyzed the impact of the chaperone BAG6 on MHC class I cell surface expression and presentation of virus-derived peptides. Although an essential role of BAG6 in antigen processing has been proposed previously, we found BAG6 to be dispensable in this pathway. Still, interaction of BAG6 and the model antigen tyrosinase was enhanced during proteasome inhibition pointing towards a role of BAG6 in antigen degradation. Redundant chaperone pathways potentially mask the contribution of BAG6 to antigen processing and presentation.

Glioblastoma Stem Cells as a New Therapeutic Target for Glioblastoma

Primary and secondary glioblastomas (GBMs) are two distinct diseases. The genetic and epigenetic background of these tumors is highly variable. The treatment procedure for these tumors is often unsuccessful because of the cellular heterogeneity and intrinsic ability of the tumor cells to invade healthy tissues. The fatal outcome of these tumors promotes researchers to find out new markers associated with the prognosis and treatment planning. In this communication, the role of glioblastoma stem cells in tumor progression and the malignant behavior of GBMs are summarized with attention to the signaling pathways and molecular regulators that are involved in maintaining the glioblastoma stem cell phenotype. A better understanding of these stem cell-like cells is necessary for designing new effective treatments and developing novel molecular strategies to target glioblastoma stem cells. We discuss hypoxia as a new therapeutic target for GBM. We focus on the inhibition of signaling pathways, which are associated with the hypoxia-mediated maintenance of glioblastoma stem cells, and the knockdown of hypoxia-inducible factors, which could be identified as attractive molecular target approaches for GBM therapeutics.

The Effects of Hsp90α1 Mutations on Myosin Thick Filament Organization

Heat shock protein 90α plays a key role in myosin folding and thick filament assembly in muscle cells. To assess the structure and function of Hsp90α and its potential regulation by post-translational modification, we developed a combined knockdown and rescue assay in zebrafish embryos to systematically analyze the effects of various mutations on Hsp90α function in myosin thick filament organization. DNA constructs expressing the Hsp90α1 mutants with altered putative ATP binding, phosphorylation, acetylation or methylation sites were co-injected with Hsp90α1 specific morpholino into zebrafish embryos. Myosin thick filament organization was analyzed in skeletal muscles of the injected embryos by immunostaining. The results showed that mutating the conserved D90 residue in the Hsp90α1 ATP binding domain abolished its function in thick filament organization. In addition, phosphorylation mimicking mutations of T33D, T33E and T87E compromised Hsp90α1 function in myosin thick filament organization. Similarly, K287Q acetylation mimicking mutation repressed Hsp90α1 function in myosin thick filament organization. In contrast, K206R and K608R hypomethylation mimicking mutations had not effect on Hsp90α1 function in thick filament organization. Given that T33 and T87 are highly conserved residues involved post-translational modification (PTM) in yeast, mouse and human Hsp90 proteins, data from this study could indicate that Hsp90α1 function in myosin thick filament organization is potentially regulated by PTMs involving phosphorylation and acetylation.

Hsp90: A New Player in DNA Repair?

Heat shock protein 90 (Hsp90) is an evolutionary conserved molecular chaperone that, together with Hsp70 and co-chaperones makes up the Hsp90 chaperone machinery, stabilizing and activating more than 200 proteins, involved in protein homeostasis (i.e., proteostasis), transcriptional regulation, chromatin remodeling, and DNA repair. Cells respond to DNA damage by activating complex DNA damage response (DDR) pathways that include: (i) cell cycle arrest; (ii) transcriptional and post-translational activation of a subset of genes, including those associated with DNA repair; and (iii) triggering of programmed cell death. The efficacy of the DDR pathways is influenced by the nuclear levels of DNA repair proteins, which are regulated by balancing between protein synthesis and degradation as well as by nuclear import and export. The inability to respond properly to either DNA damage or to DNA repair leads to genetic instability, which in turn may enhance the rate of cancer development. Multiple components of the DNA double strand breaks repair machinery, including BRCA1, BRCA2, CHK1, DNA-PKcs, FANCA, and the MRE11/RAD50/NBN complex, have been described to be client proteins of Hsp90, which acts as a regulator of the diverse DDR pathways. Inhibition of Hsp90 actions leads to the altered localization and stabilization of DDR proteins after DNA damage and may represent a cell-specific and tumor-selective radiosensibilizer. Here, the role of Hsp90-dependent molecular mechanisms involved in cancer onset and in the maintenance of the genome integrity is discussed and highlighted.

