Mannosylation of fungal glycoconjugates in the Golgi apparatus

Identification of lncRNA and weighted gene coexpression network analysis of germinating Rhizopus delemar causing mucormycosis

Rhizopus delemar, an opportunistic fungal pathogen, causes a highly fatal disease, mucormycosis. Spore germination is a crucial mechanism for disease pathogenesis. Thus, exploring the molecular mechanisms of fungal germination would underpin our knowledge of such transformation and, in turn, help control mucormycosis. To gain insight into the developmental process particularly associated with cell wall modification and synthesis, weighted gene co-expression network analysis (WGCNA) was performed including both coding and non-coding transcripts identified in the current study, to find out the module of interest in the germination stages. The module-trait relationship identified a particular module to have a high correlation only at the resting phase and further analysis revealed the module to be enriched for protein phosphorylation, carbohydrate metabolic process, and cellular response to stimulus. Moreover, co-expression network analysis of highly connected nodes revealed cell wall modifying enzymes, especially those involved in mannosylation, chitin-glucan crosslinking, and polygalacturonase activities co-expressing and interacting with the novel lncRNAs among which some of them predicted to be endogenous target mimic (eTM) lncRNAs. Hence, the present study provides an insight into the onset of spore germination and the information on the novel non-coding transcripts with key cell wall-related enzymes as potential targets against mucormycosis.

Reverse thiophosphorylase activity of a glycoside phosphorylase in the synthesis of an unnatural Manβ1,4GlcNAc library

β-Mannosides are ubiquitous in nature, with diverse roles in many biological processes. Notably, Manβ1,4GlcNAc a constituent of the core N-glycan in eukaryotes was recently identified as an immune activator, highlighting its potential for use in immunotherapy. Despite their biological significance, the synthesis of β-mannosidic linkages remains one of the major challenges in glycoscience. Here we present a chemoenzymatic strategy that affords a series of novel unnatural Manβ1,4GlcNAc analogues using the β-1,4-d-mannosyl-N-acetyl-d-glucosamine phosphorylase, BT1033. We show that the presence of fluorine in the GlcNAc acceptor facilitates the formation of longer β-mannan-like glycans. We also pioneer a "reverse thiophosphorylase" enzymatic activity, favouring the synthesis of longer glycans by catalysing the formation of a phosphorolysis-stable thioglycoside linkage, an approach that may be generally applicable to other phosphorylases.

Protein O-mannosylation across kingdoms and related diseases: From glycobiology to glycopathology

The post-translational glycosylation of proteins by O-linked α-mannose is conserved from bacteria to humans. Due to advances in high-throughput mass spectrometry-based approaches, a variety of glycoproteins are identified to be O-mannosylated. Various proteins with O-mannosylation are involved in biological processes, providing essential necessity for proper growth and development. In this review, we summarize the process and regulation of O-mannosylation. The multi-step O-mannosylation procedures are quite dynamic and complex, especially when considering the structural and functional inspection of the involved enzymes. The widely studied O-mannosylated proteins in human include α-Dystroglycan (α-DG), cadherins, protocadherins, and plexin, and their aberrant O-mannosylation are associated with many diseases. In addition, O-mannosylation also contributes to diverse functions in lower eukaryotes and prokaryotes. Finally, we present the relationship between O-mannosylation and gut microbiota (GM), and elucidate that O-mannosylation in microbiome is of great importance in the dynamic balance of GM. Our study provides an overview of the processes of O-mannosylation in mammalian cells and other organisms, and also associated regulated enzymes and biological functions, which could contribute to the understanding of newly discovered O-mannosylated glycoproteins.

