Functional similarities and differences of an archaeal Hsp70(DnaK) stress protein compared with its homologue from the bacterium Escherichia coli

Can heat shock protein 70 (HSP70) serve as biomarkers in Antarctica for future ocean acidification, warming and salinity stress?

The Antarctic Peninsula is one of the fastest-warming places on Earth. Elevated sea water temperatures cause glacier and sea ice melting. When icebergs melt into the ocean, it “freshens” the saltwater around them, reducing its salinity. The oceans absorb excess anthropogenic carbon dioxide (CO2) causing decline in ocean pH, a process known as ocean acidification. Many marine organisms are specifically affected by ocean warming, freshening and acidification. Due to the sensitivity of Antarctica to global warming, using biomarkers is the best way for scientists to predict more accurately future climate change and provide useful information or ecological risk assessments. The 70-kilodalton (kDa) heat shock protein (HSP70) chaperones have been used as biomarkers of stress in temperate and tropical environments. The induction of the HSP70 genes (Hsp70) that alter intracellular proteins in living organisms is a signal triggered by environmental temperature changes. Induction of Hsp70 has been observed both in eukaryotes and in prokaryotes as response to environmental stressors including increased and decreased temperature, salinity, pH and the combined effects of changes in temperature, acidification and salinity stress. Generally, HSP70s play critical roles in numerous complex processes of metabolism; their synthesis can usually be increased or decreased during stressful conditions. However, there is a question as to whether HSP70s may serve as excellent biomarkers in the Antarctic considering the long residence time of Antarctic organisms in a cold polar environment which appears to have greatly modified the response of heat responding transcriptional systems. This review provides insight into the vital roles of HSP70 that make them ideal candidates as biomarkers for identifying resistance and resilience in response to abiotic stressors associated with climate change, which are the effects of ocean warming, freshening and acidification in Antarctic organisms.

GmDNJ1, a type-I heat shock protein 40 (HSP40), is responsible for both Growth and heat tolerance in soybean

Global warming poses severe threats to agricultural production, including soybean. One of the major mechanisms for organisms to combat heat stress is through heat shock proteins (HSPs) that stabilize protein structures at above-optimum temperatures, by assisting in the folding of nascent, misfolded, or unfolded proteins. The HSP40 subgroups, or the J-domain proteins, functions as co-chaperones. They capture proteins that require folding or refolding and pass them on to HSP70 for processing. In this study, we have identified a type-I HSP40 gene in soybean, GmDNJ1, with high basal expression under normal growth conditions and also highly inducible under abiotic stresses, especially heat. Gmdnj1-knockout mutants had diminished growth in normal conditions, and when under heat stress, exhibited more severe browning, reduced chlorophyll contents, higher reactive oxygen species (ROS) contents, and higher induction of heat stress-responsive transcription factors and ROS-scavenging enzyme-encoding genes. Under both normal and heat-stress conditions, the mutant lines accumulated more aggregated proteins involved in protein catabolism, sugar metabolism, and membrane transportation, in both roots and leaves. In summary, GmDNJ1 plays crucial roles in the overall plant growth and heat tolerance in soybean, probably through the surveillance of misfolded proteins for refolding to maintain the full capacity of cellular functions.

The Role of Stress Proteins in Haloarchaea and Their Adaptive Response to Environmental Shifts

Over the years, in order to survive in their natural environment, microbial communities have acquired adaptations to nonoptimal growth conditions. These shifts are usually related to stress conditions such as low/high solar radiation, extreme temperatures, oxidative stress, pH variations, changes in salinity, or a high concentration of heavy metals. In addition, climate change is resulting in these stress conditions becoming more significant due to the frequency and intensity of extreme weather events. The most relevant damaging effect of these stressors is protein denaturation. To cope with this effect, organisms have developed different mechanisms, wherein the stress genes play an important role in deciding which of them survive. Each organism has different responses that involve the activation of many genes and molecules as well as downregulation of other genes and pathways. Focused on salinity stress, the archaeal domain encompasses the most significant extremophiles living in high-salinity environments. To have the capacity to withstand this high salinity without losing protein structure and function, the microorganisms have distinct adaptations. The haloarchaeal stress response protects cells against abiotic stressors through the synthesis of stress proteins. This includes other heat shock stress proteins (Hsp), thermoprotectants, survival proteins, universal stress proteins, and multicellular structures. Gene and family stress proteins are highly conserved among members of the halophilic archaea and their study should continue in order to develop means to improve for biotechnological purposes. In this review, all the mechanisms to cope with stress response by haloarchaea are discussed from a global perspective, specifically focusing on the role played by universal stress proteins.

