Biochemical and structural analysis of bacterial O-antigen chain length regulator proteins reveals a conserved quaternary structure

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.

Mutation of putative glycosyl transferases PslC and PslI confers susceptibility to antibiotics and leads to drastic reduction in biofilm formation in Pseudomonas aeruginosa

Pseudomonas aeruginosa is an opportunistic, multidrug-resistant pathogen capable of adapting to numerous environmental conditions and causing fatal infections in immunocompromised patients. The predominant lifestyle of P. aeruginosa is in the form of biofilms, which are structured communities of bacteria encapsulated in a matrix containing exopolysaccharides, extracellular DNA (eDNA) and proteins. The matrix is impervious to antibiotics, rendering the bacteria tolerant to antimicrobials. P. aeruginosa also produces a plethora of virulence factors such as pyocyanin, rhamnolipids and lipopolysaccharides among others. In this study we present the molecular characterization of pslC and pslI genes, of the exopolysaccharide operon, that code for putative glycosyltransferases. PslC is a 303 amino acid containing putative GT2 glycosyltrasferase, whereas PslI is a 367 aa long protein, possibly functioning as a GT4 glycosyltransferase. Mutation in either of these two genes results in a significant reduction in biofilm biomass with concomitant decline in c-di-GMP levels in the bacterial cells. Moreover, mutation in pslC and pslI dramatically increased susceptibility of P. aeruginosa to tobramycin, colistin and ciprofloxacin. Additionally, these mutations also resulted in an increase in rhamnolipids and pyocyanin formation. We demonstrate that elevated rhamnolipids promote a swarming phenotype in the mutant strains. Together these results highlight the importance of PslC and PslI in the biogenesis of biofilms and their potential as targets for increased antibiotic susceptibility and biofilm inhibition.

Alternating L4 loop architecture of the bacterial polysaccharide co-polymerase WzzE

Lipopolysaccharides such as the enterobacterial common antigen are important components of the enterobacterial cell envelope that act as a protective barrier against the environment and are often polymerized by the inner membrane bound Wzy-dependent pathway. By employing cryo-electron microscopy we show that WzzE, the co-polymerase component of this pathway that is responsible for the length modulation of the enterobacterial common antigen, is octameric with alternating up-down conformations of its L4 loops. The alternating up-down nature of these essential loops, located at the top of the periplasmic bell, are modulated by clashing helical faces between adjacent protomers that flank the L4 loops around the octameric periplasmic bell. This alternating arrangement and a highly negatively charged binding face create a dynamic environment in which the polysaccharide chain is extended, and suggest a ratchet-type mechanism for polysaccharide elongation.

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.

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.

(p)ppGpp-Dependent Regulation of the Nucleotide Hydrolase PpnN Confers Complement Resistance in Salmonella enterica Serovar Typhimurium

The stringent response is an essential mechanism of metabolic reprogramming during environmental stress that is mediated by the nucleotide alarmones guanosine tetraphosphate and pentaphosphate [(p)ppGpp]. In addition to physiological adaptations, (p)ppGpp also regulates virulence programs in pathogenic bacteria, including Salmonella enterica serovar Typhimurium. S Typhimurium is a common cause of acute gastroenteritis, but it may also spread to systemic tissues, resulting in severe clinical outcomes. During infection, S Typhimurium encounters a broad repertoire of immune defenses that it must evade for successful host infection. Here, we examined the role of the stringent response in S Typhimurium resistance to complement-mediated killing and found that the (p)ppGpp synthetase-hydrolase, SpoT, is required for bacterial survival in human serum. We identified the nucleotide hydrolase, PpnN, as a target of the stringent response that is required to promote bacterial fitness in serum. Using chromatography and mass spectrometry, we show that PpnN hydrolyzes purine and pyrimidine monophosphates to generate free nucleobases and ribose 5'-phosphate, and that this metabolic activity is required for conferring resistance to complement killing. In addition to PpnN, we show that (p)ppGpp is required for the biosynthesis of the very long and long O-antigen in the outer membrane, known to be important for complement resistance. Our results provide new insights into the role of the stringent response in mediating evasion of the innate immune system by pathogenic bacteria.

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.

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.

Promising discovery of beneficial Escherichia coli in the human gut

Ingested dietary fibres are hydrolysed by colon microbiota to produce energy-providing short-chain fatty acids (SCFA) that stimulate anti-inflammatory effects. SCFA-producing bacteria were screened from bacteria isolated from human faeces using bromothymol blue as an acid indicator and gas chromatography for SCFA profiling. The beneficial functions (antagonistic activity, haemolytic activities, antibiotic susceptibility, mucus adherent percentage and toxin gene detection) were evaluated for the top five SCFA-producing bacteria isolated from three healthy volunteers that identified as Escherichia coli strains. They produced acetic, propionic, isobutyric, butyric, isovaleric, valeric and caproic acids at average concentrations of 15.9, 1.8, 1.1, 1.9, 1.8, 2.7 and 3.4 mM, respectively. The SCFA production by E. coli strains was rapidly increased during the first 8 h of incubation and gradually decreased after 16 h of incubation. All E. coli strains showed acid and bile tolerance, resulting in a survival rate greater than 70% with no haemolytic activity, mucus adherence greater than 40% and susceptibility to conventional antibiotics. Hence, the selected E. coli strains exhibited promising probiotic properties with neither enterotoxin nor LPS producibility was detected. The present results confirm the existence of friendly and harmless E. coli strains in human microbiota as potential probiotics.

