Alternative Neisseria spp. type IV pilin glycosylation with a glyceramido acetamido trideoxyhexose residue

Sweet complexity: O-linked protein glycosylation in pathogenic Neisseria

The genus Neisseria, which colonizes mucosal surfaces, includes both commensal and pathogenic species that are exclusive to humans. The two pathogenic Neisseria species are closely related but cause quite different diseases, meningococcal sepsis and meningitis (Neisseria meningitidis) and sexually transmitted gonorrhea (Neisseria gonorrhoeae). Although obvious differences in bacterial niches and mechanisms for transmission exists, pathogenic Neisseria have high levels of conservation at the levels of nucleotide sequences, gene content and synteny. Species of Neisseria express broad-spectrum O-linked protein glycosylation where the glycoproteins are largely transmembrane proteins or lipoproteins localized on the cell surface or in the periplasm. There are diverse functions among the identified glycoproteins, for example type IV biogenesis proteins, proteins involved in antimicrobial resistance, as well as surface proteins that have been suggested as vaccine candidates. The most abundant glycoprotein, PilE, is the major subunit of pili which are an important colonization factor. The glycans attached can vary extensively due to phase variation of protein glycosylation (pgl) genes and polymorphic pgl gene content. The exact roles of glycosylation in Neisseria remains to be determined, but increasing evidence suggests that glycan variability can be a strategy to evade the human immune system. In addition, pathogenic and commensal Neisseria appear to have significant glycosylation differences. Here, the current knowledge and implications of protein glycosylation genes, glycan diversity, glycoproteins and immunogenicity in pathogenic Neisseria are summarized and discussed.

Cryo-EM structures of type IV pili complexed with nanobodies reveal immune escape mechanisms

Type IV pili (T4P) are prevalent, polymeric surface structures in pathogenic bacteria, making them ideal targets for effective vaccines. However, bacteria have evolved efficient strategies to evade type IV pili-directed antibody responses. Neisseria meningitidis are prototypical type IV pili-expressing Gram-negative bacteria responsible for life threatening sepsis and meningitis. This species has evolved several genetic strategies to modify the surface of its type IV pili, changing pilin subunit amino acid sequence, nature of glycosylation and phosphoforms, but how these modifications affect antibody binding at the structural level is still unknown. Here, to explore this question, we determine cryo-electron microscopy (cryo-EM) structures of pili of different sequence types with sufficiently high resolution to visualize posttranslational modifications. We then generate nanobodies directed against type IV pili which alter pilus function in vitro and in vivo. Cyro-EM in combination with molecular dynamics simulation of the nanobody-pilus complexes reveals how the different types of pili surface modifications alter nanobody binding. Our findings shed light on the impressive complementarity between the different strategies used by bacteria to avoid antibody binding. Importantly, we also show that structural information can be used to make informed modifications in nanobodies as countermeasures to these immune evasion mechanisms.

Structure of a heteropolymeric type 4 pilus from a monoderm bacterium

Type 4 pili (T4P) are important virulence factors, which belong to a superfamily of nanomachines ubiquitous in prokaryotes, called type 4 filaments (T4F). T4F are defined as helical polymers of type 4 pilins. Recent advances in cryo-electron microscopy (cryo-EM) led to structures of several T4F, revealing that the long N-terminal α-helix (α1) - the trademark of pilins - packs in the centre of the filaments to form a hydrophobic core. In diderm bacteria - all available bacterial T4F structures are from diderm species - a portion of α1 is melted (unfolded). Here we report that this architecture is conserved in phylogenetically distant monoderm species by determining the structure of Streptococcus sanguinis T4P. Our 3.7 Å resolution cryo-EM structure of S. sanguinis heteropolymeric T4P and the resulting full atomic model including all minor pilins highlight universal features of bacterial T4F and have widespread implications in understanding T4F biology.

Complete genome sequence and comparative analysis of a Vibrio vulnificus strain isolated from a clinical patient

Vibrio vulnificus is an opportunistic, global pathogen that naturally inhabits sea water and is responsible for most vibriosis-related deaths. We investigated the genetic characteristics of V. vulnificus isolated from the clinical blood culture specimen of a patient with hepatitis B virus cirrhosis in 2018 (named as V. vulnificus VV2018) by whole genome sequencing (WGS). VV2018 belonged to a novel sequencing type 620 (ST620) and comprised two circular chromosomes, containing 4,389 potential coding sequences (CDSs) and 152 RNA genes. The phylogenetic tree of single nucleotide polymorphisms (SNPs) using 26 representative genomes revealed that VV2108 grouped with two other V. vulnificus strains isolated from humans. The pan-genome of V. vulnificus was constructed using 26 representative genomes to elucidate their genetic diversity, evolutionary characteristics, and virulence and antibiotic resistance profiles. The pan-genome analysis revealed that VV2018 shared a total of 3,016 core genes (≥99% presence), including 115 core virulence factors (VFs) and 5 core antibiotic resistance-related genes, and 309 soft core genes (≥95 and <99% presence) with 25 other V. vulnificus strains. The varG gene might account for the cefazolin resistance, and comparative analysis of the genetic context of varG revealed that two genes upstream and downstream of varG were conserved. The glycosylation (pgl) like genes were found in VV2018 compared with Pgl-related proteins in Neisseria that might affect the adherence of the strain in hosts. The comparative analysis of VV2018 would contribute to a better understanding of the virulence and antibiotic resistance profiles of V. vulnificus. Meanwhile much work remains to be done to better understand the function of pgl-like genes in V. vulnificus.

Genetic, Functional, and Immunogenic Analyses of the O-Linked Protein Glycosylation System in Neisseria meningitidis Serogroup A ST-7 Isolates

Neisseria meningitidis exhibits a general O-linked protein glycosylation system in which pili and other extracytoplasmic proteins are glycosylated. To investigate glycan antigenicity in humans and the significance of high glycan diversity on immune escape mechanisms, we exploited serogroup A meningococcal strains and serum samples obtained from laboratory-confirmed Ethiopian patients with meningococcal disease. The 37 meningococcal isolates were sequenced, and their protein glycosylation (pgl) genotypes and protein glycosylation phenotypes were investigated in detail. An insertion sequence (IS1655) element in pglH reduced glycan variability in the majority of isolates, while phase variation strengthened glycan variability and microheterogeneity. Homologous recombination events within the pgl genes were identified in eight of the 37 isolates, and the phenotypic consequences ranged from none detected to altered glycoforms in two of the isolates in which the whole pgl locus was exchanged. Immunoblotting of sera against a complete panel of glycan-expressing mutant strains demonstrated that most of these patient sera had IgG antibodies against various neisserial protein glycan antigens. Furthermore, using a bactericidal assay comparing a wild-type meningococcal A strain and a glycosylation-null variant strain, we showed that these protein glycan antigens interfere with bactericidal killing by antibodies in patient sera. Altogether, we were largely able to link pgl genotype with glycosylation phenotype. Our study reveals that protein glycans seem to contribute to the ability of N. meningitidis to resist the bactericidal activity of human serum, possibly by masking protein epitopes important for bactericidal killing and thus protection against meningococcal disease. IMPORTANCE Bacterial meningitis is a serious global health problem, and one of the major causative organisms is Neisseria meningitidis. Extensive variability in protein glycan structure and antigenicity is due to phase variation of protein glycosylation genes and polymorphic gene content and function. The exact role(s) of glycosylation in Neisseria remains to be determined, but increasing evidence, supported by this study, suggests that glycan variability can be a strategy to escape the human immune system. The complexity of the O-linked protein glycosylation system requires further studies to fully comprehend how these bacteria utilize variation in pgl genes to produce such high glycoform diversity and to evade the human immune response.

