Comparative importance in vivo of conserved glutamate residues in the EX7E motif retaining glycosyltransferase Gpi3p, the UDP-GlcNAc-binding subunit of the first enzyme in glycosylphosphatidylinositol assembly

Investigation of core machinery for biosynthesis of Vi antigen capsular polysaccharides in Gram-negative bacteria

Salmonella enterica serovar Typhi causes typhoid fever. It possesses a Vi antigen capsular polysaccharide coat that is important for virulence and is the basis of a current glycoconjugate vaccine. Vi antigen is also produced by environmental Bordetella isolates, while mammal-adapted Bordetella species (such as Bordetella bronchiseptica) produce a capsule of undetermined structure that cross-reacts with antibodies recognizing Vi antigen. The Vi antigen backbone is composed of poly-α-(1→4)-linked N-acetylgalactosaminuronic acid, modified with O-acetyl residues that are necessary for vaccine efficacy. Despite its biological and biotechnological importance, some central aspects of Vi antigen production are poorly understood. Here we demonstrate that TviE and TviD, two proteins encoded in the viaB (Vi antigen production) locus, interact and are the Vi antigen polymerase and O-acetyltransferase, respectively. Structural modeling and site-directed mutagenesis reveal that TviE is a GT4-family glycosyltransferase. While TviD has no identifiable homologs beyond Vi antigen systems in other bacteria, structural modeling suggests that it belongs to the large SGNH hydrolase family, which contains other O-acetyltransferases. Although TviD possesses an atypical catalytic triad, its O-acetyltransferase function was verified by antibody reactivity and ¹³C NMR data for tviD-mutant polysaccharide. The B. bronchiseptica genetic locus predicts a mode of synthesis distinct from classical S. enterica Vi antigen production, but which still involves TviD and TviE homologs that are both active in a reconstituted S. Typhi system. These findings provide new insight into Vi antigen production and foundational information for the glycoengineering of Vi antigen production in heterologous bacteria.

Elimination of GPI2 suppresses glycosylphosphatidylinositol GlcNAc transferase activity and alters GPI glycan modification in Trypanosoma brucei

Many eukaryotic cell-surface proteins are post-translationally modified by a glycosylphosphatidylinositol (GPI) moiety that anchors them to the cell membrane. The biosynthesis of GPI anchors is initiated in the endoplasmic reticulum by transfer of GlcNAc from UDP-GlcNAc to phosphatidylinositol. This reaction is catalyzed by GPI GlcNAc transferase, a multisubunit complex comprising the catalytic subunit Gpi3/PIG-A as well as at least five other subunits, including the hydrophobic protein Gpi2, which is essential for the activity of the complex in yeast and mammals, but the function of which is not known. To investigate the role of Gpi2, we exploited Trypanosoma brucei (Tb), an early diverging eukaryote and important model organism that initially provided the first insights into GPI structure and biosynthesis. We generated insect-stage (procyclic) trypanosomes that lack TbGPI2 and found that in TbGPI2-null parasites, (i) GPI GlcNAc transferase activity is reduced, but not lost, in contrast with yeast and human cells, (ii) the GPI GlcNAc transferase complex persists, but its architecture is affected, with loss of at least the TbGPI1 subunit, and (iii) the GPI anchors of procyclins, the major surface proteins, are underglycosylated when compared with their WT counterparts, indicating the importance of TbGPI2 for reactions that occur in the Golgi apparatus. Immunofluorescence microscopy localized TbGPI2 not only to the endoplasmic reticulum but also to the Golgi apparatus, suggesting that in addition to its expected function as a subunit of the GPI GlcNAc transferase complex, TbGPI2 may have an enigmatic noncanonical role in Golgi-localized GPI anchor modification in trypanosomes.

The wclY gene of Escherichia coli serotype O117 encodes an α1,4-glucosyltransferase with strict acceptor specificity but broad donor specificity

