The C-terminal domain of the nucleotide-binding domain protein Wzt determines substrate specificity in the ATP-binding cassette transporter for the lipopolysaccharide O-antigens in Escherichia coli serotypes O8 and O9a

The polymannan O-antigenic polysaccharides (O-PSs) of Escherichia coli O8 and O9a are synthesized via an ATP-binding cassette (ABC) transporter-dependent pathway. The group 2 capsular polysaccharides of E. coli serve as prototypes for polysaccharide synthesis and export via this pathway. Here, we show that there are some fundamental differences between the ABC transporter-dependent pathway for O-PS biosynthesis and the capsular polysaccharide paradigm. In the capsule system, mutants lacking the ABC transporter are viable, and membranes isolated from these strains are no longer able to synthesize polymer using an endogenous acceptor. In contrast, E. coli strains carrying mutations in the membrane component (Wzm) and/or the nucleotide-binding component (Wzt) of the O8 and O9a polymannan transporters are nonviable under conditions permissive to O-PS biosynthesis and take on an aberrant elongated cell morphology. Whereas the ABC transporters for capsular polysaccharides with different structures are functionally interchangeable, the O8 and O9a exporters are specific for their cognate polymannan substrates. The E. coli O8 and O9a Wzt proteins contain a C-terminal domain not present in the corresponding nucleotide-binding protein (KpsT) from the capsule exporter. Whereas the Wzm components are functionally interchangeable, albeit with reduced efficiency, the Wzt components are not, indicating a specific role for Wzt in substrate specificity. Chimeric Wzt proteins were constructed in order to localize the region involved in substrate specificity to the C-terminal domain.     

In vitro reconstruction of the chain termination reaction in biosynthesis of the Escherichia coli O9a O-polysaccharide: the chain-length regulator, WbdD, catalyzes the addition of methyl phosphate to the non-reducing terminus of the growing glycan

The Escherichia coli O9a O-polysaccharide (O-PS) represents a model system for glycan biosynthesis and export by the ATP-binding cassette (ABC) transporter-dependent pathway. The polymannose O9a O-PS is synthesized using an undecaprenol-diphosphate-linked acceptor by mannosyltransferases located at the cytoplasmic membrane. An ABC-transporter subsequently exports the polymer to the periplasm where it is assembled onto lipopolysaccharide prior to translocation to the cell surface. The chain length of the O9a O-PS is regulated by the dual kinase/methyltransferase activity of the WbdD enzyme and modification of the polymer is crucial for binding and export by the ABC-transporter. Previous biochemical data provided evidence for phosphorylation/methylation at the non-reducing end of the O9a O-PS but the structure of the terminus has not been determined. Here, we describe the exploitation of a synthetic O9a O-PS repeating unit carrying a fluorescent tag as an acceptor for in vitro phosphorylation and methylation by a purified soluble form of WbdD. Phosphorylation of the acceptor was evident by both a mobility shift in thin layer chromatography and radiolabeling of the acceptor using [γ-(33)P]ATP. Methylation of the acceptor was dependent on phosphorylation and was demonstrated by radiolabeling using S-[methyl-(3)H]adenosyl-methionine as a substrate, in the presence of ATP. NMR spectroscopic and mass spectrometric methods were used to determine the precise structure of the terminal modification, leading to the conclusion that WbdD catalyzes the addition of a novel methyl phosphate group to the 3-position of the non-reducing terminal mannose of the O9a O-PS repeating unit.     

Role of Rfe and RfbF in the initiation of biosynthesis of D-galactan I, the lipopolysaccharide O antigen from Klebsiella pneumoniae serotype O1.

The 6.6-kb rfb gene cluster from Klebsiella pneumoniae serotype O1 (rfbKpO1) contains six genes whose products are required for the biosynthesis of a lipopolysaccharide O antigen with the following repeating unit structure: -->3-beta-D-Galf-1-->3-alpha-D-Galp-1-->(D-galactan I). rfbFKpO1 is the last gene in the cluster, and its gene product is required for the initiation of D-galactan I synthesis. Escherichia coli K-12 strains expressing the RfbFKpO1 polypeptide contain dual galactopyranosyl and galactofuranosyl transferase activity. This activity modifies the host lipopolysaccharide core by adding the disaccharide beta-D-Galf-1-->3-alpha-D-Galp, representing a single repeating unit of D-galactan I. The formation of the lipopolysaccharide substituted either with the disaccharide or with authentic polymeric D-galactan I is dependent on the activity of the Rfe enzyme. Rfe (UDP-GlcpNAc::undecaprenylphosphate GlcpNAc-1-phosphate transferase) catalyzes the formation of the lipid-linked biosynthetic intermediate to which galactosyl residues are transferred during the initial steps of D-galactan I synthesis. The rfbFKpO1 gene comprises 1,131 nucleotides, and the predicted polypeptide consists of 373 amino acid residues with a predicted M(r) of 42,600. A polypeptide with an M(r) of 42,000 was evident in sodium dodecyl sulfate-polyacrylamide gels when rfbKpO1 was expressed behind the T7 promoter. The carboxy-terminal region of RfbFKpO1 shares similarity with the carboxy terminus of RfpB, a galactopyranosyl transferase which is involved in the synthesis of the type 1 O antigen of Shigella dysenteriae.     

