Comparative analysis of Hsp10 and Hsp90 expression in healthy mucosa and adenocarcinoma of the large bowel

BACKGROUND

Heat shock proteins (Hsps) assist other proteins in their folding and drive the degradation of defective proteins. During evolution, these proteins have also acquired other roles. Hsp10 is involved in immunomodulation and tumor progression. Hsp90 stabilizes a range of "client" proteins involved in cell signaling. The present study evaluated the expression levels of Hsp10 and Hsp90 in normal mucosa and adenocarcinoma samples of human large bowel.

MATERIALS AND METHODS

Samples of normal mucosa and adenocarcinoma were collected and Reverse transcriptase-polymerase chain reaction RT-PCR, western blotting (WB) analyses, as well as immunohistochemistry were performed to evaluate the expression levels of Hsp10 and Hsp90.

RESULTS

RT-PCR showed a higher gene expression of Hsp10 and Hsp90 in adenocarcinoma samples compared to healthy mucosa. WB results confirmed these findings. Immunohistochemistry revealed higher levels of Hsp10 in adenocarcinoma in both the epithelium and the lamina propria, while Hsp90 expression was higher in the adenocarcinoma samples only in the lamina propria.

CONCLUSION

Hsp10 and Hsp90 may be involved in large bowel carcinogenesis.

Heat shock protein 10 regulated apoptosis of mouse ovarian granulosa cells

OBJECTIVES

To study the roles of heat shock proteins10 (HSP10) in the regulation of mouse ovarian granulose cell (GC) apoptosis, and to further define the possible roles of HSP10 in the development of polycystic ovary syndrome (PCOS).

METHODS

Mouse HSP10 small interfering RNA (siRNA) and recombinant adenoviruses overexpressing HSP10 were constructed and subsequently transfected into cultured mouse ovarian GCs. After an infection period of 48 h, the expression levels of the HSP10 gene in mouse GCs were confirmed by Western blot. The GCs were also assessed for apoptosis using flow cytometry and the TUNEL assay. Apoptosis of GCs overexpressing HSP10 was assessed by flow cytometry after cisplatin treatment.

RESULTS

Compared with control group, the expression of HSP10 was decreased in mouse GCs infected with AdCMV-siRNA/HSP10, whereas mouse GCs infected with AdCMV-HSP10 showed increased HSP10 expression p < 0.05. Knock-down of HSP10 in mouse GCs significantly increased apoptosis (p < 0.05), whereas overexpression of HSP10 significantly suppressed apoptosis induced by cisplatin (p < 0.05).

CONCLUSION

In the present primary study, we have successfully employed recombinant adenovirus technologies to modulate HSP10 gene expression in mouse GCs, and examined the effects on apoptosis. Our experiments have demonstrated that knock-down of HSP10 induces apoptosis of mouse ovarian GCs, whereas overexpression of HSP10 suppresses apoptosis. These findings suggested that HSP10 may play a role in the regulation of apoptosis of mouse ovarian GCs.

Human Hsp10 and Early Pregnancy Factor (EPF) and their relationship and involvement in cancer and immunity: current knowledge and perspectives

This article is about Hsp10 and its intracellular and extracellular forms focusing on the relationship of the latter with Early Pregnancy Factor and on their roles in cancer and immunity. Cellular physiology and survival are finely regulated and depend on the correct functioning of the entire set of proteins. Misfolded or unfolded proteins can cause deleterious effects and even cell death. The chaperonins Hsp10 and Hsp60 act together inside the mitochondria to assist protein folding. Recent studies demonstrated that these proteins have other roles inside and outside the cell, either together or independently of each other. For example, Hsp10 was found increased in the cytosol of different tumors (although in other tumors it was found decreased). Moreover, Hsp10 localizes extracellularly during pregnancy and is often indicated as Early Pregnancy Factor (EPF), which is released during the first stages of gestation and is involved in the establishment of pregnancy. Various reports show that extracellular Hsp10 and EPF modulate certain aspects of the immune response with anti-inflammatory effects in patients with autoimmune conditions improving clinically after treatment with recombinant Hsp10. Moreover, Hsp10 and EPF are involved in embryonic development, acting as a growth factor, and in cell proliferation/differentiation mechanisms. Therefore, it becomes evident that Hsp10 is not only a co-chaperonin, but an active player in its own right in various cellular functions. In this article, we present an overview of various aspects of Hsp10 and EPF as they participate in physiological and pathological processes such as the antitumor response and autoimmune diseases.

In infertile women, cells from Chlamydia trachomatis infected sites release higher levels of interferon-gamma, interleukin-10 and tumor necrosis factor-alpha upon heat-shock-protein stimulation than fertile women

BACKGROUND

The magnitude of reproductive morbidity associated with sexually transmitted Chlamydia trachomatis infection is enormous. Association of antibodies to chlamydial heat shock proteins (cHSP) 60 and 10 with various disease sequelae such as infertility or ectopic pregnancy has been reported. Cell-mediated immunity is essential in resolution and in protection to Chlamydia as well as is involved in the immunopathogenesis of chlamydial diseases. To date only peripheral cell mediated immune responses have been evaluated for cHSP60. These studies suggest cHSPs as important factors involved in immunopathological condition associated with infection. Hence study of specific cytokine responses of mononuclear cells from the infectious site to cHSP60 and cHSP10 may elucidate their actual role in the cause of immunopathogenesis and the disease outcome.

METHODS

Female patients (n = 368) attending the gynecology out patient department of Safdarjung hospital, New Delhi were enrolled for the study and were clinically characterized into two groups; chlamydia positive fertile women (n = 63) and chlamydia positive infertile women (n = 70). Uninfected healthy women with no infertility problem were enrolled as controls (n = 39). cHSP60 and cHSP10 specific cytokine responses (Interferon (IFN)-gamma, Interleukin (IL)-10, Tumor Necrosis Factor (TNF)-alpha, IL-13 and IL-4) were assessed by ELISA in stimulated cervical mononuclear cell supernatants.

