Heat-induced chaperone activity of HSP90

Novobiocin and additional inhibitors of the Hsp90 C-terminal nucleotide-binding pocket

The 90 kDa heat shock proteins (Hsp90), which are integrally involved in cell signaling, proliferation, and survival, are ubiquitously expressed in cells. Many proteins in tumor cells are dependent upon the Hsp90 protein folding machinery for their stability, refolding, and maturation. Inhibition of Hsp90 uniquely targets client proteins associated with all six hallmarks of cancer. Thus, Hsp90 has emerged as a promising target for the treatment of cancer. Hsp90 exists as a homodimer, which contains three domains. The N-terminal domain contains an ATP-binding site that binds the natural products geldanamycin and radicicol. The middle domain is highly charged and has high affinity for co-chaperones and client proteins. Initial studies by Csermely and co-workers suggested a second ATP-binding site in the C-terminus of Hsp90. This C-terminal nucleotide binding pocket has been shown to not only bind ATP, but cisplatin, novobiocin, epilgallocatechin-3-gallate (EGCG) and taxol. The coumarin antibiotics novobiocin, clorobiocin, and coumermycin A1 were isolated from several streptomyces strains and exhibit potent activity against Gram-positive bacteria. These compounds bind type II topoisomerases, including DNA gyrase, and inhibit the enzyme-catalyzed hydrolysis of ATP. As a result, novobiocin analogues have garnered the attention of numerous researchers as an attractive agent for the treatment of bacterial infection. Novobiocin was reported to bind weakly to the newly discovered Hsp90 C-terminal ATP binding site ( approximately 700 M in SkBr3 cells) and induce degradation of Hsp90 client proteins. Structural modification of this compound has led to an increase of 1000-fold in activity in anti-proliferative assays. Recent studies of structure-activity relationship (SAR) by Renoir and co-workers highlighted the crucial role of the C-4 and/or C-7 positions of the coumarin and removal of the noviose moiety, which appeared to be essential for degradation of Hsp90 client proteins. Unlike the N-terminal ATP binding site, there is no reported co-crystal structure of Hsp90 C-terminus bound to any inhibitor. The Hsp90 C-terminal domain, however, is known to contain a conserved pentapeptide sequence (MEEVD) which is recognized by co-chaperones. Cisplatin is a platinum-containing chemotherapeutic used to treat various types of cancers, including testicular, ovarian, bladder, and small cell lung cancer. Most notably, cisplatin coordinates to DNA bases, resulting in cross-linked DNA, which prohibits rapidly dividing cells from duplicating DNA for mitosis. Itoh and co-workers reported that cisplatin decreases the chaperone activity of Hsp90. This group applied bovine brain cytosol to a cisplatin affinity column, eluted with cisplatin and detected Hsp90 in the eluent. Subsequent experiments indicated that cisplatin exhibits high affinity for Hsp90. Moreover Csermely and co-workers determined that the cisplatin binding site is located proximal to the C-terminal ATP binding site. EGCG is one of the active ingredients found in green tea. EGCG is known to inhibit the activity of many Hsp90-dependent client proteins, including telomerase, several kinases, and the aryl hydrocarbon receptor (AhR). Recently Gasiewicz and co-workers reported that EGCG manifests its antagonistic activity against AhR through binding Hsp90. Similar to novobiocin, EGCG was shown to bind the C-terminus of Hsp90. Unlike previously identified N-terminal Hsp90 inhibitors, EGCG does not appear to prevent Hsp90 from forming multiprotein complexes. Studies are currently underway to determine whether EGCG competes with novobiocin or cisplatin binding. Taxol, a well-known drug for the treatment of cancer, is responsible for the stabilization of microtubules and the inhibition of mitosis. Previous studies have shown that taxol induces the activation of kinases and transcription factors, and mimics the effect of bacterial lipopolysaccharide (LPS), an attribute unrelated to its tubulin-binding properties. Rosen and co-workers prepared a biotinylated taxol derivative and performed affinity chromatography experiments with lysates from both mouse brain and macrophage cell lines. These studies led to identification of two chaperones, Hsp70 and Hsp90, by mass spectrometry. In contrast to typical Hsp90-binding drugs, taxol exhibits a stimulatory response. Recently it was reported that the geldanamycin derivative 17-AAG behaves synergistically with taxol-induced apoptosis. This review describes the different C-terminal inhibitors of Hsp90, with specific emphasis on structure-activity relationship studies of novobiocin and their effects on anti-proliferative activity.

Construction of Hsp90β gene specific silencing plasmid and its transfection efficiency

The purpose of this work was to construct the plasmid that could direct the synthesis of siRNA-like transcripts and thus mediate strong and specific repression of human heat shock protein 90β (Hsp90β) gene expression and to compare the transfection efficiency of the plasmids in varying conditions of transfection. Three 64 nt oligos corresponding to different regions of the target gene were chemically synthesized and annealed and were then ligated with pSUPER EGFP1 plasmid and double-digested with HindIII and BglII. Recombinant plasmids were transformed into Escherichia coli, DH5a, and the colonies were picked and grown in the Amp-agarose. The presence of positive clones was checked by the means of endodigestion and sequencing. Three cell strains, HepG2, Human umbilicus vein endothelium cells (HUVEC) and HeK293, were cultured. Then the plasmids were transfected into the cells at different ratios of plasmid to Lipofectamine. The transfection efficiency was measured by detection of enhanced green fluorescence protein (EGFP). The presence of positive recombinant clones were verified by double-digestion and sequencing. The bases inserted into the plasmids were correct and the positive colonies were named pSuper-Hsp90β1, pSuper-Hsp90β2 and pSuper-Hsp90β3. After optimizing the ratio of plasmid to Lipofectamine, we achieved high transfection efficiency in HeK293 cells. Transfection efficiency was still low in the HepG2 cells. In conclusion, the si-RNA-synthesizing plasmids targeting Hsp90β were constructed and transfected into cells with different transfection efficiency.

The proteasome activator PA28 functions in collaboration with Hsp90 in vivo

We have previously shown that the proteasome activator PA28 is essential to Hsp90-dependent protein refolding in vitro, where PA28 mediates transfer of the Hsp90-bound substrate protein to the Hsc70/Hsp40 chaperone machine for its correct refolding. This observation suggests that PA28 may also collaborate with Hsp90 in cells. To examine this possibility, here we have used double-stranded RNA interference (RNAi) against PA28 in Caenorhabditis elegans mutants of daf-21, which encodes Hsp90. We show that C. elegans PA28 facilitates Hsp90-initiated protein refolding, albeit with an activity lower than that of mouse PA28 proteins. RNAi-mediated knockdown of PA28 significantly suppresses the Daf-c (dauer formation constitutive) phenotype of the daf-21 mutant, but it has no affect on the distinct defects of this mutant in sensing odorants. Taking these results together, we conclude that PA28 is likely to function in collaboration with Hsp90 in vivo.

Cdc37 interacts with the glycine-rich loop of Hsp90 client kinases

Recently, we identified a client-binding site of Cdc37 that is required for its association with protein kinases. Phage display technology and liquid chromatography-tandem mass spectrometry (which identifies a total of 33 proteins) consistently identify a unique sequence, GXFG, as a Cdc37-interacting motif that occurs in the canonical glycine-rich loop (GXGXXG) of protein kinases, regardless of their dependence on Hsp90 or Cdc37. The glycine-rich motif of Raf-1 (GSGSFG) is necessary for its association with Cdc37; nevertheless, the N lobe of Raf-1 (which includes the GSGSFG motif) on its own cannot interact with Cdc37. Chimeric mutants of Cdk2 and Cdk4, which differ sharply in their affinities toward Cdc37, show that their C-terminal portions may determine this difference. In addition, a nonclient kinase, the catalytic subunit of cyclic AMP-dependent protein kinase, interacts with Cdc37 but only when a threonine residue in the activation segment of its C lobe is unphosphorylated. Thus, although a region in the C termini of protein kinases may be crucial for accomplishing and maintaining their interaction with Cdc37, we conclude that the N-terminal glycine-rich loop of protein kinases is essential for physically associating with Cdc37.

