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Dickeya solani is a pectinolytic bacterium that causes significant losses of potato crops. Interaction between bacteria and plant during infection and transmission within the host tissue are associated with a variety of challenges, including exposure to physico-chemical factors that induce osmotic, acidic, oxidative or thermal stresses. In this work, we tested the effects of various potentially adverse conditions on growth and fitness of the D. solani IPO2222 type strain. We found the bacteria were able to withstand a great variety of stressful conditions. Treatment with high concentrations of osmolytes (up to 1.6 Osm), low pH (5.0), or elevated temperature (up to 40 °C) was still tolerated. Short term exposure to these conditions did not impair the ability of bacteria to macerate potato tuber tissue. However, D. solani was sensitive to a relatively low content (above 0.5 mM) of the oxidant H2O2 and treatment with this oxidant caused a loss of culturability. All used stressful conditions are known to affect the stability, and structure of proteins and induction of the folding stress-related genes, dnaK, dnaJ and groEL, was observed in most cases. The only exception was oxidative stress for which the chaperone genes were slightly down-regulated. This observation is in line with a low tolerance of D. solani to H2O2.

Background Serine protease HtrA exhibits both proteolytic and chaperone activities, which are involved in cellular protein quality control. Moreover, HtrA is an important virulence factor in many pathogens including Helicobacter pylori, for which the crucial stage of infection is the cleavage of E-cadherin and other cell-to-cell junction proteins. Methods The in vitro study of H. pylori HtrA (HtrAHp) chaperone activity was carried out using light scattering assays and investigation of lysozyme protein aggregates. We produced H. pylori ∆htrA deletion and HtrAHp point mutants without proteolytic activity in strain N6 and investigated the survival of the bacteria under thermal, osmotic, acidic and general stress conditions as well as the presence of puromycin or metronidazole using serial dilution tests and disk diffusion method. The levels of cellular and secreted proteins were examined using biochemical fraction and Western blotting. We also studied the proteolytic activity of secreted HtrAHp using zymography and the enzymatic digestion of β-casein. Finally, the consequences of E-cadherin cleavage were determined by immunofluorescence microscopy. Results We demonstrate that HtrAHp displays chaperone activity that inhibits the aggregation of lysozyme and is stable under various pH and temperature conditions. Next, we could show that N6 expressing only HtrA chaperone activity grow well under thermal, pH and osmotic stress conditions, and in the presence of puromycin or metronidazole. In contrast, in the absence of the entire htrA gene the bacterium was more sensitive to a number of stresses. Analysing the level of cellular and secreted proteins, we noted that H. pylori lacking the proteolytic activity of HtrA display reduced levels of secreted HtrA. Moreover, we compared the amounts of secreted HtrA from several clinical H. pylori strains and digestion of β-casein. We also demonstrated a significant effect of the HtrAHp variants during infection of human epithelial cells and for E-cadherin cleavage. Conclusion Here we identified the chaperone activity of the HtrAHp protein and have proven that this activity is important and sufficient for the survival of H. pylori under multiple stress conditions. We also pinpointed the importance of HtrAHp chaperone activity for E- cadherin degradation and therefore for the virulence of this eminent pathogen.

Most bacterial secretory proteins destined to the extracytoplasmic space are secreted posttranslationally by the Sec translocase. SecA, a key component of the Sec system, is the ATPase motor protein, directly responsible for transferring the preprotein across the cytoplasmic membrane. SecA is a large protein, composed of several domains, capable of binding client preproteins and a variety of partners, including the SecYEG inner membrane channel complex, membrane phospholipids and ribosomes. SecA-mediated translocation can be divided into two major steps: (1) targeting of the preproteins to the membrane translocation apparatus and (2) transport across the membrane through the SecYEG channel. In this review we present current knowledge regarding SecA structure and function of this protein in both translocation steps. The most recent model of the SecA-dependent preprotein mechanical translocation across the bacterial cytoplasmic membrane is described. A possibility of targeting SecA with inhibitory compounds as a strategy to combat pathogenic bacteria will be discussed as well.