RESULTS

cHSP60 and cHSP10 stimulation results in significant increase in IFN-gamma (P = 0.006 and P = 0.04 respectively) and IL-10 levels (P = 0.04) in infertile group as compared to fertile group. A significant cHSP60 specific increase in TNF-alpha levels (P = 0.0008) was observed in infertile group as compared to fertile group. cHSP60 and cHSP10 specific IFN-gamma and IL-10 levels were significantly correlated (P < 0.0001, r = 0.54 and P = 0.004, r = 0.33 respectively) in infertile group.

CONCLUSION

Our results suggest that exposure to chlamydial heat shock proteins (cHSP60 and cHSP10) could significantly affect mucosal immune function by increasing the release of IFN-gamma, IL-10 and TNF-alpha by cervical mononuclear cells.

Structural Stability of Covalently Linked GroES Heptamer: Advantages in the Formation of Oligomeric Structure

In order to understand how inter-subunit association stabilizes oligomeric proteins, a single polypeptide chain variant of heptameric co-chaperonin GroES (tandem GroES) was constructed from Escherichia coli heptameric GroES by linking consecutively the C-terminal of one subunit to the N-terminal of the adjacent subunit with a small linker peptide. The tandem GroES (ESC7) showed properties similar to wild-type GroES in structural aspects and co-chaperonin activity. In unfolding and refolding equilibrium experiments using guanidine hydrochloride (Gdn-HCl) as a denaturant at a low protein concentration (50 microg ml(-1)), ESC7 showed a two-state transition with a greater resistance toward Gdn-HCl denaturation (Cm=1.95 M) compared to wild-type GroES (Cm=1.1 M). ESC7 was found to be about 10 kcal mol(-1) more stable than the wild-type GroES heptamer at 50 microg ml(-1). Kinetic unfolding and refolding experiments of ESC7 revealed that the increased stability was mainly attributed to a slower unfolding rate. Also a transient intermediate was detected in the refolding reaction. Interestingly, at the physiological GroES concentration (>1 mg ml(-1)), the free energy of unfolding for GroES heptamer exceeded that for ESC7. These results showed that at low protein concentrations (<1 mg ml(-1)), the covalent linking of subunits contributes to the stability but also complicates the refolding kinetics. At physiological concentrations of GroES, however, the oligomeric state is energetically preferred and the advantages of covalent linkage are lost. This finding highlights a possible advantage in transitioning from multi-domain proteins to oligomeric proteins with small subunits in order to improve structural and kinetic stabilities.

GroEL-mediated protein folding: making the impossible, possible

Protein folding is a spontaneous process that is essential for life, yet the concentrated and complex interior of a cell is an inherently hostile environment for the efficient folding of many proteins. Some proteins-constrained by sequence, topology, size, and function-simply cannot fold by themselves and are instead prone to misfolding and aggregation. This problem is so deeply entrenched that a specialized family of proteins, known as molecular chaperones, evolved to assist in protein folding. Here we examine one essential class of molecular chaperones, the large, oligomeric, and energy utilizing chaperonins or Hsp60s. The bacterial chaperonin GroEL, along with its co-chaperonin GroES, is probably the best-studied example of this family of protein-folding machine. In this review, we examine some of the general properties of proteins that do not fold well in the absence of GroEL and then consider how folding of these proteins is enhanced by GroEL and GroES. Recent experimental and theoretical studies suggest that chaperonins like GroEL and GroES employ a combination of protein isolation, unfolding, and conformational restriction to drive protein folding under conditions where it is otherwise not possible.

Antigen three-dimensional structure guides the processing and presentation of helper T-cell epitopes

Antigen three-dimensional structure potentially controls presentation of CD4(+) T-cell epitopes by limiting the access of proteolytic enzymes and MHC class II antigen-presenting proteins. The protease-sensitive mobile loops of Hsp10s from mycobacteria, Escherichia coli, and bacteriophage T4 (T4Hsp10) are associated with adjacent immunodominant helper T-cell epitopes, and a mobile-loop deletion in T4Hsp10 eliminated the protease sensitivity and the associated epitope immunodominance. In the present work, protease-sensitivity and epitope presentation was analyzed in a group of T4Hsp10 variants. Two mobile-loop sequence variants of T4Hsp10 were constructed by replacing different segments of the mobile loop with an irrelevant sequence from hen egg lysozyme. The variant proteins retained native-like structure, and the mobile loops retained protease sensitivity. Mobile-loop deletion and reconstruction affected the presentation of two epitopes according to whether the epitope was protease-independent or protease-dependent. The protease-independent epitope lies within the mobile loop, and the protease-dependent epitope lies in a well-ordered segment on the carboxy-terminal flank of the mobile loop. The results are consistent with a model for processing of the protease-dependent epitope in which an endoproteolytic nick in the mobile-loop unlocks T4Hsp10 three-dimensional structure, and then the epitope becomes available for binding to the MHC protein.

Co-expression of chaperonin GroEL/GroES enhances in vivo folding of yeast mitochondrial aconitase and alters the growth characteristics of Escherichia coli

Over last two decades many researchers have demonstrated the mechanisms of how the Escherichia coli chaperonin GroEL and GroES work in the binding and folding of different aggregation prone substrate proteins both in vivo and in vitro. However, preliminary aspects, such as influence of co-expressing GroEL and GroES on the over expression of other recombinant proteins in E. coli cells and subsequent growth aspects, as well as the conditions for optimum production of recombinant proteins in presence of recombinant chaperones have not been properly investigated. In the present study we have demonstrated the temperature dependent growth characteristics of E. coli cells, which are over expressing recombinant aconitase and how the co-expression of E. coli chaperonin GroEL and GroES influence the growth rate of the cells and in vivo folding of recombinant aconitase. Presence of co-expressed GroEL reduces the aconitase over-expression drastically; however, exogenous GroEL & GroES together compensate this reduction. For the aconitase over-expressing cells the growth rate decreases by 30% at 25 degrees C when compared with the M15 E. coli cells, however, there is an increase of 20% at 37 degrees C indicating the participation of endogenous chaperonin in the folding of a fraction of over expressed aconitase. However, in presence of co-expressed GroEL and GroES the growth rate of aconitase producing cells was enhanced by 30% at 37 degrees C confirming the assistance of exogenous chaperone system for the folding of recombinant aconitase. Optimum in vivo folding of aconitase requires co-production of complete E. coli chaperonin machinery GroEL and GroES together.