Refined structure of the lipophosphoglycan of Leishmania donovani.

The primary structure of the major surface glycoconjugate of Leishmania donovani parasites, a lipophosphoglycan, has been further characterized. The repeating PO4-6Galp beta 1-4Man disaccharide units, which are a salient feature of the molecule, are shown to terminate with one of several neutral structures, the most abundant of which is the branched trisaccharide Galp beta 1-4(Manp alpha 1-2)Man. The phosphosaccharide core of lipophosphoglycan, which links the disaccharide repeats to a lipid anchor, contains 2 phosphate residues. One of the core phosphates has previously been localized on O-6 of the galactosyl residue distal to the lipid anchor; the second phosphate is now shown to be on O-6 of the mannosyl residue distal to the anchor and to bear an alpha-linked glucopyranosyl residue. Also, the anomeric configuration of the unusual 3-substituted Galf residue in the phosphosaccharide core is established as beta. The complete structure of the core is thus PO4-6Galp alpha 1-6Galp alpha 1-3Galf beta 1-3[Glcp alpha 1-PO4-6]Manp alpha 1-3Manp alpha 1-4GlcN alpha 1-. This further clarification of the structure of lipophosphoglycan may prove beneficial in determining the structure-function relationships of this highly unusual glycoconjugate.     

A Membrane-located Glycosyltransferase Complex Required for Biosynthesis of the d-Galactan I Lipopolysaccharide O Antigen in Klebsiella pneumoniae

d-Galactan I is a polysaccharide with the disaccharide repeat unit structure [→3-β-d-Galf-(1→3)-α-d-Galp-(1→]. This glycan represents the lipopolysaccharide O antigen found in many Gram-negative bacteria, including several Klebsiella pneumoniae O serotypes. The polysaccharide is synthesized in the cytoplasm prior to its export via an ATP-binding cassette transporter. Sequence analysis predicts three galactosyltransferases in the d-galactan I genetic locus. They are WbbO (belonging to glycosyltransferase (GT) family 4), WbbM (GT-family 8), and WbbN (GT-family 2). The WbbO and WbbM proteins are each predicted to contain two domains, with the GT modules located toward their C termini. The N-terminal domains of WbbO and WbbM exhibit no similarity to proteins with known function. In vivo complementation assays suggest that all three glycosyltransferases are required for d-galactan I biosynthesis. Using a bacterial two-hybrid system and confirmatory co-purification strategies, evidence is provided for protein-protein interactions among the glycosyltransferases, creating a membrane-located enzyme complex dedicated to d-galactan I biosynthesis.     

Rat liver Golgi galactosyltransferases. Distinct enzymes for glycolipid and glycoprotein acceptor substrates.

Two enzymes that catalyse the transfer of galactose from UDP-galactose to GM2 ganglioside were partially purified from rat liver Golgi membranes. These preparations, designated enzyme I (basic) and enzyme II (acidic), utilized as acceptors GM2 ganglioside and asialo GM2 ganglioside as well as ovalbumin, desialodegalactofetuin, desialodegalacto-orosomucoid, desialo bovine submaxillary mucin and GM2 oligosaccharide. Enzyme II catalysed disaccharide synthesis in the presence of the monosaccharide acceptors N-acetylglucosamine and N-acetylgalactosamine. The affinity adsorbent alpha-lactalbumin-agarose, which did not retard GM2 ganglioside galactosyltransferase, was used to remove most or all of galactosyltransferase activity towards glycoprotein and monosaccharide acceptors from the extracted Golgi preparation. After treatment of the extracted Golgi preparation with alpha-lactalbumin-agarose, enzyme I and enzyme II GM2 ganglioside galactosyltransferase activities, prepared by using DEAE-Sepharose chromatography, were distinguishable from transferase activity towards GM2 oligosaccharide and glycoproteins by the criterion of thermolability. This residual galactosyltransferase activity towards glycoprotein substrates was also shown to be distinct from GM2 ganglioside galactosyltransferase in both enzyme preparations I and II by the absence of competition between the two acceptor substrates. The two types of transferase activities could be further distinguished by their response to the presence of the protein effector alpha-lactalbumin. GM2 ganglioside galactosyltransferase was stimulated in the presence of alpha-lactalbumin, whereas the transferase activity towards desialodegalactofetuin was inhibited in the presence of this protein. The results of purification studies, comparison of thermolability properties and competition analysis suggested the presence of a minimum of five galactosyltransferase species in the Golgi extract. Five peaks of galactosyltransferase activity were resolved by isoelectric focusing. Two of these peaks (pI 8.6 and 6.3) catalysed transfer of galactose to GM2 ganglioside, and three peaks (pI 8.1, 6.8 and 6.3) catalysed transfer to glycoprotein acceptors.     