Heat shock response in archaea

An adequate response to a sudden temperature rise is crucial for cellular fitness and survival. While heat shock response (HSR) is well described in bacteria and eukaryotes, much less information is available for archaea, of which many characterized species are extremophiles thriving in habitats typified by large temperature gradients. Here, we describe known molecular aspects of archaeal heat shock proteins (HSPs) as key components of the protein homeostasis machinery and place this in a phylogenetic perspective with respect to bacterial and eukaryotic HSPs. Particular emphasis is placed on structure–function details of the archaeal thermosome, which is a major element of the HSR and of which subunit composition is altered in response to temperature changes. In contrast with the structural response, it is largely unclear how archaeal cells sense temperature fluctuations and which molecular mechanisms underlie the corresponding regulation. We frame this gap in knowledge by discussing emerging questions related to archaeal HSR and by proposing methodologies to address them. Additionally, as has been shown in bacteria and eukaryotes, HSR is expected to be relevant for the control of physiology and growth in various stress conditions beyond temperature stress. A better understanding of this essential cellular process in archaea will not only provide insights into the evolution of HSR and of its sensing and regulation, but also inspire the development of biotechnological applications, by enabling transfer of archaeal heat shock components to other biological systems and for the engineering of archaea as robust cell factories.

Internal RNAs overlapping coding sequences can drive the production of alternative proteins in archaea

Prokaryotic genomes show a high level of information compaction often with different molecules transcribed from the same locus. Although antisense RNAs have been relatively well studied, RNAs in the same strand, internal RNAs (intraRNAs), are still poorly understood. The question of how common is the translation of overlapping reading frames remains open. We address this question in the model archaeon Halobacterium salinarum. In the present work we used differential RNA-seq (dRNA-seq) in H. salinarum NRC-1 to locate intraRNA signals in subsets of internal transcription start sites (iTSS) and establish the open reading frames associated to them (intraORFs). Using C-terminally flagged proteins, we experimentally observed isoforms accurately predicted by intraRNA translation for kef1, acs3 and orc4 genes. We also recovered from the literature and mass spectrometry databases several instances of protein isoforms consistent with intraRNA translation such as the gas vesicle protein gene gvpC1. We found evidence for intraRNAs in horizontally transferred genes such as the chaperone dnaK and the aerobic respiration related cydA in both H. salinarum and Escherichia coli. Also, intraRNA translation evidence in H. salinarum, E. coli and yeast of a universal elongation factor (aEF-2, fusA and eEF-2) suggests that this is an ancient phenomenon present in all domains of life.

Characterization of the molecular chaperone ClpB from the pathogenic spirochaete Leptospira interrogans