The triage of damaged proteins: degradation by the ubiquitin-proteasome pathway or repair by molecular chaperones

Accumulation of damaged proteins is causally related to many age-related diseases. The ubiquitin-proteasome pathway (UPP) plays a role in selective degradation of damaged proteins, whereas molecular chaperones, such as heat shock proteins, are involved in refolding denatured proteins. This work demonstrates for the first time that the UPP and molecular chaperones work in a competitive manner and that the fates of denatured proteins are determined by the relative activities of the UPP and molecular chaperones. Enhanced UPP activity suppresses the refolding of denatured proteins whereas elevated chaperone activity inhibits the degradation of denatured proteins. CHIP, a co-chaperone with E3 activity, plays a pivotal role in determining the fates of the damaged proteins. The delicate balance between UPP-mediated degradation and refolding of denatured proteins is governed by relative levels of CHIP and other molecular chaperones. Isopeptidases, the enzymes that reverse the actions of CHIP, also play an important role in determining the fate of denatured proteins.

Chaperoning of glucocorticoid receptors

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

Chaperones in preventing protein denaturation in living cells and protecting against cellular stress

A variety of cellular internal and external stress conditions can be classified as proteotoxic stresses. Proteotoxic stresses can be defined as stresses that increase the fraction of proteins that are in an unfolded state, thereby enhancing the probability of the formation of intracellular aggregates. These aggregates, if not disposed, can lead to cell death. In response to the appearance of damaged proteins, cells induce the expression of heat shock proteins. These can function as molecular chaperones to prevent protein aggregation and to keep proteins in a state competent for either refolding or degradation. Most knowledge of the function and regulation (by co-factors) of individual heat shock proteins comes from cell free studies on refolding of heat- or chemically denatured, purified proteins. Unlike the experimental situation in a test tube, cells contain multiple chaperones and co-factors often moving in and out different subcompartments that contain a variety of protein substrates at different folding states. Also, within cells folding competes with the degradative machinery. In this chapter, an overview will be provided on how the main cytosolic/nuclear chaperone Hsp70 is regulated, what is known about its interaction with other main cytosolic/nuclear chaperone families (Hsp27, Hsp90, and Hsp110), and how it may function as a molecular chaperone in living mammalian cells to protect against proteotoxic stresses.

Modification of heat shock protein 90 by 4-hydroxynonenal in a rat model of chronic alcoholic liver disease

Lipid peroxidation during oxidative stress leads to increased concentrations of thiol-reactive alpha,beta-unsaturated aldehyde, including 4-hydroxy-2-nonenal (4-HNE) and 4-oxo-2-nonenal (4-ONE). These aldehydes have a documented ability to disrupt protein function following adduct formation with specific residues. Therefore, to identify 4-HNE-modified proteins in a model of ethanol-induced oxidative stress, a proteomic approach was applied to liver fractions prepared from rats fed a combination high-fat/ethanol diet. The results revealed that essential 90-kDa heat shock protein (Hsp90) was consistently modified by 4-HNE in the alcohol-treated animals. In vitro chaperoning experiments using firefly luciferase as a client protein were then performed to assess the functional effect of 4-HNE modification on purified recombinant human Hsp90, modified with concentrations of this aldehyde ranging from 23 to 450 microM. Modification of Hsp90 with 4-ONE also led to significant inhibition of the chaperone. Because 4-HNE and 4-ONE react selectively with Cys, a thiol-specific mechanism of inhibition was suggested by these data. Therefore, thiol sensitivity was confirmed following treatment of Hsp90 with the specific thiol modifier N-ethylmaleimide, which resulted in more than 99% inactivation of the chaperone by concentrations as low as 6 microM (1:1 M ratio). Finally, tryptic digest of 4-HNE-modified Hsp90 followed by liquid chromatography/tandem mass spectrometry peptide analysis identified Cys 572 as a site for 4-HNE modification. The results presented here thus establish that 4-HNE consistently modifies Hsp90 in a rat model of alcohol-induced oxidative stress and that the chaperoning activity of this protein is subject to dysregulation through thiol modification.

Constitutive Expression of Small Heat Shock Protein in an htpG Disruptant of the Cyanobacterium Synechococcus sp. PCC 7942

In cyanobacteria, a disruptant of hspA encoding a small heat shock protein homologue, shows decreased cell growth rates at moderately high temperatures, and loss of both basal and acquired thermo-tolerances, which resemble the phenotype of an htpG disruptant. In vitro studies have shown that both small heat shock protein and Hsp90 can bind and keep non-native proteins in a refolding-competent state under denaturing conditions. The aim of the present study is to elucidate whether constitutive expression of HspA can functionally replace HtpG, a prokaryotic homolog of Hsp90, in the cyanobacterium Synechococcus sp. PCC 7942. HspA did not improve the viability of the htpG disruptant at a lethal temperature, although it did that of the wild type. It did not improve an iron-starved phenotype of the mutant under normal growth conditions, a novel phenotype found in the present study. These results suggest that cellular function of HtpG may differ significantly from that of HspA.

Heat-induced degradation of overexpressed glucocorticoid receptor Separate protective roles of hsp90 and hsp70

The glucocorticoid receptor (GR) occurs in cells in the form of a hormone-responsive complex (HRC) with hsp90. The HRC is dynamic, with hsp90 constantly directing disassembly, and hsp70, assisted by hsp90, driving reassembly. WCL2 cells stably overexpress GR to an extent that reduces the excess of hsp90 and hsp70 over GR by about 10-fold, compared to the ratio in HeLa cells. Yet the half-lives of the HRC in WCL2 and HeLa cells are comparable. As a result, the rate of assembly in WCL2 is overwhelmed by accumulation of the non-hormone-binding form of GR in its complex with hsp70 and hsp90. This form comprised some 50% of total GR in WCL2 cells. When the cells were heated to 44 degrees C, the hormone-binding activity and solubility of GR fell in parallel, and the receptor formed heavy aggregates by sequestering large amounts of hsp70. About 40% of this aggregated receptor was degraded in cells recovering at 37 degrees C in the presence of cycloheximide. Concentration of GR protein increased with increasing induction of hsp70 following exposure to 41-44 degrees C. However, balance between hormone-binding and inert forms of GR could shift in either direction in response to the increase or decrease of hsp90 induction, depending on the temperature. Suppression of degradation following re-exposure of the cells to 44 degrees C correlated better with induction of hsp90 than hsp70. We infer that sequestration of hsp70 by heat-unfolded receptor is the primary factor opposing degradation, while induction of hsp90 acts to further suppress degradation by accelerating HRC assembly.

Oligomeric Hsp33 with Enhanced Chaperone Activity

Hsp33, an Escherichia coli cytosolic chaperone, is inactive under normal conditions but becomes active upon oxidative stress. It was previously shown to dimerize upon activation in a concentration- and temperature-dependent manner. This dimer was thought to bind to aggregation-prone target proteins, preventing their aggregation. In the present study, we report small angle x-ray scattering (SAXS), steady state and time-resolved fluorescence, gel filtration, and glutaraldehyde cross-linking analysis of full-length Hsp33. Our circular dichroism and fluorescence results show that there are significant structural changes in oxidized Hsp33 at different temperatures. SAXS, gel filtration, and glutaraldehyde cross-linking results indicate, in addition to the dimers, the presence of oligomeric species. Oxidation in the presence of physiological salt concentration leads to significant increases in the oligomer population. Our results further show that under conditions that mimic the crowded milieu of the cytosol, oxidized Hsp33 exists predominantly as an oligomeric species. Interestingly, chaperone activity studies show that the oligomeric species is much more efficient compared with the dimers in preventing aggregation of target proteins. Taken together, these results indicate that in the cell, Hsp33 undergoes conformational and quaternary structural changes leading to the formation of oligomeric species in response to oxidative stress. Oligomeric Hsp33 thus might be physiologically relevant under oxidative stress.