Synthesis of four novel trisaccharides by induction of loose acceptor specificity in Gal beta 1----4 transferase (EC 2.4.1.22): Galp(beta 1----4)Glcp(X)Glc where X = beta 1----3: beta 1----4: beta 1----6: alpha 1----4.

Development of tandem mass spectral methods for direct linkage determination in oligosaccharides requires sets of trisaccharides differing only in one structural parameter. In this case, we chose the position of linkage to the reducing-end hexose. These sets of compounds would also be useful for the development of high-resolution separation techniques geared to resolve linkage types. Conventional organic synthesis of such a set could take as long as 2-5 months for each member of the set. Each trisaccharide would require 10-20 steps of synthesis. Instead, we utilized low pH to induce a loose acceptor specificity for bovine milk galactosyltransferase (lactose synthase: EC 2.4.1.22) and by this method, within 2 weeks, generated four novel oligosaccharides for NMR and mass spectral studies. The disaccharides cellobiose (beta 1----4), laminaribiose (beta 1----3), gentiobiose (beta 1----6) and maltose (alpha 1----4) acted as acceptors for EC 2.4.1.22 under these conditions. The beta 1----2-linked disaccharide, sophorose, was not commercially available and is not included in this study. The alpha-linked disaccharides were also examined, but except for the alpha 1----4 disaccharide maltose, were very poor acceptors under a variety of conditions. From these four acceptors, the following four novel trisaccharides were synthesized in micromole amounts, suitable for studies of linkage position using low-energy collision-induced-dissociation tandem mass spectrometry (FAB-MS-CID-MS), and for NMR: Galp(beta 1----4)Glcp(beta 1----3)-Glc, Galp(beta 1----4)Glcp(beta 1----4)Glc, Galp(beta 1----4)Glcp(beta 1----6)-Glc and Galp(beta 1----4)Glcp(alpha 1----4)Glc.     

Coordination of Polymerization, Chain Termination, and Export in Assembly of the Escherichia coli Lipopolysaccharide O9a Antigen in an ATP-binding Cassette Transporter-dependent Pathway

