Defining the roles of the periplasmic chaperones SurA, Skp, and DegP in Escherichia coli

Evolutionary conservation in biogenesis of β-barrel proteins allows mitochondria to assemble a functional bacterial trimeric autotransporter protein

Yersinia adhesin A (YadA) belongs to a class of bacterial adhesins that form trimeric structures. Their mature form contains a passenger domain and a C-terminal β-domain that anchors the protein in the outer membrane (OM). Little is known about how precursors of such proteins cross the periplasm and assemble into the OM. In the present study we took advantage of the evolutionary conservation in the biogenesis of β-barrel proteins between bacteria and mitochondria. We previously observed that upon expression in yeast cells, bacterial β-barrel proteins including the transmembrane domain of YadA assemble into the mitochondrial OM. In the current study we found that when expressed in yeast cells both the monomeric and trimeric forms of full-length YadA were detected in mitochondria but only the trimeric species was fully integrated into the OM. The oligomeric form was exposed on the surface of the organelle in its native conformation and maintained its capacity to adhere to host cells. The co-expression of YadA with a mitochondria-targeted form of the bacterial periplasmic chaperone Skp, but not with SurA or SecB, resulted in enhanced levels of both forms of YadA. Taken together, these results indicate that the proper assembly of trimeric autotransporter can occur also in a system lacking the lipoproteins of the BAM machinery and is specifically enhanced by the chaperone Skp.

Study of effect of substitution of the penultimate amino acid residue on expression, structure, and functional properties of Yersinia pseudotuberculosis OmpY porin

The purpose of the study was to compare the expression of two Yersinia pseudotuberculosis proteins, wild-type porin OmpY and the mutant porin OmpY designated as OmpY-Q having the uncharged amino acid residue Gln instead of positively charged Arg at the penultimate position in the same heterologous host. According to the literature, a similar substitution (Lys to Gln) of the penultimate amino acid residue in Neisseria meningitidis porin PorA drastically improved the assembly of the protein in the E. coli outer membrane in vivo. Site-directed mutagenesis was used to replace Arg by Gln (R338Q) in OmpY, and the conditions for optimal expression and maturation of OmpY-Q were selected. It was found that the growth rates of E. coli strains producing OmpY and OmpY-Q and the expression levels of the porins were approximately equal. Comparative analysis of recombinant OmpY and OmpY-Q did not show significant differences in structure, antigenic, and functional properties of the porins, or any noticeable effect of the R338Q substitution in OmpY on its assembly in the E. coli outer membrane in vivo. The probable causes of discrepancies between our results and the previous data on porin PorA are discussed considering the known mechanisms of biogenesis of porins at the periplasmic stage.

Protein targeting and transport as a necessary consequence of increased cellular complexity

With increasing intracellular complexity, a new cell-biological problem that is the allocation of cytoplasmically synthesized proteins to their final destinations within the cell emerged. A special challenge is thereby the translocation of proteins into or across cellular membranes. The underlying mechanisms are only in parts well understood, but it can be assumed that the course of cellular evolution had a deep impact on the design of the required molecular machines. In this article, we aim to summarize the current knowledge and concepts of the evolutionary development of protein trafficking as a necessary premise and consequence of increased cellular complexity.

Lateral opening and exit pore formation are required for BamA function

The outer membrane of Gram-negative bacteria is replete with a host of β-barrel outer membrane proteins (OMPs). Despite serving a variety of essential functions, including immune response evasion, the exact mechanism of OMP folding and membrane insertion remains largely unclear. The β-barrel assembly machinery complex is required for OMP biogenesis. Crystal structures and molecular dynamics (MD) simulations of the central and essential component, BamA, suggest a mechanism involving lateral opening of its barrel domain. MD simulations reported here reveal an additional feature of BamA: a substrate exit pore positioned above the lateral opening site. Disulfide crosslinks that prevent lateral opening and exit pore formation result in a loss of BamA function, which can be fully rescued by the reductant tris(2-carboxyethyl)phosphine. These data provide strong evidence that lateral opening and exit pore formation are required for BamA function.

Structural mapping of a chaperone-substrate interaction surface

NMR spectroscopy is used to detect site-specific intermolecular short-range contacts in a membrane-protein-chaperone complex. This is achieved by an "orthogonal" isotope-labeling scheme that permits the unambiguous detection of intermolecular NOEs between the well-folded chaperone and the unfolded substrate ensemble. The residues involved in these contacts are part of the chaperone-substrate contact interface. The approach is demonstrated for the 70 kDa bacterial Skp-tOmpA complex.

A small heat shock protein enables Escherichia coli to grow at a lethal temperature of 50°C conceivably by maintaining cell envelope integrity

It is essential for organisms to adapt to fluctuating growth temperatures. Escherichia coli, a model bacterium commonly used in research and industry, has been reported to grow at a temperature lower than 46.5°C. Here we report that the heterologous expression of the 17-kDa small heat shock protein from the nematode Caenorhabditis elegans, CeHSP17, enables E. coli cells to grow at 50°C, which is their highest growth temperature ever reported. Strikingly, CeHSP17 also rescues the thermal lethality of an E. coli mutant deficient in degP, which encodes a protein quality control factor localized in the periplasmic space. Mechanistically, we show that CeHSP17 is partially localized in the periplasmic space and associated with the inner membrane of E. coli, and it helps to maintain the cell envelope integrity of the E. coli cells at the lethal temperatures. Together, our data indicate that maintaining the cell envelope integrity is crucial for the E. coli cells to grow at high temperatures and also shed new light on the development of thermophilic bacteria for industrial application.

Diverse sequences are functional at the C-terminus of the E. coli periplasmic chaperone SurA

SurA is a major periplasmic molecular chaperone in Escherichia coli and has been shown to assist the biogenesis of several outer membrane proteins. The C-terminal fragment of SurA folds into a short β-strand, which forms a small three-stranded anti-parallel β-sheet module with the N-terminal β-hairpin. We found that the length of the C-terminal fragment, rather than its exact amino acid composition, had a big impact on SurA function. To investigate the determinant factor of the C-terminal sequence, we created a library of SurA constructs randomized in the last 10 residues. We screened the library and randomly analyzed 19 constructs that displayed SurA activity. The C-termini of these constructs shared little sequence similarity, except that β-strand-forming residues were preferentially enriched. Three SurA constructs were expressed and purified for structural characterization. Circular dichroism and fluorescence spectroscopy analyses revealed that their structures were similar to the structure of the wild-type SurA. Our results suggest that for scaffolding purpose proteins may tolerate various sequences provided certain general requirements such as hydrophobicity and secondary structure propensity are satisfied. Furthermore, the sequence tolerance of SurA at the C-terminus indicates that this area is not likely to be involved in substrate binding.