Campylobacter jejuniis a predominant zoonotic pathogen causing gastroenteritis and other diseases in humans. An important bacterial virulence factor is the secreted serine protease HtrA (HtrA(Cj)), which targets tight and adherens junctional proteins in the gut epithelium. Here we have investigated the function and structure of HtrA(Cj)using biochemical assays and cryo-electron microscopy. Mass spectrometry analysis identified differences and similarities in the cleavage site specificity for HtrA(Cj)by comparison to the HtrA counterparts fromHelicobacter pyloriandEscherichia coli. We defined the architecture of HtrA(Cj)at 5.8 angstrom resolution as a dodecamer, built of four trimers. The contacts between the trimers are quite loose, a fact that explains the flexibility and mobility of the dodecameric assembly. This flexibility has also been studied through molecular dynamics simulation, which revealed opening of the dodecamer to expose the proteolytically active site of the protease. Moreover, we examined the rearrangements at the level of oligomerization in the presence or absence of substrate using size exclusion chromatography, which revealed hexamers, dodecamers and larger oligomeric forms, as well as remarkable stability of higher oligomeric forms (> 12-mers) compared to previously tested homologs from other bacteria. Extremely dynamic decay of the higher oligomeric forms into lower forms was observed after full cleavage of the substrate by the proteolytically active variant of HtrA(Cj). Together, this is the first report on the in-depth functional and structural analysis of HtrA(Cj), which may allow the construction of therapeutically relevant HtrA(Cj)inhibitors in the near future.

The Lon protein is a protease implicated in the virulence of many pathogenic bacteria, including some plant pathogens. However, little is known about the role of Lon in bacteria from genus Dickeya. This group of bacteria includes important potato pathogens, with the most aggressive species, D. solani. To determine the importance of Lon for pathogenicity and response to stress conditions of bacteria, we constructed a D. solani Δlon strain. The mutant bacteria showed increased sensitivity to certain stress conditions, in particular osmotic and high-temperature stresses. Furthermore, qPCR analysis showed an increased expression of the lon gene in D. solani under these conditions. The deletion of the lon gene resulted in decreased motility, lower activity of secreted pectinolytic enzymes and finally delayed onset of blackleg symptoms in the potato plants. In the Δlon cells, the altered levels of several proteins, including virulence factors and proteins associated with virulence, were detected by means of Sequential Window Acquisition of All Theoretical Mass Spectra (SWATH-MS) analysis. These included components of the type III secretion system and proteins involved in bacterial motility. Our results indicate that Lon protease is important for D. solani to withstand stressful conditions and effectively invade the potato plant.

Helicobacter pylori plays an essential role in the pathogenesis of gastritis, peptic ulcer disease, and gastric cancer. The serine protease HtrA, an important secreted virulence factor, disrupts the gastric epithelium, which enables H. pylori to transmigrate across the epithelium and inject the oncogenic CagA protein into host cells. The function of periplasmic HtrA for the H. pylori cell is unknown, mainly due to unavailability of the htrA mutants. In fact, htrA has been described as an essential gene in this bacterium. We have screened 100 worldwide H. pylori isolates and show that only in the N6 strain it was possible to delete htrA or mutate the htrA gene to produce proteolytically inactive HtrA. We have sequenced the wild-type and mutant chromosomes and we found that inactivation of htrA is associated with mutations in SecA – a component of the Sec translocon apparatus used to translocate proteins from the cytoplasm into the periplasm. The cooperation of SecA and HtrA has been already suggested in Streptococcus pneumonia, in which these two proteins co-localize. Hence, our results pinpointing a potential functional relationship between HtrA and the Sec translocon in H. pylori possibly indicate for the more general mechanism responsible to maintain bacterial periplasmic homeostasis.