Activation of Hsp90 Enzymatic Activity and Conformational Dynamics through Rationally Designed Allosteric Ligands

Hsp90 is a molecular chaperone of pivotal importance for multiple cell pathways. ATP-regulated internal dynamics are critical for its function and current pharmacological approaches block the chaperone with ATP-competitive inhibitors. Herein, a general approach to perturb Hsp90 through design of new allosteric ligands aimed at modulating its functional dynamics is proposed. Based on the characterization of a first set of 2-phenylbenzofurans showing stimulatory effects on Hsp90 ATPase and conformational dynamics, new ligands were developed that activate Hsp90 by targeting an allosteric site, located 65 Å from the active site. Specifically, analysis of protein responses to first-generation activators was exploited to guide the design of novel derivatives with improved ability to stimulate ATP hydrolysis. The molecules' effects on Hsp90 enzymatic, conformational, co-chaperone and client-binding properties were characterized through biochemical, biophysical and cellular approaches. These designed probes act as allosteric activators of the chaperone and affect the viability of cancer cell lines for which proper functioning of Hsp90 is necessary.

Celastrol protects ischaemic myocardium through a heat shock response with up-regulation of haeme oxygenase-1

BACKGROUND AND PURPOSE

Celastrol, a triterpene from plants, has been used in traditional oriental medicine to treat various diseases. Here, we investigated the cardioprotective effects of celastrol against ischaemia.

EXPERIMENTAL APPROACH

Protective pathways induced by celastrol were investigated in hypoxic cultures of H9c2 rat cardiomyoblasts and in a rat model of myocardial infarction, assessed with echocardiographic and histological analysis.

KEY RESULTS

In H9c2 cells, celastrol triggered reactive oxygen species (ROS) formation within minutes, induced nuclear translocation of the transcription factor heat shock factor 1 (HSF1) resulting in a heat shock response (HSR) leading to increased expression of heat shock proteins (HSPs). ROS scavenger N-acetylcysteine reduced expression of HSP70 and HSP32 (haeme oxygenase-1, HO-1). Celastrol improved H9c2 survival under hypoxic stress, and functional analysis revealed HSF1 and HO-1 as key effectors of the HSR, induced by celastrol, in promoting cytoprotection. In the rat ischaemic myocardium, celastrol treatment improved cardiac function and reduced adverse left ventricular remodelling at 14 days. Celastrol triggered expression of cardioprotective HO-1 and inhibited fibrosis and infarct size. In the peri-infarct area, celastrol reduced myofibroblast and macrophage infiltration, while attenuating up-regulation of TGF-β and collagen genes.

CONCLUSIONS AND IMPLICATIONS

Celastrol treatment induced an HSR through activation of HSF1 with up-regulation of HO-1 as the key effector, promoting cardiomyocyte survival, reduction of injury and adverse remodelling with preservation of cardiac function. Celastrol may represent a novel potent pharmacological cardioprotective agent mimicking ischaemic conditioning that could have a valuable impact in the treatment of myocardial infarction.

Potential Anticancer Properties and Mechanisms of Action of Withanolides

Plant-based Ayurvedic medicine has been practiced in India for thousands of years for the treatment of a variety of disorders. They are rich sources of bioactive compounds potentially useful for prevention and treatment of cancer. Withania somnifera (commonly known as Ashwagandha in Ayurvedic medicine) is a widely used medicinal plant whose anticancer value was recognized after isolation of steroidal compounds withanolides from the leaves of this shrub. Withaferin A is the first member of withanolides to be isolated, and it is the most abundant withanolide present in W. somnifera. Its cancer-protective role has now been established using chemically induced and oncogene-driven rodent cancer models. The present review summarizes the key preclinical studies demonstrating anticancer effects of withaferin along with its molecular targets and mechanisms related to its anticancer effects. Anticancer potential of other withanolides is also discussed.

Heat Shock Protein 90 Associates with the Per-Arnt-Sim Domain of Heme-free Soluble Guanylate Cyclase: IMplications for Enzyme Maturation

Heat shock protein 90 (hsp90) drives heme insertion into the β1 subunit of soluble guanylate cyclase (sGC) β1, which enables it to associate with a partner sGCα1 subunit and mature into a nitric oxide (NO)-responsive active form. We utilized fluorescence polarization measurements and hydrogen-deuterium exchange mass spectrometry to define molecular interactions between the specific human isoforms hsp90β and apo-sGCβ1. hsp90β and its isolated M domain, but not its isolated N and C domains, bind with low micromolar affinity to a heme-free, truncated version of sGCβ1 (sGCβ1(1-359)-H105F). Surprisingly, hsp90β and its M domain bound to the Per-Arnt-Sim (PAS) domain of apo-sGC-β1(1-359), which lies adjacent to its heme-binding (H-NOX) domain. The interaction specifically involved solvent-exposed regions in the hsp90β M domain that are largely distinct from sites utilized by other hsp90 clients. The interaction strongly protected two regions of the sGCβ1 PAS domain and caused local structural relaxation in other regions, including a PAS dimerization interface and a segment in the H-NOX domain. Our results suggest a means by which the hsp90β interaction could prevent apo-sGCβ1 from associating with its partner sGCα1 subunit while enabling structural changes to assist heme insertion into the H-NOX domain. This mechanism would parallel that in other clients like the aryl hydrocarbon receptor and HIF1α, which also interact with hsp90 through their PAS domains to control protein partner and small ligand binding interactions.

