Detection of Wzy/Wzz interaction in Shigella flexneri

Recent insights into Wzy polymerases and lipopolysaccharide O-antigen biosynthesis

Bacteria synthesize a plethora of complex surface-associated polysaccharides which enable them to persist and thrive in distinct niches. These glycans serve an array of purposes pertaining to virulence, colonization, antimicrobial resistance, stealth, and biofilm formation. The Wzx/Wzy-dependent pathway is universally the predominant system for bacterial polysaccharide synthesis. This system is responsible for the production of lipopolysaccharide (LPS) O-antigen (Oag), enterobacterial common antigen, capsule, and exopolysaccharides, with orthologs present in both Gram-negative and Gram-positive microbes. Studies focusing principally on Pseudomonas, Shigella, and Salmonella LPS Oag synthesis have provided much of the framework underpinning the biochemical and molecular mechanism behind polysaccharide synthesis via this pathway. LPS Oag production via the Wzx/Wzy-dependent pathway occurs through the stepwise activity of multiple key biosynthetic enzymes, including primarily the polymerase, Wzy, which is responsible for the Oag assembly, and the polysaccharide co-polymerase, Wzz, which effectively modulates the length of the glycan produced. In this review, we provide a comprehensive summary of the latest genetic, structural, and mechanistic data for the main protein candidates of the Wzx/Wzy-dependent pathway, in addition to an examination of their substrate specificities. Furthermore, we have reviewed recent insights pertaining to the dynamics/kinetics of glycan synthesis by this mechanism, including the interplay of the key proteins among themselves and in complex with their substrate. Lastly, we outline key gaps in the literature and suggest future research avenues, with the aim to stimulate ongoing research into this critical pathway responsible for the production of key virulence factors for numerous debilitating and lethal pathogens.

Identification of EcpK, a bacterial tyrosine pseudokinase important for exopolysaccharide biosynthesis in Myxococcus xanthus

Bacteria synthesize chemically diverse capsular and secreted polysaccharides that function in many physiological processes and are widely used in industrial applications. In the ubiquitous Wzx/Wzy-dependent biosynthetic pathways for these polysaccharides, the polysaccharide co-polymerase (PCP) facilitates the polymerization of repeat units in the periplasm, and in Gram-negative bacteria, also polysaccharide translocation across the outer membrane. These PCPs belong to the PCP-2 family, are integral inner membrane proteins with extended periplasmic domains, and functionally depend on alternating between different oligomeric states. The oligomeric state is determined by a cognate cytoplasmic bacterial tyrosine kinase (BYK), which is either part of the PCP or a stand-alone protein. Interestingly, BYK-like proteins, which lack key catalytic residues and/or the phosphorylated Tyr residues, have been described. In Myxococcus xanthus, the exopolysaccharide (EPS) is synthesized and exported via the Wzx/Wzy-dependent EPS pathway in which EpsV serves as the PCP. Here, we confirm that EpsV lacks the BYK domain. Using phylogenomics, experiments, and computational structural biology, we identify EcpK as important for EPS biosynthesis and show that it structurally resembles canonical BYKs but lacks residues important for catalysis and Tyr phosphorylation. Using proteomic analyses, two-hybrid assays, and structural modeling, we demonstrate that EcpK directly interacts with EpsV. Based on these findings, we suggest that EcpK is a BY pseudokinase and functions as a scaffold, which by direct protein-protein interactions, rather than by Tyr phosphorylation, facilitates EpsV function. EcpK and EpsV homologs are present in other bacteria, suggesting broad conservation of this mechanism and establishing a phosphorylation-independent PCP-2 subfamily.IMPORTANCEBacteria produce a variety of polysaccharides with important biological functions. In Wzx/Wzy-dependent pathways for the biosynthesis of secreted and capsular polysaccharides in Gram-negative bacteria, the polysaccharide co-polymerase (PCP) is a key protein that facilitates repeat unit polymerization and polysaccharide translocation across the outer membrane. PCP function depends on assembly/disassembly cycles that are determined by the phosphorylation/dephosphorylation cycles of an associated bacterial tyrosine kinase (BYK). Here, we identify the BY pseudokinase EcpK as essential for exopolysaccharide biosynthesis in Myxococcus xanthus. Based on experiments and computational structural biology, we suggest that EcpK is a scaffold protein, guiding the assembly/disassembly cycles of the partner PCP via binding/unbinding cycles independently of Tyr phosphorylation/dephosphorylation cycles. We suggest that this novel mechanism is broadly conserved.

Exopolysaccharides of Paenibacillus polymyxa: A review

Paenibacillus polymyxa (P. polymyxa) is a member of the genus Paenibacillus, which is a rod-shaped, spore-forming gram-positive bacterium. P. polymyxa is a source of many metabolically active substances, including polypeptides, volatile organic compounds, phytohormone, hydrolytic enzymes, exopolysaccharide (EPS), etc. Due to the wide range of compounds that it produces, P. polymyxa has been extensively studied as a plant growth promoting bacterium which provides a direct benefit to plants through the improvement of N fixation from the atmosphere and enhancement of the solubilization of phosphorus and the uptake of iron in the soil, and phytohormones production. Among the metabolites from P. polymyxa, EPS exhibits many activities, for example, antioxidant, immunomodulating, anti-tumor and many others. EPS has various applications in food, agriculture, environmental protection. Particularly, in the field of sustainable agriculture, P. polymyxa EPS can be served as a biofilm to colonize microbes, and also can act as a nutrient sink on the roots of plants in the rhizosphere. Therefore, this paper would provide a comprehensive review of the advancements of diverse aspects of EPS from P. polymyxa, including the production, extraction, structure, biosynthesis, bioactivity and applications, etc. It would provide a direction for future research on P. polymyxa EPS.