A pathogenic lifestyle is inextricably linked with the constant necessity of facing various challenges exerted by the external environment (both within and outside the host). To successfully colonize the host and establish infection, pathogens have evolved sophisticated systems to combat the host defense mechanisms and also to be able to withstand adverse environmental conditions. Proteases, as crucial components of these systems, are involved in a variety of processes associated with infection. In phytopathogenic bacteria, they play important regulatory roles and modulate the expression and functioning of various virulence factors. Secretory proteases directly help avoid recognition by the plant immune systems, and contribute to the deactivation of the defense response pathways. Finally, proteases are important components of protein quality control systems, and thus enable maintaining homeostasis in stressed bacterial cells. In this review, we discuss the known protease functions and protease-regulated signaling processes associated with virulence of plant pathogenic bacteria.

Background An increasing resistance of bacteria to the commonly used antimicrobials forces to search for alternative or supportive ways to cure infections. Targeting virulence factors is one of such approaches. The bacterial HtrA proteins are strongly involved in virulence and the lack of functional HtrA in many cases impairs invasiveness of pathogens. HtrAs act by protecting the cells under stressful conditions as well as they take direct part in invasion of the host. The latter function is played predominantly by the recently identified extracellular fraction of HtrA. This review aims to evaluate HtrAs as therapeutic targets, including design of chemical inhibitors and vaccines. Methods We undertook a thorough search of bibliographic databases for peer-reviewed scientific literature. Results One hundred and sixty-four papers were included in the review. First, we briefly summarized key structural and functional properties of known HtrA proteins with the special focus on the extracellular HtrA fraction. Then we provided an overview of efforts and advancements to target HtrAs of pathogenic bacteria as a promising antimicrobial therapy. In some cases, encouraging results were obtained and application of HtrAspecific inhibitors protected tissues from damage and killed bacteria. Also promising reports concerning the use of HtrA as a protective antigen in several disease models have recently been published. Conclusion The findings of this review suggest that the exported HtrA proteins are very attractive therapeutic targets due to their accessibility, significance in virulence and immunogenicity. However, further extensive studies are still needed to develop a safe antimicrobial treatment.

The microaerophilic bacterium Campylobacter jejuni is the most common cause of bacterial food-borne infections in the developed world. Tolerance to environmental stress relies on proteases and chaperones in the cell envelope, such as HtrA and SurA. HtrA displays both chaperone and protease activities, but little is known about how each of these activities contributes to stress tolerance in bacteria. In vitro experiments showed temperature-dependent protease and chaperone activities of C. jejuni HtrA. A C. jejuni mutant lacking only the protease activity of HtrA was used to show that the HtrA chaperone activity is sufficient for growth at high temperature or under oxidative stress, whereas the HtrA protease activity is essential only under conditions close to the growth limit for C. jejuni . However, the protease activity was required to prevent induction of the cytoplasmic heat shock response even under optimal growth conditions. Interestingly, the requirement of HtrA at high temperatures was found to depend on the oxygen level, and our data suggest that HtrA may protect oxidatively damaged proteins. Finally, protease activity stimulates HtrA production and oligomer formation, suggesting that a regulatory role depends on the protease activity of HtrA. Studying a microaerophilic organism encoding only two known periplasmic chaperones (HtrA and SurA) revealed an efficient HtrA chaperone activity and proposed multiple roles of the protease activity, increasing our understanding of HtrA in bacterial physiology.

The HtrA family of chaperones and serine proteases is important for regulating stress responses and controlling protein quality in the periplasm of bacteria. HtrA is also associated with infectious diseases since inactivation of htrA genes results in significantly reduced virulence properties by various bacterial pathogens. These virulence features of HtrA can be attributed to reduced fitness of the bacteria, higher susceptibility to environmental stress and/or diminished secretion of virulence factors. In some Gram-negative and Gram-positive pathogens, HtrA itself can be exposed to the extracellular environment promoting bacterial colonisation and invasion of host tissues. Most of our knowledge on the function of exported HtrAs stems from research on Helicobacter pylori, Campylobacter jejuni, Borrelia burgdorferi, Bacillus anthracis, and Chlamydia species. Here, we discuss recent progress showing that extracellular HtrAs are able to cleave cell-to-cell junction factors including E-cadherin, occludin, and claudin-8, as well as extracellular matrix proteins such as fibronectin, aggrecan, and proteoglycans, disrupting the epithelial barrier and producing substantial host cell damage. We propose that the export of HtrAs is a newly discovered strategy, also applied by additional bacterial pathogens. Consequently, exported HtrA proteases represent highly attractive targets for antibacterial treatment by inhibiting their proteolytic activity or application in vaccine development.

