Domain structures and immunogenic regions of the 90-kDa heat-shock protein (HSP90). Probing with a library of anti-HSP90 monoclonal antibodies and limited proteolysis

Contact System Activation and Bradykinin Generation in Angioedema: Laboratory Assessment and Biomarker Utilization

The role of contact system activation has been clearly established in the pathogenesis of hereditary angioedema due to C1 inhibitor deficiency (HAE-C1INH). C1 inhibitor (C1INH)-protease complexes, levels of functional C1INH, plasma kallikrein activation, and cleavage of high-molecular-weight kininogen have each been associated with disease activity. More recently, HAE with normal levels of C1INH (HAE-nl-C1INH) has been recognized. Six genetic mutations have been identified which are linked to HAE-nl-C1INH phenotypes. The majority of individuals with HAE-nl-C1INH fall into the unknown category. There is substantial evidence that bradykinin generation underlies the recurrent attacks of swelling in some of these cohorts.

Hsp90 Inhibitors for the Treatment of Chronic Myeloid Leukemia

Chronic myeloid leukemia (CML) is a hematological malignancy that arises due to reciprocal translocation of 3' sequences from c-Abelson (ABL) protooncogene of chromosome 9 with 5' sequence of truncated break point cluster region (BCR) on chromosome 22. BCR-ABL is a functional oncoprotein p210 that exhibits constitutively activated tyrosine kinase causing genomic alteration of hematopoietic stem cells. BCR-ABL specific tyrosine kinase inhibitors (TKIs) successfully block CML progression. However, drug resistance owing to BCR-ABL mutations and overexpression is still an issue. Heat-shock proteins (Hsps) function as molecular chaperones facilitating proper folding of nascent polypeptides. Their increased expression under stressful conditions protects cells by stabilizing unfolded or misfolded peptides. Hsp90 is the major mammalian protein and is required by BCR-ABL for stabilization and maturation. Hsp90 inhibitors destabilize the binding of BCR-ABL protein thus leading to the formation of heteroprotein complex that is eventually degraded by the ubiquitin-proteasome pathway. Results of many novel Hsp90 inhibitors that have entered into various clinical trials are encouraging. The present review targets the current development in the CML treatment by availing Hsp90 specific inhibitors.

Expression patterns and structural modelling of Hsp70 and Hsp90 in a fish-borne zoonotic nematode Anisakis pegreffii

Heat shock proteins (HSPs) are essential molecular chaperones that are highly conserved across organisms. They have a pivotal function in responding to thermal stress and are responsible for many cellular functions. Here, we aimed to elucidate the possible roles of Hsp70 and Hsp90 in the life cycle of the parasitic nematode Anisakis, particularly third- and fourth-stage larvae, from cold-blooded fish to warm-blooded marine mammals or accidentally to human hosts. We examined the expression profiles of Hsp70 and Hsp90 in different developmental stages of Anisakis pegreffii. The open reading frame of Hsp70 of A. pegreffii was 1950 bp, and deduced amino acid sequence showed high homology with those of other nematodes. Heatmap analysis revealed sequence identity of Hsp70 and Hsp90 in 13 important parasitic species, human and yeast. On heatmap and phylogenetic analysis, ApHsp70 and ApHsp90 shared the highest amino acid sequence identity with other nematodes and formed a monophyletic clade. The three-dimensional (3D) structure prediction of the newly characterized ApHsp70 and known ApHsp90 gene showed highly conserved motifs between A. pegreffii and other species. Quantitative real-time PCR and western blot analysis revealed higher mRNA and protein expression for ApHsp70 and ApHsp90 in fourth- than third-stage larvae, with higher mRNA and protein expression for ApHsp70 than ApHsp90. ApHsp70 and ApHsp90 may play important roles in Anisakis in response to thermal stress and might be important molecules in the development of A. pegreffii, which has implications for its control.

Posttranslational modification and conformational state of heat shock protein 90 differentially affect binding of chemically diverse small molecule inhibitors

Heat shock protein 90 (Hsp90) is an essential molecular chaperone in eukaryotes that facilitates the conformational maturation and function of a diverse protein clientele, including aberrant and/or over-expressed proteins that are involved in cancer growth and survival. A role for Hsp90 in supporting the protein homeostasis of cancer cells has buoyed interest in the utility of Hsp90 inhibitors as anti-cancer drugs. Despite the fact that all clinically evaluated Hsp90 inhibitors target an identical nucleotide-binding pocket in the N domain of the chaperone, the precise determinants that affect drug binding in the cellular environment remain unclear, and it is possible that chemically distinct inhibitors may not share similar binding preferences. Here we demonstrate that two chemically unrelated Hsp90 inhibitors, the benzoquinone ansamycin geldanamycin and the purine analog PU-H71, select for overlapping but not identical subpopulations of total cellular Hsp90, even though both inhibitors bind to an amino terminal nucleotide pocket and prevent N domain dimerization. Our data also suggest that PU-H71 is able to access a broader range of N domain undimerized Hsp90 conformations than is geldanamycin and is less affected by Hsp90 phosphorylation, consistent with its broader and more potent anti-tumor activity. A more complete understanding of the impact of the cellular milieu on small molecule inhibitor binding to Hsp90 should facilitate their more effective use in the clinic.

Expression of the endoplasmic reticulum oxidoreductase Ero1α in gastro-intestinal cancer reveals a link between homocysteine and oxidative protein folding

AIM

Ero proteins are central to oxidative protein folding in the endoplasmic reticulum (ER), but their expression varies in a tissue-specific manner. The aim of this work was to establish the expression of Ero1α in the digestive system and to examine the behavior of Ero1α in premalignant Barrett's esophagus, esophageal (OE) and gastric cancers and esophageal cancer cell lines.

RESULTS

Ero1α is expressed in the columnar epithelium of Barrett's tissue, and in OE tumors and gastric tumors. Homocysteine, a precursor in the metabolism of cysteine and methionine, induces the active Ox1 form of Ero1α in the OE cancer cell line OE33.

INNOVATION

These results demonstrate for the first time that Ero1α can sense the level of an amino acid precursor, identifying a potential link between diet, antioxidants, and oxidative protein folding in the ER.