The lipopolysaccharide export pathway in Escherichia coli: structure, organization and regulated assembly of the Lpt machinery

The bacterial outer membrane (OM) is a peculiar biological structure with a unique composition that contributes significantly to the fitness of Gram-negative bacteria in hostile environments. OM components are all synthesized in the cytosol and must, then, be transported efficiently across three compartments to the cell surface. Lipopolysaccharide (LPS) is a unique glycolipid that paves the outer leaflet of the OM. Transport of this complex molecule poses several problems to the cells due to its amphipatic nature. In this review, the multiprotein machinery devoted to LPS transport to the OM is discussed together with the challenges associated with this process and the solutions that cells have evolved to address the problem of LPS biogenesis.

DegP primarily functions as a protease for the biogenesis of β-barrel outer membrane proteins in the Gram-negative bacterium Escherichia coli

DegP (also designated as HtrA) and its homologs are found in prokaryotic cells and such eukaryotic organelles as mitochondria and chloroplasts. DegP has been found to be essential for the growth of Gram-negative bacteria under heat shock conditions and arguably considered to possess both protease and chaperone activities. The function of DegP has not been clearly defined. Using genetically incorporated non-natural amino acids as photo-crosslinkers, here we identified the β-barrel outer membrane proteins (OMPs) as the major natural substrates of DegP in Escherichia coli cells. We also demonstrated that DegP primarily functions as a protease, at both low and high temperatures, to eliminate unfolded OMPs, with hardly any appreciable chaperone activity in cells. We also found that the toxic and cell membrane-damaging misfolded OMPs would accumulate in DegP-lacking cells cultured under heat shock conditions. Together, our study defines the primary function of DegP in OMP biogenesis and offers a mechanistic insight into the essentiality of DegP for cell growth under heat shock conditions.

Comparative genomic and transcriptomic analyses of NaCl-tolerant Staphylococcus sp. OJ82 isolated from fermented seafood

Bacteria belonging to the Staphylococcus genus reside in various natural environments; however, only disease-associated Staphylococcus strains have received attention while ecological function and physiologies of non-pathogenic strains were often neglected. Because high level of tolerance against NaCl is a common trait of Staphylococcus, we investigated the characteristics of halotolerance in Staphylococcus sp. OJ82 isolated from fermented seafood containing a high concentration of NaCl. Among the 292 isolates screened, OJ82 showed the highest β-galactosidase and extracellular protease activities under high-salt conditions. Comparative genomic analysis with other Staphylococcus strains showed that (a) replication origins are highly conserved, (b) the OJ82 strain has a high number of amino acid transport- and metabolism-related genes, and (c) OJ82 has many unique proteins (15 %) and 12 prophage-related genomic islands. RNA-seq analysis under high-salt conditions showed that genes involved in cell membranes, transport, osmotic stress, ATP synthesis, and translation are highly expressed. OJ82 may use the ribulose monophosphate pathway to detoxify some toxic intermediates under high-salt conditions. Six new and three known non-coding small RNAs of the OJ82 strain were also found in the RNA-seq analysis. Genomic and transcriptomic analyses identified target β-galactosidase and extracellular protease. Interestingly, the OJ82 strain became resistant to bacteriocin produced by the Bacillus strain only under high-salt conditions. Our data showed that the OJ82 strain adapted to high-salt conditions by expressing core cellular processes (translation, ATP production) and defense genes (membrane synthesis, compatible solute transports, ribulose monophosphate pathway) could survive bacteriocin exposure under high-salt conditions.

Identification of FkpA as a key quality control factor for the biogenesis of outer membrane proteins under heat shock conditions

The outer membrane proteins (OMPs) of Gram-negative bacterial cells, as well as the mitochondrion and chloroplast organelles, possess unique and highly stable β-barrel structures. Biogenesis of OMPs in Escherichia coli involves such periplasmic chaperones as SurA and Skp. In this study, we found that the ΔsurA Δskp double-deletion strain of E. coli, although lethal and defective in the biogenesis of OMPs at the normal growth temperature, is viable and effective at the heat shock temperature. We identified FkpA as the multicopy suppressor for the lethal phenotype of the ΔsurA Δskp strain. We also demonstrated that the deletion of fkpA from the ΔsurA cells resulted in only a mild decrease in the levels of folded OMPs at the normal temperature but a severe decrease as well as lethality at the heat shock temperature, whereas the deletion of fkpA from the Δskp cells had no detectable effect on OMP biogenesis at either temperature. These results strongly suggest a functional redundancy between FkpA and SurA for OMP biogenesis under heat shock stress conditions. Mechanistically, we found that FkpA becomes a more efficient chaperone for OMPs under the heat shock condition, with increases in both binding rate and affinity. In light of these observations and earlier reports, we propose a temperature-responsive OMP biogenesis mechanism in which the degrees of functional importance of the three chaperones are such that SurA > Skp > FkpA at the normal temperature but FkpA ≥ SurA > Skp at the heat shock temperature.

Folding mechanisms of periplasmic proteins

More than one fifth of the proteins encoded by the genome of Escherichia coli are destined to the bacterial cell envelope. Over the past 20years, the mechanisms by which envelope proteins reach their three-dimensional structure have been intensively studied, leading to the discovery of an intricate network of periplasmic folding helpers whose members have distinct but complementary roles. For instance, the correct assembly of ß-barrel proteins containing disulfide bonds depends both on chaperones like SurA and Skp for transport across the periplasm and on protein folding catalysts like DsbA and DsbC for disulfide bond formation. In this review, we provide an overview of the current knowledge about the complex network of protein folding helpers present in the periplasm of E. coli and highlight the questions that remain unsolved. This article is part of a Special Issue entitled: Protein trafficking and secretion in bacteria. Guest Editors: Anastassios Economou and Ross Dalbey.

