83-kilodalton heat shock proteins of trypanosomes are potent peptide-stimulated ATPases

The chaperone toolbox at the single-molecule level: From clamping to confining

Protein folding is well known to be supervised by a dedicated class of proteins called chaperones. However, the core mode of action of these molecular machines has remained elusive due to several reasons including the promiscuous nature of the interactions between chaperones and their many clients, as well as the dynamics and heterogeneity of chaperone conformations and the folding process itself. While troublesome for traditional bulk techniques, these properties make an excellent case for the use of single-molecule approaches. In this review, we will discuss how force spectroscopy, fluorescence microscopy, FCS, and FRET methods are starting to zoom in on this intriguing and diverse molecular toolbox that is of direct importance for protein quality control in cells, as well as numerous degenerative conditions that depend on it.

Review: The HSP90 molecular chaperone-an enigmatic ATPase

The HSP90 molecular chaperone is involved in the activation and cellular stabilization of a range of 'client' proteins, of which oncogenic protein kinases and nuclear steroid hormone receptors are of particular biomedical significance. Work over the last two decades has revealed a conformational cycle critical to the biological function of HSP90, coupled to an inherent ATPase activity that is regulated and manipulated by many of the co-chaperones proteins with which it collaborates. Pharmacological inhibition of HSP90 ATPase activity results in degradation of client proteins in vivo, and is a promising target for development of new cancer therapeutics. Despite this, the actual function that HSP90s conformationally-coupled ATPase activity provides in its biological role as a molecular chaperone remains obscure. © 2016 Wiley Periodicals, Inc. Biopolymers 105: 594-607, 2016.

The heat shock proteins of Trypanosoma cruzi

Trypanosoma cruzi is the causal agent of Chagas' disease, a debilitating disorder affecting millions of people in several countries. A flagellated protozoan parasite, T. cruzi has a complex life cycle that involves infecting an insect and a mammalian host. During its life cycle, the parasite undergoes several kinds of stress, prominent among which is heat stress. To deal with this environmental challenge, molecular chaperones and proteases, also known as heat shock proteins (HSPs), are induced as part of the stress response. Several families of HSPs are synthesized by T. cruzi, including members of the major HSP classes such as HSP70, HSP90, HSP100, HSP40, chaperonins and small HSPs, and these proteins show conserved and unique features. In this review we describe these proteins and the corresponding gene expression patterns and discuss their relevance to the biology of the parasite.

Potent antitrypanosomal activities of heat shock protein 90 inhibitors in vitro and in vivo

African sleeping sickness, caused by the protozoan parasite Trypanosoma brucei, is universally fatal if untreated, and current drugs are limited by severe toxicities and difficult administration. New antitrypanosomals are greatly needed. Heat shock protein 90 (Hsp90) is a conserved and ubiquitously expressed molecular chaperone essential for stress responses and cellular signaling. We investigated Hsp90 inhibitors for their antitrypanosomal activity. Geldanamycin and radicicol had nanomolar potency in vitro against bloodstream-form T. brucei; novobiocin had micromolar activity. In structure-activity studies of geldanamycin analogs, 17-AAG and 17-DMAG were most selective against T. brucei as compared to mammalian cells. 17-AAG treatment sensitized trypanosomes to heat shock and caused severe morphological abnormalities and cell cycle disruption. Both oral and parenteral 17-DMAG cured mice of a normally lethal infection of T. brucei. These promising results support the use of inhibitors to study Hsp90 function in trypanosomes and to expand current clinical development of Hsp90 inhibitors to include T. brucei.