Heat-induced oligomerization of gp96 occurs via a site distinct from substrate binding and is regulated by ATP

Gp96 (GRP94) is a dimeric glycoprotein and is the endoplasmic reticulum representative of the hsp90 family of molecular chaperones. In addition to the protein substrates it chaperones, gp96 binds weakly to both peptides and ATP, and has been shown to self-assemble into discrete oligomers upon heat shock at 50 degrees C, although physiological roles for these phenomena have not been well established. Our studies indicate that gp96 homooligomerizes irreversibly in vitro at temperatures as low as 42 degrees C and could involve pre-dissociation of dimers to monomers. Oligomerization is inhibited significantly by ATP; hydrolysis is not required, since ADP, ATP-gamma-S, and NECA inhibit self-assembly equally well. Peptide ligands do not competitively inhibit gp96 self-assembly and, in fact, bind to all oligomeric species, including the dimer. Together, these findings suggest that (1) heat-enhanced chaperone activity does not reside in oligomers per se, and (2) the regions of gp96 involved in peptide binding and oligomerization are distinct.

The power of two: protein dimerization in biology

The self-association of proteins to form dimers and higher-order oligomers is a very common phenomenon. Recent structural and biophysical studies show that protein dimerization or oligomerization is a key factor in the regulation of proteins such as enzymes, ion channels, receptors and transcription factors. In addition, self-association can help to minimize genome size, while maintaining the advantages of modular complex formation. Oligomerization, however, can also have deleterious consequences when nonnative oligomers associated with pathogenic states are generated. Specific protein dimerization is integral to biological function, structure and control, and must be under substantial selection pressure to be maintained with such frequency throughout biology.

The region adjacent to the highly immunogenic site and shielded by the middle domain is responsible for self-oligomerization/client binding of the HSP90 molecular chaperone

We here investigated the mechanism of self-oligomerization of the 90-kDa heat shock protein (HSP90) molecular chaperone, because it is known that this oligomerization reflects the client-binding activity. The transition temperatures for the self-oligomerization of the full-length forms of human HSP90alpha and HtpG (bacterial HSP90), i.e., 45 and 60 degrees C, respectively, were identical to those for the dissociation of the recombinant N domain (residues 1-400 of human HSP90alpha and residues 1-336 of HtpG in our definition) from the remainder of the molecule. The N domain of human HSP90alpha expressed in Escherichia coli was oligomeric, and the oligomerization activity was localized within residues 311-350, i.e., C-terminally adjacent to the highly immunogenic site (residues 291-304). Particularly, residues 341-350 were critical on oligomerization. On the other hand, residues 289-389 were indispensable for the interaction with the M domain (residues 401-618) of the molecule. Oligomer formation of the N domain was efficiently suppressed by its extension until Lys546, i.e., residues 401-546, which is required for the interaction with the N domain. Among highly conserved amino acids at residues 289-400, Trp297, Pro379, and Phe384 were essential for the interaction with the M domain. With these observations taken together, we propose as the activation mechanism of HSP90 molecular chaperone that heat stress induces the liberation of the oligomerization/client-binding site of residues 311-350 by disrupting the intramolecular interaction between residues 289-389 and 401-546.

Hypoxia increases Hsp90 binding to eNOS via PI3K-Akt in porcine coronary artery endothelium

This study examines the molecular mechanisms by which hypoxia regulates phosphorylated endothelial nitric oxide synthase (eNOS) activity and NO production in porcine coronary artery endothelial cells (PCAEC). Exposure to hypoxia (pO(2)=10 mmHg) for periods up to 3 h resulted in a time-dependent increase in eNOS protein expression and an early (15 min) and sustained increase in eNOS phosphorylation at Ser-1177. Exposure to hypoxia for 30 min led to a doubling in eNOS activity (control=6.2+/-4.4 vs hypoxia=14.1+/-5.0 fmol cGMP/microg protein, P<0.05) and NO release (control=5.9+/-0.8 vs hypoxia=11.8+/-1.2 nM/microg protein, P<0.05). Hypoxia also led to a significant increase in Akt phosphorylation and upregulation of Hsp90 binding to eNOS. Pretreatment of cells with either 1 microg/ml geldanamycin (a specific inhibitor of Hsp90) or 500 nM wortmannin (a specific PI3 kinase inhibitor) suppressed hypoxia-stimulated Akt and eNOS phosphorylation and significantly attenuated hypoxia-stimulated Hsp90 binding to eNOS. Both eNOS activity and NO production were inhibited by geldanamycin and wortmannin. Although hypoxia led to early activation of p42/44 mitogen-activated protein kinases (MAPK), inhibition of their pathway by PD98059 did not suppress hypoxia-stimulated eNOS phosphorylation and eNOS activity. These data demonstrate that hypoxia leads to increased eNOS phosphorylation via stimulated Hsp90 binding to eNOS and activation of the PI3-Akt pathway. We conclude that a coordinated interaction between Hsp90 and PI3-Akt may be an important mechanism by which eNOS activity and NO production is upregulated in hypoxic heart.

Dimerization and oligomerization of the chaperone calreticulin

The chaperone calreticulin is a highly conserved eukaryotic protein mainly located in the endoplasmic reticulum. It contains a free cysteine SH group but does not form disulfide-bridged dimers under physiological conditions, indicating that the SH group may not be fully accessible in the native protein. Using PAGE, urea gradient gel electrophoresis, capillary electrophoresis and MS, we show that dimerization through the SH group can be induced by lowering the pH to 5-6, heating, or under conditions that favour partial unfolding such as urea concentrations above 2.6 m or SDS concentrations above 0.025%. Moreover, we show that calreticulin also has the ability to self-oligomerize through noncovalent interactions at urea concentrations above 2.6 m at pH below 4.6 or above pH 10, at temperatures above 40 degrees C, or in the presence of high concentrations of organic solvents (25%), conditions that favour partial unfolding or an intramolecular local conformational change that allows oligomerization, resulting in a heterogeneous mixture of oligomers consisting of up to 10 calreticulin monomers. The oligomeric calreticulin was very stable, but oligomerization was partially reversed by addition of 8 m urea or 1% SDS, and heat-induced oligomerization could be inhibited by 8 m urea or 1% SDS when present during heating. Comparison of the binding properties of monomeric and oligomeric calreticulin in solid-phase assays showed increased binding to peptides and denatured proteins when calreticulin was oligomerized. Thus, calreticulin shares the ability to self-oligomerize with other important chaperones such as GRP94 and HSP90, a property possibly associated with their chaperone activity.

Role of the Conserved SRLFDQFFG Region of α-Crystallin, a Small Heat Shock Protein

Small heat shock proteins (sHsps) are necessary for several cellular functions and in stress tolerance. Most sHsps are oligomers; intersubunit interactions leading to changes in oligomeric structure and exposure of specific regions may modulate their functioning. Many sHsps, including alpha A- and alpha B-crystallin, contain a well conserved SRLFDQFFG sequence motif in the N-terminal region. Sequence-based prediction shows that it exhibits helical propensity with amphipathic character, suggesting that it plays a critical role in the structure and function of alpha-crystallins. In order to investigate the role of this motif in the structure and function of sHsps, we have made constructs deleting this sequence from alpha A- and alpha B-crystallin, overexpressed, purified, and studied these engineered proteins. Circular dichroism spectroscopic studies show changes in tertiary and secondary structure on deletion of the sequence. Glycerol density gradient centrifugation and dynamic light scattering studies show that the multimeric size of the mutant proteins is significantly reduced, indicating a role for this motif in higher order organization of the subunits. Both deletion mutants exhibit similar oligomeric size and increased chaperone-like activity. Urea-induced denaturation study shows that the SRLFDQFFG sequence contributes significantly to the structural stability. Fluorescence resonance energy transfer studies show that the rate of exchange of the subunits in the alpha Adel-crystallin oligomer is higher compared with that in the alpha A-crystallin oligomer, suggesting that this region contributes to the oligomer dynamics in addition to the higher order assembly and structural stability. Thus, our study shows that the SRLFDQFFG sequence is one of the critical motifs in structure-function regulation of alpha A- and alpha B-crystallin.

