The murein-lipoprotein linkage in the cell wall of Escherichia coli

Type 1 fimbriae-mediated collective protection against type 6 secretion system attacks

Bacterial competition may rely on secretion systems such as the type 6 secretion system (T6SS), which punctures and releases toxic molecules into neighboring cells. To subsist, bacterial targets must counteract the threats posed by T6SS-positive competitors. In this study, we used a comprehensive genome-wide high-throughput screening approach to investigate the dynamics of interbacterial competition. Our primary goal was to identify deletion mutants within the well-characterized E. coli K-12 single-gene deletion library, the Keio collection, that demonstrated resistance to T6SS-mediated killing by the enteropathogenic bacterium Cronobacter malonaticus. We identified 49 potential mutants conferring resistance to T6SS and focused our interest on a deletion mutant (∆fimE) exhibiting enhanced expression of type 1 fimbriae. We demonstrated that the presence of type 1 fimbriae leads to the formation of microcolonies and thus protects against T6SS-mediated assaults. Collectively, our study demonstrated that adhesive structures such as type 1 fimbriae confer collective protective behavior against T6SS attacks.IMPORTANCEType 6 secretion systems (T6SS) are molecular weapons employed by gram-negative bacteria to eliminate neighboring microbes. T6SS plays a pivotal role as a virulence factor, enabling pathogenic gram-negative bacteria to compete with the established communities to colonize hosts and induce infections. Gaining a deeper understanding of bacterial interactions will allow the development of strategies to control the action of systems such as the T6SS that can manipulate bacterial communities. In this context, we demonstrate that bacteria targeted by T6SS attacks from the enteric pathogen Cronobacter malonaticus, which poses a significant threat to infants, can develop a collective protective mechanism centered on the production of type I fimbriae. These adhesive structures promote the aggregation of bacterial preys and the formation of microcolonies, which protect the cells from T6SS attacks.

Phosphatase A1 accessory protein PlaS from Serratia marcescens controls cell membrane permeability, fluidity, hydrophobicity, and fatty acid composition in Escherichia coli BL21

Phospholipase A1 (PlaA) plays a pivotal role in diverse applications within the food and biochemical medical industries. Herein, we investigate the impact of the accessory protein encoded by plaS from Serratia marcescens on PlaA activity in Escherichia coli. Notably, PlaS demonstrates the ability to enhance PlaA activity while concurrently exhibiting inhibitory effects on the growth of E. coli BL21 (DE3). Our study revolves around probing the inhibitory action of PlaS on E. coli BL21 (DE3). PlaS exhibits a propensity to heighten both the permeability of outer and inner cell membranes, leading to concomitant reductions in membrane fluidity and surface hydrophobicity. This phenomenon is validated through scanning electron microscopy (SEM) analysis, which highlights PlaS's capacity to compromise membrane integrity. Moreover, through a comprehensive comparative transcriptomic sequencing approach, we identify four down-regulated genes (galM, ybhC, ldtC, and kdpB) alongside two up-regulated genes (rbsB and degP). These genes are intricately associated with processes such as cell membrane synthesis and modification, energy metabolism, and transmembrane transport. Our investigation unveils the intricate gene-level mechanisms underpinning PlaS-mediated growth inhibition and membrane disruption. Consequently, our findings serve as a significant reference for the elucidation of membrane protein mechanisms, shedding light on potential avenues for future exploration.

Characterization of Pseudomonas aeruginosa l,d-Transpeptidases and Evaluation of Their Role in Peptidoglycan Adaptation to Biofilm Growth

Peptidoglycan is an essential component of the bacterial cell envelope that sustains the turgor pressure of the cytoplasm, determines cell shape, and acts as a scaffold for the anchoring of envelope polymers such as lipoproteins. The final cross-linking step of peptidoglycan polymerization is performed by classical d,d-transpeptidases belonging to the penicillin-binding protein (PBP) family and by l,d-transpeptidases (LDTs), which are dispensable for growth in most bacterial species and whose physiological functions remain elusive. In this study, we investigated the contribution of LDTs to cell envelope synthesis in Pseudomonas aeruginosa grown in planktonic and biofilm conditions. We first assigned a function to each of the three P. aeruginosa LDTs by gene inactivation in P. aeruginosa, heterospecific gene expression in Escherichia coli, and, for one of them, direct determination of its enzymatic activity. We found that the three P. aeruginosa LDTs catalyze peptidoglycan cross-linking (LdtPae1), the anchoring of lipoprotein OprI to the peptidoglycan (LdtPae2), and the hydrolysis of the resulting peptidoglycan-OprI amide bond (LdtPae3). Construction of a phylogram revealed that LDTs performing each of these three functions in various species cannot be assigned to distinct evolutionary lineages, in contrast to what has been observed with PBPs. We showed that biofilm, but not planktonic bacteria, displayed an increase proportion of peptidoglycan cross-links formed by LdtPae1 and a greater extent of OprI anchoring to peptidoglycan, which is controlled by LdtPae2 and LdtPae3. Consistently, deletion of each of the ldt genes impaired biofilm formation and potentiated the bactericidal activity of EDTA. These results indicate that LDTs contribute to the stabilization of the bacterial cell envelope and to the adaptation of peptidoglycan metabolism to growth in biofilm. IMPORTANCE Active-site cysteine LDTs form a functionally heterologous family of enzymes that contribute to the biogenesis of the bacterial cell envelope through formation of peptidoglycan cross-links and through the dynamic anchoring of lipoproteins to peptidoglycan. Here, we report the role of three P. aeruginosa LDTs that had not been previously characterized. We show that these enzymes contribute to resistance to the bactericidal activity of EDTA and to the adaptation of cell envelope polymers to conditions that prevail in biofilms. These results indicate that LDTs should be considered putative targets in the development of drug-EDTA associations for the control of biofilm-related infections.

Deletion of a previously uncharacterized lipoprotein lirL confers resistance to an inhibitor of type II signal peptidase in Acinetobacter baumannii

Inhibiting bacterial lipoprotein biosynthesis in Enterobacteriaceae is an attractive antibacterial strategy to target multidrug resistance, and mechanisms of resistance to prolipoprotein signal peptidase (LspA) inhibitors in Escherichia coli are relatively well understood. In contrast, it has been challenging to understand the mechanisms of resistance to LspA inhibitors in Acinetobacter baumannii due to the substantially lower inhibitor potencies and the lack of a homologous lpp gene. By increasing the antibacterial potency of the LspA inhibitor, globomycin, against wild-type A. baumannii, we were able to examine resistance to LspA inhibitors, resulting in the identification of a previously uncharacterized highly abundant lipoprotein, LspA inhibitor resistance lipoprotein. This study reveals insights into resistance mechanisms of A. baumannii against inhibitors of bacterial lipoprotein biosynthesis.

Acinetobacter baumannii is a clinically important, predominantly health care–associated gram-negative bacterium with high rates of emerging resistance worldwide. Given the urgent need for novel antibacterial therapies against A. baumannii, we focused on inhibiting lipoprotein biosynthesis, a pathway that is essential for envelope biogenesis in gram-negative bacteria. The natural product globomycin, which inhibits the essential type II signal peptidase prolipoprotein signal peptidase (LspA), is ineffective against wild-type A. baumannii clinical isolates due to its poor penetration through the outer membrane. Here, we describe a globomycin analog, G5132, that is more potent against wild-type and clinical A. baumannii isolates. Mutations leading to G5132 resistance in A. baumannii map to the signal peptide of a single hypothetical gene, which we confirm encodes an alanine-rich lipoprotein and have renamed lirL (prolipoprotein signal peptidase inhibitor resistance lipoprotein). LirL is a highly abundant lipoprotein primarily localized to the inner membrane. Deletion of lirL leads to G5132 resistance, inefficient cell division, increased sensitivity to serum, and attenuated virulence. Signal peptide mutations that confer resistance to G5132 lead to the accumulation of diacylglyceryl-modified LirL prolipoprotein in untreated cells without significant loss in cell viability, suggesting that these mutations overcome a block in lipoprotein biosynthetic flux by decreasing LirL prolipoprotein substrate sensitivity to processing by LspA. This study characterizes a lipoprotein that plays a critical role in resistance to LspA inhibitors and validates lipoprotein biosynthesis as a antibacterial target in A. baumannii.

