Detecting envelope stress by monitoring ß-barrel assembly.

The cell envelope protects bacteria from their surroundings. Defects in its integrity or assembly are sensed by signal transduction systems, allowing cells to rapidly adjust. The Rcs phosphorelay responds to outer membrane (OM)- and peptidoglycan-related stress in enterobacteria. We elucidated how the OM lipoprotein RcsF, the upstream Rcs component, senses envelope stress and activates the signaling cascade. RcsF interacts with BamA, the major component of the ß-barrel assembly machinery. In growing cells, BamA continuously funnels RcsF through the ß-barrel OmpA, displaying RcsF on the cell surface. This process spatially separates RcsF from the downstream Rcs component, which we show is the inner membrane protein IgaA. The Rcs system is activated when BamA fails to bind RcsF and funnel it to OmpA. Newly synthesized RcsF then remains periplasmic, interacting with IgaA to activate the cascade. Thus RcsF senses envelope damage by monitoring the activity of the Bam machinery.

Repairing oxidized proteins in the bacterial envelope using respiratory chain electrons.

The reactive species of oxygen and chlorine damage cellular components, potentially leading to cell death. In proteins, the sulfur-containing amino acid methionine is converted to methionine sulfoxide, which can cause a loss of biological activity. To rescue proteins with methionine sulfoxide residues, living cells express methionine sulfoxide reductases (Msrs) in most subcellular compartments, including the cytosol, mitochondria and chloroplasts. Here we report the identification of an enzymatic system, MsrPQ, repairing proteins containing methionine sulfoxide in the bacterial cell envelope, a compartment particularly exposed to the reactive species of oxygen and chlorine generated by the host defence mechanisms. MsrP, a molybdo-enzyme, and MsrQ, a haem-binding membrane protein, are widely conserved throughout Gram-negative bacteria, including major human pathogens. MsrPQ synthesis is induced by hypochlorous acid, a powerful antimicrobial released by neutrophils. Consistently, MsrPQ is essential for the maintenance of envelope integrity under bleach stress, rescuing a wide series of structurally unrelated periplasmic proteins from methionine oxidation, including the primary periplasmic chaperone SurA. For this activity, MsrPQ uses electrons from the respiratory chain, which represents a novel mechanism to import reducing equivalents into the bacterial cell envelope. A remarkable feature of MsrPQ is its capacity to reduce both rectus (R-) and sinister (S-) diastereoisomers of methionine sulfoxide, making this oxidoreductase complex functionally different from previously identified Msrs. The discovery that a large class of bacteria contain a single, non-stereospecific enzymatic complex fully protecting methionine residues from oxidation should prompt a search for similar systems in eukaryotic subcellular oxidizing compartments, including the endoplasmic reticulum.

Insights into the function of YciM, a heat shock membrane protein required to maintain envelope integrity in Escherichia coli.

The cell envelope of Gram-negative bacteria is an essential organelle that is important for cell shape and protection from toxic compounds. Proteins involved in envelope biogenesis are therefore attractive targets for the design of new antibacterial agents. In a search for new envelope assembly factors, we screened a collection of Escherichia coli deletion mutants for sensitivity to detergents and hydrophobic antibiotics, a phenotype indicative of defects in the cell envelope. Strains lacking yciM were among the most sensitive strains of the mutant collection. Further characterization of yciM mutants revealed that they display a thermosensitive growth defect on low-osmolarity medium and that they have a significantly altered cell morphology. At elevated temperatures, yciM mutants form bulges containing cytoplasmic material and subsequently lyse. We also discovered that yciM genetically interacts with envC, a gene encoding a regulator of the activity of peptidoglycan amidases. Altogether, these results indicate that YciM is required for envelope integrity. Biochemical characterization of the protein showed that YciM is anchored to the inner membrane via its N terminus, the rest of the protein being exposed to the cytoplasm. Two CXXC motifs are present at the C terminus of YciM and serve to coordinate a redox-sensitive iron center of the rubredoxin type. Both the N-terminal membrane anchor and the C-terminal iron center of YciM are important for function.

