Identification of galactofuranose antigens such as galactomannoproteins and fungal-type galactomannan from the yellow koji fungus (Aspergillus oryzae)

Filamentous fungi belonging to the genus Aspergillus are known to possess galactomannan in their cell walls. Galactomannan is highly antigenic to humans and has been reported to be involved in the pathogenicity of pathogenic filamentous fungi, such as A. fumigatus, and in immune responses. In this study, we aimed to confirm the presence of D-galactofuranose-containing glycans and to clarify the biosynthesis of D-galactofuranose-containing glycans in Aspergillus oryzae, a yellow koji fungus. We found that the galactofuranose antigen is also present in A. oryzae. Deletion of ugmA, which encodes UDP-galactopyranose mutase in A. oryzae, suppressed mycelial elongation, suggesting that D-galactofuranose-containing glycans play an important role in cell wall integrity in A. oryzae. Proton nuclear magnetic resonance spectrometry revealed that the galactofuranose-containing sugar chain was deficient and that core mannan backbone structures were present in ΔugmA A. oryzae, indicating the presence of fungal-type galactomannan in the cell wall fraction of A. oryzae. The findings of this study provide new insights into the cell wall structure of A. oryzae, which is essential for the production of fermented foods in Japan.

Identification of an α-(16)-Mannosyltransferase Contributing To Biosynthesis of the Fungal-Type Galactomannan α-Core-Mannan Structure in Aspergillus fumigatus

Fungal-type galactomannan, a cell wall component of Aspergillus fumigatus, is composed of α-(1→2)-/α-(1→6)-linked mannan and β-(1→5)-/β-(1→6)-linked galactofuran side chains. Recently, CmsA and CmsB were identified as the α-(1→2)-mannosyltransferases involved in the biosynthesis of the α-core-mannan. However, the α-(1→6)-mannosyltransferase involved in the biosynthesis of the α-core-mannan has not been identified yet. In this study, we analyzed 9 putative α-(1→6)-mannosyltransferase gene disruption strains of A. fumigatus. The ΔanpA strain resulted in decreased mycelial elongation and reduced conidia formation. Proton nuclear magnetic resonance analysis revealed that the ΔanpA strain failed to produce the α-core-mannan of fungal-type galactomannan. We also found that recombinant AnpA exhibited much stronger α-(1→6)-mannosyltransferase activity toward α-(1→2)-mannobiose than α-(1→6)-mannobiose in vitro. Molecular simulations corroborated the fact that AnpA has a structure that can recognize the donor and acceptor substrates suitable for α-(1→6)-mannoside bond formation and that its catalytic activity would be specific for the elongation of the α-core-mannan structure in vivo. The identified AnpA is similar to Anp1p, which is involved in the elongation of the N-glycan outer chain in budding yeast, but the building sugar chain structure is different. The difference was attributed to the difference in substrate recognition of AnpA, which was clarified by simulations based on protein conformation. Thus, even proteins that seem to be functionally identical due to amino acid sequence similarity may be glycosyltransferase enzymes that make different glycans upon detailed analysis. This study describes an example of such a case. IMPORTANCE Fungal-type galactomannan is a polysaccharide incorporated into the cell wall of filamentous fungi belonging to the subphylum Pezizomycotina. Biosynthetic enzymes of fungal-type galactomannan are potential targets for antifungal drugs and agrochemicals. In this study, we identified an α-(1→6)-mannosyltransferase responsible for the biosynthesis of the α-core-mannan of fungal-type galactomannan, which has not been known for a long time. The findings of this study shed light on processes that shape this cellular structure while identifying a key enzyme essential for the biosynthesis of fungal-type galactomannan.