Inhibition of Escherichia coli Lipoprotein Diacylglyceryl Transferase Is Insensitive to Resistance Caused by Deletion of Braun's Lipoprotein

Lipoprotein diacylglyceryl transferase (Lgt) catalyzes the first step in the biogenesis of Gram-negative bacterial lipoproteins which play crucial roles in bacterial growth and pathogenesis. We demonstrate that Lgt depletion in a clinical uropathogenic Escherichia coli strain leads to permeabilization of the outer membrane and increased sensitivity to serum killing and antibiotics. Importantly, we identify G2824 as the first-described Lgt inhibitor that potently inhibits Lgt biochemical activity in vitro and is bactericidal against wild-type Acinetobacter baumannii and E. coli strains. While deletion of a gene encoding a major outer membrane lipoprotein, lpp, leads to rescue of bacterial growth after genetic depletion or pharmacologic inhibition of the downstream type II signal peptidase, LspA, no such rescue of growth is detected after Lgt depletion or treatment with G2824. Inhibition of Lgt does not lead to significant accumulation of peptidoglycan-linked Lpp in the inner membrane. Our data validate Lgt as a novel antibacterial target and suggest that, unlike downstream steps in lipoprotein biosynthesis and transport, inhibition of Lgt may not be sensitive to one of the most common resistance mechanisms that invalidate inhibitors of bacterial lipoprotein biosynthesis and transport.

IMPORTANCE As the emerging threat of multidrug-resistant (MDR) bacteria continues to increase, no new classes of antibiotics have been discovered in the last 50 years. While previous attempts to inhibit the lipoprotein biosynthetic (LspA) or transport (LolCDE) pathways have been made, most efforts have been hindered by the emergence of a common mechanism leading to resistance, namely, the deletion of the gene encoding a major Gram-negative outer membrane lipoprotein lpp. Our unexpected finding that inhibition of Lgt is not susceptible to lpp deletion-mediated resistance uncovers the complexity of bacterial lipoprotein biogenesis and the corresponding enzymes involved in this essential outer membrane biogenesis pathway and potentially points to new antibacterial targets in this pathway.

Brucella Outer Membrane Lipoproteins 19 and 16 Differentially Induce IL-18 Response or Pyroptosis in Human Monocytic Cells

BACKGROUND

Brucella species (B. spp.) are Gram-negative intracellular bacteria, causing severe inflammatory diseases in animals and humans. Two major lipoproteins (L19) and (L16) of Brucella outer membrane proteins (OMPs) were extensively explored in associating with inflammatory response of human monocytes (THP-1).

METHODS

Activated THP-1 cells induced with recombinant L19 and L16 were analyzed in comparison with unlipidated forms (U19 and U16) and lipopolysaccharide (LPS) of B. melitensis, respectively.

RESULTS

Secretion of inflammatory factors TNF-α, IL-6 and IL-1β was significantly increased from L19, L16 or both stimulated THP-1 cells. High secretion of IL-18 was detected only from L19-induced cells. Signaling of those cytokine responses was identified mainly through P38-MAPK pathway, and signaling of L19-induced IL-1β response was partly occurred via NF-κB. Exploration for different forms of IL-18 found that L19-induced production of active IL-18 (18 kD) was through up-regulating NLRP3 and activating caspase-1, while L16-induced production of inactive IL-18 fragments (15 kD and 16 kD) occurred through activating caspase-8/3. Additionally, L19 up-regulated phosphorylation of XIAP for inhibiting caspase-3 activity to cleave IL-18, while L16 activated caspase-3 for producing GSDME-N and leading to pyroptosis of THP-1 cells.

CONCLUSION

Brucella L19 and L16 differentially induce IL-18 response or pyroptosis in THP-1 cells, respectively.

β-Barrel proteins tether the outer membrane in many Gram-negative bacteria

Gram-negative bacteria have a cell envelope that comprises an outer membrane (OM), a peptidoglycan (PG) layer and an inner membrane (IM)¹. The OM and PG are load-bearing, selectively permeable structures that are stabilized by cooperative interactions between IM and OM proteins^2,3. In E. coli, Braun’s lipoprotein (Lpp) forms the only covalent tether between the OM and PG and is crucial for cell envelope stability⁴ but most other Gram-negative bacteria lack Lpp so it has been assumed that alternative mechanisms of OM stabilization are present⁵. We use a glycoproteomic analysis of PG to show that β-barrel OM proteins are covalently attached to PG in several Gram-negative species, including Coxiella burnetii, Agrobacterium tumefaciens and Legionella pneumophila. In C. burnetii, we found that four different types of covalent attachments occur between OM proteins and PG, with tethering of the β-barrel OM protein BbpA becoming most abundant in stationary phase and tethering of the lipoprotein LimB similar throughout the cell-cycle. Using a genetic approach, we demonstrate that the cell-cycle dependent tethering of BbpA is partly dependent on a developmentally regulated L,D transpeptidase (Ldt). We use our findings to propose a model of Gram-negative cell envelope stabilization that includes cell-cycle control and an expanded role for Ldts in covalently attaching surface proteins to PG.

Unstable Mechanisms of Resistance to Inhibitors of Escherichia coli Lipoprotein Signal Peptidase

Despite increasing evidence suggesting that antibiotic heteroresistance can lead to treatment failure, the significance of this phenomena in the clinic is not well understood, because many clinical antibiotic susceptibility testing approaches lack the resolution needed to reliably classify heteroresistant strains. Here we present G0790, a new globomycin analog and potent inhibitor of the Escherichia coli type II signal peptidase LspA. We demonstrate that in addition to previously known mechanisms of resistance to LspA inhibitors, unstable genomic amplifications containing lspA can lead to modest yet biologically significant increases in LspA protein levels that confer a heteroresistance phenotype.

Clinical development of antibiotics with novel mechanisms of action to kill pathogenic bacteria is challenging, in part, due to the inevitable emergence of resistance. A phenomenon of potential clinical importance that is broadly overlooked in preclinical development is heteroresistance, an often-unstable phenotype in which subpopulations of bacterial cells show decreased antibiotic susceptibility relative to the dominant population. Here, we describe a new globomycin analog, G0790, with potent activity against the Escherichia coli type II signal peptidase LspA and uncover two novel resistance mechanisms to G0790 in the clinical uropathogenic E. coli strain CFT073. Building on the previous finding that complete deletion of Lpp, the major Gram-negative outer membrane lipoprotein, leads to globomycin resistance, we also find that an unexpectedly modest decrease in Lpp levels mediated by insertion-based disruption of regulatory elements is sufficient to confer G0790 resistance and increase sensitivity to serum killing. In addition, we describe a heteroresistance phenotype mediated by genomic amplifications of lspA that result in increased LspA levels sufficient to overcome inhibition by G0790 in culture. These genomic amplifications are highly unstable and are lost after as few as two subcultures in the absence of G0790, which places amplification-containing resistant strains at high risk of being misclassified as susceptible by routine antimicrobial susceptibility testing. In summary, our study uncovers two vastly different mechanisms of resistance to LspA inhibitors in E. coli and emphasizes the importance of considering the potential impact of unstable and heterogenous phenotypes when developing antibiotics for clinical use.