Biosynthesis and Export of Bacterial Glycolipids

Complex carbohydrates are essential for many biological processes, from protein quality control to cell recognition, energy storage, and cell wall formation. Many of these processes are performed in topologically extracellular compartments or on the cell surface; hence, diverse secretion systems evolved to transport the hydrophilic molecules to their sites of action. Polyprenyl lipids serve as ubiquitous anchors and facilitators of these transport processes. Here, we summarize and compare bacterial biosynthesis pathways relying on the recognition and transport of lipid-linked complex carbohydrates. In particular, we compare transporters implicated in O antigen and capsular polysaccharide biosyntheses with those facilitating teichoic acid and N-linked glycan transport. Further, we discuss recent insights into the generation, recognition, and recycling of polyprenyl lipids.

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.

Concise chemical synthesis of the pentasaccharide repeating unit of the O-antigen from Escherichia albertii O2

Total chemical synthesis of the pentasaccharide repeating unit of the O-antigen from Escherichia albertii O2 is accomplished by following a [3 + 2] strategy. The target pentasaccharide in the form of its 2-aminoethyl glycoside is particularly attractive as the free amine end can be coupled with suitable aglycon to make further glycoconjugate without affecting the anomeric stereochemistry. Phthalimido derivatives were used successfully as the precursor of the desired acetamido glucose moieties and ensured the 1,2-trans linkages.

Chemical synthesis of the pentasaccharide repeating unit of the O-specific polysaccharide from Escherichia coli O132 in the form of its 2-aminoethyl glycoside

The total chemical synthesis of the pentasaccharide repeating unit of the O-polysaccharide from E. coli O132 is accomplished in the form of its 2-aminoethyl glycoside. The 2-aminoethyl glycoside is particularly important as it allows further glycoconjugate formation utilizing the terminal amine without affecting the stereochemistry of the reducing end. The target was achieved through a [3 + 2] strategy where the required monosaccharide building blocks are prepared from commercially available sugars through rational protecting group manipulation. The NIS-mediated activation of thioglycosides was used extensively for the glycosylation reactions throughout.

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.

The advance of assembly of exopolysaccharide Psl biosynthesis machinery in Pseudomonas aeruginosa

Biofilms are microbial communities embedded in extracellular matrix. Exopolysaccharide Psl (ePsl) is a key biofilm matrix component that initiates attachment, maintains biofilms architecture, and protects bacteria within biofilms of Pseudomonas aeruginosa, an opportunistic pathogen. There are at least 12 Psl proteins involved in the biosynthesis of this exopolysaccharide. However, it remains unclear about the function of each Psl protein and how these proteins work together during the biosynthesis of ePsl. PslG has been characterized as a degrader of ePsl in extracellular or periplasm and PslD is predicted to be a transporter. In this study, we found that PslG and its glycoside hydrolytic activity were also involved in the biosynthesis of ePsl. PslG localized mainly in the inner membrane and some in the periplasm. The inner membrane association of PslG was critical for the biosynthesis of ePsl. The expression of PslA, PslD, and PslE helped PslG remain in the inner membrane. The bacterial two‐hybrid results suggested that PslE could interacted with either PslA, PslD, or PslG. The strongest interaction was found between PslE and PslD. Consistently, PslD was disabled to localize on the outer membrane in the ΔpslE strain, suggesting that the PslE‐PslD interaction affected the localization of PslD. Our results shed light on the assembly of ePsl biosynthesis machinery and suggested that the membrane‐associated PslG was a part of ePsl biosynthesis proteins complex.

Chemical synthesis of the 4-amino-4,6-dideoxy-d-glucose containing pentasaccharide repeating unit of the O-specific polysaccharide from Aeromonas hydrophila strain K691 in the form of its 2-aminoethyl glycoside

Chemical synthesis of the pentasaccharide repeating unit of the O-specific polysaccharide from Aeromonas hydrophilastrain K691 is reported. Synthesis of the pentasaccharide is accomplished by using a common disaccharide in sequence and finally attaching the rare sugar unit. The target structure was made in the form of its 2-aminoethyl glycoside which is essential for further glycconjugate formation. Stereoselective glycosylations were achieved by the activation of thioglycosides in the presence of H₂SO₄-silica in conjunction with N-iodosuccinimide.

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.

The diversity of Klebsiella pneumoniae surface polysaccharides

Klebsiella pneumoniae is considered an urgent health concern due to the emergence of multi-drug-resistant strains for which vaccination offers a potential remedy. Vaccines based on surface polysaccharides are highly promising but need to address the high diversity of surface-exposed polysaccharides, synthesized as O-antigens (lipopolysaccharide, LPS) and K-antigens (capsule polysaccharide, CPS), present in K. pneumoniae. We present a comprehensive and clinically relevant study of the diversity of O- and K-antigen biosynthesis gene clusters across a global collection of over 500 K. pneumoniae whole-genome sequences and the seroepidemiology of human isolates from different infection types. Our study defines the genetic diversity of O- and K-antigen biosynthesis cluster sequences across this collection, identifying sequences for known serotypes as well as identifying novel LPS and CPS gene clusters found in circulating contemporary isolates. Serotypes O1, O2 and O3 were most prevalent in our sample set, accounting for approximately 80 % of all infections. In contrast, K serotypes showed an order of magnitude higher diversity and differ among infection types. In addition we investigated a potential association of O or K serotypes with phylogenetic lineage, infection type and the presence of known virulence genes. K1 and K2 serotypes, which are associated with hypervirulent K. pneumoniae, were associated with a higher abundance of virulence genes and more diverse O serotypes compared to other common K serotypes.