Structure and Anticoagulant Activity of a Galactofuranose-Containing Sulfated Polysaccharide from the Green Seaweed, Codium isthmocladum

A water-soluble sulfated polysaccharide, F2-1, was obtained from the marine green alga, Codium isthmocladum, using ion-exchange and size-exclusion chromatography. Structure analysis showed that the F2-1 was a sulfated arabinan comprising Ara, Rha, Man, Gal, and Xyl with an 18% sulfate content and a molecular weight of 100 kDa. Methylation analysis combined with desulfation, GC-MS, IR, and NMR spectroscopy showed that the backbone of F2-1 was →4)-β-L-Arap(1→ residue. Its 2-O and/or 3-O positions showed sulfate modification; additionally, the 2-O or 3-O position showed branch points. The side chains were composed of →5)-β-D-Galf, (1→2,6)-β-D-Galf(1→, (1→2)-β-L-Rhap4S, →4)-α-D-Glcp(1→, and terminal α-D-Galp(1→ and β-D-Xylp(1→. Polysaccharides containing β-D-galactofuranose are rarely found in seaweed. F2-1 exhibited significant anticoagulant activity in vitro. Our findings suggested that the green-tide alga, Codium isthmocladum, can be considered as a useful resource for bioactive polysaccharides.

Specific Recognition of β-Galactofuranose-Containing Glycans of Synthetic Neoglycoproteins by Sera of Chronic Chagas Disease Patients

Chagas disease (CD) can be accurately diagnosed by detecting Trypanosoma cruzi in patients’ blood using polymerase chain reaction (PCR). However, parasite-derived biomarkers are of great interest for the serological diagnosis and early evaluation of chemotherapeutic efficacy when PCR may fail, owing to a blood parasite load below the method’s limit of detection. Previously, we focused on the detection of specific anti-α-galactopyranosyl (α-Gal) antibodies in chronic CD (CCD) patients elicited by α-Gal glycotopes copiously expressed on insect-derived and mammal-dwelling infective parasite stages. Nevertheless, these stages also abundantly express cell surface glycosylphosphatidylinositol (GPI)-anchored glycoproteins and glycoinositolphospholipids (GIPLs) bearing nonreducing terminal β-galactofuranosyl (β-Galf) residues, which are equally foreign to humans and, therefore, highly immunogenic. Here we report that CCD patients’ sera react specifically with synthetic β-Galf-containing glycans. We took a reversed immunoglycomics approach that entailed: (a) Synthesis of T. cruzi GIPL-derived Galfβ1,3Manpα-(CH₂)₃SH (glycan G29_SH) and Galfβ1,3Manpα1,2-[Galfβ1,3]Manpα-(CH₂)₃SH (glycan G32_SH); and (b) preparation of neoglycoproteins NGP29b and NGP32b, and their evaluation in a chemiluminescent immunoassay. Receiver-operating characteristic analysis revealed that NGP32b can distinguish CCD sera from sera of healthy individuals with 85.3% sensitivity and 100% specificity. This suggests that Galfβ1,3Manpα1,2-[Galfβ1,3]Manpα is an immunodominant glycotope and that NGP32b could potentially be used as a novel CCD biomarker.

Galactomannan Produced by Aspergillus fumigatus: An Update on the Structure, Biosynthesis and Biological Functions of an Emblematic Fungal Biomarker

The galactomannan (GM) that is produced by the human fungal pathogen Aspergillus fumigatus is an emblematic biomarker in medical mycology. The GM is composed of two monosaccharides: mannose and galactofuranose. The furanic configuration of galactose residues, absent in mammals, is responsible for the antigenicity of the GM and has favoured the development of ELISA tests to diagnose aspergillosis in immunocompromised patients. The GM that is produced by A. fumigatus is a unique fungal polysaccharide containing a tetramannoside repeat unit and having three different forms: (i) membrane bound through a glycosylphosphatidylinositol (GPI)-anchor, (ii) covalently linked to β-1,3-glucans in the cell wall, or (iii) released in the culture medium as a free polymer. Recent studies have revealed the crucial role of the GM during vegetative and polarized fungal growth. This review highlights these recent data on its biosynthetic pathway and its biological functions during the saprophytic and pathogenic life of this opportunistic human fungal pathogen.