Protease homolog BepA (YfgC) promotes assembly and degradation of β-barrel membrane proteins in Escherichia coli

Gram-negative bacteria are equipped with quality-control systems for the outer membrane (OM) that sense and cope with defective biogenesis of its components. Accumulation of misfolded outer membrane proteins (OMPs) in Escherichia coli leads to activation of σ(E), an essential alternative σ factor that up-regulates transcription of multiple genes required to preserve OM structure and function. Disruption of bepA (formerly yfgC), a σ(E)-regulated gene encoding a putative periplasmic metalloprotease, sensitizes cells to multiple drugs, suggesting that it may be involved in maintaining OM integrity. However, the specific function of BepA remains unclear. Here, we show that BepA enhances biogenesis of LptD, an essential OMP involved in OM transport and assembly of lipopolysaccharide, by promoting rearrangement of intramolecular disulfide bonds of LptD. In addition, BepA possesses protease activity and is responsible for the degradation of incorrectly folded LptD. In the absence of periplasmic chaperone SurA, BepA also promotes degradation of BamA, the central OMP subunit of the β-barrel assembly machinery (BAM) complex. Interestingly, defective oxidative folding of LptD caused by bepA disruption was partially suppressed by expression of protease-active site mutants of BepA, suggesting that BepA functions independently of its protease activity. We also show that BepA has genetic and physical interaction with components of the BAM complex. These findings raised the possibility that BepA maintains the integrity of OM both by promoting assembly of OMPs and by proteolytically eliminating OMPs when their correct assembly was compromised.

bam Lipoproteins Assemble BamA in vitro

The Bam machine assembles β-barrel membrane proteins into the outer membranes of Gram-negative bacteria. The central component of the Bam complex, BamA, is a β-barrel that is conserved in prokaryotes and eukaryotes. We have previously reported an in vitro assay for studying the assembly of β-barrel proteins by the Bam complex and now apply this assay to identify the specific components that are required for BamA assembly. We establish that BamB and BamD, two lipoprotein components of the complex, bind to the unfolded BamA substrate and are sufficient to accelerate its assembly into the membrane.

The activity and specificity of the outer membrane protein chaperone SurA are modulated by a proline isomerase domain

SurA is a component of the periplasmic chaperone network that plays a central role in biogenesis of integral outer membrane β-barrel proteins (OMPs) in Escherichia coli. Although SurA contains two well-conserved proline isomerase (PPIase) domains, the contribution of these domains to SurA function is unclear. In the present work, we show that defects in OMP assembly caused by mutation of the β-barrel assembly factors BamA or BamB can be corrected by gain-of-function mutations in SurA that map to the first PPIase domain. These mutations apparently bypass the requirement for a stable interaction between SurA and the Bam complex and enhance SurA chaperone activity in vivo despite destabilization of the protein in vitro. Our findings suggest an autoinhibitory mechanism for regulation of SurA chaperone activity through interdomain interactions involving a PPIase domain. We propose a model in which SurA activity is modulated by an interaction between SurA and the Bam complex that alters the substrate specificity of the chaperone.

IMPORTANCE

The dominant surA mutations described here alter amino acid residues that are highly conserved in eukaryotic homologs of SurA, including Pin 1, the human proline isomerase (PPIase) implicated in Alzheimer's disease and certain cancers. Consequently, a mechanistic description of SurA function may enhance our understanding of clinically important PPIases and their role(s) in disease. In addition, the virulence of Gram-negative bacterial pathogens, such as Salmonella, Shigella, and Escherichia coli O157:H7, is largely dependent on SurA, making this PPIase/chaperone an attractive antibiotic target. Investigating the function of SurA in outer membrane (OM) biogenesis will be useful in the development of novel therapeutic strategies for the disruption of the OM or the processes that are essential for its assembly.

Predicting functionally informative mutations in Escherichia coli BamA using evolutionary covariance analysis

The essential outer membrane β-barrel protein BamA forms a complex with four lipoprotein partners BamBCDE that assembles β-barrel proteins into the outer membrane of Escherichia coli. Detailed genetic studies have shown that BamA cycles through multiple conformations during substrate assembly, suggesting that a complex network of residues may be involved in coordinating conformational changes and lipoprotein partner function. While genetic analysis of BamA has been informative, it has also been slow in the absence of a straightforward selection for mutants. Here we take a bioinformatic approach to identify candidate residues for mutagenesis using direct coupling analysis. Starting with the BamA paralog FhaC, we show that direct coupling analysis works well for large β-barrel proteins, identifying pairs of residues in close proximity in tertiary structure with a true positive rate of 0.64 over the top 50 predictions. To reduce the effects of noise, we designed and incorporated a novel structured prior into the empirical correlation matrix, dramatically increasing the FhaC true positive rate from 0.64 to 0.88 over the top 50 predictions. Our direct coupling analysis of BamA implicates residues R661 and D740 in a functional interaction. We find that the substitutions R661G and D740G each confer OM permeability defects and destabilize the BamA β-barrel. We also identify synthetic phenotypes and cross-suppressors that suggest R661 and D740 function in a similar process and may interact directly. We expect that the direct coupling analysis approach to informed mutagenesis will be particularly useful in systems lacking adequate selections and for dynamic proteins with multiple conformations.

Dissecting the effects of periplasmic chaperones on the in vitro folding of the outer membrane protein PagP

Although many periplasmic folding factors have been identified, the mechanisms by which they interact with unfolded outer membrane proteins (OMPs) to promote correct folding and membrane insertion remain poorly understood. Here, we have investigated the effect of two chaperones, Skp and SurA, on the folding kinetics of the OMP, PagP. Folding kinetics of PagP into both zwitterionic diC12:0PC (1,2-dilauroyl-sn-glycero-3-phosphocholine) liposomes and negatively charged 80:20 diC12:0PC:diC12:0PG [1,2-dilauroyl-sn-glycero-3-phospho-(1'-rac-glycerol)] liposomes were investigated using a combination of spectroscopic and SDS-PAGE assays. The results indicate that Skp modulates the observed rate of PagP folding in a manner that is dependent on the composition of the membrane and the ionic strength of the buffer used. These data suggest that electrostatic interactions play an important role in Skp-assisted substrate delivery to the membrane. In contrast, SurA showed no effect on the observed folding rates of PagP, consistent with the view that these chaperones act by distinct mechanisms in partially redundant parallel chaperone pathways that facilitate OMP assembly. In addition to delivery of the substrate protein to the membrane, the ability of Skp to prevent OMP aggregation was investigated. The results show that folding and membrane insertion of PagP can be restored, in part, by Skp in conditions that strongly favour PagP aggregation. These results illustrate the utility of in vitro systems for dissecting the complex folding environment encountered by OMPs in the periplasm and demonstrate the key role of Skp in holding aggregation-prone OMPs prior to their direct or indirect delivery to the membrane.

Role for Skp in LptD assembly in Escherichia coli

The periplasmic chaperone Skp has long been implicated in the assembly of outer membrane proteins (OMPs) in Escherichia coli. It has been shown to interact with unfolded OMPs, and the simultaneous loss of Skp and the main periplasmic chaperone in E. coli, SurA, results in synthetic lethality. However, a Δskp mutant displays only minor OMP assembly defects, and no OMPs have been shown to require Skp for their assembly. Here, we report a role for Skp in the assembly of the essential OMP LptD. This role may be compensated for by other OMP assembly proteins; in the absence of both Skp and FkpA or Skp and BamB, LptD assembly is impaired. Overexpression of SurA does not restore LptD levels in a Δskp ΔfkpA double mutant, nor does the overexpression of Skp or FkpA restore LptD levels in the ΔsurA mutant, suggesting that Skp acts in concert with SurA to efficiently assemble LptD in E. coli. Other OMPs, including LamB, are less affected in the Δskp ΔfkpA and Δskp bamB::kan double mutants, suggesting that Skp is specifically necessary for the assembly of certain OMPs. Analysis of an OMP with a domain structure similar to that of LptD, FhuA, suggests that common structural features may determine which OMPs require Skp for their assembly.

