Transmembrane transport of peptidoglycan precursors across model and bacterial membranes

Host defence peptide plectasin targets bacterial cell wall precursor lipid II by a calcium-sensitive supramolecular mechanism

Antimicrobial resistance is a leading cause of mortality, calling for the development of new antibiotics. The fungal antibiotic plectasin is a eukaryotic host defence peptide that blocks bacterial cell wall synthesis. Here, using a combination of solid-state nuclear magnetic resonance, atomic force microscopy and activity assays, we show that plectasin uses a calcium-sensitive supramolecular killing mechanism. Efficient and selective binding of the target lipid II, a cell wall precursor with an irreplaceable pyrophosphate, is achieved by the oligomerization of plectasin into dense supra-structures that only form on bacterial membranes that comprise lipid II. Oligomerization and target binding of plectasin are interdependent and are enhanced by the coordination of calcium ions to plectasin's prominent anionic patch, causing allosteric changes that markedly improve the activity of the antibiotic. Structural knowledge of how host defence peptides impair cell wall synthesis will likely enable the development of superior drug candidates.

Insights into Peptidoglycan-Targeting Radiotracers for Imaging Bacterial Infections: Updates, Challenges, and Future Perspectives

The unique structural architecture of the peptidoglycan allows for the stratification of bacteria as either Gram-negative or Gram-positive, which makes bacterial cells distinguishable from mammalian cells. This classification has received attention as a potential target for diagnostic and therapeutic purposes. Bacteria's ability to metabolically integrate peptidoglycan precursors during cell wall biosynthesis and recycling offers an opportunity to target and image pathogens in their biological state. This Review explores the peptidoglycan biosynthesis for bacteria-specific targeting for infection imaging. Current and potential radiolabeled peptidoglycan precursors for bacterial infection imaging, their development status, and their performance in vitro and/or in vivo are highlighted. We conclude by providing our thoughts on how to shape this area of research for future clinical translation.

Breaking down the cell wall: Still an attractive antibacterial strategy

Since the advent of penicillin, humans have known about and explored the phenomenon of bacterial inhibition via antibiotics. However, with changes in the global environment and the abuse of antibiotics, resistance mechanisms have been selected in bacteria, presenting huge threats and challenges to the global medical and health system. Thus, the study and development of new antimicrobials is of unprecedented urgency and difficulty. Bacteria surround themselves with a cell wall to maintain cell rigidity and protect against environmental insults. Humans have taken advantage of antibiotics to target the bacterial cell wall, yielding some of the most widely used antibiotics to date. The cell wall is essential for bacterial growth and virulence but is absent from humans, remaining a high-priority target for antibiotic screening throughout the antibiotic era. Here, we review the extensively studied targets, i.e., MurA, MurB, MurC, MurD, MurE, MurF, Alr, Ddl, MurI, MurG, lipid A, and BamA in the cell wall, starting from the very beginning to the latest developments to elucidate antimicrobial screening. Furthermore, recent advances, including MraY and MsbA in peptidoglycan and lipopolysaccharide, and tagO, LtaS, LspA, Lgt, Lnt, Tol-Pal, MntC, and OspA in teichoic acid and lipoprotein, have also been profoundly discussed. The review further highlights that the application of new methods such as macromolecular labeling, compound libraries construction, and structure-based drug design will inspire researchers to screen ideal antibiotics.

Metabolic labeling of the bacterial peptidoglycan by functionalized glucosamine

N-Acetylglucosamine (GlcNAc) is an essential monosaccharide required in almost all organisms. Fluorescent labeling of the peptidoglycan (PG) on N-acetylglucosamine has been poorly explored. Here, we report on the labeling of the PG with a bioorthogonal handle on the GlcNAc. We developed a facile one-step synthesis of uridine diphosphate N-azidoacetylglucosamine (UDP-GlcNAz) using the glycosyltransferase OleD, followed by in vitro incorporation of GlcNAz into the peptidoglycan precursor Lipid II and fluorescent labeling of the azido group via click chemistry. In a PG synthesis assay, fluorescent GlcNAz-labeled Lipid II was incorporated into peptidoglycan by the DD-transpeptidase activity of bifunctional class A penicillin-binding proteins. We further demonstrate the incorporation of GlcNAz into the PG layer of OleD-expressed bacteria by feeding with 2-chloro-4-nitrophenyl GlcNAz (GlcNAz-CNP). Hence, our labeling method using the heterologous expression of OleD is useful to study PG synthesis and possibly other biological processes involving GlcNAc metabolism in vivo.

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Peptidoglycan consists of N-acetylglucosamine, N-acetylmuramic acid, and amino acids

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We developed a one-step synthesis of azide-labeled UDP-N-acetylglucosamine

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In vivo generated azide-labeled UDP-N-acetylglucosamine gets incorporated into peptidoglycan

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Bacteria were fluorescently labeled on N-acetylglucosamine of peptidoglycan

The Bacterial Cell Wall: From Lipid II Flipping to Polymerization

The peptidoglycan (PG) cell wall is an extra-cytoplasmic glycopeptide polymeric structure that protects bacteria from osmotic lysis and determines cellular shape. Since the cell wall surrounds the cytoplasmic membrane, bacteria must add new material to the PG matrix during cell elongation and division. The lipid-linked precursor for PG biogenesis, Lipid II, is synthesized in the inner leaflet of the cytoplasmic membrane and is subsequently translocated across the bilayer so that the PG building block can be polymerized and cross-linked by complex multiprotein machines. This review focuses on major discoveries that have significantly changed our understanding of PG biogenesis in the past decade. In particular, we highlight progress made toward understanding the translocation of Lipid II across the cytoplasmic membrane by the MurJ flippase, as well as the recent discovery of a novel class of PG polymerases, the SEDS (shape, elongation, division, and sporulation) glycosyltransferases RodA and FtsW. Since PG biogenesis is an effective target of antibiotics, these recent developments may lead to the discovery of much-needed new classes of antibiotics to fight bacterial resistance.

