Functional Analysis of CbpA, a DnaJ Homolog and Nucleoid-associated DNA-binding Protein

J-domain Proteins form Binary Complexes with Hsp90 and Ternary Complexes with Hsp90 and Hsp70

Hsp90 and Hsp70 are highly conserved molecular chaperones that help maintain proteostasis by participating in protein folding, unfolding, remodeling and activation of proteins. Both chaperones are also important for cellular recovery following environmental stresses. Hsp90 and Hsp70 function collaboratively for the remodeling and activation of some client proteins. Previous studies using E. coli and S. cerevisiae showed that residues in the Hsp90 middle domain directly interact with a region in the Hsp70 nucleotide binding domain, in the same region known to bind J-domain proteins. Importantly, J-domain proteins facilitate and stabilize the interaction between Hsp90 and Hsp70 both in E. coli and S. cerevisiae. To further explore the role of J-domain proteins in protein reactivation, we tested the hypothesis that J-domain proteins participate in the collaboration between Hsp90 and Hsp70 by simultaneously interacting with Hsp90 and Hsp70. Using E. coli Hsp90, Hsp70 (DnaK), and a J-domain protein (CbpA), we detected a ternary complex containing all three proteins. The interaction involved the J-domain of CbpA, the DnaK binding region of E. coli Hsp90, and the J-domain protein binding region of DnaK where Hsp90 also binds. Additionally, results show that E. coli Hsp90 interacts with E. coli J-domain proteins, DnaJ and CbpA, and that yeast Hsp90, Hsp82, interacts with a yeast J-domain protein, Ydj1. Together these results suggest that the complexes may be transient intermediates in the pathway of collaborative protein remodeling by Hsp90 and Hsp70.

Enhancement of acid tolerance of Escherichia coli by introduction of molecule chaperone CbpA from extremophile

Molecular chaperone CbpA from extreme acidophile Acidithiobacillus caldus was applied to improve acid tolerance of Escherichia coli via CRISPR/Cas9. Cell growth and viability of plasmid complementary strain indicated the importance of cbpA^Ac for bacteria acid tolerance. With in situ gene replacement by CRISPR/Cas9 system, colony formation unit (CFU) of genome recombinant strain BL21-ΔcbpA/AccbpA showed 7.7 times higher cell viability than deficient strain BL21-ΔcbpA and 2.3 times higher than wild type. Cell morphology observation using Field Emission Scanning Electron Microscopy (FESEM) revealed cell breakage of BL21-ΔcbpA and significant recovery of BL21-ΔcbpA/AccbpA. The intracellular ATP level of all strains gradually decreased along with the increased stress time. Particularly, the value of recombinant strain was 56.0% lower than that of deficient strain after 5 h, indicating that the recombinant strain consumed a lot of energy to resist acid stress. The arginine concentration in BL21-ΔcbpA/AccbpA was double that of BL21-ΔcbpA, while the aspartate and glutamate contents were 14.8% and 6.2% higher, respectively, compared to that of wild type. Moreover, RNA-Seq analysis examined 93 genes down-regulated in BL21-ΔcbpA compared to wild type strain, while 123 genes were up-regulated in BL21-ΔcbpA/AccbpA compared to BL21-ΔcbpA, with an emphasis on energy metabolism, transport, and cell components. Finally, the working model in response to acid stress of cbpA from A. caldus was developed. This study constructed a recombinant strain resistant to acid stress and also provided a reference for enhancing microorganisms' robustness to various conditions.

J-domain protein chaperone circuits in proteostasis and disease

The J-domain proteins (JDP) form the largest protein family among cellular chaperones. In cooperation with the Hsp70 chaperone system, these co-chaperones orchestrate a plethora of distinct functions, including those that help maintain cellular proteostasis and development. JDPs evolved largely through the fusion of a J-domain with other protein subdomains. The highly conserved J-domain facilitates the binding and activation of Hsp70s. How JDPs (re)wire Hsp70 chaperone circuits and promote functional diversity remains insufficiently explained. Here, we discuss recent advances in our understanding of the JDP family with a focus on the regulation built around J-domains to ensure correct pairing and assembly of JDP-Hsp70 machineries that operate on different clientele under various cellular growth conditions.