Leptospira interrogans is a spirochaete responsible for leptospirosis in mammals. The molecular mechanisms of the Leptospira virulence remain mostly unknown. Recently, it has been demonstrated that an AAA+ chaperone ClpB (a member of the Hsp100 family) from L. interrogans (ClpBLi) is not only essential for survival of Leptospira under the thermal and oxidative stresses, but also during infection of a host. The aim of this study was to provide further insight into the role of ClpB in the pathogenic spirochaetes and explore its biochemical properties. We found that a non-hydrolysable ATP analogue, ATPγS, but not AMP-PNP induces the formation of ClpBLi hexamers and stabilizes the associated form of the chaperone. ADP also induces structural changes in ClpBLi and promotes its self-assembly, but does not produce full association into the hexamers. We also demonstrated that ClpBLi exhibits a weak ATPase activity that is stimulated by κ-casein and poly-lysine, and may mediate protein disaggregation independently from the DnaK chaperone system. Unexpectedly, the presence of E. coli DnaK/DnaJ/GrpE did not significantly affect the disaggregation activity of ClpBLi and ClpBLi did not substitute for the ClpBEc function in the clpB-null E. coli strain. This result underscores the species-specificity of the ClpB cooperation with the co-chaperones and is most likely due to a loss of interactions between the ClpBLi middle domain and the E. coli DnaK. We also found that ClpBLi interacts more efficiently with the aggregated G6PDH in the presence of ATPγS rather than ATP. Our results indicate that ClpB's importance during infection might be due to its role as a molecular chaperone involved in reactivation of protein aggregates.

Cloning and expression analysis of HSP70 gene from mangrove plant Kandelia obovata under cold stress

Heat shock protein 70 (HSP70), the primary member of the HSPs that play various stress-protective roles in plants. In this study, a hsp70 gene of Kandelia obovata (KoHsp70) was cloned by rapid amplification of cDNA ends (RACE). The full-length of KoHsp70 was 2255 bp, consisting of a 5'-terminal untranslated region (UTR) of 118 bp, a 3'-terminal UTR of 178 bp, and an open reading frame (ORF) of 1959 bp. The ORF (KoHSP70) was predicted to encode a polypeptide of 652 amino acids with a theoretical molecular weight (MW) of 71.40 kDa and a pI of 5.16. The amino acid sequence analysis revealed that the KoHSP70 contained three conserved regions of HSP70 family, a bipartite nuclear localization signal sequences (NLS), an ATP/GTP-binding site motif and a cytoplasmic characteristic motif (EEVD). Homology analysis indicated that KoHSP70 shared 96.0 % identity with the known HSP70 (Gossypium hirsutum). Bioinformatics analysis indicated that the KoHSP70 was hydrophilic and had no signal peptide or transmembrane region. The mRNA expression of KoHsp70 kept relatively stable at first and then increased significantly after 48 h cold stress, and reached the highest level at 168 h after cold treatment. The results indicated that the KoHsp70 was a stress-inducible gene that might play a role in cold stress-protective response and in coping with environmental and biological stresses for K. obovata. This study provided a basis to further study the mechanism of anti-adverseness and expression characteristics under stress conditions of K. obovata.

In vitro characterization of bacterial and chloroplast Hsp70 systems reveals an evolutionary optimization of the co-chaperones for their Hsp70 partner

The chloroplast Hsp70 (heat-shock protein of 70 kDa) system involved in protein folding in Chlamydomonas reinhardtii consists of HSP70B, the DnaJ homologue CDJ1 and the GrpE-type nucleotide-exchange factor CGE1. The finding that HSP70B needs to be co-expressed with HEP2 (Hsp70 escort protein 2) to become functional allowed the reconstitution of the chloroplast Hsp70 system in vitro and comparison with the homologous Escherichia coli system. Both systems support luciferase refolding and display ATPase and holdase activities. Steady-state activities are low and strongly stimulated by the co-chaperones, whose concentrations need to be balanced to optimally support luciferase refolding. Although the co-chaperones of either system generally stimulate ATPase and folding-assistance activities of the other, luciferase refolding is reduced ~10-fold and <2-fold if either Hsp70 is supplemented with the foreign DnaJ and GrpE protein respectively, suggesting an evolutionary specialization of the co-chaperones for their Hsp70 partner. Distinct features are that HSP70B's steady-state ATPase exhibits ~20-fold higher values for Vmax and Km and that the HSP70B system displays a ~6-fold higher folding assistance on denatured luciferase. Although truncating up to 16 N-terminal amino acids of CGE1 does not affect HSP70B's general ATPase and folding-assistance activities in the physiological temperature range, further deletions hampering dimerization of CGE1 via its N-terminal coiled coil do.

Multiple hsp70 isoforms in the eukaryotic cytosol: mere redundancy or functional specificity?