Heat-shock protein 90 (Hsp90) as anticancer target for drug discovery: an ample computational perspective

There are over 100 different types of cancer, and each is classified based on the type of cell that is initially affected. If left untreated, cancer can result in serious health problems and eventually death. Recently, the paradigm of cancer chemotherapy has evolved to use a combination approach, which involves the use of multiple drugs each of which targets an individual protein. Inhibition of heat-shock protein 90 (Hsp90) is one of the novel key cancer targets. Because of its ability to target several signaling pathways, Hsp90 inhibition emerged as a useful strategy to treat a wide variety of cancers. Molecular modeling approaches and methodologies have become 'close counterparts' to experiments in drug design and discovery workflows. A wide range of molecular modeling approaches have been developed, each of which has different objectives and outcomes. In this review, we provide an up-to-date systematic overview on the different computational models implemented toward the design of Hsp90 inhibitors as anticancer agents. Although this is the main emphasis of this review, different topics such as background and current statistics of cancer, different anticancer targets including Hsp90, and the structure and function of Hsp90 from an experimental perspective, for example, X-ray and NMR, are also addressed in this report. To the best of our knowledge, this review is the first account, which comprehensively outlines various molecular modeling efforts directed toward identification of anticancer drugs targeting Hsp90. We believe that the information, methods, and perspectives highlighted in this report would assist researchers in the discovery of potential anticancer agents.

Hsp70 and the Cochaperone StiA (Hop) Orchestrate Hsp90-Mediated Caspofungin Tolerance in Aspergillus fumigatus

Aspergillus fumigatus is the primary etiologic agent of invasive aspergillosis (IA), a major cause of death among immunosuppressed patients. Echinocandins (e.g., caspofungin) are increasingly used as second-line therapy for IA, but their activity is only fungistatic. Heat shock protein 90 (Hsp90) was previously shown to trigger tolerance to caspofungin and the paradoxical effect (i.e., decreased efficacy of caspofungin at higher concentrations). Here, we demonstrate the key role of another molecular chaperone, Hsp70, in governing the stress response to caspofungin via Hsp90 and their cochaperone Hop/Sti1 (StiA in A. fumigatus). Mutation of the StiA-interacting domain of Hsp70 (C-terminal EELD motif) impaired thermal adaptation and caspofungin tolerance with loss of the caspofungin paradoxical effect. Impaired Hsp90 function and increased susceptibility to caspofungin were also observed following pharmacologic inhibition of the C-terminal domain of Hsp70 by pifithrin-μ or after stiA deletion, further supporting the links among Hsp70, StiA, and Hsp90 in governing caspofungin tolerance. StiA was not required for the physical interaction between Hsp70 and Hsp90 but had distinct roles in the regulation of their function in caspofungin and heat stress responses. In conclusion, this study deciphering the physical and functional interactions of the Hsp70-StiA-Hsp90 complex provided new insights into the mechanisms of tolerance to caspofungin in A. fumigatus and revealed a key C-terminal motif of Hsp70, which can be targeted by specific inhibitors, such as pifithrin-μ, to enhance the antifungal activity of caspofungin against A. fumigatus.

Hsp70 forms antiparallel dimers stabilized by post-translational modifications to position clients for transfer to Hsp90

Protein folding in cells is regulated by networks of chaperones, including the heat shock protein 70 (Hsp70) system, which consists of the Hsp40 cochaperone and a nucleotide exchange factor. Hsp40 mediates complex formation between Hsp70 and client proteins prior to interaction with Hsp90. We used mass spectrometry (MS) to monitor assemblies formed between eukaryotic Hsp90/Hsp70/Hsp40, Hop, p23, and a client protein, a fragment of the glucocorticoid receptor (GR). We found that Hsp40 promotes interactions between the client and Hsp70, and facilitates dimerization of monomeric Hsp70. This dimerization is antiparallel, stabilized by post-translational modifications (PTMs), and maintained in the stable heterohexameric client-loading complex Hsp90₂Hsp70₂HopGR identified here. Addition of p23 to this client-loading complex induces transfer of GR onto Hsp90 and leads to expulsion of Hop and Hsp70. Based on these results, we propose that Hsp70 antiparallel dimerization, stabilized by PTMs, positions the client for transfer from Hsp70 to Hsp90.

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Antiparallel dimerization of Hsp70 is stabilized by PTMs

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Hsp40 catalyzes Hsp70 dimerization and client transfer to Hsp70

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Hsp70 antiparallel dimerization is maintained in the client-loading complex

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Addition of p23 induces transfer of GR onto Hsp90 and loss of Hop and Hsp70

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.