ddcP, pstB, and excess D-lactate impact synergism between vancomycin and chlorhexidine against Enterococcus faecium 1,231,410

Vancomycin-resistant enterococci (VRE) are important nosocomial pathogens that cause life-threatening infections. To control hospital-associated infections, skin antisepsis and bathing utilizing chlorhexidine is recommended for VRE patients in acute care hospitals. Previously, we reported that exposure to inhibitory chlorhexidine levels induced the expression of vancomycin resistance genes in VanA-type Enterococcus faecium. However, vancomycin susceptibility actually increased for VanA-type E. faecium in the presence of chlorhexidine. Hence, a synergistic effect of the two antimicrobials was observed. In this study, we used multiple approaches to investigate the mechanism of synergism between chlorhexidine and vancomycin in the VanA-type VRE strain E. faecium 1,231,410. We generated clean deletions of 7 of 11 pbp, transpeptidase, and carboxypeptidase genes in this strain (ponA, pbpF, pbpZ, pbpA, ddcP, ldt_fm, and vanY). Deletion of ddcP, encoding a membrane-bound carboxypeptidase, altered the synergism phenotype. Furthermore, using in vitro evolution, we isolated a spontaneous synergy escaper mutant and utilized whole genome sequencing to determine that a mutation in pstB, encoding an ATPase of phosphate-specific transporters, also altered synergism. Finally, addition of excess D-lactate, but not D-alanine, enhanced synergism to reduce vancomycin MIC levels. Overall, our work identified factors that alter chlorhexidine and vancomycin synergism in a model VanA-type VRE strain.

Peptidoglycan: Structure, Synthesis, and Regulation

Peptidoglycan is a defining feature of the bacterial cell wall. Initially identified as a target of the revolutionary beta-lactam antibiotics, peptidoglycan has become a subject of much interest for its biology, its potential for the discovery of novel antibiotic targets, and its role in infection. Peptidoglycan is a large polymer that forms a mesh-like scaffold around the bacterial cytoplasmic membrane. Peptidoglycan synthesis is vital at several stages of the bacterial cell cycle: for expansion of the scaffold during cell elongation and for formation of a septum during cell division. It is a complex multifactorial process that includes formation of monomeric precursors in the cytoplasm, their transport to the periplasm, and polymerization to form a functional peptidoglycan sacculus. These processes require spatio-temporal regulation for successful assembly of a robust sacculus to protect the cell from turgor and determine cell shape. A century of research has uncovered the fundamentals of peptidoglycan biology, and recent studies employing advanced technologies have shed new light on the molecular interactions that govern peptidoglycan synthesis. Here, we describe the peptidoglycan structure, synthesis, and regulation in rod-shaped bacteria, particularly Escherichia coli, with a few examples from Salmonella and other diverse organisms. We focus on the pathway of peptidoglycan sacculus elongation, with special emphasis on discoveries of the past decade that have shaped our understanding of peptidoglycan biology.

Squalamine and Aminosterol Mimics Inhibit the Peptidoglycan Glycosyltransferase Activity of PBP1b

Peptidoglycan (PG) is an essential polymer of the bacterial cell wall and a major antibacterial target. Its synthesis requires glycosyltransferase (GTase) and transpeptidase enzymes that, respectively, catalyze glycan chain elongation and their cross-linking to form the protective sacculus of the bacterial cell. The GTase domain of bifunctional penicillin-binding proteins (PBPs) of class A, such as Escherichia coli PBP1b, belong to the GTase 51 family. These enzymes play an essential role in PG synthesis, and their specific inhibition by moenomycin was shown to lead to bacterial cell death. In this work, we report that the aminosterol squalamine and mimic compounds present an unexpected mode of action consisting in the inhibition of the GTase activity of the model enzyme PBP1b. In addition, selected compounds were able to specifically displace the lipid II from the active site in a fluorescence anisotropy assay, suggesting that they act as competitive inhibitors.

Fluorescence anisotropy assays for high throughput screening of compounds binding to lipid II, PBP1b, FtsW and MurJ

Lipid II precursor and its processing by a flippase and peptidoglycan polymerases are considered key hot spot targets for antibiotics. We have developed a fluorescent anisotropy (FA) assay using a unique and versatile probe (fluorescent lipid II) and monitored direct binding between lipid II and interacting proteins (PBP1b, FtsW and MurJ), as well as between lipid II and interacting antibiotics (vancomycin, nisin, ramoplanin and a small molecule). Competition experiments performed using unlabelled lipid II, four lipid II-binding antibiotics and moenomycin demonstrate that the assay can detect compounds interacting with lipid II or the proteins. These results provide a proof-of-concept for the use of this assay in a high-throughput screening of compounds against all these targets. In addition, the assay constitutes a powerful tool in the study of the mode of action of compounds that interfere with these processes. Interestingly, FA assay with lipid II probe has the advantage over moenomycin based probe to potentially identify compounds that interfere with both donor and acceptor sites of the aPBPs GTase as well as compounds that bind to lipid II. In addition, this assay would allow the screening of compounds against SEDS proteins and MurJ which do not interact with moenomycin.