YejM Controls LpxC Levels by Regulating Protease Activity of the FtsH/YciM Complex of Escherichia coli

LpxC is a deacetylase that catalyzes the first committed step of lipid A biosynthesis in Escherichia coli LpxC competes for a common precursor, R-3-hydroxymyristoyl-UDP-GlcNAc, with FabZ, whose dehydratase activity catalyzes the first committed step of phospholipid biosynthesis. To maintain the optimum flow of the common precursor to these two competing pathways, the LpxC level is controlled by FtsH/YciM-mediated proteolysis. It is not known whether this complex or another protein senses the status of lipid A synthesis to control LpxC proteolysis. The work carried out in this study began with a novel mutation, yejM1163, which causes hypersensitivity to large antibiotics such as vancomycin and erythromycin. Isolates resistant to these antibiotics carried suppressor mutations in the ftsH and yciM genes. Western blot analysis showed a dramatically reduced LpxC level in the yejM1163 background, while the presence of ftsH or yciM suppressor mutations restored LpxC levels to different degrees. Based on these observations, it is proposed that YejM is a sensor of lipid A synthesis and controls LpxC levels by modulating the activity of the FtsH/YciM complex. The truncation of the periplasmic domain in the YejM1163 protein causes unregulated proteolysis of LpxC, thus diverting a greater pool of R-3-hydroxymyristoyl-UDP-GlcNAc toward phospholipid synthesis. This imbalance in lipid synthesis perturbs the outer membrane permeability barrier, causing hypersensitivity toward vancomycin and erythromycin. yejM1163 suppressor mutations in ftsH and yciM lower the proteolytic activity toward LpxC, thus restoring lipid homeostasis and the outer membrane permeability barrier.IMPORTANCE Lipid homeostasis is critical for proper envelope functions. The level of LpxC, which catalyzes the first committed step of lipopolysaccharide (LPS) synthesis, is controlled by an essential protease complex comprised of FtsH and YciM. Work carried out here suggests YejM, an essential envelope protein, plays a central role in sensing the state of LPS synthesis and controls LpxC levels by regulating the activity of FtsH/YciM. All four essential proteins are attractive targets of therapeutic development.

Establishment of a Protein Concentration Gradient in the Outer Membrane Requires Two Diffusion-Limiting Mechanisms

OmpA-like proteins are involved in the stabilization of the outer membrane, resistance to osmotic stress, and pathogenesis. In Caulobacter crescentus, OmpA2 forms a physiologically relevant concentration gradient that forms by an uncharacterized mechanism, in which the gradient orientation depends on the position of the gene locus. This suggests that OmpA2 is synthesized and translocated to the periplasm close to the position of the gene and that the gradient forms by diffusion of the protein from this point. To further understand how the OmpA2 gradient is established, we determined the localization and mobility of the full protein and of its two structural domains. We show that OmpA2 does not diffuse and that both domains are required for gradient formation. The C-terminal domain binds tightly to the cell wall and the immobility of the full protein depends on the binding of this domain to the peptidoglycan; in contrast, the N-terminal membrane β-barrel diffuses slowly. Our results support a model in which once OmpA2 is translocated to the periplasm, the N-terminal membrane β-barrel is required for an initial fast restriction of diffusion until the position of the protein is stabilized by the binding of the C-terminal domain to the cell wall. The implications of these results on outer membrane protein diffusion and organization are discussed.IMPORTANCE Protein concentration gradients play a relevant role in the organization of the bacterial cell. The Caulobacter crescentus protein OmpA2 forms an outer membrane polar concentration gradient. To understand the molecular mechanism that determines the formation of this gradient, we characterized the mobility and localization of the full protein and of its two structural domains an integral outer membrane β-barrel and a periplasmic peptidoglycan binding domain. Each domain has a different role in the formation of the OmpA2 gradient, which occurs in two steps. We also show that the OmpA2 outer membrane β-barrel can diffuse, which is in contrast to what has been reported previously for several integral outer membrane proteins in Escherichia coli, suggesting a different organization of the outer membrane proteins.

Chemical Changes On, and Through, The Bacterial Envelope in Escherichia coli Mutants Exhibiting Impaired Plasmid Transfer Identified Using Time-of-Flight Secondary Ion Mass Spectrometry

Time-of-flight secondary ion mass spectrometry (ToF-SIMS) using a (CO₂)_6k⁺ gas cluster ion beam (GCIB) was used to analyze Escherichia coli mutants previously identified as having impaired plasmid transfer capability related to the spread of antibiotic resistance. The subset of mutants selected were expected to result in changes in the bacterial envelope composition through the deletion of genes encoding for FabF, DapF, and Lpp, where the surface sensitivity of ToF-SIMS can be most useful. Analysis of arrays of spotted bacteria allowed changes in the lipid composition of the bacteria to be elucidated using multivariate analysis and confirmed through imaging of individual ion signals. Significant changes in chemical composition were observed, including a surprising loss of cyclopropanated fatty acids in the fabF mutant where FabF is associated with the elongation of FA(16:1) to FA(18:1) and not cyclopropane formation. The ability of the GCIB to generate increased higher mass signals from biological samples allowed intact lipid A (m/z 1796) to be detected on the bacteria and, despite a 40 keV impact energy, depth profiled through the bacterial envelope along with other high mass ions including species at m/z 1820 and 2428, attributed to ECA_CYC, that were only observed below the surface of the bacteria and were notably absent in the depth profile of the lpp mutant. The analysis provides new insights into the action of the specific pathways targeted in this study and paves the way for whole new avenues for the characterization of intact molecules within the bacterial envelope.

Details of hydrophobic entanglement between small molecules and Braun's lipoprotein within the cavity of the bacterial chaperone LolA

The cell envelope of Gram-negative bacteria is synthesized and maintained via mechanisms that are targets for development of novel antibiotics. Here we focus on the process of moving Braun's lipoprotein (BLP) from the periplasmic space to the outer membrane of E. coli, via the LolA protein. In contrast to current thinking, we show that binding of multiple inhibitor molecules inside the hydrophobic cavity of LolA does not prevent subsequent binding of BLP inside the same cavity. Rather, based on our atomistic simulations we propose the theory that once inhibitors and BLP are bound inside the cavity of LolA, driven by hydrophobic interactions, they become entangled with each other. Our umbrella sampling calculations show that on the basis of energetics, it is more difficult to dislodge BLP from the cavity of LolA when it is uncomplexed compared to complexed with inhibitor. Thus the inhibitor reduces the affinity of BLP for the LolA cavity.

The Role of Outer Membrane Proteins and Lipopolysaccharides for the Sensitivity of Escherichia coli to Antimicrobial Peptides

Bacterial resistance to classical antibiotics is emerging worldwide. The number of infections caused by multidrug resistant bacteria is increasing and becoming a serious threat for human health globally. In particular, Gram-negative pathogens including multidrug resistant Escherichia coli are of serious concern being resistant to the currently available antibiotics. All Gram-negative bacteria are enclosed by an outer membrane which acts as an additional protection barrier preventing the entry of toxic compounds including antibiotics and antimicrobial peptides (AMPs). In this study we report that the outer membrane component lipopolysaccharide (LPS) plays a crucial role for the antimicrobial susceptibility of E. coli BW25113 against the cationic AMPs Cap18, Cap11, Cap11-1-18m², melittin, indolicidin, cecropin P1, cecropin B, and the polypeptide antibiotic colistin, whereas the outer membrane protease OmpT and the lipoprotein Lpp only play a minor role for the susceptibility against cationic AMPs. Increased susceptibility toward cationic AMPs was found for LPS deficient mutants of E. coli BW25113 harboring deletions in any of the genes required for the inner part of core-oligosaccharide of the LPS, waaC, waaE, waaF, waaG, and gmhA. In addition, our study demonstrates that the antimicrobial activity of Cap18, Cap11, Cap11-1-18m², cecropin B, and cecropin P1 is not only dependent on the inner part of the core oligosaccharide, but also on the outer part and its sugar composition. Finally, we demonstrated that the antimicrobial activity of selected Cap18 derivatives harboring amino acid substitutions in the hydrophobic interface, are non-active against wild-type E. coli ATCC29522. By deleting waaC, waaE, waaF, or waaG the antimicrobial activity of the non-active derivatives can be partially or fully restored, suggesting a very close interplay between the LPS core oligosaccharide and the specific Cap18 derivative. Summarizing, this study implicates that the nature of the outer membrane component LPS has a big impact on the antimicrobial activity of cationic AMPs against E. coli. In particular, the inner as well as the outer part of the core oligosaccharide are important elements determining the antimicrobial susceptibility of E. coli against cationic AMPs.

Lipid moieties on lipoproteins of commensal and non-commensal staphylococci induce differential immune responses

Lipoproteins (Lpp) of Gram-positive bacteria are major players in alerting our immune system. Here, we show that the TLR2 response induced by commensal species Staphylococcus aureus and Staphylococcus epidermidis is almost ten times lower than that induced by noncommensal Staphylococcus carnosus, and this is at least partially due to their different modifications of the Lpp lipid moieties. The N terminus of the lipid moiety is acylated with a long-chain fatty acid (C17) in S. aureus and S. epidermidis, while it is acylated with a short-chain fatty acid (C2) in S. carnosus. The long-chain N-acylated Lpp, recognized by TLR2-TLR1 receptors, silences innate and adaptive immune responses, while the short-chain N-acetylated Lpp, recognized by TLR2-TLR6 receptors, boosts it.