Modelling evolutionary pathways for commensalism and hypervirulence in Neisseria meningitidis

Neisseria meningitidis, the meningococcus, resides exclusively in humans and causes invasive meningococcal disease (IMD). The population of N. meningitidis is structured into stable clonal complexes by limited horizontal recombination in this naturally transformable species. N. meningitidis is an opportunistic pathogen, with some clonal complexes, such as cc53, effectively acting as commensal colonizers, while other genetic lineages, such as cc11, are rarely colonizers but are over-represented in IMD and are termed hypervirulent. This study examined theoretical evolutionary pathways for pathogenic and commensal lineages by examining the prevalence of horizontally acquired genomic islands (GIs) and loss-of-function (LOF) mutations. Using a collection of 4850 genomes from the BIGSdb database, we identified 82 GIs in the pan-genome of 11 lineages (10 hypervirulent and one commensal lineage). A new computational tool, Phaser, was used to identify frameshift mutations, which were examined for statistically significant association with genetic lineage. Phaser identified a total of 144 frameshift loci of which 105 were shown to have a statistically significant non-random distribution in phase status. The 82 GIs, but not the LOF loci, were associated with genetic lineage and invasiveness using the disease carriage ratio metric. These observations have been integrated into a new model that infers the early events of the evolution of the human adapted meningococcus. These pathways are enriched for GIs that are involved in modulating attachment to the host, growth rate, iron uptake and toxin expression which are proposed to increase competition within the meningococcal population for the limited environmental niche of the human nasopharynx. We surmise that competition for the host mucosal surface with the nasopharyngeal microbiome has led to the selection of isolates with traits that enable access to cell types (non-phagocytic and phagocytic) in the submucosal tissues leading to an increased risk for IMD.

Recent Advances in Biological Functions of Thick Pili in the Cyanobacterium Synechocystis sp. PCC 6803

Cyanobacteria have evolved various strategies to sense and adapt to biotic and abiotic stresses including active movement. Motility in cyanobacteria utilizing the type IV pili (TFP) is useful to cope with changing environmental conditions. The model cyanobacterium Synechocystis sp. PCC 6803 (hereafter named Synechocystis) exhibits motility via TFP called thick pili, and uses it to seek out favorable light/nutrition or escape from unfavorable conditions. Recently, a number of studies on Synechocystis thick pili have been undertaken. Molecular approaches support the role of the pilin in motility, cell adhesion, metal utilization, and natural competence in Synechocystis. This review summarizes the most recent studies on the function of thick pili as well as their formation and regulation in this cyanobacterium.

Identification of the Wzx flippase, Wzy polymerase and sugar-modifying enzymes for spore coat polysaccharide biosynthesis in Myxococcus xanthus

The rod-shaped cells of Myxococcus xanthus, a Gram-negative deltaproteobacterium, differentiate to environmentally resistant spores upon starvation or chemical stress. The environmental resistance depends on a spore coat polysaccharide that is synthesised by the ExoA-I proteins, some of which are part of a Wzx/Wzy-dependent pathway for polysaccharide synthesis and export; however, key components of this pathway have remained unidentified. Here, we identify and characterise two additional loci encoding proteins with homology to enzymes involved in polysaccharide synthesis and export, as well as sugar modification and show that six of the proteins encoded by these loci are essential for the formation of environmentally resistant spores. Our data support that MXAN_3260, renamed ExoM and MXAN_3026, renamed ExoJ, are the Wzx flippase and Wzy polymerase, respectively, responsible for translocation and polymerisation of the repeat unit of the spore coat polysaccharide. Moreover, we provide evidence that three glycosyltransferases (MXAN_3027/ExoK, MXAN_3262/ExoO and MXAN_3263/ExoP) and a polysaccharide deacetylase (MXAN_3259/ExoL) are important for formation of the intact spore coat, while ExoE is the polyisoprenyl-phosphate hexose-1-phosphate transferase responsible for initiating repeat unit synthesis, likely by transferring N-acetylgalactosamine-1-P to undecaprenyl-phosphate. Together, our data generate a more complete model of the Exo pathway for spore coat polysaccharide biosynthesis and export.

Glycoengineering bioconjugate vaccines, therapeutics, and diagnostics in E. coli

The first, general glycosylation pathway in bacteria, the N-linked glycosylation system of Campylobacter jejuni, was discovered two decades ago. Since then, many diverse prokaryotic glycosylation systems have been characterized, including O-linked glycosylation systems that have no homologous counterparts in eukaryotic organisms. Shortly after these discoveries, glycosylation pathways were recombinantly introduced into E. coli creating the field of bacterial glycoengineering. Bacterial glycoengineering is an emerging biotechnological tool that harnesses prokaryotic glycosylation systems for the generation of recombinantly glycosylated proteins using E. coli as a host. Over the last decade, as our understanding of prokaryotic glycosylation systems has advanced, so too has the glycoengineering toolbox. Currently, glycoengineering utilizes two broad approaches to recombinantly glycosylate proteins, both of which can generate N- or O-linkages: oligosaccharyltransferase (OTase)-dependent and OTase-independent. This review discusses the applications of these bacterial glycoengineering techniques as they relate to the development of glycoconjugate vaccines, therapeutic proteins, and diagnostics.

Genetic determinants of genus-level glycan diversity in a bacterial protein glycosylation system

The human pathogens N. gonorrhoeae and N. meningitidis display robust intra- and interstrain glycan diversity associated with their O-linked protein glycosylation (pgl) systems. In an effort to better understand the evolution and function of protein glycosylation operating there, we aimed to determine if other human-restricted, Neisseria species similarly glycosylate proteins and if so, to assess the levels of glycoform diversity. Comparative genomics revealed the conservation of a subset of genes minimally required for O-linked protein glycosylation glycan and established those pgl genes as core genome constituents of the genus. In conjunction with mass spectrometric–based glycan phenotyping, we found that extant glycoform repertoires in N. gonorrhoeae, N. meningitidis and the closely related species N. polysaccharea and N. lactamica reflect the functional replacement of a progenitor glycan biosynthetic pathway. This replacement involved loss of pgl gene components of the primordial pathway coincident with the acquisition of two exogenous glycosyltransferase genes. Critical to this discovery was the identification of a ubiquitous but previously unrecognized glycosyltransferase gene (pglP) that has uniquely undergone parallel but independent pseudogenization in N. gonorrhoeae and N. meningitidis. We suggest that the pseudogenization events are driven by processes of compositional epistasis leading to gene decay. Additionally, we documented instances where inter-species recombination influences pgl gene status and creates discordant genetic interactions due ostensibly to the multi-locus nature of pgl gene networks. In summary, these findings provide a novel perspective on the evolution of protein glycosylation systems and identify phylogenetically informative, genetic differences associated with Neisseria species.