The endogenous galactofuranosidase GlfH1 hydrolyzes mycobacterial arabinogalactan

Despite impressive progress made over the past 20 years in our understanding of mycolylarabinogalactan-peptidoglycan (mAGP) biogenesis, the mechanisms by which the tubercle bacillus Mycobacterium tuberculosis adapts its cell wall structure and composition to various environmental conditions, especially during infection, remain poorly understood. Being the central portion of the mAGP complex, arabinogalactan (AG) is believed to be the constituent of the mycobacterial cell envelope that undergoes the least structural changes, but no reports exist supporting this assumption. Herein, using recombinantly expressed mycobacterial protein, bioinformatics analyses, and kinetic and biochemical assays, we demonstrate that the AG can be remodeled by a mycobacterial endogenous enzyme. In particular, we found that the mycobacterial GlfH1 (Rv3096) protein exhibits exo-β-d-galactofuranose hydrolase activity and is capable of hydrolyzing the galactan chain of AG by recurrent cleavage of the terminal β-(1,5) and β-(1,6)-Galf linkages. The characterization of this galactosidase represents a first step toward understanding the remodeling of mycobacterial AG.

Biosynthesis of β-(1→5)-Galactofuranosyl Chains of Fungal-Type and O-Mannose-Type Galactomannans within the Invasive Pathogen Aspergillus fumigatus

β-(1→5)-Galactofuranosyl residues are widely distributed in the subphylum Pezizomycotina of the phylum Ascomycota. Pezizomycotina includes many plant and animal pathogens. Although the structure of β-(1→5)-galactofuranosyl residues of galactomannans in filamentous fungi was discovered long ago, it remains unclear which enzyme is responsible for biosynthesis of this glycan. Fungal cell wall formation processes are complicated, and information concerning glycosyltransferases is essential for understanding them. In this study, we showed that GfsA and GfsC are responsible for the biosynthesis of all β-(1→5)-galactofuranosyl residues of fungal-type and O-mannose-type galactomannans. The data presented here indicate that β-(1→5)-galactofuranosyl residues are involved in cell growth, conidiation, polarity, and cell surface hydrophobicity. Our new understanding of β-(1→5)-galactofuranosyl residue biosynthesis provides important novel insights into the formation of the complex cell wall structure and the virulence of the members of the subphylum Pezizomycotina.

The pathogenic fungus Aspergillus fumigatus contains galactomannans localized on the surface layer of its cell walls, which are involved in various biological processes. Galactomannans comprise α-(1→2)-/α-(1→6)-mannan and β-(1→5)-/β-(1→6)-galactofuranosyl chains. We previously revealed that GfsA is a β-galactofuranoside β-(1→5)-galactofuranosyltransferase involved in the biosynthesis of β-(1→5)-galactofuranosyl chains. In this study, we clarified the biosynthesis of β-(1→5)-galactofuranosyl chains in A. fumigatus. Two paralogs exist within A. fumigatus: GfsB and GfsC. We show that GfsB and GfsC, in addition to GfsA, are β-galactofuranoside β-(1→5)-galactofuranosyltransferases by biochemical and genetic analyses. GfsA, GfsB, and GfsC can synthesize β-(1→5)-galactofuranosyl oligomers at up to lengths of 7, 3, and 5 galactofuranoses within an established in vitro highly efficient assay of galactofuranosyltransferase activity. Structural analyses of galactomannans extracted from ΔgfsB, ΔgfsC, ΔgfsAC, and ΔgfsABC strains revealed that GfsA and GfsC synthesized all β-(1→5)-galactofuranosyl residues of fungal-type and O-mannose-type galactomannans and that GfsB exhibited limited function in A. fumigatus. The loss of β-(1→5)-galactofuranosyl residues decreased the hyphal growth rate and conidium formation ability and increased the abnormal hyphal branching structure and cell surface hydrophobicity, but this loss is dispensable for sensitivity to antifungal agents and virulence toward immunocompromised mice.

IMPORTANCE β-(1→5)-Galactofuranosyl residues are widely distributed in the subphylum Pezizomycotina of the phylum Ascomycota. Pezizomycotina includes many plant and animal pathogens. Although the structure of β-(1→5)-galactofuranosyl residues of galactomannans in filamentous fungi was discovered long ago, it remains unclear which enzyme is responsible for biosynthesis of this glycan. Fungal cell wall formation processes are complicated, and information concerning glycosyltransferases is essential for understanding them. In this study, we showed that GfsA and GfsC are responsible for the biosynthesis of all β-(1→5)-galactofuranosyl residues of fungal-type and O-mannose-type galactomannans. The data presented here indicate that β-(1→5)-galactofuranosyl residues are involved in cell growth, conidiation, polarity, and cell surface hydrophobicity. Our new understanding of β-(1→5)-galactofuranosyl residue biosynthesis provides important novel insights into the formation of the complex cell wall structure and the virulence of the members of the subphylum Pezizomycotina.