Conserved transmembrane glycine residues in the Shigella flexneri polysaccharide co-polymerase protein WzzB influence protein-protein interactions

The O antigen (Oag) component of lipopolysaccharides (LPS) is crucial for virulence and Oag chain-length regulation is controlled by the polysaccharide co-polymerase class 1 (PCP1) proteins. Crystal structure analyses indicate that structural conservation among PCP1 proteins is highly maintained, however the mechanism of Oag modal-chain-length control remains to be fully elucidated. Shigella flexneri PCP1 protein WzzBSF confers a modal-chain length of 10-17 Oag repeat units (RUs), whereas the Salmonella enterica Typhimurium PCP1 protein WzzBST confers a modal-chain length of ~16-28 Oag RUs. Both proteins share >70 % overall sequence identity and contain two transmembrane (TM1 and TM2) regions, whereby a conserved proline-glycine-rich motif overlapping the TM2 region is identical in both proteins. Conserved glycine residues within TM2 are functionally important, as glycine to alanine substitutions at positions 305 and 311 confer very short Oag modal-chain length (~2-6 Oag RUs). In this study, WzzBSF was co-expressed with WzzBST in S. flexneri and a single intermediate modal-chain length of ~11-21 Oag RUs was observed, suggesting the presence of Wzz:Wzz interactions. Interestingly, co-expression of WzzBSF with WzzBG305A/G311A conferred a bimodal LPS Oag chain length (despite over 99 % protein sequence identity), and we hypothesized that the proteins fail to interact. Co-purification assays detected His6-WzzBSF co-purifying with FLAG-tagged WzzBST but not with FLAG-tagged WzzBG305A/G311A, supporting our hypothesis. These data indicate that the conserved glycine residues in TM2 are involved in Wzz:Wzz interactions, and provide insight into key interactions that drive Oag modal length control.

Structural and Biochemical Analysis of a Single Amino-Acid Mutant of WzzBSF That Alters Lipopolysaccharide O-Antigen Chain Length in Shigella flexneri

Lipopolysaccharide (LPS), a surface polymer of Gram-negative bacteria, helps bacteria survive in different environments and acts as a virulence determinant of host infection. The O-antigen (Oag) component of LPS exhibits a modal chain-length distribution that is controlled by polysaccharide co-polymerases (PCPs). The molecular basis of the regulation of Oag chain-lengths remains unclear, despite extensive mutagenesis and structural studies of PCPs from Escherichia coli and Shigella. Here, we identified a single mutation (A107P) of the Shigella flexneri WzzBSF, by a random mutagenesis approach, that causes a shortened Oag chain-length distribution in bacteria. We determined the crystal structures of the periplasmic domains of wild-type WzzBSF and the A107P mutant. Both structures form a highly similar open trimeric assembly in the crystals, and show a similar tendency to self-associate in solution. Binding studies by bio-layer interferometry reveal cooperative binding of very short (VS)-core-plus-O-antigen polysaccharide (COPS) to the periplasmic domains of both proteins, but with decreased affinity for the A107P mutant. Our studies reveal that subtle and localized structural differences in PCPs can have dramatic effects on LPS chain-length distribution in bacteria, for example by altering the affinity for the substrate, which supports the role of the structure of the growing Oag polymer in this process.

Challenges and perspectives in combinatorial assembly of novel exopolysaccharide biosynthesis pathways

Because of their rheological properties various microbial polysaccharides are applied as thickeners and viscosifiers both in food and non-food industries. A broad variety of microorganisms secrete structurally diverse exopolysaccharides (EPS) that contribute to their surface attachment, protection against abiotic or biotic stress factors, and nutrient gathering. Theoretically, a massive number of EPS structures are possible through variations in monosaccharide sequences, condensation linkages and non-sugar decorations. Given the already-high diversity of EPS structures, taken together with the principal of combinatorial biosynthetic pathways, microbial polysaccharides are an attractive class of macromolecules with which to generate novel structures via synthetic biology approaches. However, previous manipulations primarily focused on increasing polysaccharide yield, with structural modifications restricted to removal of side chains or non-sugar decorations. This article outlines the biosynthetic pathways of the bacterial heteroexopolysaccharides xanthan and succinoglycan, which are used as thickening and stabilizing agents in food and non-food industries. Challenges and perspectives of combining synthetic biology approaches with directed evolution to overcome obstacles in assembly of novel EPS biosynthesis pathways are discussed.