Hsp70 molecular chaperones play a variety of functions in every organism, cell type and organelle, and their activities have been implicated in a number of human pathologies, ranging from cancer to neurodegenerative diseases. The functions, regulations and structure of Hsp70s were intensively studied for about three decades, yet much still remains to be learned about these essential folding enzymes. Genome sequencing efforts revealed that most genomes contain multiple members of the Hsp70 family, some of which co-exist in the same cellular compartment. For example, the human cytosol and nucleus contain six highly homologous Hsp70 proteins while the yeast Saccharomyces cerevisiae contains four canonical Hsp70s and three fungal-specific ribosome-associated and specialized Hsp70s. The reasons and significance of the requirement for multiple Hsp70s is still a subject of debate. It has been postulated for a long time that these Hsp70 isoforms are functionally redundant and differ only by their spatio-temporal expression patterns. However, several studies in yeast and higher eukaryotic organisms challenged this widely accepted idea by demonstrating functional specificity among Hsp70 isoforms. Another element of complexity is brought about by specific cofactors, such as Hsp40s or nucleotide exchange factors that modulate the activity of Hsp70s and their binding to client proteins. Hence, a dynamic network of chaperone/co-chaperone interactions has evolved in each organism to efficiently take advantage of the multiple cellular roles Hsp70s can play. We summarize here our current knowledge of the functions and regulations of these molecular chaperones, and shed light on the known functional specificities among isoforms.

Residues Leu52 and Leu134 are important for the structural integrity of a nucleotide exchange factor GrpE from Bacillus licheniformis

A DNA fragment encoding Bacillus licheniformis GrpE (BlGrpE) with double mutations at codons 52 and 134 was obtained during PCR cloning. Leu52 and Leu134 in BlGrpE were individually replaced with Pro and His to generate BlGrpE-L52P and BlGrpE-L134H. BlGrpE and BlGrpE-L52P synergistically stimulated the ATPase activity of B. licheniformis DnaK (BlDnaK); however, BlGrpE-L134H and the double-mutated protein (BlGrpE-L52P/L134H) had no co-chaperone function. BlGrpE, BlGrpE-L52P, and BlGrpE-L134H mainly interacted with the monomer of BlDnaK but non-specific interaction was observed for BlGrpE-L52P/L134H. Measurement of intrinsic fluorescence revealed a significant alteration of the microenvironment of aromatic acid residues in the mutant proteins. As compared with BlGrpE, quenching of 208-nm and 222-nm signals were observed in the mutant BlGrpEs and the single-mutated proteins were more sensitive to thermal denaturation.

Walker-A threonine couples nucleotide occupancy with the chaperone activity of the AAA+ ATPase ClpB

Hexameric AAA+ ATPases induce conformational changes in a variety of macromolecules. AAA+ structures contain the nucleotide-binding P-loop with the Walker A sequence motif: GxxGxGK(T/S). A subfamily of AAA+ sequences contains Asn in the Walker A motif instead of Thr or Ser. This noncanonical subfamily includes torsinA, an ER protein linked to human dystonia and DnaC, a bacterial helicase loader. Role of the noncanonical Walker A motif in the functionality of AAA+ ATPases has not been explored yet. To determine functional effects of introduction of Asn into the Walker A sequence, we replaced the Walker-A Thr with Asn in ClpB, a bacterial AAA+ chaperone which reactivates aggregated proteins. We found that the T-to-N mutation in Walker A partially inhibited the ATPase activity of ClpB, but did not affect the ClpB capability to associate into hexamers. Interestingly, the noncanonical Walker A sequence in ClpB induced preferential binding of ADP vs. ATP and uncoupled the linkage between the ATP-bound conformation and the high-affinity binding to protein aggregates. As a consequence, ClpB with the noncanonical Walker A sequence showed a low chaperone activity in vitro and in vivo. Our results demonstrate a novel role of the Walker-A Thr in sensing the nucleotide's gamma-phosphate and in maintaining an allosteric linkage between the P-loop and the aggregate binding site of ClpB. We postulate that AAA+ ATPases with the noncanonical Walker A might utilize distinct mechanisms to couple the ATPase cycle with their substrate-remodeling activity.