Shigella flexneri remodeling and consumption of host lipids during infection

IMPORTANCE

Bacterial pathogens have vastly distinct sites that they inhabit during infection. This requires adaptation due to changes in nutrient availability and antimicrobial stress. The bacterial surface is a primary barrier, and here, we show that the bacterial pathogen Shigella flexneri increases its surface decorations when it transitions to an intracellular lifestyle. We also observed changes in bacterial and host cell fatty acid homeostasis. Specifically, intracellular S. flexneri increased the expression of their fatty acid degradation pathway, while the host cell lipid pool was significantly depleted. Importantly, bacterial proliferation could be inhibited by fatty acid supplementation of host cells, thereby providing novel insights into the possible link between human malnutrition and susceptibility to S. flexneri.

Repeat-Unit Elongations To Produce Bacterial Complex Long Polysaccharide Chains, an O-Antigen Perspective

The O-antigen, a long polysaccharide that constitutes the distal part of the outer membrane-anchored lipopolysaccharide, is one of the critical components in the protective outer membrane of Gram-negative bacteria. Most species produce one of the structurally diverse O-antigens, with nearly all the polysaccharide components having complex structures made by the Wzx/Wzy pathway. This pathway produces repeat-units of mostly 3-8 sugars on the cytosolic face of the cytoplasmic membrane that is translocated by Wzx flippase to the periplasmic face and polymerized by Wzy polymerase to give long-chain polysaccharides. The Wzy polymerase is a highly diverse integral membrane protein typically containing 10-14 transmembrane segments. Biochemical evidence confirmed that Wzy polymerase is the sole driver of polymerization, and recent progress also began to demystify its interacting partner, Wzz, shedding some light to speculate how the proteins may operate together during polysaccharide biogenesis. However, our knowledge of how the highly variable Wzy proteins work as part of the O-antigen processing machinery remains poor. Here, we discuss the progress to the current understanding of repeat-unit polymerization and propose an updated model to explain the formation of additional short chain O-antigen polymers found in the lipopolysaccharide of diverse Gram-negative species and their importance in the biosynthetic process.

Shigella flexneri Adapts to Niche-Specific Stresses through Modifications in Cell Envelope Composition and Decoration

Shigella flexneri is the primary causative agent of worldwide shigellosis. As the pathogen transverses the distinct niches of the gastrointestinal tract it necessitates dynamic adaptation strategies to mitigate host antimicrobials such as dietary fatty acids (FAs) and the bile salt, deoxycholate (DOC). This study investigates the dynamics of the S. flexneri cell envelope, by interrogating adaptations following FA or DOC exposure. We deciphered the effects of FAs and DOC on bacterial membrane fatty acid and lipopolysaccharide (LPS) compositions. We identified novel LPS-based strategies by the pathogen to support resistance to these host compounds. In particular, expression of S. flexneri very-long O antigen (VL-Oag) LPS was found to play a central role in stress mitigation, as VL-Oag protects against antimicrobial FAs, but its presence rendered S. flexneri susceptible to DOC stress. Collectively, this work underpins the importance for S. flexneri to maintain appropriate regulation of cell envelope constituents, in particular VL-Oag LPS, to adequately adapt to diverse stresses during infection.

Hypermucoviscosity Regulator RmpD Interacts with Wzc and Controls Capsular Polysaccharide Chain Length

Klebsiella pneumoniae is a leading cause of nosocomial infections, including pneumonia, bacteremia, and urinary tract infections. Treatment options are increasingly restricted by the high prevalence of resistance to frontline antibiotics, including carbapenems, and the recently identified plasmid-conferred colistin resistance. The classical pathotype (cKp) is responsible for most of the nosocomial infections observed globally, and these isolates are often multidrug resistant. The hypervirulent pathotype (hvKp) is a primary pathogen capable of causing community-acquired infections in immunocompetent hosts. The hypermucoviscosity (HMV) phenotype is strongly associated with the increased virulence of hvKp isolates. Recent studies demonstrated that HMV requires capsule (CPS) synthesis and the small protein RmpD but is not dependent on the increased amount of capsule associated with hvKp. Here, we identified the structure of the capsular and extracellular polysaccharide isolated from hvKp strain KPPR1S (serotype K2) with and without RmpD. We found that the polymer repeat unit structure is the same in both strains and that it is identical to the K2 capsule. However, the chain length of CPS produced by strains expressing rmpD demonstrates more uniform length. This property was reconstituted in CPS from Escherichia coli isolates that possess the same CPS biosynthesis pathway as K. pneumoniae but naturally lack rmpD. Furthermore, we demonstrate that RmpD binds Wzc, a conserved capsule biosynthesis protein required for CPS polymerization and export. Based on these observations, we present a model for how the interaction of RmpD with Wzc could impact CPS chain length and HMV. IMPORTANCE Infections caused by Klebsiella pneumoniae continue to be a global public health threat; the treatment of these infections is complicated by the high frequency of multidrug resistance. K. pneumoniae produces a polysaccharide capsule required for virulence. Hypervirulent isolates also have a hypermucoviscous (HMV) phenotype that increases virulence, and we recently demonstrated that a horizontally acquired gene, rmpD, is required for HMV and hypervirulence but that the identity of the polymeric product(s) in HMV isolates is uncertain. Here, we demonstrate that RmpD regulates capsule chain length and interacts with Wzc, a part of the capsule polymerization and export machinery shared by many pathogens. We further show that RmpD confers HMV and regulates capsule chain length in a heterologous host (E. coli). As Wzc is a conserved protein found in many pathogens, it is possible that RmpD-mediated HMV and increased virulence may not be restricted to K. pneumoniae.