Quaternary structure of WzzB and WzzE polysaccharide copolymerases

Bacteria have evolved cellular control mechanisms to ensure proper length specification for surface-bound polysaccharides. Members of the Polysaccharide Copolymerase (PCP) family are central to this process. PCP-1 family members are anchored to the inner membrane through two transmembrane helices and contain a large periplasm-exposed domain. PCPs are known to form homooligomers but their exact stoichiometry is controversial in view of conflicting structural and biochemical data. Several prior investigations addressing this question indicated a nonameric, hexameric, or tetrameric organization of several PCP-1 family members. In this work, we gathered additional evidence that E.coli WzzB and WzzE PCPs form octameric homo-oligomeric complexes. Detergent-solubilized PCPs were purified to homogeneity and subjected to blue native gel analysis, which indicated the presence of a predominant high-molecular product of over 500 kDa in mass. Molecular mass of WzzE and WzzB-detergent oligomers was estimated to be 550 kDA by size-exclusion coupled to multiangle laser light scattering (SEC-MALLS). Oligomeric organization of purified WzzB and WzzE was further investigated by negative stain electron microscopy and by X-ray crystallography, respectively. Analysis of EM-derived molecular envelope of WzzB indicated that the full-length protein is composed of eight protomers. Crystal structure of LDAO-solubilized WzzE was solved to 6 Å resolutions and revealed its octameric subunit stoichiometry. In summary, we identified a possible biological unit utilized for the glycan chain length determination by two PCP-1 family members. This provides an important step toward further unraveling of the mechanistic basis of chain length control of the O-antigen and the enterobacterial common antigen.

Synthesis of bacterial polysaccharides via the Wzx/Wzy-dependent pathway

The surfaces of bacteria mediate a multitude of functions in the environment and in an infected host, including adhesion to both biotic and abiotic substrata, motility, immune system interaction and (or) activation, biofilm formation, and cell–cell communication, with many of these features directly influenced by cell-surface glycans. In both Gram-negative and Gram-positive bacteria, the majority of cell-surface polysaccharides are produced via the Wzx/Wzy-dependent assembly pathway; these glycans include heteropolymeric O-antigen, enterobacterial common antigen, exopolysaccharide, spore coat, and capsule in diverse bacteria. The key components of this assembly pathway are the integral inner membrane Wzx flippase, Wzy polymerase, and Wzz chain-length regulator proteins, which until recently have resisted detailed structural and functional characterization. In this review, we have provided a comprehensive synthesis of the latest structural and mechanistic data for each protein, as well as an examination of substrate specificity for each assembly step and complex formation between the constituent proteins. To complement the unprecedented explosion of genomic-sequencing data for bacteria, we have also highlighted both classical and state-of-the-art methods by which encoded Wzx, Wzy, and Wzz proteins can be reliably identified and annotated, using the model Gram-negative bacterium Pseudomonas aeruginosa as an example data set. Lastly, we outline future avenues of research, with the aim of stimulating researchers to take the next steps in investigating the function of, and interplay between, the constituents of this widespread assembly scheme.

Bacterial cell-envelope glycoconjugates

Prokaryotic glycosylation fulfills an important role in maintaining and protecting the structural integrity and function of the bacterial cell wall, as well as serving as a flexible adaption mechanism to evade environmental and host-induced pressure. The scope of bacterial and archaeal protein glycosylation has considerably expanded over the past decade(s), with numerous examples covering the glycosylation of flagella, pili, glycosylated enzymes, as well as surface-layer proteins. This article addresses structure, analysis, function, genetic basis, biosynthesis, and biomedical and biotechnological applications of cell-envelope glycoconjugates, S-layer glycoprotein glycans, and "nonclassical" secondary-cell wall polysaccharides. The latter group of polymers mediates the important attachment and regular orientation of the S-layer to the cell wall. The structures of these glycopolymers reveal an enormous diversity, resembling the structural variability of bacterial lipopolysaccharides and capsular polysaccharides. While most examples are presented for Gram-positive bacteria, the S-layer glycan of the Gram-negative pathogen Tannerella forsythia is also discussed. In addition, archaeal S-layer glycoproteins are briefly summarized.

Biophysical characterization of the outer membrane polysaccharide export protein and the polysaccharide co-polymerase protein from Xanthomonas campestris

This study investigated the structural and biophysical characteristics of GumB and GumC, two Xanthomonas campestris membrane proteins that are involved in xanthan biosynthesis. Xanthan is an exopolysaccharide that is thought to be a virulence factor that contributes to bacterial in planta growth. It also is one of the most important industrial biopolymers. The first steps of xanthan biosynthesis are well understood, but the polymerization and export mechanisms remain unclear. For this reason, the key proteins must be characterized to better understand these processes. Here we characterized, by biochemical and biophysical techniques, GumB, the outer membrane polysaccharide export protein, and GumC, the polysaccharide co-polymerase protein of the xanthan biosynthesis system. Our results suggested that recombinant GumB is a tetrameric protein in solution. On the other hand, we observed that both native and recombinant GumC present oligomeric conformation consistent with dimers and higher-order oligomers. The transmembrane segments of GumC are required for GumC expression and/or stability. These initial results provide a starting point for additional studies that will clarify the roles of GumB and GumC in the xanthan polymerization and export processes and further elucidate their functions and mechanisms of action.

Salmonellae PhoPQ regulation of the outer membrane to resist innate immunity

Salmonellae sense host cues to regulate properties important for bacterial survival and replication within host tissues. The PhoPQ two-component regulatory system senses phagosome acidification and cationic antimicrobial peptides (CAMP) to regulate the protein and lipid contents of the bacterial envelope that comprises an inner and outer membrane. PhoPQ-regulated lipid components of the outer membrane include lipopolysaccharides and glycerophospholipids. Envelope proteins regulated by PhoPQ, include: components of virulence associated secretion systems, the flagellar apparatus, membrane transport systems, and proteins that are likely structural components of the outer membrane. PhoPQ alteration of the bacterial surface results in increased bacterial resistance to CAMP and decreased detection by the innate immune system. This review details the molecular complexity of the bacterial cell envelope and highlights the outer membrane lipid bilayer as an environmentally regulated bacterial organelle.