Chaperonin GroEL meets the substrate protein as a "load" of the rings

Chaperonin GroEL is an essential molecular chaperone that assists protein folding in the cell. With the aid of cochaperonin GroES and ATP, double ring-shaped GroEL encapsulates non-native substrate proteins inside the cavity of the GroEL-ES complex. Although extensive studies have revealed the outline of GroEL mechanism over the past decade, central questions remain: What are the in vivo substrate proteins? How does GroEL encapsulate the substrates inside the cavity in spite of an apparent entropic difficulty? Is the folding inside the GroEL-ES cavity the same as bulk spontaneous folding? In this review I summarize the recent progress on in vivo and in vitro aspects of GroEL. In particular, emerging evidence shows that the substrate protein itself influences the chaperonin GroEL structure and reaction cycle. Finally I propose the mechanistic similarity between GroEL and kinesin, a molecular motor that moves along a microtubule in an ATP-dependent manner.

Low temperature of GroEL/ES overproduction permits growth of Escherichia coli cells lacking trigger factor DnaK

Escherichia coli trigger factor (TF) and DnaK cooperate in the folding of newly synthesized proteins. The combined deletion of the TF-encoding tig gene and the dnaK gene causes protein aggregation and synthetic lethality at 30 degrees C. Here we show that the synthetic lethality of deltatigdeltadnaK52 cells is abrogated either by growth below 30 degrees C or by overproduction of GroEL/GroES. At 23 degrees C deltatigdeltadnaK52 cells were viable and showed only minor protein aggregation. Overproduction of GroEL/GroES, but not of other chaperones, restored growth of deltatigdeltadnaK52 cells at 30 degrees C and suppressed protein aggregation including proteins >/= 60 kDa, which normally require TF and DnaK for folding. GroEL/GroES thus influences the folding of proteins previously identified as DnaK/TF substrates.

Monitoring macromolecular complexes involved in the chaperonin-assisted protein folding cycle by mass spectrometry

We have used native mass spectrometry to analyze macromolecular complexes involved in the chaperonin-assisted refolding of gp23, the major capsid protein of bacteriophage T4. Adapting the instrumental methods allowed us to monitor all intermediate complexes involved in the chaperonin folding cycle. We found that GroEL can bind up to two unfolded gp23 substrate molecules. Notably, when GroEL is in complex with the cochaperonin gp31, it binds exclusively one gp23. We also demonstrated that the folding and assembly of gp23 into 336-kDa hexamers by GroEL-gp31 can be monitored directly by electrospray ionization mass spectrometry (ESI-MS). These data reinforce the great potential of ESI-MS as a technique to investigate structure-function relationships of protein assemblies in general and the chaperonin-protein folding machinery in particular. A major advantage of native mass spectrometry is that, given sufficient resolution, it allows the analysis at the picomole level of sensitivity of heterogeneous protein complexes with molecular masses up to several million daltons.

Error-prone DNA polymerase IV is regulated by the heat shock chaperone GroE in Escherichia coli

An insertion in the promoter of the operon that encodes the molecular chaperone GroE was isolated as an antimutator for stationary-phase or adaptive mutation. The groE operon consists of two genes, groES and groEL; point mutations in either gene conferred the same phenotype, reducing Lac+ adaptive mutation 10- to 20-fold. groE mutant strains had 1/10 the amount of error-prone DNA polymerase IV (Pol IV). In recG+ strains, the reduction in Pol IV was sufficient to account for their low rate of adaptive mutation, but in recG mutant strains, a deficiency of GroE had some additional effect on adaptive mutation. Pol IV is induced as part of the SOS response, but the effect of GroE on Pol IV was independent of LexA. We were unable to show that GroE interacts directly with Pol IV, suggesting that GroE may act indirectly. Together with previous results, these findings indicate that Pol IV is a component of several cellular stress responses.

Co-translational involvement of the chaperonin GroEL in the folding of newly translated polypeptides

A large fraction of the newly translated polypeptides emerging from the ribosome require certain proteins, the so-called molecular chaperones, to assist in their folding. In Escherichia coli, three major chaperone systems are considered to contribute to the folding of newly synthesized cytosolic polypeptides. Trigger factor (TF), a ribosome-tethered chaperone, and DnaK are known to exhibit overlapping co-translational roles, whereas the cage-shaped GroEL, with the aid of the co-chaperonin, GroES, and ATP, is believed to be implicated in folding only after the polypeptides are released from the ribosome. However, the recent finding that GroEL-GroES overproduction permits the growth of E. coli cells lacking both TF and DnaK raised questions regarding the separate roles of these chaperones. Here, we report the puromycin-sensitive association of GroEL-GroES with translating ribosomes in vivo. Further experiments in vitro, using a reconstituted cell-free translation system, clearly demonstrate that GroEL associates with the translation complex and accomplishes proper folding by encapsulating the newly translated polypeptides in the central cavity formed by GroES. Therefore, we propose that GroEL is a versatile chaperone, which participates in the folding pathway co-translationally and also achieves correct folding post-translationally.

Functional consequences of single:double ring transitions in chaperonins: life in the cold