Crystal structure of the outer membrane protein RcsF, a new substrate for the periplasmic protein-disulfide isomerase DsbC.

The bacterial Rcs phosphorelay is a stress-induced defense mechanism that controls the expression of numerous genes, including those for capsular polysaccharides, motility, and virulence factors. It is a complex multicomponent system that includes the histidine kinase (RcsC) and the response regulator (RcsB) and also auxiliary proteins such as RcsF. RcsF is an outer membrane lipoprotein that transmits signals from the cell surface to RcsC. The physiological signals that activate RcsF and how RcsF interacts with RcsC remain unknown. Here, we report the three-dimensional structure of RcsF. The fold of the protein is characterized by the presence of a central 4-stranded ß sheet, which is conserved in several other proteins, including the copper-binding domain of the amyloid precursor protein. RcsF, which contains four conserved cysteine residues, presents two nonconsecutive disulfides between Cys(74) and Cys(118) and between Cys(109) and Cys(124), respectively. These two disulfides are not functionally equivalent; the Cys(109)-Cys(124) disulfide is particularly important for the assembly of an active RcsF. Moreover, we show that formation of the nonconsecutive disulfides of RcsF depends on the periplasmic disulfide isomerase DsbC. We trapped RcsF in a mixed disulfide complex with DsbC, and we show that deletion of dsbC prevents the activation of the Rcs phosphorelay by signals that function through RcsF. The three-dimensional structure of RcsF provides the structural basis to understand how this protein triggers the Rcs signaling cascade.

Contribution of proteomics toward solving the fascinating mysteries of the biogenesis of the envelope of Escherichia coli.

The cell envelope of Gram-negative bacteria is a complex macromolecular structure that is essential for their viability. Little is known on how the proteins which are secreted to the envelope fold into their unique three-dimensional structure. Several folding factors, including chaperones and protein folding catalysts involved in disulfide bond formation, have been identified in the periplasm. The characterization of these proteins has advanced our understanding of envelope biogenesis, although many fundamental questions remain unanswered. In particular, we still do not know how beta-barrel proteins are transported through the periplasm and inserted into the outer membrane. Here, we discuss the recent discoveries that have shed new light on the mechanisms that ensure the correct folding of envelope proteins. We have paid particular attention to the significant contribution of proteomic studies.

Data from a proteomic analysis highlight different osmoadaptations in two strain of Propionibacterium freudenreichii.

The article presents a proteomic data set generated by a comparative analysis of the proteomes of Propionibacterium freudenreichii, comparing the CIRM-BIA 129 and CIRM-BIA 1025 strains. The two strains were cultivated until the beginning of the stationary phase in a chemical defined medium (MMO), and in this medium in the presence of NaCl, with or without glycine betaine. Whole-cell proteins were extracted, trypsinolyzed and analyzed by nano LC-MS/MS, prior to identification and classification by function using the X!Tandem pipeline software and the proteomic data from NCBI.nlm.nigh.gov. Quantification of proteins was then carried out in order to detect change in their expression depending on the culture medium. This article is related to the research article entitled "Benefits and drawbacks of osmotic adjustment in Propionibacterium freudenreichii". The comparative proteomic analysis of the two strains reveal strain-dependent and medium-dependent stress proteomes in the probiotic P. freudenreichii.

Characterization of the role of the Escherichia coli periplasmic chaperone SurA using differential proteomics.