Biogenesis of β-barrel integral proteins of bacterial outer membrane

Gram-negative bacteria are enveloped by two membranes, the inner (cytoplasmic) (CM) and the outer (OM). The majority of integral outer membrane proteins are arranged in β-barrels of cylindrical shape composed of amphipathic antiparallel β-strands. In bacteria, β-barrel proteins function as water-filled pores, active transporters, enzymes, receptors, and structural proteins. Proteins of bacterial OM are synthesized in the cytoplasm as unfolded polypeptides with an N-terminal sequence that marks them for transport across the CM. Precursors of membrane proteins move through the aqueous medium of the cytosol and periplasm under the protection of chaperones (SecB, Skp, SurA, and DegP), then cross the CM via the Sec system composed of a polypeptide-conducting channel (SecYEG) and ATPase (SecA), the latter providing the energy for the translocation of the pre-protein. Pre-protein folding and incorporation in the OM require the participation of the Bam-complex, probably without the use of energy. This review summarizes current data on the biogenesis of the β-barrel proteins of bacterial OM. Data on the structure of the proteins included in the multicomponent system for delivery of the OM proteins to their destination in the cell and on their complexes with partners, including pre-proteins, are presented. Molecular models constructed on the basis of structural, genetic, and biochemical studies that describe the mechanisms of β-barrel protein assembly by this molecular transport machinery are also considered.

Protective anti-outer membrane protein immunity against Pasteurella pneumotropica infection of mice

The ability of recombinant outer membrane proteins of Pasteurella pneumotropica to vaccinate against the infections of mice was studied. The proteins examined were the homologues of the P4, P6, P26, and D15 proteins of Haemophilus influenzae. Intranasal vaccination with P4 and P6 produced protection against pneumonia. P6 vaccination, which was most studied, reduced the peak bacteria load in lungs by 50-fold and caused a rapid resolution of an infection that lasted for at least 5 days in unvaccinated animals. Protection could be partially transferred with CD4(+) T cells and pulmonary challenge with the P6 antigen induced interferon-γ and the Th17 cytokine IL-21. This is the first demonstration of the ability of a recombinant P6 to mediate protective immunity to a pathogen in its natural host and it is proposed that it would not only have utility for mouse breeding but also for investigating how to improve the efficacy of vaccination with homologous proteins for related species.

Demarcating SurA activities required for outer membrane targeting of Yersinia pseudotuberculosis adhesins

SurA is a periplasmic protein folding factor involved in chaperoning and trafficking of outer membrane proteins across the Gram-negative bacterial periplasm. In addition, SurA also possesses peptidyl-prolyl cis/trans isomerase activity. We have previously reported that in enteropathogenic Yersinia pseudotuberculosis, SurA is needed for bacterial virulence and envelope integrity. In this study, we investigated the role of SurA in the assembly of important Yersinia adhesins. Using genetic mutation, biochemical characterization, and an in vitro-based bacterial host cell association assay, we confirmed that surface localization of the invasin adhesin is dependent on SurA. As a surA deletion also has some impact on the levels of individual components of the BAM complex in the Yersinia outer membrane, abolished invasin surface assembly could reflect both a direct loss of SurA-dependent periplasmic targeting and a potentially compromised BAM complex assembly platform in the outer membrane. To various degrees, the assembly of two other adhesins, Ail and the pH 6 antigen fibrillum PsaA, also depends on SurA. Consequently, loss of SurA leads to a dramatic reduction in Yersinia attachment to eukaryotic host cells. Genetic complementation of surA deletion mutants indicated a prominent role for SurA chaperone function in outer membrane protein assembly. Significantly, the N terminus of SurA contributed most of this SurA chaperone function. Despite a dominant chaperoning role, it was also evident that SurA isomerization activity did make a modest contribution to this assembly process.

Structure, Function and Regulation of Outer Membrane Proteins Involved in Drug Transport in Enterobactericeae: the OmpF/C - TolC Case

Antibiotic translocation across membranes of Gram-negative bacteria is a key step for the activity on their specific intracellular targets. Resistant bacteria control their membrane permeability as a first line of defense to protect themselves against external toxic compounds such as antibiotics and biocides. On one hand, resistance to small hydrophilic antibiotics such as ß-lactams and fluoroquinolones frequently results from the « closing » of their way in: the general outer membrane porins. On the other hand, an effective way out for a wide range of antibiotics is provided by TolC-like proteins, which are outer membrane components of multidrug efflux pumps. Accordingly, altered membrane permeability, including porin modifications and/or efflux pumps’ overexpression, is always associated to multidrug resistance (MDR) in a number of clinical isolates.

Several recent studies have highlighted our current understanding of porins/TolC structures and functions in Enterobacteriaceae. Here, we review the transport of antibiotics through the OmpF/C general porins and the TolC-like channels with regards to recent data on their structure, function, assembly, regulation and contribution to bacterial resistance.

Because MDR strains have evolved global strategies to identify and fight our antibiotic arsenal, it is important to constantly update our global knowledge on antibiotic transport.

Insights into the function and structural flexibility of the periplasmic molecular chaperone SurA

SurA is the primary periplasmic molecular chaperone that facilitates the folding and assembling of outer membrane proteins (OMPs) in Gram-negative bacteria. Deletion of the surA gene in Escherichia coli leads to a decrease in outer membrane density and an increase in bacterial drug susceptibility. Here, we conducted mutational studies on SurA to identify residues that are critical for function. One mutant, SurA(V37G), significantly reduced the activity of SurA. Further characterization indicated that SurA(V37G) was structurally similar to, but less stable than, the wild-type protein. The loss of activity in SurA(V37G) could be restored through the introduction of a pair of Cys residues and the subsequent formation of a disulfide bond. Inspired by this success, we created three additional SurA constructs, each containing a disulfide bond at different regions of the protein between two rigid secondary structural elements. The formation of disulfide bond in these mutants has no observable detrimental effect on protein activity, indicating that SurA does not undergo large-scale conformational change while performing its function.