Interaction of Neuropeptide Y and Hsp90 through a Novel Peptide Binding Region†

Hsp90 is a molecular chaperone that binds and assists refolding of non-native and/or labile polypeptides and also bind various peptides. However, the rules of how Hsp90 recognizes substrates have not been well characterized. By surface plasmon resonance measurements, a physiologically active peptide, neuropeptide Y (NPY), with a strong binding property to Hsp90 was identified from screening of 38 randomly selected peptide candidates. We showed that the carboxy-terminal fragment of NPY (NPY13-36), which forms an amphipathic alpha-helix structure, preserved the strong binding to Hsp90. Immunoprecipitation and immunoblotting using HeLa cell extracts revealed that newly synthesized NPY precursors bound to Hsp90, suggesting that the in vitro binding experiments identified an interactive peptide in vivo. Proteolytic cleavage of the NPY13-36/Hsp90 complex, as well as binding site analysis using deletion mutants of Hsp90, revealed the NPY binding locus on Hsp90alpha as the 192 amino acid region following the N-terminal domain. By electron microscopic analysis using an anti-Hsp90 antibody against the sequence proximal to the highly charged region, we showed that the Hsp90 dimer bound to NPY13-36 at both ends. Mutation of arginine residues in NPY13-36 to alanine abrogated binding to Hsp90. Our studies indicate that the hinge region after the N-terminal domain of Hsp90 and the positive charges on NPY are important for this interaction.

Maintenance of glucocorticoid receptor function following severe heat-shock of heat-conditioned cells

The competence of the glucocorticoid receptor to regulate gene expression is thought to depend on Hsp70-driven continuous reactivation following spontaneous inactivation of its hormone-binding state. We show here that the glucocorticoid-binding capacity of HeLa cells fell with increasing temperature in the range 43-45 degrees C in a manner that closely paralleled the loss of soluble receptor protein. Receptor activity was maintained during moderate (43 degrees C) but not severe (45 degrees C) heat shock. Hsp70 was rapidly rendered insoluble and was replenished by soluble chaperone at 43 but not 45 degrees C. In heat-conditioned cells expressing different levels of Hsp70, we observed a positive correlation between the concentration of active receptor and the amount of Hsp70 rendered insoluble by heat shock. Much higher amounts of Hsp70 were rendered insoluble and receptor competence to regulate gene expression was preserved after severe heat shock of appropriately heat-conditioned cells. An excess of Hsp90 was found associated with resolubilized heat-inactivated receptor from severely heat-shocked cells. The data indicate that GR activity is maintained, provided that denaturation and/or aggregation of the receptor is prevented by Hsp70; and that the concentration of the chaperone is the limiting determinant of receptor activity in heat-shocked HeLa cells.

Stabilization and re-activation of trapped enzyme by immobilized heat shock protein and molecular chaperones

The potential of using immobilized Heat Shock Protein 70 (HSP 70) in combination with other molecular chaperones to ameliorate problems of enzyme denaturation was investigated. Firefly luciferase was used as a model enzyme due to its sensitivity to thermal denaturation, and the availability of a sensitive chemiluminescent assay method for determination of relative activity of this enzyme. Control experiments and development of effective combinations of HSP with other chaperones involved re-activation of enzyme in bulk solution. A combination of HSP 70, alpha-crystallin and reticulocyte lysate (RL) in bulk solution were found to re-activate soluble firefly luciferase to about 60% of the initial activity after the enzyme activity had been reduced to less than 2% by thermal denaturation. HSP 70 that was covalently immobilized onto glass surfaces was also able to re-activate denatured enzyme that was in bulk solution. Over 30% of the initial activity could be regained from heat denatured enzyme when using immobilized HSP in the presence of other chaperones. The activity of soluble enzyme decayed to negligible values in a period of days when stored at room temperature. In the presence of immobilized HSP and chaperones, activity stabilized at about 10% of the initial activity even after many weeks. The results suggest that immobilized molecular chaperones such as HSP 70 may provide some potential for stabilization and re-activation of enzymes that are trapped in thin aqueous films for applications in biosensors and reactors.

Overexpressed glucocorticoid receptor negatively regulates gene expression under conditions that favour accumulation of non-hormone-binding forms of the receptor

Previous reports have suggested that the native hormone-responsive glucocorticoid receptor is a heterocomplex with hsp90 and that the receptor constantly cycles between the hormone-responsive and an inactive state, with complex assembly and turnover being driven by hsp70 and hsp90, respectively. Since hsp70 appears to be titrated in cells that transiently overexpress the receptor, assembly intermediates may accumulate when more receptor is produced than can be assembled to hormone-responsive complex. Comparison of receptor protein and hormone-binding levels in extracts from transiently transfected COS-7 cells revealed the presence of non-hormone-binding receptor forms in addition to the native heterocomplex. The receptor was predominantly nuclear in the majority of the transfected cells even in the absence of hormone, with the DNA-binding domain (DBD) being necessary for nuclear localisation. Moreover, the unliganded receptor exhibited constitutive DNA-binding activity and reactivity towards antibodies against the hinge region where NLS1 is known to reside. By comparing fluorography to immunoblotting of two-dimensional SDS-PAGE of cross-linked [3H]dexamethasone-mesylate-labelled receptor, we detected non-hormone-binding receptor species capable of binding DNA in vitro. In addition, using a constitutively active receptor mutant, we found that the overexpressed wild-type receptor was capable of repressing mutant-activated transcription of transiently and stably transfected reporter genes alike in a DBD-dependent manner.

A hydrophobic segment within the C-terminal domain is essential for both client-binding and dimer formation of the HSP90-family molecular chaperone

The alpha isoform of human 90-kDa heat shock protein (HSP90alpha) is composed of three domains: the N-terminal (residues 1-400); middle (residues 401-615) and C-terminal (residues 621-732). The middle domain is simultaneously associated with the N- and C-terminal domains, and the interaction with the latter mediates the dimeric configuration of HSP90. Besides one in the N-terminal domain, an additional client-binding site exists in the C-terminal domain of HSP90. The aim of the present study is to elucidate the regions within the C-terminal domain responsible for the bindings to the middle domain and to a client protein, and to define the relationship between the two functions. A bacterial two-hybrid system revealed that residues 650-697 of HSP90alpha were essential for the binding to the middle domain. An almost identical region (residues 657-720) was required for the suppression of heat-induced aggregation of citrate synthase, a model client protein. Replacement of either Leu665-Leu666 or Leu671-Leu672 to Ser-Ser within the hydrophobic segment (residues 662-678) of the C-terminal domain caused the loss of bindings to both the middle domain and the client protein. The interaction between the middle and C-terminal domains was also found in human 94-kDa glucose-regulated protein. Moreover, Escherichia coli HtpG, a bacterial HSP90 homologue, formed heterodimeric complexes with HSP90alpha and the 94-kDa glucose-regulated protein through their middle-C-terminal domains. Taken together, it is concluded that the identical region including the hydrophobic segment of the C-terminal domain is essential for both the client binding and dimer formation of the HSP90-family molecular chaperone and that the dimeric configuration appears to be similar in the HSP90-family proteins.

The chlorate-resistant and photomorphogenesis-defective mutant cr88 encodes a chloroplast-targeted HSP90

The cr88 mutant of Arabidopsis is a novel chlorate-resistant mutant that displays long hypocotyls in red light, but not in far red or blue light, and is delayed in the greening process. In cotyledons and young leaves, plastids are less developed compared with those of the wild type. In addition, a subset of light-regulated genes are under-expressed in this mutant. To understand the pleiotropic phenotypes of cr88, we isolated the CR88 gene through map-based cloning. We found that CR88 encodes a chloroplast-targeted 90-kDa heat shock protein (HSP90). The CR88 gene is expressed at highest levels during early post-germination stages and in leaves and reproductive organs. It is constitutively expressed but is also light and heat shock inducible. Chloroplast import experiments showed that the protein is localized to the stroma compartment of the chloroplast. The possible function of an HSP90 in the chloroplast and a plausible explanation of the pleiotropic phenotypes observed in cr88 are discussed.