The Escherichia coli O9a O-polysaccharide (O-PS) is a prototype for O-PS synthesis and export by the ATP-binding cassette transporter-dependent pathway. Comparable systems are widespread in Gram-negative bacteria. The polymannose O9a O-PS is assembled on a polyisoprenoid lipid intermediate by mannosyltransferases located at the cytoplasmic membrane, and the final polysaccharide chain length is determined by the chain terminating dual kinase/methyltransferase, WbdD. The WbdD protein is tethered to the membrane via a C-terminal region containing amphipathic helices located between residues 601 and 669. Here, we establish that the C-terminal domain of WbdD plays an additional pivotal role in assembly of the O-PS by forming a complex with the chain-extending mannosyltransferase, WbdA. Membrane preparations from a ΔwbdD mutant had severely diminished mannosyltransferase activity in vitro, and no significant amounts of the WbdA protein are targeted to the membrane fraction. Expression of a polypeptide comprising the WbdD C-terminal region was sufficient to restore both proper localization of WbdA and mannosyltransferase activity. In contrast to WbdA, the other required mannosyltransferases (WbdBC) are targeted to the membrane independent of WbdD. A bacterial two-hybrid system confirmed the interaction of WbdD and WbdA and identified two regions in the C terminus of WbdD that contributed to the interaction. Therefore, in the O9a assembly export system, the WbdD protein orchestrates the critical localization and coordination of activities involved in O-PS chain extension and termination at the cytoplasmic membrane.Lipopolysaccharide (LPS)³ is a glycolipid unique to the outer membranes of Gram-negative bacteria. LPS has three structural domains in most bacteria (1). Hydrophobic lipid A is a major component of the outer leaflet of the outer membrane. A short core oligosaccharide (OS) serves as a linker between lipid A and a repeat unit polymer termed the O-polysaccharide (O-PS; O-antigen). The structure of lipid A is conserved among Gram-negative bacteria, whereas limited variability is observed among the core OSs of a given species. For example, five closely related core oligosaccharides have been described for Escherichia coli (2). In contrast, the O-PS structures vary extensively within species. O-PS structural variations include differences in the number and type of sugars in the repeat unit and the nature of the glycosidic linkages within and between repeat units. O-PS variations provide the basis for the O-antigen serotyping system, and there are over 180 O-antigen serogroups proposed for E. coli (3, 4).Lipid A-core OS and O-PS are synthesized independently at the cytoplasmic membrane and are subsequently linked together in the periplasm (reviewed in Ref. 1). O-PS assembly is initiated by transfer of a sugar-1-phosphate from a nucleotide sugar precursor to the 55-carbon lipid acceptor, undecaprenol phosphate. In the majority of E. coli serotypes, the initiating reaction is performed by the GlcNAc:Und-P GlcNAc-1-P transferase, WecA (5, 6). WecA is an integral membrane protein and is also essential for initiating synthesis of the enterobacterial common antigen (7). In E. coli, elongation and export of the undecaprenol-PP-linked intermediate proceeds through one of two fundamentally different O-PS assembly pathways. These pathways have been termed Wzy (polymerase)-dependent and ATP-binding cassette (ABC) transporter-dependent biosynthesis, respectively (reviewed in Ref. 1). In Wzy-dependent O-PS biosynthesis, single repeat units are assembled on the undecaprenol-PP-linked intermediate at the cytoplasmic face of the inner membrane. The lipid-linked repeat units are subsequently reoriented to the periplasm where they are assembled into polysaccharide by a process involving Wzy and a chain length regulator, Wzz. In contrast, in the ABC transporter-dependent pathway, the O-PS is elongated on the undecaprenol-PP-linked intermediate in the cytoplasm by sequential glycosyl transfer. Depending on the system, chain extension is terminated by the addition of a nonreducing terminal residue or by interaction with the ABC transporter (8). Full-length O-PS chains are then translocated across the inner membrane by the ABC transporter. The two O-PS assembly pathways converge at a ligation reaction, which transfers the O-PS from undecaprenol-PP to lipid A-core OS at the periplasmic face of the inner membrane. Once assembled, LPS molecules are shuttled to the outer membrane through a process involving the LptABCDE complex (reviewed in Ref. 9).The polymannose O-PS of E. coli O9a provides a model system for ABC transporter-dependent O-PS biosynthesis. The E. coli O9a PS biosynthesis gene cluster (see Fig. 1A) encodes three GDP-mannose-dependent mannosyltransferases (WbdA, WbdB, and WbdC) that assemble the O-PS on undecaprenol-PP-GlcNAc (10). Structural studies identify terminal capping residues in a number of O-PSs synthesized by the ABC transporter-dependent pathway (11). It has been proposed that the addition of a capping residue to the nonreducing end of the undecaprenol-PP-linked PS serves to regulate O-PS chain length by terminating elongation. In the case of the O9a PS, termination involves methylation and phosphorylation. The chain length of the O9a PS is strictly controlled by the activity of WbdD, and O-PS-substituted LPS molecules expressed on the cell surface exhibit a narrow size distribution. The E. coli O9a WbdD protein contains putative kinase and methyltransferase domains, and these activities have been confirmed in biochemical studies (12). In addition to the role in O-PS chain regulation, methyl and/or phosphoryl modification is required for binding of the O9a PS to the nucleotide-binding component (Wzt) of the ABC transporter (13, 14), a crucial initial step in O-PS export. Unmodified O9a PS does not bind to Wzt, and a wbdD mutant accumulates unmodified polysaccharide in the cytoplasm.Open in a separate windowFIGURE 1.Structure and biosynthesis of the E. coli O9a PS and schematic showing WbdD and mutant derivatives. A, the structure of the O9a PS shows the adaptor region, repeat unit, and terminating residues. The nonreducing end of the O-PS is capped by methylation and phosphorylation, but the nature of the linkage between capping residues and the repeat unit is unknown (11, 12). The O9a-PS biosynthesis and export genes are shown together with the functions of the encoded proteins. B, a linear representation of the wild-type WbdD protein from CWG634 is shown in context with the genomic wbdD mutations in CWG635 and CWG900. The methyltransferase (MTase) and kinase domains are shown within WbdD and have been described previously (12). In CWG635, the chromosomal wbdD ORF was disrupted by replacing a 500-bp SmaI restriction fragment with the aacC1 cassette. A potential ribosomal-binding site, initiation codon, and stop codon are shown and together define an ORF encoding amino acids 501–708 of WbdD. In CWG900, the entire wbdD ORF has been removed from the chromosome. C, a schematic of the truncated WbdD polypeptide derivatives encoded by plasmids used in this study. The numbers shown above the polypeptides refer to amino acid positions in the native WbdD protein. Each polypeptide contained either an N-terminal His₆ tag or the T25 fragment of B. pertussis adenylate cyclase (see plasmids in 15, 16). However, the variability of specific assembly proteins among different biosynthetic systems precludes development of a generalized model for a polysaccharide assembly complex. Here we present data revealing the mechanisms that target the O9a mannosyltransferases to the cytoplasmic membrane and identify essential protein-protein interactions within the biosynthesis complex.     