The lipid linked oligosaccharide polymerase Wzy and its regulating co-polymerase, Wzz, from enterobacterial common antigen biosynthesis form a complex

The enterobacterial common antigen (ECA) is a carbohydrate polymer that is associated with the cell envelope in the Enterobacteriaceae. ECA contains a repeating trisaccharide which is polymerized by WzyE, a member of the Wzy membrane protein polymerase superfamily. WzyE activity is regulated by a membrane protein polysaccharide co-polymerase, WzzE. Förster resonance energy transfer experiments demonstrate that WzyE and WzzE from Pectobacterium atrosepticum form a complex in vivo, and immunoblotting and cryo-electron microscopy (cryo-EM) analysis confirm a defined stoichiometry of approximately eight WzzE to one WzyE. Low-resolution cryo-EM reconstructions of the complex, aided by an antibody recognizing the C-terminus of WzyE, reveals WzyE sits in the central membrane lumen formed by the octameric arrangement of the transmembrane helices of WzzE. The pairing of Wzy and Wzz is found in polymerization systems for other bacterial polymers, including lipopolysaccharide O-antigens and capsular polysaccharides. The data provide new structural insight into a conserved mechanism for regulating polysaccharide chain length in bacteria.

Identification of the Shigella flexneri Wzy Domain Modulating WzzpHS-2 Interaction and Detection of the Wzy/Wzz/Oag Complex

Shigella flexneri implements the Wzy-dependent pathway to biosynthesize the O antigen (Oag) component of its surface lipopolysaccharide. The inner membrane polymerase Wzy_SF catalyzes the repeat addition of undecaprenol-diphosphate-linked Oag (Und-PP-RUs) to produce a polysaccharide, the length of which is tightly regulated by two competing copolymerase proteins, Wzz_SF (short-type Oag; 10 to 17 RUs) and Wzz_pHS-2 (very-long-type Oag; >90 RUs). The nature of the interaction between Wzy_SF and Wzz_SF/Wzz_pHS-2 in Oag polymerization remains poorly characterized, with the majority of the literature characterizing the individual protein constituents of the Wzy-dependent pathway. Here, we report instead a major investigation into the specific binding interactions of Wzy_SF with its copolymerase counterparts. For the first time, a region of Wzy_SF that forms a unique binding site for Wzz_pHS-2 has been identified. Specifically, this work has elucidated key Wzy_SF moieties at the N- and C-terminal domains (NTD and CTD) that form an intramolecular pocket modulating the Wzz_pHS-2 interaction. Novel copurification data highlight that disruption of residues within this NTD-CTD pocket impairs the interaction with Wzz_pHS-2 without affecting Wzz_SF binding, thereby specifically disrupting polymerization of longer polysaccharide chains. This study provides a novel understanding of the molecular interaction of Wzy_SF with Wzz_SF/Wzz_pHS-2 in the Wzy-dependent pathway and, furthermore, detects the Wzy/Wzz/Und-PP-Oag complex for the first time. Beyond S. flexneri, this work may be extended to provide insight into the interactions between protein homologues expressed by related species, especially members of Enterobacteriaceae, that produce dual Oag chain length determinants. IMPORTANCE Shigella flexneri is a pathogen causing significant morbidity and mortality, predominantly devastating the pediatric age group in developing countries. A major virulence factor contributing to S. flexneri pathogenesis is its surface lipopolysaccharide, which is comprised of three domains: lipid A, core oligosaccharide, and O antigen (Oag). The Wzy-dependent pathway is the most common biosynthetic mechanism implemented for Oag biosynthesis by Gram-negative bacteria, including S. flexneri. The nature of the interaction between the polymerase, Wzy_SF, and the polysaccharide copolymerases, Wzz_SF and Wzz_pHS-2, in Oag polymerization is poorly characterized. This study investigates the molecular interplay between Wzy_SF and its copolymerases, deciphering key interactions in the Wzy-dependent pathway that may be extended beyond S. flexneri, providing insight into Oag biosynthesis in Gram-negative bacteria.