Lipid Phases and Cell Geometry During the Cell Cycle of Streptococcus pneumoniae

The coexistence of different lipid phases is well-known in vitro, but evidence for their presence and function in cellular membranes remains scarce. Using a combination of fluorescent lipid probes, we observe segregation of domains that suggests the coexistence of liquid and gel phases in the membrane of Streptococcus pneumoniae, where they are localized to minimize bending stress in the ellipsoid geometry defined by the cell wall. Gel phase lipids with high bending rigidity would be spontaneously organized at the equator where curvature is minimal, thus marking the future division site, while liquid phase membrane maps onto the oblong hemispheres. In addition, the membrane-bound cell wall precursor with its particular dynamic acyl chain localizes at the division site where the membrane is highly curved. We propose a complete “chicken-and-egg” model where cell geometry determines the localization of lipid phases that positions the cell division machinery, which in turn alters the localization of lamellar phases by assembling the cell wall with a specific geometry.

Transport of organic substances through the cytoplasmic membrane of cyanobacteria

Cyanobacteria are mainly known to incorporate inorganic molecules like carbon dioxide and ammonia from the environment into organic material within the cell. Nevertheless cyanobacteria do import and export organic substances through the cytoplasmic membrane and these processes are essential for all cyanobacteria. In addition understanding the mechanisms of transport of organic molecules through the cytoplasmic membrane might become very important. Genetically modified strains of cyanobacteria could serve as producers and exporters of commercially important substances. In this review we attempt to present all data of transport of organic molecules through the cytoplasmic membrane of cyanobacteria that are currently available with the transported molecules ordered according to their chemical classes.

Enzyme structures of the bacterial peptidoglycan and wall teichoic acid biogenesis pathways

The bacterial cell wall is a complex polymeric structure with essential roles in defence, survival and pathogenesis. Common to both Gram-positive and Gram-negative bacteria is the mesh-like peptidoglycan sacculus that surrounds the outer leaflet of the cytoplasmic membrane. Recent crystallographic studies of enzymes that comprise the peptidoglycan biosynthetic pathway have led to significant new understanding of all stages. These include initial multi-step cytosolic formation of sugar-pentapeptide precursors, transfer of the precursors to activated polyprenyl lipids at the membrane inner leaflet and flippase mediated relocalization of the resulting lipid II precursors to the outer leaflet where glycopolymerization and subsequent peptide crosslinking are finalized. Additional, species-specific enzymes allow customized peptidoglycan modifications and biosynthetic regulation that are important to bacterial virulence and survival. These studies have reinforced the unique and specific catalytic mechanisms at play in cell wall biogenesis and expanded the atomic foundation to develop novel, structure guided, antibacterial agents.

Targeting a cell wall biosynthesis hot spot

Covering: up to 2017History points to the bacterial cell wall biosynthetic network as a very effective target for antibiotic intervention, and numerous natural product inhibitors have been discovered. In addition to the inhibition of enzymes involved in the multistep synthesis of the macromolecular layer, in particular, interference with membrane-bound substrates and intermediates essential for the biosynthetic reactions has proven a valuable antibacterial strategy. A prominent target within the peptidoglycan biosynthetic pathway is lipid II, which represents a particular "Achilles' heel" for antibiotic attack, as it is readily accessible on the outside of the cytoplasmic membrane. Lipid II is a unique non-protein target that is one of the structurally most conserved molecules in bacterial cells. Notably, lipid II is more than just a target molecule, since sequestration of the cell wall precursor may be combined with additional antibiotic activities, such as the disruption of membrane integrity or disintegration of membrane-bound multi-enzyme machineries. Within the membrane bilayer lipid II is likely organized in specific anionic phospholipid patches that form a particular "landing platform" for antibiotics. Nature has invented a variety of different "lipid II binders" of at least 5 chemical classes, and their antibiotic activities can vary substantially depending on the compounds' physicochemical properties, such as amphiphilicity and charge, and thus trigger diverse cellular effects that are decisive for antibiotic activity.

Genetics of Peptidoglycan Biosynthesis

The complex cell envelope is a hallmark of mycobacteria and is anchored by the peptidoglycan layer, which is similar to that of Escherichia coli and a number of other bacteria but with modifications to the monomeric units and other structural complexities that are likely related to a role for the peptidoglycan in stabilizing the mycolyl-arabinogalactan-peptidoglycan complex (MAPc). In this article, we will review the genetics of several aspects of peptidoglycan biosynthesis in mycobacteria, including the production of monomeric precursors in the cytoplasm, assembly of the monomers into the mature wall, cell wall turnover, and cell division. Finally, we will touch upon the resistance of mycobacteria to β-lactam antibiotics, an important class of drugs that, until recently, have not been extensively exploited as potential antimycobacterial agents. We will also note areas of research where there are still unanswered questions.

Lipid Flippases for Bacterial Peptidoglycan Biosynthesis

The biosynthesis of cellular polysaccharides and glycoconjugates often involves lipid-linked intermediates that need to be translocated across membranes. Essential pathways such as N-glycosylation in eukaryotes and biogenesis of the peptidoglycan (PG) cell wall in bacteria share a common strategy where nucleotide-sugars are used to build a membrane-bound oligosaccharide precursor that is linked to a phosphorylated isoprenoid lipid. Once made, these lipid-linked intermediates must be translocated across a membrane so that they can serve as substrates in a different cellular compartment. How translocation occurs is poorly understood, although it clearly requires a transporter or flippase. Identification of these transporters is notoriously difficult, and, in particular, the identity of the flippase of lipid II, an intermediate required for PG biogenesis, has been the subject of much debate. Here, I will review the body of work that has recently fueled this controversy, centered on proposed flippase candidates FtsW, MurJ, and AmJ.