Polymerization of mycobacterial arabinogalactan and ligation to peptidoglycan

The cell wall of Mycobacterium spp. consists predominately of arabinogalactan chains linked at the reducing ends to peptidoglycan via a P-GlcNAc-(alpha1-3)-Rha linkage unit (LU) and esterified to a variety of mycolic acids at the nonreducing ends. Several aspects of the biosynthesis of this complex have been defined, including the initial formation of the LU on a polyprenyl phosphate (Pol-P) molecule followed by the sequential addition of galactofuranosyl (Galf) units to generate Pol-P-P-LU-(Galf)1,2,3, etc. and Pol-P-P-LU-galactan, catalyzed by a bifunctional galactosyltransferase (Rv3808c) capable of adding alternating 5- and 6-linked Galf units. By applying cell-free extracts of Mycobacterium smegmatis, containing cell wall and membrane fragments, and differential labeling with UDP-[14C]Galp and recombinant UDP-Galp mutase as the source of [14C]Galf for galactan biosynthesis and 5-P-[14C]ribosyl-P-P as a donor of [14C]Araf for arabinan synthesis, we now demonstrate sequential synthesis of the simpler Pol-P-P-LU-(Galf)n glycolipid intermediates followed by the Pol-P-P-LU-arabinogalactan and, finally, ligation of the P-LU-arabinogalactan to peptidoglycan. This first time demonstration of in vitro ligation of newly synthesized P-LU-arabinogalactan to newly synthesized peptidoglycan is a necessary forerunner to defining the genetics and enzymology of cell wall polymer-peptidoglycan ligation in Mycobacterium spp. and examining this step as a target for new antibacterial drugs.     

Molecular characterisation of a membrane-bound galactosyltransferase of plant cell wall matrix polysaccharide biosynthesis

Galactomannan biosynthesis in vitro is catalysed by membrane preparations from developing fenugreek seed endosperms. Two enzymes interact: a GDP-mannose dependent (1-->4)-beta-D-mannan synthase and a UDP-galactose dependent (1-->6)-alpha-D-galactosyltransferase. The statistical distribution of galactosyl substituents along the mannan backbone, and the degree of galactose substitution of the primary product of galactomannan biosynthesis appear to be regulated by the specificity of the galactosyltransferase. We now report the detergent solubilisation of the fenugreek galactosyltransferase with retention of activity, the identification on gels of a putative 51 kDa galactosyltransferase protein, and the isolation, cloning and sequencing of the corresponding cDNA. The solubilised galactosyltransferase has an absolute requirement for added acceptor substrates. Beta-(1-->4)-linked D-manno-oligosaccharides with chain lengths greater than or equal to 5 acted as acceptors, as did galactomannans of low to medium galactose-substitution. The putative galactosyltransferase cDNA encodes a 51282 Da protein, with a single transmembrane alpha helix near the N terminus. We have also confirmed the identity of the galactosyltransferase by inserting the cDNA in frame into the genome of the methylotrophic yeast Pichia pastoris under the control of an AOX promoter and the yeast alpha secretion factor and observing the secretion of galactomannan alpha-galactosyltransferase activity. Particularly high activities were observed when a truncated sequence, lacking the membrane-spanning helix, was expressed.     