Cloning and expression of heat shock protein 70 gene in the haemocytes of pearl oyster (Pinctada fucata, Gould 1850) responding to bacterial challenge

The cDNA of pearl oyster Pinctada fucata Hsp70 (designated PFHsp70) was cloned by EST and rapid amplification of cDNA ends (RACE) techniques. The full length of PFHsp70 cDNA was 2376 bp, consisting of a 5'-terminal untranslated region (UTR) of 89 bp, a 3' terminal UTR of 328 bp, and an open reading frame (ORF) of 1959 bp encoding a polypeptide of 652 amino acids with a theoretical molecular weight of 71.42 kDa and an estimated isoelectric point of 5.18. BLAST analysis revealed that the PFHsp70 gene shared high similarity with other Hsp70 genes. PFHsp70 contained all the three classical Hsp70 family signatures. The results indicated that the PFHsp70 was a member of the heat shock protein 70 family. Fluorescent real-time quantitative RT-PCR was used to examine the expression of PFHsp70 gene in haemocytes of P. fucata after the challenge of bacteria Vibrio alginolyticus. There was a clear time-dependent expression pattern of PFHsp70 after bacterial challenge, and the mRNA expression reached a maximum level at 4 h post-challenge, which returned to control level after 32 h. The up-regulated mRNA expression of PFHsp70 in P. fucata after bacterial challenge indicates that the Hsp70 gene is inducible and involved in immune response.

Functional characterization of the DnaK chaperone system from the archaeon Methanothermobacter thermautotrophicus DeltaH

We characterized the biochemical and functional properties of the DnaK system from the archaeon Methanothermobacter thermautotrophicus DeltaH. In contrast to the eubacterial chaperone components the archaeal Hsp70 system shows thermal transitions only slightly above the optimal environmental temperature (65 degrees C). Nevertheless, it prevents aggregation of luciferase in the physiological temperature range of the organism, but is also fully functional at 30 degrees C in luciferase refolding. Additionally, GrpE(M.th.) and DnaJ(M.th.) substitute their eubacterial counterparts whereas DnaK(M.th.) is only functional with its native cochaperones which could be attributed to a functional specialization of the eubacterial chaperones during evolution.

Cloning and characterization of a haloarchaeal heat shock protein 70 functionally expressed in Escherichia coli

The Hsp70 molecular chaperone machine is constituted by the 70-kDa heat shock protein Hsp70 (DnaK), cochaperone protein Hsp40 (DnaJ) and a nucleotide-exchange factor GrpE. Although it is one of the best-characterized molecular chaperone machines, little is known about it in archaea. A 5.2-kb region containing the hsp70 (dnaK) gene was cloned from Natrinema sp. J7 strain and sequenced. It contained the Hsp70 chaperone machine gene locus arranged unidirectionally in the order of grpE, hsp70 and hsp40 (dnaJ). The hsp70 gene from Natrinema sp. J7 was overexpressed in Escherichia coli BL21 (DE3). The recombinant Hsp70 protein was in a soluble and active form, and its ATPase activity was optimally active in 2.0 M KCl, whereas NaCl had less effect. In vivo, the haloarchaeal hsp70 gene allowed an E. coli dnak-null mutant to propagate lambda phages and grow at 42 degrees C. The results suggested that haloarchaeal Hsp70 should be beneficial for extreme halophiles survival in low-salt environments.