Small Prokaryotic DNA-Binding Proteins Protect Genome Integrity throughout the Life Cycle

Genomes of all organisms are persistently threatened by endogenous and exogenous assaults. Bacterial mechanisms of genome maintenance must provide protection throughout the physiologically distinct phases of the life cycle. Spore-forming bacteria must also maintain genome integrity within the dormant endospore. The nucleoid-associated proteins (NAPs) influence nucleoid organization and may alter DNA topology to protect DNA or to alter gene expression patterns. NAPs are characteristically multifunctional; nevertheless, Dps, HU and CbpA are most strongly associated with DNA protection. Archaea display great variety in genome organization and many inhabit extreme environments. As of yet, only MC1, an archaeal NAP, has been shown to protect DNA against thermal denaturation and radiolysis. ssDNA are intermediates in vital cellular processes, such as DNA replication and recombination. Single-stranded binding proteins (SSBs) prevent the formation of secondary structures but also protect the hypersensitive ssDNA against chemical and nuclease degradation. Ionizing radiation upregulates SSBs in the extremophile Deinococcus radiodurans.

The Helicobacter pylori HspR-Modulator CbpA Is a Multifunctional Heat-Shock Protein

The medically important human pathogen Helicobacter pylori relies on a collection of highly conserved heat-shock and chaperone proteins to preserve the integrity of cellular polypeptides and to control their homeostasis in response to external stress and changing environmental conditions. Among this set of chaperones, the CbpA protein has been shown to play a regulatory role in heat-shock gene regulation by directly interacting with the master stress-responsive repressor HspR. Apart from this regulatory role, little is known so far about CbpA functional activities. Using biochemistry and molecular biology approaches, we have started the in vitro functional characterization of H. pylori CbpA. Specifically, we show that CbpA is a multifunctional protein, being able to bind DNA and to stimulate the ATPase activity of the major chaperone DnaK. In addition, we report a preliminary observation suggesting that CbpA DNA-binding activity can be affected by the direct interaction with the heat-shock master repressor HspR, supporting the hypothesis of a reciprocal crosstalk between these two proteins. Thus, our work defines novel functions for H. pylori CbpA and stimulates further studies aimed at the comprehension of the complex regulatory interplay among chaperones and heat-shock transcriptional regulators.

The Interplay between Two Transcriptional Repressors and Chaperones Orchestrates Helicobacter pylori Heat-Shock Response

The ability to gauge the surroundings and modulate gene expression accordingly is a crucial feature for the survival bacterial pathogens. In this respect, the heat-shock response, a universally conserved mechanism of protection, allows bacterial cells to adapt rapidly to hostile conditions and to survive during environmental stresses. The important and widespread human pathogen Helicobacter pylori enrolls a collection of highly conserved heat-shock proteins to preserve cellular proteins and to maintain their homeostasis, allowing the pathogen to adapt and survive in the hostile niche of the human stomach. Moreover, various evidences suggest that some chaperones of H. pylori may play also non-canonical roles as, for example, in the interaction with the extracellular environment. In H. pylori, two dedicated transcriptional repressors, named HspR and HrcA, homologues to well-characterized regulators found in many other bacterial species, orchestrate the regulation of heat-shock proteins expression. Following twenty years of intense research, characterized by molecular, as well as genome-wide, approaches, it is nowadays possible to appreciate the complex picture representing the heat-shock regulation in H. pylori. Specifically, the HspR and HrcA repressors combine to control the transcription of target genes in a way that the HrcA regulon results embedded within the HspR regulon. Moreover, an additional level of control of heat-shock genes' expression is exerted by a posttranscriptional feedback regulatory circuit in which chaperones interact and modulate HspR and HrcA DNA-binding activity. This review recapitulates our understanding of the roles and regulation of the most important heat-shock proteins of H. pylori, which represent a crucial virulence factor for bacterial infection and persistence in the human host.