Functional expression of enterobacterial O-polysaccharide biosynthesis enzymes in Bacillus subtilis

The expression of heterologous bacterial glycosyltransferases is of interest for potential application in the emerging field of carbohydrate engineering in gram-positive organisms. To assess the feasibility of using enzymes from gram-negative bacteria, the functional expression of the genes wbaP (formerly rfbP), wecA (formerly rfe), and wbbO (formerly rfbF) from enterobacterial lipopolysaccharide O-polysaccharide biosynthesis pathways was examined in Bacillus subtilis. WbaP and WecA are initiation enzymes for O-polysaccharide formation, catalyzing the transfer of galactosyl 1-phosphate from UDP-galactose and N-acetylglucosaminyl 1-phosphate from UDP-N-acetylglucosamine, respectively, to undecaprenylphosphate. The WecA product (undecaprenylpyrophosphoryl GlcNAc) is used as an acceptor to which the bifunctional wbbO gene product sequentially adds a galactopyranose and a galactofuranose residue from the corresponding UDP sugars to form a lipid-linked trisaccharide. Genes were cloned into the shuttle vectors pRB374 and pAW10. In B. subtilis hosts, the genes were effectively transcribed under the vegII promoter control of pRB374, but the plasmids were susceptible to rearrangements and deletion. In contrast, pAW10-based constructs, in which genes were cloned downstream of the tet resistance cassette, were stable but yielded lower levels of enzyme activity. In vitro glycosyltransferase assays were performed in Escherichia coli and B. subtilis, using membrane preparations as sources of enzymes and endogenous undecaprenylphosphate as an acceptor. Incorporation of radioactivity from UDP-alpha-D-(14)C-sugar into reaction products verified the functionality of WbaP, WecA, and WbbO in either host. Enzyme activities in B. subtilis varied between 20 and 75% of those measured in E. coli.     

Galactan biosynthesis in Mycobacterium tuberculosis. Identification of a bifunctional UDP-galactofuranosyltransferase

The cell wall of Mycobacterium tuberculosis and related genera is unique among prokaryotes, consisting of a covalently bound complex of mycolic acids, D-arabinan and D-galactan, which is linked to peptidoglycan via a special linkage unit consisting of Rhap-(1-->3)-GlcNAc-P. Information concerning the biosynthesis of this entire polymer is now emerging with the promise of new drug targets against tuberculosis. Accordingly, we have developed a galactosyltransferase assay that utilizes the disaccharide neoglycolipid acceptors beta-d-Galf-(1-->5)-beta-D-Galf-O-C(10:1) and beta-D-Galf-(1-->6)-beta-D-Galf-O-C(10:1), with UDP-Gal in conjunction with isolated membranes. Chemical analysis of the subsequent reaction products established that the enzymatically synthesized products contained both beta-D-Galf linkages ((1-->5) and (1-->6)) found within the mycobacterial cell, as well as in an alternating (1-->5) and (1-->6) fashion consistent with the established structure of the cell wall. Furthermore, through a detailed examination of the M. tuberculosis genome, we have shown that the gene product of Rv3808c, now termed glfT, is a novel UDP-galactofuranosyltransferase. This enzyme possesses dual functionality in performing both (1-->5) and (1-->6) galactofuranosyltransferase reactions with the above neoglycolipid acceptors, using membranes isolated from the heterologous host Escherichia coli expressing Rv3808c. Thus, at a biochemical and genetic level, the polymerization of the galactan region of the mycolyl-arabinogalactan complex has been defined, allowing the possibility of further studies toward substrate recognition and catalysis and assay development. Ultimately, this may also lead to a more rational approach to drug design to be explored in the context of mycobacterial infections.     

Functional characterization of the initiation enzyme of S-layer glycoprotein glycan biosynthesis in Geobacillus stearothermophilus NRS 2004/3a

The glycan chain of the S-layer glycoprotein of Geobacillus stearothermophilus NRS 2004/3a is composed of repeating units [-->2)-alpha-l-Rhap-(1-->3)-beta-l-Rhap-(1-->2)-alpha-l-Rhap-(1-->], with a 2-O-methyl modification of the terminal trisaccharide at the nonreducing end of the glycan chain, a core saccharide composed of two or three alpha-l-rhamnose residues, and a beta-d-galactose residue as a linker to the S-layer protein. In this study, we report the biochemical characterization of WsaP of the S-layer glycosylation gene cluster as a UDP-Gal:phosphoryl-polyprenol Gal-1-phosphate transferase that primes the S-layer glycoprotein glycan biosynthesis of Geobacillus stearothermophilus NRS 2004/3a. Our results demonstrate that the enzyme transfers in vitro a galactose-1-phosphate from UDP-galactose to endogenous phosphoryl-polyprenol and that the C-terminal half of WsaP carries the galactosyltransferase function, as already observed for the UDP-Gal:phosphoryl-polyprenol Gal-1-phosphate transferase WbaP from Salmonella enterica. To confirm the function of the enzyme, we show that WsaP is capable of reconstituting polysaccharide biosynthesis in WbaP-deficient strains of Escherichia coli and Salmonella enterica serovar Typhimurium.     