Cell Wall Mannoproteins from Yeast Affect Salivary Protein-Flavanol Interactions through Different Molecular Mechanisms

It is known that interactions between wine flavanols and salivary proline-rich proteins (PRPs) are one of the main factors responsible for wine astringency. The addition of commercial yeast mannoproteins (MPs) to wines has been pointed to as a possible tool to modulate the excessive astringency due to a lack of phenolic maturity at harvest time that might occur as a consequence of global climate change. The aim of this work was to study by isothermal titration calorimetry and molecular dynamics simulation the molecular mechanisms by which mannoproteins could modulate astringency elicited by tannins and if it can be influenced by mannoprotein composition. Results obtained indicate that the MPs assayed had an important impact on astringency through the formation of ternary aggregates with different solubilities or by preventing the flavanol-PRP interaction by a competitive mechanism, although in a different strength, depending on the size and the compositional characteristic of the mannoprotein.

Differences in fungal immune recognition by monocytes and macrophages: N-mannan can be a shield or activator of immune recognition

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Cytokine response to N-mannan mutants was dependent on the immune cell type used.

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N-mannan mutants stimulated less cytokines from monocytes but more from macrophages.

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N-mannan can therefore act as both an immune agonist or an immune shield.

We designed experiments to assess whether fungal cell wall mannans function as an immune shield or an immune agonist. Fungal cell wall β-(1,3)-glucan normally plays a major and dominant role in immune activation. The outer mannan layer has been variously described as an immune shield, because it has the potential to mask the underlying β-(1,3)-glucan, or an immune activator, as it also has the potential to engage with a wide range of mannose detecting PRRs. To resolve this conundrum we examined species-specific differences in host immune recognition in the och1Δ N-mannosylation-deficient mutant background in four species of yeast-like fungi. Irrespective of the fungal species, the cytokine response (TNFα and IL-6) induced by the och1Δ mutants in human monocytes was reduced compared to that of the wild type. In contrast, TNFα production induced by och1Δ was increased, relative to wild type, due to increased β-glucan exposure, when mouse or human macrophages were used. These observations suggest that N-mannan is not a major PAMP for macrophages and that in these cells mannan does shield the fungus from recognition of the inner cell wall β-glucan. However, N-mannan is a significant inducer of cytokine for monocytes. Therefore the metaphor of the fungal “mannan shield” can only be applied to some, but not all, myeloid cells used in immune profiling experiments of fungal species.

N-glycosylation of the protein disulfide isomerase Pdi1 ensures full Ustilago maydis virulence

Fungal pathogenesis depends on accurate secretion and location of virulence factors which drive host colonization. Protein glycosylation is a common posttranslational modification of cell wall components and other secreted factors, typically required for correct protein localization, secretion and function. Thus, the absence of glycosylation is associated with animal and plant pathogen avirulence. While the relevance of protein glycosylation for pathogenesis has been well established, the main glycoproteins responsible for the loss of virulence observed in glycosylation-defective fungi have not been identified. Here, we devise a proteomics approach to identify such proteins and use it to demonstrate a role for the highly conserved protein disulfide isomerase Pdi1 in virulence. We show that efficient Pdi1 N-glycosylation, which promotes folding into the correct protein conformation, is required for full pathogenic development of the corn smut fungus Ustilago maydis. Remarkably, the observed virulence defects are reminiscent of those seen in glycosylation-defective cells suggesting that the N-glycosylation of Pdi1 is necessary for the full secretion of virulence factors. All these observations, together with the fact that Pdi1 protein and RNA expression levels rise upon virulence program induction, suggest that Pdi1 glycosylation is important for normal pathogenic development in U. maydis. Our results provide new insights into the role of glycosylation in fungal pathogenesis.

Evolution of protein N-glycosylation process in Golgi apparatus which shapes diversity of protein N-glycan structures in plants, animals and fungi

Protein N-glycosylation (PNG) is crucial for protein folding and enzymatic activities, and has remarkable diversity among eukaryotic species. Little is known of how unique PNG mechanisms arose and evolved in eukaryotes. Here we demonstrate a picture of onset and evolution of PNG components in Golgi apparatus that shaped diversity of eukaryotic protein N-glycan structures, with an emphasis on roles that domain emergence and combination played on PNG evolution. 23 domains were identified from 24 known PNG genes, most of which could be classified into a single clan, indicating a single evolutionary source for the majority of the genes. From 153 species, 4491 sequences containing the domains were retrieved, based on which we analyzed distribution of domains among eukaryotic species. Two domains in GnTV are restricted to specific eukaryotic domains, while 10 domains distribute not only in species where certain unique PNG reactions occur and thus genes harboring these domains are supoosed to be present, but in other ehkaryotic lineages. Notably, two domains harbored by β-1,3 galactosyltransferase, an essential enzyme in forming plant-specific Le^a structure, were present in separated genes in fungi and animals, suggesting its emergence as a result of domain shuffling.