Regulation of heat-shock genes in bacteria: from signal sensing to gene expression output

The heat-shock response is a mechanism of cellular protection against sudden adverse environmental growth conditions and results in the prompt production of various heat-shock proteins. In bacteria, specific sensory biomolecules sense temperature fluctuations and transduce intercellular signals that coordinate gene expression outputs. Sensory biomolecules, also known as thermosensors, include nucleic acids (DNA or RNA) and proteins. Once a stress signal is perceived, it is transduced to invoke specific molecular mechanisms controlling transcription of genes coding for heat-shock proteins. Transcriptional regulation of heat-shock genes can be under either positive or negative control mediated by dedicated regulatory proteins. Positive regulation exploits specific alternative sigma factors to redirect the RNA polymerase enzyme to a subset of selected promoters, while negative regulation is mediated by transcriptional repressors. Interestingly, while various bacteria adopt either exclusively positive or negative mechanisms, in some microorganisms these two opposite strategies coexist, establishing complex networks regulating heat-shock genes. Here, we comprehensively summarize molecular mechanisms that microorganisms have adopted to finely control transcription of heat-shock genes.

The Escherichia coli Nucleoid in Stationary Phase

Compaction of DNA is an essential phenomenon that affects all facets of cellular biology. Surprisingly, given the abundance and apparent simplicity of bacteria, our understanding of chromosome organization in these ancient organisms is inadequate. In this chapter we will focus on arguably the best understood aspect of DNA folding in the model bacterium Escherichia coli: the supercondensation of the chromosome that occurs during periods of starvation and stress.

DNA recognition by Escherichia coli CbpA protein requires a conserved arginine-minor-groove interaction

Curved DNA binding protein A (CbpA) is a co-chaperone and nucleoid associated DNA binding protein conserved in most γ-proteobacteria. Best studied in Escherichia coli, CbpA accumulates to >2500 copies per cell during periods of starvation and forms aggregates with DNA. However, the molecular basis for DNA binding is unknown; CbpA lacks motifs found in other bacterial DNA binding proteins. Here, we have used a combination of genetics and biochemistry to elucidate the mechanism of DNA recognition by CbpA. We show that CbpA interacts with the DNA minor groove. This interaction requires a highly conserved arginine side chain. Substitution of this residue, R116, with alanine, specifically disrupts DNA binding by CbpA, and its homologues from other bacteria, whilst not affecting other CbpA activities. The intracellular distribution of CbpA alters dramatically when DNA binding is negated. Hence, we provide a direct link between DNA binding and the behaviour of CbpA in cells.

Synergism between a foldase and an unfoldase: reciprocal dependence between the thioredoxin-like activity of DnaJ and the polypeptide-unfolding activity of DnaK

The role of bacterial Hsp40, DnaJ, is to co-chaperone the binding of misfolded or alternatively folded proteins to bacterial Hsp70, DnaK, which is an ATP-fuelled unfolding chaperone. In addition to its DnaK targeting activity, DnaJ has a weak thiol-reductase activity. In between the substrate-binding domain and the J-domain anchor to DnaK, DnaJ has a unique domain with four conserved CXXC motives that bind two Zn²⁺ and partly contribute to polypeptide binding. Here, we deleted in DnaJ this Zn-binding domain, which is characteristic to type I but not of type II or III J-proteins. This caused a loss of the thiol-reductase activity and strongly reduced the ability of DnaJ to mediate the ATP- and DnaK-dependent unfolding/refolding of mildly oxidized misfolded polypeptides, an inhibition that was alleviated in the presence of thioredoxin or DTT. We suggest that in addition to their general ability to target misfolded polypeptide substrates to the Hsp70/Hsp110 chaperone machinery, Type I J-proteins carry an ancillary protein dithiol-isomerase function that can synergize the unfolding action of the chaperone, in the particular case of substrates that are further stabilized by non-native disulfide bonds. Whereas the unfoldase can remain ineffective without the transient untying of disulfide bonds by the foldase, the foldase can remain ineffective without the transient ATP-fuelled unfolding of wrong local structures by the unfoldase.