Bacteria express a remarkable diversity of sugars and oligosaccharides in conjunction with protein glycosylation systems. Currently however, little is known about the evolutionary processes and selective forces shaping glycan biosynthetic pathways. The closely related bacterial pathogens Neisseria gonorrhoeae and Neisseria meningitidis remain serious sources of human disease and these species express antigenically variable oligosaccharides as components of their broad-spectrum, O‐linked protein glycosylation (pgl) systems. With the exception of isolates of Neisseria elongata subspecies glycolytica, the status of such post-translational modifications in related commensal species colonizing humans remains largely undefined. Here, we exploit new data from further studies of protein glycosylation in Neisseria elongata subspecies glycolytica to address these concerns. Employing comparative genomics and glycan phenotyping, we show that related pgl systems are indeed expressed by all human-restricted Neisseria species but identify unique gene gain and loss events as well as loss-of-function polymorphisms that accommodate a dramatic shift in glycoform structure occurring across the genus. These findings constitute novel perspectives on both the evolution of protein glycosylation systems in general and the macroevolutionary processes occurring in related bacterial species residing within a single host.

Deep mutational scanning of the Neisseria meningitidis major pilin reveals the importance of pilus tip-mediated adhesion

Type IV pili (TFP) are multifunctional micrometer-long filaments expressed at the surface of many prokaryotes. In Neisseria meningitidis, TFP are crucial for virulence. Indeed, these homopolymers of the major pilin PilE mediate interbacterial aggregation and adhesion to host cells. However, the mechanisms behind these functions remain unclear. Here, we simultaneously determined regions of PilE involved in pilus display, auto-aggregation, and adhesion by using deep mutational scanning and started mining this extensive functional map. For auto-aggregation, pili must reach a minimum length to allow pilus-pilus interactions through an electropositive cluster of residues centered around Lys140. For adhesion, results point to a key role for the tip of the pilus. Accordingly, purified pili interacting with host cells initially bind via their tip-located major pilin and then along their length. Overall, these results identify functional domains of PilE and support a direct role of the major pilin in TFP-dependent aggregation and adhesion.

Sample Preparation for Proteomic Analysis of Neisseria meningitidis

Mass spectrometry (MS) proteomics allows systematic identification, characterization, and relative quantification of the full suite of proteins in a biological sample, and is a powerful analytical approach for investigation of many aspects of the biology of Neisseria meningitidis. Here, we describe methods for robust and efficient sample preparation of the proteome of N. meningitidis suitable for diverse MS proteomics workflows.

Disrupted Synthesis of a Di-N-acetylated Sugar Perturbs Mature Glycoform Structure and Microheterogeneity in the O-Linked Protein Glycosylation System of Neisseria elongata subsp. glycolytica

The genus Neisseria includes three major species of importance to human health and disease (Neisseria gonorrhoeae, Neisseria meningitidis, and Neisseria lactamica) that express broad-spectrum O-linked protein glycosylation (Pgl) systems. The potential for related Pgl systems in other species in the genus, however, remains to be determined. Using a strain of Neisseria elongata subsp. glycolytica, a unique tetrasaccharide glycoform consisting of di-N-acetylbacillosamine and glucose as the first two sugars followed by a rare sugar whose mass spectrometric fragmentation profile was most consistent with di-N-acetyl hexuronic acid and a N-acetylhexosamine at the nonreducing end has been identified. Based on established mechanisms for UDP-di-N-acetyl hexuronic acid biosynthesis found in other microbes, we searched for genes encoding related pathway components in the N. elongata subsp. glycolytica genome. Here, we detail the identification of such genes and the ensuing glycosylation phenotypes engendered by their inactivation. While the findings extend the conservative nature of microbial UDP-di-N-acetyl hexuronic acid biosynthesis, mutant glycosylation phenotypes reveal unique, relaxed specificities of the glycosyltransferases and oligosaccharyltransferases to incorporate pathway intermediate UDP-sugars into mature glycoforms.IMPORTANCE Broad-spectrum protein glycosylation (Pgl) systems are well recognized in bacteria and archaea. Knowledge of how these systems relate structurally, biochemically, and evolutionarily to one another and to others associated with microbial surface glycoconjugate expression is still incomplete. Here, we detail reverse genetic efforts toward characterization of protein glycosylation mutants of N. elongata subsp. glycolytica that define the biosynthesis of a conserved but relatively rare UDP-sugar precursor. The results show both a significant degree of intra- and transkingdom conservation in the utilization of UDP-di-N-acetyl-glucuronic acid and singular properties related to the relaxed specificities of the N. elongata subsp. glycolytica system.

Structural and genetic analyses of glycan O-acetylation in a bacterial protein glycosylation system: evidence for differential effects on glycan chain length

O-acetylation is a common modification of bacterial glycoconjugates. By modifying oligosaccharide structure and chemistry, O-acetylation has important consequences for biotic and abiotic recognition events and thus bacterial fitness in general. Previous studies of the broad-spectrum O-linked protein glycosylation in pathogenic Neisseria species (including N. gonorrhoeae and N. meningitidis) have revealed O-acetylation of some of their diverse glycoforms and identified the committed acetylase, PglI. Herein, we extend these observations by using mass spectrometry to examine a complete set of all glycan variants identified to date. Regardless of composition, all glycoforms and all sugars in the oligosaccharide are subject to acetylation in a PglI-dependent fashion with the only exception of di-N-acetyl-bacillosamine. Moreover, multiple sugars in a single oligosaccharide could be simultaneously modified. Interestingly, O-acetylation status was found to be correlated with altered chain lengths of oligosaccharides expressed in otherwise isogenic backgrounds. Models for how this unprecedented phenomenon might arise are discussed with some having potentially important implications for the membrane topology of glycan O-acetylation. Together, the findings provide better insight into how O-acetylation can both directly and indirectly govern glycoform structure and diversity.

Cryoelectron Microscopy Reconstructions of the Pseudomonas aeruginosa and Neisseria gonorrhoeae Type IV Pili at Sub-nanometer Resolution

We report here cryoelectron microscopy reconstructions of type IV pili (T4P) from two important human pathogens, Pseudomonas aeruginosa and Neisseria gonorrhoeae, at ∼ 8 and 5 Å resolution, respectively. The two structures reveal distinct arrangements of the pilin globular domains on the pilus surfaces, which impart different helical parameters, but similar packing of the conserved N-terminal α helices, α1, in the filament core. In contrast to the continuous α helix seen in the X-ray crystal structures of the P. aeruginosa and N. gonorrhoeae pilin subunits, α1 in the pilus filaments has a melted segment located between conserved helix-breaking residues Gly14 and Pro22, as seen for the Neisseria meningitidis T4P. Using mutagenesis we show that Pro22 is critical for pilus assembly, as are Thr2 and Glu5, which are positioned to interact in the hydrophobic filament core. These structures provide a framework for understanding T4P assembly, function, and biophysical properties.

The sweet side of the pathogenic Neisseria: the role of glycan interactions in colonisation and disease

Glycomics is a rapidly growing field that focuses on the structure and function of carbohydrates (glycans) in biological systems. Glycan interactions play a major role in infectious disease, at all stages of colonisation and disease progression. Neisseria meningitidis, the cause of meningococcal sepsis and meningitis, and Neisseria gonorrhoeae, which causes the sexually transmitted infection gonorrhoea, are responsible for significant morbidity and mortality worldwide. Neisseria meningitidis displays a range of surface glycosylations including capsule polysaccharide, lipooligosaccharide and O-linked glycoproteins. While N. gonorrhoeae does not have a capsule, it does express both lipooligosaccharide and O-linked glycoproteins. Neisseria gonorrhoeae also has the ability to scavenge host sialic acids, while several N. meningitidis serogroups can synthesise sialic acid. Surface expressed sialic acid is key in serum resistance and survival in the host. On the host side, the pathogenic Neisseria protein adhesins such as Opc and NHBA bind to host glycans for adherence and colonisation of host cells. Essentially, from both the bacterial and host perspective, glycan interactions are fundamental in colonisation and disease of pathogenic Neisseria. The key aspects of glycobiology of the pathogenic Neisseria are reviewed herein.