Little is known on how beta-barrel proteins are assembled in the outer membrane (OM) of Gram-negative bacteria. SurA has been proposed to be the primary chaperone escorting the bulk mass of OM proteins across the periplasm. However, the impact of SurA deletion on the global OM proteome has not been determined, limiting therefore our understanding of the function of SurA. By using a differential proteomics approach based on 2-D LC-MS(n), we compared the relative abundance of 64 OM proteins, including 23 beta-barrel proteins, in wild-type and surA strains. Unexpectedly, we found that the loss of SurA affects the abundance of eight beta-barrel proteins. Of all the decreased proteins, FhuA and LptD are the only two for which the decreased protein abundance cannot be attributed, at least in part, to decreased mRNA levels in the surA strain. In the case of LptD, an essential protein involved in OM biogenesis, our data support a role for SurA in the assembly of this protein and suggest that LptD is a true SurA substrate. Based on our results, we propose a revised model in which only a subset of OM proteins depends on SurA for proper folding and insertion in the OM.

Benefits and drawbacks of osmotic adjustment in Propionibacterium freudenreichii.

Propionibacterium freudenreichii is a beneficial bacterium used as a cheese starter and as a probiotic. Indeed, selected strains of P. freudenreichii combine both technological and health-promoting abilities. Moreover, during large-scale industrial production of dried bacteria and during consumption, P. freudenreichii may undergo different stressful processes. Osmotic adaptation was shown to enhance P. freudenreichii tolerance towards stresses, which are encountered during freeze-drying and during digestion. In this report, we compared the osmoadaptation molecular mechanisms of two P. freudenreichii strains. Both osmotolerance and osmoadaptation were strain-dependent and had different effects on multiple stress tolerance, depending on the presence of osmoprotectants. Availability of glycine betaine (GB) restored the growth of one of the two strains. In this strain, osmotic preadaptation enhanced heat, oxidative and acid stresses tolerance, as well as survival upon freeze-drying. However, addition of GB in the medium had deleterious effects on stress tolerance, while restoring optimal growth under hyperosmotic constraint. In the other strain, neither salt nor GB enhanced stress tolerance, which was constitutively low. Accordingly, whole cell proteomics revealed that mechanisms triggered by salt in the presence and in the absence of GB are different between strains. Osmotic adjustment may thus have deleterious effects on industrial abilities of P. freudenreichii. BIOLOGICAL SIGNIFICANCE: Propionibacteria are found in various niches including fodder, silage, rumen, milk and cheeses. This means adaptation towards different ecological environments with different physicochemical parameters. Propionibacterium freudenreichii, in particular, is furthermore used both as dairy starter and as probiotic and is thus submitted to high scale industrial production. Production and subsequent stabilization still need optimization. Drying processes like freeze-drying are stressful. Osmotic adjustments may modulated tolerance towards drying. However, they are strain-dependent, medium-dependent and may either reduce or increase stress tolerance. A case-by-case study, for each strain-medium thus seems necessary. In this work, we identify key proteins involved in osmoadaptation and give new insights into adaptation mechanisms in P. freudenreichii. This opens new perspectives for the selections of strains and for the choice of the growth medium composition.

Changes in protein synthesis during thermal adaptation of Propionibacterium freudenreichii subsp. shermanii.

Dairy propionibacteria are present in Graviera Kritis, a traditional Gruyère-type cheese made without added propionic starter. Ten isolated strains were identified by a combination of SDS-PAGE, species-specific PCR and according to their ability to ferment lactose. They were all found to belong to the Propionibacterium freudenreichii subsp. shermanii species. Because of the stressing Gruyère technology, which includes cooking at 52 to 53 degrees C their thermotolerance was investigated at 55 degrees C. Thermotolerant and thermosensitive strains were clearly discriminated. Interestingly, the reference strain CIP 103027 belongs to the sensitive subset. One sensitive strain, ACA-DC 1305 and one tolerant, ACA-DC 1451, were selected for further study and compared to CIP 103027. For the sensitive strains ACA-DC 1305 and CIP 103027, heat pre-treatment at 42 degrees C conferred thermoprotection of cells at the lethal temperature of 55 degrees C, while there was less effect on the tolerant ACA-DC 1451. No cross-protection of salt-adapted cells against heat stress was observed for none of the strains. Differential proteomic analysis revealed distinct but overlapping cell responses to heat stress between sensitive and tolerant strains. Thermal adaptation upregulated typical HSPs involved in protein repair or turnover in the sensitive one. In the tolerant one, a distinct subset of proteins was overexpressed, whatever the temperature used, in addition to HSPs. This included enzymes involved in propionic fermentation, amino acid metabolism, oxidative stress remediation and nucleotide phosphorylation. These results bring new insights into thermoprotection in propionibacteria and the occurrence of divergent phenotypes within a same subspecies.