Core Steps of Membrane-Bound Peptidoglycan Biosynthesis: Recent Advances, Insight and Opportunities

We are entering an era where the efficacy of current antibiotics is declining, due to the development and widespread dispersion of antibiotic resistance mechanisms. These factors highlight the need for novel antimicrobial discovery. A large number of antimicrobial natural products elicit their effect by directly targeting discrete areas of peptidoglycan metabolism. Many such natural products bind directly to the essential cell wall precursor Lipid II and its metabolites, i.e., preventing the utlisation of vital substrates by direct binding rather than inhibiting the metabolising enzymes themselves. Concurrently, there has been an increase in the knowledge surrounding the proteins essential to the metabolism of Lipid II at and across the cytoplasmic membrane. In this review, we draw these elements together and look to future antimicrobial opportunities in this area.

Activities and regulation of peptidoglycan synthases

Peptidoglycan (PG) is an essential component in the cell wall of nearly all bacteria, forming a continuous, mesh-like structure, called the sacculus, around the cytoplasmic membrane to protect the cell from bursting by its turgor. Although PG synthases, the penicillin-binding proteins (PBPs), have been studied for 70 years, useful in vitro assays for measuring their activities were established only recently, and these provided the first insights into the regulation of these enzymes. Here, we review the current knowledge on the glycosyltransferase and transpeptidase activities of PG synthases. We provide new data showing that the bifunctional PBP1A and PBP1B from Escherichia coli are active upon reconstitution into the membrane environment of proteoliposomes, and that these enzymes also exhibit DD-carboxypeptidase activity in certain conditions. Both novel features are relevant for their functioning within the cell. We also review recent data on the impact of protein-protein interactions and other factors on the activities of PBPs. As an example, we demonstrate a synergistic effect of multiple protein-protein interactions on the glycosyltransferase activity of PBP1B, by its cognate lipoprotein activator LpoB and the essential cell division protein FtsN.

One-pot protection-glycosylation reactions for synthesis of lipid II analogues

(2,6-Dichloro-4-methoxyphenyl)(2,4-dichlorophenyl)methyl trichloroacetimidate (3) and its polymer-supported reagent 4 can be successfully applied to a one-pot protection-glycosylation reaction to form the disaccharide derivative 7 d for the synthesis of lipid II analogues. The temporary protecting group or linker at the C-6 position and N-Troc protecting group of 7 d can be cleaved simultaneously through a reductive condition. Overall yields of syntheses of lipid II (1) and neryl-lipid II N(ε)-dansylthiourea are significantly improved by using the described methods.

Specificity of the transport of lipid II by FtsW in Escherichia coli

Synthesis of biogenic membranes requires transbilayer movement of lipid-linked sugar molecules. This biological process, which is fundamental in prokaryotic cells, remains as yet not clearly understood. In order to obtain insights into the molecular basis of its mode of action, we analyzed the structure-function relationship between Lipid II, the important building block of the bacterial cell wall, and its inner membrane-localized transporter FtsW. Here, we show that the predicted transmembrane helix 4 of Escherichia coli FtsW (this protein consists of 10 predicted transmembrane segments) is required for the transport activity of the protein. We have identified two charged residues (Arg(145) and Lys(153)) within this segment that are specifically involved in the flipping of Lipid II. Mutating these two amino acids to uncharged ones affected the transport activity of FtsW. This was consistent with loss of in vivo activity of the mutants, as manifested by their inability to complement a temperature-sensitive strain of FtsW. The transport activity of FtsW could be inhibited with a Lipid II variant having an additional size of 420 Da. Reducing the size of this analog by about 274 Da resulted in the resumption of the transport activity of FtsW. This suggests that the integral membrane protein FtsW forms a size-restricted porelike structure, which accommodates Lipid II during transport across the bacterial cytoplasmic membrane.

A Burkholderia cenocepacia MurJ (MviN) homolog is essential for cell wall peptidoglycan synthesis and bacterial viability

The cell wall peptidoglycan (PG) of Burkholderia cenocepacia, an opportunistic pathogen, has not yet been characterized. However, the B. cenocepacia genome contains homologs of genes encoding PG biosynthetic functions in other bacteria. PG biosynthesis involves the formation of the undecaprenyl-pyrophosphate-linked N-acetyl glucosamine-N-acetyl muramic acid-pentapeptide, known as lipid II, which is built on the cytosolic face of the cell membrane. Lipid II is then translocated across the membrane and its glycopeptide moiety becomes incorporated into the growing cell wall mesh; this translocation step is critical to PG synthesis. We have investigated candidate flippase homologs of the MurJ family in B. cenocepacia. Our results show that BCAL2764, herein referred to as murJBc, is indispensable for viability. Viable B. cenocepacia could only be obtained through a conditional mutagenesis strategy by placing murJBc under the control of a rhamnose-inducible promoter. Under rhamnose depletion, the conditional strain stopped growing and individual cells displayed morphological abnormalities consistent with a defect in PG synthesis. Bacterial cells unable to express MurJBc underwent cell lysis, while partial MurJBc depletion sensitized the mutant to the action of β-lactam antibiotics. Depletion of MurJBc caused accumulation of PG precursors consistent with the notion that this protein plays a role in lipid II flipping to the periplasmic compartment. Reciprocal complementation experiments of conditional murJ mutants in B. cenocepacia and Escherichia coli with plasmids expressing MurJ from each strain indicated that MurJBc and MurJEc are functional homologs. Together, our results are consistent with the notion that MurJBc is a PG lipid II flippase in B. cenocepacia.