The polymorphisms in the promoter of HSP90 gene and their association with heat tolerance of bay scallop

The heat shock protein 90 (HSP90) is a highly abundant and ubiquitous molecular chaperone which plays essential roles in many cellular processes. In the present study, the messenger RNA (mRNA) expressions of HSP90 after acute heat stress were investigated in two bay scallop populations (Argopecten irradians irradians and Argopecten irradians concentricus). The heat-resistant scallop A. i. concentricus, which is distributed in Zhanjiang, China, exhibited significantly higher induction of HSP90 compared with that of the heat-sensitive scallop A. i. irradians, which is distributed in Qinhuangdao, China. The promoter sequence of HSP90 gene from bay scallop (AiHSP90) was cloned, and the polymorphisms within this region were investigated by sequencing to analyze their association with heat tolerance. A total of six single nucleotide polymorphisms (SNPs), including -1167 T-C, -1023 A-C, -799 C-T, -774 A-G, -686 C-T, and -682 A-C, were identified in the amplified promoter region, and most of them affected the putative transcription factor binding sites except for locus -1167. All the six SNP sites were found to be associated with heat tolerance after Hardy-Weinberg equilibrium (HWE) and association analysis. Moreover, haplotypes CACACC and TCTATC were also found to be associated with heat tolerance based on the result of linkage disequilibrium and association analysis. The results provided insights into the molecular mechanisms underlying the thermal adaptation of different congener endemic bay scallops, which suggested that the increased heat tolerance of A. i. concentricus (compared with A. i. irradians) was associated with the higher expression of AiHSP90. Meanwhile, the six genotypes (-1167 TT, -1023 CC, -799 TT, -774 GG, -686 CC, and -682 AA) and two haplotypes (CACACC and TCTATC) could be used as potential markers for scallop selection breeding with higher heat tolerance.

Prolyl isomerization and its catalysis in protein folding and protein function

Prolyl isomerizations are intrinsically slow processes. They determine the rates of many protein folding reactions and control regulatory events in folded proteins. Prolyl isomerases are able to catalyze these isomerizations, and thus, they have the potential to assist protein folding and to modulate protein function. Here, we provide examples for how prolyl isomerizations limit protein folding and are accelerated by prolyl isomerases and how native-state prolyl isomerizations regulate protein functions. The roles of prolines in protein folding and protein function are closely interrelated because both of them depend on the coupling between cis/trans isomerization and conformational changes that can involve extended regions of a protein.

Discovery of novel oxazepine and diazepine carboxamides as two new classes of heat shock protein 90 inhibitors

Two novel series of oxazepine and diazepine based HSP90 inhibitors are reported. This effort relied on structure based design and isothermal calorimetry to identify small drug like macrocycles. Computational modelling was used to build into a solvent exposed pocket near the opening of the ATP binding site, which led to potent inhibitors of HSP90 (25-30).

Identification of proteins associated with Aha1 in HeLa cells by quantitative proteomics

The identification of the activator of heat shock protein 90 (Hsp90) ATPase's (Aha1) protein-protein interaction (PPI) network will provide critical insights into the relationship of Aha1 with multi-molecular complexes and shed light onto Aha1's interconnections with Hsp90-regulated biological functions. Flag-tagged Aha1 was over-expressed in HeLa cells and isolated by anti-Flag affinity pull downs, followed by trypsin digestion and identification co-adsorbing proteins by liquid chromatography-tandem mass spectroscopy (LC-MS/MS). A probability-based identification of Aha1 PPIs was generated from the LC-MS/MS analysis by using a relative quantification strategy, spectral counting (SC). By comparing the SC-based protein levels between Aha1 pull-down samples and negative controls, 164 Aha1-interacting proteins were identified that were quantitatively enriched in the pull-down samples over the controls. The identified Aha1-interacting proteins are involved in a wide number of intracellular bioprocesses, including DNA maintenance, chromatin structure, RNA processing, translation, nucleocytoplasmic and vesicle transport, among others. The interactions of 33 of the identified proteins with Aha1 were further confirmed by Western blotting, demonstrating the reliability of our affinity-purification-coupled quantitative SC-MS strategy. Our proteomic data suggests that Aha1 may participate in diverse biological pathways to facilitate Hsp90 chaperone functions in response to stress.

Antifungal activity of compounds targeting the Hsp90-calcineurin pathway against various mould species

OBJECTIVES

Invasive mould infections are associated with a high mortality rate and the emergence of MDR moulds is of particular concern. Calcineurin and its chaperone, the heat shock protein 90 (Hsp90), represent an important pathway for fungal virulence that can be targeted at different levels. We investigated the antifungal activity of compounds directly or indirectly targeting the Hsp90-calcineurin axis against different mould species.

METHODS

The in vitro antifungal activity of the anticalcineurin drug FK506 (tacrolimus), the Hsp90 inhibitor geldanamycin, the lysine deacetylase inhibitor trichostatin A and the Hsp70 inhibitor pifithrin-μ was assessed by the standard broth dilution method against 62 clinical isolates of Aspergillus spp. and non-Aspergillus moulds (Mucoromycotina, Fusarium spp., Scedosporium spp., Purpureocillium/Paecilomyces spp. and Scopulariopsis spp.)