Conserved-residue mutations in Wzy affect O-antigen polymerization and Wzz-mediated chain-length regulation in Pseudomonas aeruginosa PAO1

O antigen (O-Ag) in many bacteria is synthesized via the Wzx/Wzy-dependent pathway in which Wzy polymerizes lipid-linked O-Ag subunits to modal lengths regulated by Wzz. Characterization of 83 site-directed mutants of Wzy from Pseudomonas aeruginosa PAO1 (Wzy_Pa) in topologically-mapped periplasmic (PL) and cytoplasmic loops (CL) verified the functional importance of PL3 and PL5, with the former shown to require overall cationic properties. Essential Arg residues in the RX₁₀G motifs of PL3 and PL5 were found to be conserved in putative homologues of Wzy_Pa, as was the overall sequence homology between these two periplasmic loops in each protein. Amino acid substitutions in CL6 were found to alter Wzz-mediated O-antigen modality, with evidence suggesting that these changes may perturb the C-terminal Wzy_Pa tertiary structure. Together, these data suggest that the catch-and-release mechanism of O-Ag polymerization is widespread among bacteria and that regulation of polymer length is affected by interaction of Wzz with Wzy.

The Wzy O-antigen polymerase of Yersinia pseudotuberculosis O:2a has a dependence on the Wzz chain-length determinant for efficient polymerization

Lipopolysaccharide is a major immunogenic structure for the pathogen Yersinia pseudotuberculosis, which contains the O-specific polysaccharide (OPS) that is presented on the cell surface. The OPS contains many repeats of the oligosaccharide O-unit and exhibits a preferred modal chain length that has been shown to be crucial for cell protection in Yersinia. It is well established that the Wzz protein determines the preferred chain length of the OPS, and in its absence, the polymerization of O units by the Wzy polymerase is uncontrolled. However, for Y. pseudotuberculosis, a wzz mutation has never been described. In this study, we examine the effect of Wzz loss in Y. pseudotuberculosis serotype O:2a and compare the lipopolysaccharide chain-length profile to that of Escherichia coli serotype O111. In the absence of Wzz, the lipopolysaccharides of the two species showed significant differences in Wzy polymerization. Yersinia pseudotuberculosis O:2a exhibited only OPS with very short chain lengths, which is atypical of wzz-mutant phenotypes that have been observed for other species. We hypothesise that the Wzy polymerase of Y. pseudotuberculosis O:2a has a unique default activity in the absence of the Wzz, revealing the requirement of Wzz to drive O-unit polymerization to greater lengths.

The D3 bacteriophage α-polymerase inhibitor (Iap) peptide disrupts O-antigen biosynthesis through mimicry of the chain length regulator Wzz in Pseudomonas aeruginosa

Lysogenic bacteriophage D3 causes seroconversion of Pseudomonas aeruginosa PAO1 from serotype O5 to O16 by inverting the linkage between O-specific antigen (OSA) repeat units from α to β. The OSA units are polymerized by Wzy to modal lengths regulated by Wzz1 and Wzz2. A key component of the D3 seroconversion machinery is the inhibitor of α-polymerase (Iap) peptide, which is able to solely suppress α-linked long-chain OSA production in P. aeruginosa PAO1. To establish the target specificity of Iap for Wzyα, changes in OSA phenotypes were examined via Western immunoblotting for wzz1 and wzz2 single-knockout strains, as well as a wzz1 wzz2 double knockout, following the expression of iap from a tuneable vector. Increased induction of Iap expression completely abrogated OSA production in the wzz1 wzz2 double mutant, while background levels of OSA production were still observed in either of the single mutants. Therefore, Iap inhibition of OSA biosynthesis was most effective in the absence of both Wzz proteins. Sequence alignment analyses revealed a high degree of similarity between Iap and the first transmembrane segment (TMS) of either Wzz1 or Wzz2. Various topology prediction analyses of the Iap sequence consistently predicted the presence of a single TMS, suggesting a propensity for Iap to insert itself into the inner membrane (IM). The compromised ability of Iap to abrogate Wzyα function in the presence of Wzz1 or Wzz2 provides compelling evidence that inhibition occurs after Wzyα inserts itself into the IM and is achieved through mimicry of the first TMS from the Wzz proteins of P. aeruginosa PAO1.

Residues located inside the Escherichia coli FepE protein oligomer are essential for lipopolysaccharide O-antigen modal chain length regulation

The Escherichia coli O157 : H7 FepE protein regulates lipopolysaccharide (LPS) O-antigen (Oag) chain length to confer a very long modal chain length of >80 Oag repeat units (RUs). The mechanism by which FepE regulates Oag modal chain length and the regions within it that are important for its function remain unclear. Studies on the structure of FepE show that the protein oligomerizes. However, the exact size of the oligomer is in dispute, further hampering our understanding of its mechanism. Guided by information previously obtained for regions known to be important for Oag modal chain length determination in the homologous Shigella flexneri WzzBSF protein, a set of FepE mutant constructs with single amino acid substitutions was created. Analysis of the resulting LPS conferred by these mutant His6-FepE proteins showed that amino acid substitutions of leucine 168 (L168) and aspartic acid 268 (D268) resulted in LPS with consistently shortened Oag chain lengths of <80 Oag RUs. Substitution of FepE's transmembrane cysteine residues did not affect function. Chemical cross-linking experiments on mutant FepE proteins showed no consistent correlation between oligomer size and functional activity, and MS analysis of FepE oligomers indicated that the in vivo size of FepE is consistent with a maximum size of a hexamer. Our findings suggest that different FepE residues, mainly located within the internal cavity of the oligomer, contribute to Oag modal chain length determination but not the oligomeric state of the protein.