Structure of the Neisseria meningitidis Type IV pilus

Neisseria meningitidis use Type IV pili (T4P) to adhere to endothelial cells and breach the blood brain barrier, causing cause fatal meningitis. T4P are multifunctional polymers of the major pilin protein, which share a conserved hydrophobic N terminus that is a curved extended α-helix, α1, in X-ray crystal structures. Here we report a 1.44 Å crystal structure of the N. meningitidis major pilin PilE and a ∼6 Å cryo-electron microscopy reconstruction of the intact pilus, from which we built an atomic model for the filament. This structure reveals the molecular arrangement of the N-terminal α-helices in the filament core, including a melted central portion of α1 and a bridge of electron density consistent with a predicted salt bridge necessary for pilus assembly. This structure has important implications for understanding pilus biology.

Type IV pili are present on a wide range of bacterial pathogens and mediate diverse functions. Here the authors report a high resolution crystal structure of the pilin subunit PilE, and a cryoEM reconstruction of the Type IV pilus filament from N. meningitidis that offer insight into pilus assembly and functions.

Characterization of a Unique Tetrasaccharide and Distinct Glycoproteome in the O-Linked Protein Glycosylation System of Neisseria elongata subsp. glycolytica

Broad-spectrum O-linked protein glycosylation is well characterized in the major Neisseria species of importance to human health and disease. Within strains of Neisseria gonorrhoeae, N. meningitidis, and N. lactamica, protein glycosylation (pgl) gene content and the corresponding oligosaccharide structure are fairly well conserved, although intra- and interstrain variability occurs. The status of such systems in distantly related commensal species, however, remains largely unexplored. Using a strain of deeply branching Neisseria elongata subsp. glycolytica, a heretofore unrecognized tetrasaccharide glycoform consisting of di-N-acetylbacillosamine-glucose-di-N-acetyl hexuronic acid-N-acetylhexosamine (diNAcBac-Glc-diNAcHexA-HexNAc) was identified. Directed mutagenesis, mass spectrometric analysis, and glycan serotyping confirmed that the oligosaccharide is an extended version of the diNAcBac-Glc-based structure seen in N. gonorrhoeae and N. meningitidis generated by the successive actions of PglB, PglC, and PglD and glucosyltransferase PglH orthologues. In addition, a null mutation in the orthologue of the broadly conserved but enigmatic pglG gene precluded expression of the extended glycoform, providing the first evidence that its product is a functional glycosyltransferase. Despite clear evidence for a substantial number of glycoprotein substrates, the major pilin subunit of the endogenous type IV pilus was not glycosylated. The latter finding raises obvious questions as to the relative distribution of pilin glycosylation within the genus, how protein glycosylation substrates are selected, and the overall structure-function relationships of broad-spectrum protein glycosylation. Together, the results of this study provide a foundation upon which to assess neisserial O-linked protein glycosylation diversity at the genus level.

IMPORTANCE

Broad-spectrum protein glycosylation systems are well characterized in the pathogenic Neisseria species N. gonorrhoeae and N. meningitidis. A number of lines of evidence indicate that the glycan components in these systems are subject to diversifying selection and suggest that glycan variation may be driven in the context of glycosylation of the abundant and surface-localized pilin protein PilE, the major subunit of type IV pili. Here, we examined protein glycosylation in a distantly related, nonpathogenic neisserial species, Neisseria elongata subsp. glycolytica. This system has clear similarities to the systems found in pathogenic species but makes novel glycoforms utilizing a glycosyltransferase that is widely conserved at the genus level but whose function until now remained unknown. Remarkably, PilE pilin is not glycosylated in this species, a finding that raises important questions about the evolutionary trajectories and overall structure-function relationships of broad-spectrum protein glycosylation systems in bacteria.

Top-Down Mass Spectrometry: Proteomics to Proteoforms

This chapter highlights many of the fundamental concepts and technologies in the field of top-down mass spectrometry (TDMS), and provides numerous examples of contributions that TD is making in biology, biophysics, and clinical investigations. TD workflows include variegated steps that may include non-specific or targeted preparative strategies, orthogonal liquid chromatography techniques, analyte ionization, mass analysis, tandem mass spectrometry (MS/MS) and informatics procedures. This diversity of experimental designs has evolved to manage the large dynamic range of protein expression and diverse physiochemical properties of proteins in proteome investigations, tackle proteoform microheterogeneity, as well as determine structure and composition of gas-phase proteins and protein assemblies.

Neisseria meningitidis Type IV Pili Composed of Sequence Invariable Pilins Are Masked by Multisite Glycosylation

The ability of pathogens to cause disease depends on their aptitude to escape the immune system. Type IV pili are extracellular filamentous virulence factors composed of pilin monomers and frequently expressed by bacterial pathogens. As such they are major targets for the host immune system. In the human pathogen Neisseria meningitidis, strains expressing class I pilins contain a genetic recombination system that promotes variation of the pilin sequence and is thought to aid immune escape. However, numerous hypervirulent clinical isolates express class II pilins that lack this property. This raises the question of how they evade immunity targeting type IV pili. As glycosylation is a possible source of antigenic variation it was investigated using top-down mass spectrometry to provide the highest molecular precision on the modified proteins. Unlike class I pilins that carry a single glycan, we found that class II pilins display up to 5 glycosylation sites per monomer on the pilus surface. Swapping of pilin class and genetic background shows that the pilin primary structure determines multisite glycosylation while the genetic background determines the nature of the glycans. Absence of glycosylation in class II pilins affects pilus biogenesis or enhances pilus-dependent aggregation in a strain specific fashion highlighting the extensive functional impact of multisite glycosylation. Finally, molecular modeling shows that glycans cover the surface of class II pilins and strongly decrease antibody access to the polypeptide chain. This strongly supports a model where strains expressing class II pilins evade the immune system by changing their sugar structure rather than pilin primary structure. Overall these results show that sequence invariable class II pilins are cloaked in glycans with extensive functional and immunological consequences.

During infection pathogens and their host engage in a series of measures and counter-measures to promote their own survival: pathogens express virulence factors, the immune system targets these surface structures and pathogens modify them to evade detection. Like numerous bacterial pathogens, Neisseria meningitidis express type IV pili, long filamentous adhesive structures composed of pilins. Intriguingly the amino acid sequences of pilins from most hypervirulent strains do not vary, raising the question of how they evade the immune system. This study shows that the pilus structure is completely coated with sugars thus limiting access of antibodies to the pilin polypeptide chain. We propose that multisite glycosylation and thus variation in the type of sugar mediates immune evasion in these strains.