Vancomycin: ligand recognition, dimerization and super-complex formation

The antibiotic vancomycin targets lipid II, blocking cell wall synthesis in Gram-positive bacteria. Despite extensive study, questions remain regarding how it recognizes its primary ligand and what is the most biologically relevant form of vancomycin. In this study, molecular dynamics simulation techniques have been used to examine the process of ligand binding and dimerization of vancomycin. Starting from one or more vancomycin monomers in solution, together with different peptide ligands derived from lipid II, the simulations predict the structures of the ligated monomeric and dimeric complexes to within 0.1 nm rmsd of the structures determined experimentally. The simulations reproduce the conformation transitions observed by NMR and suggest that proposed differences between the crystal structure and the solution structure are an artifact of the way the NMR data has been interpreted in terms of a structural model. The spontaneous formation of both back-to-back and face-to-face dimers was observed in the simulations. This has allowed a detailed analysis of the origin of the cooperatively between ligand binding and dimerization and suggests that the formation of face-to-face dimers could be functionally significant. The work also highlights the possible role of structural water in stabilizing the vancomycin ligand complex and its role in the manifestation of vancomycin resistance.

Structural perspective of peptidoglycan biosynthesis and assembly

The peptidoglycan biosynthetic pathway is a critical process in the bacterial cell and is exploited as a target for the design of antibiotics. This pathway culminates in the production of the peptidoglycan layer, which is composed of polymerized glycan chains with cross-linked peptide substituents. This layer forms the major structural component of the protective barrier known as the cell wall. Disruption in the assembly of the peptidoglycan layer causes a weakened cell wall and subsequent bacterial lysis. With bacteria responsible for both properly functioning human health (probiotic strains) and potentially serious illness (pathogenic strains), a delicate balance is necessary during clinical intervention. Recent research has furthered our understanding of the precise molecular structures, mechanisms of action, and functional interactions involved in peptidoglycan biosynthesis. This research is helping guide our understanding of how to capitalize on peptidoglycan-based therapeutics and, at a more fundamental level, of the complex machinery that creates this critical barrier for bacterial survival.

Characterization of the elongasome core PBP2 : MreC complex of Helicobacter pylori

The definition of bacterial cell shape is a complex process requiring the participation of multiple components of an intricate macromolecular machinery. We aimed at characterizing the determinants involved in cell shape of the helical bacterium Helicobacter pylori. Using a yeast two-hybrid screen with the key cell elongation protein PBP2 as bait, we identified an interaction between PBP2 and MreC. The minimal region of MreC required for this interaction ranges from amino acids 116 to 226. Using recombinant proteins, we showed by affinity and size exclusion chromatographies and surface plasmon resonance that PBP2 and MreC form a stable complex. In vivo, the two proteins display a similar spatial localization and their complex has an apparent 1:1 stoichiometry; these results were confirmed in vitro by analytical ultracentrifugation and chemical cross-linking. Small angle X-ray scattering analyses of the PBP2 : MreC complex suggest that MreC interacts directly with the C-terminal region of PBP2. Depletion of either PBP2 or MreC leads to transition into spherical cells that lose viability. Finally, the specific expression in trans of the minimal interacting domain of MreC with PBP2 in the periplasmic space leads to cell rounding, suggesting that the PBP2/MreC complex formation in vivo is essential for cell morphology.

A novel in vivo cell-wall labeling approach sheds new light on peptidoglycan synthesis in Escherichia coli

Peptidoglycan synthesis and turnover in relation to cell growth and division has been studied by using a new labeling method. This method involves the incorporation of fluorescently labeled peptidoglycan precursors into the cell wall by means of the cell-wall recycling pathway. We show that Escherichia coli is able to import exogenous added murein tripeptide labeled with N-7-nitro-2,1,3-benzoxadiazol-4-yl (AeK-NBD) into the cytoplasm where it enters the peptidoglycan biosynthesis route, resulting in fluorescent labels specifically located in the cell wall. When wild-type cells were grown in the presence of the fluorescent peptide, peptidoglycan was uniformly labeled in cells undergoing elongation. Cells in the process of division displayed a lack of labeled peptidoglycan at mid-cell. Analysis of labeling patterns in cell division mutants showed that the occurrence of unlabeled peptidoglycan is dependent on the presence of FtsZ, but independent of FtsQ and FtsI. Accumulation of fluorescence at the division sites of a triple amidase mutant (ΔamiABC) revealed that AeK-NBD is incorporated into septal peptidoglycan. AmiC was shown to be involved in the rapid removal of labeled peptidoglycan side chains at division sites in wild-type cells. Because septal localization of AmiC is dependent on FtsQ and FtsI, this points to the presence of another peptidoglycan hydrolase activity directly dependent on FtsZ.

Identification of FtsW as a transporter of lipid-linked cell wall precursors across the membrane

This study identifies FtsW as the flippase that translocates lipid-linked peptidoglycan precursors across the cell membrane during bacterial cell wall synthesis.

Bacterial cell growth necessitates synthesis of peptidoglycan. Assembly of this major constituent of the bacterial cell wall is a multistep process starting in the cytoplasm and ending in the exterior cell surface. The intracellular part of the pathway results in the production of the membrane-anchored cell wall precursor, Lipid II. After synthesis this lipid intermediate is translocated across the cell membrane. The translocation (flipping) step of Lipid II was demonstrated to require a specific protein (flippase). Here, we show that the integral membrane protein FtsW, an essential protein of the bacterial division machinery, is a transporter of the lipid-linked peptidoglycan precursors across the cytoplasmic membrane. Using Escherichia coli membrane vesicles we found that transport of Lipid II requires the presence of FtsW, and purified FtsW induced the transbilayer movement of Lipid II in model membranes. This study provides the first biochemical evidence for the involvement of an essential protein in the transport of lipid-linked cell wall precursors across biogenic membranes.