Detection of a disulphide bond and conformational changes in Shigella flexneri Wzy, and the role of cysteine residues in polymerase activity

Shigella flexneri utilises the Wzy-dependent pathway for the production of a plethora of complex polysaccharides, including the lipopolysaccharide O-antigen (Oag) component. The inner membrane protein Wzy_SF polymerises Oag repeat units, whilst two co-polymerase proteins, Wzz_SF and Wzz_pHS-2, together interact with Wzy_SF to regulate production of short- (S-Oag) and very long- (VL-Oag) Oag modal lengths, respectively. The 2D arrangement of Wzy_SF transmembrane and soluble regions has been previously deciphered, however, attaining information on the 3D structural and conformational arrangement of Wzy_SF, or any homologue, has proven difficult. For the first time, the current study detected insights into the in situ Wzy_SF arrangement. In vitro assays using thiol-reactive PEG-maleimide were used to probe Wzy_SF conformation, which additionally detected novel, unique conformational changes in response to interaction with intrinsic factors, including Wzz_SF and Wzz_pHS-2, and extrinsic factors, such as temperature. Site-directed mutagenesis of Wzy_SF cysteine residues revealed the presence of a putative intramolecular disulphide bond, between cysteine moieties 13 and 60. Subsequent analyses highlighted both the structural and functional importance of Wzy_SF cysteines. Substitution of Wzy_SF cysteine residues significantly decreased biosynthesis of the VL-Oag modal length, without disruption to S-Oag production. This phenotype was corroborated in the absence of co-polymerase competition for Wzy_SF interaction. These data suggest Wzy_SF cysteine substitutions directly impair the interaction between Wzy/Wzz_pHS-2, without altering the Wzy/Wzz_SF interplay, and in combination with structural data, we propose that the N- and C-termini of Wzy_SF are arranged in close proximity, and together may form the unique Wzz_pHS-2 interaction site.

Topology of the Shigella flexneri Enterobacterial Common Antigen polymerase WzyE

Enterobacteriales have evolved a specialized outer membrane polysaccharide [Enterobacterial Common Antigen (ECA)] which allows them to persist in various environmental niches. Biosynthesis of ECA initiates on the cytoplasmic leaflet of the inner membrane (IM) where glycosyltransferases assemble ECA repeat units (RUs). Complete RUs are then translocated across the IM and assembled into polymers by ECA-specific homologues of the Wzy-dependent pathway. Consisting of the membrane proteins Wzx, Wzy and Wzz, the Wzy-dependent pathway is the most common polysaccharide biosynthetic pathway in Gram-negative bacteria where it is most notably involved in LPS O antigen (Oag) biosynthesis. As such, the majority of research directed towards these proteins has been orientated towards Oag biosynthetic homologues with little directed towards ECA homologues. Belonging to the Shape, Elongation, Division and Sporulation (SEDS) protein family, Wzy proteins are polymerases, and are characterized as possessing little or no peptide homology among homologues as well as being polytopic membrane proteins with functionally relevant residues within periplasmic loops, as defined by C-terminal reporter fusion topology mapping. Here, we present the first the first major study into the ECA polymerase WzyE. Multiple sequence alignments and topology mapping showed that WzyE is unlike WzyB proteins involved with Oag biosynthesis WzyE displays high peptide conservation across Enterobacteriales. In silico structures and reporter mapping allowed us to identify possible functionally conserved residues with WzyE_SF's periplasmic loops, which we showed were crucial for its function. This work provides novel insight into Wzy proteins and suggests that WzyE is an optimal model to investigate Wzy proteins and the Wzy-dependent pathway.

Structural elucidation of the fucose containing polysaccharide of Paenibacillus polymyxa DSM 365

Paenibacillus polymyxa is an avid producer of exopolysaccharides of industrial interest. However, due to the complexity of the polymer composition, structural elucidation of the polysaccharide remained unfeasible for a long time. By using a CRISPR-Cas9 mediated knock-out strategy, all single glycosyltransferases as well as the Wzy polymerases were individually deleted in the corresponding gene cluster for the first time. Thereby, it was observed that the main polymer fraction was completely suppressed (or deleted) and a pure minor fucose containing polysaccharide could be isolated, which was named paenan II. Applying this combinatorial approach, the monosaccharide composition, sequence and linkage pattern of this novel polymer was determined via HPLC-MS, GC-MS and NMR. Furthermore, we demonstrated that the knock-out of the glycosyltransferases PepQ, PepT, PepU and PepV as well as of the Wzy polymerase PepG led to the absence of paenan II, attributing those enzymes to the assembly of the repeating unit.

Capsules and Extracellular Polysaccharides in Escherichia coli and Salmonella

Escherichia coli and Salmonella isolates produce a range of different polysaccharide structures that play important roles in their biology. E. coli isolates often possess capsular polysaccharides (K antigens), which form a surface structural layer. These possess a wide range of repeat-unit structures. In contrast, only one capsular polymer (Vi antigen) is found in Salmonella, and it is confined to typhoidal serovars. In both genera, capsules are vital virulence determinants and are associated with the avoidance of host immune defenses. Some isolates of these species also produce a largely secreted exopolysaccharide called colanic acid as part of their complex Rcs-regulated phenotypes, but the precise function of this polysaccharide in microbial cell biology is not fully understood. E. coli isolates produce two additional secreted polysaccharides, bacterial cellulose and poly-N-acetylglucosamine, which play important roles in biofilm formation. Cellulose is also produced by Salmonella isolates, but the genes for poly-N-acetylglucosamine synthesis appear to have been lost during its evolution toward enhanced virulence. Here, we discuss the structures, functions, relationships, and sophisticated assembly mechanisms for these important biopolymers.

