Protein O-mannosylation

Regulation of cell wall biogenesis in Saccharomyces cerevisiae: the cell wall integrity signaling pathway

The yeast cell wall is a strong, but elastic, structure that is essential not only for the maintenance of cell shape and integrity, but also for progression through the cell cycle. During growth and morphogenesis, and in response to environmental challenges, the cell wall is remodeled in a highly regulated and polarized manner, a process that is principally under the control of the cell wall integrity (CWI) signaling pathway. This pathway transmits wall stress signals from the cell surface to the Rho1 GTPase, which mobilizes a physiologic response through a variety of effectors. Activation of CWI signaling regulates the production of various carbohydrate polymers of the cell wall, as well as their polarized delivery to the site of cell wall remodeling. This review article centers on CWI signaling in Saccharomyces cerevisiae through the cell cycle and in response to cell wall stress. The interface of this signaling pathway with other pathways that contribute to the maintenance of cell wall integrity is also discussed.

Proteomic analysis of ripening tomato fruit infected by Botrytis cinerea

Botrytis cinerea, a model necrotrophic fungal pathogen that causes gray mold as it infects different organs on more than 200 plant species, is a significant contributor to postharvest rot in fresh fruit and vegetables, including tomatoes. By describing host and pathogen proteomes simultaneously in infected tissues, the plant proteins that provide resistance and allow susceptibility and the pathogen proteins that promote colonization and facilitate quiescence can be identified. This study characterizes fruit and fungal proteins solubilized in the B. cinerea-tomato interaction using shotgun proteomics. Mature green, red ripe wild type and ripening inhibited (rin) mutant tomato fruit were infected with B. cinerea B05.10, and the fruit and fungal proteomes were identified concurrently 3 days postinfection. One hundred eighty-six tomato proteins were identified in common among red ripe and red ripe-equivalent ripening inhibited (rin) mutant tomato fruit infected by B. cinerea. However, the limited infections by B. cinerea of mature green wild type fruit resulted in 25 and 33% fewer defense-related tomato proteins than in red and rin fruit, respectively. In contrast, the ripening stage of genotype of the fruit infected did not affect the secreted proteomes of B. cinerea. The composition of the collected proteins populations and the putative functions of the identified proteins argue for their role in plant-pathogen interactions.

Glycoconjugates and polysaccharides from the Scedosporium/Pseudallescheria boydii complex: structural characterisation, involvement in cell differentiation, cell recognition and virulence

Peptidorhamnomannans (PRMs), rhamnomannans and α-glucans are especially relevant for the architecture of the Scedosporium/Pseudallescheria boydii cell wall, but many of them are immunologically active, with great potential as regulators of pathogenesis and the immune response of the host. In addition, some of them can be specifically recognised by antibodies from the sera of patients, suggesting that they could also be useful in diagnosis of fungal infections. Their primary structures have been determined, based on a combination of techniques including gas chromatography, electrospray ionization - mass spectrometry (ESI-MS), (1)H-COSY and TOCSY, (13)C and (1)H/(13)C NMR spectroscopy. Using monoclonal antibodies to PRM, we showed that it is involved in germination and viability of P. boydii conidia, in the phagocytosis of P. boydii conidia by macrophages and non-phagocytic cells and in the survival of mice with P. boydii infection. Also, components of the fungal cell wall, such as α-glucans, are involved. Rhamnomannans are immunostimulatory and participate in the recognition and uptake of fungal cells by the immune system. These glycosylated polymers, being present in the fungal cell wall, are mostly absent from mammalian cells, and are excellent targets for the design of new agents capable of inhibiting fungal growth and differentiation of pathogens.