Broad-spectrum antimicrobial peptide resistance by MprF-mediated aminoacylation and flipping of phospholipids

Bacteria are frequently exposed to cationic antimicrobial peptides (CAMPs) from eukaryotic hosts (host defence peptides) or from prokaryotic competitors (bacteriocins). However, many bacteria, among them most of the major human pathogens, achieve CAMP resistance by MprF, a unique enzyme that modifies anionic phospholipids with l-lysine or l-alanine thereby introducing positive charges into the membrane surface and reducing the affinity for CAMPs. The lysyl or alanyl groups are derived from aminoacyl tRNAs and are usually transferred to phosphatidylglycerol (PG). Recent studies with MprF from Staphylococcus aureus demonstrated that production of Lys-PG only leads to CAMP resistance when an additional flippase domain of MprF is present that translocates Lys-PG and exposes it at the outer leaflet of the membrane. Thus, MprF exerts two specific functions that have hardly been found in other bacterial proteins. MprF proteins are crucial virulence factors of many human pathogens, which recommends them as targets for new anti-virulence drugs. Intriguingly, specific point mutations in mprF cause resistance to the CAMP-like antibiotic daptomycin in a yet unclear way that may involve altered Lys-PG synthesis and/or Lys-PG flipping capacities. Thus, a thorough characterization of MprF domains and functions will help to unravel how bacteria maintain and protect their cytoplasmic membranes.

Deciphering the catalytic domain of colicin M, a peptidoglycan lipid II-degrading enzyme

Colicin M inhibits Escherichia coli peptidoglycan synthesis through cleavage of its lipid-linked precursors. It has a compact structure, whereas other related toxins are organized in three independent domains, each devoted to a particular function: translocation through the outer membrane, receptor binding, and toxicity, from the N to the C termini, respectively. To establish whether colicin M displays such an organization despite its structural characteristics, protein dissection experiments were performed, which allowed us to delineate an independent toxicity domain encompassing exactly the C-terminal region conserved among colicin M-like proteins and covering about half of colicin M (residues 124-271). Surprisingly, the in vitro activity of the isolated domain was 45-fold higher than that of the full-length protein, suggesting a mechanism by which the toxicity of this domain is revealed following primary protein maturation. In vivo, the isolated toxicity domain appeared as toxic as the full-length protein under conditions where the reception and translocation steps were by-passed. Contrary to the full-length colicin M, the isolated domain did not require the presence of the periplasmic FkpA protein to be toxic under these conditions, demonstrating that FkpA is involved in the maturation process. Mutational analysis further identified five residues that are essential for cytotoxicity as well as in vitro lipid II-degrading activity: Asp-229, His-235, Asp-226, Tyr-228, and Arg-236. Most of these residues are surface-exposed and located relatively close to each other, hence suggesting they belong to the colicin M active site.

Flipping lipids: why an' what's the reason for?

The biosynthesis of glycoconjugates such as N-glycoproteins and GPI-anchored proteins in eukaryotes and cell wall peptidoglycan and lipopolysaccharide in bacteria requires lipid intermediates to be flipped rapidly across the endoplasmic reticulum or bacterial cytoplasmic membrane (so-called biogenic membranes). Rapid flipping is also required to normalize the number of glycerophospholipids in the two leaflets of the bilayer as the membrane expands in a growing cell. Although lipids diffuse rapidly in the plane of the membrane, the intrinsic rate at which they flip across membranes is very low. Biogenic membranes possess dedicated lipid transporters or flippases to increase flipping to a physiologically sufficient rate. The flippases are "ATP-independent" and facilitate "downhill" transport. Most predicted biogenic membrane flippases have not been identified at the molecular level, and the few flippases that have been identified by genetic approaches have not been biochemically validated. Here we summarize recent progress on this fundamental topic and speculate on the mechanism(s) by which biogenic membrane flippases facilitate transbilayer lipid movement.

Bacillus subtilis homologs of MviN (MurJ), the putative Escherichia coli lipid II flippase, are not essential for growth

Although peptidoglycan synthesis is one of the best-studied metabolic pathways in bacteria, the mechanism underlying the membrane translocation of lipid II, the undecaprenyl-disaccharide pentapeptide peptidoglycan precursor, remains mysterious. Recently, it was proposed that the essential Escherichia coli mviN gene encodes the lipid II flippase. Bacillus subtilis contains four proteins that are putatively homologous to MviN, including SpoVB, previously reported to be necessary for spore cortex peptidoglycan synthesis during sporulation. MviN complemented the sporulation defect of a DeltaspoVB mutation, and SpoVB and another of the B. subtilis homologs, YtgP, complemented the growth defect of an E. coli strain depleted for MviN. Thus, these B. subtilis proteins are likely to be MviN homologs. However, B. subtilis strains lacking these four proteins have no defects in growth, indicating that they likely do not serve as lipid II flippases in this organism.

Specific labeling of peptidoglycan precursors as a tool for bacterial cell wall studies

Because of its importance for bacterial cell survival, the bacterial cell wall is an attractive target for new antibiotics in a time of great demand for new antibiotic compounds. Therefore, more knowledge about the diverse processes related to bacterial cell wall synthesis is needed. The cell wall is located on the exterior of the cell and consists mainly of peptidoglycan, a large macromolecule built up from a three-dimensional network of aminosugar strands interlinked with peptide bridges. The subunits of peptidoglycan are synthesized inside the cell before they are transported to the exterior in order to be incorporated into the growing peptidoglycan. The high flexibility of the cell wall synthesis machinery towards unnatural derivatives of these subunits enables research on the bacterial cell wall using labeled compounds. This review highlights the high potential of labeled cell wall precursors in various areas of cell wall research. Labeled precursors can be used in investigating direct cell wall-antibiotic interactions and in cell wall synthesis and localization studies. Moreover, these compounds can provide a powerful tool in the elucidation of the cell wall proteome, the "wallosome," and thus, might provide new targets for antibiotics.