Identification of a Region in Shigella flexneri WzyB Disrupting the Interaction with WzzpHS2

Shigella flexneri can synthesize polysaccharide chains having complex sugars and a regulated number of repeating units. S. flexneri lipopolysaccharide O antigen (Oag) is synthesized by the Wzy-dependent pathway, which is the most common pathway used in bacteria for polysaccharide synthesis. The inner membrane protein WzyB polymerizes the Oag repeat units into chains, while the polysaccharide copolymerases WzzB and Wzz_pHS2 determine the average number of repeat units or "the modal length," termed short type and very long type. Our data show for the first time a direct interaction between WzyB and Wzz_pHS2, with and without the use of the chemical cross-linker dithiobis (succinimidyl propionate) (DSP). Additionally, mutations generated via random and site-directed mutagenesis identify a region of WzyB that caused diminished function and significantly decreased very long Oag chain polymerization, and that affected the aforementioned interaction. These results provide insight into the mechanisms underlying the regulation of Oag biosynthesis. IMPORTANCE Complex polysaccharide chains are synthesized by bacteria, usually at a regulated number of repeating units, which has broad implications for bacterial pathogenesis. One example is the O antigen (Oag) component of lipopolysaccharide that is predominantly synthesized by the Wzy-dependent pathway. Our findings show for the first time a direct physical interaction between WzyB and Wzz_pHS2. Additionally, a set of Wzy mutant constructs were generated, revealing a proposed active site/switch region involved in the activity of WzyB and the physical interaction with Wzz_pHS2. Combined, these findings further understanding of the Wzy-dependent pathway. The identification of a novel interaction with the polysaccharide copolymerase Wzz_pHS2 and the region of WzyB that is involved in this aforementioned interaction and its impact on WzyB Oag synthesis activity have significant implication for the prevention/treatment of bacterial diseases and discovery of novel biotechnologies.

Structure of a full-length bacterial polysaccharide co-polymerase

Lipopolysaccharides are important components of the bacterial cell envelope that among other things act as a protective barrier against the environment and toxic molecules such as antibiotics. One of the most widely disseminated pathways of polysaccharide biosynthesis is the inner membrane bound Wzy-dependent pathway. Here we present the 3.0 Å structure of the co-polymerase component of this pathway, WzzB from E. coli solved by single-particle cryo-electron microscopy. The overall architecture is octameric and resembles a box jellyfish containing a large bell-shaped periplasmic domain with the 2-helix transmembrane domain from each protomer, positioned 32 Å apart, encircling a large empty transmembrane chamber. This structure also reveals the architecture of the transmembrane domain, including the location of key residues for the Wzz-family of proteins and the Wzy-dependent pathway present in many Gram-negative bacteria, explaining several of the previous biochemical and mutational studies and lays the foundation for future investigations.

Polysaccharide co-polymerase WzzB/WzzE chimeras reveal transmembrane 2 region of WzzB is important for interaction with WzyB

The ability of bacteria to synthesise complex polysaccharide chains at a controlled number of repeating units has wide implications for a range of biological activities that include: symbiosis, biofilm formation and immune system avoidance. Complex polysaccharide chains such as the O antigen (Oag) component of lipopolysaccharide and the enterobacterial common antigen (ECA) are synthesised by the most common polysaccharide synthesis pathway used in bacteria, known as the Wzy-dependent pathway. The Oag and ECA are polymerized into chains via the inner membrane proteins WzyB and WzyE, respectively, while the respective co-polymerases WzzB and WzzE modulate the number of repeat units in the chains or "the modal length" of the polysaccharide via a hypothesised interaction. Our data shows for the first time "cross-talk" between Oag and ECA synthesis in that WzzE is able to partially regulate Oag modal length via a potential interaction with WzyB. To investigate this, one or both of the transmembrane regions (TM1 and TM2) of WzzE and WzzB were swapped creating six chimera proteins. Several chimeric proteins showed significant increases Oag modal length control, while others reduced control. Additionally, co-purification experiments show an interaction between WzyB and WzzB for the first time without the use of a chemical cross-linker, and a novel interaction between WzyB and WzzE. These results suggest the TM2 region of Wzz proteins plays a critical role in Oag and ECA modal length control, presumably via the interaction with respective Wzy proteins, thus providing insight into the complex mechanism underlying the control of polysaccharide biosynthesis.ImportanceBacteria synthesise complex polysaccharide chains at a controlled number of repeating units, this has wide implications for a range of bacterial activities involved in virulence. Examples of complex polysaccharide chains include, the O antigen (Oag) component of lipopolysaccharide and the enterobacterial common antigen (ECA), both of these examples are predominantly synthesised by their own independent Wzy-dependent pathway. Our data show for the first time "cross-talk" between Oag and ECA synthesis and identifies novel physical protein-protein interactions between proteins in these systems. These findings further the understanding of how the system functions to control polysaccharide chain length which has great implications for novel biotechnologies and/or the combat of bacterial diseases.