Hydrodynamic properties and quaternary structure of the 90 kDa heat-shock protein: effects of divalent cations

The 90 kDa heat-shock protein (Hsp90) is one of the major stress proteins whose overall structure remains unknown. In this study, we investigated the influence of divalent cations Mg(2+) and Ca(2+) on the hydrodynamic properties and quaternary structure of Hsp90. Using analytical ultracentrifugation, size-exclusion chromatography, and polyacrylamide gel electrophoresis, we showed that native Hsp90 was mostly dimeric. The Hsp90 dimer had a sedimentation coefficient, s(w,20) degrees, of 6.10 +/- 0.03 S, which slightly deviated from the hydrodynamics of a globular protein. Using chemical cross-linking and analytical ultracentrifugation, we showed that Mg(2+) and Ca(2+) induced a tertiary conformational change of Hsp90, leading to a self-association process. In the presence of divalent cations, Hsp90 existed as a mixture of monomers, dimers, and tetramers at equilibrium. Finally, to identify Hsp90 domains involved in this divalent cation-dependent self-association, we studied the oligomerization state of the N-terminal (positions 1-221) of Hsp90, the influence of an N-terminal specific ligand, geldanamycin (GA), and the effect of C-terminal truncation on the ability of Hsp90 to oligomerize in the presence of divalent cations. We previously showed that GA inhibits Hsp90 heat-induced oligomerization [Garnier, C., Protasevich, I., Gilli, R., Tsvetkov, P., Lobachov, V., Peyrot, V., Briand, C., and Makarov, A. (1998) Biochem. Biophys. Res. Commun. 249, 197-201], but now we observed that GA does not influence divalent cation-dependent oligomerization of Hsp90, suggesting another mechanism. This mechanism involved the C-terminal part of the protein since C-terminally truncated Hsp90 did not oligomerize in the presence of divalent cations.

Protein-protein interactions: mechanisms and modification by drugs

Protein-protein interactions form the proteinaceous network, which plays a central role in numerous processes in the cell. This review highlights the main structures, properties of contact surfaces, and forces involved in protein-protein interactions. The properties of protein contact surfaces depend on their functions. The characteristics of contact surfaces of short-lived protein complexes share some similarities with the active sites of enzymes. The contact surfaces of permanent complexes resemble domain contacts or the protein core. It is reasonable to consider protein-protein complex formation as a continuation of protein folding. The contact surfaces of the protein complexes have unique structure and properties, so they represent prospective targets for a new generation of drugs. During the last decade, numerous investigations have been undertaken to find or design small molecules that block protein dimerization or protein(peptide)-receptor interaction, or on the other hand, induce protein dimerization.

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

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

Distinct roles of the N-terminal-binding domain and the C-terminal-solubilizing domain of alpha-synuclein, a molecular chaperone

alpha-Synuclein, an acidic neuronal protein of 140 amino acids, is extremely heat-resistant and is natively unfolded. Recent studies have demonstrated that alpha-synuclein has chaperone activity both in vitro and in vivo, and that this activity is lost upon removing its C-terminal acidic tail. However, the detailed mechanism of the chaperone action of alpha-synuclein remains unknown. In this study, we investigated the molecular mechanism of the chaperone action of alpha-synuclein by analyzing the roles of its N-terminal and C-terminal domains. The N-terminal domain (residues 1-95) was found to bind to substrate proteins to form high molecular weight complexes, whereas the C-terminal acidic tail (residues 96-140) appears to be primarily involved in solubilizing the high molecular weight complexes. Because the substrate-binding domain and the solubilizing domain for chaperone function are well separated in alpha-synuclein, the N-terminal-binding domain can be substituted by other proteins or peptides. Interestingly, the resultant engineered chaperone proteins appeared to display differential efficiency and specificity in terms of the chaperone function, which depended upon the nature of the binding domain. This finding implies that the C-terminal acidic tail of alpha-synuclein can be fused with other proteins or peptides to engineer synthetic chaperones for specific purposes.

Involvement of human heat shock protein 90 alpha in nicotine-induced apoptosis

There have been conflicting reports of the apoptotic effects of nicotine on human cells and those studies reporting nicotine-induced apoptosis have not unequivocally clarified the molecular mechanisms underlying the effect. However, we found here that human RSa cells, established from embryonic fibroblastic cells doubly infected with Rous sarcoma virus and Simian virus 40, underwent apoptosis when cultured with medium containing 0.06-0.6 microM nicotine. The apoptosis was assessed by cellular DNA fragmentation and caspase-3 protease activation. Viability of RSa cells was reduced by nicotine treatment, as analyzed by MTT assay and the reduction was lessened by combination treatment with a caspase-3 inhibitor, acetyl-L-aspartyl-L-glutamyl-L-valyl-L-aspart-1-al (Ac-DEVD-CHO). Levels of expression of heat shock protein 90 alpha (Hsp90 alpha) were found to be increased 20 min after the nicotine treatment, as analyzed by polymerase chain reaction-based mRNA differential display after Northern blotting analysis of mRNA amounts. Cellular contents of Hsp90 alpha were furthermore increased in the nicotine-treated RSa cells, as quantitated by Western immunoblot analysis. By contrast, in RSa cells treated with nicotine in combination with geldanamycin (GA), an inhibitor of Hsp90 alpha function, DNA fragmentation was not detected and caspase-3 protease activity levels were the same as those of mock-treated cells. Nicotine-induced caspase-3 activation and Hsp90 alpha expression, as well as suppression of the induction by GA, were also observed in a xeroderma pigmentosum patient-derived cell line, XP2OS cells. Thus, it was suggested that nicotine induces apoptosis, possibly via Hsp90 alpha expression, in human cells tested.

Calreticulin recognizes misfolded HLA-A2 heavy chains

Our studies investigated functional interactions between calreticulin, an endoplasmic reticulum chaperone, and major histocompatibility complex (MHC) class I molecules. Using in vitro thermal aggregation assays, we established that calreticulin can inhibit heat-induced aggregation of soluble, peptide-deficient HLA-A2 purified from supernatants of insect cells. The presence of HLA-A2-specific peptides also inhibits heat-induced aggregation. Inhibition of heat-induced aggregation of peptide-deficient HLA-A2 by calreticulin correlates with a rescue of the HLA-A2 heavy chain from precipitation, by forming high-molecular-weight complexes with calreticulin. Complex formation between HLA-A2 heavy chains and calreticulin occurs at 50 degrees C but not 37 degrees C, suggesting polypeptide-based interactions between the HLA-A2 heavy chain and calreticulin. Once complexes are formed, the addition of peptide is not sufficient to trigger efficient assembly of heavy chain/beta2m/peptide complexes. Using a fluorescent peptide-based binding assay, we show that calreticulin does not enhance peptide binding by HLA-A2 at 37 degrees C. We also show that calreticulin itself is converted to oligomeric species on exposure to 37 degrees C or higher temperatures, and that oligomeric forms of calreticulin are active in inhibiting thermal aggregation of peptide-deficient HLA-A2. Taken together, these results suggest that calreticulin functions in the recognition of misfolded MHC class I heavy chains in the endoplasmic reticulum. However, in the absence of other endoplasmic reticulum components, calreticulin by itself does not enhance the assembly of misfolded MHC class I heavy chains with beta2m and peptides.

Folding of newly translated proteins in vivo: the role of molecular chaperones

Recent years have witnessed dramatic advances in our understanding of how newly translated proteins fold in the cell and the contribution of molecular chaperones to this process. Folding in the cell must be achieved in a highly crowded macromolecular environment, in which release of nonnative polypeptides into the cytosolic solution might lead to formation of potentially toxic aggregates. Here I review the cellular mechanisms that ensure efficient folding of newly translated proteins in vivo. De novo protein folding appears to occur in a protected environment created by a highly processive chaperone machinery that is directly coupled to translation. Genetic and biochemical analysis shows that several distinct chaperone systems, including Hsp70 and the cylindrical chaperonins, assist the folding of proteins upon translation in the cytosol of both prokaryotic and eukaryotic cells. The cellular chaperone machinery is specifically recruited to bind to ribosomes and protects nascent chains and folding intermediates from nonproductive interactions. In addition, initiation of folding during translation appears to be important for efficient folding of multidomain proteins.