The HtrA (DegP) protein from Escherichia coli is a periplasmic protease whose function is to protect cells from the deleterious effects of various stress conditions. At temperatures below 28 degrees C the proteolytic activity of HtrA was regarded as negligible and it was believed that the protein mainly plays the role of a chaperone. In the present work we provide evidence that HtrA can in fact act as a protease at low temperatures. Under folding stress, caused by disturbances in the disulfide bond formation, the lack of proteolytic activity of HtrA lowered the survival rates of mutant strains deprived of a functional DsbA/DsbB oxidoreductase system. HtrA degraded efficiently the unfolded, reduced alkaline phosphatase at 20 degrees C, both in vivo and in vitro. The cleavage was most efficient in the case of HtrA deprived of its internal S-S bond; therefore we expect that the reduction of HtrA may play a regulatory role in proteolysis.

Bacterial HtrAs are proteases engaged in extracytoplasmic activities during stressful conditions and pathogenesis. A model prokaryotic HtrA (HtrA/DegP from Escherichia coli) requires activation to cleave its substrates efficiently. In the inactive state of the enzyme, one of the regulatory loops, termed LA, forms inhibitory contacts in the area of the active center. Reduction of the disulfide bond located in the middle of LA stimulates HtrA activity in vivo suggesting that this S-S bond may play a regulatory role, although the mechanism of this stimulation is not known. Here, we show that HtrA lacking an S-S bridge cleaved a model peptide substrate more efficiently and exhibited a higher affinity for a protein substrate. An LA loop lacking the disulfide was more exposed to the solvent; hence, at least some of the interactions involving this loop must have been disturbed. The protein without S-S bonds demonstrated lower thermal stability and was more easily converted to a dodecameric active oligomeric form. Thus, the lack of the disulfide within LA affected the stability and the overall structure of the HtrA molecule. In this study, we have also demonstrated that in vitro human thioredoxin 1 is able to reduce HtrA; thus, reduction of HtrA can be performed enzymatically.

Bacterial HtrAs are serine proteases engaged in extracytoplasmic protein quality control and are required for the virulence of several pathogenic species. The proteolytic activity of HtrA (DegP) from Escherichia coli, a model prokaryotic HtrA, is stimulated by stressful conditions; the regulation of this process is mediated by the LA, LD, L1, L2, and L3 loops. The precise mechanism of action of the LA loop is not known due to a lack of data concerning its three-dimensional structure as well as its mode of interaction with other regulatory elements. To address these issues we generated a theoretical model of the three-dimensional structure of the LA loop as per the resting state of HtrA and subsequently verified its correctness experimentally. We identified intra- and intersubunit contacts that formed with the LA loops; these played an important role in maintaining HtrA in its inactive conformation. The most significant proved to be the hydrophobic interactions connecting the LA loops of the hexamer and polar contacts between the LA' (the LA loop on an opposite subunit) and L1 loops on opposite subunits. Disturbance of these interactions caused the stimulation of HtrA proteolytic activity. We also demonstrated that LA loops contribute to the preservation of the integrity of the HtrA oligomer and to the stability of the monomer. The model presented in this work explains the regulatory role of the LA loop well; it should also be applicable to numerous Enterobacteriaceae pathogenic species as the amino acid sequences of the members of this bacterial family are highly conserved.