CONCLUSION

The high expression of Ero1α in cancers of the esophagus and stomach demonstrates the importance of ER redox regulation in the gastro-intestinal (GI) tract in health and disease. Proteins and metabolites involved in disulfide bond formation and redox regulation may be suitable targets for both biomarker and drug development in GI cancer.

Charged linker sequence modulates eukaryotic heat shock protein 90 (Hsp90) chaperone activity

Hsp90 is an essential and highly conserved modular molecular chaperone whose N and middle domains are separated by a disordered region termed the charged linker. Although its importance has been previously disregarded, because a minimal linker length is sufficient for Hsp90 activity, the evolutionary persistence of extensive charged linkers of divergent sequence in Hsp90 proteins of most eukaryotes remains unexplained. To examine this question further, we introduced human and plasmodium native and length-matched artificial linkers into yeast Hsp90. After evaluating ATPase activity and biophysical characteristics in vitro, and chaperone function in vivo, we conclude that linker sequence affects Hsp90 function, cochaperone interaction, and conformation. We propose that the charged linker, in addition to providing the flexibility necessary for Hsp90 domain rearrangements--likely its original purpose--has evolved in eukaryotes to serve as a rheostat for the Hsp90 chaperone machine.

To fold or not to fold: modulation and consequences of Hsp90 inhibition

BACKGROUND

The 90-kDa heat-shock proteins (Hsp90) have rapidly evolved into promising therapeutic targets for the treatment of several diseases, including cancer and neurodegenerative diseases. Hsp90 is a molecular chaperone that aids in the conformational maturation of nascent polypeptides, as well as the rematuration of denatured proteins.

DISCUSSION

Many of the Hsp90-dependent client proteins are associated with cellular growth and survival and, consequently, inhibition of Hsp90 represents a promising approach for the treatment of cancer. Conversely, stimulation of heat-shock protein levels has potential therapeutic applications for the treatment of neurodegenerative diseases that result from misfolded and aggregated proteins.

CONCLUSION

Hsp90 modulation exhibits the potential to treat unrelated disease states, from cancer to neurodegenerative diseases, and, thus, to fold or not to fold, becomes a question of great value.

Coexposure to mercury increases immunotoxicity of trichloroethylene

We have shown previously that chronic (32 weeks) exposure to occupationally relevant concentrations of the environmental pollutant trichloroethylene (TCE) induced autoimmune hepatitis (AIH) in autoimmune-prone MRL+/+ mice. In real-life, individuals are never exposed to only one chemical such as TCE. However, very little is known about the effects of chemical mixtures on the immune system. The current study examined whether coexposure to another known immunotoxicant, mercuric chloride (HgCl(2)), altered TCE-induced AIH. Female MRL+/+ mice were treated for only 8 weeks with TCE (9.9 or 186.9 mg/kg/day in drinking water) and/or HgCl(2) (260 μg/kg/day, sc). Unlike mice exposed to either TCE or HgCl(2) alone, mice exposed to both toxicants for 8 weeks developed significant liver pathology commensurate with early stages of AIH. Disease development in the coexposed mice was accompanied by a unique pattern of anti-liver and anti-brain antibodies that recognized, among others, a protein of approximately 90 kDa. Subsequent immunoblotting showed that sera from the coexposed mice contained antibodies specific for heat shock proteins, a chaperone protein targeted by antibodies in patients with AIH. Thus, although TCE can promote autoimmune disease following chronic exposure, a shorter exposure to a binary mixture of TCE and HgCl(2) accelerated disease development. Coexposure to TCE and HgCl(2) also generated a unique liver-specific antibody response not found in mice exposed to a single toxicant. This finding stresses the importance of including mixtures in assessments of chemical immunotoxicity.

Key motifs in EBV (Epstein-Barr virus)-encoded protein kinase for phosphorylation activity and nuclear localization

A sole EBV (Epstein-Barr virus)-encoded protein kinase (EBV-PK) (the BGLF4 gene product) plays important roles in viral infection. Although a number of targets of this protein have been identified, the kinase itself remains largely unstudied with regard to its enzymology and structure. In the present study, site-directed mutagenesis has been employed to generate mutations targeting residues involved in nuclear localization of the EBV-PK, core residues in subdomain III of the protein kinase domain conserved in most protein kinases or residues in subdomain VIa conserved only within the HPK (herpesvirus-encoded protein kinase) group. Deletion of amino acids 389-391 resulted in exclusive cytoplasmic localization of the protein, indicating the involvement of this region in nuclear translocation of the EBV-PK. Mutations at the amino acids Glu113 (core component), Phe175, Leu178, Phe184, Leu185 and Asn186 (conserved in HPKs) resulted in loss of EBV-PK autophosphorylation, protein substrate [EBV EA-D (early antigen diffused)] phosphorylation, and ability to facilitate ganciclovir phosphorylation. These results reiterate the unique features of this group of kinases and present an opportunity for designing more specific antiviral compounds.

Hsp90 charged-linker truncation reverses the functional consequences of weakened hydrophobic contacts in the N domain

Heat shock protein 90 (Hsp90) is an essential molecular chaperone in eukaryotes, as it regulates diverse signal transduction nodes that integrate numerous environmental cues to maintain cellular homeostasis. Hsp90 also is secreted from normal and transformed cells and regulates cell motility. Here, we have identified a conserved hydrophobic motif in a beta-strand at the boundary between the N domain and charged linker of Hsp90, whose mutation not only abrogated Hsp90 secretion but also inhibited its function. These Hsp90 mutants lacked chaperone activity in vitro and failed to support yeast viability. Notably, truncation of the charged linker reduced solvent accessibility of this beta-strand and restored chaperone activity to these mutants. These data underscore the importance of beta-strand 8 for Hsp90 function and demonstrate that the functional consequences of weakened hydrophobic contacts in this region are reversed by charged-linker truncation.