RESULTS

FK506 had variable antifungal activity against different Aspergillus spp. and was particularly active against Mucor spp. Geldanamycin had moderate antifungal activity against Fusarium spp. and Paecilomyces variotii. Importantly, trichostatin A had good activity against the triazole-resistant Aspergillus ustus and the amphotericin B-resistant Aspergillus terreus as well as the MDR Scedosporium prolificans. Moreover, trichostatin A exhibited synergistic interactions with caspofungin against A. ustus and with geldanamycin against Rhizopus spp. for which none of the other agents showed activity. Pifithrin-μ exhibited little antifungal activity.

CONCLUSIONS

Targeting the Hsp90-calcineurin axis at different levels resulted in distinct patterns of susceptibility among different fungal species. Lysine deacetylase inhibition may represent a promising novel antifungal strategy against emerging resistant moulds.

Terazosin activates Pgk1 and Hsp90 to promote stress resistance

Drugs that can protect against organ damage are urgently needed, especially for diseases such as sepsis and brain stroke. We discovered that terazosin (TZ), a widely marketed α1-adrenergic receptor antagonist, alleviated organ damage and improved survival in rodent models of stroke and sepsis. Through combined studies of enzymology and X-ray crystallography, we discovered that TZ binds a new target, phosphoglycerate kinase 1 (Pgk1), and activates its enzymatic activity, probably through 2,4-diamino-6,7-dimethoxyisoquinoline's ability to promote ATP release from Pgk1. Mechanistically, the ATP generated from Pgk1 may enhance the chaperone activity of Hsp90, an ATPase known to associate with Pgk1. Upon activation, Hsp90 promotes multistress resistance. Our studies demonstrate that TZ has a new protein target, Pgk1, and reveal its corresponding biological effect. As a clinical drug, TZ may be quickly translated into treatments for diseases including stroke and sepsis.

Heat shock protein 90 maintains the stability and function of transcription factor Broad Z7 by interacting with its Broad-Complex-Tramtrack-Bric-a-brac domain

Heat shock protein 90 (Hsp90) is a highly conserved chaperone protein that interacts with various client proteins to mediate their folding and stability. The Broad-Complex-Tramtrack-Bric-a-brac (BTB) domain, also known as poxvirus and zinc finger (POZ) domain, exists widely in different proteins and is highly conserved. However, the stability mechanism of BTB domain-containing proteins has not been fully understood. Co-immunoprecipitation and a protein pull-down assay were performed to investigate the interaction between Hsp90 and the transcription factor Broad isoform Z7 (BrZ7) in vivo and in vitro. The middle domain of Hsp90 directly associated with the BTB domain of BrZ7. The Hsp90 inhibitor 17-(Allylamino)-17-demethoxygeldanamycin (17-AAG) interrupted the interaction between Hsp90 and BrZ7 and decreased the protein level of BrZ7 but did not affect the mRNA level of BrZ7. The addition of the proteasome inhibitor peptide aldehyde Cbz-leu-leu leucinal suppressed the 17-AAG-induced degradation of BrZ7. BTB domain deletion and 17-AAG treatment resulted in inhibition of BrZ7 function in gene expression in the 20-hydroxyecdysone and juvenile hormone pathways. These results reveal that the middle domain of Hsp90 associates with the BTB domain of BrZ7 to prevent BrZ7 degradation and maintain BrZ7 function in gene expression in the lepidopteran insect Helicoverpa armigera.

Structural characterization of the substrate transfer mechanism in Hsp70/Hsp90 folding machinery mediated by Hop

In eukarya, chaperones Hsp70 and Hsp90 act coordinately in the folding and maturation of a range of key proteins with the help of several co-chaperones, especially Hop. Although biochemical data define the Hop-mediated Hsp70-Hsp90 substrate transfer mechanism, the intrinsic flexibility of these proteins and the dynamic nature of their complexes have limited the structural studies of this mechanism. Here we generate several complexes in the Hsp70/Hsp90 folding pathway (Hsp90:Hop, Hsp90:Hop:Hsp70 and Hsp90:Hop:Hsp70 with a fragment of the client protein glucocorticoid receptor (GR-LBD)), and determine their 3D structure using electron microscopy techniques. Our results show that one Hop molecule binds to one side of the Hsp90 dimer in both extended and compact conformations, through Hop domain rearrangement that take place when Hsp70 or Hsp70:GR-LBD bind to Hsp90:Hop. The compact conformation of the Hsp90:Hop:Hsp70:GR-LBD complex shows that GR-LBD binds to the side of the Hsp90 dimer opposite the Hop attachment site.