Crystal structure of BamB from Pseudomonas aeruginosa and functional evaluation of its conserved structural features

The assembly of β-barrel Outer Membrane Proteins (OMPs) in the outer membrane is essential for gram-negative bacteria. The process requires the β-Barrel Assembly Machine (BAM), a multiprotein complex that, in E. coli, is composed of the OMP BamA and four lipoproteins BamB-E. Whereas BamA and BamD are essential, deletion of BamB, C or E produce membrane permeability defects. Here we present the high-resolution structure of BamB from Pseudomonas aeruginosa. This protein can complement the deletion of bamB in E. coli indicating that they are functionally equivalent. Conserved structural features include an eight-bladed β-propeller fold stabilized by tryptophan docking motifs with a central pore about 8 Å in diameter at the narrowest point. This pore distinguishes BamB from related β-propellers, such as quinoprotein dehydrogenases. However, a double mutation designed to block this pore was fully functional indicating that the opening is not essential. Two loops protruding from the bottom of the propeller are conserved and mediate binding to BamA. Conversely, an additional loop only present in E. coli BamB is not required for function. A cluster of highly conserved residues in a groove between blades 6 and 7 is crucial for proper BamB folding or biogenesis. It has been proposed that BamB may bind nascent OMPs by β-augmentation to its propeller outer strands, or recognize the aromatic residue signature at the C-terminus of OMPs. However, Isothermal Titration Calorimetry experiments and structural analysis do not support these proposals. The structural and mutagenesis analysis suggests that the main function of BamB is to bind and modulate BamA, rather than directly interact with nascent OMPs.

Assembly of the β-Barrel Outer Membrane Proteins in Gram-Negative Bacteria, Mitochondria, and Chloroplasts

In the last decade, there has been an explosion of publications on the assembly of β-barrel outer membrane proteins (OMPs), which carry out diverse cellular functions, including solute transport, protein secretion, and assembly of protein and lipid components of the outer membrane. Of the three outer membrane model systems—Gram-negative bacteria, mitochondria and chloroplasts—research on bacterial and mitochondrial systems has so far led the way in dissecting the β-barrel OMP assembly pathways. Many exciting discoveries have been made, including the identification of β-barrel OMP assembly machineries in bacteria and mitochondria, and potentially the core assembly component in chloroplasts. The atomic structures of all five components of the bacterial β-barrel assembly machinery (BAM) complex, except the β-barrel domain of the core BamA protein, have been solved. Structures reveal that these proteins contain domains/motifs known to facilitate protein-protein interactions, which are at the heart of the assembly pathways. While structural information has been valuable, most of our current understanding of the β-barrel OMP assembly pathways has come from genetic, molecular biology, and biochemical analyses. This paper provides a comparative account of the β-barrel OMP assembly pathways in Gram-negative bacteria, mitochondria, and chloroplasts.

Intimin and invasin export their C-terminus to the bacterial cell surface using an inverse mechanism compared to classical autotransport

Invasin and intimin are major virulence factors of enteropathogenic Yersiniae and Escherichia coli, mediating invasion into and intimate adherence to host cells, respectively. Several studies have hinted that extracellular portion of these homologous proteins might be exported via an autotransport mechanism, but rigorous experimental proof has been lacking. Here, we present a topology model for invasin and intimin, consistent with the hypothesis that the N-terminal β-barrel domain acts as a translocation pore to secrete the C-terminal passenger domain. We confirmed this topology model by inserting epitope tags into the loops of the β-barrel. We further show that obstructing the pore of β-barrel hinders the export of the passenger domain. As for classical autotransport, the biogenesis of invasin and intimin is dependent on the Bam complex and the periplasmic chaperone SurA, whereas the chaperone/protease DegP is involved in quality control. However, compared to classical autotransporters (Type Va secretion), the domain structure of intimin and invasin is inverted. We conclude that proteins of the intimin and invasin family constitute a novel group of autotransported proteins, and propose that this class of autotransporters be termed Type Ve secretion.

Is the C-terminal insertional signal in Gram-negative bacterial outer membrane proteins species-specific or not?

BACKGROUND

In Gram-negative bacteria, the outer membrane is composed of an asymmetric lipid bilayer of phopspholipids and lipopolysaccharides, and the transmembrane proteins that reside in this membrane are almost exclusively β-barrel proteins. These proteins are inserted into the membrane by a highly conserved and essential machinery, the BAM complex. It recognizes its substrates, unfolded outer membrane proteins (OMPs), through a C-terminal motif that has been speculated to be species-specific, based on theoretical and experimental results from only two species, Escherichia coli and Neisseria meningitidis, where it was shown on the basis of individual sequences and motifs that OMPs from the one cannot easily be over expressed in the other, unless the C-terminal motif was adapted. In order to determine whether this species specificity is a general phenomenon, we undertook a large-scale bioinformatics study on all predicted OMPs from 437 fully sequenced proteobacterial strains.

RESULTS

We were able to verify the incompatibility reported between Escherichia coli and Neisseria meningitidis, using clustering techniques based on the pairwise Hellinger distance between sequence spaces for the C-terminal motifs of individual organisms. We noticed that the amino acid position reported to be responsible for this incompatibility between Escherichia coli and Neisseria meningitidis does not play a major role for determining species specificity of OMP recognition by the BAM complex. Instead, we found that the signal is more diffuse, and that for most organism pairs, the difference between the signals is hard to detect. Notable exceptions are the Neisseriales, and Helicobacter spp. For both of these organism groups, we describe the specific sequence requirements that are at the basis of the observed difference.

CONCLUSIONS

Based on the finding that the differences between the recognition motifs of almost all organisms are small, we assume that heterologous overexpression of almost all OMPs should be feasible in E. coli and other Gram-negative bacterial model organisms. This is relevant especially for biotechnology applications, where recombinant OMPs are used e.g. for the development of vaccines. For the species in which the motif is significantly different, we identify the residues mainly responsible for this difference that can now be changed in heterologous expression experiments to yield functional proteins.

SurA is involved in the targeting to the outer membrane of a Tat signal sequence-anchored protein

The twin arginine translocation (Tat) pathway exports folded proteins from the cytoplasm to the periplasm of bacteria. The targeting of the exported proteins to the Tat pathway relies on a specific amino-terminal signal sequence, which is cleaved after exportation. In the phytopathogen Dickeya dadantii, the pectin lyase homologue PnlH is exported by the Tat pathway without cleavage of its signal sequence, which anchors PnlH into the outer membrane. In proteobacteria, the vast majority of outer membrane proteins consists of β-barrel proteins and lipoproteins. Thus, PnlH represents a new kind of outer membrane protein. In Escherichia coli, periplasmic chaperones SurA, Skp, and DegP work together with the β-barrel assembly machinery (Bam) to target and insert β-barrel proteins into the outer membrane. In this work, we showed that SurA is required for an efficient targeting of PnlH to the outer membrane. Moreover, we were able to detect an in vitro interaction between SurA and the PnlH signal sequence. Since the PnlH signal sequence contains a highly hydrophobic region, we propose that SurA protects it from the hydrophobic periplasm during targeting of PnlH to the outer membrane. We also studied the nature of the information carried by the PnlH signal sequence responsible for its targeting to the outer membrane after exportation by the Tat system.