Transfer specificity of detergent-solubilized fenugreek galactomannan galactosyltransferase

The current experimental model for galactomannan biosynthesis in membrane-bound enzyme systems from developing legume-seed endosperms involves functional interaction between a GDP-mannose (Man) mannan synthase and a UDP-galactose (Gal) galactosyltransferase. The transfer specificity of the galactosyltransferase to the elongating mannan chain is critical in regulating the distribution and the degree of Gal substitution of the mannan backbone of the primary biosynthetic product. Detergent solubilization of the galactosyltransferase of fenugreek (Trigonella foenum-graecum) with retention of activity permitted the partial purification of the enzyme and the cloning and sequencing of the corresponding cDNA with proof of functional identity. We now document the positional specificity of transfer of ((14)C)Gal from UDP-((14)C)Gal to manno-oligosaccharide acceptors, chain lengths 5 to 8, catalyzed by the detergent-solubilized galactosyltransferase. Enzymatic fragmentation analyses of the labeled products showed that a single Gal residue was transferred per acceptor molecule, that the linkage was (1-->6)-alpha, and that there was transfer to alternative Man residues within the acceptor molecules. Analysis of the relative frequencies of transfer to alternative Man residues within acceptor oligosaccharides of different chain length allowed the deduction of the substrate subsite recognition requirement of the galactosyltransferase. The enzyme has a principal recognition sequence of six Man residues, with transfer of Gal to the third Man residue from the nonreducing end of the sequence. These observations are incorporated into a refined model for enzyme interaction in galactomannan biosynthesis.     

Biosynthesis of a substituted cellulose from a mutant strain of Xanthomonas campestris

In Xanthomonas campestris the genes involved in polysaccharide (xanthan) biosynthesis are located in a gene cluster (gum) of 16 kb. A Tn5 insertion mutant with a reduced slimy phenotype has been characterized. This mutant failed to produce the pentasaccharide repeating-unit of xanthan. Only three sugars were transferred to the prenyl phosphate intermediate. Several lines of evidence suggested that the lipid-associated saccharide was the trisaccharide reducing end of the pentasaccharide from the wild-type strain. This trisaccharide was built up from UDP-Glc and GDP-Man, and a glucose residue was at the reducing end, linked to an allylic prenol through a diphosphate bridge. Results from one- or two-stage reactions showed that the trisaccharide-P-P-polyprenol was the precursor of the polymer. This new polymer, a polytrisaccharide, was detected also in vivo. The transposon responsible for the mutation was located within gumK gene. Therefore, this gene encodes for the glycosyltransferase IV, which catalyses the transfer of glucuronic acid to the lipid-linked beta-D-Manp-(1-->3)-beta-D-Glcp-(1-->4)-beta-D-Glcp trisaccharide. A recombinant plasmid with the whole gum cluster restored the wild type phenotype.     

Production of recombinant xenotransplantation antigen in Escherichia coli

The synthesis of sufficient amounts of oligosaccharides is the bottleneck for the study of their biological function and their possible use as drug. As an alternative for chemical synthesis, we propose to use Escherichia coli as a "living factory." We have addressed the production of the Galp alpha(1-3)Galp beta(1-4)GlcNAc epitope, the major porcine antigen responsible for xenograft rejection. An E. coli strain was generated which simultaneously expresses NodC (to provide the chitin-pentaose acceptor), beta(1-4) galactosyltransferase LgtB, and bovine alpha(1-3) galactosyltransferase GstA. This strain produced 0.68 g/L of the heptasaccharide Galp alpha(1-3)Galp beta(1-4)(GlcNAc)(5), which harbours the xenoantigen at its non-reducing end, establishing the feasibility of this approach.     