Hsp90α forms a stable complex at the cilium neck for the interaction of signalling molecules in IGF-1 receptor signalling

The primary cilium is composed of an axoneme that protrudes from the cell surface, a basal body beneath the membrane and a transition neck in between. It is a sensory organelle on the plasma membrane, involved in mediating extracellular signals. In the transition neck region of the cilium, the microtubules change from triplet to doublet microtubules. This region also contains the transition fibres that crosslink the axoneme with the membrane and the necklace proteins that regulate molecules being transported into and out of the cilium. In this protein-enriched, complex area it is important to maintain the correct assembly of all of these proteins. Here, through immunofluorescent staining and protein isolation, we identify the molecular chaperone Hsp90α clustered at the periciliary base. At the transition neck region, phosphorylated Hsp90α forms a stable ring around the axoneme. Heat shock treatment causes Hsp90α to dissipate and induces resorption of cilia. We further identify that Hsp90α at the transition neck region represents a signalling platform on which IRS-1 interacts with intracellular downstream signalling molecules involved in IGF-1 receptor signalling.

Heat shock protein 90 (Hsp90): A novel antifungal target against Aspergillus fumigatus

Invasive aspergillosis is a life-threatening and difficult to treat infection in immunosuppressed patients. The efficacy of current anti-Aspergillus therapies, targeting the cell wall or membrane, is limited by toxicity (polyenes), fungistatic activity and some level of basal resistance (echinocandins), or the emergence of acquired resistance (triazoles). The heat shock protein 90 (Hsp90) is a conserved molecular chaperone involved in the rapid development of antifungal resistance in the yeast Candida albicans. Few studies have addressed its role in filamentous fungi such as Aspergillus fumigatus, in which mechanisms of resistance may differ substantially. Hsp90 is at the center of a complex network involving calcineurin, lysine deacetylases (KDAC) and other client proteins, which orchestrate compensatory repair mechanisms of the cell wall in response to the stress induced by antifungals. In A. fumigatus, Hsp90 is a trigger for resistance to high concentrations of caspofungin, known as the paradoxical effect. Disrupting Hsp90 circuitry by different means (Hsp90 inhibitors, KDAC inhibitors and anti-calcineurin drugs) potentiates the antifungal activity of caspofungin, thus representing a promising novel antifungal approach. This review will discuss the specific features of A. fumigatus Hsp90 and the potential for antifungal strategies of invasive aspergillosis targeting this essential chaperone.

Profiling Hsp90 differential expression and the molecular effects of the Hsp90 inhibitor IPI-504 in high-grade glioma models

Retaspimycin hydrochloride (IPI-504), an Hsp90 (heat shock protein 90) inhibitor, has shown activity in multiple preclinical cancer models, such as lung, breast and ovarian cancers. However, its biological effects in gliomas and normal brain derived cellular populations remain unknown. In this study, we profiled the expression pattern of Hsp90α/β mRNA in stable glioma cell lines, multiple glioma-derived primary cultures and human neural stem/progenitor cells. The effects of IPI-504 on cell proliferation, apoptosis, motility and expression of Hsp90 client proteins were evaluated in glioma cell lines. In vivo activity of IPI-504 was investigated in subcutaneous glioma xenografts. Our results showed Hsp90α and Hsp90β expression levels to be patient-specific, higher in high-grade glioma-derived primary cells than in low-grade glioma-derived primary cells, and strongly correlated with CD133 expression and differentiation status of cells. Hsp90 inhibition by IPI-504 induced apoptosis, blocked migration and invasion, and significantly decreased epidermal growth factor receptor levels, mitogen-activated protein kinase and/or Akt activities, and secretion of vascular endothelial growth factor in glioma cell lines. In vivo study showed that IPI-504 could mildly attenuate tumor growth in immunocompromised mice. These findings suggest that targeting Hsp90 by IPI-504 has the potential to become an active therapeutic strategy in gliomas in a selective group of patients, but further research into combination therapies is still needed.

Heat shock protein 83 (Hsp83) facilitates methoprene-tolerant (Met) nuclear import to modulate juvenile hormone signaling

Juvenile hormone (JH) receptors, methoprene-tolerant (Met) and Germ-cell expressed (Gce), transduce JH signals to induce Kr-h1 expression in Drosophila. Dual luciferase assay identified a 120-bp JH response region (JHRR) in the Kr-h1α promoter. Both in vitro and in vivo experiments revealed that Met and Gce transduce JH signals to induce Kr-h1 expression through the JHRR. DNA affinity purification identified chaperone protein Hsp83 as one of the proteins bound to the JHRR in the presence of JH. Interestingly, Hsp83 physically interacts with PAS-B and basic helix-loop-helix domains of Met, and JH induces Met-Hsp83 interaction. As determined by immunohistochemistry, Met is mainly distributed in the cytoplasm of fat body cells of the larval when the JH titer is low and JH induces Met nuclear import. Hsp83 was accumulated in the cytoplasm area adjunct to the nucleus in the presence of JH and Met/Gce. Loss-of-function of Hsp83 attenuated JH binding and JH-induced nuclear import of Met, resulting in a decrease in the JHRR-driven reporter activity leading to reduction of Kr-h1 expression. These data show that Hsp83 facilitates the JH-induced nuclear import of Met that induces Kr-h1 expression through the JHRR.