HSP90 at the hub of protein homeostasis: emerging mechanistic insights

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

Trypanosomatid protein phosphatases

Protein phosphorylation is one of the most important post-translational modifications regulating various signaling processes in all known living organisms. In the cell, protein phosphatases and protein kinases play a dynamic antagonistic role, controlling the phosphorylation state of tyrosine (Tyr), serine (Ser) and threonine (Thr) side chains of proteins. The reversible phosphorylation modulates protein function, through initiating conformational changes, which influences protein complex formation, alteration of enzyme activity and changes in protein stability and subcellular localization. These molecular changes affect signaling cascades regulating the cell cycle, differentiation, cell-cell and cell-substrate interactions, cell motility, the immune response, ion-channel and transporter activities, gene transcription, mRNA translation, and basic metabolism. In addition to these processes, in unicellular parasites, like Trypanosoma brucei, Trypanosoma cruzi and Leishmania spp., additional signaling pathways have evolved to enable the survival of parasites in the changing environment of the vector and host organism. In recent years the genome of five trypanosomatid genomes have been sequenced and annotated allowing complete definition of the composition of the trypanosomatid phosphatomes. The very diverse environments involved in the different stages of the kinetoplastids' life cycle might have played a role to develop a set of trypanosomatid-specific phosphatases in addition to orthologues of many higher eukaryote protein phosphatases present in the kinetoplastid phosphatomes. In spite of their well-described phosphatomes, few trypanosomatid protein phosphatases have been characterized and studied in vivo. The aim of this review is to give an up to date scope of the research, which has been carried out on trypanosomatid protein phosphatases.

Stability of the Hsp90 inhibitor 17AAG hydroquinone and prevention of metal-catalyzed oxidation

17-(allylamino)-17-demethoxygeldanamycin (17AAG) is a benzoquinone ansamycin Hsp90 inhibitor which has promising anticancer activity in vitro, in animal models and in clinical trials. 17AAG has poor water-solubility which is a potential problem for clinical formulation. The hydroquinone derivative of 17AAG, 17AAG hydroquinone (17AAGH(2)), is considerably more water soluble and since we previously demonstrated that 17AAGH(2) was a more potent Hsp90 inhibitor than its parent quinone, it is a good candidate for clinical use and is currently in clinical trials. However, 17AAGH(2) can be oxidized back to 17AAG under aerobic conditions so we tested the relative stability of 17AAGH(2) and the effect of different metal ions and metal chelators on the oxidation of 17AAGH(2). We found that copper could accelerate 17AAGH(2) oxidation while copper chelators such as D-penicillamine could inhibit oxidation. Human serum albumin (HA) has copper-binding ability and we found that HA diminished the rate of 17AAGH(2) oxidation. Although we found that 17AAG could associate with HA, no association was observed between 17AAGH(2) and HA. In summary, our data demonstrates that copper chelators can prevent 17AAGH(2) oxidation and suggests that HA prevents 17AAGH(2) oxidation via a copper chelation mechanism. Agents that prevent oxidation may be useful in clinical formulations of 17AAGH(2.)

Protein phosphatase 5 is required for Hsp90 function during proteotoxic stresses in Trypanosoma brucei

Trypanosoma brucei, a parasitic protozoan that causes African trypanosomiasis in human and domestic animals, adapt in various environments during their digenetic life cycle. In this study, we found that Hsp90 is crucial for the survival of this parasite. Inhibition of Hsp90 activity by geldanamycin (GA) reduced cell growth and increased the level of Hsp90. Both the bloodstream and procyclic forms of T. brucei showed a several-fold greater sensitivity than the mammalian cells to GA and also to 17-AAG, a less toxic derivative of GA, suggesting that Hsp90 could be a potential chemotherapeuric target for African trypanosomiasis. T. brucei Hsp90 interacts with the protein phosphatase 5 (PP5) in vivo. Under normal growth conditions, T. brucei PP5 (TbPP5) and Hsp90 are primarily localized in the cytosol. However, with increase in growth temperature and GA treatment, these proteins translocate to the nucleus. Overproduction of TbPP5 by genetic manipulation reduced the growth inhibitory effect of GA, while knockdown of TbPP5 reduced cell growth more in the presence of GA, as compared to parental control. Depletion of TbPP5, however, did not prevent the induction of Hsp90 protein level during GA treatment. Together, these results suggest that TbPP5 positively regulates the function of Hsp90 to maintain cellular homeostasis during proteotoxic stresses in T. brucei.