Structure of CbpA J-domain bound to the regulatory protein Cbpm explains its specificity and suggests evolutionary link between Cbpm and transcriptional regulators

CbpA is one of the six E. coli DnaJ/Hsp40 homologues of DnaK co-chaperones and the only one that is additionally regulated by a small protein CbpM, conserved in γ-proteobacteria. CbpM inhibits the co-chaperone and DNA binding activities of CbpA. This regulatory function of CbpM is accomplished through reversible interaction with the N-terminal J-domain of CbpA, which is essential for the interaction with DnaK. CbpM is highly specific for CbpA and does not bind DnaJ despite the high degree of structural and functional similarity between the J-domains of CbpA and DnaJ. Here we report the crystal structure of the complex of CbpM with the J-domain of CbpA. CbpM forms dimers and the J-domain of CbpA interacts with both CbpM subunits. The CbpM-binding surface of CbpA is highly overlapping with the CbpA interface for DnaK, providing a competitive model for regulation through forming mutually exclusive complexes. The structure also provides the explanation for the strict specificity of CbpM for CbpA, which we confirmed by making mutants of DnaJ that became regulated by CbpM. Interestingly, the structure of CbpM reveals a striking similarity to members of the MerR family of transcriptional regulators, suggesting an evolutionary connection between the functionally distinct bacterial co-chaperone regulator CbpM and the transcription regulator HspR.

Nucleoid localization of Hsp40 Mdj1 is important for its function in maintenance of mitochondrial DNA

Faithful replication and propagation of mitochondrial DNA (mtDNA) is critical for cellular respiration. Molecular chaperones, ubiquitous proteins involved in protein folding and remodeling of protein complexes, have been implicated in mtDNA transactions. In particular, cells lacking Mdj1, an Hsp40 co-chaperone of Hsp70 in the mitochondrial matrix, do not maintain functional mtDNA. Here we report that the great majority of Mdj1 is associated with nucleoids, DNA-protein complexes that are the functional unit of mtDNA transactions. Underscoring the importance of Hsp70 chaperone activity in the maintenance of mtDNA, an Mdj1 variant having an alteration in the Hsp70-interacting J-domain does not maintain mtDNA. However, a J-domain containing fragment expressed at the level that Mdj1 is normally present is not competent to maintain mtDNA, suggesting a function of Mdj1 beyond that carried out by its J-domain. Nevertheless, loss of mtDNA function upon Mdj1 depletion is retarded when the J-domain, is overexpressed. Analysis of Mdj1 variants revealed a correlation between nucleoid association and DNA maintenance activity, suggesting that localization is functionally important. We found that Mdj1 has DNA binding activity and that variants retaining DNA-binding activity also retained nucleoid association. Together, our results are consistent with a model in which Mdj1, tethered to the nucleoid via DNA binding, thus driving a high local concentration of the Hsp70 machinery, is important for faithful DNA maintenance and propagation.

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J-protein co-chaperone Mdj1 is critical for maintenance of functional mtDNA.

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Majority of Mdj1 localizes to the nucleoid, likely via interaction with mtDNA.

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Nucleoid localization of Mdj1 is necessary for mtDNA maintenance.

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Function of Mdj1 in mtDNA maintenance requires cooperation with Hsp70.

E. coli Fis protein insulates the cbpA gene from uncontrolled transcription

The Escherichia coli curved DNA binding protein A (CbpA) is a poorly characterised nucleoid associated factor and co-chaperone. It is expressed at high levels as cells enter stationary phase. Using genetics, biochemistry, and genomics, we have examined regulation of, and DNA binding by, CbpA. We show that Fis, the dominant growth-phase nucleoid protein, prevents CbpA expression in growing cells. Regulation by Fis involves an unusual “insulation” mechanism. Thus, Fis protects cbpA from the effects of a distal promoter, located in an adjacent gene. In stationary phase, when Fis levels are low, CbpA binds the E. coli chromosome with a preference for the intrinsically curved Ter macrodomain. Disruption of the cbpA gene prompts dramatic changes in DNA topology. Thus, our work identifies a novel role for Fis and incorporates CbpA into the growing network of factors that mediate bacterial chromosome structure.