Dissecting the Escherichia coli periplasmic chaperone network using differential proteomics

β-Barrel proteins, or outer membrane proteins (OMPs), perform many essential functions in Gram-negative bacteria, but questions remain about the mechanism by which they are assembled into the outer membrane (OM). In Escherichia coli, β-barrels are escorted across the periplasm by chaperones, most notably SurA and Skp. However, the contributions of these two chaperones to the assembly of the OM proteome remained unclear. We used differential proteomics to determine how the elimination of Skp and SurA affects the assembly of many OMPs. We have shown that removal of Skp has no impact on the levels of the 63 identified OM proteins. However, depletion of SurA in the skp strain has a marked impact on the OM proteome, diminishing the levels of almost all β-barrel proteins. Our results are consistent with a model in which SurA plays a primary chaperone role in E. coli. Furthermore, they suggest that while no OMPs prefer the Skp chaperone pathway in wild-type cells, most can use Skp efficiently when SurA is absent. Our data, which provide a unique glimpse into the protein content of the nonviable surA skp mutant, clarify the roles of the periplasmic chaperones in E. coli.

Absence of O antigen suppresses Shigella flexneri IcsA autochaperone region mutations

The Shigella flexneri IcsA (VirG) protein is a polarly distributed autotransporter protein. IcsA functions as a virulence factor by interacting with the host actin regulatory protein N-WASP, which in turn activates the Arp2/3 complex, initiating actin polymerization. Formation of F-actin comet tails allows bacterial cell-to-cell spreading. Although various accessory proteins such as periplasmic chaperones and the β-barrel assembly machine (BAM) complex have been shown to be involved in the export of IcsA, the IcsA translocation mechanism remains to be fully elucidated. A putative autochaperone (AC) region (amino acids 634-735) located at the C-terminal end of the IcsA passenger domain, which forms part of the self-associating autotransporter (SAAT) domain, has been suggested to be required for IcsA biogenesis, as well as for N-WASP recruitment, based on mutagenesis studies. IcsA(i) proteins with linker insertion mutations within the AC region have a significant reduction in production and are defective in N-WASP recruitment when expressed in smooth LPS (S-LPS) S. flexneri. In this study, we have found that the LPS O antigen plays a role in IcsA(i) production based on the use of an rmlD (rfbD) mutant having rough LPS (R-LPS) and a novel assay in which O antigen is depleted using tunicamycin treatment and then regenerated. In addition, we have identified a new N-WASP binding/interaction site within the IcsA AC region.

The Skp chaperone helps fold soluble proteins in vitro by inhibiting aggregation

The periplasmic seventeen kilodalton protein (Skp) chaperone has been characterized primarily for its role in outer membrane protein (OMP) biogenesis, during which the jellyfish-like trimeric protein encapsulates partially folded OMPs, protecting them from the aqueous environment until delivery to the BAM outer membrane protein insertion complex. However, Skp is increasingly recognized as a chaperone that also assists in folding soluble proteins in the bacterial periplasm. In this capacity, Skp coexpression increases the active yields of many recombinant proteins and bacterial virulence factors. Using a panel of single-chain antibodies and a single-chain T-cell receptor (collectively termed scFvs) possessing varying stabilities and biophysical characteristics, we performed in vivo expression and in vitro folding and aggregation assays in the presence or absence of Skp. For Skp-sensitive scFvs, the presence of Skp during in vitro refolding assays reduced aggregation but did not alter the observed folding rates, resulting in a higher overall yield of active protein. Of the proteins analyzed, Skp sensitivity in all assays correlated with the presence of folding intermediates, as observed with urea denaturation studies. These results are consistent with Skp acting as a holdase, sequestering partially folded intermediates and thereby preventing aggregation. Because not all soluble proteins are sensitive to Skp coexpression, we hypothesize that the presence of a long-lived protein folding intermediate renders a protein sensitive to Skp. Improved understanding of the bacterial periplasmic protein folding machinery may assist in high-level recombinant protein expression and may help identify novel approaches to block bacterial virulence.

Structure and conservation of the periplasmic targeting factor Tic22 protein from plants and cyanobacteria

Mitochondria and chloroplasts are of endosymbiotic origin. Their integration into cells entailed the development of protein translocons, partially by recycling bacterial proteins. We demonstrate the evolutionary conservation of the translocon component Tic22 between cyanobacteria and chloroplasts. Tic22 in Anabaena sp. PCC 7120 is essential. The protein is localized in the thylakoids and in the periplasm and can be functionally replaced by a plant orthologue. Tic22 physically interacts with the outer envelope biogenesis factor Omp85 in vitro and in vivo, the latter exemplified by immunoprecipitation after chemical cross-linking. The physical interaction together with the phenotype of a tic22 mutant comparable with the one of the omp85 mutant indicates a concerted function of both proteins. The three-dimensional structure allows the definition of conserved hydrophobic pockets comparable with those of ClpS or BamB. The results presented suggest a function of Tic22 in outer membrane biogenesis.

The bacterial outer membrane β-barrel assembly machinery

β-Barrel proteins found in the outer membrane of Gram-negative bacteria serve a variety of cellular functions. Proper folding and assembly of these proteins are essential for the viability of bacteria and can also play an important role in virulence. The β-barrel assembly machinery (BAM) complex, which is responsible for the proper assembly of β-barrels into the outer membrane of Gram-negative bacteria, has been the focus of many recent studies. This review summarizes the significant progress that has been made toward understanding the structure and function of the bacterial BAM complex.

BB0324 and BB0028 are constituents of the Borrelia burgdorferi β-barrel assembly machine (BAM) complex

Background

Similar to Gram-negative bacteria, the outer membrane (OM) of the pathogenic spirochete, Borrelia burgdorferi, contains integral OM-spanning proteins (OMPs), as well as membrane-anchored lipoproteins. Although the mechanism of OMP biogenesis is still not well-understood, recent studies have indicated that a heterooligomeric OM protein complex, known as BAM (β-barrel assembly machine) is required for proper assembly of OMPs into the bacterial OM. We previously identified and characterized the essential β-barrel OMP component of this complex in B. burgdorferi, which we determined to be a functional BamA ortholog.