Drug-target identification from total cellular lysate by drug-induced conformational changes

Identification of drug targets is a key step in the development of novel pharmaceuticals. To this end, chemical probes or affinity matrices are often used, requiring substantial structure-activity relationship (SAR) studies. Here we report on the development of a novel technique for drug-target identification from total cellular lysate conducted independently of SAR information. This technique relies on binding of a drug to its target inducing a conformational change in target protein, thereby altering its susceptibility to proteolysis and resulting in specific degradation in some cases or in protection of target protein in others. As proof of concept, three drugs with identified targets were used. First, incubation of cellular lysates with okadaic acid elicited a specific protective effect on its target, protein phosphatase 2A catalytic subunit. Second, specific protection from exogenous protease of FKBP12 by FK506 and Hsp90 fragments by radicicol were observed. We then used the method to validate the targets of UCS15A, an Src signaling inhibitor. UCS15A induced proteolysis of a number of proteins, one of which was identified as Sam68. These studies suggest that the technology may be generally useful for identification and validation of drug targets.

Transforming growth factor alpha (TGFalpha)-stimulated secretion of HSP90alpha: using the receptor LRP-1/CD91 to promote human skin cell migration against a TGFbeta-rich environment during wound healing

Jump-starting and subsequently maintaining epidermal and dermal cell migration are essential processes for skin wound healing. These events are often disrupted in nonhealing wounds, causing patient morbidity and even fatality. Currently available treatments are unsatisfactory. To identify novel wound-healing targets, we investigated secreted molecules from transforming growth factor alpha (TGFalpha)-stimulated human keratinoytes, which contained strong motogenic, but not mitogenic, activity. Protein purification allowed us to identify the heat shock protein 90alpha (hsp90alpha) as the factor fully responsible for the motogenic activity in keratinocyte secretion. TGFalpha causes rapid membrane translocation and subsequent secretion of hsp90alpha via the unconventional exosome pathway in the cells. Secreted hsp90alpha promotes both epidermal and dermal cell migration through the surface receptor LRP-1 (LDL receptor-related protein 1)/CD91. The promotility activity resides in the middle domain plus the charged sequence of hsp90alpha but is independent of the ATPase activity. Neutralizing the extracellular function of hsp90alpha blocks TGFalpha-induced keratinicyte migration. Most intriguingly, unlike the effects of canonical growth factors, the hsp90alpha signaling overrides the inhibition of TGFbeta, an abundant inhibitor of dermal cell migration in skin wounds. This finding provides a long-sought answer to the question of how dermal cells migrate into the wound environment to build new connective tissues and blood vessels. Thus, secreted hsp90alpha is potentially a new agent for wound healing.

Substitution of only two residues of human Hsp90alpha causes impeded dimerization of Hsp90beta

Two isoforms of the 90-kDa heat-shock protein (Hsp90), i.e., Hsp90alpha and Hsp90beta, are expressed in the cytosol of mammalian cells. Although Hsp90 predominantly exists as a dimer, the dimer-forming potential of the beta isoform of human and mouse Hsp90 is less than that of the alpha isoform. The 16 amino acid substitutions located in the 561-685 amino acid region of the C-terminal dimerization domain should be responsible for this impeded dimerization of Hsp90beta (Nemoto T, Ohara-Nemoto Y, Ota M, Takagi T, Yokoyama K. Eur J Biochem 233: 1-8, 1995). The present study was performed to define the amino acid substitutions that cause the impeded dimerization of Hsp90beta. Bacterial two-hybrid analysis revealed that among the 16 amino acids, the conversion from Ala(558) of Hsp90beta to Thr(566) of Hsp90alpha and that from Met(621) of Hsp90beta to Ala(629) of Hsp90alpha most efficiently reversed the dimeric interaction, and that the inverse changes from those of Hsp90alpha to Hsp90beta primarily explained the impeded dimerization of Hsp90beta We conclude that taken together, the conversion of Thr(566) and Ala(629) of Hsp90alpha to Ala(558) and Met(621) is primarily responsible for impeded dimerization of Hsp90beta.

Src-mediated phosphorylation of Hsp90 in response to vascular endothelial growth factor (VEGF) is required for VEGF receptor-2 signaling to endothelial NO synthase

Nitric oxide (NO) release from endothelial cells, via endothelial NO synthase (eNOS) activation, is central to the proangiogenic actions of vascular endothelial growth factor (VEGF). VEGF signaling to eNOS is principally mediated by an Akt-dependent phosphorylation of eNOS and by increased association of eNOS to the molecular chaperone, heat-shock protein 90 kDa (Hsp90). Herein, we report that VEGFR-2 activation induces tyrosine phosphorylation of VEGF receptor 2 (VEGFR-2)-associated Hsp90beta. Tyrosine phosphorylation of Hsp90beta in response to VEGF is dependent on internalization of the VEGFR-2 and on Src kinase activation. Furthermore, we demonstrate that c-Src directly phosphorylates Hsp90 on tyrosine 300 residue and that this event is essential for VEGF-stimulated eNOS association to Hsp90 and thus NO release from endothelial cells. Our work identifies Y300 phosphorylation of Hsp90 as a novel regulated posttranslational modification of the chaperone and demonstrates its importance in the proangiogenic actions of VEGF, namely by regulating NO release from endothelial cells.

An acetylation site in the middle domain of Hsp90 regulates chaperone function

Heat-shock protein 90 (Hsp90) chaperones a key subset of signaling proteins and is necessary for malignant transformation. Hsp90 is subject to an array of posttranslational modifications that affect its function, including acetylation. Histone deacetylase (HDAC) inhibitors and knockdown of HDAC6 induce Hsp90 acetylation and inhibit its activity. However, direct determination of the functional consequences of Hsp90 acetylation has awaited mapping of specific sites. We now demonstrate that Hsp90 K294 is acetylated. Mutational analysis of K294 shows that its acetylation status is a strong determinant of client protein and cochaperone binding. In yeast, Hsp90 mutants that cannot be acetylated at K294 have reduced viability and chaperone function compared to WT or to mutants that mimic constitutive acetylation. These data suggest that acetylation/deacetylation of K294 plays an important role in regulating the Hsp90 chaperone cycle.