Compaction of chromosomal DNA is a fundamental process that impacts on all aspects of cellular biology. However, our understanding of chromosome organisation in bacteria is poorly developed. Since bacteria are amongst the most abundant living organisms on the planet, this represents a startling gap in our knowledge. Despite our lack of understanding, it has long been known that Escherichia coli, and other bacteria, radically re-model their chromosomes in response to environmental stress. This is most notable during periods of starvation, when the E. coli chromosome is super compacted. In dissecting the molecular mechanisms that control this phenomenon, we have found that regulatory cross-talk between DNA–organising proteins plays an essential role. Thus, the major DNA folding protein from growing E. coli inhibits production of the major chromosome organisers in starved cells. Our findings illustrate the highly dynamic nature of bacterial chromosomes. Thus, DNA topology, gene transcription, and chromosome folding proteins entwine to create a web of interactions that define the properties of the chromosome.

CbpA acts as a modulator of HspR repressor DNA binding activity in Helicobacter pylori

The ability of pathogens to cope with disparate environmental stresses is a crucial feature for bacterial survival and for the establishment of a successful infection and colonization of the host; in this respect, chaperones and heat shock proteins (HSPs) play a fundamental role in host-pathogen interactions. In Helicobacter pylori, the expression of the major HSPs is tightly regulated through dedicated transcriptional repressors (named HspR and HrcA), as well as via a GroESL-dependent posttranscriptional feedback control acting positively on the DNA binding affinity of the HrcA regulator itself. In the present work we show that the CbpA chaperone also participates in the posttranscriptional feedback control of the H. pylori heat shock regulatory network. Our experiments suggest that CbpA specifically modulates HspR in vitro binding to DNA without affecting HrcA regulator activity. In particular, CbpA directly interacts with HspR, preventing the repressor from binding to its target operators. This interaction takes place only when HspR is not bound to DNA since CbpA is unable to affect HspR once the repressor is bound to its operator site. Accordingly, in vivo overexpression of CbpA compromises the response kinetics of the regulatory circuit, inducing a failure to restore HspR-dependent transcriptional repression after heat shock. The data presented in this work support a model in which CbpA acts as an important modulator of HspR regulation by fine-tuning the shutoff response of the regulatory circuit that governs HSP expression in H. pylori.

A conserved acidic amino acid mediates the interaction between modulators and co-chaperones in enterobacteria

Hsp40-like co-chaperones are ubiquitous enzymes that stimulate the protein refolding activity of Hsp70 family chaperones. They are widespread in prokaryotic and eukaryotic systems. In bacteria, the best characterized co-chaperone is the Escherichia coli DnaJ protein. Many γ-proteobacteria encode a functional homologue of DnaJ, known as CbpA, which is expressed in response to starvation and environmental stress. The activity of CbpA is regulated by the "modulator" protein CbpM. Here, we have used a combination of genetics and biochemistry to identify the co-chaperone contact determinant of CbpM. We show that the nature of the interaction is conserved in enterobacteria.

Heat shock protein 90 from Escherichia coli collaborates with the DnaK chaperone system in client protein remodeling

Molecular chaperones are proteins that assist the folding, unfolding, and remodeling of other proteins. In eukaryotes, heat shock protein 90 (Hsp90) proteins are essential ATP-dependent molecular chaperones that remodel and activate hundreds of client proteins with the assistance of cochaperones. In Escherichia coli, the activity of the Hsp90 homolog, HtpG, has remained elusive. To explore the mechanism of action of E. coli Hsp90, we used in vitro protein reactivation assays. We found that E. coli Hsp90 promotes reactivation of heat-inactivated luciferase in a reaction that requires the prokaryotic Hsp70 chaperone system, known as the DnaK system. An Hsp90 ATPase inhibitor, geldanamycin, inhibits luciferase reactivation demonstrating the importance of the ATP-dependent chaperone activity of E. coli Hsp90 during client protein remodeling. Reactivation also depends upon the ATP-dependent chaperone activity of the DnaK system. Our results suggest that the DnaK system acts first on the client protein, and then E. coli Hsp90 and the DnaK system collaborate synergistically to complete remodeling of the client protein. Results indicate that E. coli Hsp90 and DnaK interact in vivo and in vitro, providing additional evidence to suggest that E. coli Hsp90 and the DnaK system function together.