In vivo role of capsular polysaccharide in Mycoplasma mycoides

Capsular polysaccharides have been confirmed to be an important virulence trait in many gram-positive and gram-negative bacteria. Similarly, they are proposed to be virulence traits in minimal Mycoplasma that cause disease in humans and animals. In the current study, goats were infected with the caprine pathogen Mycoplasma mycoides subsp. capri or an engineered mutant lacking the capsular polysaccharide, galactofuranose. Goats infected with the mutant strain showed only transient fever. In contrast, 5 of 8 goats infected with the parental strain reached end-point criteria after infection. These findings confirm that galactofuranose is a virulence factor in M. mycoides.

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

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

Monoclonal Antibody AP3 Binds Galactomannan Antigens Displayed by the Pathogens Aspergillus flavus, A. fumigatus, and A. parasiticus

Aspergillus fumigatus and A. flavus are the fungal pathogens responsible for most cases of invasive aspergillosis (IA). Early detection of the circulating antigen galactomannan (GM) in serum allows the prompt application of effective antifungal therapy, thus improving the survival rate of IA patients. However, the use of monoclonal antibodies (mAbs) for the diagnosis of IA is often associated with false positives due to cross-reaction with bacterial polysaccharides. More specific antibodies are therefore needed. Here we describe the characterization of the Aspergillus-specific mAb AP3 (IgG1κ), including the precise identification of its corresponding antigen. The antibody was generated using A. parasiticus cell wall fragments and was shown to bind several Aspergillus species. Immunofluorescence microscopy revealed that AP3 binds a cell wall antigen, but immunoprecipitation and enzyme-linked immunosorbent assays showed that the antigen is also secreted into the culture medium. The inability of AP3 to bind the A. fumigatus galactofuranose (Galf )-deficient mutant ΔglfA confirmed that Galf residues are part of the epitope. Several lines of evidence strongly indicated that AP3 recognizes the Galf residues of O-linked glycans on Aspergillus proteins. Glycoarray analysis revealed that AP3 recognizes oligo-[β-D-Galf-1,5] sequences containing four or more residues with longer chains more efficiently. We also showed that AP3 captures GM in serum, suggesting it may be useful as a diagnostic tool for patients with IA.

Distribution of the O-acetyl groups and β-galactofuranose units in galactoxylomannans of the opportunistic fungus Cryptococcus neoformans

Galactoxylomannans (GalXMs) are a mixture of neutral and acidic capsular polysaccharides produced by the opportunistic fungus Cryptococcus neoformans that exhibit potent suppressive effects on the host immune system. Previous studies describing the chemical structure of C. neoformans GalXMs have reported species without O-acetyl substituents. Herein we describe that C. neoformans grown in capsule-inducing medium produces highly O-acetylated GalXMs. The location of the O-acetyl groups was determined by nuclear magnetic resonance (NMR) spectroscopy. In the neutral GalXM (NGalXM), 80% of 3-linked mannose (α-Manp) residues present in side chains are acetylated at the O-2 position. In the acidic GalXM also termed glucuronoxylomannogalactan (GXMGal), 85% of the 3-linked α-Manp residues are acetylated either in the O-2 (75%) or in the O-6 (25%) position, but O-acetyl groups are not present at both positions simultaneously. In addition, NMR spectroscopy and methylation analysis showed that β-galactofuranose (β-Galf) units are linked to O-2 and O-3 positions of nonbranched α-galactopyranose (α-Galp) units present in the GalXMs backbone chain. These findings highlight new structural features of C. neoformans GalXMs. Among these features, the high degree of O-acetylation is of particular interest, since O-acetyl group-containing polysaccharides are known to possess a range of immunobiological activities.