Troglitazone induces differentiation in Trypanosoma brucei

Trypanosoma brucei, a protozoan parasite causing sleeping sickness, is transmitted by the tsetse fly and undergoes a complex lifecycle including several defined stages within the insect vector and its mammalian host. In the latter, differentiation from the long slender to the short stumpy form is induced by a yet unknown factor of trypanosomal origin. Here we describe that some thiazolidinediones are also able to induce differentiation. In higher eukaryotes, thiazolidinediones are involved in metabolism and differentiation processes mainly by binding to the intracellular receptor peroxisome proliferator activated receptor gamma. Our studies focus on the effects of troglitazone on bloodstream form trypanosomes. Differentiation was monitored using mitochondrial markers (membrane potential, succinate dehydrogenase activity, inhibition of oxygen uptake by KCN, amount of cytochrome transcripts), morphological changes (Transmission EM and light microscopy), and transformation experiments (loss of the Variant Surface Glycoprotein coat and increase of dihydroliponamide dehydrogenase activity). To further investigate the mechanisms responsible for these changes, microarray analyses were performed, showing an upregulation of expression site associated gene 8 (ESAG8), a potential differentiation regulator.

Structure and mechanism of the Hsp90 molecular chaperone machinery

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

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.

Hsp70 and Hsp90--a relay team for protein folding

Molecular chaperones are a functionally defined set of proteins which assist the structure formation of proteins in vivo. Without certain protective mechanisms, such as binding nascent polypeptide chains by molecular chaperones, cellular protein concentrations would lead to misfolding and aggregation. In the mammalian system, the molecular chaperones Hsp70 and Hsp90 are involved in the folding and maturation of key regulatory proteins, like steroid hormone receptors, transcription factors, and kinases, some of which are involved in cancer progression. Hsp70 and Hsp90 form a multichaperone complex, in which both are connected by a third protein called Hop. The connection of and the interplay between the two chaperone machineries is of crucial importance for cell viability. This review provides a detailed view of the Hsp70 and Hsp90 machineries, their cofactors and their mode of regulation. It summarizes the current knowledge in the field, including the ATP-dependent regulation of the Hsp70/Hsp90 multichaperone cycle and elucidates the complex interplay and their synergistic interaction.

Nucleotide binding and hydrolysis properties of Neurospora crassa cytosolic molecular chaperones, Hsp70 and Hsp80, heat-inducible members of the eukaryotic stress-70 and stress-90 families

Formation of a hetero-oligomeric complex between Hsp70 and Hsp80 of Neurospora crassa was observed previously by means of chemical crosslinking and enzyme-linked immunosorbent assays (ELISA). The present study documents the effect of nucleotides on the subunit structure of Hsp70 and Hsp80 by crosslinking with bifunctional reagents: glutaraldehyde, dimethyl adipimidate (DMA), and dimethyl suberimidate (DMS). The inter-protomer crosslinking of Hsp80 with DMA and DMS was suppressed by ATP and to a lesser extent by ADP, CTP, and NAD. Crosslinking of purified Hsp70 by glutaraldehyde yielded dimers and higher order oligomers. Binding of ATP, ADP, CTP, and NAD, but not NADH, led to a marked reduction in the yield of oligomers. Similarly, crosslinking by DMA and DMS was suppressed by ADP, ATP, and CTP. Both Hsp70 and Hsp80 exhibited intrinsic ATPase activity. Interestingly, ATP levels exceeding 25 microM resulted in pronounced inhibition of the ATPase activity of Hsp80 and 0.5 mM and 0.25 mM ATP led to a prolonged lag in the reaction. Addition of NAD resulted in the abolition of the lag period. The binding of 2-p-toluidinylnapthalene-6-sulfonate (TNS) to Hsp70 and its displacement by ATP and other nucleotides demonstrated the hydrophobic nature of the nucleotide-binding region.