Protein disulfide bond formation in the periplasm: determination of the in vivo redox state of cysteine residues.

Many proteins secreted to the bacterial cell envelope contain cysteine residues that are involved in disulfide bonds. These disulfides either play a structural role, increasing protein stability, or reversibly form in the catalytic site of periplasmic oxidoreductases. Monitoring the in vivo redox state of cysteine residues, i.e., determining whether those cysteines are oxidized to a disulfide bond or not, is therefore required to fully characterize the function and structural properties of numerous periplasmic proteins. Here, we describe a reliable and rapid method based on trapping reduced cysteine residues with 4'-acetamido-4'-maleimidylstilbene-2,2'-disulfonic acid (AMS), a maleimide compound. We use the Escherichia coli DsbA protein to illustrate the method, which can be applied to all envelope proteins.

Dissecting the machinery that introduces disulfide bonds in Pseudomonas aeruginosa.

UNLABELLED: Disulfide bond formation is required for the folding of many bacterial virulence factors. However, whereas the Escherichia coli disulfide bond-forming system is well characterized, not much is known on the pathways that oxidatively fold proteins in pathogenic bacteria. Here, we report the detailed unraveling of the pathway that introduces disulfide bonds in the periplasm of the human pathogen Pseudomonas aeruginosa. The genome of P. aeruginosa uniquely encodes two DsbA proteins (P. aeruginosa DsbA1 [PaDsbA1] and PaDsbA2) and two DsbB proteins (PaDsbB1 and PaDsbB2). We found that PaDsbA1, the primary donor of disulfide bonds to secreted proteins, is maintained oxidized in vivo by both PaDsbB1 and PaDsbB2. In vitro reconstitution of the pathway confirms that both PaDsbB1 and PaDsbB2 shuttle electrons from PaDsbA1 to membrane-bound quinones. Accordingly, deletion of both P. aeruginosa dsbB1 (PadsbB1) and PadsbB2 is required to prevent the folding of several P. aeruginosa virulence factors and to lead to a significant decrease in pathogenicity. Using a high-throughput proteomic approach, we also analyzed the impact of PadsbA1 deletion on the global periplasmic proteome of P. aeruginosa, which allowed us to identify more than 20 new potential substrates of this major oxidoreductase. Finally, we report the biochemical and structural characterization of PaDsbA2, a highly oxidizing oxidoreductase, which seems to be expressed under specific conditions. By fully dissecting the machinery that introduces disulfide bonds in P. aeruginosa, our work opens the way to the design of novel antibacterial molecules able to disarm this pathogen by preventing the proper assembly of its arsenal of virulence factors. IMPORTANCE: The human pathogen Pseudomonas aeruginosa causes life-threatening infections in immunodepressed and cystic fibrosis patients. The emergence of P. aeruginosa strains resistant to all of the available antibacterial agents calls for the urgent development of new antibiotics active against this bacterium. The pathogenic power of P. aeruginosa is mediated by an arsenal of extracellular virulence factors, most of which are stabilized by disulfide bonds. Thus, targeting the machinery that introduces disulfide bonds appears to be a promising strategy to combat P. aeruginosa. Here, we unraveled the oxidative protein folding system of P. aeruginosa in full detail. The system uniquely consists of two membrane proteins that generate disulfide bonds de novo to deliver them to P. aeruginosa DsbA1 (PaDsbA1), a soluble oxidoreductase. PaDsbA1 in turn donates disulfide bonds to secreted proteins, including virulence factors. Disruption of the disulfide bond formation machinery dramatically decreases P. aeruginosa virulence, confirming that disulfide formation systems are valid targets for the design of antimicrobial drugs.