The O antigen of enterotoxigenic Escherichia coli serotype O117 consists of repeating units with the structure [-D-GalNAcβ1-3-L-Rhaα1-4-D-Glcα1-4-D-Galβ1-3-D-GalNAcα1-4]n. A related structure is found in E. coli O107 where Glc is replaced by a GlcNAc residue. The O117 and O107 antigen biosynthesis gene clusters are homologous and reveal the presence of four putative glycosyltransferase (GT) genes, wclW, wclX, wclY and wclZ, but the enzymes have not yet been biochemically characterized. We show here that the His6-tagged WclY protein expressed in E. coli Lemo21(DE3) cells is an α1,4-Glc-transferase that transfers Glc to the Gal moiety of Galβ1-3GalNAcα-OPO3-PO3-phenoxyundecyl as a specific acceptor and that the diphosphate moiety of this acceptor is required. WclY utilized UDP-Glc, TDP-Glc, ADP-Glc, as well as UDP-GlcNAc, UDP-Gal or UDP-GalNAc as donor substrates, suggesting an unusual broad donor specificity. Activity using GDP-Man suggested the presence of a novel Man-transferase in Lemo21(DE3) cells. Mutations of WclY revealed that both Glu residues of the Ex7E motif within the predicted GT domain are essential for activity. High GlcNAc-transferase (GlcNAc-T) activities of WclY were created by mutating Arg194 to Cys. A triple mutant identical to WclY in E. coli O107 was identified as an α1,4 GlcNAc-T. The characterization of WclY opens the door for the development of antibacterial approaches.

Alternative routes for synthesis of N-linked glycans by Alg2 mannosyltransferase

Asparagine ( N)-linked glycosylation requires the ordered, stepwise synthesis of lipid-linked oligosaccharide (LLO) precursor Glc₃Man₉GlcNAc₂-pyrophosphate-dolichol (Glc₃Man₉Gn₂-PDol) on the endoplasmic reticulum. The fourth and fifth steps of LLO synthesis are catalyzed by Alg2, an unusual mannosyltransferase (MTase) with two different MTase activities; Alg2 adds both an α1,3- and α1,6-mannose onto ManGlcNAc₂-PDol to form the trimannosyl core Man₃GlcNAc₂-PDol. The biochemical properties of Alg2 are controversial and remain undefined. In this study, a liquid chromatography/mass spectrometry-based quantitative assay was established and used to analyze the MTase activities of purified yeast Alg2. Alg2-dependent Man₃GlcNAc₂-PDol production relied on net-neutral lipids with a propensity to form bilayers. We further showed addition of the α1,3- and α1,6-mannose can occur independently in either order but at differing rates. The conserved C-terminal EX₇E motif, N-terminal cytosolic tail, and 3 G-rich loop motifs in Alg2 play crucial roles for these activities, both in vitro and in vivo. These findings provide insight into the unique bifunctionality of Alg2 during LLO synthesis and lead to a new model in which alternative, independent routes exist for Alg2 catalysis of the trimannosyl core oligosaccharide.-Li, S.-T., Wang, N., Xu, X.-X., Fujita, M., Nakanishi, H., Kitajima, T., Dean, N., Gao, X.-D. Alternative routes for synthesis of N-linked glycans by Alg2 mannosyltransferase.

Identification and biochemical characterization of WbwB, a novel UDP-Gal: Neu5Ac-R α1,4-galactosyltransferase from the intestinal pathogen Escherichia coli serotype O104

The intestinal pathogen Escherichia coli serotype O104:H4 (ECO104) can cause bloody diarrhea and haemolytic uremic syndrome. The ECO104 O antigen has the unique repeating unit structure [4Galα1-4Neu5,7,9Ac₃α2-3Galβ1-3GalNAcβ1-], which includes the mammalian sialyl-T antigen as an internal structure. Previously, we identified WbwC from ECO104 as the β3Gal-transferase that synthesizes the T antigen, and showed that α3-sialyl-transferase WbwA transfers sialic acid to the T antigen. Here we identify the wbwB gene product as a unique α1,4-Gal-transferase WbwB that transfers Gal from UDP-Gal to the terminal sialic acid residue of Neu5Acα2-3Galβ1-3GalNAcα-diphosphate-lipid acceptor. NMR analysis of the WbwB enzyme reaction product indicated that Galα1-4Neu5Acα2-3Galβ1-3GalNAcα-diphosphate-lipid was synthesized. WbwB from ECO104 has a unique acceptor specificity for terminal sialic acid as well as the diphosphate group in the acceptor. The characterization studies showed that WbwB does not require divalent metal ion as a cofactor. Mutagenesis identified Lys243 within an RKR motif and both Glu315 and Glu323 of the fourth EX₇E motif as essential for the activity. WbwB is the final glycosyltransferase in the biosynthesis pathway of the ECO104 antigen repeating unit. This work contributes to knowledge of the biosynthesis of bacterial virulence factors.