UDP-4-Keto-6-Deoxyglucose, a Transient Antifungal Metabolite, Weakens the Fungal Cell Wall Partly by Inhibition of UDP-Galactopyranose Mutase

Can accumulation of a normally transient metabolite affect fungal biology? UDP-4-keto-6-deoxyglucose (UDP-KDG) represents an intermediate stage in conversion of UDP-glucose to UDP-rhamnose. Normally, UDP-KDG is not detected in living cells, because it is quickly converted to UDP-rhamnose by the enzyme UDP-4-keto-6-deoxyglucose-3,5-epimerase/-4-reductase (ER). We previously found that deletion of the er gene in Botrytis cinerea resulted in accumulation of UDP-KDG to levels that were toxic to the fungus due to destabilization of the cell wall. Here we show that these negative effects are at least partly due to inhibition by UDP-KDG of the enzyme UDP-galactopyranose mutase (UGM), which reversibly converts UDP-galactopyranose (UDP-Galp) to UDP-galactofuranose (UDP-Galf). An enzymatic activity assay showed that UDP-KDG inhibits the B. cinerea UGM enzyme with a K_i of 221.9 µM. Deletion of the ugm gene resulted in strains with weakened cell walls and phenotypes that were similar to those of the er deletion strain, which accumulates UDP-KDG. Galf residue levels were completely abolished in the Δugm strain and reduced in the Δer strain, while overexpression of the ugm gene in the background of a Δer strain restored Galf levels and alleviated the phenotypes. Collectively, our results show that the antifungal activity of UDP-KDG is due to inhibition of UGM and possibly other nucleotide sugar-modifying enzymes and that the rhamnose metabolic pathway serves as a shunt that prevents accumulation of UDP-KDG to toxic levels. These findings, together with the fact that there is no Galf in mammals, support the possibility of developing UDP-KDG or its derivatives as antifungal drugs.IMPORTANCE Nucleotide sugars are donors for the sugars in fungal wall polymers. We showed that production of the minor sugar rhamnose is used primarily to neutralize the toxic intermediate compound UDP-KDG. This surprising finding highlights a completely new role for minor sugars and other secondary metabolites with undetermined function. Furthermore, the toxic potential of predicted transition metabolites that never accumulate in cells under natural conditions are highlighted. We demonstrate that UDP-KDG inhibits the UDP-galactopyranose mutase enzyme, thereby affecting production of Galf, which is one of the components of cell wall glycans. Given the structural similarity, UDP-KDG likely inhibits additional nucleotide sugar-utilizing enzymes, a hypothesis that is also supported by our findings. Our results suggest that UDP-KDG could serve as a template to develop antifungal drugs.

Galactofuranose antigens, a target for diagnosis of fungal infections in humans

The use of biomarkers for the detection of fungal infections is of interest to complement histopathological and culture methods. Since the production of antibodies in immunocompromised patients is scarce, detection of a specific antigen could be effective for early diagnosis. D-Galactofuranose (Galf) is the antigenic epitope in glycoconjugates of several pathogenic fungi. Since Galf is not biosynthesized by mammals, it is an attractive candidate for diagnosis of infection. A monoclonal antibody that recognizes Galf is commercialized for detection of aspergillosis. The linkage of Galf in the natural glycans and the chemical structures of the synthesized Galf-containing oligosaccharides are described in this paper. The oligosaccharides could be used for the synthesis of artificial carbohydrate-based antigens, not enough exploited for diagnosis.

D-Galactofuranose (Galf) is the unit in polysaccharides and glycoconjugates of several pathogenic fungi that is recognized by the immune system. Since Galf is not synthesized by mammals, it is an attractive candidate for diagnosis of infection. Since the production of antibodies in immunocompromised patients is scarce, detection of a specific antigen could be effective for early diagnosis. An antibody that recognizes Galf is commercialized for the detection of aspergillosis. Chemically synthesized Galf-containing oligosaccharides, reviewed in this paper, could therefore be used for the synthesis of artificial carbohydrate-based antigens and in diagnosis.