The disulphide isomerase DsbC cooperates with the oxidase DsbA in a DsbD-independent manner.

In Escherichia coli, DsbA introduces disulphide bonds into secreted proteins. DsbA is recycled by DsbB, which generates disulphides from quinone reduction. DsbA is not known to have any proofreading activity and can form incorrect disulphides in proteins with multiple cysteines. These incorrect disulphides are thought to be corrected by a protein disulphide isomerase, DsbC, which is kept in the reduced and active configuration by DsbD. The DsbC/DsbD isomerization pathway is considered to be isolated from the DsbA/DsbB pathway. We show that the DsbC and DsbA pathways are more intimately connected than previously thought. dsbA(-)dsbC(-) mutants have a number of phenotypes not exhibited by either dsbA(-), dsbC(-) or dsbA(-)dsbD(-) mutations: they exhibit an increased permeability of the outer membrane, are resistant to the lambdoid phage Phi80, and are unable to assemble the maltoporin LamB. Using differential two-dimensional liquid chromatographic tandem mass spectrometry/mass spectrometry analysis, we estimated the abundance of about 130 secreted proteins in various dsb(-) strains. dsbA(-)dsbC(-) mutants exhibit unique changes at the protein level that are not exhibited by dsbA(-)dsbD(-) mutants. Our data indicate that DsbC can assist DsbA in a DsbD-independent manner to oxidatively fold envelope proteins. The view that DsbC's function is limited to the disulphide isomerization pathway should therefore be reinterpreted.

Metal binding to ligands: cadmium complexes with glutathione revisited.

We studied the interaction of gamma-L-glutamyl-L-cysteinyl-glycine (glutathione, GSH) with cadmium ions (Cd(2+)) by first performing classical potentiometric pH titration measurements and then turning to additional spectroscopic methods. To estimate the residual concentrations of free cadmium, we studied the competition of glutathione with a Cd(2+)-sensitive dye, either an absorbing dye (murexide) or a fluorescent one (FluoZin-1), and consistent results were obtained with the two dyes. In KCl-containing Tes, Mops, or Tris buffer at pH 7.0 to 7.1 and 37 degrees C (and at a total Cd(2+) concentration of 0.01 mM), results suggest that free cadmium concentration is halved when the concentration of glutathione is approximately 0.05 mM; this mainly reflects the combined apparent dissociation constant for the Cd(glutathione) 1:1 complex under these conditions. To identify the other complexes formed, we used far-UV spectroscopy of the ligand-to-metal charge transfer absorption bands. The Cd(glutathione)(2) 1:2 complex predominated over the 1:1 complex only at high millimolar concentrations of total glutathione and not at low submillimolar concentrations of total glutathione. The apparent conditional constants derived from these spectroscopy results made it possible to discriminate between sets of absolute constants that would otherwise have simulated the pH titration data similarly well in this complicated system. Related experiments showed that although the Cl(-) ions in our media competed (modestly) with glutathione for binding to Cd(2+), the buffers we had chosen did not bind Cd(2+) significantly under our conditions. Our experiments also revealed that Cd(2+) may be adsorbed onto quartz or glass vessel walls, reducing the accuracy of theoretical predictions of the concentrations of species in solution. Lastly, the experiments confirmed the rapid kinetics of formation and dissociation of the UV-absorbing Cd(glutathione)(2) 1:2 complexes. The methods described here may be useful for biochemists needing to determine conditional binding constants for charge transfer metal-ligand complexes under their own conditions.

Susceptibility and adaptive response to bile salts in Propionibacterium freudenreichii: physiological and proteomic analysis.