Liberation of the intramolecular interaction as the mechanism of heat-induced activation of HSP90 molecular chaperone

The molecular chaperone function of HSP90 is activated under heat-stress conditions. In the present study, we investigated the role of the interactions in the heat-induced activation of HSP90 molecular chaperone. The preceding paper demonstrated two domain-domain interactions of HtpG, an Escherichia coli homologue of mammalian HSP90, i.e. an intra-molecular interaction between the N-terminal and middle domains and an intermolecular one between the middle and C-terminal domains. A bacterial two-hybrid system revealed that the two interactions also existed in human HSP90alpha. Partners of the interaction between the N-terminal and middle domains of human HSP90alpha could, but those between the middle and C-terminal domains could not, be replaced by the domains of HtpG. Thus, the interface between the N-terminal and middle domains is essentially unvaried from bacterial to human members of the HSP90-family proteins. The citrate synthase-binding activity of HtpG at an elevated temperature was solely localized in the N-terminal domain, but HSP90alpha possessed two sites in the N-terminal and other domains. The citrate-synthase-binding activity of the N-terminal domain was suppressed by the association of the middle domain. The complex between the N-terminal and middle domains is labile at elevated temperatures, but the other is stable even at 70 degrees C. Taken together, we propose the liberation of the N-terminal client-binding domain from the middle suppressor domain is involved in the temperature-dependent activation mechanism of HSP90 molecular chaperone.

Both the N- and C-terminal chaperone sites of Hsp90 participate in protein refolding

Hsp90 is able to bind partially unfolded firefly luciferase and maintain it in a refoldable state; the subsequent successive action of the 20S proteasome activator PA28, Hsc70 and Hsp40 enables its refolding. Hsp90 possesses two chaperone sites in the N- and C-terminal domains that prevent the aggregation of denatured proteins. Here we show that both chaperone sites of Hsp90 are effective not only in capturing thermally denatured luciferase, but also in holding it in a state prerequisite for the successful refolding process mediated by PA28, Hsc70 and Hsp40. In contrast, the heat-induced activity of Hsp90 to bind chemically denature dihydrofolate reductase efficiently and prevent its rapid spontaneous refolding was detected in the N-terminal site of Hsp90 only, while the C-terminal site was without effect. Thus it is most likely that both the N- and C-terminal chaperone sites may contribute to Hsp90 function as holder chaperones, however, in a significantly distinct manner.

Phosphorylation and oligomerization states of native pig brain HSP90 studied by mass spectrometry

HSP90 is one of the most abundant proteins in the cytosol of eukaryotic cells. HSP90 forms transient or stable complexes with several key proteins involved in signal transduction including protooncogenic protein kinases and nuclear receptors, it interacts with cellular structural elements such as actin-microfilament, tubulin-microtubule and intermediate filaments, and also exhibits conventional chaperone functions. This protein exists in two isoforms alpha-HSP90 and beta-HSP90, and it forms dimers which are crucial species for its biological activity. PAGE, ESI-MS and MALDI-MS were used to study HSP90 purified from pig brain. The two protein isoforms were clearly distinguished by ESI-MS, the alpha isoform being approximately six times more abundant than the beta isoform. ESI-MS in combination with lambda phosphatase treatment provided direct evidence of the existence of four phosphorylated forms of native pig brain alpha-HSP90, with the diphosphorylated form being the most abundant. For the beta isoform, the di-phosphorylated was also the most abundant. MALDI mass spectra of HSP90 samples after chemical cross-linking showed a high percentage of alpha-alpha homodimers. In addition, evidence for the existence of higher HSP90 oligomers was obtained.

Substrate-binding characteristics of proteins in the 90 kDa heat shock protein family

In the present study we investigated the substrate-binding characteristics of three members of the 90 kDa heat shock protein (HSP90) family, namely the alpha isoform of human HSP90 (HSP90alpha), human GRP94 (94 kDa glucose-regulated protein, a form of HSP90 from endoplasmic reticulum), and HtpG (the Escherichia coli homologue of HSP90) and the domain responsible for these characteristics. The recombinant forms of HSP90alpha, GRP94 and HtpG existed as dimers and became oligomerized at higher temperatures. Among the three family members, HtpG required the highest temperature (65 degrees C) for its transition to oligomeric forms. The precipitation of the substrate protein, glutathione S-transferase, which occurred at 55 degrees C, was efficiently prevented by the simultaneous presence of a sufficient amount of HSP90alpha or GRP94, but not by HtpG, which was still present as a dimer at that temperature. However, precipitation was stopped completely at 65-70 degrees C, at which temperature HtpG was oligomerized. Thus the transition of HSP90-family proteins to a state with self-oligomerization ability is essential for preventing the precipitation of substrate proteins. We then investigated the domain responsible for the substrate binding of HtpG on the basis of the three domain structures. The self-oligomerizing and substrate-binding activities towards glutathione S-transferase and citrate synthase were both located in a single domain, the N-terminal domain (residues 1-336) of HtpG. We therefore propose that the primary peptide-binding site is located in the N-terminal domain of HSP90-family proteins.

Molecular cloning of horse Hsp90 cDNA and its comparative analysis with other vertebrate Hsp90 sequences

Heat shock protein 90 (Hsp90), a molecular chaperone, is ubiquitous and involved in numerous cellular processes. To contribute to the relatively small collection of vertebrate Hsp90 sequences in the gene data bank, we cloned and sequenced horse (Equus caballus) Hsp90 alpha and beta cDNAs. This enabled identification of horse-specific primers for development of a convenient PCR-based method that could monitor horse stress tolerance. We analyzed the sequence data comparatively and phylogenetically with other Hsp90 cDNA sequences, and identified vertebrate-specific and isoform-specific conserved regions to facilitate future molecular investigations of Hsp90 functions. We found 4 highly conserved regions to vertebrate Hsp90 exclusively and 27 amino acids conserved among but differing between Hsp90 alpha and Hsp90 beta sequences. Protein-based phylogenetic trees revealed high conservation between mammal species within Hsp90 alpha and beta clusters. Comparison of nucleotide and amino acid substitution levels suggests that horse Hsp90 beta has undergone strong purifying selection, while rat Hsp90 beta and hamster Hsp90 alpha have been positively selected. Surprisingly, fish Hsp90 alpha genes clearly clustered with Hsp90 beta genes, and no distinct placement of fish Hsp90 alpha protein was found. The Hsp90 alpha isoform is apparently the result of beta gene duplication. Our results highlight the importance of organism- and isoform-specific Hsp90 functional analyses in describing the role of Hsp90 in cells.

Perinatal expression of heat-shock protein 90 in different regions of the brain and in non-neural tissues of the piglet

Important stressful events occur at birth or within the few hours that follow. To establish a possible involvement of stress proteins, expression of heat-shock protein 90 was determined by Western blotting in several regions of the brain and in non-neural tissues of the developing piglet (fetal to 10 days and adult). Expression was found in all the tissues studied. While comparable values were found in the whole brain during development, decreased expressions were observed from 4 to 8 h to 2 days after birth in cerebellum, cortex, hypothalamus and striatum. In hippocampus, low expression was observed from 4 h postnatally onward. In non-neural tissues, low expression was observed after birth and in the adult for heart, liver and lungs. In kidney, low values were found from birth to 1 day of age. Changes in environmental parameters like temperature and/or hypoxia can be related to differential expressions of heat-shock proteins and they possibly result in severe developmental outcomes. The results are discussed in terms of using the newborn piglet as a model for the study of different forms of stress on the heat-shock protein expression during postnatal development.