Identification of the Pseudomonas aeruginosa O17 and O15 O-Specific Antigen Biosynthesis Loci Reveals an ABC Transporter-Dependent Synthesis Pathway and Mechanisms of Genetic Diversity

Many bacterial cell surface glycans, such as the O antigen component of lipopolysaccharide (LPS), are produced via the so-called Wzx/Wzy- or ABC transporter-dependent pathways. O antigens are highly diverse polysaccharides that protect bacteria from their environment and engage in important host-pathogen interactions. The specific structure and composition of O antigens are the basis of classifying bacteria into O serotypes. In the opportunistic pathogen Pseudomonas aeruginosa, there are currently 20 known O-specific antigen (OSA) structures. The clusters of genes responsible for 18 of these O antigens have been identified, all of which follow the Wzx/Wzy-dependent pathway and are located at a common locus. In this study, we located the two unidentified O antigen biosynthesis clusters responsible for the synthesis of the O15 and the O17 OSA structures by analyzing published whole-genome sequence data. Intriguingly, these clusters were found outside the conserved OSA biosynthesis locus and were likely acquired through multiple horizontal gene transfer events. Based on data from knockout and overexpression studies, we determined that the synthesis of these O antigens follows an ABC transporter-dependent rather than a Wzx/Wzy-dependent pathway. In addition, we collected evidence to show that the O15 and O17 polysaccharide chain lengths are regulated by molecular rulers with distinct and variable domain architectures. The findings in this report are critical for a comprehensive understanding of O antigen biosynthesis in P. aeruginosa and provide a framework for future studies.IMPORTANCE P. aeruginosa is a problematic opportunistic pathogen that causes diseases in those with compromised host defenses, such as those suffering from cystic fibrosis. This bacterium produces a number of virulence factors, including a serotype-specific O antigen. Here, we identified and characterized the gene clusters that produce the O15 and O17 O antigens and show that they utilize a pathway for synthesis that is distinct from that of the 18 other known serotypes. We also provide evidence that these clusters have acquired mutations in specific biosynthesis genes and have undergone extensive horizontal gene transfer within the P. aeruginosa population. These findings expand on our understanding of O antigen biosynthesis in Gram-negative bacteria and the mechanisms that drive O antigen diversity.

Enterobacterial Common Antigen: Synthesis and Function of an Enigmatic Molecule

The outer membrane (OM) of Gram-negative bacteria poses a barrier to antibiotic entry due to its high impermeability. Thus, there is an urgent need to study the function and biogenesis of the OM. In Enterobacterales, an order of bacteria with many pathogenic members, one of the components of the OM is enterobacterial common antigen (ECA). We have known of the presence of ECA on the cell surface of Enterobacterales for many years, but its properties have only more recently begun to be unraveled. ECA is a carbohydrate antigen built of repeating units of three amino sugars, the structure of which is conserved throughout Enterobacterales. There are three forms of ECA, two of which (ECAPG and ECALPS) are located on the cell surface, while one (ECACYC) is located in the periplasm. Awareness of the importance of ECA has increased due to studies of its function that show it plays a vital role in bacterial physiology and interaction with the environment. Here, we review the discovery of ECA, the pathways for the biosynthesis of ECA, and the interactions of its various forms. In addition, we consider the role of ECA in the host immune response, as well as its potential roles in host-pathogen interaction. Furthermore, we explore recent work that offers insights into the cellular function of ECA. This review provides a glimpse of the biological significance of this enigmatic molecule.

Structural analysis of the O-antigen polysaccharide from Escherichia coli O188

The structure of the O-antigen from Escherichia coli reference strain O188 (E. coli O188:H10) has been investigated. The lipopolysaccharide shows a typical nonrandom modal chain-length distribution and the sugar and absolute configuration analysis revealed d-Man, d-Glc, d-GlcN and d-GlcA as major components. The structure of the O-specific polysaccharide was determined using one- and two-dimensional ¹H and ¹³C NMR spectroscopy experiments, where inter-residue correlations were identified by ¹H,¹³C-heteronuclear multiple-bond correlation and ¹H,¹H-NOESY experiments, which revealed that it consists of pentasaccharide repeating units with the following structure: Biosynthetic aspects and NMR analysis are consistent with the presented structure as the biological repeating unit. The O-antigen of Shigella boydii type 16 differs only in that it carries O-acetyl groups to ~50% at O6 of the branch-point mannose residues. A molecular model of the E. coli O188 O-antigen containing 20 repeating units extends ~100 Å, which is similar to the height of the periplasmic portion of polysaccharide co-polymerase Wzz proteins that regulate the O-antigen chain length of lipopolysaccharides in the Wzx/Wzy biosynthetic pathway.

Lipopolysaccharide O-antigens-bacterial glycans made to measure

Lipopolysaccharides are critical components of bacterial outer membranes. The more conserved lipid A part of the lipopolysaccharide molecule is a major element in the permeability barrier imposed by the outer membrane and offers a pathogen-associated molecular pattern recognized by innate immune systems. In contrast, the long-chain O-antigen polysaccharide (O-PS) shows remarkable structural diversity and fulfills a range of functions, depending on bacterial lifestyles. O-PS production is vital for the success of clinically important Gram-negative pathogens. The biological properties and functions of O-PSs are mostly independent of specific structures, but the size distribution of O-PS chains is particularly important in many contexts. Despite the vast O-PS chemical diversity, most are produced in bacterial cells by two assembly strategies, and the different mechanisms employed in these pathways to regulate chain-length distribution are emerging. Here, we review our current understanding of the mechanisms involved in regulating O-PS chain-length distribution and discuss their impact on microbial cell biology.