Xanthomonas albilineans HtpG is required for biosynthesis of the antibiotic and phytotoxin albicidin

Xanthomonas albilineans, the causal agent of leaf scald disease of sugarcane, produces a highly potent polyketide-peptide antibiotic and phytotoxin called albicidin. Previous studies established the involvement of a large cluster of genes in the biosynthesis of this toxin. We report here the sub-cloning and sequencing of an additional gene outside of the main cluster and essential for albicidin biosynthesis. This gene encodes a 634-amino-acid protein that shows high identity with the Escherichia coli heat shock protein HtpG. Complementation studies of X. albilineans Tox- mutants confirmed the requirement of htpG for albicidin biosynthesis and revealed functional interchangeability between E. coli and X. albilineans htpG genes. HtpG was co-localised with albicidin in the cellular membrane, i.e., the cellular fraction where the toxin is most probably biosynthesised. Here we show the requirement of an HtpG protein for the biosynthesis of a polyketide-peptide antibiotic.

Antibodies against heat shock proteins in environmental stresses and diseases: friend or foe?

Heat shock proteins (Hsps) can be found in two forms, intracellular and extracellular. The intracellular Hsps are induced as a result of stress and have been found to be cytoprotective in many instances due to their chaperone functions in protein folding and in protein degradation. The origin and role of extracellular Hsps is less clear. Although they were suspected originally to be released from damaged cells (necrosis), their presence in most normal individuals rather suggests that they have regulatory functions in circulation. As immunodominant molecules, Hsps can stimulate the immune system, leading to the production of autoantibodies recognizing epitopes shared by microbial and human Hsps. Thus, extracellular Hsps can influence the inflammatory response as evidenced by the production of inflammatory cytokines. Antibodies to Hsps have been found under normal conditions but seem to be increased in certain stresses and diseases. Such antibodies could regulate the inflammatory response positively or negatively. Here, we review the literature on the findings of antibodies to Hsps in situations of environmental or occupational stress and in a number of diseases and discuss their possible significance for the diagnosis, prognosis, or pathogenesis of these diseases.

Structures of the N-terminal and middle domains of E. coli Hsp90 and conformation changes upon ADP binding

Hsp90 is an abundant molecular chaperone involved in many biological systems. We report here the crystal structures of the unliganded and ADP bound fragments containing the N-terminal and middle domains of HtpG, an E. coli Hsp90. These domains are not connected through a flexible linker, as often portrayed in models, but are intimately associated with one another. The individual HtpG domains have similar folding to those of DNA gyrase B but assemble differently, suggesting somewhat different mechanisms for the ATPase superfamily. ADP binds to a subpocket of a large site that is jointly formed by the N-terminal and middle domains and induces conformational changes of the N-terminal domain. We speculate that this large pocket serves as a putative site for binding of client proteins/cochaperones. Modeling shows that ATP is not exposed to the molecular surface, thus implying that ATP activation of hsp90 chaperone activities is accomplished via conformational changes.

Identification of the pentapeptide constituting a dominant epitope common to all eukaryotic heat shock protein 90 molecular chaperones

We previously reported that, in human heat shock protein (Hsp) 90 (hHsp90), there are 4 highly immunogenic sites, designated sites Ia, Ib, Ic, and II. This study was performed to further characterize their epitopes and to identify the epitope that is potentially common to all members of the Hsp90 family. Panning of a bacterial library carrying randomized dodecapeptides revealed that Glu251-Ser-X-Asp254 constituted site Ia and Pro295-Ile-Trp-Thr-Arg299, site Ic. Site II (Asp701-Pro717) was composed of several epitopes. When 19 anti-hHsp90 monoclonal antibodies (mAbs) were subjected to immunoblotting against recombinant forms of 7 Hsp90-family members, 2 mAbs (K41110 and K41116C) that recognized site Ic bound to yeast Hsp90 with affinity identical to that for hHsp90, and 1 mAb (K3729) that recognized Glu222-Ala23, of hHsp90beta could bind to human 94-kDa glucose-regulated protein (Grp94), an endoplasmic reticulum paralog of Hsp90. Among the 5 amino acids constituting site Ic, Trp297 and Pro295 were essential for recognition by all anti-site-Ic mAbs, and Arg299 was important for most of them. The necessity of Ile296, Thr298, and Arg299, which are replaced by Leu, Met/Leu, and Lys, respectively, in some eukaryotic Hsp90, was dependent on the mAbs, and K41110 and K41116C could react with Hsp90s carrying these substitutions. From these data taken together, we propose that the pentapeptide Pro295-Ile-Trp-Thr-Arg299 of hHsp90 functions as an immunodominant epitope common to all eukaryotic Hsp90.

Redirecting the humoral immune response against Streptococcus mutans antigen P1 with monoclonal antibodies

The adhesin P1 of Streptococcus mutans has been studied as an anticaries vaccine antigen. An anti-P1 monoclonal antibody (MAb) bound to S. mutans prior to mucosal immunization of mice was shown previously to alter the amount, specificity, isotype, and biological activity of anti-P1 antibodies. The present study was undertaken to screen this and four additional anti-P1 MAbs for immunomodulatory activity when complexed with S. mutans and administered by a systemic route and to evaluate sera from immunized mice for the ability to inhibit adherence of S. mutans to immobilized human salivary agglutinin. All five MAbs tested influenced murine anti-P1 serum antibody responses in terms of subclass distribution and/or specificity. The effects varied depending on which MAb was used and its coating concentration. Two MAbs promoted a more effective, and two others a less effective, adherence inhibition response. An inverse relationship was observed between the ability of the MAbs themselves to inhibit adherence and the ability of antibodies elicited following immunization with immune complexes to inhibit adherence. Statistically significant correlations were demonstrated between the levels of anti-P1 serum immunoglobulin G2a (IgG2a) and IgG2b, but not of IgG1 or IgG3, and the ability of sera from immunized animals to inhibit bacterial adherence. These results indicate that multiple anti-P1 MAbs can mediate changes in the immune response and that certain alterations are potentially more biologically relevant than others. Immunomodulation by anti-P1 MAbs represents a useful strategy to improve the beneficial immune response against S. mutans.