Comparing Galactan Biosynthesis in Mycobacterium tuberculosis and Corynebacterium diphtheriae

The suborder Corynebacterineae encompasses species like Corynebacterium glutamicum, which has been harnessed for industrial production of amino acids, as well as Corynebacterium diphtheriae and Mycobacterium tuberculosis, which cause devastating human diseases. A distinctive component of the Corynebacterineae cell envelope is the mycolyl-arabinogalactan (mAG) complex. The mAG is composed of lipid mycolic acids, and arabinofuranose (Araf) and galactofuranose (Galf) carbohydrate residues. Elucidating microbe-specific differences in mAG composition could advance biotechnological applications and lead to new antimicrobial targets. To this end, we compare and contrast galactan biosynthesis in C. diphtheriae and M. tuberculosis In each species, the galactan is constructed from uridine 5'-diphosphate-α-d-galactofuranose (UDP-Galf), which is generated by the enzyme UDP-galactopyranose mutase (UGM or Glf). UGM and the galactan are essential in M. tuberculosis, but their importance in Corynebacterium species was not known. We show that small molecule inhibitors of UGM impede C. glutamicum growth, suggesting that the galactan is critical in corynebacteria. Previous cell wall analysis data suggest the galactan polymer is longer in mycobacterial species than corynebacterial species. To explore the source of galactan length variation, a C. diphtheriae ortholog of the M. tuberculosis carbohydrate polymerase responsible for the bulk of galactan polymerization, GlfT2, was produced, and its catalytic activity was evaluated. The C. diphtheriae GlfT2 gave rise to shorter polysaccharides than those obtained with the M. tuberculosis GlfT2. These data suggest that GlfT2 alone can influence galactan length. Our results provide tools, both small molecule and genetic, for probing and perturbing the assembly of the Corynebacterineae cell envelope.

Carboxylate Surrogates Enhance the Antimycobacterial Activity of UDP-Galactopyranose Mutase Probes

Uridine diphosphate galactopyranose mutase (UGM also known as Glf) is a biosynthetic enzyme required for construction of the galactan, an essential mycobacterial cell envelope polysaccharide. Our group previously identified two distinct classes of UGM inhibitors; each possesses a carboxylate moiety that is crucial for potency yet likely detrimental for cell permeability. To enhance the antimycobacterial potency, we sought to replace the carboxylate with a functional group mimic-an N-acylsulfonamide group. We therefore synthesized a series of N-acylsulfonamide analogs and tested their ability to inhibit UGM. For each inhibitor scaffold tested, the N-acylsulfonamide group functions as an effective carboxylate surrogate. Although the carboxylates and their surrogates show similar activity against UGM in a test tube, several N-acylsulfonamide derivatives more effectively block the growth of Mycobacterium smegmatis. These data suggest that the replacement of a carboxylate with an N-acylsulfonamide group could serve as a general strategy to augment antimycobacterial activity.

Structures of Xenopus Embryonic Epidermal Lectin Reveal a Conserved Mechanism of Microbial Glycan Recognition

Intelectins (X-type lectins), broadly distributed throughout chordates, have been implicated in innate immunity. Xenopus laevis embryonic epidermal lectin (XEEL), an intelectin secreted into environmental water by the X. laevis embryo, is postulated to function as a defense against microbes. XEEL is homologous (64% identical) to human intelectin-1 (hIntL-1), which is also implicated in innate immune defense. We showed previously that hIntL-1 binds microbial glycans bearing exocyclic vicinal diol groups. It is unknown whether XEEL has the same ligand specificity. Also unclear is whether XEEL and hIntL-1 have similar quaternary structures, as XEEL lacks the corresponding cysteine residues in hIntL-1 that stabilize the disulfide-linked trimer. These observations prompted us to further characterize XEEL. We found that hIntL-1 and XEEL have similar structural features. Even without the corresponding intermolecular disulfide bonds present in hIntL-1, the carbohydrate recognition domain of XEEL (XEELCRD) forms a stable trimer in solution. The structure of XEELCRD in complex with d-glycerol-1-phosphate, a residue present in microbe-specific glycans, indicated that the exocyclic vicinal diol coordinates to a protein-bound calcium ion. This ligand-binding mode is conserved between XEEL and hIntL-1. The domain architecture of full-length XEEL is reminiscent of a barbell, with two sets of three glycan-binding sites oriented in opposite directions. This orientation is consistent with our observation that XEEL can promote the agglutination of specific serotypes of Streptococcus pneumoniae. These data support a role for XEEL in innate immunity, and they highlight structural and functional conservation of X-type lectins among chordates.