Polysaccharide co-polymerases: the enigmatic conductors of the O-antigen assembly orchestra

The O-antigen lipopolysaccharides on bacterial surface contain variable number of oligosaccharide repeat units with their length having a modal distribution specific to the bacterial strain. The polysaccharide length distribution is controlled by the proteins called polysaccharide co-polymerases (PCPs), which are embedded in the inner membrane in Gram-negative bacteria and form homo oligomers. The 3D structures of periplasmic domains of several PCPs have been determined and provided the first insights into the possible mechanism of polysaccharide length determination mechanism. Here we review the current knowledge of structure and function of these polysaccharide length regulators.

O-antigen modulates infection-induced pain states

The molecular initiators of infection-associated pain are not understood. We recently found that uropathogenic E. coli (UPEC) elicited acute pelvic pain in murine urinary tract infection (UTI). UTI pain was due to E. coli lipopolysaccharide (LPS) and its receptor, TLR4, but pain was not correlated with inflammation. LPS is known to drive inflammation by interactions between the acylated lipid A component and TLR4, but the function of the O-antigen polysaccharide in host responses is unknown. Here, we examined the role of O-antigen in pain using cutaneous hypersensitivity (allodynia) to quantify pelvic pain behavior and using sacral spinal cord excitability to quantify central nervous system manifestations in murine UTI. A UPEC mutant defective for O-antigen biosynthesis induced chronic allodynia that persisted long after clearance of transient infections, but wild type UPEC evoked only acute pain. E. coli strains lacking O-antigen gene clusters had a chronic pain phenotype, and expressing cloned O-antigen gene clusters altered the pain phenotype in a predictable manner. Chronic allodynia was abrogated in TLR4-deficient mice, but inflammatory responses in wild type mice were similar among E. coli strains spanning a wide range of pain phenotypes, suggesting that O-antigen modulates pain independent of inflammation. Spinal cords of mice with chronic allodynia exhibited increased spontaneous firing and compromised short-term depression, consistent with centralized pain. Taken together, these findings suggest that O-antigen functions as a rheostat to modulate LPS-associated pain. These observations have implications for an infectious etiology of chronic pain and evolutionary modification of pathogens to alter host behaviors.

Structural characterization of closely related O-antigen lipopolysaccharide (LPS) chain length regulators

The surface O-antigen polymers of gram-negative bacteria exhibit a modal length distribution that depends on dedicated chain length regulator periplasmic proteins (polysaccharide co-polymerases, PCPs) anchored in the inner membrane by two transmembrane helices. In an attempt to determine whether structural changes underlie the O-antigen modal length specification, we have determined the crystal structures of several closely related PCPs, namely two chimeric PCP-1 family members solved at 1.6 and 2.8 Å and a wild-type PCP-1 from Shigella flexneri solved at 2.8 Å. The chimeric proteins form circular octamers, whereas the wild-type WzzB from S. flexneri was found to be an open trimer. We also present the structure of a Wzz(FepE) mutant, which exhibits severe attenuation in its ability to produce very long O-antigen polymers. Our findings suggest that the differences in the modal length distribution depend primarily on the surface-exposed amino acids in specific regions rather than on the differences in the oligomeric state of the PCP protomers.

Site-directed mutagenesis reveals key residue for O antigen chain length regulation and protein stability in Pseudomonas aeruginosa Wzz2

The production of preferred lipopolysaccharide O antigen chain lengths is important for the survival of pathogenic Gram-negative bacteria in different environments, yet how Wzz proteins regulate these lengths is not well understood. The Wzz2 proteins from two different serotype O11 Pseudomonas aeruginosa strains are responsible for the expression of different very long chain lengths despite high sequence homology. Site-directed mutagenesis was performed to determine whether a specific amino acid was responsible for this difference in chain length; the residue present in position 321 within the second predicted coiled-coil region was able to determine which chain length was produced. A panel of site-directed mutants introducing different amino acids at this position implicated that the charge of the amino acid affected chain length, with positively charged residues associated with shorter chain lengths. Expression data also suggested this site was important for overall stability of the protein because mutants predicted to disrupt proper folding of the α helix led to lower protein levels. Cross-linking studies found that Wzz2 proteins producing shorter chain lengths had more stable higher-order oligomers. Mapping residue 321 onto the solved Escherichia coli Wzz FepE crystal structure predicted it to be located within α helix 8, which participates in intermonomeric interactions. These data further support the observation that Wzz oligomerization is necessary for chain length regulating activity but also provide evidence that differences in complex stability or changes in the conformation of the oligomer can lead to shifts in the length of the O antigen side chain.