The sweet tooth of bacteria: common themes in bacterial glycoconjugates

Humans have been increasingly recognized as being superorganisms, living in close contact with a microbiota on all their mucosal surfaces. However, most studies on the human microbiota have focused on gaining comprehensive insights into the composition of the microbiota under different health conditions (e.g., enterotypes), while there is also a need for detailed knowledge of the different molecules that mediate interactions with the host. Glycoconjugates are an interesting class of molecules for detailed studies, as they form a strain-specific barcode on the surface of bacteria, mediating specific interactions with the host. Strikingly, most glycoconjugates are synthesized by similar biosynthesis mechanisms. Bacteria can produce their major glycoconjugates by using a sequential or an en bloc mechanism, with both mechanistic options coexisting in many species for different macromolecules. In this review, these common themes are conceptualized and illustrated for all major classes of known bacterial glycoconjugates, with a special focus on the rather recently emergent field of glycosylated proteins. We describe the biosynthesis and importance of glycoconjugates in both pathogenic and beneficial bacteria and in both Gram-positive and -negative organisms. The focus lies on microorganisms important for human physiology. In addition, the potential for a better knowledge of bacterial glycoconjugates in the emerging field of glycoengineering and other perspectives is discussed.

The biosynthesis of UDP-d-FucNAc-4N-(2)-oxoglutarate (UDP-Yelosamine) in Bacillus cereus ATCC 14579: Pat and Pyl, an aminotransferase and an ATP-dependent Grasp protein that ligates 2-oxoglutarate to UDP-4-amino-sugars

Surface glycan switching is often observed when micro-organisms transition between different biotic and abiotic niches, including biofilms, although the advantages of this switching to the organism are not well understood. Bacillus cereus grown in a biofilm-inducing medium has been shown to synthesize an unusual cell wall polysaccharide composed of the repeating subunit →6)Gal(α1-2)(2-R-hydroxyglutar-5-ylamido)Fuc2NAc4N(α1-6)GlcNAc(β1→, where galactose is linked to the hydroxyglutarate moiety of FucNAc-4-amido-(2)-hydroxyglutarate. The molecular mechanism involved in attaching 2-hydroxyglutarate to 4-amino-FucNAc has not been determined. Here, we show two genes in B. cereus ATCC 14579 encoding enzymes involved in the synthesis of UDP-FucNAc-4-amido-(2)-oxoglutarate (UDP-Yelosamine), a modified UDP-sugar not previously reported to exist. Using mass spectrometry and real time NMR spectroscopy, we show that Bc5273 encodes a C4″-aminotransferase (herein referred to as Pat) that, in the presence of pyridoxal phosphate, transfers the primary amino group of l-Glu to C-4″ of UDP-4-keto-6-deoxy-d-GlcNAc to form UDP-4-amino-FucNAc and 2-oxoglutarate. Pat also converts 4-keto-xylose, 4-keto-glucose, and 4-keto-2-acetamido-altrose to their corresponding UDP-4-amino-sugars. Bc5272 encodes a carboxylate-amine ligase (herein referred as Pyl) that, in the presence of ATP and Mg(II), adds 2-oxoglutarate to the 4-amino moiety of UDP-4-amino-FucNAc to form UDP-Yelosamine and ADP. Pyl is also able to ligate 2-oxoglutarate to other 4-amino-sugar derivatives to form UDP-Yelose, UDP-Solosamine, and UDP-Aravonose. Characterizing the metabolic pathways involved in the formation of modified nucleotide sugars provides a basis for understanding some of the mechanisms used by bacteria to modify or alter their cell surface polysaccharides in response to changing growth and environmental challenges.

Extended glycan diversity in a bacterial protein glycosylation system linked to allelic polymorphisms and minimal genetic alterations in a glycosyltransferase gene

Glycans manifest in conjunction with the broad spectrum O-linked protein glycosylation in species within the genus Neisseria display intra- and interstrain diversity. Variability in glycan structure and antigenicity are attributable to differences in the content and expression status of glycan synthesis genes. Given the high degree of standing allelic polymorphisms in these genes, the level of glycan diversity may exceed that currently defined. Here, we identify unique protein-associated disaccharide glycoforms that carry N-acetylglucosamine (GlcNAc) at their non-reducing end. This altered structure was correlated with allelic variants of pglH whose product was previously demonstrated to be responsible for the expression of glucose (Glc)-containing disaccharides. Allele comparisons and site-specific mutagenesis showed that the presence of a single residue, alanine at position 303 in place of a glutamine, was sufficient for GlcNAc versus Glc incorporation. Phylogenetic analyses revealed that GlcNAc-containing disaccharides may be widely distributed within the pgl systems of Neisseria particularly in strains of N. meningitidis. Although analogous minimal structural alterations in glycosyltransferases have been documented in association with lipopolysaccharide and capsular polysaccharide variability, this appears to be the first example in which such changes have been implicated in glycan diversification within a bacterial protein glycosylation system.

Complete posttranslational modification mapping of pathogenic Neisseria meningitidis pilins requires top-down mass spectrometry

In pathogenic bacteria, posttranslationally modified proteins have been found to promote bacterial survival, replication, and evasion from the host immune system. In the human pathogen Neisseria meningitidis, the protein PilE (15-18 kDa) is the major building block of type IV pili, extracellular filamentous organelles that play a major role in mediating pathogenesis. Previous reports have shown that PilE can be expressed as a number of different proteoforms, each harboring its own set of PTMs and that specific proteoforms are key in promoting bacterial virulence. Efficient tools that allow complete PTM mapping of proteins involved in bacterial infection are therefore strongly needed. As we show in this study, a simple combination of mass profiling and bottom-up proteomics is fundamentally unable to achieve this goal when more than two proteoforms are present simultaneously. In a N. meningitidis strain isolated from a patient with meningitis, mass profiling revealed the presence of four major proteoforms of PilE, in a 1:1:1:1 ratio. Due to the complexity of the sample, a top-down approach was required to achieve complete PTM mapping for all four proteoforms, highlighting an unprecedented extent of glycosylation. Top-down MS therefore appears to be a promising tool for the analysis of highly posttranslationally modified proteins involved in bacterial virulence.

Type IV pilin proteins: versatile molecular modules

Type IV pili (T4P) are multifunctional protein fibers produced on the surfaces of a wide variety of bacteria and archaea. The major subunit of T4P is the type IV pilin, and structurally related proteins are found as components of the type II secretion (T2S) system, where they are called pseudopilins; of DNA uptake/competence systems in both Gram-negative and Gram-positive species; and of flagella, pili, and sugar-binding systems in the archaea. This broad distribution of a single protein family implies both a common evolutionary origin and a highly adaptable functional plan. The type IV pilin is a remarkably versatile architectural module that has been adopted widely for a variety of functions, including motility, attachment to chemically diverse surfaces, electrical conductance, acquisition of DNA, and secretion of a broad range of structurally distinct protein substrates. In this review, we consider recent advances in this research area, from structural revelations to insights into diversity, posttranslational modifications, regulation, and function.

A combined mass spectrometry strategy for complete posttranslational modification mapping of Neisseria meningitidis major pilin

Herein, we report a new approach, based on the combination of mass profiling and tandem mass spectrometry, to address the issue of localising all post-translational modifications (PTMs) on the major pilin protein PiIE expressed by the pathogenic Neisseria species. PilE is the main component of type IV pili; filamentous organelles expressed at the surface of many bacterial pathogens and important virulence factors. Previous reports have shown that PilE can harbour various combinations of PTMs and have established strong links between PTM and pathogenesis. Complete PTM mapping of proteins involved in bacterial infection is therefore highly desirable. The methodology we propose here allowed us to fully characterise the PilE proteoforms of Neisseria meningitidis strain 8013, definitively identifying all PTMs present on all proteoforms and localising their position on the protein backbone. These modifications include a processed and methylated N-terminus, disulfide bridge, glycosylation and glycerophosphorylation at two different sites. A key element of our approach is high resolution, intact mass measurement of the proteoforms, a piece of information completely lacking in all classical bottom-up proteomics strategies used for PTM analysis and without which it is difficult to ensure complete PTM mapping.