GfsA encodes a novel galactofuranosyltransferase involved in biosynthesis of galactofuranose antigen of O-glycan in Aspergillus nidulans and Aspergillus fumigatus

The cells walls of filamentous fungi in the genus Aspergillus have galactofuranose (Galf)-containing polysaccharides and glycoconjugates, including O-glycans, N-glycans, fungal-type galactomannan and glycosylinositolphosphoceramide, which are important for cell wall integrity. Here, we attempted to identify galactofuranosyltransferases that couple Galf monomers onto other wall components in Aspergillus nidulans. Using reverse-genetic and biochemical approaches, we identified that the AN8677 gene encoded a galactofuranosyltransferase, which we called GfsA, involved in Galf antigen biosynthesis. Disruption of gfsA reduced binding of β-Galf-specific antibody EB-A2 to O-glycosylated WscA protein and galactomannoproteins. The results of an in-vitro Galf antigen synthase assay revealed that GfsA has β1,5- or β1,6-galactofuranosyltransferase activity for O-glycans in glycoproteins, uses UDP-d-Galf as a sugar donor, and requires a divalent manganese cation for activity. GfsA was found to be localized at the Golgi apparatus based on cellular fractionation experiments. ΔgfsA cells exhibited an abnormal morphology characterized by poor hyphal extension, hyphal curvature and limited formation of conidia. Several gfsA orthologues were identified in members of the Pezizomycotina subphylum of Ascomycota, including the human pathogen Aspergillus fumigatus. To our knowledge, this is the first characterization of a fungal β-galactofuranosyltransferase, which was shown to be involved in Galf antigen biosynthesis of O-glycans in the Golgi.

Carbasugar analogues of galactofuranosides: α-O-linked derivatives

Using an indirect method, we have synthesised α-linked carbasugar analogues of galactofuranosides for the first time. Ring opening of a β-talo configured carbasugar 1,2-epoxide by alcohol nucleophiles under Lewis acidic conditions proceeded with very good regioselectivity to give α-talo configured C1-substituted ethers with a free OH-group at the C2 position. Inversion of configuration at C2 by an oxidation-reduction sequence gave the α-galacto configured carbahexofuranose C1 ethers. A carbadisaccharide corresponding to the Galf(α1→3)Manp substructure from Apodus deciduus galactomannan was synthesised to exemplify the method.

Synthesis of galactofuranose-based acceptor substrates for the study of the carbohydrate polymerase GlfT2

Despite the prevalence and importance of carbohydrate polymers, the molecular details of their biosynthesis remain elusive. Many enzymes responsible for the synthesis of carbohydrate polymers require a 'primer' or 'initiator' carbohydrate sequence. One example of such an enzyme is the mycobacterial galactofuranosyltransferase GlfT2 (Rv3808c), which generates an essential cell wall building block. We recently demonstrated that recombinant GlfT2 is capable of producing a polymer composed of alternating beta-(1,5) and beta-(1,6)-linked galactofuranose (Galf) residues. Intriguingly, the length of the polymers produced from a synthetic glycosyl acceptor is consistent with those found in the cell wall. To probe the mechanism by which polymer length is controlled, a collection of initiator substrates has been assembled. The central feature of the synthetic route is a ruthenium-catalyzed cross-metathesis as the penultimate transformation. Access to synthetic substrates has led us to postulate a new mechanism for length control in this template-independent polymerization. Moreover, our investigations indicate that lipids possessing but a single galactofuranose residue can act as substrates for GlfT2.