SWATH-MS Glycoproteomics Reveals Consequences of Defects in the Glycosylation Machinery

Glycan macro- and microheterogeneity have profound impacts on protein folding and function. This heterogeneity can be regulated by physiological or environmental factors. However, unregulated heterogeneity can lead to disease, and mutations in the glycosylation process cause a growing number of Congenital Disorders of Glycosylation. We systematically studied how mutations in the N-glycosylation pathway lead to defects in mature proteins using all viable Saccharomyces cerevisiae strains with deletions in genes encoding Endoplasmic Reticulum lumenal mannosyltransferases (Alg3, Alg9, and Alg12), glucosyltransferases (Alg6, Alg8, and Die2/Alg10), or oligosaccharyltransferase subunits (Ost3, Ost5, and Ost6). To measure the changes in glycan macro- and microheterogeneity in mature proteins caused by these mutations we developed a SWATH-mass spectrometry glycoproteomics workflow. We measured glycan structures and occupancy on mature cell wall glycoproteins, and relative protein abundance, in the different mutants. All mutants showed decreased glycan occupancy and altered cell wall proteomes compared with wild-type cells. Mutations in earlier mannosyltransferase or glucosyltransferase steps of glycan biosynthesis had stronger hypoglycosylation phenotypes, but glucosyltransferase defects were more severe. ER mannosyltransferase mutants displayed substantial global changes in glycan microheterogeneity consistent with truncations in the glycan transferred to protein in these strains. Although ER glucosyltransferase and oligosaccharyltransferase subunit mutants broadly showed no change in glycan structures, ost3Δ cells had shorter glycan structures at some sites, consistent with increased protein quality control mannosidase processing in this severely hypoglycosylating mutant. This method allows facile relative quantitative glycoproteomics, and our results provide insights into global regulation of site-specific glycosylation.

Initiation of phospholipomannan β-1,2 mannosylation involves Bmts with redundant activity, influences its cell wall location and regulates β-glucans homeostasis but is dispensable for Candida albicans systemic infection

Pathogenic and non-pathogenic fungi synthesize glycosphingolipids, which have a crucial role in growth and viability. Glycosphingolipids also contribute to fungal-associated pathogenesis. The opportunistic yeast pathogen Candida albicans synthesizes phospholipomannan (PLM), which is a glycosphingolipid of the mannosylinositol phosphorylceramide family. Through its lipid and glycan moieties, PLM contributes to the initial recognition of the yeast, causing immune system disorder and persistent fungal disease through activation of host signaling pathways. The lipid moiety of PLM activates the deregulation signaling pathway involved in yeast phagocytosis whereas its glycan moiety, composed of β-1,2 mannosides (β-Mans), participates to inflammatory processes through a mechanism involving Galectin-3. Biosynthesis of PLM β-Mans involves two β-1,2 mannosyltransferases (Bmts) that initiate (Bmt5) and elongate (Bmt6) the glycan chains. After generation of double bmtsΔ mutants, we show that Bmt5 has redundant activity with Bmt2, which can replace Bmt5 in bmt5Δ mutant. We also report that PLM is located in the inner layer of the yeast cell wall. PLM seems to be not essential for systemic infection of the yeast. However, defect of PLM β-mannosylation increases resistance of C. albicans to inhibitors of β-glucans and chitin synthesis, highlighting a role of PLM in cell wall homeostasis.