Tolerance to digestive stresses is one of the main factors limiting the use of microorganisms as live probiotic agents. Susceptibility to bile salts and tolerance acquisition in the probiotic strain Propionibacterium freudenreichii SI41 were characterized. We showed that pretreatment with a moderate concentration of bile salts (0.2 g/liter) greatly increased its survival during a subsequent lethal challenge (1.0 g/liter, 60 s). Bile salts challenge led to drastic morphological changes, consistent with intracellular material leakage, for nonadapted cells but not for preexposed ones. Moreover, the physiological state of the cells during lethal treatment played an important role in the response to bile salts, as stationary-phase bacteria appeared much less sensitive than exponentially growing cells. Either thermal or detergent pretreatment conferred significantly increased protection toward bile salts challenge. In contrast, some other heterologous pretreatments (hypothermic and hyperosmotic) had no effect on tolerance to bile salts, while acid pretreatment even might have sensitized the cells. Two-dimensional electrophoresis experiments revealed that at least 24 proteins were induced during bile salts adaptation. Identification of these polypeptides suggested that the bile salts stress response involves signal sensing and transduction, a general stress response (also triggered by thermal denaturation, oxidative toxicity, and DNA damage), and an alternative sigma factor. Taken together, our results provide new insights into the tolerance of P. freudenreichii to bile salts, which must be taken into consideration for the use of probiotic strains and the improvement of technological processes.

Mass spectrometry proteomic analysis of stress adaptation reveals both common and distinct response pathways in Propionibacterium freudenreichii.

Microorganisms used in food technology and probiotics are exposed to technological and digestive stresses, respectively. Traditionally used as Swiss-type cheese starters, propionibacteria also constitute promising human probiotics. Stress tolerance and cross-protection in Propionibacterium freudenreichii were thus examined after exposure to heat, acid, or bile salts stresses. Adapted cells demonstrated acquired homologous tolerance. Cross-protection between bile salts and heat adaptation was demonstrated. By contrast, bile salts pretreatment sensitized cells to acid challenge and vice versa. Surprisingly, heat and acid responses did not present significant cross-protection in P. freudenreichii. During adaptations, important changes in cellular protein synthesis were observed using two-dimensional electrophoresis. While global protein synthesis decreased, several proteins were overexpressed during stress adaptations. Thirty-four proteins were induced by acid pretreatment, 34 by bile salts pretreatment, and 26 by heat pretreatment. Six proteins are common to all stresses and represent general stress-response components. Among these polypeptides, general stress chaperones, and proteins involved in energetic metabolism, oxidative stress response, or SOS response were identified. These results bring new insight into the tolerance of P. freudenreichii to heat, acid, and bile salts, and should be taken into consideration in the development of probiotic preparations.

Anti-stress activity of Propionibacterium freudenreichii : identification of a reactivative protein.

Propionibacterium freudenreichii subsp. shermanii is known to prevent mutations caused by various agents such as N-methyl-N'-nitro-N-nitrosoguanidine, 9-aminoacridine, 4-nitro-quinoline-1-oxide and by UV radiation in both prokaryotic and eukaryotic cells. It was also shown to prevent or repair damage caused by H(2)O(2) or UV radiation in Salmonella typhimurium and Escherichia coli, a characteristic previously designated as reactivative effect. In order to characterise this effect at the molecular level, we have purified the active component from a P. freudenreichii cell-free extract using a combination of ammonium sulfate precipitation, anion-exchange and size-exclusion chromatography. The isolated 35 kDa protein was then identified using both N-terminal and internal peptide sequencing as a cysteine synthase. The latter was localised in the P. freudenreichii proteomic map. It is constitutively expressed but also clearly induced during adaptation to detergent and heat, but not acid, stresses. The biological meaning of cysteine synthase in the context of adaptation to oxidative and non-oxidative stresses is discussed.