Braun's Lipoprotein Facilitates OmpA Interaction with the Escherichia coli Cell Wall

Gram-negative bacteria such as Escherichia coli are protected by a complex cell envelope. The development of novel therapeutics against these bacteria necessitates a molecular level understanding of the structure-dynamics-function relationships of the various components of the cell envelope. We use atomistic MD simulations to reveal the details of covalent and noncovalent protein interactions that link the outer membrane to the aqueous periplasmic region. We show that the Braun's lipoprotein tilts and bends, and thereby lifts the cell wall closer to the outer membrane. Both monomers and dimers of the outer membrane porin OmpA can interact with peptidoglycan in the presence of Braun's lipoprotein, but in the absence of the latter, only dimers of OmpA show a propensity to form contacts with peptidoglycan. Our study provides a glimpse of how the molecular components of the bacterial cell envelope interact with each other to mediate cell wall attachment in E. coli.

Peptidoglycan Association of Murein Lipoprotein Is Required for KpsD-Dependent Group 2 Capsular Polysaccharide Expression and Serum Resistance in a Uropathogenic Escherichia coli Isolate

Murein lipoprotein (Lpp) and peptidoglycan-associated lipoprotein (Pal) are major outer membrane lipoproteins in Escherichia coli. Their roles in cell-envelope integrity have been documented in E. coli laboratory strains, and while Lpp has been linked to serum resistance in vitro, the underlying mechanism has not been established. Here, lpp and pal mutants of uropathogenic E. coli strain CFT073 showed reduced survival in a mouse bacteremia model, but only the lpp mutant was sensitive to serum killing in vitro. The peptidoglycan-bound Lpp form was specifically required for preventing complement-mediated bacterial lysis in vitro and complement-mediated clearance in vivo. Compared to the wild-type strain, the lpp mutant had impaired K2 capsular polysaccharide production and was unable to respond to exposure to serum by elevating capsular polysaccharide amounts. These properties correlated with altered cellular distribution of KpsD, the predicted outer membrane translocon for “group 2” capsular polysaccharides. We identified a novel Lpp-dependent association between functional KpsD and peptidoglycan, highlighting important interplay between cell envelope components required for resistance to complement-mediated lysis in uropathogenic E. coli isolates.

Uropathogenic E. coli (UPEC) isolates represent a significant cause of nosocomial urinary tract and bloodstream infections. Many UPEC isolates are resistant to serum killing. Here, we show that a major cell-envelope lipoprotein (murein lipoprotein) is required for serum resistance in vitro and for complement-mediated bacterial clearance in vivo. This is mediated, in part, through a novel mechanism by which murein lipoprotein affects the proper assembly of a key component of the machinery involved in production of “group 2” capsules. The absence of murein lipoprotein results in impaired production of the capsule layer, a known participant in complement resistance. These results demonstrate an important role for murein lipoprotein in complex interactions between different outer membrane biogenesis pathways and further highlight the importance of lipoprotein assembly and transport in bacterial pathogenesis.

Lipoproteins of Gram-Positive Bacteria: Key Players in the Immune Response and Virulence

Since the discovery in 1973 of the first of the bacterial lipoproteins (Lpp) in Escherichia coli, Braun's lipoprotein, the ever-increasing number of publications indicates the importance of these proteins. Bacterial Lpp belong to the class of lipid-anchored proteins that in Gram-negative bacteria are anchored in both the cytoplasmic and outer membranes and in Gram-positive bacteria are anchored only in the cytoplasmic membrane. In contrast to the case for Gram-negative bacteria, in Gram-positive bacteria lipoprotein maturation and processing are not vital. Physiologically, Lpp play an important role in nutrient and ion acquisition, allowing particularly pathogenic species to better survive in the host. Bacterial Lpp are recognized by Toll-like receptor 2 (TLR2) of the innate immune system. The important role of Lpp in Gram-positive bacteria, particularly in the phylum Firmicutes, as key players in the immune response and pathogenicity has emerged only in recent years. In this review, we address the role of Lpp in signaling and modulating the immune response, in inflammation, and in pathogenicity. We also address the potential of Lpp as promising vaccine candidates.

Outer membrane lipoprotein biogenesis: Lol is not the end

Bacterial lipoproteins are lipid-anchored proteins that contain acyl groups covalently attached to the N-terminal cysteine residue of the mature protein. Lipoproteins are synthesized in precursor form with an N-terminal signal sequence (SS) that targets translocation across the cytoplasmic or inner membrane (IM). Lipid modification and SS processing take place at the periplasmic face of the IM. Outer membrane (OM) lipoproteins take the localization of lipoproteins (Lol) export pathway, which ends with the insertion of the N-terminal lipid moiety into the inner leaflet of the OM. For many lipoproteins, the biogenesis pathway ends here. We provide examples of lipoproteins that adopt complex topologies in the OM that include transmembrane and surface-exposed domains. Biogenesis of such lipoproteins requires additional steps beyond the Lol pathway. In at least one case, lipoprotein sequences reach the cell surface by being threaded through the lumen of a beta-barrel protein in an assembly reaction that requires the heteropentomeric Bam complex. The inability to predict surface exposure reinforces the importance of experimental verification of lipoprotein topology and we will discuss some of the methods used to study OM protein topology.

Surface-Exposed Lipoproteins: An Emerging Secretion Phenomenon in Gram-Negative Bacteria

Bacterial lipoproteins are hydrophilic proteins that are anchored to a cell membrane by N-terminally linked fatty acids. It is widely believed that nearly all lipoproteins produced by Gram-negative bacteria are either retained in the inner membrane (IM) or transferred to the inner leaflet of the outer membrane (OM). Lipoproteins that are exposed on the cell surface have also been reported but are generally considered to be rare. Results from a variety of recent studies, however, now suggest that the prevalence of surface-exposed lipoproteins has been underestimated. In this review we describe the evidence that the surface exposure of lipoproteins in Gram-negative bacteria is a widespread phenomenon and discuss possible mechanisms by which these proteins might be transported across the OM.

Escherichia coli lipoprotein binds human plasminogen via an intramolecular domain

Escherichia coli lipoprotein (Lpp) is a major cellular component that exists in two distinct states, bound-form and free-form. Bound-form Lpp is known to interact with the periplasmic bacterial cell wall, while free-form Lpp is localized to the bacterial cell surface. A function for surface-exposed Lpp has yet to be determined. We hypothesized that the presence of C-terminal lysinses in the surface-exposed region of Lpp would facilitate binding to the host zymogen plasminogen (Plg), a protease commandeered by a number of clinically important bacteria. Recombinant Lpp was synthesized and the binding of Lpp to Plg, the effect of various inhibitors on this binding, and the effects of various mutations of Lpp on Lpp-Plg interactions were examined. Additionally, the ability of Lpp-bound Plg to be converted to active plasmin was analyzed. We determined that Lpp binds Plg via an atypical domain located near the center of mature Lpp that may not be exposed on the surface of intact E. coli according to the current localization model. Finally, we found that Plg bound by Lpp can be converted to active plasmin. While the consequences of Lpp binding Plg are unclear, these results prompt further investigation of the ability of surface exposed Lpp to interact with host molecules such as extracellular matrix components and complement regulators, and the role of these interactions in infections caused by E. coli and other bacteria.

From cells to muropeptide structures in 24 h: peptidoglycan mapping by UPLC-MS

Peptidoglycan (PGN) is ubiquitous in nearly all bacterial species. The PGN sacculus protects the cells against their own internal turgor making PGN one of the most important targets for antibacterial treatment. Within the last sixty years PGN composition has been intensively studied by various methods. The breakthrough was the application of HPLC technology on the analysis of muropeptides. However, preparation of pure PGN relied on a very time consuming method of about one week. We established a purification protocol for both Gram-positive and Gram-negative bacteria which can be completely performed in plastic reaction tubes yielding pure muropeptides within 24 hours. The muropeptides can be analyzed by UPLC-MS, allowing their immediate determination. This new rapid method provides the feasibility to screen PGN composition even in high throughput, making it a highly useful tool for basic research as well as for the pharmaceutical industry.