The HtrA (DegP) protein of Escherichia coli is a heat shock serine protease, essential for cell survival only at temperatures above 42 degrees C. It has been shown by genetic experiments that HtrA is an envelope protease, functioning in the periplasmic space. To clarify the cellular localization of HtrA, E. coli cells were fractionated, and HtrA was not detected by the immunoblotting technique in the periplasm or in the fraction of soluble proteins but was found in the inner membrane. The protein could be partially eluted from the total membrane fraction by a high ionic strength solution, whereas solutions affecting protein conformation released HtrA almost completely. These results, taken together with the evidence showing that HtrA functions in the periplasm, indicate that HtrA is a peripheral membrane protein, localized on the periplasmic side of the inner membrane. As the first step toward solving the problem of HtrA-membrane interactions, the structure of HtrA in the presence of phosphatidylglycerol (PG), phosphatidylethanolamine (PE), or cardiolipin (CL) was analyzed by fluorescence and Fourier-transform infrared spectroscopy. The infrared and fluorescence data indicated an interaction of HtrA with PG and CL but not with PE suspensions. Fluorescence spectroscopy revealed that this interaction was at the level of the polar head group of the phospholipid. In the PG/HtrA system, small changes were observed in the HtrA secondary structure and a remarkable decrease of the thermal stability of the protein, which suggested changes in HtrA tertiary structure. This suggestion was supported by fluorescence data that showed a shift of the fluorescence emission spectrum of HtrA tyrosine residues in the presence of PG and a reduced fluorescence intensity, phenomena not observed in the presence of PE or CL suspensions. Infrared data revealed also that the interaction of HtrA with PG leads to a protection of unfolded protein against aggregation at relatively low temperatures. The conformational changes of HtrA in the presence of PG influenced the proteolytic activity of HtrA by increasing it at the temperatures 37-45 degrees C and inhibiting it at 50-55 degrees C. CL inhibited HtrA activity at all of the temperatures tested.

Thermally aggregated, endogenous proteins of Escherichia coli form a distinct fraction, denoted S, which is separable by sucrose-density-gradient centrifugation. It was shown earlier that DnaK, DnaJ, IbpA and IbpB heat-shock proteins are associated with the S fraction. Comparison of the rise and decay of the S fraction in mutants defective for heat-shock proteases Lon (La), Clp, HtrA (DegP, Do) and in wild-type strains made studies of proteolysis and the function of the heat-shock response possible in vivo. Different timing and the extent of action of particular proteases was revealed by the initial size and decay kinetics of the S fraction. The proteases Lon, Clp, and HtrA all participated in removal of the aggregated proteins. Mutation in the gene encoding ClpB caused the most prominent effect (47% stabilization of the S fraction). The correlation between the disappearance of the S fraction and proteolytic activity was supported by the result of the in vitro reaction. Approximately one third of the isolated S fraction was converted to trichloroacetic acid-soluble products by the purified HtrA protease. Mg2+ ions stimulated the reaction, in contrast to the reaction of the HtrA protease with casein. The digestion of the aggregated proteins, unlike the digestion of casein, by HtrA protease in vitro was inhibited by added DnaJ, which might reflect protection of the aggregated proteins in vivo by DnaJ from excessive degradation. One might expect that such an activity of DnaJ would promote denatured protein renaturation versus proteolysis. Moreover, among the aggregated proteins that are discernible by electrophoresis, none could be identified as being more susceptible than any other to HtrA degradation. The separation pattern of these proteins before and after the in vitro digestion did not show a difference corresponding to the loss of about 30% of constituting proteins. This was interpreted as recognition by the HtrA protease of a state of protein denaturation rather than specific amino acid sequences in particular proteins. We conclude that the fraction consisting of proteins heat-aggregated in vivo (i.e. the S fraction) contains endogenous substrates for the heat-shock proteases tested. Their use for in vitro reaction reveals information that is in some respects different from that obtained with exogenous substrates such as casein.

The HtrA(DegP) 48-kDa serine protease of Escherichia coli is indispensable for bacterial survival at elevated temperatures. It contains the amino-acid sequence Gly208AnsSerGlyGlyAlaLeu, which is similar to the consensus sequence GlyAspSerGlyGlyProLys surrounding the active Ser residue of trypsin-like proteases. Mutational alteration of Ser210 eliminated proteolytic activity of HtrA. An identical effect was observed when His105 was mutated. The mutated HtrA were unable to suppress thermosensitivity of the htrA bacteria. These results suggest that Ser210 and His105 may be important elements of the catalytic domain and indicate that the proteolytic activity of HtrA is essential for the survival of cells at elevated temperatures.