Biosynthesis of the Pseudomonas aeruginosa Extracellular Polysaccharides, Alginate, Pel, and Psl

Pseudomonas aeruginosa thrives in many aqueous environments and is an opportunistic pathogen that can cause both acute and chronic infections. Environmental conditions and host defenses cause differing stresses on the bacteria, and to survive in vastly different environments, P. aeruginosa must be able to adapt to its surroundings. One strategy for bacterial adaptation is to self-encapsulate with matrix material, primarily composed of secreted extracellular polysaccharides. P. aeruginosa has the genetic capacity to produce at least three secreted polysaccharides; alginate, Psl, and Pel. These polysaccharides differ in chemical structure and in their biosynthetic mechanisms. Since alginate is often associated with chronic pulmonary infections, its biosynthetic pathway is the best characterized. However, alginate is only produced by a subset of P. aeruginosa strains. Most environmental and other clinical isolates secrete either Pel or Psl. Little information is available on the biosynthesis of these polysaccharides. Here, we review the literature on the alginate biosynthetic pathway, with emphasis on recent findings describing the structure of alginate biosynthetic proteins. This information combined with the characterization of the domain architecture of proteins encoded on the Psl and Pel operons allowed us to make predictive models for the biosynthesis of these two polysaccharides. The results indicate that alginate and Pel share certain features, including some biosynthetic proteins with structurally or functionally similar properties. In contrast, Psl biosynthesis resembles the EPS/CPS capsular biosynthesis pathway of Escherichia coli, where the Psl pentameric subunits are assembled in association with an isoprenoid lipid carrier. These models and the environmental cues that cause the cells to produce predominantly one polysaccharide over the others are subjects of current investigation.

Structure-guided investigation of lipopolysaccharide O-antigen chain length regulators reveals regions critical for modal length control

The O-antigen component of the lipopolysaccharide (LPS) represents a population of polysaccharide molecules with nonrandom (modal) chain length distribution. The number of the repeat O units in each individual O-antigen polymer depends on the Wzz chain length regulator, an inner membrane protein belonging to the polysaccharide copolymerase (PCP) family. Different Wzz proteins confer vastly different ranges of modal lengths (4 to >100 repeat units), despite having remarkably conserved structural folds. The molecular mechanism responsible for the selective preference for a certain number of O units is unknown. Guided by the three-dimensional structures of PCPs, we constructed a panel of chimeric molecules containing parts of two closely related Wzz proteins from Salmonella enterica and Shigella flexneri which confer different O-antigen chain length distributions. Analysis of the O-antigen length distribution imparted by each chimera revealed the region spanning amino acids 67 to 95 (region 67 to 95), region 200 to 255, and region 269 to 274 as primarily affecting the length distribution. We also showed that there is no synergy between these regions. In particular, region 269 to 274 also influenced chain length distribution mediated by two distantly related PCPs, WzzB and FepE. Furthermore, from the 3 regions uncovered in this study, region 269 to 274 appeared to be critical for the stability of the oligomeric form of Wzz, as determined by cross-linking experiments. Together, our data suggest that chain length determination depends on regions that likely contribute to stabilize a supramolecular complex.

Functional and structural characterization of polysaccharide co-polymerase proteins required for polymer export in ATP-binding cassette transporter-dependent capsule biosynthesis pathways

Neisseria meningitidis serogroup B and Escherichia coli K1 bacteria produce a capsular polysaccharide (CPS) that is composed of α2,8-linked polysialic acid (PSA). Biosynthesis of PSA in these bacteria occurs via an ABC (ATP-binding cassette) transporter-dependent pathway. In N. meningitidis, export of PSA to the surface of the bacterium requires two proteins that form an ABC transporter (CtrC and CtrD) and two additional proteins, CtrA and CtrB, that are proposed to form a cell envelope-spanning export complex. CtrA is a member of the outer membrane polysaccharide export (OPX) family of proteins, which are proposed to form a pore to mediate export of CPSs across the outer membrane. CtrB is an inner membrane protein belonging to the polysaccharide co-polymerase (PCP) family. PCP proteins involved in other bacterial polysaccharide assembly systems form structures that extend into the periplasm from the inner membrane. There is currently no structural information available for PCP or OPX proteins involved in an ABC transporter-dependent CPS biosynthesis pathway to support their proposed roles in polysaccharide export. Here, we report cryo-EM images of purified CtrB reconstituted into lipid bilayers. These images contained molecular top and side views of CtrB and showed that it formed a conical oligomer that extended ∼125 Å from the membrane. This structure is consistent with CtrB functioning as a component of an envelope-spanning complex. Cross-complementation of CtrA and CtrB in E. coli mutants with defects in genes encoding the corresponding PCP and OPX proteins show that PCP-OPX pairs require interactions with their cognate partners to export polysaccharide. These experiments add further support for the model of an ABC transporter-PCP-OPX multiprotein complex that functions to export CPS across the cell envelope.

Endotoxins: lipopolysaccharides of gram-negative bacteria

Endotoxin refers lipopolysaccharide that constitutes the outer leaflet of the outer membrane of most Gram-negative bacteria. Lipopolysaccharide is comprised of a hydrophilic polysaccharide and a hydrophobic component known as lipid A which is responsible for the major bioactivity of endotoxin. Lipopolysaccharide can be recognized by immune cells as a pathogen-associated molecule through Toll-like receptor 4. Most enzymes and genes related to the biosynthesis and export of lipopolysaccharide have been identified in Escherichia coli, and they are shared by most Gram-negative bacteria based on available genetic information. However, the detailed structure of lipopolysaccharide differs from one bacterium to another, suggesting that additional enzymes that can modify the basic structure of lipopolysaccharide exist in bacteria, especially some pathogens. These structural modifications of lipopolysaccharide are sometimes tightly regulated. They are not required for survival but closely related to the virulence of bacteria. In this chapter we will focus on the mechanism of biosynthesis and export of lipopolysaccharide in bacteria.