Expression, purification and preliminary X-ray crystallographic analysis of UDP-galactopyranose mutase from Deinococcus radiodurans

UDP-galactopyranose mutase (UGM) catalyzes the interconversion of UDP-galactopyranose and UDP-galactofuranose. A UGM-substrate complex from Deinococccus radiodurans has been expressed, purified and crystallized. Crystals were obtained by the microbatch-under-oil method at room temperature. The crystals diffracted to 2.36 A resolution at the Canadian Light Source. The space group was found to be P2(1)2(1)2(1), with unit-cell parameters a = 134.0, b = 176.6, c = 221.6 A. The initial structure solution was determined by molecular replacement using UGM from Mycobacterium tuberculosis (PDB code 1v0j) as a template model.

The diverse roles of flavin coenzymes--nature's most versatile thespians

Flavin coenzymes play a variety of roles in biological systems. This Perspective highlights the chemical versatility of flavins by reviewing research on five flavoenzymes that have been studied in our laboratory. Each of the enzymes discussed in this review [the acyl-CoA dehydrogenases (ACDs), CDP-6-deoxy-l-threo-d-glycero-4-hexulose-3-dehydrase reductase (E3), CDP-4-aceto-3,6-dideoxygalactose synthase (YerE), UDP-galactopyranose mutase (UGM), and type II isopentenyl diphosphate:dimethylallyl diphosphate isomerase (IDI-2)] utilizes flavin in a distinct role. In particular, the catalytic mechanisms of two of these enzymes, UGM and IDI-2, may involve novel flavin chemistry.

Disaccharide analogs as probes for glycosyltransferases in Mycobacterium tuberculosis

Glycosyltransferases (GTs) play a crucial role in mycobacterial cell wall biosynthesis and are necessary for the survival of mycobacteria. Hence, these enzymes are potential new drug targets for the treatment of tuberculosis (TB), especially multiple drug-resistant TB (MDR-TB). Herein, we report the efficient syntheses of Araf(alpha 1-->5)Araf, Galf(beta 1-->5)Galf, and Galf(beta 1-->6)Galf disaccharides possessing a 5-N,N-dimethylaminonaphthalene-1-sulfonamidoethyl (dansyl) unit that were prepared as fluorescent disaccharide acceptors for arabinosyl- and galactosyl-transferases, respectively. Such analogs may offer advantages relative to radiolabeled acceptors or donors for studying the enzymes and for assay development and compound screening. Additionally, analogs possessing a 5-azidonaphthalene-1-sulfonamidoethyl unit were prepared as photoaffinity probes for their potential utility in studying active site labeling of the GTs (arabinosyl and galactosyl) in Mycobacterium tuberculosis (MTB). Beyond their preparation, initial biological testing and kinetic analysis of these disaccharides as acceptors toward glycosyltransferases are also presented.

Lipophosphoglycan is a virulence factor distinct from related glycoconjugates in the protozoan parasite Leishmania major

Protozoan parasites of the genus Leishmania undergo a complex life cycle involving transmission by biting sand flies and replication within mammalian macrophage phagolysosomes. A major component of the Leishmania surface coat is the glycosylphosphatidylinositol (GPI)-anchored polysaccharide called lipophosphoglycan (LPG). LPG has been proposed to play many roles in the infectious cycle, including protection against complement and oxidants, serving as the major ligand for macrophage adhesion, and as a key factor mitigating host responses by deactivation of macrophage signaling pathways. However, all structural domains of LPG are shared by other major surface or secretory products, providing a biochemical redundancy that compromises the ability of in vitro tests to establish whether LPG itself is a virulence factor. To study truly lpg(-) parasites, we generated Leishmania major lacking the gene LPG1 [encoding a putative galactofuranosyl (Gal(f)) transferase] by targeted gene disruption. The lpg1(-) parasites lacked LPG but contained normal levels of related glycoconjugates and GPI-anchored proteins. Infections of susceptible mice and macrophages in vitro showed that these lpg(-) Leishmania were highly attenuated. Significantly and in contrast to previous LPG mutants, reintroduction of LPG1 into the lpg(-) parasites restored virulence. Thus, genetic approaches allow dissection of the roles of this complex family of interrelated parasite virulence factors, and definitively establish the role of LPG itself as a parasite virulence factor. Because the lpg1(-) mutant continue to synthesize bulk GPI-anchored Gal(f)-containing glycolipids other than LPG, a second pathway distinct from the Golgi-associated LPG synthetic compartment must exist.