Inhibition of adjuvant-induced arthritis by DNA vaccination with the 70-kd or the 90-kd human heat-shock protein: immune cross-regulation with the 60-kd heat-shock protein

OBJECTIVE

Adjuvant arthritis can be induced in Lewis rats by immunization with Mycobacterium tuberculosis (Mt). The mycobacterial 65-kd heat-shock protein (Hsp65) is targeted by arthritogenic T cells. However, Hsp65 and the mycobacterial 71-kd heat-shock protein are also recognized by T cells that can down-regulate adjuvant-induced arthritis (AIA). We have recently demonstrated that vaccination with human Hsp60 DNA inhibits AIA. The present study was undertaken to analyze the role of the T cell responses to self HSP molecules other than Hsp60 in the control of AIA.

METHODS

Lewis rats were immunized with DNA vaccines coding for human Hsp70 or Hsp90 (Hsp70 plasmid [pHsp70] or pHsp90), and AIA was induced. The T cell response to Mt, Hsp60, Hsp70, and Hsp90 (proliferation and cytokine release) was studied, and the T cell response to Hsp60 was mapped with overlapping peptides.

RESULTS

The Hsp70 or Hsp90 DNA vaccines shifted the arthritogenic T cell response from a Th1 to a Th2/3 phenotype and inhibited AIA. We detected immune crosstalk between Hsp70/90 and Hsp60: both the Hsp70 and Hsp90 DNA vaccines induced Hsp60-specific T cell responses. Similarly, DNA vaccination with Hsp60 induced Hsp70-specific T cell immunity. Epitope mapping studies revealed that Hsp60-specific T cells induced by pHsp70 vaccination reacted with known regulatory Hsp60 epitopes.

CONCLUSION

T cell immunity to Hsp70 and to Hsp90, like Hsp60-specific immunity, can modulate the arthritogenic response in AIA. In addition, our results suggest that the regulatory mechanisms induced by Hsp60, Hsp70, and Hsp90 are reinforced by an immune network that connects their reactivities.

Biochemical and structural studies of the interaction of Cdc37 with Hsp90

The heat shock protein Hsp90 plays a key, but poorly understood role in the folding, assembly and activation of a large number of signal transduction molecules, in particular kinases and steroid hormone receptors. In carrying out these functions Hsp90 hydrolyses ATP as it cycles between ADP- and ATP-bound forms, and this ATPase activity is regulated by the transient association with a variety of co-chaperones. Cdc37 is one such co-chaperone protein that also has a role in client protein recognition, in that it is required for Hsp90-dependent folding and activation of a particular group of protein kinases. These include the cyclin-dependent kinases (Cdk) 4/6 and Cdk9, Raf-1, Akt and many others. Here, the biochemical details of the interaction of human Hsp90 beta and Cdc37 have been characterised. Small angle X-ray scattering (SAXS) was then used to study the solution structure of Hsp90 and its complexes with Cdc37. The results suggest a model for the interaction of Cdc37 with Hsp90, whereby a Cdc37 dimer binds the two N-terminal domain/linker regions in an Hsp90 dimer, fixing them in a single conformation that is presumably suitable for client protein recognition.

Further characterization of immunomodulation by a monoclonal antibody against Streptococcus mutans antigen P1

We demonstrated previously that mucosal immunization of mice with Streptococcus mutans coated with the monoclonal antibody (MAb) 6-11A directed against the major surface adhesin protein P1 results in changes in the amount, isotype distribution, and specificity of serum antibodies compared with animals immunized with bacteria only. We now show that the specificity of the mucosal secretory IgA response was also influenced by this MAb. Changes in antibody specificity were associated with changes in biological activity. Serum samples which differed in antibody reactivity with P1 polypeptides generated by partial digestion with N-chlorosuccinimide but not in isotype distribution or overall reactivity with S. mutans or intact P1 demonstrated a statistically significant difference in the ability to inhibit bacterial adherence to salivary-agglutinin-coated hydroxyapatite beads. Serum IgG antibodies against P1 from mice immunized with either S. mutans alone or S. mutans coated with 6-11A were shown to recognize antigenic determinants dependent on the presence of the central proline-rich repeat domain, a segment necessary for the structural integrity of the molecule. However, no statistically significant differences were observed in antibody reactivity with a panel of six partial P1 polypeptides encoded by overlapping spaP subclones, suggesting that the targets of biologically relevant antibodies involve complex epitopes not reconstituted by the recombinant products tested. Lastly, we show that binding of MAb 6-11A to P1 on the surface of S. mutans alters P1's susceptibility to proteolytic digestion. Hence, changes in antigen processing and presentation may contribute to the immunomodulatory effects of this MAb.

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

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

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.

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.

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

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

Platelet-derived growth factor-BB and basic fibroblast growth factor directly interact in vitro with high affinity

Platelet-derived growth factor-BB (PDGF-BB) and basic fibroblast growth factor (bFGF) are potent growth factors active on many cell types. The present study indicates that they directly interact in vitro. The interaction was investigated with overlay experiments, surface plasmon resonance experiments, and solid-phase immunoassays by immobilizing one factor or the other and by steady-state fluorescence analysis. The interaction observed was specific, dose-dependent, and saturable, and the bFGF/PDGF-BB binding stoichiometry was found to be 2:1. K(D)(1) for the first step equilibrium and the overall K(D) values were found to be in the nanomolar and in the picomolar range, respectively. Basic FGF/PDGF-BB interaction was strongly reduced as a function of time of PDGF-BB proteolysis. Furthermore, docking analysis suggested that the PDGF-BB region interacting with bFGF may overlap, at least in part, with the PDGF-BB receptor-binding site. This hypothesis was supported by surface plasmon resonance experiments showing that an anti-PDGF-BB antibody, known to inhibit PDGF-BB binding with its receptor, strongly reduced bFGF/PDGF-BB interaction, whereas a control antibody was ineffective. According to these data, the observed bFGF.PDGF-BB complex formation might explain, at least in part, previous observations showing that PDGF-BB chemotactic and mitogenic activity on smooth muscle cells are strongly inhibited in the presence of bFGF.

