Expression of Campylobacter hyoilei lipo-oligosaccharide (LOS) antigens in Escherichia coli

Amino acid 310 determines the donor substrate specificity of serogroup W-135 and Y capsule polymerases of Neisseria meningitidis

The capsular polysaccharides of serogroup W-135 and Y meningococci are sialic acid-containing heteropolymers, with either galactose or glucose as the second sugar residue. As shown previously, sequences of the predicted enzymes that catalyse capsule polymerization, i.e. SiaD(W-135) and SiaD(Y), differ in only a few amino acids. By in vitro assays with purified recombinant proteins, SiaD(W-135) and SiaD(Y) were now confirmed to be the capsule polymerases harbouring both hexosyltransferase and sialyltransferase activity. In order to identify amino acids crucial for substrate specificity of the capsule polymerases, polymorphic sites were narrowed down by DNA sequence comparisons and subsequent site-directed mutagenesis. Serogroup-specific amino acids were restricted to the N-terminal part of the proteins. Exclusively amino acid 310, located within the nucleotide recognition domain of the enzymes' predicted hexosyltransferase moiety, accounted for substrate specificity as shown by immunochemistry and in vitro activity assay. Pro-310 determined galactosyltransferase activity that resulted in a serogroup W-135 capsule and Gly-310 determined glucosyltransferase activity that resulted in a serogroup Y capsule. In silico analysis revealed a similar amino acid-based association in other members of the same glycosyltransferase family irrespective of the bacterial species.

Protein glycosylation in bacterial mucosal pathogens

In eukaryotes, glycosylated proteins are ubiquitous components of extracellular matrices and cellular surfaces. Their oligosaccharide moieties are implicated in a wide range of cell-cell and cell-matrix recognition events that are required for biological processes ranging from immune recognition to cancer development. Glycosylation was previously considered to be restricted to eukaryotes; however, through advances in analytical methods and genome sequencing, there have been increasing reports of both O-linked and N-linked protein glycosylation pathways in bacteria, particularly amongst mucosal-associated pathogens. Studying glycosylation in relatively less-complicated bacterial systems provides the opportunity to elucidate and exploit glycoprotein biosynthetic pathways. We will review the genetic organization, glycan structures and function of glycosylation systems in mucosal bacterial pathogens, and speculate on how this knowledge may help us to understand glycosylation processes in more complex eukaryotic systems and how it can be used for glycoengineering.

Campylobacter--a tale of two protein glycosylation systems

Post-translational glycosylation is a universal modification of proteins in eukarya, archaea and bacteria. Two recent publications describe the first confirmed report of a bacterial N-linked glycosylation pathway in the human gastrointestinal pathogen Campylobacter jejuni. In addition, an O-linked glycosylation pathway has been identified and characterized in C. jejuni and the related species Campylobacter coli. Both pathways have similarity to the respective N- and O-linked glycosylation processes in eukaryotes. In bacteria, homologues of the genes in both pathways are found in other organisms, the complex glycans linked to the glycoproteins share common biosynthetic precursors and these modifications could play similar biological roles. Thus, Campylobacter provides a unique model system for the elucidation and exploitation of glycoprotein biosynthesis.

Characterization of the Campylobacter jejuni heptosyltransferase II gene, waaF, provides genetic evidence that extracellular polysaccharide is lipid A core independent

Campylobacter jejuni produces both lipooligosaccharide (LOS) and a higher-molecular-weight polysaccharide that is believed to form a capsule. The role of these surface polysaccharides in C. jejuni-mediated enteric disease is unclear; however, epitopes associated with the LOS are linked to the development of neurological complications. In Escherichia coli and Salmonella enterica serovar Typhimurium the waaF gene encodes a heptosyltransferase, which catalyzes the transfer of the second L-glycero-D-manno-heptose residue to the core oligosaccharide moiety of lipopolysaccharide (LPS), and mutation of waaF results in a truncated core oligosaccharide. In this report we confirm experimentally that C. jejuni gene Cj1148 encodes the heptosyltransferase II enzyme, WaaF. The Campylobacter waaF gene complements an S. enterica serovar Typhimurium waaF mutation and restores the ability to produce full-sized lipopolysaccharide. To examine the role of WaaF in C. jejuni, waaF mutants were constructed in strains NCTC 11168 and NCTC 11828. Loss of heptosyltransferase activity resulted in the production of a truncated core oligosaccharide, failure to bind specific ligands, and loss of serum reactive GM(1), asialo-GM(1), and GM(2) ganglioside epitopes. The mutation of waaF did not affect the higher-molecular-weight polysaccharide supporting the production of a LOS-independent capsular polysaccharide by C. jejuni. The exact structural basis for the truncation of the core oligosaccharide was verified by comparative chemical analysis. The NCTC 11168 core oligosaccharide differs from that known for HS:2 strain CCUG 10936 in possessing an extra terminal disaccharide of galactose-beta(1,3) N-acetylgalactosamine. In comparison, the waaF mutant possessed a truncated molecule consistent with that observed with waaF mutants in other bacterial species.