Medium-dependent phenotypes of Streptomyces coelicolor with mutations in ftsI or ftsW

Streptomyces coelicolor A3(2) ftsI- and ftsW-null mutants produced aerial hyphae with no evidence of septation when grown on a traditional osmotically enhanced medium. This phenotype was partially suppressed when cultures were grown on media prepared without sucrose. We infer that functional FtsZ rings can form in ftsI- and ftsW-null mutants under certain growth conditions.

Bioinformatics identification of MurJ (MviN) as the peptidoglycan lipid II flippase in Escherichia coli

Peptidoglycan is a cell-wall glycopeptide polymer that protects bacteria from osmotic lysis. Whereas in gram-positive bacteria it also serves as scaffold for many virulence factors, in gram-negative bacteria, peptidoglycan is an anchor for the outer membrane. For years, we have known the enzymes required for the biosynthesis of peptidoglycan; what was missing was the flippase that translocates the lipid-anchored precursors across the cytoplasmic membrane before their polymerization into mature peptidoglycan. Using a reductionist bioinformatics approach, I have identified the essential inner-membrane protein MviN (renamed MurJ) as a likely candidate for the peptidoglycan flippase in Escherichia coli. Here, I present genetic and biochemical data that confirm the requirement of MurJ for peptidoglycan biosynthesis and that are in agreement with a role of MurJ as a flippase. Because of its essential nature, MurJ could serve as a target in the continuing search for antimicrobial compounds.

Undecaprenyl Phosphate Synthesis

Undecaprenyl phosphate (C55-P) is an essential 55-carbon long-chain isoprene lipidinvolved in the biogenesis of bacterial cell wall carbohydrate polymers: peptidoglycan, O antigen, teichoic acids, and other cell surface polymers. It functions as a lipid carrier that allows the traffic of sugar intermediates across the plasma membrane, towards the periplasm,where the polymerization of the different cellwall components occurs. At the end of these processes, the lipid is released in a pyrophosphate form (C55-PP). C55-P arises from the dephosphorylation of C55-PP, which itself originates from either a recycling event or a de novo synthesis. In Escherichia coli, the formation of C55-PP is catalyzed by the essential UppS synthase, a soluble cis-prenyltransferase, whichadds eight isoprene units ontofarnesyl pyrophosphate. Severalapo- and halo-UppSthree-dimensional structures have provided a high level of understanding of this enzymatic step. The following dephosphorylationstep is required before the lipid carrier can accept a sugar unit at the cytoplasmic face of the membrane. Four integralmembrane proteins have been shown to catalyzethis reaction in E. coli:BacA and three members of the PAP2 super-family:YbjG, LpxT, and PgpB. None of these enzymes is essential,but the simultaneous inactivation of bacA, ybjG, and pgpB genes gave rise to a lethal phenotype, raising the question of the relevance of such a redundancy of activity. It was alsorecently shown that LpxTcatalyzes the specific transfer of the phosphate group arising from C55-PP to the lipidA moiety of lipopolysaccharides, leading to a lipid-A 1-diphosphate form whichaccounts for one-third of the total lipidA in wild-type E. coli cells. The active sites of LpxT, PgpB,andYbjG were shown to face the periplasm, suggesting that PAP2 enzymes arerather involved in C55-PP recycling. These recent discoveries have opened the way to the elucidation of the functional and structural characterization of these different phosphatases.

Lipid II: a central component in bacterial cell wall synthesis and a target for antibiotics

The bacterial cell wall is mainly composed of peptidoglycan, which is a three-dimensional network of long aminosugar strands located on the exterior of the cytoplasmic membrane. These strands consist of alternating MurNAc and GlcNAc units and are interlinked to each other via peptide moieties that are attached to the MurNAc residues. Peptidoglycan subunits are assembled on the cytoplasmic side of the bacterial membrane on a polyisoprenoid anchor and one of the key components in the synthesis of peptidoglycan is Lipid II. Being essential for bacterial cell survival, it forms an attractive target for antibacterial compounds such as vancomycin and several lantibiotics. Lipid II consists of one GlcNAc-MurNAc-pentapeptide subunit linked to a polyiosoprenoid anchor 11 subunits long via a pyrophosphate linker. This review focuses on this special molecule and addresses three questions. First, why are special lipid carriers as polyprenols used in the assembly of peptidoglycan? Secondly, how is Lipid II translocated across the bacterial cytoplasmic membrane? And finally, how is Lipid II used as a receptor for lantibiotics to kill bacteria?

Involvement of an essential gene, mviN, in murein synthesis in Escherichia coli

We isolated a temperature-sensitive mutant with a mutation in mviN, an essential gene in Escherichia coli. At the nonpermissive temperature, mviN mutant cells swelled and burst. An intermediate in murein synthesis, polyprenyl diphosphate-N-acetylmuramic acid-(pentapeptide)-N-acetyl-glucosamine, accumulated in mutant cells. These results indicated that MviN is involved in murein synthesis.

Substrate specificity and membrane topology of Escherichia coli PgpB, an undecaprenyl pyrophosphate phosphatase

The synthesis of the lipid carrier undecaprenyl phosphate (C(55)-P) requires the dephosphorylation of its precursor, undecaprenyl pyrophosphate (C(55)-PP). The latter lipid is synthesized de novo in the cytosol and is also regenerated after its release from the C(55)-PP-linked glycans in the periplasm. In Escherichia coli the dephosphorylation of C(55)-PP was shown to involve four integral membrane proteins, BacA, and three members of the type 2 phosphatidic acid phosphatase family, PgpB, YbjG, and YeiU. Here, the PgpB protein was purified to homogeneity, and its phosphatase activity was examined. This enzyme was shown to catalyze the dephosphorylation of C(55)-PP with a relatively low efficiency compared with diacylglycerol pyrophosphate and farnesyl pyrophosphate (C(15)-PP) lipid substrates. However, the in vitro C(55)-PP phosphatase activity of PgpB was specifically enhanced by different phospholipids. We hypothesize that the phospholipids are important determinants to ensure proper conformation of the atypical long axis C(55) carrier lipid in membranes. Furthermore, a topological analysis demonstrated that PgpB contains six transmembrane segments, a large periplasmic loop, and the type 2 phosphatidic acid phosphatase signature residues at a periplasmic location.