Domain-domain interactions of HtpG, an Escherichia coli homologue of eukaryotic HSP90 molecular chaperone

In the present study, we investigated the domain structure and domain-domain interactions of HtpG, an Escherichia coli homologue of eukaryotic HSP90. Limited proteolysis of recombinant HtpG, revealed three major tryptic sites, i.e. Arg7-Gly8, Arg336-Glu337 and Lys552-Leu553, of which the latter two were located at the positions equivalent to the major cleavage sites of human HSP90alpha. A similar pattern was obtained by papain treatment under nondenaturing conditions but not under denaturing conditions. Thus, HtpG consists of three domains, i.e. Domain A, Met1-Arg336; domain B, Glu337-Lys552; and domain C, Leu553-Ser624, as does HSP90. The domains of HtpG were expressed and their interactions were estimated on polyacrylamide gel electrophoresis under nondenaturing conditions. As a result, two kinds of domain-domain interactions were revealed: domain B interaction with domain A of the same polypeptide and domain C of one partner with domain B of the other in the dimer. Domain B could be structurally and functionally divided into two subdomains, the N-terminal two-thirds (subdomain BI) that interacted with domain A and the C-terminal one-third (subdomain BII) that interacted with domain C. The C-terminal two-thirds of domain A, i.e. Asp116-Arg336, were sufficient for the binding to domain B. We finally propose the domain organization of an HtpG dimer.

Monoclonal antibodies to mycobacterial DNA gyrase A inhibit DNA supercoiling activity

DNA gyrase is an essential type II topoisomerase found in bacteria. We have previously characterized DNA gyrase from Mycobacterium tuberculosis and Mycobacterium smegmatis. In this study, several monoclonal antibodies were generated against the gyrase A subunit (GyrA) of M. smegmatis. Three, MsGyrA:C3, MsGyrA:H11 and MsGyrA:E9, were further analyzed for their interaction with the enzyme. The monoclonal antibodies showed high degree of cross-reactivity with both fast-growing and slow-growing mycobacteria. In contrast, none recognized Escherichia coli GyrA. All the three monoclonal antibodies were of IgG1 isotype falling into two distinct types with respect to epitope recognition and interaction with the enzyme. MsGyrA:C3 and MsGyrA:H11 IgG, and their respective Fab fragments, inhibited the DNA supercoiling activity catalyzed by mycobacterial DNA gyrase. The epitope for the neutralizing monoclonal antibodies appeared to involve the region towards the N-terminus (residues 351-415) of the enzyme in a conformation-dependent manner. These monoclonal antibodies would serve as valuable tools for structure-function analysis and immunocytological studies of mycobacterial DNA gyrase. In addition, they would be useful for designing peptide inhibitors against DNA gyrase.

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.

Regulation of the Hsp90-binding immunophilin, cyclophilin 40, is mediated by multiple sites for GA-binding protein (GABP)

Within steroid receptor heterocomplexes the large tetratricopeptide repeat-containing immunophilins, cyclophilin 40 (CyP40), FKBP51, and FKBP52, target a common interaction site in heat shock protein 90 (Hsp90) and act coordinately with Hsp90 to modulate receptor activity. The reversible nature of the interaction between the immunophilins and Hsp90 suggests that relative cellular abundance might be a key determinant of the immunophilin component within steroid receptor complexes. To investigate CyP40 gene regulation, we have isolated a 5-kilobase (kb) 5'-flanking region of the human gene and demonstrated that a approximately 50 base pair (bp) sequence adjacent to the transcription start site is essential for CyP40 basal expression. Three tandemly arranged Ets sites within this critical region were identified as binding elements for the multimeric Ets-related transcription factor, GA binding protein (GABP). Functional studies of this proximal promoter sequence, in combination with mutational analysis, confirmed these sites to be crucial for basal promoter function. Furthermore, overexpression of both GABP alpha and GABP beta subunits in Cos1 cells resulted in increased endogenous CyP40 mRNA levels. Significantly, a parallel increase in FKBP52 mRNA expression was not observed, highlighting an important difference in the mode of regulation of the CyP40 and FKBP52 genes. Our results identify GABP as a key regulator of CyP40 expression. GABP is a common target of mitogen and stress-activated pathways and may integrate these diverse extracellular signals to regulate CyP40 gene expression.

HBV polymerase interacts independently with N-terminal and C-terminal fragments of Hsp90beta

Hsp90 is an abundant chaperone protein that assists the folding of specific proteins, such as steroid receptors, protein kinases, and so on, for their proper function. TP and RT domains of HBV polymerase have been also shown to be associated with Hsp90. Therefore, the identification of the binding sites within Hsp90, responsible for forming Hsp90/HBV Pol complex, is important for the understanding of HBV replication. In this study, cotransfection and immunoprecipitation experiments were performed to localize the binding sites of HBV pol to Hsp90. Our data show that HBV pol interact independently with both N-terminal and C-terminal fragments of Hsp90. Further analysis showed that N-terminal fragment (1-302) of Hsp90 interacts with both TP and RT domains of HBV pol, whereas C-terminal fragment (438-723) interacts with only RT domain. In conclusion, we showed that HBV pol independently interacts with N-terminal and C-terminal fragments, but not the middle fragment (327-438) of Hsp90.

Characterization of heat-inducible expression and cloning of HtpG (Hsp90 homologue) of Porphyromonas gingivalis

Porphyromonas gingivalis is implicated in the etiology of periodontal disease. Associations between microbial virulence and stress protein expression have been identified in other infections. For example, Hsp90 homologues in several microbial species have been shown to contribute to virulence. We previously reported that P. gingivalis possessed an Hsp90 homologue (HtpG) which cross-reacts with human Hsp90. In addition, we found that elevated levels of serum antibody to Hsp90 stress protein in individuals colonized with this microorganism were associated with periodontal health. However, the role of HtpG in P. gingivalis has not been explored. Therefore, we cloned the htpG gene and investigated the characteristics of HtpG localization and expression in P. gingivalis. htpG exists as a single gene of 2,052 bp from which a single message encoding a mature protein of approximately 68 kDa is transcribed. Western blot analysis revealed that the 68-kDa polypeptide was stress inducible and that a major band at 44 kDa and a minor band at 40 kDa were present at constitutive levels. Cellular localization studies revealed that the 44- and 40-kDa species were associated with membrane and vesicle fractions, while the 68-kDa polypeptide was localized to the cytosolic fractions.