Pathogenesis of meningococcemia

Neisseria meningitidis is responsible for two major diseases: cerebrospinal meningitis and/or septicemia. The latter can lead to a purpura fulminans, an often-fatal condition owing to the associated septic shock. These two clinical aspects of the meningococcal infection are consequences of a tight interaction of meningococci with host endothelial cells. This interaction, mediated by the type IV pili, is responsible for the formation of microcolonies on the apical surface of the cells. This interaction is followed by the activation of signaling pathways in the host cells leading to the formation of a microbiological synapse. A low level of bacteremia is likely to favor the colonization of brain vessels, leading to bacterial meningitis, whereas the colonization of a large number of vessels by a high number of bacteria is responsible for one of the most severe forms of septic shock observed.

Allelic variation in a simple sequence repeat element of neisserial pglB2 and its consequences for protein expression and protein glycosylation

Neisseria species express an O-linked glycosylation system in which functionally distinct proteins are elaborated with variable glycans. A major source of glycan diversity in N. meningitidis results from two distinct pglB alleles responsible for the synthesis of either N,N'-diacetylbacillosamine or glyceramido-acetamido trideoxyhexose that occupy the reducing end of the oligosaccharides. Alternative modifications at C-4 of the precursor UDP-4-amino are attributable to distinct C-terminal domains that dictate either acetyltransferase or glyceramidotransferase activity, encoded by pglB and pglB2, respectively. Naturally occurring alleles of pglB2 have homopolymeric tracts of either 7 or 8 adenosines (As) bridging the C-terminal open reading frame (ORF) and the ORF encompassing the conserved N-terminal domain associated with phosphoglycosyltransferase activity. In the work presented here, we explored the consequences of such pglB2 allele variation and found that, although both alleles are functional vis-à-vis glycosylation, the 7A form results in the expression of a single, multidomain protein, while the 8A variant elicits two single-domain proteins. We also found that the glyceramidotransferase activity-encoding domain is essential to protein glycosylation, showing the critical role of the C-4 modification of the precursor UDP-4-amino in the pathway. These findings were further extended and confirmed by examining the phenotypic consequences of extended poly(A) tract length variation. Although ORFs related to those of pglB2 are broadly distributed in eubacteria, they are primarily found as two distinct, juxtaposed ORFs. Thus, the neisserial pglB2 system provides novel insights into the potential influence of hypermutability on modular evolution of proteins by providing a unique snapshot of the progression of ongoing gene fusion.

Dual pili post-translational modifications synergize to mediate meningococcal adherence to platelet activating factor receptor on human airway cells

Pili of pathogenic Neisseria are major virulence factors associated with adhesion, twitching motility, auto-aggregation, and DNA transformation. Pili of N. meningitidis are subject to several different post-translational modifications. Among these pilin modifications, the presence of phosphorylcholine (ChoP) and a glycan on the pilin protein are phase-variable (subject to high frequency, reversible on/off switching of expression). In this study we report the location of two ChoP modifications on the C-terminus of N. meningitidis pilin. We show that the surface accessibility of ChoP on pili is affected by phase variable changes to the structure of the pilin-linked glycan. We identify for the first time that the platelet activating factor receptor (PAFr) is a key, early event receptor for meningococcal adherence to human bronchial epithelial cells and tissue, and that synergy between the pilin-linked glycan and ChoP post-translational modifications is required for pili to optimally engage PAFr to mediate adherence to human airway cells.

Neisseria meningitidis is an important human pathogen that can cause rapidly progressing, life threatening meningitis and sepsis in humans. There is no fully protective vaccine against this pathogen in current use and the key processes that dictate the transition from harmless carriage of the bacterium in the airway (the case for the vast majority of colonised hosts) to invasive disease are largely undefined. A key missing link in this organism's interaction with the human host is the identity of the receptor that is the first point of contact for the organism within the airway. In this study, we report that the receptor for this important human pathogen on airway epithelial cells is the platelet activating factor receptor (PAFr), an immunomodulatory molecule shown by others to play a role in promoting bacterial sepsis. We also show that two post-translational modifications, glycosylation and phosphorylcholine, are subject to phase-variation (high frequency, reversible switching of gene expression). They are closely associated on adjacent pilin subunits, and synergy between both are required for the efficient engagement with the PAFr. These data define a new role for these post-translational modifications in meningococcal adherence and also provide an insight into the selective pressures that underlie their phase variable expression.

De novo synthesis of D- and L-fucosamine containing disaccharides

The availability of rare monosaccharides that cannot be isolated from natural sources is currently limiting the access to the synthesis and the biological evaluation of complex bacterial cell-surface glycans. Here, we report the synthesis of D- and L-fucosamine building blocks by a de novo approach from L- and D-Garner aldehydes. These differentially protected monosaccharide building blocks were utilized to prepare disaccharides present on the surface of Pseudomonas aeruginosa bacteria.

Attachment and invasion of Neisseria meningitidis to host cells is related to surface hydrophobicity, bacterial cell size and capsule

We compared exemplar strains from two hypervirulent clonal complexes, strain NMB-CDC from ST-8/11 cc and strain MC58 from ST-32/269 cc, in host cell attachment and invasion. Strain NMB-CDC attached to and invaded host cells at a significantly greater frequency than strain MC58. Type IV pili retained the primary role for initial attachment to host cells for both isolates regardless of pilin class and glycosylation pattern. In strain MC58, the serogroup B capsule was the major inhibitory determinant affecting both bacterial attachment to and invasion of host cells. Removal of terminal sialylation of lipooligosaccharide (LOS) in the presence of capsule did not influence rates of attachment or invasion for strain MC58. However, removal of either serogroup B capsule or LOS sialylation in strain NMB-CDC increased bacterial attachment to host cells to the same extent. Although the level of inhibition of attachment by capsule was different between these strains, the regulation of the capsule synthesis locus by the two-component response regulator MisR, and the level of surface capsule determined by flow cytometry were not significantly different. However, the diplococci of strain NMB-CDC were shown to have a 1.89-fold greater surface area than strain MC58 by flow cytometry. It was proposed that the increase in surface area without changing the amount of anchored glycolipid capsule in the outer membrane would result in a sparser capsule and increase surface hydrophobicity. Strain NMB-CDC was shown to be more hydrophobic than strain MC58 using hydrophobicity interaction chromatography and microbial adhesion-to-solvents assays. In conclusion, improved levels of adherence of strain NMB-CDC to cell lines was associated with increased bacterial cell surface and surface hydrophobicity. This study shows that there is diversity in bacterial cell surface area and surface hydrophobicity within N. meningitidis which influence steps in meningococcal pathogenesis.

Bacterial protein N-glycosylation: new perspectives and applications

Protein glycosylation is widespread throughout all three domains of life. Bacterial protein N-glycosylation and its application to engineering recombinant glycoproteins continue to be actively studied. Here, we focus on advances made in the last 2 years, including the characterization of novel bacterial N-glycosylation pathways, examination of pathway enzymes and evolution, biological roles of protein modification in the native host, and exploitation of the N-glycosylation pathways to create novel vaccines and diagnostics.