The HtrA protease of Escherichia coli, identical with the DegP protease, is a 48-kDa heat shock protein, indispensable for bacterial survival only at temperatures above 42 °C. Proteolytic activity of HtrA is inhibited by diisopropyl fluorophosphate, suggesting that HtrA is a serine protease. We have recently found that mutational alteration of serine in position 210 of the mature HtrA or of histidine in position 105 totally eliminated proteolytic activity of HtrA. However, little was known about the consequences of the mutations on HtrA conformation. In this work, Fourier transform infrared spectroscopy has been used to examine the conformation in aqueous solution of wild-type HtrA and mutant HtrAS210 and HtrAH105 proteins. The spectra were collected at different temperatures in order to gain information also on the thermal stability of the three proteins. The analysis of HtrA protein spectrum, by resolution-enhancement methods, revealed that β-sheet is the major structural element of the conformation of HtrA. Deconvoluted as well as second derivative spectra of wild-type HtrA and mutant HtrAS210 and HtrAH105 collected at 20 °C were identical, indicating no differences in the secondary structure of these proteins. The analysis of spectra obtained at different temperatures revealed a maximum of protein denaturation within 65–70 °C for wild-type HtrA as well as for the HtrAS210 and HtrAH105 mutant proteins. However, the thermal denaturation pattern of wild-type HtrA revealed a lower cooperativity in the denaturation process as compared to the mutant proteins which instead behaved similarly. These data suggest that the mutations in HtrA protein induced minor changes in the tertiary structure of the protein (most likely located at the mutation sites). Our results strongly support the idea that Ser210 and His105 may represent two elements of the active-site triad (Ser, His, and Asp), found in most serine proteases. We have also found that in vitro, in the range from 37 to 55 °C, the proteolytic activity of HtrA rapidly increased with temperature and that HtrA activity remained unchanged for at least 4 h at 45 °C.

Review on HTRA1 (HtrA serine peptidase 1), with data on DNA, on the protein encoded, and where the gene is implicated.

The aim of this study was to characterize the role of particular elements of the regulatory loop L2 in the activation process and maintaining the proteolytic activity of HtrA (DegP) from Escherichia coli . We measured the effects of various mutations introduced to the L2 loop’s region (residues 228–238) on the stability of HtrA molecule and its proteolytic activity. We demonstrated that most mutations affected the activity of HtrA. In the case of the following substitutions: L229N, N235I, I238N, the proteolytic activity was undetectable. Thus, the majority of interactions mediated by the studied amino-acid residues seem to play important role in maintaining the active conformation. Formation of contacts between the apical parts (residues 231–234) of the L2 loops within the HtrA trimer, in particular the residues D232, was shown to play a crucial role in the activation process of HtrA. Stabilization of these intermolecular interactions by substitution of D232 with valine caused a stimulation of proteolytic activity whereas deletion of this region abolished the activity. Since the pathogenic E. coli strains require active HtrA for virulence, the apical part of L2 is of particular interest in terms of structure-based drug design for treatment E. coli infections.

The protective action of chaperone-like activity of HtrA protease against protein aggregation was studied. High levels of proteolytically inactive HtrAS210A (active center serine replaced by alanine) suppressed the temperature-sensitive phenotype of the htrA mutants. The ability of HtrAS210A to alleviate the lethality of htrA bacteria at high temperatures correlated well with the observed decrease of cellular level of large protein aggregates in cells overproducing HtrAS210A. The in vitro experiments proved that HtrA was very efficient in inhibiting the unfolded substrate (lysozyme) aggregation over a wide range of temperatures (30-45 degrees C). HtrA was able to bind to the denatured polypeptides and as a consequence limited their ability to form large aggregates. Our results suggest that HtrA may protect the bacterial cells from deleterious effects of heat shock not only by degrading the damaged proteins but by combination of the proteolytic and chaperoning activities.