Results

In the current study, we report on the identification of two additional protein components of the B. burgdorferi BAM complex, which were identified as putative lipoproteins encoded by ORFs BB0324 and BB0028. Biochemical assays with a BamA-depleted B. burgdorferi strain indicate that BB0324 and BB0028 do not readily interact with the BAM complex without the presence of BamA, suggesting that the individual B. burgdorferi BAM components may associate only when forming a functional BAM complex. Cellular localization assays indicate that BB0324 and BB0028 are OM-associated subsurface lipoproteins, and in silico analyses indicate that BB0324 is a putative BamD ortholog.

Conclusions

The combined data suggest that the BAM complex of B. burgdorferi contains unique protein constituents which differ from those found in other proteobacterial BAM complexes. The novel findings now allow for the B. burgdorferi BAM complex to be further studied as a model system to better our understanding of spirochetal OM biogenesis in general.

Genetic analysis of 15 protein folding factors and proteases of the Escherichia coli cell envelope

Each cell hosts thousands of proteins that vary greatly in abundance, structure, and chemical properties. To ensure that all proteins are biologically active and properly localized, efficient quality control systems have evolved. While the structure, function, and regulation of some individual protein folding factors and proteases were resolved up to atomic resolution, others remain poorly characterized. In addition, little is known about which factors are required for viability under specific stress conditions. We therefore determined the physiological implications of 15 factors of the E. coli cell envelope by an integrated genetic approach comprising phenotypic analyses. Our data indicate that surA and tsp null mutations are a lethal combination in rich medium, that surA dsbA and surA dsbC double mutants are temperature sensitive, and that surA ptrA, surA yfgC, dsbA fkpA, degP tsp, degP ppiD, tsp ppiD, and degP dsbA double mutants are temperature sensitive in rich medium containing 0.5 M NaCl, while degP dsbA, degP yfgC, tsp ydgD, and degP tsp double mutants do not grow in the presence of SDS/EDTA. Furthermore, we show that in degP dsbA, degP tsp, and degP yfgC double mutants a subpopulation of LamB exists as unfolded monomers. In addition, dsbA null mutants expressed lower levels of the outer membrane proteins LptD, LamB, FhuA, and OmpW while FhuA levels were reduced in surA single and degP ppiD double mutants. Lower FhuA levels in degP ppiD strains depend on Tsp, since in a tsp degP ppiD triple mutant FhuA levels are restored.

Outer membrane biogenesis in Escherichia coli, Neisseria meningitidis, and Helicobacter pylori: paradigm deviations in H. pylori

The bacterial pathogen Helicobacter pylori is capable of colonizing the gastric mucosa of the human stomach using a variety of factors associated with or secreted from its outer membrane (OM). Lipopolysaccharide (LPS) and numerous OM proteins have been shown to be involved in adhesion and immune stimulation/evasion. Many of these factors are essential for colonization and/or pathogenesis in a variety of animal models. Despite this wide array of potential targets present on the bacterial surface, the ability of H. pylori to vary its OM profile limits the effectiveness of vaccines or therapeutics that target any single one of these components. However, it has become evident that the proteins comprising the complexes that transport the majority of these molecules to the OM are highly conserved and often essential. The field of membrane biogenesis has progressed remarkably in the last few years, and the possibility now exists for targeting the mechanisms by which β-barrel proteins, lipoproteins, and LPS are transported to the OM, resulting in loss of bacterial fitness and significant altering of membrane permeability. In this review, the OM transport machinery for LPS, lipoproteins, and outer membrane proteins (OMPs) are discussed. While the principal investigations of these transport mechanisms have been conducted in Escherichia coli and Neisseria meningitidis, here these systems will be presented in the genetic context of ε proteobacteria. Bioinformatic analysis reveals that minimalist genomes, such as that of Helicobacter pylori, offer insight into the smallest number of components required for these essential pathways to function. Interestingly, in the majority of ε proteobacteria, while the inner and OM associated apparatus of LPS, lipoprotein, and OMP transport pathways appear to all be intact, most of the components associated with the periplasmic compartment are either missing or are almost unrecognizable when compared to their E. coli counterparts. Eventual targeting of these pathways would have the net effect of severely limiting the delivery/transport of components to the OM and preventing the bacterium's ability to infect its human host.

PpiA, a surface PPIase of the cyclophilin family in Lactococcus lactis

Background

Protein folding in the envelope is a crucial limiting step of protein export and secretion. In order to better understand this process in Lactococcus lactis, a lactic acid bacterium, genes encoding putative exported folding factors like Peptidyl Prolyl Isomerases (PPIases) were searched for in lactococcal genomes.

Results

In L. lactis, a new putative membrane PPIase of the cyclophilin subfamily, PpiA, was identified and characterized. ppiA gene was found to be constitutively expressed under normal and stress (heat shock, H₂O₂) conditions. Under normal conditions, PpiA protein was synthesized and released from intact cells by an exogenously added protease, showing that it was exposed at the cell surface. No obvious phenotype could be associated to a ppiA mutant strain under several laboratory conditions including stress conditions, except a very low sensitivity to H₂O₂. Induction of a ppiA copy provided in trans had no effect i) on the thermosensitivity of an mutant strain deficient for the lactococcal surface protease HtrA and ii) on the secretion and stability on four exported proteins (a highly degraded hybrid protein and three heterologous secreted proteins) in an otherwise wild-type strain background. However, a recombinant soluble form of PpiA that had been produced and secreted in L. lactis and purified from a culture supernatant displayed both PPIase and chaperone activities.

Conclusions

Although L. lactis PpiA, a protein produced and exposed at the cell surface under normal conditions, displayed a very moderate role in vivo, it was found, as a recombinant soluble form, to be endowed with folding activities in vitro.

Engineered oligomerization state of OmpF protein through computational design decouples oligomer dissociation from unfolding

Biogenesis of β-barrel membrane proteins is a complex, multistep, and as yet incompletely characterized process. The bacterial porin family is perhaps the best-studied protein family among β-barrel membrane proteins that allows diffusion of small solutes across the bacterial outer membrane. In this study, we have identified residues that contribute significantly to the protein-protein interaction (PPI) interface between the chains of outer membrane protein F (OmpF), a trimeric porin, using an empirical energy function in conjunction with an evolutionary analysis. By replacing these residues through site-directed mutagenesis either with energetically favorable residues or substitutions that do not occur in natural bacterial outer membrane proteins, we succeeded in engineering OmpF mutants with dimeric and monomeric oligomerization states instead of a trimeric oligomerization state. Moreover, our results suggest that the oligomerization of OmpF proceeds through a series of interactions involving two distinct regions of the extensive PPI interface: two monomers interact to form a dimer through the PPI interface near G19. This dimer then interacts with another monomer through the PPI interface near G135 to form a trimer. We have found that perturbing the PPI interface near G19 results in the formation of the monomeric OmpF only. Thermal denaturation of the designed dimeric OmpF mutant suggests that oligomer dissociation can be separated from the process of protein unfolding. Furthermore, the conserved site near G57 and G59 is important for the PPI interface and might provide the essential scaffold for PPIs.