Dimerization and DNA-dependent aggregation of the Escherichia coli nucleoid protein and chaperone CbpA

The Escherichia coli curved DNA-binding protein A (CbpA) is a nucleoid-associated DNA-binding factor and chaperone that is expressed at high levels as cells enter stationary phase. Using a combination of genetics, biochemistry, structural modelling and single-molecule atomic force microscopy we have examined dimerization of, and DNA binding by, CbpA. Our data show that CbpA dimerization is driven by a hydrophobic surface comprising amino acid side chains W287 and L290 located on the same side of an α helix close to the C-terminus of CbpA. Derivatives of CbpA that are unable to dimerize are also unable to bind DNA. Free in solution, CbpA can exist as either a monomer or dimer. However, when bound to DNA, CbpA forms large aggregates that can protect DNA from degradation by nucleases. These CbpA–DNA aggregates are similar in morphology to protein–DNA complexes formed by the DNA-binding protein from starved cells (Dps), the only other stationary phase-specific nucleoid protein. Conversely, protein–DNA complexes formed by Fis, the major growth phase nucleoid protein, have a markedly different appearance.

Nuclear-encoded DnaJ homologue of Plasmodium falciparum interacts with replication ori of the apicoplast genome

The apicoplast of Plasmodium falciparum carries a 35 kb circular genome (plDNA) that replicates at the late trophozoite stage of the parasite intraerythocytic cycle. plDNA replication proceeds predominantly via a d-loop/bi-directional ori mechanism with replication ori localized within inverted repeat region. Although replication of the apicoplast genome is a validated drug target, the proteins involved in the replication process are only partially characterized. We analysed DNA-protein interactions at a plDNA replication ori region and report the identification of a nuclear-encoded DnaJ homologue that binds directly to ori elements of the plDNA molecule. PfDnaJ(A) interacted with the minor groove of the DNA double-helix and recognized a 13 bp sequence within the ori. Inhibition of binding with anti-PfDnaJ(A) antibodies confirmed identity of the protein in DNA-binding experiments with organellar protein fractions. The DNA-binding domain of the approximately 69 kDa PfDnaJ(A) lay within the N-terminal 38 kDa region that carries DnaJ signature motifs. In contrast to PfDnaJ(A) in parasite organellar fractions, the recombinant protein interacted with DNA in a sequence non-specific manner. Our results suggest a role for PfDnaJ(A) in replication/repair of the apicoplast genome.

Structural basis of the regulation of the CbpA co-chaperone by its specific modulator CbpM

CbpA, one of the Escherichia coli DnaJ homologues, acts as a co-chaperone in the DnaK chaperone system. Despite its extensive similarity in domain structure and function to DnaJ, CbpA has a unique and specific regulatory mechanism mediated through the small protein CbpM. Both CbpA and CbpM are highly conserved in bacteria. Earlier studies showed that CbpM interacts with the N-terminal J-domain of CbpA inhibiting its co-chaperone activity but the structural basis of this interaction is not known. Here, we have combined NMR spectroscopy, site-directed mutagenesis and surface plasmon resonance to characterize the CbpA/CbpM interaction at the molecular level. We have determined the solution structure of the CbpA J-domain and mapped the residues that are perturbed upon CbpM binding. The NMR data defined a broad region on helices alpha2 and alpha 3 as involved in the interactions. Site-directed mutagenesis has been used to further delineate the CbpA J-domain/CbpM interface. We show that the binding sites of CbpM and DnaK on CbpA J-domain overlap, which suggests a competition between DnaK and CbpM for binding to CbpA as a mechanism for CbpA regulation. This study also provides the explanation for the specificity of CbpM for CbpA versus DnaJ, by identifying the key residues for differential binding.