The cpn60 and cpn10 genes from psychrophilic bacterium, Oleispira antarctica RB8, showed a positive effect in Escherichia coli growth at low temperature, shifting its theoretical minimal growth temperature from +7.5 degrees C to -13.7 degrees C [Ferrer, M., Chernikova, T.N., Yakimov, M., Golyshin, P.N., and Timmis, K.N. (2003) Nature Biotechnol 21: 1266-1267]. To provide experimental support for this finding, Cpn60 and 10 were overproduced in E. coli and purified to apparent homogeneity. Recombinant O.Cpn60 was identical to the native protein based on tetradecameric structure, and it dissociates during native PAGE. Gel filtration and native PAGE revealed that, in vivo and in vitro, (O.Cpn60)(7) was the active oligomer at 4-10 degrees C, whereas at > 10 degrees C, this complex was converted to (O.Cpn60)(14). The dissociation reduces the ATP consumption (energy-saving mechanism) and increases the refolding capacity at low temperatures. In order for this transition to occur, we demonstrated that K468 and S471 may play a key role in conforming the more advantageous oligomeric state in O.Cpn60. We have proved this hypothesis by showing that single and double mutations in K468 and S471 for T and G, as in E.GroEL, produced a more stable double-ring oligomer. The optimum temperature for ATPase and chaperone activity for the wild-type chaperonin was 24-28 degrees C and 4-18 degrees C, whereas that for the mutants was 45-55 degrees C and 14-36 degrees C respectively. The temperature inducing unfolding (T(M)) increased from 45 degrees C to more than 65 degrees C. In contrast, a single ring mutant, O.Cpn60(SR), with three amino acid substitutions (E461A, S463A and V464A) was as stable as the wild type but possessed refolding activity below 10 degrees C. Above 10 degrees C, this complex lost refolding capacity to the detriment of the double ring, which was not an efficient chaperone at 4 degrees C as the single ring variant. We demonstrated that expression of O.Cpn60(WT) and O.Cpn60(SR) leads to a higher growth of E. coli at 4 degrees C ( micro (max), 0.22 and 0.36 h(-1) respectively), whereas at 10-15 degrees C, only E. coli cells expressing O.Cpn60 or O.Cpn60(DR) grew better than parental cells (-cpn). These results clearly indicate that the single-to-double ring transition in Oleispira chaperonin is a wild-type mechanism for its thermal acclimation. Although previous studies have also reported single-to-double ring transitions under many circumstances, this is the first clear indication that single-ring chaperonins are necessary to support growth when the temperature falls from 37 degrees C to 4 degrees C.

Heat shock protein 10 inhibits lipopolysaccharide-induced inflammatory mediator production

Heat shock protein 10 (Hsp10) and heat shock protein 60 (Hsp60) were originally described as essential mitochondrial proteins involved in protein folding. However, both proteins have also been shown to have a number of extracellular immunomodulatory activities. Here we show that purified recombinant human Hsp10 incubated with cells in vitro reduced lipopolysaccharide (LPS)-induced nuclear factor-kappaB activation and secretion of several inflammatory mediators from RAW264.7 cells, murine macrophages, and human peripheral blood mononuclear cells. Induction of tolerance by contaminating LPS was formally excluded as being responsible for Hsp10 activity. Treatment of mice with Hsp10 before endotoxin challenge resulted in the reduction of serum tumor necrosis factor-alpha and RANTES (regulated upon activation, normal T cell expressed and secreted) levels and an elevation of serum interleukin-10 levels. Hsp10 treatment also delayed mortality in a murine graft-versus-host disease model, where gut-derived LPS contributes to pathology. We were unable to confirm previous reports that Hsp10 has tumor growth factor properties and suggest that Hsp10 exerts anti-inflammatory activity by inhibiting Toll-like receptor signaling possibly by interacting with extracellular Hsp60.

Transcriptional analysis of the groES-groEL1, groEL2, and dnaK genes in Corynebacterium glutamicum: characterization of heat shock-induced promoters

The appropriate conditions to switch on the heat shock promoters in Corynebacterium glutamicum were defined by Northern blot analysis. Transcriptional patterns were characterized for the groEL2 gene and the groES-groEL1 and dnaK operons. Transcriptional start points of these genes were determined by primer extension analysis, allowing the identification of CIRCE and HAIR boxes close to the -10 and -35 regions of the promoters. The presence of both CIRCE and HAIR sequences within a single promoter (P-groEL2) in bacteria is described for the first time. In addition, the dnaK promoter showed -10 and -35 sequences similar to those recognized by SigH of Mycobacterium and SigR of Streptomyces close to a second transcription start region with -10 and -35 boxes typical of promoters for housekeeping genes.

Low temperature or GroEL/ES overproduction permits growth of Escherichia coli cells lacking trigger factor and DnaK

Escherichia coli trigger factor (TF) and DnaK cooperate in the folding of newly synthesized proteins. The combined deletion of the TF-encoding tig gene and the dnaK gene causes protein aggregation and synthetic lethality at 30 degrees C. Here we show that the synthetic lethality of DeltatigDeltadnaK52 cells is abrogated either by growth below 30 degrees C or by overproduction of GroEL/GroES. At 23 degrees C DeltatigDeltadnaK52 cells were viable and showed only minor protein aggregation. Overproduction of GroEL/GroES, but not of other chaperones, restored growth of DeltatigDeltadnaK52 cells at 30 degrees C and suppressed protein aggregation including proteins >/=60 kDa, which normally require TF and DnaK for folding. GroEL/GroES thus influences the folding of proteins previously identified as DnaK/TF substrates.

Hsp10 and Hsp60 suppress ubiquitination of insulin-like growth factor-1 receptor and augment insulin-like growth factor-1 receptor signaling in cardiac muscle: implications on decreased myocardial protection in diabetic cardiomyopathy

We have investigated the effects of two heat shock proteins, Hsp10 and Hsp60, on insulin-like growth factor-1 receptor (IGF-1R) signaling in cardiac muscle cells. Neonatal cardiomyocytes were transduced with Hsp10 or Hsp60 via adenoviral vector. Compared with the cells transduced with a control vector, overexpression of Hsp10 or Hsp60 increased the abundance of IGF-1R and IGF-1-stimulated receptor autophosphorylation. Thus, Hsp10 and Hsp60 overexpression increased the number of functioning receptors and amplified activation of IGF-1R signaling. IGF-1 stimulation of MEK, Erk, p90Rsk, and Akt were accordingly augmented. Transducing cardiomyocytes with antisense Hsp60 oligonucleotides reduced Hsp60 expression, decreased the abundance of IGF-1R, attenuated IGF-1R autophosphorylation, and suppressed the pro-survival action of IGF-1 in cardiomyocytes. Using cycloheximide to inhibit protein synthesis did not alter the effect of Hsp60 on IGF-1R signaling, and IGF-1R mRNA levels were not up-regulated by Hsp10 or Hsp60. Additional experiments showed that Hsp10 and Hsp60 suppressed polyubiquitination of IGF-1 receptor. These data indicate that Hsp10 and Hsp60 can modulate IGF-1R signaling through post-translational modification. In animal models of diabetes, diabetic myocardium is associated with decreased abundance of Hsp60, increased ubiquitination of IGF-1R, and lower level of IGF-1R protein. Declined myocardial protection is a major feature of diabetic cardiomyopathy. These data suggest that decreased Hsp60 expression and subsequent decline of IGF-1R signaling may be a fundamental mechanism underlying the development of diabetic cardiomyopathy.