Membrane topology mapping of the O-antigen flippase (Wzx), polymerase (Wzy), and ligase (WaaL) from Pseudomonas aeruginosa PAO1 reveals novel domain architectures

Biosynthesis of B-band lipopolysaccharide (LPS) in Pseudomonas aeruginosa follows the Wzy-dependent pathway, requiring the integral inner membrane proteins Wzx (O-antigen [O-Ag] flippase), Wzy (O-Ag polymerase), and WaaL (O-Ag ligase). For an important first step in deciphering the mechanisms of LPS assembly, we set out to map the membrane topology of these proteins. Random and targeted 3′wzx, wzy, and waaL truncations were fused to a phoA-lacZα dual reporter capable of displaying both alkaline phosphatase and β-galactosidase activity. The results from truncation fusion expression and the corresponding differential enzyme activity ratios allowed for the assignment of specific regions of the proteins to cytoplasmic, transmembrane (TM), or periplasmic loci. Protein orientation in the inner membrane was confirmed via C-terminal fusion to green fluorescent protein. Our data revealed unique TM domain properties in these proteins, particularly for Wzx, indicating the potential for a charged pore. Novel periplasmic and cytoplasmic loop domains were also uncovered, with the latter in Wzy and WaaL revealing tracts consistent with potential Walker A/B motifs.

The opportunistic pathogen Pseudomonas aeruginosa synthesizes its virulence factor lipopolysaccharide via the Wzy-dependent pathway, requiring translocation, polymerization, and ligation of lipid-linked polysaccharide repeat units by the integral inner membrane proteins Wzx, Wzy, and WaaL, respectively. However, structural evidence to help explain the function of these proteins is lacking. Since membrane proteins are difficult to crystallize, topological mapping is an important first step in identifying exposed and membrane-embedded domains. We mapped the topologies of Wzx, Wzy, and WaaL from P. aeruginosa PAO1 by use of truncation libraries of a randomly fused C-terminal reporter capable of different enzyme activities in the periplasm and cytoplasm. Topology maps were created based directly on residue localization data, eliminating the bias associated with reliance on multiple topology prediction algorithms for initial generation of consensus transmembrane domain localizations. Consequently, we have identified novel periplasmic, cytoplasmic, and transmembrane domain properties that would help to explain the proposed functions of Wzx, Wzy, and WaaL.

Mutagenesis and chemical cross-linking suggest that Wzz dimer stability and oligomerization affect lipopolysaccharide O-antigen modal chain length control

In Shigella flexneri, the polysaccharide copolymerase (PCP) protein Wzz(SF) confers a modal length of 10 to 17 repeat units (RUs) to the O-antigen (Oag) component of lipopolysaccharide (LPS). PCPs form oligomeric structures believed to be related to their function. To identify functionally important regions within Wzz(SF), random in-frame linker mutagenesis was used to create mutants with 5-amino-acid insertions (termed Wzz(i) proteins), and DNA sequencing was used to locate the insertions. Analysis of the resulting LPS conferred by Wzz(i) proteins identified five mutant classes. The class I mutants were inactive, resulting in nonregulated LPS Oag chains, while classes II and III conferred shorter LPS Oag chains of 2 to 10 and 8 to 14 RUs, respectively. Class IV mutants retained near-wild-type function, and class V mutants increased the LPS Oag chain length to 16 to 25 RUs. In vivo formaldehyde cross-linking indicated class V mutants readily formed high-molecular-mass oligomers; however, class II and III Wzz(i) mutants were not effectively cross-linked. Wzz dimer stability was also investigated by heating cross-linked oligomers at 100 degrees C in the presence of SDS. Unlike the Wzz(SF) wild type and class IV and V Wzz(i) mutants, the class II and III mutant dimers were not detectable. The location of each insertion was mapped onto available PCP three-dimensional (3D) structures, revealing that class V mutations were most likely located within the inner cavity of the PCP oligomer. These data suggest that the ability to produce stable dimers may be important in determining Oag modal chain length.

In vitro bacterial polysaccharide biosynthesis: defining the functions of Wzy and Wzz

Polysaccharides constitute a major component of bacterial cell surfaces and play critical roles in bacteria-host interactions. The biosynthesis of such molecules, however, has mainly been characterized through in vivo genetic studies, thus precluding discernment of the details of this pathway. Accordingly, we present a chemical approach that enabled reconstitution of the E. coli O-polysaccharide biosynthetic pathway in vitro. Starting with chemically prepared undecaprenyl-diphospho-N-acetyl-D-galactosamine, the E. coli O86 oligosaccharide repeating unit was assembled by means of sequential enzymatic glycosylation. Successful expression of the putative polymerase Wzy using a chaperone coexpression system then allowed demonstration of polymerization in vitro using this substrate. Analysis of more substrates revealed a defined mode of recognition for Wzy toward the lipid moiety. Specific polysaccharide chain length modality was furthermore demonstrated to result from the action of Wzz. Collectively, polysaccharide biosynthesis was chemically reconstituted in vitro, providing a well defined system for further underpinning molecular details of this biosynthetic pathway.