Localization of the outer membrane protein OmpA2 in Caulobacter crescentus depends on the position of the gene in the chromosome

The outer membrane of Gram-negative bacteria is an essential structure involved in nutrient uptake, protection against harmful substances, and cell growth. Different proteins keep the outer membrane from blebbing out by simultaneously interacting with it and with the cell wall. These proteins have been mainly studied in enterobacteria, where OmpA and the Braun and Pal lipoproteins stabilize the outer membrane. Some degree of functional redundancy exists between these proteins, since none of them is essential but the absence of two of them results in a severe phenotype. Caulobacter crescentus has a different strategy to maintain its outer membrane, since it lacks the Braun lipoprotein and Pal is essential. In this work, we characterized OmpA2, an OmpA-like protein, in this bacterium. Our results showed that this protein is required for normal stalk growth and that it plays a minor role in the stability of the outer membrane. An OmpA2 fluorescent fusion protein showed that the concentration of this protein decreases from the stalk to the new pole. This localization pattern is important for its function, and it depends on the position of the gene locus in the chromosome and, as a consequence, in the cell. This result suggests that little diffusion occurs from the moment that the gene is transcribed until the mature protein attaches to the cell wall in the periplasm. This mechanism reveals the integration of different levels of information from protein function down to genome arrangement that allows the cell to self-organize.

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.

Phenotypic analysis of Eschericia coli mutants lacking L,D-transpeptidases

Escherichia coli has five genes encoding L,D-transpeptidases (Ldt) with varied functions. Three of these enzymes (YbiS, ErfK, YcfS) have been shown to cross-link Braun's lipoprotein to the peptidoglycan (PG), while the other two (YnhG, YcbB) form direct meso-diaminopimelate (DAP-DAP, or 3-3) cross-links within the PG. In addition, Ldt enzymes can also incorporate non-canonical D-amino acids, such as D-methionine, into the PG. To further investigate the role of these enzymes and, in particular, 3-3 linkages in cell envelope physiology we constructed and phenotypically characterized a variety of multiple Ldt deletion mutants of E. coli. We report that a triple deletion mutant lacking ybiS, erfK and ycfS is hypersusceptible to the metal-chelating agent EDTA, leaks periplasmic proteins and is resistant to the toxic effect of D-methionine. A double ynhG ycbB mutant had no discernible phenotype; however, examination of the phenotypes of various Ldt mutants bearing an additional DAP auxotrophic mutation (dapA : : Cm) showed that a quintuple mutant strain lacking all Ldt genes was severely impaired for growth on media with limited DAP. These data demonstrate that loss of the E. coli Ldt enzymes involved with coupling the PG to Braun's lipoprotein resulted in the loss of outer membrane stability while loss of the Ldt enzymes involved with DAP-DAP linkages had no observable effect on the cell envelope. Loss of all Ldt enzymes proved detrimental to growth when cells were starved for DAP, indicating a combined role for both 3-3 and Braun's lipoprotein cross-links in cell viability only under a specific PG stress.

Envelope control of outer membrane vesicle production in Gram-negative bacteria

All Gram-negative bacteria studied to date have been shown to produce outer membrane vesicles (OMVs), which are budded, released spheres of outer membrane with periplasmic content. OMVs have been implicated in the delivery of virulence factors in pathogenesis. However, OMVs also benefit nonpathogenic species by delivering degradative enzymes to defend an ecological niche against competing bacterial species, and they can serve as an envelope stress response. Despite these important roles, very little is known about the mechanism of production of OMVs. Here we review the advantage of vesiculation, particularly in a nonpathogenic context, as well as the hurdles that have to be overcome in Gram-negative envelope architecture before a vesicle can form and bud. Lastly, we address the question of whether OMV production is a stochastic or regulated process.

The free and bound forms of Lpp occupy distinct subcellular locations in Escherichia coli

The lipoprotein Lpp is the most numerically abundant protein in Escherichia coli, has been investigated for over 40 years, and has served as the paradigmatic bacterial lipoprotein since its initial discovery. It exists in two distinct forms: a 'bound-form', which is covalently bound to the cell's peptidoglycan layer, and a 'free-form', which is not. Although it is known that the carboxyl-terminus of bound-form Lpp is located in the periplasm, the precise location of free-form Lpp has never been determined. For decades, it has been widely assumed that free-form Lpp is associated with bound-form. In this work, we show that the free and bound forms of Lpp are not largely associated with each other, but are found in distinct subcellular locations. Our results indicate that free-form Lpp spans the outer membrane and is surface-exposed, whereas bound-form Lpp resides in the periplasm. Thus, Lpp represents a novel example of a single lipoprotein that is able to occupy distinct subcellular locations, and challenges models in which the free and bound forms of Lpp are assumed to be associated with each other.

Biogenesis and Membrane Targeting of Lipoproteins

Bacterial lipoproteins represent a unique class of membrane proteins, which are anchored to membranes through triacyl chains attached to the amino-terminal cysteine. They are involved in various functions localized in cell envelope. Escherichia coli possesses more than 90 species of lipoproteins, most of which are localized in the outer membrane, with others being in the inner membrane. All lipoproteins are synthesized in the cytoplasm with an N-terminal signal peptide, translocated across the inner membrane by the Sec translocon to the periplasmic surface of the inner membrane, and converted to mature lipoproteins through sequential reactions catalyzed by three lipoprotein-processing enzymes: Lgt, LspA, and Lnt. The sorting of lipoproteins to the outer membrane requires a system comprising five Lol proteins. An ATP-binding cassette transporter, LolCDE, initiates the sorting by mediating the detachment of lipoproteins from the inner membrane. Formation of the LolA-lipoprotein complex is coupled to this LolCDE-dependent release reaction. LolA accommodates the amino-terminal acyl chain of lipoproteins in its hydrophobic cavity, thereby generating a hydrophilic complex that can traverse the periplasmic space by diffusion. Lipoproteins are then transferred to LolB on the outer membrane and anchored to the inner leaflet of the outer membrane by the action of LolB. In contrast, since LolCDE does not recognize lipoproteins possessing Asp at position +2, these lipoproteins remain anchored to the inner membrane. Genes for Lol proteins are widely conserved among gram-negative bacteria, and Lol-mediated outer membrane targeting of lipoproteins is considered to be the general lipoprotein localization mechanism.

In vitro synthesis of cellulose II from a cytoplasmic membrane fraction of Acetobacter xylinum

The cytoplasmic and outer membranes of Acetobacter xylinum (ATCC 53582) were isolated by discontinuous sucrose density ultracentrifugation. Both lysozyme (EC 3.2.1.17) and trypsin (EC 3.4.21.4) were required for efficient crude membrane separation. Primary dehydrogenases and NADH oxidase were used as cytoplasmic membrane markers, and 2-keto-3-deoxyoctulosonic acid was used to identify the outer membranes. Cellulose synthetase (UDP-glucose:1,4-beta-D-glucan 4-beta-D-glucosyltransferase; EC 2.4.1.12) activity was assayed as the conversion of radioactivity from UDP-[(14)C]glucose into an alkali-insoluble beta-1,4-D-[(14)C]glucan. This activity was predominantly found in the cytoplasmic membrane. The cellulose nature of the product was demonstrated by (i) enzymatic hydrolysis followed by TLC, (ii) methylation analysis followed by TLC, and (iii) GC/MS. Further, the weight-average and number-average degree of polymerization of the in vitro product, determined by high-performance gel permeation chromatography, were 4820 and 5270, respectively. In addition, x-ray diffraction analysis indicated that the in vitro product is cellulose II, which is in contrast to the in vivo product-namely, cellulose I.