Cellular localization of a Hsp90 homologue in Porphyromonas gingivalis

We previously reported an association between elevated serum antibody titers to the 90-kDa human heat shock protein (Hsp90), periodontal health and colonization by Porphyromonas gingivalis. In this study, we examined the cellular localization of the Hsp90 homologue of P. gingivalis. Cultures of P. gingivalis were heat-stressed (45 degrees C) and examined for localization of the Hsp90 homologue. Heat stress induced a 4-5-fold increase in anti-Hsp90 antibody reactivity over that of the unstressed controls. Western blot analysis revealed two bands (44 and 68 kDa) that reacted with anti-Hsp90 antibodies. The 68-kDa band was heat-inducible, while the 44-kDa band was not. Immunogold staining revealed that the Hsp90 homologue localized principally to the membrane and extracellular vesicles. Subcellular fractionation confirmed that the Hsp90 homologue was primarily membrane-associated.

The beta-isoform of heat shock protein hsp-90 is structurally related with human microtubule-interacting protein Mip-90

Through major research advances in the study of cytoskeletal organization, an integrated view of the complexity of this system has emerged. Recent findings on the microtubule-interacting protein Mip-90, which associates with microtubules and actin filaments in different cell domains, have shed light on its roles in cytoskeletal regulation. In order to study structural features of Mip-90, we sequenced several peptide fragments. A comparative sequence analysis revealed a high degree of similarity between the primary structure of this protein and the human heat shock protein of 90 kDa (hsp-90). Taken together, the present studies indicate the identity between Mip-90 and the the beta-isoform of hsp-90 (hsp-90beta). Western blot assays with an anti-hsp-90 monoclonal antibody showed cross-reactivity of hsp-90 and Mip-90 affinity purified from HeLa cells. Furthermore, the observed structural identity of Mip-90 with the hsp-90beta was sustained by immunoblot assays using monoclonal antibodies that specifically recognize the alpha- and beta-forms of hsp-90. Comparative fingerprinting analysis, along with the evidence of a remarkably similar biochemical behavior of both hsp-90 and Mip-90 in different affinity chromatographic systems, supported these observations. These studies, along with previous investigations, provide new data to elucidate the functional significance of these interesting cellular components and its relationships with other proteins linked to the cell architecture.

Localization of calponin binding sites in the structure of 90 kDa heat shock protein (Hsp90)

The structure of rabbit liver Hsp90 was reevaluated by limited trypsinolysis, N-terminal sequencing and determination of the site that is phosphorylated by casein kinase II. Limited proteolysis results in formation of four groups of large peptides with M(r) in the range of 26-41 kDa. Peptides with M(r) 39-41 kDa were represented by large N-terminal and central peptides starting at residue 283 of the alpha-isoform of Hsp90. All sites phosphorylated by casein kinase II were located in the large 39-41 kDa peptides. Peptides with M(r) 26-27 kDa were represented by short N-terminal and central peptides starting at Glu-400 of the alpha-isoform of Hsp90. The data of affinity chromatography and light scattering indicate that smooth muscle calponin interacts with Hsp90. The calponin binding sites are located in the large (37-41 kDa) N-terminal and in a short (26-27 kDa) central peptide starting at Glu-400 of the alpha-isoform of Hsp90. Phosphorylation by casein kinase II up to 2 mol of phosphate per mol of Hsp90 does not affect interaction of Hsp90 with calponin.

Delimitation of two regions in the 90-kDa heat shock protein (Hsp90) able to interact with the glucocorticosteroid receptor (GR)

The role of the 90-kDa heat shock protein (Hsp90) as a chaperone and its regulatory functions for cellular proteins such as the glucocorticosteroid receptor (GR) depends on the direct interaction of the Hsp90 with the corresponding protein as part of a multiprotein complex. The search for the amino acid sequence(s) in Hsp90 involved in interaction with the human GR has been carried out by mutational deletion analysis in whole cells, studying the effects of interaction on the nucleocytoplasmic distributions of transiently expressed Hsp90 and GR derivatives in COS-7 cells. Using a recently developed confocal microscopic immunofluorescence method that allows quantification of the nucleocytoplasmic ratios of the proteins in individual cells and statistical comparison of cell populations, two subregions of the Hsp90 molecule have been defined that allow interaction with GR (residues 206-291 and 446-581). The latter region may play a fundamental role in the interaction, while the former may merely stabilize the binding to GR of the intact Hsp90 molecule. Moreover, the dissection of the Hsp90 molecule allowed us to define two regions displaying nuclear localization activity (residues 1-206 and 381-581), followed by two regions having a predominantly cytoplasmic localization activity (residues 287-381 and 581-728) and counteracting the nuclear localization activities.

The common tetratricopeptide repeat acceptor site for steroid receptor-associated immunophilins and hop is located in the dimerization domain of Hsp90

Structurally related tetratricopeptide repeat motifs in steroid receptor-associated immunophilins and the STI1 homolog, Hop, mediate the interaction with a common cellular target, hsp90. We have identified the binding domain in hsp90 for cyclophilin 40 (CyP40) using a two-hybrid system screen of a mouse cDNA library. All isolated clones encoded the intact carboxyl terminus of hsp90 and overlapped with a common region corresponding to amino acids 558-724 of murine hsp84. The interaction was confirmed in vitro with bacterially expressed CyP40 and deletion mutants of hsp90beta and was delineated further to a 124-residue COOH-terminal segment of hsp90. Deletion of the conserved MEEVD sequence at the extreme carboxyl terminus of hsp90 precludes interaction with CyP40, signifying an important role for this motif in hsp90 function. We show that CyP40 and Hop display similar interaction profiles with hsp90 truncation mutants and present evidence for the direct competition of Hop and FK506-binding protein 52 with CyP40 for binding to the hsp90 COOH-terminal region. Our results are consistent with a common tetratricopeptide repeat interaction site for Hop and steroid receptor-associated immunophilins within a discrete COOH-terminal domain of hsp90. This region of hsp90 mediates ATP-independent chaperone activity, overlaps the hsp90 dimerization domain, and includes structural elements important for steroid receptor interaction.