Genetic characterization of pilin glycosylation in Neisseria meningitidis

Pili of Neisseria meningitidis are a key virulence factor, being the major adhesin of this capsulate organism and contributing to specificity for the human host. Pili are post-translationally modified by addition of an O-linked trisaccharide, Gal(beta1-4)Gal(alpha1-3)2,4-diacetimido-2,4,6-trideoxyhexose++ +. In a previous study the authors identified and characterized a gene, pglA, encoding a galactosyltransferase involved in pilin glycosylation. In this study a set of random genomic sequences from N. meningitidis strain MC58 was used to search for further genes involved in pilin glycosylation. Initially, an open reading frame was identified, and designated pglD (pilin glycosylation gene D), which was homologous to genes involved in polysaccharide biosynthesis. The region adjacent to this gene was cloned and nucleotide sequence analysis revealed two further genes, pglB and pglC, which were also homologous with genes involved in polysaccharide biosynthesis. Insertional mutations were constructed in pglB, pglC and pglD in N. meningitidis C311#3, a strain with well-defined LPS and pilin-linked glycan structures, to determine whether these genes had a role in the biosynthesis of either of these molecules. Analysis of these mutants revealed that there was no alteration in the phenotype of LPS in any of the mutant strains as judged by SDS-PAGE gel migration. In contrast, increased gel migration of the pilin subunit molecules of pglB, pglC and pglD mutants by Western analysis was observed. Pilin from each of the pglB, pglC and pglD mutants did not react with a terminal-galactose-specific stain, confirming that the gel migration differences were due to the alteration or absence of the pilin-linked trisaccharide structure in these mutants. In addition, antisera specific for the C311#3 trisaccharide failed to react with pilin from the pglB, pglC, pglD and galE mutants. Analysis of nucleotide sequence homologies has suggested specific roles for pglB, pglC and pglD in the biosynthesis of the 2,4-diacetimido-2,4,6-trideoxyhexose structure.

Genetic and biochemical evidence of a Campylobacter jejuni capsular polysaccharide that accounts for Penner serotype specificity

Campylobacter jejuni, a Gram-negative spiral bacterium, is the most common bacterial cause of acute human gastroenteritis and is increasingly recognized for its association with the serious post-infection neurological complications of the Miller-Fisher and Guillain-Barré syndromes. C. jejuni lipopolysaccharide (LPS) is thought to be involved in the pathogenesis of both uncomplicated infection and more serious sequelae, yet the LPS remains poorly characterized. Current studies on C. jejuni suggest that all strains produce lipooligosaccharide (LOS), with about one-third of strains also producing high-molecular-weight LPS (referred to as O-antigen). In this report, we demonstrate the presence of the high-molecular-weight LPS in all C. jejuni strains tested. Furthermore, we show that this LPS is biochemically and genetically unrelated to LOS and is similar to group II and group III capsular polysaccharides. All tested kpsM, kpsS and kpsC mutants of C. jejuni lost the ability to produce O-antigen. Moreover, this correlated with serotype changes. We demonstrate for the first time that the previously described O-antigen of C. jejuni is a capsular polysaccharide and a common component of the thermostable antigen used for serotyping of C. jejuni.

Conserved domains of glycosyltransferases

Glycosyltransferases catalyze the synthesis of glycoconjugates by transferring a properly activated sugar residue to an appropriate acceptor molecule or aglycone for chain initiation and elongation. The acceptor can be a lipid, a protein, a heterocyclic compound, or another carbohydrate residue. A catalytic reaction is believed to involve the recognition of both the donor and acceptor by suitable domains, as well as the catalytic site of the enzyme. To elucidate the structural requirements for substrate recognition and catalytic reactions of glycosyltransferases, we have searched the databases for homologous sequences, identified conserved amino acid residues, and proposed potential domain motifs for these enzymes. Depending on the configuration of the anomeric functional group of the glycosyl donor molecule and of the resulting glycoconjugate, all known glycosyltransferases can be divided into two major types: retaining glycosyltransferases, which transfer sugar residue with the retention of anomeric configuration, and inverting glycosyltransferases, which transfer sugar residue with the inversion of anomeric configuration. One conserved domain of the inverting glycosyltransferases identified in the database is responsible for the recognition of a pyrimidine nucleotide, which is either the UDP or the TDP portion of a donor sugar-nucleotide molecule. This domain is termed "Nucleotide Recognition Domain 1 beta," or NRD1 beta, since the type of nucleotide is the only common structure among the sugar donors and acceptors. NRD1 beta is present in 140 glycosyltransferases. The central portion of the NRD1 beta domain is very similar to the domain that is present in one family of retaining glycosyltransferases. This family is termed NRD1 alpha to designate the similarity and stereochemistry of sugar transfer, and it consists of 77 glycosyltransferases identified thus far. In the central portion there is a homologous region for these two families and this region probably has a catalytic function. A third conserved domain is found exclusively in membrane-bound glycosyltransferases and is termed NRD2; this domain is present in 98 glycosyltransferases. All three identified NRDs are present in archaebacterial, eubacterial, viral, and eukaryotic glycosyltransferases. The present article presents the alignment of conserved NRD domains and also presents a brief overview of the analyzed glycosyltransferases which comprise about 65% of all known sugar-nucleotide dependent (Leloir-type) and putative glycosyltransferases in different databases. A potential mechanism for the catalytic reaction is also proposed. This proposed mechanism should facilitate the design of experiments to elucidate the regulatory mechanisms of glycosylation reactions. Amino acid sequence information within the conserved domain may be utilized to design degenerate primers for identifying DNA encoding new glycosyltransferases.