Redox-regulated chaperone function and conformational changes of Escherichia coli Hsp33

We have studied the chaperone activity and conformation of Escherichia coli heat shock protein (Hsp)33, whose activity is known to be switched on by oxidative conditions. While oxidized Hsp33 completely prevents the heat-induced aggregation of zeta-crystallin at 42 degrees C at a ratio of 1:1 (w/w), the reduced form exhibits only a marginal effect on the aggregation. Far UV-circular dichroism (CD) spectra show that reduced Hsp33 contains a significant alpha-helical component. Oxidation results in significant changes in the far UV-CD spectrum. Near UV-CD spectra show changes in tertiary structural packing upon oxidation. Polarity-sensitive fluorescent probes report enhanced hydrophobic surfaces in the oxidized Hsp33. Our studies show that the oxidative activation of the chaperone function of Hsp33 involves observable conformational changes accompanying increased exposure of hydrophobic pockets.

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

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

Structural changes in alpha-synuclein affect its chaperone-like activity in vitro

Alpha-synuclein, a major constituent of Lewy bodies (LBs) in Parkinson's disease (PD), has been implicated to play a critical role in synaptic events, such as neuronal plasticity during development, learning, and degeneration under pathological conditions, although the physiological function of alpha-synuclein has not yet been established. We here present biochemical evidence that recombinant alpha-synuclein has a chaperone-like function against thermal and chemical stress in vitro. In our experiments, alpha-synuclein protected glutathione S-transferase (GST) and aldolase from heat-induced precipitation, and alpha-lactalbumin and bovine serum albumin from dithiothreitol (DTT)-induced precipitation like other molecular chaperones. Moreover, preheating of alpha-synuclein, which is believed to reorganize the molecular surface of alpha-synuclein, increased the chaperone-like activity. Interestingly, in organic solvents, which promotes the formation of secondary structure, alpha-synuclein aggregated more easily than in its native condition, which eventually might abrogate the chaperone-like function of the protein. In addition, alpha-synuclein was also rapidly and significantly precipitated by heat in the presence of Zn2+ in vitro, whereas it was not affected by the presence of Ca2+ or Mg2+. Circular dichroism spectra confirmed that alpha-synuclein underwent conformational change in the presence of Zn2+. Taken together, our data suggest that alpha-synuclein could act as a molecular chaperone, and that the conformational change of the alpha-synuclein could explain the aggregation kinetics of alpha-synuclein, which may be related to the abolishment of the chaperonic-like activity.

Acute pancreatitis results in induction of heat shock proteins 70 and 27 and heat shock factor-1

Heat shock proteins (HSPs) 70 and 27 are stress-responsive proteins that are important for cell survival after injury; the expression of these HSPs is regulated primarily by the transcription factor heat shock factor-1 (HSF-1). The purpose of this study was to determine the effect of acute pancreatitis on pancreatic HSPs (70, 27, 60, and 90) expression and to assess potential mechanisms for HSP induction using a murine model of cerulein-induced pancreatitis. We found an increase of both HSP70 and HSP27 levels with expression noted throughout the pancreas after induction of pancreatitis. HSP60 and HSP90 levels were constitutively expressed in the pancreas and did not significantly change with acute pancreatitis. HSF-1 DNA binding activity increased in accordance with increased HSP expression. We conclude that acute pancreatitis results in a marked increase in the expression of HSP70 and HSP27. Furthermore, the induction of HSP70 and HSP27 expression was associated with a concomitant increase in HSF-1 binding activity. The increased expression of both HSP70 and HSP27 noted with pancreatic inflammation may confer a protective effect for the remaining acini after acute pancreatitis.

Co-expression of human chaperone Hsp70 and Hsdj or Hsp40 co-factor increases solubility of overexpressed target proteins in insect cells

The insect-baculovirus expression system has proved particularly useful for producing recombinant proteins that are biologically active. Overexpression of foreign proteins using the recombinant baculovirus system is often accompanied by aggregation of the overexpressed protein, which is thought to be due to a limitation of the translated protein folding in the infected cells. Co-infection of a recombinant baculovirus capable of expressing the human chaperone Hsp70 slightly increased the solubility of the overexpressed Epstein-Barr virus replication protein, BZLF1. Co-expression of Hsp70 and its co-factor, Hsdj or Hsp40, was here found to improve the solubility of the target protein several fold. Thus, a baculovirus expression system producing these molecular chaperones may find application for improved production of target foreign gene products in insect cells.

Two inbred strains of rats, Fischer 344 and Lewis, showed differential behavior and brain expression of corticosterone receptor mRNA induced by methamphetamine

Recently, a role of the hypothalamo-pituitary-adrenocortical (HPA) axis in facilitating the behavioral and neurochemical effects of psychostimulants has been proposed. Two inbred strains of rats, Fischer 344/N (F344) and Lewis/N (LEW), have markedly different HPA axes as well as behavioral responses to psychostimulants: F344 rats show hyperresponsive HPA axis and no significant sensitization to cocaine, whereas LEW rats display blunted response in HPA axis and develop cocaine-induced locomotor sensitization. Corticosterone exerts its biological effects via intracellular receptors, termed type I (mineralocorticoid receptor: MR) and type II (glucocorticoid receptor: GR). The present study examines the development of stereotypy sensitization and the brain expression of mRNAs for MR, GR, and heat shock protein 90 (HSP90) in methamphetamine (MAP)-treated F344 and LEW rats. Animals received i.p. injections with chronic saline (SAL: once daily for 21 days), chronic saline and acute MAP (AM: saline for 20 days and 4 mg/kg MAP on the 21st day), or chronic MAP (CM: 4 mg/kg MAP for 21 days) and were sacrificed three hours after the last injections. Striatum, hippocampus, and cerebellum were quickly dissected on ice and total RNA was isolated for northern analyses. LEW rats developed stereotypy sensitization significantly earlier than F344 rats. AM significantly decreased GR and MR mRNA expression in hippocampus of LEW, compared to SAL. CM significantly increased GR and MR mRNA expression in striatum of F344 compared to SAL and AM, while it decreased GR mRNA in striatum of LEW and MR mRNA in hippocampus, compared to SAL. AM significantly increased HSP90 mRNA in all brain regions examined, without the hippocampus in LEW. CM significantly increased the expression of HSP90 mRNA in the striatum and cerebellum of F344, but significantly decreased in the striatum and hippocampus of LEW. These contrasting differences between F344 and LEW, in their susceptibility to stereotypy sensitization and striatal expression of GR mRNA by chronic MAP, suggest that some striatal genes, whose transcription is regulated by GR, play a crucial role in the development of MAP-induced behavioral sensitization.

Negative regulation of cytochrome c-mediated oligomerization of Apaf-1 and activation of procaspase-9 by heat shock protein 90

The release of cytochrome c from mitochondria results in the formation of an Apaf-1-caspase-9 apoptosome and induces the apoptotic protease cascade by activation of procaspase-3. The present studies demonstrate that heat shock protein 90 (Hsp90) forms a cytosolic complex with Apaf-1 and thereby inhibits the formation of the active complex. Immunodepletion of Hsp90 depletes Apaf-1 and thereby inhibits cytochrome c-mediated activation of caspase-9. Addition of purified Apaf-1 to Hsp90-depleted cytosolic extracts restores cytochrome c-mediated activation of procaspase-9. We also show that Hsp90 inhibits cytochrome c-mediated oligomerization of Apaf-1 and thereby activation of procaspase-9. Furthermore, treatment of cells with diverse DNA-damaging agents dissociates the Hsp90-Apaf-1 complex and relieves the inhibition of procaspase-9 activation. These findings provide the first evidence for a negative cytosolic regulator of cytochrome c-dependent apoptosis and for involvement of a chaperone in the caspase cascade.