Type I chaperonins: not all are created equal

Type I chaperonins play an essential role in the folding of newly translated and stress-denatured proteins in eubacteria, mitochondria and chloroplasts. Since their discovery, the bacterial chaperonins have provided an excellent model system for investigating the mechanism by which chaperonins mediate protein folding. Due to the high conservation of the primary sequence among Type I chaperonins, it is generally accepted that organellar chaperonins function similar to the bacterial ones. However, recent studies indicate that the chloroplast and mitochondrial chaperonins possess unique structural and functional properties that distinguish them from their bacterial homologs. This review focuses on the unique properties of organellar chaperonins.

Molecular chaperones--cellular machines for protein folding

Proteins are linear polymers synthesized by ribosomes from activated amino acids. The product of this biosynthetic process is a polypeptide chain, which has to adopt the unique three-dimensional structure required for its function in the cell. In 1972, Christian Anfinsen was awarded the Nobel Prize for Chemistry for showing that this folding process is autonomous in that it does not require any additional factors or input of energy. Based on in vitro experiments with purified proteins, it was suggested that the correct three-dimensional structure can form spontaneously in vivo once the newly synthesized protein leaves the ribosome. Furthermore, proteins were assumed to maintain their native conformation until they were degraded by specific enzymes. In the last decade this view of cellular protein folding has changed considerably. It has become clear that a complicated and sophisticated machinery of proteins exists which assists protein folding and allows the functional state of proteins to be maintained under conditions in which they would normally unfold and aggregate. These proteins are collectively called molecular chaperones, because, like their human counterparts, they prevent unwanted interactions between their immature clients. In this review, we discuss the principal features of this peculiar class of proteins, their structure-function relationships, and the underlying molecular mechanisms.

Archaeal group II chaperonin mediates protein folding in the cis-cavity without a detachable GroES-like co-chaperonin

Group II chaperonins of archaea and eukaryotes are distinct from group I chaperonins of bacteria. Whereas group I chaperonins require the co-chaperonin Cpn-10 or GroES for protein folding, no co-chaperonin has been known for group II. The protein folding mechanism of group II chaperonins is not yet clear. To understand this mechanism, we examined protein refolding by the recombinant alpha or beta-subunit chaperonin homo-oligomer (alpha16mer and beta16mer) from a hyperthermoplilic archaeum, Thermococcus strain KS-1, using a model substrate, green fluorescent protein (GFP). The alpha16mer and beta16mer captured the non-native GFP and promoted its refolding without any co-chaperonin in an ATP dependent manner. A non-hydrolyzable ATP analog, AMP-PNP, induced the GFP refolding mediated by beta16mer but not by the alpha16mer. A mutant alpha-subunit chaperonin homo-oligomer (trap-alpha) could capture the non-native protein but lacked the ability to refold it. Although trap-alpha suppressed ATP-dependent refolding of GFP mediated by alpha16mer or beta16mer, it did not affect the AMP-PNP-dependent refolding. This indicated that the GFP refolding mediated by beta16mer with AMP-PNP was not accessible to the trap-alpha. Gel filtration chromatography and a protease protection experiment revealed that this refolded GFP, in the presence of AMP-PNP, was associated with beta16mer. After the completion of GFP refolding mediated by beta16mer with AMP-PNP, addition of ATP induced an additional refolding of GFP. Furthermore, the beta16mer preincubated with AMP-PNP showed the ability to capture the non-native GFP. These suggest that AMP-PNP induced one of two chaperonin rings (cis-ring) to close and induced protein refolding in this ring, and that the other ring (trans-ring) could capture the unfolded GFP which was refolded by adding ATP. The present data indicate that, in the group II chaperonin of Thermococcus strain KS-1, the protein folding proceeds in its cis-ring in an ATP-dependent fashion without any co-chaperonin.

Chaperonin-assisted folding of glutamine synthetase under nonpermissive conditions: off-pathway aggregation propensity does not determine the co-chaperonin requirement

One of the proposed roles of the GroEL-GroES cavity is to provide an "infinite dilution" folding chamber where protein substrate can fold avoiding deleterious off-pathway aggregation. Support for this hypothesis has been strengthened by a number of studies that demonstrated a mandatory GroES requirement under nonpermissive solution conditions, i.e., the conditions where proteins cannot spontaneously fold. We have found that the refolding of glutamine synthetase (GS) does not follow this pattern. In the presence of natural osmolytes trimethylamine N-oxide (TMAO) or potassium glutamate, refolding GS monomers readily aggregate into very large inactive complexes and fail to reactivate even at low protein concentration. Surprisingly, under these "nonpermissive" folding conditions, GS can reactivate with GroEL and ATP alone and does not require the encapsulation by GroES. In contrast, the chaperonin dependent reactivation of GS under another nonpermissive condition of low Mg2+ (<2 mM MgCl2) shows an absolute requirement of GroES. High-performance liquid chromatography gel filtration analysis and irreversible misfolding kinetics show that a major species of the GS folding intermediates, generated under these "low Mg2+" conditions exist as long-lived metastable monomers that can be reactivated after a significantly delayed addition of the GroEL. Our results indicate that the GroES requirement for refolding of GS is not simply dictated by the aggregation propensity of this protein substrate. Our data also suggest that the GroEL-GroES encapsulated environment is not required under all nonpermissive folding conditions.

High salt-induced conversion of Escherichia coli GroEL into a fully functional thermophilic chaperonin

The GroE chaperonin system can adapt to and function at various environmental folding conditions. To examine chaperonin-assisted protein folding at high salt concentrations, we characterized Escherichia coli GroE chaperonin activity in 1.2 m ammonium sulfate. Our data are consistent with GroEL undergoing a conformational change at this salt concentration, characterized by elevated ATPase activity and increased exposure of hydrophobic surface, as indicated by increased binding of the fluorophore bis-(5, 5')-8-anilino-1-naphthalene sulfonic acid to the chaperonin. The presence of the salt results in increased substrate stringency and dependence on the full GroE system for release and productive folding of substrate proteins. Surprisingly, GroEL is fully functional as a thermophilic chaperonin in high concentrations of ammonium sulfate and is stable at temperatures up to 75 degrees C. At these extreme conditions, GroEL can suppress aggregation and mediate refolding of non-native proteins.