Growth of Escherichia coli: significance of peptidoglycan degradation during elongation and septation

We have found a striking difference between the modes of action of amdinocillin (mecillinam) and compound A22, both of which inhibit cell elongation. This was made possible by employment of a new method using an Escherichia coli peptidoglycan (PG)-recycling mutant, lacking ampD, to analyze PG degradation during cell elongation and septation. Using this method, we have found that A22, which is known to prevent MreB function, strongly inhibited PG synthesis during elongation. In contrast, treatment of elongating cells with amdinocillin, which inhibits penicillin-binding protein 2 (PBP2), allowed PG glycan synthesis to proceed at a nearly normal rate with concomitant rapid degradation of the new glycan strands. By treating cells with A22 to inhibit sidewall synthesis, the method could also be applied to study septum synthesis. To our surprise, over 30% of newly synthesized septal PG was degraded during septation. Thus, excess PG sufficient to form at least one additional pole was being synthesized and rapidly degraded during septation. We propose that during cell division, rapid removal of the excess PG serves to separate the new poles of the daughter cells. We have also employed this new method to demonstrate that PBP2 and RodA are required for the synthesis of glycan strands during elongation and that the periplasmic amidases that aid in cell separation are minor players, cleaving only one-sixth of the PG that is turned over by the lytic transglycosylases.

Lipid intermediates in the biosynthesis of bacterial peptidoglycan

This review is an attempt to bring together and critically evaluate the now-abundant but dispersed data concerning the lipid intermediates of the biosynthesis of bacterial peptidoglycan. Lipid I, lipid II, and their modified forms play a key role not only as the specific link between the intracellular synthesis of the peptidoglycan monomer unit and the extracytoplasmic polymerization reactions but also in the attachment of proteins to the bacterial cell wall and in the mechanisms of action of antibiotics with which they form specific complexes. The survey deals first with their detection, purification, structure, and preparation by chemical and enzymatic methods. The recent important advances in the study of transferases MraY and MurG, responsible for the formation of lipids I and II, are reported. Various modifications undergone by lipids I and II are described, especially those occurring in gram-positive organisms. The following section concerns the cellular location of the lipid intermediates and the translocation of lipid II across the cytoplasmic membrane. The great efforts made since 2000 in the study of the glycosyltransferases catalyzing the glycan chain formation with lipid II or analogues are analyzed in detail. Finally, examples of antibiotics forming complexes with the lipid intermediates are presented.

The biosynthesis of peptidoglycan lipid-linked intermediates

The biosynthesis of bacterial cell wall peptidoglycan is a complex process involving many different steps taking place in the cytoplasm (synthesis of the nucleotide precursors) and on the inner and outer sides of the cytoplasmic membrane (assembly and polymerization of the disaccharide-peptide monomer unit, respectively). This review summarizes the current knowledge on the membrane steps leading to the formation of the lipid II intermediate, i.e. the substrate of the polymerization reactions. It makes the point on past and recent data that have significantly contributed to the understanding of the biosynthesis of undecaprenyl phosphate, the carrier lipid required for the anchoring of the peptidoglycan hydrophilic units in the membrane, and to the characterization of the MraY and MurG enzymes which catalyze the successive transfers of the N-acetylmuramoyl-peptide and N-acetylglucosamine moieties onto the carrier lipid, respectively. Enzyme inhibitors and antibacterial compounds interfering with these essential metabolic steps and interesting targets are presented.

Expanding role of lipid II as a target for lantibiotics

Lipid II is an essential cell-wall precursor required for the growth and replication of both Gram-positive and Gram-negative bacteria. Compounds that use lipid II to selectively target bacterial cells for destruction represent an important class of antibiotics. Clinically, vancomycin is the most important example of an antibiotic that operates in this manner. Despite being considered the 'antibiotic drug of last resort', significant bacterial resistance to vancomycin now manifests itself worldwide. In this paper we review recent progress made in understanding the lipid II-associated antibacterial characteristics of various naturally occurring compounds, with particular focus on the lantibiotic peptides.

Determinants for the subcellular localization and function of a nonessential SEDS protein

The Bacillus subtilis SpoVE integral membrane protein is essential for the heat resistance of spores, probably because of its involvement in spore peptidoglycan synthesis. We found that an SpoVE-yellow fluorescent protein (YFP) fusion protein becomes localized to the forespore during the earliest stages of engulfment, and this pattern is maintained throughout sporulation. SpoVE belongs to a well-conserved family of proteins that includes the FtsW and RodA proteins of B. subtilis. These proteins are involved in bacterial shape determination, although their function is not known. FtsW is necessary for the formation of the asymmetric septum in sporulation, and we found that an FtsW-YFP fusion localized to this structure prior to the initiation of engulfment in a nonoverlapping pattern with SpoVE-cyan fluorescent protein. Since FtsW and RodA are essential for normal growth, it has not been possible to identify loss-of-function mutations that would greatly facilitate analysis of their function. We took advantage of the fact that SpoVE is not required for growth to obtain point mutations in SpoVE that block the development of spore heat resistance but that allow normal protein expression and targeting to the forespore. These mutant proteins will be invaluable tools for future experiments aimed at elucidating the function of members of the SEDS ("shape, elongation, division, and sporulation") family of proteins.