Mapping the functional domains of nucleolar protein B23

Protein B23 is a multifunctional nucleolar protein whose cellular location and characteristics strongly suggest that it is a ribosome assembly factor. The protein has nucleic acid binding, ribonuclease, and molecular chaperone activities. To determine the contributions of unique polypeptide segments enriched in certain classes of amino acid residues to the respective activities, several constructs that produced N- and C-terminal deletion mutant proteins were prepared. The C-terminal quarter of the protein was shown to be necessary and sufficient for nucleic acid binding. Basic and aromatic segments at the N- and C-terminal ends, respectively, of the nucleic acid binding region were required for activity. The molecular chaperone activity was contained in the N-terminal half of the molecule, with important contributions from both nonpolar and acidic regions. The chaperone activity also correlated with the ability of the protein to form oligomers. The central portion of the molecule was required for ribonuclease activity and possibly contains the catalytic site; this region overlapped with the chaperone-containing segment of the molecule. The C-terminal, nucleic acid-binding region enhanced the ribonuclease activity but was not essential for it. These data suggest that the three activities reside in mainly separate but partially overlapping segments of the polypeptide chain.

Ligand interactions in the adenosine nucleotide-binding domain of the Hsp90 chaperone, GRP94. II. Ligand-mediated activation of GRP94 molecular chaperone and peptide binding activity

The N-terminal domain of eukaryotic Hsp90 proteins contains a conserved adenosine nucleotide binding pocket that also serves as the binding site for the Hsp90 inhibitors geldanamycin and radicicol. Although this domain is essential for Hsp90 function, the molecular basis for adenosine nucleotide-dependent regulation of GRP94, the endoplasmic reticulum paralog of Hsp90, remains to be established. We report that bis-ANS (1,1'-bis(4-anilino-5-napthalenesulfonic acid), an environment sensitive fluorophore known to interact with nucleotide-binding domains, binds to the adenosine nucleotide-binding domain of GRP94 and thereby activates its molecular chaperone and peptide binding activities. bis-ANS was observed to elicit a tertiary conformational change in GRP94 similar to that occurring upon heat shock, which also activates GRP94 function. bis-ANS activation of GRP94 function was efficiently blocked by radicicol, an established inhibitory ligand for the adenosine nucleotide binding pocket. Confirmation of the N-terminal nucleotide binding pocket as the bis-ANS-binding site was obtained following covalent incorporation of bis-ANS into GRP94, trypsinolysis, and sequencing of bis-ANS-labeled limit digestion products. These data identify a ligand dependent regulation of GRP94 function and suggest a model whereby GRP94 function is regulated through a ligand-dependent conversion of GRP94 from an inactive to an active conformation.

Modulation of the chaperone activities of Hsc70/Hsp40 by Hsp105alpha and Hsp105beta

Hsp105alpha and Hsp105beta are stress proteins found in various mammals including human, mouse, and rat, which belong to the Hsp105/Hsp110 protein family. To elucidate their physiological functions, we examined here the chaperone activity of these stress proteins. Hsp105alpha and Hsp105beta prevented the aggregation of firefly luciferase during thermal denaturation, whereas the thermally denatured luciferase was not reactivated by itself or by rabbit reticulocyte lysate (RRL). On the other hand, Hsp105alpha and Hsp105beta suppressed the reactivation of thermally denatured luciferase by RRL and of chemically denatured luciferase by Hsc70/Hsp40 or RRL. Furthermore, although Hsp105alpha and Hsp105beta did not show ATPase activity, the addition of Hsp105alpha or Hsp105beta to Hsc70/Hsp40 enhanced the amount of hydrolysis of ATP greater than that of the Hsp40-stimulated Hsc70 ATPase activity. These findings suggest that Hsp105alpha and Hsp105beta are not only chaperones that prevent thermal aggregation of proteins, but also regulators of the Hsc70 chaperone system in mammalian cells.

A critical role for the proteasome activator PA28 in the Hsp90-dependent protein refolding

The 90-kDa heat shock protein, Hsp90, was previously shown to capture firefly luciferase during thermal inactivation and prevent it from undergoing an irreversible off-pathway aggregation, thereby maintaining it in a folding-competent state. While Hsp90 by itself was not sufficient to refold the denatured luciferase, addition of rabbit reticulocyte lysate remarkably restored the luciferase activity. Here we demonstrate that Hsc70, Hsp40, and the 20 S proteasome activator PA28 are the effective components in reticulocyte lysate. Purified Hsc70, Hsp40, and PA28 were necessary and sufficient to fully reconstitute Hsp90-initiated refolding. Kinetics of substrate binding support the idea that PA28 acts as the molecular link between the Hsp90-dependent capture of unfolded proteins and the Hsc70- and ATP-dependent refolding process.

Induction of heat-shock (stress) protein gene expression by selected natural and anthropogenic disturbances in the octocoral Dendronephthya klunzingeri

Previously it was found that the expression of selected heat-shock proteins is upregulated in corals after exposure to elevated temperature. We published that HSPs are suitable markers in sponges to monitor the degree of environmental stress on these animals. In the present study the heat-shock proteins (HSPs) with a molecular weight of 90 kDa have been selected to prove their potential usefulness as biomarkers under controlled laboratory conditions and in the field. The studies have been performed with the octocoral Dendronephthya klunzingeri4.5-fold higher steady-state level of the respective mRNA. Also animals taken from stressed locations in the field showed an increased expression. The amount of HSP90 protein in D. klunzingeri was found to be strongly increased under thermal stress, or exposure to polychlorinated biphenyl (congener 118), but not after treatment with cadmium. Field studies revealed that samples taken from a nonstressed area have a low level of HSP90, but those collected from locations at which the corals are under physical stress (sedimentation through landfilling) show a high expression of HSP90. It is concluded that the chaperone HSP90 might become a suitable biomarker to monitor environmental stress on corals.

Alpha-complemented beta-galactosidase. An in vivo model substrate for the molecular chaperone heat-shock protein 90 in yeast

Intracistronic complementation of N-terminally truncated beta-galactosidase mutants such as M15 by coexpressed alpha-peptide was originally discovered in Escherichia coli and exploited for plasmid cloning as the well-known blue-white screening method. We show here that alpha-complementation also works in the budding yeast Saccharomyces cerevisiae, and that it can be used as a simple nonselective enzymatic marker for a variety of in vivo studies, for example, on the role of molecular chaperones in protein folding and assembly. To be able to induce alpha-complementation post-translationally, we have constructed a hormone-inducible alpha-peptide by fusion of the DNA encoding the alpha-peptide to that of to the hormone binding domain of the estrogen receptor. The accumulation of both subunits, the alpha-peptide and M15, is severely compromised when they are expressed separately, presumably because their hydrophobic surfaces remain exposed. Moreover, alpha-complementation is defective in a strain of S. cerevisiae carrying a point mutant of the molecular chaperone heat-shock protein 90. Heat-shock protein 90, which coprecipitates with M15, might be required in vivo to prevent the degradation of unassembled M15 and to hold it in an interaction-competent conformation.

Hsc70/Hsp40 chaperone system mediates the Hsp90-dependent refolding of firefly luciferase

BACKGROUND

The 90-kDa heat shock protein, Hsp90, was previously shown to capture firefly luciferase during thermal inactivation, thereby preventing its irreversible off-pathway aggregation and maintaining it in a folding-competent state. However, subsequent refolding of the luciferase required addition of rabbit reticulocyte lysate.

RESULTS

Here we demonstrate that Hsc70 (cytosolic Hsp70) and Hsp40/Hdj1 (cytosolic DnaJ homologue) are the effective components in a reticulocyte lysate, while other unidentified factor in the lysate is also required for the refolding of Hsp90-captured luciferase. Though another cytosolic DnaJ homologue, Hdj2/HSDJ, was more efficient than Hsp40 in suppressing the aggregation of rhodanese, Hdj2 was less effective for the refolding of luciferase than Hsp40. In the absence of the third factor, Hsp40 could bind to the luciferase captured by Hsp90, which suggested that Hsp40 on its own was able to bind the substrate protein, but Hsc70 could not.

CONCLUSIONS

Hsc70, Hsp40 and at least another additional component in the reticulocyte lysate are necessary for full accomplishment of the refolding of Hsp90-captured luciferase. The third factor may be required for the loading of Hsc70 on to the substrate protein bound to Hsp90.