Temperature-dependent regulation of Thermus thermophilus DnaK/DnaJ chaperones by DafA protein

DafA, a unique 8-kDa protein found in Thermus thermophilus, assembles the chaperones DnaK and DnaJ to produce a DnaK(3)-DnaJ(3)-DafA(3) complex (KJA complex). Although, it is known that DafA is denatured irreversibly at nonphysiological 89 degrees C and the KJA complex dissociates into fully active DnaK and DnaJ, the function of the KJA complex is not fully understood. In this article, we report that the reversible dissociation of the KJA complex occurs in a temperature-dependent manner even below physiological 75 degrees C and that excess DafA completely inhibits the chaperone activities of the DnaK system. The inhibited activities are not rescued by supplementing DnaK or DnaJ. The results indicate that DafA inhibits the chaperone activities of both DnaK and DnaJ by forming the KJA complex and can act as a thermosensor under both heat stress and optimal growth conditions.

Mycoplasma pneumoniae J-domain protein required for terminal organelle function

The cell wall-less prokaryote Mycoplasma pneumoniae causes tracheobronchitis and primary atypical pneumonia in humans. Colonization of the respiratory epithelium requires proper assembly of a complex, multifunctional, polar terminal organelle. Loss of a predicted J-domain protein also having domains unique to mycoplasma terminal organelle proteins (TopJ) resulted in a non-motile, adherence-deficient phenotype. J-domain proteins typically stimulate ATPase activity of Hsp70 chaperones to bind nascent peptides for proper folding, translocation or macromolecular assembly, or to resolve stress-induced protein aggregates. By Western immunoblotting all defined terminal organelle proteins examined except protein P24 remained at wild-type levels in the topJ mutant; previous studies established that P24 is required for normal initiation of terminal organelle formation. Nevertheless, terminal organelle proteins P1, P30, HMW1 and P41 failed to localize to a cell pole, and when evaluated quantitatively, P30 and HMW1 foci were undetectable in >40% of cells. Complementation of the topJ mutant with the recombinant wild-type topJ allele largely restored terminal organelle development, gliding motility and cytadherence. We propose that this J-domain protein, which localizes to the base of the terminal organelle in wild-type M. pneumoniae, functions in the late stages of assembly, positioning, or both, of nascent terminal organelles.

The Hsp70 chaperone machines of Escherichia coli: a paradigm for the repartition of chaperone functions

Molecular chaperones are highly conserved in all free-living organisms. There are many types of chaperones, and most are conveniently grouped into families. Genome sequencing has revealed that many organisms contain multiple members of both the DnaK (Hsp70) family and their partner J-domain protein (JDP) cochaperone, belonging to the DnaJ (Hsp40) family. Escherichia coli K-12 encodes three Hsp70 genes and six JDP genes. The coexistence of these chaperones in the same cytosol suggests that certain chaperone-cochaperone interactions are permitted, and that chaperone tasks and their regulation have become specialized over the course of evolution. Extensive genetic and biochemical analyses have greatly expanded knowledge of chaperone tasking in this organism. In particular, recent advances in structure determination have led to significant insights of the underlying complexities and functional elegance of the Hsp70 chaperone machine.

In vivo modulation of a DnaJ homolog, CbpA, by CbpM

CbpA, an Escherichia coli DnaJ homolog, can function as a cochaperone for the DnaK/Hsp70 chaperone system, and its in vitro activity can be modulated by CbpM. We discovered that CbpM specifically inhibits the in vivo activity of CbpA, preventing it from functioning in cell growth and division. Furthermore, we have shown that CbpM interacts with CbpA in vivo during stationary phase, suggesting that the inhibition of activity is a result of the interaction. These results reveal that the activity of the E. coli DnaK system can be regulated in vivo by a specific inhibitor.