ATP binding and hydrolysis are essential to the function of the Hsp90 molecular chaperone in vivo

Hsp90 is an abundant molecular chaperone essential to the establishment of many cellular regulation and signal transduction systems, but remains one of the least well described chaperones. The biochemical mechanism of protein folding by Hsp90 is poorly understood, and the direct involvement of ATP has been particularly contentious. Here we demonstrate in vitro an inherent ATPase activity in both yeast Hsp90 and the Escherichia coli homologue HtpG, which is sensitive to inhibition by the Hsp90-specific antibiotic geldanamycin. Mutations of residues implicated in ATP binding and hydrolysis by structural studies abolish this ATPase activity in vitro and disrupt Hsp90 function in vivo. These results show that Hsp90 is directly ATP dependent in vivo, and suggest an ATP-coupled chaperone cycle for Hsp90-mediated protein folding.

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.

ATP-binding properties of human Hsp90

Hsp90 is one of the most abundant proteins in the cytosol of eukaryotic cells. Under physiological conditions Hsp90 has been shown to play a major role in several specific signaling pathways, including maturation of various kinases and maintenance of steroid receptors in an activable state. It is well established that the level of Hsp90 increases severalfold under stress conditions, and it has been shown that the chaperone function of Hsp90 is ATP-independent. Although yeast Hsp90 does not bind ATP, as determined by a number of methods monitoring tight binding, ATP-dependent functions of Hsp90 in the presence of co-factors and elevated temperatures are still under discussion. Here, we have reinvestigated ATP-binding properties and ATPase activity of human Hsp90 under various conditions. We show that human Hsp90 does not bind ATP tightly and does not exhibit detectable ATPase activity. However, using electron spin resonance spectroscopy, weak binding of spin-labeled ATP analogues with half-maximal binding at 400 microM ATP was detected. The functional significance of this weak interaction remains enigmatic.

Identification and structural characterization of the ATP/ADP-binding site in the Hsp90 molecular chaperone

Hsp90 molecular chaperones in eukaryotic cells play essential roles in the folding and activation of a range of client proteins involved in cell cycle regulation, steroid hormone responsiveness, and signal transduction. The biochemical mechanism of Hsp90 is poorly understood, and the involvement of ATP in particular is controversial. Crystal structures of complexes between the N-terminal domain of the yeast Hsp90 chaperone and ADP/ATP unambiguously identify a specific adenine nucleotide binding site homologous to the ATP-binding site of DNA gyrase B. This site is the same as that identified for the antitumor agent geldanamycin, suggesting that geldanamycin acts by blocking the binding of nucleotides to Hsp90 and not the binding of incompletely folded client polypeptides as previously suggested. These results finally resolve the question of the direct involvement of ATP in Hsp90 function.

Steroid receptor interactions with heat shock protein and immunophilin chaperones