Monomer arrangement in HSP90 dimer as determined by decoration with N and C-terminal region specific antibodies

Electron microscopy using the low-angle rotary shadowing replica method showed that the HSP90 dimer consists of four globular domains aligning in a tandem fashion. When decorated with two monoclonal antibodies against epitopes mapped on the N-terminal region of HSP90, these antibodies bound to both ends of the HSP90 dimer. A C-terminal region specific antibody was shown to bind to the side of HSP90. These results support a model for HSP90 dimer whereby two HSP90 monomers are arranged in an antiparallel fashion and dimerize through the C-terminal domain. Treatment of HSP90 at elevated temperatures or with ATP at room temperature, though not with ADP, induces molecular transformation of the linear HSP90 dimer into an O-ring-shaped structure.

Molecular cloning and characterization of porcine cDNA encoding a 90-kDa heat shock protein and its expression following hyperthermia

We have isolated and sequenced cDNA clones encoding a 90-kDa heat shock protein (HSP90) from a porcine brain cDNA library. The sequence of the 2202-nucleotide coding region showed 88.6% homology with that of the human homologue. Moreover, the deduced amino acid sequence of the porcine hsp90 cDNA was 99.7% identical to that of the human counterpart, with a difference of only three amino acids in a total of 733 residues. Expression of the gene was greatly increased in cultured cells during recovery from heat shock treatment at 45 degrees C for 60 min. Three major transcripts 2.2, 3.0, and 4.1kb in size were detected by Northern blot hybridization. These transcripts were further identified in a whole-pig hyperthermia experiment. These three hsp90 transcripts were constitutively expressed in porcine tissues including kidney, liver, brain, and heart, and their levels were markedly enhanced during recovery from 30-min hyperthermia treatment at 43 degrees C. Furthermore, we found that HSP90 was preferentially expressed in pituitary gland, brain, adrenal gland, and testis, in comparison to the other tissues.

The two-state process of the heat shock protein 90 thermal denaturation: effect of calcium and magnesium

Scanning microcalorimetry, native PAG electrophoresis, and circular dichroism were used to characterize thermal denaturation and oligomerization of heat shock protein 90 (hsp90) and the calcium and magnesium effect on these processes. The calorimetric curve of the hsp90 dimer consists of two transitions centered at 53.8 and 63.1 degrees C. Using specific ligand geldanamycin, we have found that N-terminal domains in the hsp90 dimer are melted independently in the lower-temperature peak, while the higher-temperature one comprises unfolding of two non-interacting parts of the middle domains and dimerization region. Unfolding of the N-terminal domain gives start to oligomerization of dimers; oligomers consist of dimers not dissociating upon denaturation. Calcium and magnesium strongly decrease the hsp90 thermostability and thereby cause oligomerization at lower temperature. We suggest that calcium affects the hsp90 oligomerization, known to be important for its chaperone activity, by shifting the unfolding temperature of the hsp90 N-terminal domain close to the heat shock temperature range.

The 90-kDa molecular chaperone family: structure, function, and clinical applications. A comprehensive review

The 90-kDa molecular chaperone family (which comprises, among other proteins, the 90-kDa heat-shock protein, hsp90 and the 94-kDa glucose-regulated protein, grp94, major molecular chaperones of the cytosol and of the endoplasmic reticulum, respectively) has become an increasingly active subject of research in the past couple of years. These ubiquitous, well-conserved proteins account for 1-2% of all cellular proteins in most cells. However, their precise function is still far from being elucidated. Their involvement in the aetiology of several autoimmune diseases, in various infections, in recognition of malignant cells, and in antigen-presentation already demonstrates the essential role they likely will play in clinical practice of the next decade. The present review summarizes our current knowledge about the cellular functions, expression, and clinical implications of the 90-kDa molecular chaperone family and some approaches for future research.

Oligomeric forms of the 90-kDa heat shock protein

Two isoforms of the 90-kDa heat shock protein, HSP90alpha and HSP90beta, are present in the cytosol of mammalian cells. Analysis by polyacrylamide gel electrophoresis under nondenaturing conditions (native PAGE) revealed that HSP90alpha predominantly exists as a homodimer and that HSP90beta is present mainly as a monomer [Minami, Kawasaki, Miyata, Suzuki and Yahara (1991) J. Biol. Chem. 266, 10099-10103]. However, only the dimeric form has been observed under other analytical conditions such as gradient centrifugation. In this study, therefore, we investigated native forms of HSP90 by use of immunochemical techniques with isoform-specific monoclonal antibodies recently developed in our laboratory. Glycerol gradient centrifugation at the physiological salt concentration as well as native PAGE analysis of rat liver cytosol revealed oligomeric forms of HSP90alpha sedimenting at 8-10S as predominant ones. On the other hand, the glycerol gradient centrifugation revealed multiple forms of HSP90beta oligomers sedimenting at 6-12S. All of the HSP90beta oligomers, however, migrated at 100-kDa monomer and 190-kDa dimer positions on native PAGE. A novel two-dimensional double native PAGE revealed that the entity was converted from the HSP90beta dimer to monomers during the electrophoresis. The same PAGE further revealed that the HSP90alpha oligomer also dissociated into dimers during the electrophoresis. Full-length form of bacterially-expressed human HSP90alpha migrated as dimers, but a considerable amount did not penetrate into the gel under native PAGE conditions, indicating the existence of oligomeric forms. Electrophoretic studies of deletion mutants of HSP90 demonstrated that the C-terminal 200 amino acids were capable of forming oligomers. Taken together, we conclude that both of the HSP90 isoforms predominantly exist as oligomeric forms in the cytosol even under unstressed conditions but that they artificially dissociate into smaller forms when subjected to native PAGE.