New insight into the catalytic properties of rice sucrose synthase

Sucrose synthase (SuS), which catalyzes the reversible conversion of sucrose and uridine diphosphate (UDP) into fructose and UDP-glucose, is a key enzyme in sucrose metabolism in higher plants. SuS belongs to family 4 of the glycosyltransferases (GT4) and contains an E-X7-E motif that is conserved in members of GT4 and two other GT families. To gain insight into the roles of this motif in rice sucrose synthase 3 (RSuS3), the two conserved glutamate residues (E678 and E686) in this motif and a phenylalanine residue (F680) that resides between the two glutamate residues were changed by site-directed mutagenesis. All mutant proteins maintained their tetrameric conformation. The mutants E686D and F680Y retained partial enzymatic activity and the mutants E678D, E678Q, F680S, and E686Q were inactive. Substrate binding assays indicated that UDP and fructose, respectively, were the leading substrates in the sucrose degradation and synthesis reactions of RSuS3. Mutations on E678, F680, and E686 affected the binding of fructose, but not of UDP. The results indicated that E678, F680, and E686 in the E-X7-E motif of RSuS3 are essential for the activity of the enzyme and the sequential binding of substrates. The sequential binding of the substrates implied that the reaction catalyzed by RSuS can be controlled by the availability of fructose and UDP, depending on the metabolic status of a tissue.

Role of PelF in pel polysaccharide biosynthesis in Pseudomonas aeruginosa

Pseudomonas aeruginosa produces three exopolysaccharides, Psl, Pel, and alginate, that play vital roles in biofilm formation. Pel is a glucose-rich, cellulose-like exopolysaccharide. The essential Pel biosynthesis proteins are encoded by seven genes, pelA to pelG. Bioinformatics analysis suggests that PelF is a cytosolic glycosyltransferase. Here, experimental evidence was provided to support this PelF function. A UDP-glucose dehydrogenase-based assay was developed to quantify UDP-glucose. UDP-glucose was proposed as the substrate for PelF. The isogenic pelF deletion mutant accumulated 1.8 times more UDP-glucose in its cytosol than the wild type. This suggested that PelF, which was found localized in the cystosol, uses UDP-glucose as substrate. Additionally, in vitro experiments confirmed that PelF uses UDP-glucose as substrate. To analyze the functional roles of conserved residues in PelF, site-directed mutagenesis was performed. The presence of the EX7E motif is characteristic for various glycosyltransferase families, and in PelF, E405/E413 are the conserved residues in this motif. Replacement of E405 with A resulted in a reduction of PelF activity to 30.35% ± 3.15% (mean ± standard deviation) of the wild-type level, whereas replacement of the second E, E413, with A did not produce a significant change in the activity of PelF. Moreover, replacement of both E residues did not result in a loss of PelF function, but replacement of the conserved R325 or K330 with A resulted in a complete loss of PelF activity. Overall, our data show that PelF is a soluble glycosyltransferase that uses UDP-glucose as the substrate for Pel synthesis and that conserved residues R325 and K330 are important for the activity of PelF.

Domain organization of the polymerizing mannosyltransferases involved in synthesis of the Escherichia coli O8 and O9a lipopolysaccharide O-antigens

The Escherichia coli O9a and O8 polymannose O-polysaccharides (O-PSs) serve as model systems for the biosynthesis of bacterial polysaccharides by ATP-binding cassette transporter-dependent pathways. Both O-PSs contain a conserved primer-adaptor domain at the reducing terminus and a serotype-specific repeat unit domain. The repeat unit domain is polymerized by the serotype-specific WbdA mannosyltransferase. In serotype O9a, WbdA is a bifunctional α-(1→2)-, α-(1→3)-mannosyltransferase, and its counterpart in serotype O8 is trifunctional (α-(1→2), α-(1→3), and β-(1→2)). Little is known about the detailed structures or mechanisms of action of the WbdA polymerases, and here we establish that they are multidomain enzymes. WbdA(O9a) contains two separable and functionally active domains, whereas WbdA(O8) possesses three. In WbdC(O9a) and WbdB(O9a), substitution of the first Glu of the EX(7)E motif had detrimental effects on the enzyme activity, whereas substitution of the second had no significant effect on activity in vivo. Mutation of the Glu residues in the EX(7)E motif of the N-terminal WbdA(O9a) domain resulted in WbdA variants unable to synthesize O-PS. In contrast, mutation of the Glu residues in the motif of the C-terminal WbdA(O9a) domain generated an enzyme capable of synthesizing an altered O-PS repeat unit consisting of only α-(1→2) linkages. In vitro assays with synthetic acceptors unequivocally confirmed that the N-terminal domain of WbdA(O9a) possesses α-(1→2)-mannosyltransferase activity. Together, these studies form a framework for detailed structure-function studies on individual domains and a strategy applicable for dissection and analysis of other multidomain glycosyltransferases.