We have provided a historical perspective on a body of steroid receptor research dealing with the structure and physiological significance of the untransformed 9S receptor that has often confused both novice and expert investigators. The frequent controversies and equivocations of earlier studies were due to the fact that the native, hormone-free state of these receptors is a large multiprotein complex that resisted description for many years because of its unstable and dynamic nature. The untransformed 9S state of the steroid and dioxin receptors has provided a unique system for studying the function of the ubiquitous, abundant, and conserved heat shock protein, hsp90. The hormonal control of receptor association with hsp90 provided a method of manipulating the receptor heterocomplex in a manner that was physiologically meaningful. For several steroid receptors, binding to hsp90 was required for the receptor to be in a native hormone-binding state, and for all of the receptors, hormone binding promoted dissociation of the receptor from hsp90 and conversion of the receptor to the DNA-binding state. Although the complexes between tyrosine kinases and hsp90 were discovered earlier, the hormonal regulation or steroid receptor association with hsp90 permitted much more rapid and facile study of hsp90 function. The observations that hsp90 binds to the receptors through their HBDs and that these domains can be fused to structurally different proteins bringing their function under hormonal control provided a powerful linkage between the hormonal regulation of receptor binding to hsp90 and the initial step in steroid hormone action. Because the 9S receptor hsp90 heterocomplexes could be physically stabilized by molybdate, their protein composition could be readily studied, and it became clear that these complexes are multiprotein structures containing a number of unique proteins, such as FKBP51, FKBP52, CyP-40, and p23, that were discovered because of their presence in these structures. Further analysis showed that hsp90 itself exists in a variety of native multiprotein heterocomplexes independent of steroid receptors and other 'substrate' proteins. Cell-free systems can now be used to study the formation of receptor heterocomplexes. As we outlined in the scheme of Fig. 1, the multicomponent receptor-hsp90 heterocomplex assembly system is being reconstituted, and the importance of individual proteins, such as hsp70, p60, and p23, in the assembly process is becoming recognized. It should be noted that our understanding of the mechanism and purpose of steroid receptor heterocomplex assembly is still at an early stage. We can now speculate on the roles of receptor-associated proteins in receptor action, both as individuals and as a group, but their actual functions are still vague or unknown. We can make realistic models about the chaperoning and trafficking of steroid receptors, but we don't yet know how these processes occur, we don't know where chaperoning occurs in the cell (e.g. Is it limited to the cytoplasm? Is it a diffuse process or does chaperoning occur in association with structural elements?), and, with the exception of the requirement for hormone binding, we don't know the extent to which the hsp90-based chaperone system impacts on steroid hormone action. It is not yet clear how far the discovery of this hsp90 heterocomplex assembly system will be extended to the development of a general understanding of protein processing in the cell. Because this assembly system is apparently present in all eukaryotic cells, it probably performs an essential function for many proteins. The bacterial homolog of hsp90 is not an essential protein, but hsp90 is essential in eukaryotes, and recent studies indicate that the development of the cell nucleus from prokaryotic progenitors was accompanied by the duplication of genes for hsp90 and hsp70 (698). (ABSTRACT TRUNCATED)

Nucleotides and two functional states of hsp90

Previous studies have demonstrated the ATP-dependent formation of a complex containing the heat shock protein hsp90, the unique hsp90 binding protein p23, and one of three high molecular weight immunophilins. In the present study, hsp90 and p23 are shown to form a complex that requires elevated temperature and ATP/Mg2+. Complex formation is strongly promoted by molybdate and by the nonionic detergent Nonidet P-40. ADP and the benzoquinone ansamycin, geldanamycin, are potent inhibitors of complex formation. The ATP-dependent process alters the state of hsp90, not p23, and influences the ability of hsp90 to bind to phenyl-Sepharose. Conversion of hsp90 to the ATP-bound state lowers its affinity for phenyl-Sepharose. These results show that hsp90 can exist in at least two functional states, one able to bind p23 and the other with a high affinity for hydrophobic resins. A model is presented where these states are dictated by the binding of either ATP or ADP.

Assessment of the ATP binding properties of Hsp90

Hsp90, one of the most prominent proteins in eucaryotic cells under physiological and stress conditions, chaperones protein folding reactions in an ATP-independent way. Surprisingly, ATP binding and ATPase activity of Hsp90 has been reported by several groups. To clarify this important issue, we have reinvestigated the potential ATP binding properties and ATPase activity of highly purified Hsp90 using a number of different techniques. Hsp90 was compared to the well characterized ATP-binding chaperone Hsc70 and to two control proteins, immunoglobulin G and bovine serum albumin, that are known to not bind ATP. Hsp90 behaved very similarly to the non-ATP-binding proteins and very differently from the ATP-binding protein Hsc70. Like bovine serum albumin and immunoglobulin G, Hsp90 (i) did not bind to immobilized ATP, (ii) could not be specifically photocross-linked with azido-ATP, (iii) failed to exhibit significant changes in intrinsic protein fluorescence upon ATP addition, and (iv) did not bind to three fluorescent ADP analogues. In contrast, Hsc70 strongly bound ATP and ADP, specifically cross-linked with azido-ATP, and exhibited major shifts in fluorescence upon addition of ATP. Finally, reexamination of the amino acid sequence of Hsp90 failed to reveal any significant homologies to known ATP-binding motifs. Taken together, we conclude that highly purified Hsp90 does not bind ATP. Weak ATPase activities associated with Hsp90 preparations may be due to minor impurities or kinases copurifying with Hsp90.

