Identification of the galactosyltransferase of Cryptococcus neoformans involved in the biosynthesis of basidiomycete-type glycosylinositolphosphoceramide

Identification of a putative α-galactoside β-(1 → 3)-galactosyltransferase involved in the biosynthesis of galactomannan side chain of glucuronoxylomannogalactan in Cryptococcus neoformans

The cell surface of Cryptococcus neoformans is covered by a thick capsular polysaccharide. The capsule is the most important virulence factor of C. neoformans; however, the complete mechanism of its biosynthesis is unknown. The capsule is composed of glucuronoxylomannan (GXM) and glucuronoxylomannogalactan (GXMGal). As GXM is the most abundant component of the capsule, many studies have focused on GXM biosynthesis. However, although GXMGal has an important role in virulence, studies on its biosynthesis are scarce. Herein, we have identified a GT31 family β-(1 → 3)-galactosyltransferase Ggt2, which is involved in the biosynthesis of the galactomannan side chain of GXMGal. Comparative analysis of GXMGal produced by a ggt2 disruption strain revealed that Ggt2 is a glycosyltransferase that catalyzes the initial reaction in the synthesis of the galactomannan side chain of GXMGal. The ggt2 disruption strain showed a temperature-sensitive phenotype at 37°C, indicating that the galactomannan side chain of GXMGal is important for high-temperature stress tolerance in C. neoformans. Our findings provide insights into complex capsule biosynthesis in C. neoformans.

Analysis of carbohydrates and glycoconjugates by matrix-assisted laser desorption/ionization mass spectrometry: An update for 2013-2014

This review is the eighth update of the original article published in 1999 on the application of Matrix-assisted laser desorption/ionization mass spectrometry (MALDI) mass spectrometry to the analysis of carbohydrates and glycoconjugates and brings coverage of the literature to the end of 2014. Topics covered in the first part of the review include general aspects such as theory of the MALDI process, matrices, derivatization, MALDI imaging, fragmentation, and arrays. The second part of the review is devoted to applications to various structural types such as oligo- and poly- saccharides, glycoproteins, glycolipids, glycosides, and biopharmaceuticals. Much of this material is presented in tabular form. The third part of the review covers medical and industrial applications of the technique, studies of enzyme reactions, and applications to chemical synthesis. © 2018 Wiley Periodicals, Inc. Mass Spec Rev 37:353-491, 2018.

Genetic alteration of UDP-rhamnose metabolism in Botrytis cinerea leads to the accumulation of UDP-KDG that adversely affects development and pathogenicity

Botrytis cinerea is a model plant-pathogenic fungus that causes grey mould and rot diseases in a wide range of agriculturally important crops. A previous study has identified two enzymes and corresponding genes (bcdh, bcer) that are involved in the biochemical transformation of uridine diphosphate (UDP)-glucose, the major fungal wall nucleotide sugar precursor, to UDP-rhamnose. We report here that deletion of bcdh, the first biosynthetic gene in the metabolic pathway, or of bcer, the second gene in the pathway, abolishes the production of rhamnose-containing glycans in these mutant strains. Deletion of bcdh or double deletion of both bcdh and bcer has no apparent effect on fungal development or pathogenicity. Interestingly, deletion of the bcer gene alone adversely affects fungal development, giving rise to altered hyphal growth and morphology, as well as reduced sporulation, sclerotia production and virulence. Treatments with wall stressors suggest the alteration of cell wall integrity. Analysis of nucleotide sugars reveals the accumulation of the UDP-rhamnose pathway intermediate UDP-4-keto-6-deoxy-glucose (UDP-KDG) in hyphae of the Δbcer strain. UDP-KDG could not be detected in hyphae of the wild-type strain, indicating fast conversion to UDP-rhamnose by the BcEr enzyme. The correlation between high UDP-KDG and modified cell wall and developmental defects raises the possibility that high levels of UDP-KDG result in deleterious effects on cell wall composition, and hence on virulence. This is the first report demonstrating that the accumulation of a minor nucleotide sugar intermediate has such a profound and adverse effect on a fungus. The ability to identify molecules that inhibit Er (also known as NRS/ER) enzymes or mimic UDP-KDG may lead to the development of new antifungal drugs.

Cryptococcus neoformans UGT1 encodes a UDP-Galactose/UDP-GalNAc transporter

Cryptococcus neoformans, an opportunistic fungal pathogen, produces a glycan capsule to evade the immune system during infection. This definitive virulence factor is composed mainly of complex polysaccharides, which are made in the secretory pathway by reactions that utilize activated nucleotide sugar precursors. Although the pathways that synthesize these precursors are known, the identity and the regulation of the nucleotide sugar transporters (NSTs) responsible for importing them into luminal organelles remain elusive. The UDP-galactose transporter, Ugt1, was initially identified by homology to known UGTs and glycan composition analysis of ugt1Δ mutants. However, sequence is an unreliable predictor of NST substrate specificity, cells may express multiple NSTs with overlapping specificities, and NSTs may transport multiple substrates. Determining NST activity thus requires biochemical demonstration of function. We showed that Ugt1 transports both UDP-galactose and UDP-N-acetylgalactosamine in vitro. Deletion of UGT1 resulted in growth and mating defects along with altered capsule and cellular morphology. The mutant was also phagocytosed more readily by macrophages than wild-type cells and cleared more quickly in vivo and in vitro, suggesting a mechanism for the lack of virulence observed in mouse models of infection.

Characterization of an N-acetylglucosaminyltransferase involved in Aspergillus fumigatus zwitterionic glycoinositolphosphoceramide biosynthesis

Glycoinositolphosphoceramides (GIPCs) are complex sphingolipids present at the plasma membrane of various eukaryotes with the important exception of mammals. In fungi, these glycosphingolipids commonly contain an α-mannose residue (Man) linked at position 2 of the inositol. However, several pathogenic fungi additionally synthesize zwitterionic GIPCs carrying an α-glucosamine residue (GlcN) at this position. In the human pathogen Aspergillus fumigatus, the GlcNα1,2IPC core (where IPC is inositolphosphoceramide) is elongated to Manα1,3Manα1,6GlcNα1,2IPC, which is the most abundant GIPC synthesized by this fungus. In this study, we identified an A. fumigatus N-acetylglucosaminyltransferase, named GntA, and demonstrate its involvement in the initiation of zwitterionic GIPC biosynthesis. Targeted deletion of the gene encoding GntA in A. fumigatus resulted in complete absence of zwitterionic GIPC; a phenotype that could be reverted by episomal expression of GntA in the mutant. The N-acetylhexosaminyltransferase activity of GntA was substantiated by production of N-acetylhexosamine-IPC in the yeast Saccharomyces cerevisiae upon GntA expression. Using an in vitro assay, GntA was furthermore shown to use UDP-N-acetylglucosamine as donor substrate to generate a glycolipid product resistant to saponification and to digestion by phosphatidylinositol-phospholipase C as expected for GlcNAcα1,2IPC. Finally, as the enzymes involved in mannosylation of IPC, GntA was localized to the Golgi apparatus, the site of IPC synthesis.