HSP90s are required for NLR immune receptor accumulation in Arabidopsis

Heat shock proteins (HSPs) serve as molecular chaperones for diverse client proteins in many biological processes. In plant immunity, cytosolic HSP90s participate in the assembly, stability control and/or activation of immune receptor complexes. In this paper we report that in addition to the well-established positive roles that HSP90 isoforms play in plant immunity, they are also involved in the negative regulation of immune receptor accumulation. Point mutations in two HSP90 genes, HSP90.2 and HSP90.3, were identified from a forward genetic screen designed to isolate mutants with enhanced disease resistance. We found that specific mutations in HSP90.2 and HSP90.3 lead to heightened accumulation of immune receptors, including SNC1, RPS2 and RPS4. HSP90s may assist SGT1 in the formation of SCF E3 ubiquitin ligase complexes that target immune receptors for degradation. Such regulation is critical for maintaining appropriate levels of immune receptor proteins to avoid autoimmunity.

Cytosolic quality control of mislocalized proteins requires RNF126 recruitment to Bag6

Approximately 30% of eukaryotic proteins contain hydrophobic signals for localization to the secretory pathway. These proteins can be mislocalized in the cytosol due to mutations in their targeting signals, certain stresses, or intrinsic inefficiencies in their translocation. Mislocalized proteins (MLPs) are protected from aggregation by the Bag6 complex and degraded by a poorly characterized proteasome-dependent pathway. Here, we identify the ubiquitin ligase RNF126 as a key component of the MLP degradation pathway. In vitro reconstitution and fractionation studies reveal that RNF126 is the primary Bag6-dependent ligase. RNF126 is recruited to the N-terminal Ubl domain of Bag6 and preferentially ubiquitinates juxtahydrophobic lysine residues on Bag6-associated clients. Interfering with RNF126 recruitment in vitro prevents ubiquitination, and RNF126 depletion in cells partially stabilizes a Bag6 client. Bag6-dependent ubiquitination can be recapitulated with purified components, paving the way for mechanistic analyses of downstream steps in this cytosolic quality control pathway.

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The chaperone Bag6 recruits the ubiquitin ligase RNF126 to its Ubl domain

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RNF126 is necessary and sufficient for optimal ubiquitination of Bag6 clients

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Purified Bag6-client complex supports ubiquitination by recombinant RNF126

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Bag6-associated mislocalized protein is stabilized by loss of RNF126 in cells

Four-colour FRET reveals directionality in the Hsp90 multicomponent machinery

In living organisms, most proteins work in complexes to form multicomponent protein machines. The function of such multicomponent machines is usually addressed by dividing them into a collection of two state systems at equilibrium. Many molecular machines, like Hsp90, work far from equilibrium by utilizing the energy of ATP hydrolysis. In these cases, important information is gained from the observation of the succession of more than two states in a row. We developed a four-colour single-molecule FRET system to observe the succession of states in the heat shock protein 90 (Hsp90) system, consisting of an Hsp90 dimer, the cochaperone p23 and nucleotides. We show that this multicomponent system is a directional ATP-dependent machinery. This reveals a previously undescribed mechanism on how cochaperones can modify Hsp90, namely by strengthening of the coupling between ATP hydrolysis and a kinetic step involved in the Hsp90 system resulting in a stronger directionality.

The effect of celastrol, a triterpene with antitumorigenic activity, on conformational and functional aspects of the human 90kDa heat shock protein Hsp90α, a chaperone implicated in the stabilization of the tumor phenotype

BACKGROUND

Hsp90 is a molecular chaperone essential for cell viability in eukaryotes that is associated with the maturation of proteins involved in important cell functions and implicated in the stabilization of the tumor phenotype of various cancers, making this chaperone a notably interesting therapeutic target. Celastrol is a plant-derived pentacyclic triterpenoid compound with potent antioxidant, anti-inflammatory and anticancer activities; however, celastrol's action mode is still elusive.

RESULTS

In this work, we investigated the effect of celastrol on the conformational and functional aspects of Hsp90α. Interestingly, celastrol appeared to target Hsp90α directly as the compound induced the oligomerization of the chaperone via the C-terminal domain as demonstrated by experiments using a deletion mutant. The nature of the oligomers was investigated by biophysical tools demonstrating that a two-fold excess of celastrol induced the formation of a decameric Hsp90α bound throughout the C-terminal domain. When bound, celastrol destabilized the C-terminal domain. Surprisingly, standard chaperone functional investigations demonstrated that neither the in vitro chaperone activity of protecting against aggregation nor the ability to bind a TPR co-chaperone, which binds to the C-terminus of Hsp90α, were affected by celastrol.