Autophosphorylation of grp94 (endoplasmin)

The 94-kDa glucose-regulated protein (endoplasmin, grp94) is an abundant member of the 90-kDa molecular chaperone family in the endoplasmic reticulum. We have found earlier that the 50% homologous 90-kDa heat shock protein, hsp90, has ATP-binding site(s) and autophosphorylating activity (Csermely, P., and Kahn, C. R. (1991) J. Biol. Chem. 266, 4943-4950). In the present paper we demonstrate that highly purified grp94 is also able to autophosphorylate itself on serine and threonine residues. grp94 can be freed from the co-purifying casein kinase II by concanavalin A affinity chromatography, and its phosphorylation is unaffected by activators and inhibitors of numerous protein kinases known to associate with the homologous hsp90. The autophosphorylation persists in immunoprecipitates and in SDS-polyacrylamide gel-purified and renatured grp94. Autophosphorylation displays a monomolecular kinetics, is activated by micromolar calcium concentrations, has an extreme heat stability, and can utilize both ATP and GTP with relatively high km values of 243 +/- 14 microM and 116 +/- 23 microM, respectively. Sequence analysis of grp94 shows the presence of two ATP-binding sites. The major product of limited proteolysis of grp94 by chymotrypsin or papain is an N-terminal 85-kDa fragment that can bind to ATP-agarose but does not show autophosphorylation. Our data suggest that grp94 has an enzymatic function analogous in many respects to the similar activity of hsp70, hsp90, and grp78 (BiP). Autophosphorylation may participate in/regulate the complex formation of these proteins, so it may be involved in their chaperone function.

Molecular characterization of the heat shock protein 90 gene of the human malaria parasite Plasmodium falciparum

We report here the nucleotide sequence of hsp90 (heat shock protein 90) of Plasmodium falciparum. Computer analysis of the deduced protein sequence revealed an unusually large region of charged amino acids when compared to hsp90 from other species. This region shows striking homology to the calcium binding domain of calreticulin, the major calcium binding protein of endoplasmic reticulum. Phylogenetic tree analysis indicates that P. falciparum hsp90 is more closely related to hsp90 from plants than to hsp90 from vertebrates or other parasites. The malaria hsp90 is an ATP binding protein encoded by a single gene constitutively expressed in both asexual (trophozoite) and sexual (gametocyte) stage parasites. The hsp90 protein is homologous to a previously identified 90-kDa antigen strongly recognised by both sera from vaccinated monkeys and monoclonal antibody XIV/7.

Molecular cloning and characterization of the 78-kilodalton glucose-regulated protein of Trypanosoma cruzi

The protozoan Trypanosoma cruzi is the etiologic agent of Chagas' disease, an illness responsible for morbidity and death among millions of Latin Americans. Mice also develop this disease when infected with T. cruzi and are a useful model organism for the study of parasite-specific immune responses. To identify immunogenic T. cruzi antigens, serum from an infected mouse was used to isolate clones from a T. cruzi epimastigote cDNA expression library. One of these clones was found to encode the 78-kDa glucose-regulated protein (grp78), the endoplasmic reticular member of the 70-kDa heat shock protein (hsp70) family. Like the mammalian and yeast grp78s, the T. cruzi protein contains an endoplasmic reticular leader peptide and a carboxyl-terminal endoplasmic reticular retention sequence. T. cruzi grp78 is encoded by a tandemly arranged family of three genes located on a chromosome of 1.6 Mb. The effects on grp78 expression of heat shock and tunicamycin treatment, the latter of which specifically stimulates mammalian grp78, were investigated. While the level of the grp78 protein remained constant under all circumstances, grp78 mRNA was unaffected by heat shock but induced fivefold by tunicamycin. Finally, we found that grp78 is the most immunogenic of the T. cruzi heat shock proteins we have characterized, reacting strongly in immunoblots with sera from infected mice.