Capsule anchoring in Bacillus anthracis occurs by a transpeptidation reaction that is inhibited by capsidin

Bacillus anthracis, the causative agent of anthrax, is a dangerous biological weapon, as spores derived from drug-resistant strains cause infections for which antibiotic therapy is no longer effective. We sought to develop an anti-infective therapy for anthrax and targeted CapD, an enzyme that cleaves poly-gamma-D-glutamate capsule and generates amide bonds with peptidoglycan cross-bridges to deposit capsular material into the envelope of B. anthracis. In agreement with the model that capsule confers protection from phagocytic clearance, B. anthracis capD variants failed to deposit capsule into the envelope and displayed defects in anthrax pathogenesis. By screening chemical libraries, we identified the CapD inhibitor capsidin, 4-[(4-bromophenyl)thio]-3-(diacetylamino)benzoic acid), which covalently modifies the active-site threonine of the transpeptidase. Capsidin treatment blocked capsular assembly by B. anthracis and enabled phagocytic killing of non-encapsulated vegetative forms.

Insight from TonB hybrid proteins into the mechanism of iron transport through the outer membrane

We created hybrid proteins to study the functions of TonB. We first fused the portion of Escherichia coli tonB that encodes the C-terminal 69 amino acids (amino acids 170 to 239) of TonB downstream from E. coli malE (MalE-TonB69C). Production of MalE-TonB69C in tonB(+) bacteria inhibited siderophore transport. After overexpression and purification of the fusion protein on an amylose column, we proteolytically released the TonB C terminus and characterized it. Fluorescence spectra positioned its sole tryptophan (W213) in a weakly polar site in the protein interior, shielded from quenchers. Affinity chromatography showed the binding of the TonB C-domain to other proteins: immobilized TonB-dependent (FepA and colicin B) and TonB-independent (FepADelta3-17, OmpA, and lysozyme) proteins adsorbed MalE-TonB69C, revealing a general affinity of the C terminus for other proteins. Additional constructions fused full-length TonB upstream or downstream of green fluorescent protein (GFP). TonB-GFP constructs had partial functionality but no fluorescence; GFP-TonB fusion proteins were functional and fluorescent. The activity of the latter constructs, which localized GFP in the cytoplasm and TonB in the cell envelope, indicate that the TonB N terminus remains in the inner membrane during its biological function. Finally, sequence analyses revealed homology in the TonB C terminus to E. coli YcfS, a proline-rich protein that contains the lysin (LysM) peptidoglycan-binding motif. LysM structural mimicry occurs in two positions of the dimeric TonB C-domain, and experiments confirmed that it physically binds to the murein sacculus. Together, these findings infer that the TonB N terminus remains associated with the inner membrane, while the downstream region bridges the cell envelope from the affinity of the C terminus for peptidoglycan. This architecture suggests a membrane surveillance model of action, in which TonB finds occupied receptor proteins by surveying the underside of peptidoglycan-associated outer membrane proteins.

Covalent attachment of proteins to peptidoglycan

Bacterial surface proteins are key players in host-symbiont or host-pathogen interactions. How these proteins are targeted and displayed at the cell surface are challenging issues of both fundamental and clinical relevance. While surface proteins of Gram-negative bacteria are assembled in the outer membrane, Gram-positive bacteria predominantly utilize their thick cell wall as a platform to anchor their surface proteins. This surface display involves both covalent and noncovalent interactions with either the peptidoglycan or secondary wall polymers such as teichoic acid or lipoteichoic acid. This review focuses on the role of enzymes that covalently link surface proteins to the peptidoglycan, the well-known sortases in Gram-positive bacteria, and the recently characterized l,d-transpeptidases in Gram-negative bacteria.

Murein (peptidoglycan) structure, architecture and biosynthesis in Escherichia coli

The periplasmic murein (peptidoglycan) sacculus is a giant macromolecule made of glycan strands cross-linked by short peptides completely surrounding the cytoplasmic membrane to protect the cell from lysis due to its internal osmotic pressure. More than 50 different muropeptides are released from the sacculus by treatment with a muramidase. Escherichia coli has six murein synthases which enlarge the sacculus by transglycosylation and transpeptidation of lipid II precursor. A set of twelve periplasmic murein hydrolases (autolysins) release murein fragments during cell growth and division. Recent data on the in vitro murein synthesis activities of the murein synthases and on the interactions between murein synthases, hydrolases and cell cycle related proteins are being summarized. There are different models for the architecture of murein and for the incorporation of new precursor into the sacculus. We present a model in which morphogenesis of the rod-shaped E. coli is driven by cytoskeleton elements competing for the control over the murein synthesis multi-enzyme complexes.

Physicochemical and biological analysis of synthetic bacterial lipopeptides: validity of the concept of endotoxic conformation

The importance of the biological function and activity of lipoproteins from the outer or cytoplasmic membranes of Gram-positive and Gram-negative bacteria is being increasingly recognized. It is well established that they are like the endotoxins (lipopolysaccharide (LPS)), which are the main amphiphilic components of the outer membrane of Gram-negative bacteria, potent stimulants of the human innate immune system, and elicit a variety of proinflammatory immune responses. Investigations of synthetic lipopeptides corresponding to N-terminal partial structures of bacterial lipoproteins defined the chemical prerequisites for their biological activity and in particular the number and length of acyl chains and sequence of the peptide part. Here we present experimental data on the biophysical mechanisms underlying lipopeptide bioactivity. Investigation of selected synthetic diacylated and triacylated lipopeptides revealed that the geometry of these molecules (i.e. the molecular conformations and supramolecular aggregate structures) and the preference for membrane intercalation provide an explanation for the biological activities of the different lipopeptides. This refers in particular to the agonistic or antagonistic activity (i.e. their ability to induce cytokines in mononuclear cells or to block this activity, respectively). Biological activity of lipopeptides was hardly affected by the LPS-neutralizing antibiotic polymyxin B, and the biophysical interaction characteristics were found to be in sharp contrast to that of LPS with polymyxin B. The analytical data show that our concept of "endotoxic conformation," originally developed for LPS, can be applied also to the investigated lipopeptide and suggest that the molecular mechanisms of cell activation by amphiphilic molecules are governed by a general principle.

Lipoprotein computational prediction in spirochaetal genomes

Lipoproteins are of great interest in understanding the molecular pathogenesis of spirochaetes. Because spirochaete lipobox sequences exhibit more plasticity than those of other bacteria, application of existing prediction algorithms to emerging sequence data has been problematic. In this paper a novel lipoprotein prediction algorithm is described, designated SpLip, constructed as a hybrid of a lipobox weight matrix approach supplemented by a set of lipoprotein signal peptide rules allowing for conservative amino acid substitutions. Both the weight matrix and the rules are based on a training set of 28 experimentally verified spirochaetal lipoproteins. The performance of the SpLip algorithm was compared to that of the hidden Markov model-based LipoP program and the rules-based algorithm Psort for all predicted protein-coding genes of Leptospira interrogans sv. Copenhageni, L. interrogans sv. Lai, Borrelia burgdorferi, Borrelia garinii, Treponema pallidum and Treponema denticola. Psort sensitivity (13-35 %) was considerably less than that of SpLip (93-100 %) or LipoP (50-84 %) due in part to the requirement of Psort for Ala or Gly at the -1 position, a rule based on E. coli lipoproteins. The percentage of false-positive lipoprotein predictions by the LipoP algorithm (8-30 %) was greater than that of SpLip (0-1 %) or Psort (4-27 %), due in part to the lack of rules in LipoP excluding unprecedented amino acids such as Lys and Arg in the -1 position. This analysis revealed a higher number of predicted spirochaetal lipoproteins than was previously known. The improved performance of the SpLip algorithm provides a more accurate prediction of the complete lipoprotein repertoire of spirochaetes. The hybrid approach of supplementing weight matrix scoring with rules based on knowledge of protein secretion biochemistry may be a general strategy for development of improved prediction algorithms.