CONCLUSION

Celastrol interferes with specific biological functions of Hsp90α. Our results suggest a model in which celastrol binds directly to the C-terminal domain of Hsp90α causing oligomerization. However, the ability to protect against protein aggregation (supported by our results) and to bind to TPR co-chaperones are not affected by celastrol. Therefore celastrol may act primarily by inducing specific oligomerization that affects some, but not all, of the functions of Hsp90α.

GENERAL SIGNIFICANCE

To the best of our knowledge, this study is the first work to use multiple probes to investigate the effect that celastrol has on the stability and oligomerization of Hsp90α and on the binding of this chaperone to Tom70. This work provides a novel mechanism by which celastrol binds Hsp90α.

Computational modeling of allosteric regulation in the hsp90 chaperones: a statistical ensemble analysis of protein structure networks and allosteric communications

A fundamental role of the Hsp90 chaperone in regulating functional activity of diverse protein clients is essential for the integrity of signaling networks. In this work we have combined biophysical simulations of the Hsp90 crystal structures with the protein structure network analysis to characterize the statistical ensemble of allosteric interaction networks and communication pathways in the Hsp90 chaperones. We have found that principal structurally stable communities could be preserved during dynamic changes in the conformational ensemble. The dominant contribution of the inter-domain rigidity to the interaction networks has emerged as a common factor responsible for the thermodynamic stability of the active chaperone form during the ATPase cycle. Structural stability analysis using force constant profiling of the inter-residue fluctuation distances has identified a network of conserved structurally rigid residues that could serve as global mediating sites of allosteric communication. Mapping of the conformational landscape with the network centrality parameters has demonstrated that stable communities and mediating residues may act concertedly with the shifts in the conformational equilibrium and could describe the majority of functionally significant chaperone residues. The network analysis has revealed a relationship between structural stability, global centrality and functional significance of hotspot residues involved in chaperone regulation. We have found that allosteric interactions in the Hsp90 chaperone may be mediated by modules of structurally stable residues that display high betweenness in the global interaction network. The results of this study have suggested that allosteric interactions in the Hsp90 chaperone may operate via a mechanism that combines rapid and efficient communication by a single optimal pathway of structurally rigid residues and more robust signal transmission using an ensemble of suboptimal multiple communication routes. This may be a universal requirement encoded in protein structures to balance the inherent tension between resilience and efficiency of the residue interaction networks.

Functional versatility and structural adaptability of the Hsp90 chaperones are regulated by allosteric interactions that allow for diverse functions including modulation of ATP hydrolysis and binding with cochaperones and client proteins. By integrating molecular simulations and network-based approaches we have characterized conformational dynamics and allosteric interactions in different functional forms of Hsp90. The network centrality analysis and structural mapping of allosteric communications have revealed a small-world organization of the interaction network that is mediated by functionally important residues of the Hsp90 activity. We have found that effective allosteric communications in the Hsp90 chaperone may be provided by structurally stable residues that exhibit high centrality properties. Nucleotide-specific rewiring of the network topology and assortative organization of functional residues may protect the active form of the chaperone from random perturbations and detrimental mutations. These results have confirmed that allosteric interactions in the Hsp90 chaperone may be determined by a small-world network of functional residues that can regulate the network efficiency and resiliency by modulating the statistical ensemble of communication pathways in response to functional requirements of the ATPase cycle.

A G protein-coupled receptor and the intracellular synthase of its agonist functionally cooperate

The GPCR DP1 promotes the activity of L-PGDS, the enzyme that produces the DP1 agonist PGD₂, while at the same time L-PGDS promotes the export and activity of DP1 in response to PGD₂.

Export of newly synthesized G protein–coupled receptors (GPCRs) remains poorly characterized. We show in this paper that lipocalin-type prostaglandin D₂ (PGD₂) synthase (L-PGDS) interacts intracellularly with the GPCR DP1 in an agonist-independent manner. L-PGDS promotes cell surface expression of DP1, but not of other GPCRs, in HEK293 and HeLa cells, independent of L-PGDS enzyme activity. In addition, formation of a DP1–Hsp90 complex necessary for DP1 export to the cell surface is dependent on the interaction between L-PGDS and the C-terminal MEEVD residues of Hsp90. Surprisingly, PGD₂ synthesis by L-PGDS is promoted by coexpression of DP1, suggesting a possible intracrine/autocrine signaling mechanism. In this regard, L-PGDS increases the formation of a DP1–ERK1/2 complex and increases DP1-mediated ERK1/2 signaling. Our findings define a novel cooperative mechanism in which a GPCR (DP1) promotes the activity of the enzyme (L-PGDS) that produces its agonist (PGD₂) and in which this enzyme in turn acts as a cofactor (of Hsp90) to promote export and agonist-dependent activity of the receptor.