Evolution of a Protein-Folding Machine: Genomic and Evolutionary Analyses Reveal Three Lineages of the Archaeal hsp70(dnaK) Gene

The stress chaperone protein Hsp70 (DnaK) (abbreviated DnaK) and its co-chaperones Hsp40(DnaJ) (or DnaJ) and GrpE are universal in bacteria and eukaryotes but occur only in some archaea clustered in the order 5'-grpE-dnaK-dnaJ-3' in a locus termed Locus I. Three structural varieties of Locus I, termed Types I, II, and III, were identified, respectively, in Methanosarcinales, in Thermoplasmatales and Methanothermobacter thermoautotrophicus, and in Halobacteriales. These Locus I types corresponded to three groups identified by phylogenetic trees of archaeal DnaK proteins including the same archaeal subdivisions. These archaeal DnaK groups were not significantly interrelated, clustering instead with DnaKs from three bacterial lineages, Methanosarcinales with Firmicutes, Thermoplasmatales and M. thermoautotrophicus with Thermotoga, and Halobacteriales with Actinobacteria, suggesting that the three archaeal types of Locus I were acquired by independent events of lateral gene transfer. These associations, however, lacked strong bootstrap support and were sensitive to dataset choice and tree-reconstruction method. Structural features of dnaK loci in bacteria revealed that Methanosarcinales and Firmicutes shared a similar structure, also common to most other bacterial groups. Structural differences were observed instead in Thermotoga compared to Thermoplasmatales and M. thermoautotrophicus, and in Actinobacteria compared to Halobacteriales. It was also found that the association between the DnaK sequences from Halobacteriales and Actinobacteria likely reflects common biases in their amino acid compositions. Although the loci structural features and the DnaK trees suggested the possibility of lateral gene transfer between Firmicutes and Methanosarcinales, the similarity between the archaeal and the ancestral bacterial loci favors the more parsimonious hypothesis that all archaeal sequences originated from a unique prokaryotic ancestor.

Low temperature-induced systems failure inEscherichia coli: Insights from rescue by cold-adapted chaperones

The growth of Escherichia coli cells is impaired at temperatures below 21 degrees C and stops at 7.5 degrees C; however, growth of a transgenic strain producing the cold-adapted chaperones Cpn60 and Cpn10 from the psychrophilic bacterium Oleispira antarctica is good at low temperatures. The E. coli cpn(+) transgene offers a novel opportunity for examining the essential protein for cell viability at low temperatures. By screening a large-scale protein map (proteome) of cells of K-12 and its Cpn(+) transgene incubated at 4 degrees C, we identified 22 housekeeping proteins involved in systems failure of E. coli when confronted with low temperature. Through co-immunoprecipitation of Cpn60, Northern blot, and in vitro refolding, we systematically identified that protein-chaperone interactions are key determinants of their protein functions at low temperatures. Furthermore, chromosomal gene deletion experiments suggest that the mechanism of cold-induced systems failure in E. coli is cold-induced inactivation of the GroELS chaperonins and the resulting failure to refold cold-inactivated Dps, ClpB, DnaK and RpsB proteins. These findings: (1) indicate the potential importance of chaperones in cold sensitivity, cold adaptation and cold tolerance in cellular systems, and (2) suggest the identity of a few key cold-sensitive chaperone-interacting proteins that get inactivated and ultimately cause systems failure in E. coli cells at low temperatures.

Interactions within the ClpB/DnaK bi-chaperone system from Escherichia coli

ClpB and DnaK form a bi-chaperone system that reactivates strongly aggregated proteins in vivo and in vitro. Previously observed interaction between purified ClpB and DnaK suggested that one of the chaperones might recruit its partner during substrate reactivation. We show that ClpB from Escherichia coli binds at the substrate binding site of DnaK and the interaction is supported by the N-terminal domain and the middle domain of ClpB. Moreover, the interaction between ClpB and DnaK depends on the nucleotide-state of DnaK: it is stimulated by ADP and inhibited by ATP. These observations indicate that DnaK recognizes selected structural motifs in ClpB as "pseudo-substrates" and that ClpB may compete with bona fide substrates of DnaK. We conclude that direct interaction between ClpB and DnaK does not mediate a substrate transfer between the chaperones, it may, however, play a role in the recruitment of the bi-chaperone system to specific recognition sites in aggregated particles.