Periplasmic localization of a GroES homologue in Escherichia coli transformed with groESx cloned from Legionella-like endosymbionts in Amoeba proteus

Escherichia coli MC4100 transformed with a groE homologous operon cloned from X-bacteria accumulated large amounts of the gene product when cultured at 30 or 37 degrees C. Heat shock for 10-30 min at 42 degrees C or ethanol (5%) shock for 2 h increased GroESx levels to about twice that in E. coli grown at 30 degrees C. The subcellular localization of GroESx in transformed E. coli was determined by several subcellular fractionation methods, by the analysis of extracted proteins in SDS polyacrylamide gels and by assays of marker enzymes. The GroESx protein was detected in both the periplasmic and cytoplasmic extracts and a large amount of the protein was accumulated in the periplasm. The GroEL protein and recombinant beta-galactosidase were exclusively localized in the cytoplasmic fraction, eliminating the possibility that periplasmic GroESx might be due to simple overproduction. N-terminal amino acid sequencing confirmed that the protein resolved on a 2-D gel was GroESx. This work represents the first report of the periplasmic location of GroES homologues in E. coli.

Molecular chaperones: containers and surfaces for folding, stabilising or unfolding proteins

Newly solved chaperone structures include the thermosome, a group II chaperonin, and a small heat-shock protein. Novel ideas on chaperone mechanism are presented in the forced unfolding hypothesis of GroEL action. Structures of chaperone-pilin complexes reveal the mechanism of chaperone interaction in bacterial pilus assembly and there have been major advances in understanding the structure and function of Hsp100 unfoldases.

Multivalent binding of nonnative substrate proteins by the chaperonin GroEL

The chaperonin GroEL binds nonnative substrate protein in the central cavity of an open ring through exposed hydrophobic residues at the inside aspect of the apical domains and then mediates productive folding upon binding ATP and the cochaperonin GroES. Whether nonnative proteins bind to more than one of the seven apical domains of a GroEL ring is unknown. We have addressed this using rings with various combinations of wild-type and binding-defective mutant apical domains, enabled by their production as single polypeptides. A wild-type extent of binary complex formation with two stringent substrate proteins, malate dehydrogenase or Rubisco, required a minimum of three consecutive binding-proficient apical domains. Rhodanese, a less-stringent substrate, required only two wild-type domains and was insensitive to their arrangement. As a physical correlate, multivalent binding of Rubisco was directly observed in an oxidative cross-linking experiment.

Cloning, sequencing, and functional expression in Escherichia coli of chaperonin (groESL) genes from Vibrio cholerae

Using a series of oligonucleotides synthesized on the basis of conserved nucleotide motifs in heat-shock genes, the groESL heat-shock operon from a Vibrio cholerae TSI-4 strain has been cloned and sequenced, revealing that the presence of two open reading frames (ORFs) of 291 nucleotides and 1,632 nucleotides separated by 54 nucleotides. The first ORF encoded a polypeptide of 97 amino acids, GroES homologue, and the second ORF encoded a polypeptide of 544 amino acids, GroEL homologue. A comparison of the deduced amino acid sequences revealed that the primary structures of the V. cholerae GroES and GroEL proteins showed significant homology with those of the GroES and GroEL proteins of other bacteria. Complementation experiments were performed using Escherichia coli groE mutants which have the temperature-sensitive growth phenotype. The results showed that the groES and groEL from V. cholerae were expressed in E. coli, and groE mutants harboring V. cholerae groESL genes regained growth ability at high temperature. The evolutionary analysis indicates a closer relationship between V. cholerae chaperonins and those of the Haemophilus and Yersinia species.

Chaperonin function: folding by forced unfolding

The ability of the GroEL chaperonin to unfold a protein trapped in a misfolded condition was detected and studied by hydrogen exchange. The GroEL-induced unfolding of its substrate protein is only partial, requires the complete chaperonin system, and is accomplished within the 13 seconds required for a single system turnover. The binding of nucleoside triphosphate provides the energy for a single unfolding event; multiple turnovers require adenosine triphosphate hydrolysis. The substrate protein is released on each turnover even if it has not yet refolded to the native state. These results suggest that GroEL helps partly folded but blocked proteins to fold by causing them first to partially unfold. The structure of GroEL seems well suited to generate the nonspecific mechanical stretching force required for forceful protein unfolding.

The role of DnaK/DnaJ and GroEL/GroES systems in the removal of endogenous proteins aggregated by heat-shock from Escherichia coli cells

The submission of Escherichia coli cells to heat-shock (45 degrees C, 15 min) caused the intracellular aggregation of endogenous proteins. In the wt cells the aggregates (the S fraction) disappeared 10 min after transfer to 37 degrees C. In contrast, the S fraction in the dnaK and dnaJ mutant strains was stable during approximately one generation time (45 min). This demonstrated that neither the renaturation nor the degradation of the denatured proteins was possible in the absence of DnaK and DnaJ. The groEL44 and groES619 mutations stabilised the aggregates to a lesser extent. It was shown by the use of cloned genes, dnaK/dnaJ or groEL/groES, producing the corresponding proteins in about 4-fold excess, that the appearance of the S fraction in the wt strain resulted from a transiently insufficient supply of the heat-shock proteins. Overproduction of the GroEL/GroES proteins in dnaK756 or dnaJ259 background prevented the aggregation, however, overproduction of the DnaK/DnaJ proteins did not prevent the aggregation in the groEL44 or groES619 mutant cells although it accelerated the disappearance of the aggregates. The properties of the aggregated proteins are discussed from the point of view of their competence to renaturation/degradation by the heat-shock system.