Analogues of the mycobacterial arabinogalactan linkage disaccharide as cell wall biosynthesis inhibitors

The mycobacterial arabinogalactan linkage disaccharide [alpha-L-Rha-(1-->3)-alpha-D-GlcNAc] provides a basis for the design of new antitubercular drugs, since it supports a key skeletal structure in the bacterial cell wall. A series of analogues of the linker was synthesized by coupling appropriate thiorhamnosyl donors modified at their 4-positions, with an N-acetyl glucosamine acceptor. In a cell-free enzyme inhibition assay, three analogues inhibited the activity of the galactosyltransferase that adds a Galf residue to the linkage disaccharide. Although the compounds were modest inhibitors, these data confirm the viability of this approach to anti-mycobacterial agents. It is especially significant that the three effective compounds are modified at the site of the acceptor atom in the natural substrate.     

Exopolysaccharide Biosynthesis in Lactococcus lactis NIZO B40: Functional Analysis of the Glycosyltransferase Genes Involved in Synthesis of the Polysaccharide Backbone

We used homologous and heterologous expression of the glycosyltransferase genes of the Lactococcus lactis NIZO B40 eps gene cluster to determine the activity and substrate specificities of the encoded enzymes and established the order of assembly of the trisaccharide backbone of the exopolysaccharide repeating unit. EpsD links glucose-1-phosphate from UDP-glucose to a lipid carrier, EpsE and EpsF link glucose from UDP-glucose to lipid-linked glucose, and EpsG links galactose from UDP-galactose to lipid-linked cellobiose. Furthermore, EpsJ appeared to be involved in EPS biosynthesis as a galactosyl phosphotransferase or an enzyme which releases the backbone oligosaccharide from the lipid carrier.     

Structural identification of the lipopolysaccharide O-antigen produced by Yersinia enterocolitica serotype O:28.

The structure of the O-antigenic polysaccharide (O-PS) component of the lipopolysaccharide produced by Yersinia enterocolitica serotype O:28 has been elucidated. From chemical methods involving glycose analysis, periodate oxidation, methylation and the use of one- and two-dimensional NMR spectroscopy, the O-PS was found to be a polymer of repeating branched hexasaccharide units composed of L-rhamnose (four parts), 2-acetamido-2-deoxy-D-glucose (one part), and 2-acetamido-2-deoxy-D-galacturonic acid (one part) having the following structure:     

Structures of the glycoinositolphospholipids from Leishmania major. A family of novel galactofuranose-containing glycolipids

Structures of the major glycolipids isolated from the protozoan parasite Leishmania major (strains V121 and LRC-L119), were elucidated by fast atom bombardment-mass spectrometry, two-dimensional proton NMR, methylation analysis, exoglycosidase digestions and mild acid hydrolysis. These glycolipids belong to a family of glycoinositolphospholipids (GIPLs), which contain 4-6 saccharide residues linked to alkylacylphosphatidylinositol (alkylacyl-PI) or lyso alkyl-PI. The general structure of the elucidated GIPLs can be expressed as follows: R-3Galf(alpha 1-3)Manp(alpha 1-3)Manp(alpha 1-4)GlcNp(alpha 1-6) alkylacyl-PI or lyso alkyl-PI where R = OH for GIPL-1; R = Galp(alpha 1- for GIPL-2; R = Galp(alpha 1-6)Galp (alpha 1- for GIPL-3 and R = Galp(alpha 1-3)Galf(alpha 1- for GIPL-A. The alkylacyl-PI lipid moieties are unusual in containing predominantly 18:0, 22:0, 24:0, or 26:0 alkyl chains and 12:0, 14:0, or 16:0 acyl chains. Remodeling of the lipid moieties may occur based on the finding that 1) lyso derivatives account for approximately 35% of the GIPL-3 fraction in strain V121 and 2) there is an increase in the proportion of 24:0 and 26:0 alkyl chains with elongation of the carbohydrate chain. Together with the elucidated structures, these properties are consistent with some of the GIPLs having a role as biosynthetic precursors to the major cell surface glycoconjugate, lipophosphoglycan. In particular, the saccharide sequences of GIPL-3, lyso-GIPL-3, and the glycan core of lipophosphoglycan (Turco, S. J., Orlandi, P. A., Homans, S. W., Ferguson, M. A. J., Dwek, R. A., and Rademacher, T. W. (1989) J. Biol. Chem. 264, 6711-6715) are identical. Finally, immunostaining of thin layer chromatograms with antibodies from patients with cutaneous leishmaniasis suggests that the major GIPLs are highly immunogenic and that the elevated anti-Gal antibodies, commonly seen in leishmaniasis patients, may be directed against terminal Galp(alpha 1-3)Galf residues.