Outer membrane protein A, peptidoglycan-associated lipoprotein, and murein lipoprotein are released by Escherichia coli bacteria into serum

Complexes containing lipopolysaccharide (LPS) and three outer membrane proteins (OMPs) are released by gram-negative bacteria incubated in human serum and into the circulation in an experimental model of sepsis. The same OMPs are bound by immunoglobulin G (IgG) in the cross-protective antiserum raised to Escherichia coli J5 (anti-J5 IgG). This study was performed to identify the three OMPs. The 35-kDa OMP was identified as outer membrane protein A (OmpA) by immunoblotting studies using OmpA-deficient bacteria and recombinant OmpA protein. The 18-kDa OMP was identified as peptidoglycan-associated lipoprotein (PAL) based on peptide sequences from the purified protein and immunoblotting studies using PAL-deficient bacteria. The 5- to 9-kDa OMP was identified as murein lipoprotein (MLP) based on immunoblotting studies using MLP-deficient bacteria. The studies identify the OMPs released into human serum and into the circulation in an experimental model of sepsis as OmpA, PAL, and MLP.

Topology and transport of membrane lipids in bacteria

The last two decades have witnessed a break-through in identifying and understanding the functions of both the proteins and lipids of bacterial membranes. This development was parallelled by increasing insights into the biogenesis, topology, transport and sorting of membrane proteins. However, progress in research on the membrane distribution and transport of lipids in bacteria has been slow in that period. The development of novel biochemical in vitro approaches and recent genetic studies have increased our understanding of these subjects. The aim of this review is to present an overview of the current knowledge of the distribution and transport of lipids in both Gram-positive and Gram-negative bacteria. Special attention is paid to recently obtained results, which are expected to inspire further research to finally unravel these poorly understood phenomena.

The mxi-Spa type III secretory pathway of Shigella flexneri requires an outer membrane lipoprotein, MxiM, for invasin translocation

Invasion of epithelial cells by Shigella flexneri is mediated by a set of translocated bacterial invasins, the Ipa proteins, and its dedicated type III secretion system, called Mxi-Spa. We show here that mxiM, part of the mxi-spa locus in the S. flexneri virulence plasmid, encodes an indispensable type III secretion apparatus component, required for both Ipa translocation and tissue culture cell invasion. We demonstrated that mature MxiM, first identified as a putative lipoprotein, is lipidated in vivo. Consistent with features of known lipoproteins, MxiM (i) can be labeled with [3H]palmitate and [2-3H]glycerol, (ii) is associated with the cell envelope, (iii) is secreted independently of the type III pathway, and (iv) requires an intact lipoprotein modification and processing site for full activity. The lipidated form of MxiM was detected primarily in the outer membrane, where it establishes a peripheral association with the inner leaflet. Through analysis of subcellular Ipa distribution in a mxiM null mutant background, MxiM was found to be required for the assembly and/or function of outer, but not inner, membrane regions of Mxi-Spa. This function probably requires interactions with other Mxi-Spa subunits within the periplasmic space. We discuss implications of these findings with respect to the function of MxiM and the structure of Mxi-Spa as a whole.

Surface proteins of gram-positive bacteria and mechanisms of their targeting to the cell wall envelope

The cell wall envelope of gram-positive bacteria is a macromolecular, exoskeletal organelle that is assembled and turned over at designated sites. The cell wall also functions as a surface organelle that allows gram-positive pathogens to interact with their environment, in particular the tissues of the infected host. All of these functions require that surface proteins and enzymes be properly targeted to the cell wall envelope. Two basic mechanisms, cell wall sorting and targeting, have been identified. Cell well sorting is the covalent attachment of surface proteins to the peptidoglycan via a C-terminal sorting signal that contains a consensus LPXTG sequence. More than 100 proteins that possess cell wall-sorting signals, including the M proteins of Streptococcus pyogenes, protein A of Staphylococcus aureus, and several internalins of Listeria monocytogenes, have been identified. Cell wall targeting involves the noncovalent attachment of proteins to the cell surface via specialized binding domains. Several of these wall-binding domains appear to interact with secondary wall polymers that are associated with the peptidoglycan, for example teichoic acids and polysaccharides. Proteins that are targeted to the cell surface include muralytic enzymes such as autolysins, lysostaphin, and phage lytic enzymes. Other examples for targeted proteins are the surface S-layer proteins of bacilli and clostridia, as well as virulence factors required for the pathogenesis of L. monocytogenes (internalin B) and Streptococcus pneumoniae (PspA) infections. In this review we describe the mechanisms for both sorting and targeting of proteins to the envelope of gram-positive bacteria and review the functions of known surface proteins.

Growth of the stress-bearing and shape-maintaining murein sacculus of Escherichia coli

To withstand the high intracellular pressure, the cell wall of most bacteria is stabilized by a unique cross-linked biopolymer called murein or peptidoglycan. It is made of glycan strands [poly-(GlcNAc-MurNAc)], which are linked by short peptides to form a covalently closed net. Completely surrounding the cell, the murein represents a kind of bacterial exoskeleton known as the murein sacculus. Not only does the sacculus endow bacteria with mechanical stability, but in addition it maintains the specific shape of the cell. Enlargement and division of the murein sacculus is a prerequisite for growth of the bacterium. Two groups of enzymes, hydrolases and synthases, have to cooperate to allow the insertion of new subunits into the murein net. The action of these enzymes must be well coordinated to guarantee growth of the stress-bearing sacculus without risking bacteriolysis. Protein-protein interaction studies suggest that this is accomplished by the formation of a multienzyme complex, a murein-synthesizing machinery combining murein hydrolases and synthases. Enlargement of both the multilayered murein of gram-positive and the thin, single-layered murein of gram-negative bacteria seems to follow an inside-to-outside growth strategy. New material is hooked in a relaxed state underneath the stress-bearing sacculus before it becomes inserted upon cleavage of covalent bonds in the layer(s) under tension. A model is presented that postulates that maintenance of bacterial shape is achieved by the enzyme complex copying the preexisting murein sacculus that plays the role of a template.

Demonstration of leptospiral antigens on tissues using monoclonal antibodies and avidin-biotin peroxidase staining

Glycolipoprotein (GLP) cytotoxin was extracted from Leptospira interrogans serovar canicola. The silver staining profile of GLP subjected to SDS-PAGE under denaturing conditions showed a number of bands in the mol. weight range of 14-66 kDa. Mouse Monoclonal Antibodies (MAbs) IgG3 recognizing a band near to 24 kDa of leptospiral GLP were produced (clone number MGLP-01). The agglutinating property of MAbs was established by microscopic agglutination test (MAT) using 25 different serovars as antigens. Only the homologous serovar was agglutinated by MAbs suggesting that the recognized epitope is a specific surface-exposed antigen. The MAbs were applied to demonstration of leptospiral antigens in tissue damage by avidin-biotin immunoperoxidase staining. Golden hamsters were experimentally infected with a virulent strain of L. interrogans serovar canicola. Histologically kidneys stained by routine hematoxylin and eosin showed changes characterized by injury of tubular epithelial cells leading to acute tubular necrosis (ATN). Typical, well-defined morphologic leptospires or finely granular deposits were found by immunoperoxidase staining near to blood vessels, within inflammatory infiltrates and intraluminal in proximal and distal parts of the nephron. Binding of leptospiral antigens to capillary endothelial cells, tubular epithelial cells and macrophages were also demonstrated. This entails a basis for further studies either in research or in diagnostic histopathology.

Anchor structure of staphylococcal surface proteins. A branched peptide that links the carboxyl terminus of proteins to the cell wall

Surface proteins of Staphylococcus aureus are anchored to the cell wall by a mechanism requiring a COOH-terminal sorting signal. Previous work demonstrated that the sorting signal is cleaved at the conserved LPXTG motif and that the carboxyl of threonine (T) is linked to the staphylococcal cell wall. By employing different cell wall lytic enzymes, surface proteins were released from the staphylococcal peptidoglycan and their COOH-terminal anchor structure was revealed by a combination of mass spectrometry and chemical analysis. The results demonstrate that surface proteins are linked to a branched peptide (NH2-Ala-gamma-Gln-Lys-(NH2-Gly5)-Ala-COOH) by an amide bond between the carboxyl of threonine and the amino of the pentaglycine cross-bridge that is attached to the epsilon-amino of lysyl. This branched anchor peptide is amide-linked to the carboxyl of N-acetylmuramic acid, thereby tethering the COOH-terminal end of surface proteins to the staphylococcal peptidoglycan.