The oligomeric structure of GroEL/GroES is required for biologically significant chaperonin function in protein folding

Two models are being considered for the mechanism of chaperonin-assisted protein folding in E. coli: (i) GroEL/GroES act primarily by enclosing substrate polypeptide in a folding cage in which aggregation is prevented during folding. (ii) GroEL mediates the repetitive unfolding of misfolded polypeptides, returning them onto a productive folding track. Both models are not mutually exclusive, but studies with the polypeptide-binding domain of GroEL have suggested that unfolding is the primary mechanism, enclosure being unnecessary. Here we investigate the capacity of the isolated apical polypeptide-binding domain to functionally replace the complete GroEL/GroES system. We show that the apical domain binds aggregation-sensitive polypeptides but cannot significantly assist their refolding in vitro and fails to replace the groEL gene or to complement defects of groEL mutants in vivo. A single-ring version of GroEL cannot substitute for GroEL. These results strongly support the view that sequestration of aggregation-prone intermediates in a folding cage is an important element of the chaperonin mechanism.

A single ring is sufficient for productive chaperonin-mediated folding in vivo

Facilitated protein folding by the double toroidal bacterial chaperonin, GroEL/GroES, proceeds by a "two-stroke engine" mechanism in which an allosteric interaction between the two rings synchronizes the reaction cycle by controlling the binding and release of cochaperonin. Using chimeric chaperonin molecules assembled by fusing equatorial and apical domains derived from GroEL and its mammalian mitochondrial homolog, Hsp60, we show that productive folding by Hsp60 and its cognate cochaperonin, Hsp10, proceeds in vitro and in vivo without the formation of a two-ring structure. This simpler "one-stroke" engine works because Hsp60 has a different mechanism for the release of its cochaperonin cap and bound target protein.

Chaperone coexpression plasmids: differential and synergistic roles of DnaK-DnaJ-GrpE and GroEL-GroES in assisting folding of an allergen of Japanese cedar pollen, Cryj2, in Escherichia coli

Plasmids that can be used for controlled expression of the DnaK-DnaJ-GrpE and/or GroEL-GroES chaperone team were constructed in order to facilitate assessment of the effects of these chaperone teams on folding or assembly or recombinant proteins in Escherichia coli. A typical pACYC184-based plasmid which was obtained could express the major DnaK-DnaJ-GrpE and GroEL-GroES chaperone teams from separate promoters when L-arabinose and tetracycline, respectively, were added in a dose-dependent fashion. The model protein used to determine whether this system was useful was an allergen of Japanese cedar pollen, Cryj2, which was unstable when it was produced in E. coli K-12. The effects of chaperone coexpression on the folding, aggregation, and stability of Cryj2 were examined in the wild type and in several mutant bacteria. Coexpression of the DnaK-DnaJ-GrpE and/or GroEL-GroES chaperone team at appropriate levels resulted in marked stabilization and accumulation of Cryj2 without extensive aggregation. Experiments performed with mutants that lack each of the chaperone proteins (DnaK, DnaJ, GrpE, GroEL, and GroES) or heat shock transcription factor sigma 32 revealed that both chaperone teams are critically involved in Cryj2 folding but that they are involved in distinct ways. In addition, it was observed that the two chaperone teams have synergistic roles in preventing aggregation of Cryj2 in the absence of sigma 32 at certain temperatures.

Chaperones in bacteriophage T4 assembly

Protein folding in the cell is controlled at the levels of translation and post-translational modification, depends on a number of conserved proteins known as chaperones, and is catalyzed by specific enzymes, such as protein disulfide isomerase and peptidyl prolyl cis-trans isomerase. The chaperones stabilize folding intermediates and participate in assembly and disaggregation of supramolecular structures. Bacteriophage T4 is an especially convenient system for studying of protein folding mechanisms, since its genome encodes several virus-specific chaperones. In this review, the chaperones of phage T4 that take part in capsid formation (gp31 and gp40) and in folding and assembly of virion tail fibers (gp38, gp57A) have been considered. Protein encoded by gene 31 completely substitutes co-chaperonin GroES of the host cell in folding of the major capsid protein, gp23, aided by chaperonin GroEL. The product of gene 40, which is homologous to analogs of eukaryotic GroEL and peptidyl prolyl cis-trans isomerase, participates in assembly of gp20 while the formation of procapsid connector. The chaperone encoded by gene 57A is essential for folding and oligomerization of both long and short phage tail fibers. gp38, together with gp57A, participates in the formation of the distal part of the long fibers. This protein seems to represent a principally new group of chaperones that change steric structure of folded polypeptide. One phage chaperone, fibritin, encoded by gene wac (whiskers antigen control) and taking part in assembly the subunits of the long tail fibers is a constituent of the virion. Fibritin is a convenient model for studying mechanisms of folding and oligomerization of fibrous proteins due to its labile triple-stranded alpha-helical coiled-coil structure.

Assembly of both the head and tail of bacteriophage Mu is blocked in Escherichia coli groEL and groES mutants

Like several other Escherichia coli bacteriophages, transposable phage Mu does not develop normally in groE hosts (M. Pato, M. Banerjee, L. Desmet, and A. Toussaint, J. Bacteriol. 169:5504-5509, 1987). We show here that lysates obtained upon induction of groE Mu lysogens contain free inactive tails and empty heads. GroEL and GroES are thus essential for the correct assembly of both Mu heads and Mu tails. Evidence is presented that groE mutations inhibit processing of the phage head protein gpH as well as the formation of a 25S complex suspected to be an early Mu head assembly intermediate.

Molecular chaperones in biology and medicine at Obernai

No Abstract Available

Assembly of membrane-containing bacteriophage PRD1 is dependent on GroEL and GroES

Assembly of the broad-host-range bacteriophage PRD1 involves translocation of the virus-specific membrane to the inside of the icosahedral protein shell formed of trimeric coat proteins. The formation of PRD1 particles is, in addition to the virus-encoded assembly factors P10 and P17, dependent on GroEL/GroES chaperonins. The chaperonins assist in the folding of the capsid proteins P3 and P5 and in the assembly of viral membrane proteins.