HtrA heat shock protease is highly conserved in evolution, and in Escherichia coli, it protects the cell by degradation of proteins denatured by heat and oxidative stress, and also degrades misfolded proteins with reduced disulfide bonds. The mature, 48-kDa HtrA undergoes partial autocleavage with formation of two approximately 43 kDa truncated polypeptides. We showed that under reducing conditions, the HtrA level in cells was increased and efficient autocleavage occurred, while heat shock and oxidative shock caused the increase of HtrA level, but not the autocleavage. Purified HtrA cleaved itself during proteolysis of substrates but only under reducing conditions. These results indicate that the autocleavage is triggered specifically by proteolysis under reducing conditions, and is a physiological process occurring in cells. Conformations of reduced and oxidized forms of HtrA differed as judged by SDS-PAGE, indicating presence of a disulfide bridge in native protein. HtrA mutant protein lacking Cys57 and Cys69 was autocleaved even without the reducing agents, which indicates that the cysteines present in the N-terminal region are necessary for stabilization of HtrA peptide. Autocleavage caused the native, hexameric HtrA molecules dissociate into monomers that were still proteolytically active. This shows that the N-terminal part of HtrA is essential for maintaining quaternary structure of HtrA.

Hsp70 (DnaK) is a highly conserved molecular chaperone present in bacteria, eukaryotes, and some archaea. In a previous work we demonstrated that DnaK from the archaeon Methanosarcina mazei (DnaK(Mm)) and the DnaK from the bacterium Escherichia coli (DnaK(Ec)) were functionally similar when assayed in vitro but DnaK(Mm) failed to substitute for DnaK(Ec) in vivo. Searching for the molecular basis of the observed DnaK species specificity we compared substrate binding by DnaK(Mm) and DnaK(Ec). DnaK(Mm) showed a lower affinity for the model peptide (a-CALLQSRLLS) compared to DnaK(Ec). Furthermore, it was unable to negatively regulate the E. coli sigma32 transcription factor level under heat shock conditions and poorly bound purified sigma32, which is a native substrate of DnaK(Ec). These observations taken together indicate differences in substrate specificity of archaeal and bacterial DnaKs. Structural modeling of DnaK(Mm) showed some structural differences in the substrate-binding domains of DnaK(Mm) and DnaK(Ec), which may be responsible, at least partially, for the differences in peptide binding. Size-exclusion chromatography and native gel electrophoresis revealed that DnaK(Mm) was found preferably in high molecular mass oligomeric forms, contrary to DnaK(Ec). Oligomers of DnaK(Mm) could be dissociated in the presence of ATP and a substrate (peptide) but not ADP, which may suggest that monomer is the active form of DnaK(Mm).

Hsp70s are chaperone proteins that are conserved in evolution and present in all prokaryotic and eukaryotic organisms. In the archaea, which form a distinct kingdom, the Hsp70 chaperones have been found in some species only, including Methanosarcina mazei. Both the bacterial and archaeal Hsp70(DnaK) chaperones cooperate with a GrpE co-chaperone which stimulates the ATPase activity of the DnaK protein. It is currently believed that the archaeal Hsp70 system was obtained by the lateral transfer of chaperone genes from bacteria. Our previous finding that the DnaK and GrpE proteins of M. mazei can functionally cooperate with the Escherichia coli GrpE and DnaK supported this hypothesis. However, the cooperation was surprising, considering the very low identity of the GrpE proteins (26%) and the relatively low identity of the DnaK proteins (56%). The aim of this work was to investigate the molecular basis of the observed interspecies chaperone interaction. Infrared resolution-enhanced spectra of the M. mazei and E. coli DnaK proteins were almost identical, indicating high similarity of their secondary structures, however, some small differences in band position and in the intensity of amide I' band components were observed and discussed. Profiles of thermal denaturation of both proteins were similar, although they indicated a higher thermostability of the M. mazei DnaK compared to the E. coli DnaK. Electrophoresis under non-denaturing conditions demonstrated that purified DnaK and GrpE of E. coli and M. mazei formed mixed complexes. Protein modeling revealed high similarity of the 3-dimensional structures of the archaeal and bacterial DnaK and GrpE proteins.