Interactions between Lactobacillus plantarum NCU116 and its environments based on extracellular proteins and polysaccharides prediction by comparative analysis

Lactic acid bacteria (LAB) play a significant role in food industry and artisan fermented-food. Most of the applicable LABs were commonly obtained from natural fermented food or human gut. And Lactobacillus plantarum NCU116 was screened from a LAB-dominated traditional Chinese sauerkraut (TCS). In order to comprehend the interaction between NCU116 and its environments, comparative genomics were performed to identify genes involved in extracellular protein biosynthesis and secretion. Four secretory pathways were identified, including Sec and FPE pathways, holins and efflux ABC transporter system. Then 348 potential secretory proteins were identified, including 11 alpha-amylases responsible for degradation of macromolecules, and 8 mucus binding proteins which attribute to adherence to intestine epithelium. Besides, EPS clusters of NCU116 (EPS116) were identified and analyzed by comparing to other strains, which suggested a novel genotype of EPS clusters. These findings could be critical to extend the application of NCU116 in food and pharmaceuticals industries.

Unique Regions of the Polysaccharide Copolymerase Wzz2 from Pseudomonas aeruginosa Are Essential for O-Specific Antigen Chain Length Control

The outer leaflet of the outer membrane of nearly all Gram-negative bacteria contains lipopolysaccharide (LPS). The distal end of LPS may be capped with O antigen, a long polysaccharide that can range from a few to hundreds of sugars in length. The chain length of the polysaccharide has many implications for bacterial survival and consequently is tightly controlled. In the Wzx/Wzy-dependent route of O antigen synthesis, one or more Wzz proteins determine the chain length via an unknown mechanism. To gain insight into this mechanism, we identified and characterized important regions of two Wzz proteins in Pseudomonas aeruginosa serotype O13, which confer the production of "long" (Wzz₁) and "very long" (Wzz₂) chain lengths, respectively. We found that compared to Wzz₁, Wzz₂ has distinct amino acid insertions in the central α-helices (ins_α6 and ins_α7) and in membrane-distal (ins_L4) and -proximal (ins_IL) loops. When these regions were deleted in Wzz₂, the mutant proteins conferred drastically shortened chain lengths. Within these regions we identified several conserved amino acid residues that were then targeted for site-directed mutagenesis. Our results implicate an RTE motif in loop 4 and a "hot spot" of charged and polar residues in ins_α7 in the function of Wzz₂ We present evidence that the functionally important residues of ins_α7 are likely involved in stabilizing Wzz through coiled-coil interactions.IMPORTANCE O antigen is an important virulence factor presented on the cell surface of Gram-negative bacteria that is critical for bacterial physiology and pathogenesis. However, some aspects of O antigen biosynthesis, such as the mechanisms for determining polysaccharide chain length, are poorly understood. In this study, we identified unique regions in the O antigen chain length regulators (termed Wzz) of the problematic opportunistic pathogen Pseudomonas aeruginosa We show that these regions are critical for determining O antigen chain length, which provides new insight into the model of the Wzz mechanism. Ultimately, our work adds knowledge toward understanding an important step in the biosynthesis of this virulence factor, which is applicable to a wide range of Gram-negative pathogens.

Function and Biogenesis of Lipopolysaccharides

The cell envelope is the first line of defense between a bacterium and the world-at-large. Often, the initial steps that determine the outcome of chemical warfare, bacteriophage infections, and battles with other bacteria or the immune system greatly depend on the structure and composition of the bacterial cell surface. One of the most studied bacterial surface molecules is the glycolipid known as lipopolysaccharide (LPS), which is produced by most Gram-negative bacteria. Much of the initial attention LPS received in the early 1900s was owed to its ability to stimulate the immune system, for which the glycolipid was commonly known as endotoxin. It was later discovered that LPS also creates a permeability barrier at the cell surface and is a main contributor to the innate resistance that Gram-negative bacteria display against many antimicrobials. Not surprisingly, these important properties of LPS have driven a vast and still prolific body of literature for more than a hundred years. LPS research has also led to pioneering studies in bacterial envelope biogenesis and physiology, mostly using Escherichia coli and Salmonella as model systems. In this review, we will focus on the fundamental knowledge we have gained from studies of the complex structure of the LPS molecule and the biochemical pathways for its synthesis, as well as the transport of LPS across the bacterial envelope and its assembly at the cell surface.