Biochemical characterization, membrane association and identification of amino acids essential for the function of Alg11 from Saccharomyces cerevisiae, an alpha1,2-mannosyltransferase catalysing two sequential glycosylation steps in the formation of the lipid-linked core oligosaccharide

The biosynthesis of asparagine-linked glycans occurs in an evolutionarily conserved manner with the assembly of the unique lipid-linked oligosaccharide precursor Glc3Man9GlcNAc2-PP-Dol at the ER (endoplasmic reticulum). In the present study we characterize Alg11 from yeast as a mannosyltransferase catalysing the sequential transfer of two alpha1,2-linked mannose residues from GDP-mannose to Man3GlcNAc2-PP-Dol and subsequently to Man4GlcNAc2-PP-Dol forming the Man5GlcNAc2-PP-Dol intermediate at the cytosolic side of the ER before flipping to the luminal side. Alg11 is predicted to contain three hydrophobic transmembrane-spanning helices. Using Alg11 topology reporter fusion constructs, we show that only the N-terminal domain fulfils this criterion. Surprisingly, this domain can be deleted without disturbing glycosyltransferase function and membrane association, indicating also that the other two hydrophobic domains contribute to ER localization, but in a non-transmembrane manner. By site-directed mutagenesis we investigated amino acids important for transferase activity. We demonstrate that the first glutamate residue in the EX7E motif, conserved in a variety of glycosyltransferases, is more critical than the second, and loss of Alg11 function occurs only when both glutamate residues are exchanged, or when the mutation of the first glutamate residue is combined with replacement of another amino acid in the motif. This indicates that perturbations in EX7E are not restricted to the second glutamate residue. Moreover, Gly85 and Gly87, within a glycine-rich domain as part of a potential flexible loop, were found to be required for Alg11 function. Similarly, a conserved lysine residue, Lys319, was identified as being important for activity, which could be involved in the binding of the phosphate of the glycosyl donor.

Biochemical characterization and membrane topology of Alg2 from Saccharomyces cerevisiae as a bifunctional alpha1,3- and 1,6-mannosyltransferase involved in lipid-linked oligosaccharide biosynthesis

N-Linked glycosylation involves the ordered, stepwise synthesis of the unique lipid-linked oligosaccharide precursor Glc(3)Man(9) GlcNAc(2)-PP-Dol on the endoplasmic reticulum (ER), catalyzed by a series of glycosyltransferases. Here we characterize Alg2 as a bifunctional enzyme that is required for both the transfer of the alpha1,3- and the alpha1,6-mannose-linked residue from GDP-mannose to Man(1)GlcNAc(2)-PP-Dol forming the Man(3)GlcNAc(2)-PP-Dol intermediate on the cytosolic side of the ER. Alg2 has a calculated mass of 58 kDa and is predicted to contain four transmembrane-spanning helices, two at the N terminus and two at the C terminus. Contradictory to topology predictions, we prove that only the two N-terminal domains fulfill this criterion, whereas the C-terminal hydrophobic sequences contribute to ER localization in a nontransmembrane manner. Surprisingly, none of the four domains is essential for transferase activity because truncated Alg2 variants can exert their function as long as Alg2 is associated with the ER by either its N- or C-terminal hydrophobic regions. By site-directed mutagenesis we demonstrate that an EX(7)E motif, conserved in a variety of glycosyltransferases, is not important for Alg2 function in vivo and in vitro. Instead, we identify a conserved lysine residue, Lys(230), as being essential for activity, which could be involved in the binding of the phosphate of the glycosyl donor.