From self sufficiency to dependence: mechanisms and factors important for autotransporter biogenesis

Autotransporters are a superfamily of proteins that use the type V secretion pathway for their delivery to the surface of Gram-negative bacteria. At first glance, autotransporters look to contain all the functional elements required to promote their own secretion: an amino-terminal signal peptide to mediate translocation across the inner membrane, a central passenger domain that is the secreted functional moiety, and a channel-forming carboxyl terminus that facilitates passenger domain translocation across the outer membrane. However, recent discoveries of common structural themes, translocation intermediates and accessory interactions have challenged the perceived simplicity of autotransporter secretion. Here, we discuss how these studies have led to an improved understanding of the mechanisms responsible for autotransporter biogenesis.

Activation of the Escherichia coli β-barrel assembly machine (Bam) is required for essential components to interact properly with substrate

The outer membrane (OM) of gram-negative bacteria such as Escherichia coli contains lipoproteins and integral β-barrel proteins (outer-membrane proteins, OMPs) assembled into an asymmetrical lipid bilayer. Insertion of β-barrel proteins into the OM is mediated by a protein complex that contains the OMP BamA and four associated lipoproteins (BamBCDE). The mechanism by which the Bam complex catalyzes the assembly of OMPs is not known. We report here the isolation and characterization of a temperature-sensitive lethal mutation, bamAE373K, which alters the fifth polypeptide transport-associated domain and disrupts the interaction between the BamAB and BamCDE subcomplexes. Suppressor mutations that map to codon 197 in bamD restore Bam complex function to wild-type levels. However, these suppressors do not restore the interaction between BamA and BamD; rather, they bypass the requirement for stable holocomplex formation by activating BamD. These results imply that BamA and BamD interact directly with OMP substrates.

Insights into the structure and assembly of Escherichia coli outer membrane protein A

Outer membrane protein A (OmpA) of Escherichia coli is a paradigm for the biogenesis of outer membrane proteins; however, the structure and assembly of OmpA have remained controversial. A review of studies to date supports the hypothesis that native OmpA is a single-domain large pore, while a two-domain narrow-pore structure is a folding intermediate or minor conformer. The in vitro refolding of OmpA to the large-pore conformation requires isolation of the protein from outer membranes with retention of an intact disulfide bond followed by sufficient incubation in lipids at temperatures of ≥ 26 °C to overcome the high energy of activation for refolding. The in vivo maturation of the protein involves covalent modification of serines in the eighth β-barrel of the N-terminal domain by oligo-(R)-3-hydroxybutyrates as the protein is escorted across the cytoplasm by SecB for post-translational secretion across the secretory translocase in the inner membrane. After cleavage of the signal sequence, protein chaperones, such as Skp, DegP and SurA, guide OmpA across the periplasm to the β-barrel assembly machinery (BAM) complex in the outer membrane. During this passage, a disulfide bond is formed between C290 and C302 by DsbA, and the hydrophobicity of segments of the C-terminal domain, which are destined for incorporation as β-barrels in the outer membrane bilayer, is increased by covalent attachment of oligo-(R)-3-hydroxybutyrates. With the aid of the BAM complex, OmpA is then assembled into the outer membrane as a single-domain large pore.

Inactivation of Haemophilus influenzae lipopolysaccharide biosynthesis genes interferes with outer membrane localization of the hap autotransporter

Nontypeable Haemophilus influenzae is a major cause of localized respiratory tract disease and initiates infection by colonizing the nasopharynx. Colonization requires adherence to host epithelial cells, which is mediated by surface proteins such as the Hap adhesin. In this study, we identified a relationship between Hap levels in the outer membrane and lipopolysaccharide (LPS) biosynthesis enzymes. We found that mutation of the rfaF, pgmB, lgtC, kfiC, orfE, rfbP, lsgB, or lsgD genes, which are involved in the synthesis of the LPS oligosaccharide core in H. influenzae strain Rd/HapS243A, resulted in loss of Hap in the bacterial outer membrane and a decrease in hap transcript levels. In contrast, the same mutations had no effect on outer membrane localization of H. influenzae P5 or IgA1 protease or levels of p5 or iga1 transcripts, suggesting a Hap-specific effect. Elimination of the HtrA periplasmic protease resulted in a return of Hap to the outer membrane and restoration of hap transcript levels. Consistently, in lgtC phase-off bacteria, Hap was absent from the outer membrane, and hap transcript levels were reduced. Hap localization and hap transcript levels were not related to LPS size but to the functions of the LPS biosynthesis enzymes themselves. We speculate that the lack of certain LPS biosynthesis enzymes causes Hap to mislocalize and accumulate in the periplasm, where it is degraded by HtrA. This degradation then leads to a decrease in hap transcript levels. Together, these data highlight a novel interplay between Hap and LPS biosynthesis that can influence H. influenzae interactions with the host.

Making a beta-barrel: assembly of outer membrane proteins in Gram-negative bacteria

The outer membrane (OM) of Gram-negative bacteria is an essential organelle that serves as a selective permeability barrier by keeping toxic compounds out of the cell while allowing vital nutrients in. How the OM and its constituent lipid and protein components are assembled remains an area of active research. In this review, we describe our current understanding of how outer membrane proteins (OMPs) are delivered to and then assembled in the OM of the model Gram-negative organism Escherichia coli.

ATP-independent control of autotransporter virulence protein transport via the folding properties of the secreted protein

Autotransporter (AT) proteins are the largest class of extracellular virulence proteins secreted from Gram-negative bacteria. The mechanism by which AT proteins cross the bacterial outer membrane (OM), in the absence of ATP or another external energy source, is unknown. Here we demonstrate a linear correlation between localized regions of stability (ΔG(folding)) in the mature virulence protein (the AT "passenger") and OM secretion efficiency. Destabilizing the C-terminal β-helical domain of a passenger reduced secretion efficiency. In contrast, destabilizing the globular N-terminal domain of a passenger produced a linearly correlated increase in secretion efficiency. Thus, C-terminal passenger stability facilitates OM secretion, whereas N-terminal stability hinders it. The contributions of regional passenger stability to OM secretion demonstrate a crucial role for the passenger itself in directing its secretion, suggesting a novel type of ATP-independent, folding-driven transporter.