Novel Role of VisP and the Wzz System during O-Antigen Assembly in Salmonella enterica Serovar Typhimurium Pathogenesis

Salmonella enterica serovars are associated with diarrhea and gastroenteritis and are a helpful model for understanding host-pathogen mechanisms. Salmonella enterica serovar Typhimurium regulates the distribution of O antigen (OAg) and presents a trimodal distribution based on Wzy polymerase and the Wzz_ST (long-chain-length OAg [L-OAg]) and Wzz_fepE (very-long-chain-length OAg [VL-OAg]) copolymerases; however, several mechanisms regulating this process remain unclear. Here, we report that LPS modifications modulate the infectious process and that OAg chain length determination plays an essential role during infection. An increase in VL-OAg is dependent on Wzy polymerase, which is promoted by a growth condition resembling the environment of Salmonella-containing vacuoles (SCVs). The virulence- and stress-related periplasmic protein (VisP) participates in OAg synthesis, as a ΔvisP mutant presents a semirough OAg phenotype. The ΔvisP mutant has greatly decreased motility and J774 macrophage survival in a colitis model of infection. Interestingly, the phenotype is restored after mutation of the wzz_ST or wzz_fepE gene in a ΔvisP background. Loss of both the visP and wzz_ST genes promotes an imbalance in flagellin secretion. L-OAg may function as a shield against host immune systems in the beginning of an infectious process, and VL-OAg protects bacteria during SCV maturation and facilitates intramacrophage replication. Taken together, these data highlight the roles of OAg length in generating phenotypes during S Typhimurium pathogenesis and show the periplasmic protein VisP as a novel protein in the OAg biosynthesis pathway.

Progress in Our Understanding of Wzx Flippase for Translocation of Bacterial Membrane Lipid-Linked Oligosaccharide

Translocation of lipid-linked oligosaccharides is a common theme across prokaryotes and eukaryotes. For bacteria, such activity is used in cell wall construction, polysaccharide synthesis, and the relatively recently discovered protein glycosylation. To the best of our knowledge, the Gram-negative inner membrane flippase Wzx was the first protein identified as being involved in oligosaccharide translocation, and yet we still have only a limited understanding of this protein after 3 decades of research. At present, Wzx is known to be a multitransmembrane protein with enormous sequence diversity that flips oligosaccharide substrates with varied degrees of preference. In this review, we provide an overview of the major findings for this protein, with a particular focus on substrate preference.

Synthesis of Rhizobial Exopolysaccharides and Their Importance for Symbiosis with Legume Plants

Rhizobia dwell and multiply in the soil and represent a unique group of bacteria able to enter into a symbiotic interaction with plants from the Fabaceae family and fix atmospheric nitrogen inside de novo created plant organs, called nodules. One of the key determinants of the successful interaction between these bacteria and plants are exopolysaccharides, which represent species-specific homo- and heteropolymers of different carbohydrate units frequently decorated by non-carbohydrate substituents. Exopolysaccharides are typically built from repeat units assembled by the Wzx/Wzy-dependent pathway, where individual subunits are synthesized in conjunction with the lipid anchor undecaprenylphosphate (und-PP), due to the activity of glycosyltransferases. Complete oligosaccharide repeat units are transferred to the periplasmic space by the activity of the Wzx flippase, and, while still being anchored in the membrane, they are joined by the polymerase Wzy. Here we have focused on the genetic control over the process of exopolysaccharides (EPS) biosynthesis in rhizobia, with emphasis put on the recent advancements in understanding the mode of action of the key proteins operating in the pathway. A role played by exopolysaccharide in Rhizobium-legume symbiosis, including recent data confirming the signaling function of EPS, is also discussed.

Full-length, Oligomeric Structure of Wzz Determined by Cryoelectron Microscopy Reveals Insights into Membrane-Bound States

Wzz is an integral inner membrane protein involved in regulating the length of lipopolysaccharide O-antigen glycans and essential for the virulence of many Gram-negative pathogens. In all Wzz homologs, the large periplasmic domain is proposed to be anchored by two transmembrane helices, but no information is available for the transmembrane and cytosolic domains. Here we have studied purified oligomeric Wzz complexes using cryoelectron microscopy and resolved the transmembrane regions within a semi-continuous detergent micelle. The transmembrane helices of each monomer display a right-handed super-helical twist, and do not interact with the neighboring transmembrane domains. Modeling, flexible fitting and multiscale simulation approaches were used to study the full-length complex and to provide explanations for the influence of the lipid bilayer on its oligomeric status. Based on structural and in silico observations, we propose a new mechanism for O-antigen chain-length regulation that invokes synergy of Wzz and its polymerase partner, Wzy.

Model for the Controlled Synthesis of O-Antigen Repeat Units Involving the WaaL Ligase

The Wzx/Wzy O-antigen pathway involves synthesis of a repeat unit (O unit) consisting of 3 to 8 sugars on an inner-membrane-embedded lipid carrier. These O units are translocated across the membrane to its periplasmic face by Wzx, while retaining linkage to the carrier, and then polymerized by Wzy to O-antigen polymer, which WaaL ligase transfers to a lipopolysaccharide precursor to complete lipopolysaccharide synthesis, concomitantly releasing the lipid carrier. This lipid carrier is also used for peptidoglycan assembly, and sequestration is known to be toxic. Thus, O-unit synthesis must involve precise regulation to meet demand but avoid overproduction. Here we show that loss of WaaL reverses a known growth defect in a Salmonella mutant that otherwise accumulates O-unit intermediates and propose that WaaL is also involved in a novel feedback mechanism to regulate O-unit synthesis, based on the availability of O units on the periplasmic face of the membrane.