Role of the Synthase Domain of Ags1p in Cell Wall α-Glucan Biosynthesis in Fission Yeast

The cell wall is important for maintenance of the structural integrity and morphology of fungal cells. Besides beta-glucan and chitin, alpha-glucan is a major polysaccharide in the cell wall of many fungi. In the fission yeast Schizosaccharomyces pombe, cell wall alpha-glucan is an essential component, consisting mainly of (1,3)-alpha-glucan with approximately 10% (1,4)-linked alpha-glucose residues. The multidomain protein Ags1p is required for alpha-glucan biosynthesis and is conserved among cell wall alpha-glucan-containing fungi. One of its domains shares amino acid sequence motifs with (1,4)-alpha-glucan synthases such as bacterial glycogen synthases and plant starch synthases. Whether Ags1p is involved in the synthesis of the (1,4)-alpha-glucan constituent of cell wall alpha-glucan had remained unclear. Here, we show that overexpression of Ags1p in S. pombe cells results in accumulation of (1,4)-alpha-glucan. To determine whether the synthase domain of Ags1p is responsible for this activity, we overexpressed Ags1p-E1526A, which carries a mutation in a putative catalytic residue of the synthase domain, but observed no accumulation of (1,4)-alpha-glucan. Compared with wild-type Ags1p, this mutant Ags1p showed a markedly reduced ability to complement the cell lysis phenotype of the temperature-sensitive ags1-1 mutant. Therefore, we conclude that, in S. pombe, the production of (1,4)-alpha-glucan by the synthase domain of Ags1p is important for the biosynthesis of cell wall alpha-glucan.

Thematic review series: lipid posttranslational modifications. GPI anchoring of protein in yeast and mammalian cells, or: how we learned to stop worrying and love glycophospholipids

Glycosylphosphatidylinositol (GPI) anchoring of cell surface proteins is the most complex and metabolically expensive of the lipid posttranslational modifications described to date. The GPI anchor is synthesized via a membrane-bound multistep pathway in the endoplasmic reticulum (ER) requiring >20 gene products. The pathway is initiated on the cytoplasmic side of the ER and completed in the ER lumen, necessitating flipping of a glycolipid intermediate across the membrane. The completed GPI anchor is attached to proteins that have been translocated across the ER membrane and that display a GPI signal anchor sequence at the C terminus. GPI proteins transit the secretory pathway to the cell surface; in yeast, many become covalently attached to the cell wall. Genes encoding proteins involved in all but one of the predicted steps in the assembly of the GPI precursor glycolipid and its transfer to protein in mammals and yeast have now been identified. Most of these genes encode polytopic membrane proteins, some of which are organized in complexes. The steps in GPI assembly, and the enzymes that carry them out, are highly conserved. GPI biosynthesis is essential for viability in yeast and for embryonic development in mammals. In this review, we describe the biosynthesis of mammalian and yeast GPIs, their transfer to protein, and their subsequent processing.

Investigation of the structural requirements in the lipopolysaccharide core acceptor for ligation of O antigens in the genus Salmonella: WaaL "ligase" is not the sole determinant of acceptor specificity

The ligation of O antigen polysaccharide to lipid A-core oligosaccharide is a late step in the formation of the complex glycolipid known as lipopolysaccharide. Although the process has been localized to the periplasmic face of the inner membrane, details of the ligation mechanism have not been resolved. To date, there is only one gene product (WaaL, often referred to as "ligase") known to be required. There exists a requirement for a specific lipid A-core oligosaccharide acceptor structure for ligation activity, and it has been proposed that the WaaL protein imparts this acceptor specificity. Here the structural requirements in the core oligosaccharide acceptor for O antigen ligation are investigated in prototype serovars of Salmonella enterica. Complementation experiments in mutants with defined core oligosaccharide structure indicate that the specificity of the ligation reaction for a particular core oligosaccharide structure is not dependent on the WaaL protein alone. The data provide the first indication of a more complicated recognition process involving additional cellular components.

Overexpression, purification, biochemical characterization, and molecular modeling of recombinant GDP-mannosyltransferase (GumH) from Xylella fastidiosa

The GumH enzyme from Xylella fastidiosa catalyzes the transfer reaction of a mannose from GDP-mannose to the carrier lipid cellobiose-pyrophosphate-polyprenol (Glc(2)-PP-Lip), an intermediary in the reaction for the synthesis of the exopolysaccharide (EPS) fastidian gum. The gumH gene was subcloned in the pMal-c2x vector, allowing the expression of the GumH-MBP fusion protein. Various attempts were made to obtain protein with the necessary degree of purity for crystallographic studies but the yield was very low. The gumH gene was then subcloned in the pET28a vector allowing the expression of the GumH enzyme in fusion with a histidine-rich peptide. The protein was purified and characterized. The three-dimensional structure of the X. fastidiosa GumH enzyme was modeled by threading studies. The model consists of N- and C-terminal domains similar in size and topology and separated by a deep cleft, which includes the EX(7)E motif that can be involved in the catalysis of GumH.