Approaches to the isolation and characterization of molecular chaperones

Molecular chaperones are integral components of the cellular machinery involved in ensuring correct protein folding and the continued maintenance of protein structure. An understanding of these ubiquitous molecules is key to finding cures to protein misfolding diseases such as Alzheimer's and Creutzfeldt-Jacob diseases. In addition, further understanding of chaperones will enhance our comprehension of the way the body copes with the environmental stresses that humans encounter daily. Our laboratory and our collaborators specialize in the production and characterization of chaperones from a wide variety of sources in order to gain a fuller understanding of how chaperones function in the cell. In this review, we primarily use the Hsp70/Hsp40 chaperone pair as an example to discuss recent advances in technology and reductions in cost that lend themselves to chaperone purification from both native and recombinant sources. Common assays to assess purified chaperone activity are also discussed.

The amino-terminal domain of ClpB supports binding to strongly aggregated proteins

Bacterial heat-shock proteins, ClpB and DnaK form a bichaperone system that efficiently reactivates aggregated proteins. ClpB undergoes nucleotide-dependent self-association and forms ring-shaped oligomers. The ClpB-assisted dissociation of protein aggregates is linked to translocation of substrates through the central channel in the oligomeric ClpB. Events preceding the translocation step, such as recognition of aggregates by ClpB, have not yet been explored, and the location of the aggregate-binding site in ClpB has been under discussion. We investigated the reactivation of aggregated glucose-6-phosphate dehydrogenase (G6PDH) by ClpB and its N-terminally truncated variant ClpBDeltaN in the presence of DnaK, DnaJ, and GrpE. We found that the chaperone activity of ClpBDeltaN becomes significantly lower than that of the full-length ClpB as the size of G6PDH aggregates increases. Using a "substrate trap" variant of ClpB with mutations of Walker B motifs in both ATP-binding modules (E279Q/E678Q), we demonstrated that ClpBDeltaN binds to G6PDH aggregates with a significantly lower affinity than the full-length ClpB. Moreover, we identified two conserved acidic residues at the surface of the N-terminal domain of ClpB that support binding to G6PDH aggregates. Those N-terminal residues (Asp-103, Glu-109) contribute as much substrate-binding capability to ClpB as the conserved Tyr located at the entrance to the ClpB channel. In summary, we provided evidence for an essential role of the N-terminal domain of ClpB in recognition and binding strongly aggregated proteins.

Complementation studies of the DnaK-DnaJ-GrpE chaperone machineries from Vibrio harveyi and Escherichia coli, both in vivo and in vitro

The marine bacterium Vibrio harveyi is a potential indicator organism for evaluating marine environmental pollution. The DnaK-DnaJ-GrpE chaperone machinery of V. harveyi has been studied as a model of response to stress conditions and compared to the Escherichia coli DnaK system. The genes encoding DnaK, DnaJ and GrpE of V. harveyi were cloned into expression vectors and grpE was sequenced. It was found that V. harveyi possesses a unique organization of the hsp gene cluster (grpE-gltP-dnaK-dnaJ), which is present exclusively in marine Vibrio species. In vivo experiments showed that suppression of the E. coli dnaK mutation by V. harveyi DnaK protein was weak or absent, while suppression of the dnaJ and grpE mutations by V. harveyi DnaJ and GrpE proteins was efficient. These results suggest higher species-specificity of the DnaK chaperone than the GrpE and DnaJ cochaperones. Proteins of the DnaK chaperone machinery of V. harveyi were purified to homogeneity and their efficient cooperation with the E. coli chaperones in the luciferase refolding reaction and in stimulation of DnaK ATPase activity was demonstrated. Compared to the E. coli system, the purified DnaK-DnaJ-GrpE system of V. harveyi exhibited about 20% lower chaperoning activity in the luciferase reactivation assay. ATPase activity of V. harveyi DnaK protein was at least twofold higher than that of the E. coli model DnaK but its stimulation by the cochaperones DnaJ and GrpE was significantly (10 times) weaker. These results indicate that, despite their high structural identity (approximately 80%) and similar mechanisms of action, the DnaK chaperones of closely related V. harveyi and E.coli bacteria differ functionally.