Characterization of the highly conserved VFMGD motif in a bacterial polyisoprenyl-phosphate N-acetylaminosugar-1-phosphate transferase

Polyisoprenyl-phosphate N-acetylaminosugar-1-phosphate transferases (PNPTs) constitute a family of eukaryotic and prokaryotic membrane proteins that catalyze the transfer of a sugar-1-phosphate to a phosphoisoprenyl lipid carrier. All PNPT members share a highly conserved 213-Valine-Phenylalanine-Methionine-Glycine-Aspartic acid-217 (VFMGD) motif. Previous studies using the MraY protein suggested that the aspartic acid residue in this motif, D267, is a nucleophile for a proposed double-displacement mechanism involving the cleavage of the phosphoanhydride bond of the nucleoside. Here, we demonstrate that the corresponding residue in the E. coli WecA, D217, is not directly involved in catalysis, as its replacement by asparagine results in a more active enzyme. Kinetic data indicate that the D217N replacement leads to more than twofold increase in V(max) without significant change in the K(m) for the nucleoside sugar substrate. Furthermore, no differences in the binding of the reaction intermediate analog tunicamycin were found in D217N as well as in other replacement mutants at the same position. We also found that alanine substitutions in various residues of the VFMGD motif affect to various degrees the enzymatic activity of WecA in vivo and in vitro. Together, our data suggest that the highly conserved VFMGD motif defines a common region in PNPT proteins that contributes to the active site and is likely involved in the release of the reaction product.

Hypomorphic glycosyltransferase alleles and recoding at contingency loci influence glycan microheterogeneity in the protein glycosylation system of Neisseria species

As more bacterial protein glycosylation systems are identified and characterized, a central question that arises is, what governs the prevalence of particular glycans associated with them? In addition, accumulating evidence shows that bacterial protein glycans can be subject to the phenomenon of microheterogeneity, in which variant glycan structures are found at specific attachment sites of a given glycoprotein. Although factors underlying microheterogeneity in reconstituted expression systems have been identified and modeled, those impacting natural systems largely remain enigmatic. On the basis of a sensitive and specific glycan serotyping system, microheterogeneity has been reported for the broad-spectrum, O-linked protein glycosylation system in species within the genus Neisseria. To elucidate the mechanisms involved, a genetic approach was used to identify a hypomorphic allele of pglA (encoding the PglA galactosyltransferase) as a significant contributor to simultaneous expression of multiple glycoforms. Moreover, this phenotype was mapped to a single amino acid polymorphism in PglA. Further analyses revealed that many pglA phase-off variants (containing out-of-frame configurations in simple nucleotide repeats within the open reading frame) were associated with disproportionally high levels of the N,N'-diacetylbacillosamine-Gal disaccharide glycoform generated by PglA. This phenotype is emblematic of nonstandard decoding involving programmed ribosomal frameshifting and/or programmed transcriptional realignment. Together, these findings provide new information regarding the mechanisms of neisserial protein glycan microheterogeneity and the anticipatory nature of contingency loci.

Mechanism of meningeal invasion by Neisseria meningitidis

The blood-cerebrospinal fluid barrier physiologically protects the meningeal spaces from blood-borne bacterial pathogens, due to the existence of specialized junctional interendothelial complexes. Few bacterial pathogens are able to reach the subarachnoidal space and among those, Neisseria meningitidis is the one that achieves this task the most constantly when present in the bloodstream. Meningeal invasion is a consequence of a tight interaction of meningococci with brain endothelial cells. This interaction, mediated by the type IV pili, is responsible for the formation of microcolonies on the apical surface of the cells. This interaction is followed by the activation of signaling pathways in the host cells leading to the formation of endothelial docking structures resembling those elicited by the interaction of leukocytes with endothelial cells during extravasation. The consequence of these bacterial-induced signaling events is the recruitment of intercellular junction components in the docking structure and the subsequent opening of the intercellular junctions.

Analysis of carbohydrates and glycoconjugates by matrix-assisted laser desorption/ionization mass spectrometry: an update for 2007-2008

This review is the fifth update of the original review, published in 1999, on the application of MALDI mass spectrometry to the analysis of carbohydrates and glycoconjugates and brings coverage of the literature to the end of 2008. The first section of the review covers fundamental studies, fragmentation of carbohydrate ions, use of derivatives and new software developments for analysis of carbohydrate spectra. Among newer areas of method development are glycan arrays, MALDI imaging and the use of ion mobility spectrometry. The second section of the review discusses applications of MALDI MS to the analysis of different types of carbohydrate. Specific compound classes that are covered include carbohydrate polymers from plants, N- and O-linked glycans from glycoproteins, biopharmaceuticals, glycated proteins, glycolipids, glycosides and various other natural products. There is a short section on the use of MALDI mass spectrometry for the study of enzymes involved in glycan processing and a section on the use of MALDI MS to monitor products of the chemical synthesis of carbohydrates with emphasis on carbohydrate-protein complexes and glycodendrimers. Corresponding analyses by electrospray ionization now appear to outnumber those performed by MALDI and the amount of literature makes a comprehensive review on this technique impractical. However, most of the work relating to sample preparation and glycan synthesis is equally relevant to electrospray and, consequently, those proposing analyses by electrospray should also find material in this review of interest.

Novel protein substrates of the phospho-form modification system in Neisseria gonorrhoeae and their connection to O-linked protein glycosylation

The zwitterionic phospho-form moieties phosphoethanolamine (PE) and phosphocholine (PC) are important components of bacterial membranes and cell surfaces. The major type IV pilus subunit protein of Neisseria gonorrhoeae, PilE, undergoes posttranslational modifications with these moieties via the activity of the pilin phospho-form transferase PptA. A number of observations relating to colocalization of phospho-form and O-linked glycan attachment sites in PilE suggested that these modifications might be either functionally or mechanistically linked or interact directly or indirectly. Moreover, it was unknown whether the phenomenon of phospho-form modification was solely dedicated to PilE or if other neisserial protein targets might exist. In light of these concerns, we screened for evidence of phospho-form modification on other membrane glycoproteins targeted by the broad-spectrum O-linked glycosylation system. In this way, two periplasmic lipoproteins, NGO1043 and NGO1237, were identified as substrates for PE addition. As seen previously for PilE, sites of PE modifications were clustered with those of glycan attachment. In the case of NGO1043, evidence for at least six serine phospho-form attachment sites was found, and further analyses revealed that at least two of these serines were also attachment sites for glycan. Finally, mutations altering glycosylation status led to the presence of pptA-dependent PC modifications on both proteins. Together, these results reinforce the associations established in PilE and provide evidence for dynamic interplay between phospho-form modification and O-linked glycosylation. The observations also suggest that phospho-form modifications likely contribute biologically at both intracellular and extracellular levels.