Evidence for a system of general protein glycosylation in Campylobacter jejuni

A genetic locus from Campylobacter jejuni 81-176 (O:23, 36) has been characterized that appears to be involved in glycosylation of multiple proteins, including flagellin. The lipopolysaccharide (LPS) core of Escherichia coli DH5alpha containing some of these genes is modified such that it becomes immunoreactive with O:23 and O:36 antisera and loses reactivity with the lectin wheat germ agglutinin (WGA). Site-specific mutation of one of these genes in the E. coli host causes loss of O:23 and O:36 antibody reactivity and restores reactivity with WGA. However, site-specific mutation of each of the seven genes in 81-176 failed to show any detectable changes in LPS. Multiple proteins from various cellular fractions of each mutant showed altered reactivity by Western blot analyses using O:23 and O:36 antisera. The changes in protein antigenicity could be restored in one of the mutants by the presence of the corresponding wild-type allele in trans on a shuttle vector. Flagellin, which is known to be a glycoprotein, was one of the proteins that showed altered reactivity with O:23 and O:36 antiserum in the mutants. Chemical deglycosylation of protein fractions from the 81-176 wild type suggests that the other proteins with altered antigenicity in the mutants are also glycosylated.

Identification of a novel gene involved in pilin glycosylation in Neisseria meningitidis

The pili of Neisseria meningitidis are a key virulence factor, being major adhesins of this capsulate organism that contribute to specificity for the human host. Recently it has been reported that meningococcal pili are post-translationally modified by the addition of an O-linked trisaccharide, Gal (beta1-4) Gal (alpha1-3) 2,4-diacetimido-2,4,6-trideoxyhexose. Using a set of random genomic sequences from N. meningitidis strain MC58, we have identified a novel gene homologous to a family of glycosyltransferases. A plasmid clone containing the gene was isolated from a genomic library of N. meningitidis strain MC58 and its nucleotide sequence determined. The clone contained a complete copy of the gene, here designated pglA (pilin glycosylation). Insertional mutations were constructed in pglA in a range of meningococcal strains with well-defined lipopolysaccharide (LPS) or pilin-linked glycan structures to determine whether pglA had a role in the biosynthesis of these molecules. There was no alteration in the phenotype of LPS from pglA mutant strains as judged by gel migration and the binding of monoclonal antibodies. In contrast, decreased gel migration of the pilin subunit molecules of pglA mutants was observed, which was similar to the migration of pilins of galE mutants of same strains, supporting the notion that pglA is a glycosyltransferase involved in the biosynthesis of the pilin-linked trisaccharide structure. The pglA mutation, like the galE mutation reported previously, had no effect on pilus-mediated adhesion to human epithelial or endothelial cells. Pilin from pglA mutants were unable to bind to monospecific antisera recognizing the Gal (beta1-4) Gal structure, suggesting that PglA is a glycosyltransferase involved in the addition of galactose of the trisaccharide substituent of pilin.

The lipopolysaccharide biosynthesis locus of Campylobacter jejuni 81116

Most Campylobacter jejuni strains express lipo-oligosaccharides. Some strains also express lipopolysaccharides (LPS), with O-antigen-like carbohydrate repeats. C. jejuni 81116 expresses an LPS containing both lipo-oligosaccharides and O-antigen-like repeats, but nothing is known about the structure or sugar composition of these LPS species. A cosmid library of the genome of C. jejuni 81116 was constructed and probed with Campylobacter hyoilei genes involved in LPS synthesis. Five cosmids hybridized with the probe and two of these expressed C. jejuni 81116 LPS in Escherichia coli. By subcloning, a 16 kb DNA region was identified which contains the genetic information required to express C. jejuni LPS. DNA sequence analysis revealed 11 ORFs homologous to genes involved in LPS synthesis of other bacteria. They consisted of three homologues of sugar biosynthesis genes, two homologues of transport genes and six homologues of sugar transferases.