Evidence for GroES acting as a transcriptional regulator

Cochaperonins (cpn10) assist chaperonins (cpn60) in promoting folding and assembly of other proteins. Upon expression of Mycobacterium tuberculosis cpn10 in Escherichia coli we have purified a polypeptide which, through amino acid sequencing, was identified as the endogenous E. coli 10K-S protein. Subsequent studies showed that its expression was specifically upregulated upon cloning of different members of the cpn10 family, including GroES, the E. coli cpn10. Pulse-chase experiments demonstrated that 10K-S is but one of several proteins whose expression is modulated upon cloning of cpn10. Up-regulation of 10K-S was also observed after exposure of normal cells, but not of groES- mutants, to elevated temperatures (42 degrees C). This allowed us to define 10K-S as a heat-shock protein (hsp) whose expression is dependent on the production of another hsp, GroES. Northern blot experiments showed that enhanced expression of 10K-S was consequent to increased accumulation of transcripts and that groES- mutants were devoid even of baseline levels of transcripts both at 37 degrees C and after temperature upshift. These results show that GroES, in addition to its established role in assisting protein folding may act as a transcriptional regulator and is likely to play an important role in modulating gene expression particularly in those conditions, like the stress response, in which its production is greatly enhanced.

The chaperonin ATPase cycle: mechanism of allosteric switching and movements of substrate-binding domains in GroEL

Chaperonin-assisted protein folding proceeds through cycles of ATP binding and hydrolysis by the large chaperonin GroEL, which undergoes major allosteric rearrangements. Interaction between the two back-to-back seven-membered rings of GroEL plays an important role in regulating binding and release of folding substrates and of the small chaperonin GroES. Using cryo-electron microscopy, we have obtained three-dimensional reconstructions to 30 A resolution for GroEL and GroEL-GroES complexes in the presence of ADP, ATP, and the nonhydrolyzable ATP analog, AMP-PNP. Nucleotide binding to the equatorial domains of GroEL causes large rotations of the apical domains, containing the GroES and substrate protein-binding sites. We propose a mechanism for allosteric switching and describe conformational changes that may be involved in critical steps of folding for substrates encapsulated by GroES.

Evidence for involvement of Escherichia coli genes pmbA, csrA and a previously unrecognized gene tldD, in the control of DNA gyrase by letD (ccdB) of sex factor F

The letA (ccdA) and letD (ccdB) genes of the F plasmid, located just outside the sequence essential for replication, contribute to stable maintenance of the plasmid in Escherichia coli cells. The letD gene product acts to inhibit partitioning of chromosomal DNA and cell division by inhibiting DNA gyrase activity, whereas the letA gene product acts to reverse the inhibitory activity of the letD gene product. To identify the host factor(s) involved in this process, we analyzed the mutants that escaped letD expression and their suppressor, and found that the three E. coli genes tldD, tldE and zfiA participate in the process, in addition to the groE genes we reported previously. The tldD and tldE mutations made cells tolerant for letD expression, as did groES mutations, while the mutation in the zfiA gene made tldD, tldE and groES mutants LetD sensitive. We hypothesize that these gene products are factors that modulate activity of DNA gyrase along with the letD gene product; the zfiA gene product acts to inhibit interaction between the LetD protein and the A subunit of DNA gyrase, while the tldD, tldE and groE gene products act to suppress the inhibitory activity of the zfiA gene product. The tldD, tldE, and zfiA genes are located at 70.4, 96.0 and 58.2 minutes on the E. coli chromosome, respectively, and code for proteins with relative molecular masses of 51,000, 48,000 and 6800, respectively. tldD is a novel gene, but the tldE and zfiA genes proved to be the pmbA gene (production of Microcin B17) and the csrA gene (carbon storage regulator), respectively.

Simulations of chaperone-assisted folding

We investigated a chaperone mechanism of protein folding using a 36-mer model on a cubic lattice. The mechanism simulates folding, which proceeds with repetitive cycles of binding, unfolding, and releasing of misfolded metastable states. We measured the yield enhancement due to this mechanism for sequences selected by evolutionary design and showed that the binding and releasing mechanism is efficient for the yield enhancement of folding for sequences that are poorly designed, i.e., where selection is not adequately strong. From this it follows that the chaperone mechanism can be considered as the evolutionary alternative to compensate for poor sequence design. On the other hand, random sequences show a decrease in yield and no effect on the total mean first passage time when the proposed chaperone mechanism is implemented, thus implying that sequence optimization is a necessary condition for the efficiency of the proposed mechanism. We qualitatively reproduced experimental results for folding in the presence of GroEL/GroES, fit our results with the aid of a double-exponential model of folding kinetics, and characterized the conditions under which this mechanism of chaperone action affects folding.

The crystal structure of the GroES co-chaperonin at 2.8 A resolution

The GroES heptamer forms a dome, approximately 75 A in diameter and 30 A high, with an 8 A orifice in the centre of its roof. The 'mobile loop' segment, previously identified as a GroEL binding determinant, is disordered in the crystal structure in six subunits; the single well-ordered copy extends from the bottom outer rim of the GroES dome, suggesting that the cavity within the dome is continuous with the polypeptide binding chamber of GroEL in the chaperonin complex.

Identification of early pregnancy factor as chaperonin 10: implications for understanding its role

Early pregnancy factor (EPF) is a secreted substance with growth regulatory and immunomodulatory properties. It is required for successful establishment of pregnancy and for proliferation of both normal and neoplastic cells, in vivo and in vitro. The rosette inhibition test was used as a bioassay, and the appearance of EPF in serum in the very early stages of pregnancy (in mice, within 4-6 h of mating) was first described two decades ago. However, because of the difficulty of this bioassay and the paucity of EPF in biological materials, the primary structure of the molecule has been identified only recently. Seventy per cent of the amino acid sequence of EPF derived from human platelets was determined. With the exception of a single residue, this was identical to the sequence of rat mitochondrial chaperonin 10 (cpn10). Cpn10 is a heat shock protein that functions as a molecular chaperone. It binds to and stabilizes cpn60 and, in concert, these molecules mediate protein folding in mitochondria, chloroplasts and bacteria. Characterizing EPF as an extracellular form of cpn10 raises unprecedented questions about the mechanism of action. It may be that, as a molecular chaperone in the extracellular compartment, EPF can functionally modify other proteins, serving as a regulator of regulators.