A molecular chaperone mediates a two-protein enzyme complex and glycosylation of serine-rich streptococcal adhesins

Serine-rich repeat glycoproteins identified from streptococci and staphylococci are important for bacterial adhesion and biofilm formation. Two putative glycosyltransferases, Gtf1 and Gtf2, from Streptococcus parasanguinis form a two-protein enzyme complex that is required for glycosylation of a serine-rich repeat adhesin, Fap1. Gtf1 is a glycosyltransferase; however, the function of Gtf2 is unknown. Here, we demonstrate that Gtf2 enhances the enzymatic activity of Gtf1 by its chaperone-like property. Gtf2 interacted with Gtf1, mediated the subcellular localization of Gtf1, and stabilized Gtf1. Deletion of invariable amino acid residues in a conserved domain of unknown function (DUF1975) at the N terminus of Gtf2 had a greater impact on Fap1 glycosylation than deletion of the C-terminal non-DUF1975 residues. The DUF1975 deletions concurrently reduced the interaction between Gtf1 and Gtf2, altered the subcellular localization of Gtf1, and destabilized Gtf1, suggesting that DUF1975 is crucial for the chaperone activity of Gtf2. Homologous GtfA and GtfB from Streptococcus agalactiae rescued the glycosylation defect in the gtf1gtf2 mutant; like Gtf2, GtfB also possesses chaperone-like activity. Taken together, our studies suggest that Gtf2 and its homologs possess the conserved molecular chaperone activity that mediates protein glycosylation of bacterial adhesins.

The C-terminal domain of the Salmonella enterica WbaP (UDP-galactose:Und-P galactose-1-phosphate transferase) is sufficient for catalytic activity and specificity for undecaprenyl monophosphate

Two families of membrane enzymes catalyze the initiation of the synthesis of O-antigen lipopolysaccharide. The Salmonella enterica Typhimurium WbaP is a prototypic member of one of these families. We report here the purification and biochemical characterization of the WbaP C-terminal (WbaP(CT)) domain harboring one putative transmembrane helix and a large cytoplasmic tail. An N-terminal thioredoxin fusion greatly improved solubility and stability of WbaP(CT) allowing us to obtain highly purified protein. We demonstrate that WbaP(CT) is sufficient to catalyze the in vitro transfer of galactose (Gal)-1-phosphate from uridine monophosphate (UDP)-Gal to the lipid carrier undecaprenyl monophosphate (Und-P). We optimized the in vitro assay to determine steady-state kinetic parameters with the substrates UDP-Gal and Und-P. Using various purified polyisoprenyl phosphates of increasing length and variable saturation of the isoprene units, we also demonstrate that the purified enzyme functions highly efficiently with Und-P, suggesting that the WbaP(CT) domain contains all the essential motifs to catalyze the synthesis of the Und-P-P-Gal molecule that primes the biosynthesis of bacterial surface glycans.

Glycosylation regulates specific induction of rice immune responses by Acidovorax avenae flagellin

Plants have a sensitive system that detects various pathogen-derived molecules to protect against infection. Flagellin, a main component of the bacterial flagellum, from the rice avirulent N1141 strain of the Gram-negative phytopathogenic bacterium Acidovorax avenae induces plant immune responses including H₂O generation, whereas flagellin from the rice virulent K1 strain of A. avenae does not induce these immune responses. To clarify the molecular mechanism that leads to these differing responses between the K1 and N1141 flagellins, recombinant K1 and N1141 flagellins were generated using an Escherichia coli expression system. When cultured rice cells were treated with recombinant K1 or N1141 flagellin, both flagellins equally induced H₂O₂ generation, suggesting that post-translational modifications of the flagellins are involved in the specific induction of immune responses. Mass spectrometry analyses using glycosyltransferase-deficient mutants showed that 1,600- and 2,150-Da glycans were present on the flagellins from N1141 and K1, respectively. A deglycosylated K1 flagellin induced immune responses in the same manner as N1141 flagellin. Site-directed mutagenesis revealed that glycans were attached to four amino acid residues (Ser¹⁷⁸, Ser¹⁸³, Ser²¹², and Thr³⁵¹) in K1 flagellin. Among mutant K1 flagellins in which each glycan-attached amino acid residue was changed to alanine, S178A and S183A, K1 flagellin induced a strong immune response in cultured rice cells, indicating that the glycans at Ser¹⁷⁸ and Ser¹⁸³ in K1 flagellin prevent epitope recognition in rice.

Genetic and molecular analyses reveal an evolutionary trajectory for glycan synthesis in a bacterial protein glycosylation system

Although protein glycosylation systems are becoming widely recognized in bacteria, little is known about the mechanisms and evolutionary forces shaping glycan composition. Species within the genus Neisseria display remarkable glycoform variability associated with their O-linked protein glycosylation (pgl) systems and provide a well developed model system to study these phenomena. By examining the potential influence of two ORFs linked to the core pgl gene locus, we discovered that one of these, previously designated as pglH, encodes a glucosyltransferase that generates unique disaccharide products by using polyprenyl diphosphate-linked monosaccharide substrates. By defining the function of PglH in the glycosylation pathway, we identified a metabolic conflict related to competition for a shared substrate between the opposing glycosyltransferases PglA and PglH. Accordingly, we propose that the presence of a stereotypic, conserved deletion mutation inactivating pglH in strains of Neisseria gonorrhoeae, Neisseria meningitidis, and related commensals, reflects a resolution of this conflict with the consequence of reduced glycan diversity. This model of genetic détente is supported by the characterization of pglH "missense" alleles encoding proteins devoid of activity or reduced in activity such that they cannot exert their effect in the presence of PglA. Thus, glucose-containing glycans appear to be a trait undergoing regression at the genus level. Together, these findings document a role for intrinsic genetic interactions in shaping glycan evolution in protein glycosylation systems.

Biochemical characterization of the O-linked glycosylation pathway in Neisseria gonorrhoeae responsible for biosynthesis of protein glycans containing N,N'-diacetylbacillosamine

The O-linked protein glycosylation pathway in Neisseria gonorrhoeae is responsible for the synthesis of a complex oligosaccharide on undecaprenyl diphosphate and subsequent en bloc transfer of the glycan to serine residues of select periplasmic proteins. Protein glycosylation (pgl) genes have been annotated on the basis of bioinformatics and top-down mass spectrometry analysis of protein modifications in pgl-null strains [Aas, F. E., et al. (2007) Mol. Microbiol. 65, 607-624; Vik, A., et al. (2009) Proc. Natl. Acad. Sci. U.S.A. 106, 4447-4452], but relatively little biochemical analysis has been performed to date. In this report, we present the expression, purification, and functional characterization of seven Pgl enzymes. Specifically, the enzymes studied are responsible for synthesis of an uncommon uridine diphosphate (UDP)-sugar (PglD, PglC, and PglB-acetyltransferase domain), glycan assembly (PglB-phospho-glycosyltransferase domain, PglA, PglE, and PglH), and final oligosaccharide transfer (PglO). UDP-2,4-diacetamido-2,4,6-trideoxy-α-d-hexose (DATDH), which is the first sugar in glycan biosynthesis, was produced enzymatically, and the stereochemistry was assigned as uridine diphosphate N'-diacetylbacillosamine (UDP-diNAcBac) by nuclear magnetic resonance characterization. In addition, the substrate specificities of the phospho-glycosyltransferase, glycosyltransferases, and oligosaccharyltransferase (OTase) were analyzed in vitro, and in most cases, these enzymes exhibited strong preferences for the native substrates relative to closely related glycans. In particular, PglO, the O-linked OTase, and PglB(Cj), the N-linked OTase from Campylobacter jejuni, preferred the native N. gonorrhoeae and C. jejuni substrates, respectively. This study represents the first comprehensive biochemical characterization of this important O-linked glycosylation pathway and provides the basis for further investigations of these enzymes as antibacterial targets.