Candida albicans β-1,2-mannosyltransferase Bmt3 prompts the elongation of the cell-wall phosphopeptidomannan

β-1,2-Linked mannosides are expressed on numerous cell-wall glycoconjugates of the opportunistic pathogen yeast Candida albicans. Several studies evidenced their implication in the host-pathogen interaction and virulence mechanisms. In the present study, we characterized the in vitro activity of CaBmt3, a β-1,2-mannosyltransferase involved in the elongation of β-1,2-oligomannosides oligomers onto the cell-wall polymannosylated N-glycans. A recombinant soluble enzyme Bmt3p was produced in Pichia pastoris and its enzyme activity was investigated using natural and synthetic oligomannosides as potential acceptor substrates. Bmt3p was shown to exhibit an exquisite enzymatic specificity by adding a single terminal β-mannosyl residue to α-1,2-linked oligomannosides capped by a Manβ1-2Man motif. Furthermore, we demonstrated that the previously identified CaBmt1 and CaBmt3 efficiently act together to generate Manβ1-2Manβ1-2[Manα1-2]n sequence from α-1,2-linked oligomannosides onto exogenous and endogenous substrates.

β-1,2-Mannosyltransferases 1 and 3 Participate in Yeast and Hyphae O- and N-Linked Mannosylation and Alter Candida albicans Fitness During Infection

β-1,2-mannosylation of Candida albicans glycoconjugates has been investigated through the identification of enzymes involved in the addition of β-1,2-oligomannosides (β-Mans) to phosphopeptidomannan and phospholipomannan. β-1,2-oligomannosides are supposed to have virulence properties that they confer to these glycoconjugates. In a previous study, we showed that cell wall mannoproteins (CWMPs) harbor β-Mans in their O-mannosides; therefore, we analyzed their biosynthesis and impact on virulence. In this study, we demonstrate that O-mannans are heterogeneous and that α-mannosylated O-mannosides, which are biosynthesized by Mnt1 and Mnt2 α-1,2-mannosyltransferases, can be modified with β-Mans but only at the nonreducing end of α-1,2-mannotriose. β-1,2-mannosylation of this O-mannotriose depends on growth conditions, and it involves 2 β-1,2-mannosyltransferases, Bmt1 and Bmt3. These Bmts are essential for β-1,2-mannosylation of CWMPs and expression of β-Mans on germ tubes. A bmt1Δ mutant and a mutant expressing no β-Mans unexpectedly disseminated more in BALB/c mice, whereas they had neither attenuated nor enhanced virulence in C57BL/6 mice. In galectin (Gal)3 knockout mice, the reference strain was more virulent than in C57BL/6 mice, suggesting that the β-Mans innate receptor Gal3 is involved in C. albicans fitness during infection.

Role of the unfolded protein response in regulating the mucin-dependent filamentous-growth mitogen-activated protein kinase pathway

Signaling mucins are evolutionarily conserved regulators of signal transduction pathways. The signaling mucin Msb2p regulates the Cdc42p-dependent mitogen-activated protein kinase (MAPK) pathway that controls filamentous growth in yeast. The cleavage and release of the glycosylated inhibitory domain of Msb2p is required for MAPK activation. We show here that proteolytic processing of Msb2p was induced by underglycosylation of its extracellular domain. Cleavage of underglycosylated Msb2p required the unfolded protein response (UPR), a quality control (QC) pathway that operates in the endoplasmic reticulum (ER). The UPR regulator Ire1p, which detects misfolded/underglycosylated proteins in the ER, controlled Msb2p cleavage by regulating transcriptional induction of Yps1p, the major protease that processes Msb2p. Accordingly, the UPR was required for differentiation to the filamentous cell type. Cleavage of Msb2p occurred in conditional trafficking mutants that trap secretory cargo in the endomembrane system. Processed Msb2p was delivered to the plasma membrane, and its turnover by the ubiquitin ligase Rsp5p and ESCRT attenuated the filamentous-growth pathway. We speculate that the QC pathways broadly regulate signaling glycoproteins and their cognate pathways by recognizing altered glycosylation patterns that can occur in response to extrinsic cues.