Lethality of the covalent linkage between mislocalized major outer membrane lipoprotein and the peptidoglycan of Escherichia coli

The major outer membrane lipoprotein (Lpp) of Escherichia coli possesses serine at position 2, which is thought to function as the outer membrane sorting signal, and lysine at the C terminus, through which Lpp covalently associates with peptidoglycan. Arginine (R) is present before the C-terminal lysine in the wild-type Lpp (LppSK). By replacing serine (S) at position 2 with aspartate (D), the putative inner membrane sorting signal, and by deleting lysine (K) at the C terminus, Lpp mutants with a different residue at either position 2 (LppDK) or the C terminus (LppSR) or both (LppDR) were constructed. Expression of LppSR and LppDR little affected the growth of E. coli. In contrast, the number of viable cells immediately decreased when LppDK was expressed. Prolonged expression of LppDK inhibited separation of the inner and outer membranes by sucrose density gradient centrifugation, whereas short-term expression did not. Pulse-labeled LppDK and LppDR were localized in the inner membrane, indicating that the amino acid residue at position 2 functions as a sorting signal for the membrane localization of Lpp. LppDK accumulated in the inner membrane covalently associated with the peptidoglycan and thus prevented the separation of the two membranes. Globomycin, an inhibitor of lipoprotein-specific signal peptidase II, was lethal for E. coli only when Lpp possessed the C-terminal lysine. Taken together, these results indicate that the inner membrane accumulation of Lpp per se is not lethal for E. coli. Instead, a covalent linkage between the inner membrane Lpp having the C-terminal lysine and the peptidoglycan is lethal for E. coli, presumably due to the disruption of the cell surface integrity.

Na,K-ATPase: a molecular target for Leptospira interrogans endotoxin

On the basis of our report that a glycolipoprotein fraction (GLP) extracted from Leptospira interrogans contains a potent inhibitor of renal Na,K-ATPase, we proposed that GLP-induced inhibition of Na,K-ATPase might be the primary cellular defect in the physiopathology of leptospirosis. The present study was designed to test this hypothesis by determining whether or not 1). GLP inhibits all the isoforms of Na,K-ATPase which are expressed in the tissues affected by leptospirosis, 2) Na,K-ATPase from leptospirosis-resistant species, such as the rat, is sensitive to GLP, 3) GLP inhibits Na,K-ATPase from intact cells, and 4) GLP inhibits ouabain-sensitive H,K-ATPase. The results indicate that in the rabbit, a leptospirosis-sensitive species, GLP inhibits with similar efficiency (apparent IC50: 120-220 micrograms protein GLP/ml) all isoforms of Na,K-ATPase known to be expressed in target tissues for the disease. Na,K-ATPase from rat kidney displays a sensitivity to GLP similar to that of the rabbit kidney enzyme (apparent IC50: 25-80 and 50-150 micrograms protein GLP/ml for rat and rabbit, respectively), indicating that resistance to the disease does not result from the resistance of Na,K-ATPase to GLP. GLP also reduces ouabain-sensitive rubidium uptake in rat thick ascending limbs (pmol mm-1 min-1 +/- SEM; control: 23.8 +/- 1.8; GLP, 88 micrograms protein/ml: 8.2 +/- 0.9), demonstrating that it is active in intact cells. Finally, GLP had no demonstrable effect on renal H,K-ATPase activity, even on the ouabain-sensitive form, indicating that the active principle of GLP is more specific for Na,K-ATPase than ouabain itself. Although the hypothesis remains to be demonstrated in vivo, the present findings are compatible with the putative role of GLP-induced inhibition of Na,K-ATPase as an initial mechanism in the physiopathology of leptospirosis.

Physiological and biochemical analyses of FlgH, a lipoprotein forming the outer membrane L ring of the flagellar basal body of Salmonella typhimurium

The FlgH protein of Salmonella typhimurium, from which the outer membrane L ring of the flagellar basal body is constructed, has a consensus motif (LTG C) for lipoylation and signal peptide cleavage. We have confirmed the previous finding (M. Homma, K. Ohnishi, T. Iino, and R. M. Macnab, J. Bacteriol. 169:3617-3624, 1987) that it is synthesized in precursor form and processed to a mature form with an apparent molecular mass of ca. 25 kDa. flgH alleles with an in-frame deletion or a 3' truncation still permitted processing. The deletion permitted partial restoration of motility in complementation tests, whereas the truncation did not. Globomycin, an antibiotic which inhibits signal peptide cleavage of prolipoproteins, caused accumulation of precursor forms of FlgH. When cells transformed with a plasmid containing the flgH gene were grown in the presence of [3H]palmitate, a 25-kDa protein doublet was found to be radiolabeled; its identity as FlgH was confirmed by shifts in mobility when the internally deleted and truncated alleles of the gene were used. Hook-basal body complexes from cells grown in the presence of [3H]palmitate demonstrated that FlgH incorporated into flagellar structure was also labeled. An in-frame fusion between the leader sequence of the periplasmic protein PeIB and the mature FlgH sequence, with the putative N-terminal cysteine replaced by glycine, resulted in production of a fusion protein that was processed to its mature form. With a low-copy-number plasmid, the ability of this pelB-flgH fusion to complement a flgH mutant was poor, but with a high-copy-number plasmid, it was comparable to that of the wild type. Although lacking the N-terminal cysteine and therefore being incapable of lipoylation via a thioether linkage, the mutant protein still incorporated [3H]palmitate at low levels, perhaps through acylation of the N-terminal alpha-amino group. We conclude that FlgH is a lipoprotein and that under normal physiological conditions the lipoyl modification is necessary for FlgH to function properly as the L-ring protein of the flagellar basal body. We suggest that the N terminus of FlgH is responsible for anchoring the basal body in the outer membrane and that the C terminus may be responsible for binding to the P ring to form the L,P-ring complex.

Cell wall sorting of lipoproteins in Staphylococcus aureus

Many surface proteins are thought to be anchored to the cell wall of gram-positive organisms via their C termini, while the N-terminal domains of these molecules are displayed on the bacterial surface. Cell wall anchoring of surface proteins in Staphylococcus aureus requires both an N-terminal leader peptide and a C-terminal cell wall sorting signal. By fusing the cell wall sorting of protein A to the C terminus of staphylococcal beta-lactamase, we demonstrate here that lipoproteins can also be anchored to the cell wall of S. aureus. The topology of cell wall-anchored beta-lactamase is reminiscent of that described for Braun's murein lipoprotein in that the N terminus of the polypeptide chain is membrane anchored whereas the C-terminal end is tethered to the bacterial cell wall.

Characterization of braun's lipoprotein and determination of its attachment sites to peptidoglycan by (252)Cf-PD and MALDI time-of-flight mass spectrometry

A strategy for the characterization of bacterial lipoprotein-in this case Braun's lipoprotein (an outer membrane 7-ku lipoprotein) isolated from Escherichia coli-is described by time-of-flight mass spectrometric (TOF/MS) techniques [(252)Cf plasma desorption (PD) TOF/MS and matrix-assisted laser desorption-ionization (MALDI) TOF/MS]. Covalent linkage of lipid at the N-terminal cysteine (posttranslationally modified to a S-[2,3-bis(acyloxy)-propyl]-N-acylcysteine) and, therefore, strict insolubility in aqueous solution constitute common features for this class of proteins. Relative molecular mass determination of the major molecular species of Braun's lipoprotein was obtained by selection of an appropriate mixture of organic solvents compatible with matrix/support materials useful for the mass spectrometric techniques applied. Minor components of this lipoprotein that differ only in the fatty acid composition of the lipid anchor were detected by PD TOF/MS after enzymatic release of the extremely hydrophobic N-terminal amino acid followed by selective extraction with chloroform. Part of the primary sequence of this lipoprotein was confirmed based on peptide fragment ions observed in the positive ion PD mass spectra of cyanogen bromide-generated peptide fragments that had been isolated previously by reverse phase high-performance liquid chromatography (HPLC). Peptidoglycan fragments that represent the attachment sites of lipoprotein to peptidoglycan were enzymatically released, separated by reverse phase HPLC, and finally characterized by time-of-flight mass spectrometric techniques ((252)Cf-PD TOF/MS, MALDI TOF/MS). The results obtained with both techniques differed only in the better sensitivity obtained with MALDI TOF/MS, which consumed a factor of 100 to 1000 less material than with PD TOF/MS.