Influence of solvent and intramolecular hydrogen bonding on the conformational properties of o-linked glycopeptides

A detailed investigation of the conformational properties of all the biologically relevant O-glycosidic linkages using the Hamiltonian replica exchange (HREX) simulation methodology and the recently developed CHARMM carbohydrate force field parameters is presented. Fourteen biologically relevant O-linkages between the five sugars N-acetylgalactosamine (GalNAc), N-acetylglucosamine (GlcNAc), D-glucose (Glc), D-mannose (Man), and L-fucose (Fuc) and the amino acids serine and threonine were studied. The force field was tested by comparing the simulation results of the model glycopeptides to various NMR (3)J coupling constants, NOE distances, and data from molecular dynamics with time-averaged restraints (tar-MD). The results show the force field to be in overall agreement with experimental and previous tar-MD simulations, although some small limitations are identified. An in-depth hydrogen bond and bridging water analysis revealed an interplay of hydrogen bonding and bridge water interactions influencing the geometry of the underlying peptide backbone, with the O-linkages favoring extended β-sheet and polyproline type II (PPII) conformations over the compact α(R)-helical conformation. The newly developed parameters were also able to identify hydrogen bonding and water mediated interactions between O-linked sugars and proteins. These results indicate that the newly developed parameters in tandem with HREX conformational sampling provide the means to study glycoproteins in the absence of targeted NMR restraint data.

Protein Glycosylation in Aspergillus fumigatus Is Essential for Cell Wall Synthesis and Serves as a Promising Model of Multicellular Eukaryotic Development

Glycosylation is a conserved posttranslational modification that is found in all eukaryotes, which helps generate proteins with multiple functions. Our knowledge of glycosylation mainly comes from the investigation of the yeast Saccharomyces cerevisiae and mammalian cells. However, during the last decade, glycosylation in the human pathogenic mold Aspergillus fumigatus has drawn significant attention. It has been revealed that glycosylation in A. fumigatus is crucial for its growth, cell wall synthesis, and development and that the process is more complicated than that found in the budding yeast S. cerevisiae. The present paper implies that the investigation of glycosylation in A. fumigatus is not only vital for elucidating the mechanism of fungal cell wall synthesis, which will benefit the design of new antifungal therapies, but also helps to understand the role of protein glycosylation in the development of multicellular eukaryotes. This paper describes the advances in functional analysis of protein glycosylation in A. fumigatus.

Different roles of the two components of human protein O-mannosyltransferase, POMT1 and POMT2

Protein O-mannosyltransferase 1 (POMT1) and its homolog, POMT2, are responsible for the catalysis of the first step in O-mannosyl glycan synthesis. Mutations in their genes are associated with a type of congenital muscular dystrophy called Walker-Warburg syndrome. Arg(64), Glu(78) and Arg(138) in the N-terminus region of ScPmt1p, a POMT homolog in Saccharomyces cerevisiae, are important for transferase activity. Arg(138) is also essential for complex formation with ScPmt2p. Here we examined the effects of replacing the corresponding residues in human POMT1 and POMT2 with Ala on complex formation and enzymatic activity. The human POMT1 mutants lost almost all transferase activity while the POMT2 mutants retained enzymatic activity. Neither mutant lost its ability to form complexes with the native counter component. These results indicate that ScPmtps and human POMTs have different mechanisms of complex formation. They also suggest that human POMT1 and POMT2 have discrete functions since the effect of amino acid substitutions on enzymatic activity are different.

Cloning and functional analysis of the dpm2 and dpm3 genes from Trichoderma reesei expressed in a Saccharomyces cerevisiae dpm1Δ mutant strain

InTrichoderma reesei, dolichyl phosphate mannose (dpm) synthase, a key enzyme in the O-glycosylation process, requires three proteins for full activity. In this study, thedpm2anddpm3genes coding for the DPMII and DPMIII subunits ofT. reeseiDPM synthase were cloned and functionally analyzed after expression in theSaccharomyces cerevisiae dpm1Δ[genotype (BY4743;his3Δ1; /leu2Δ0; lys2Δ0; /ura3Δ0; YPR183w::kanMX4] mutant. It was found that apart from the catalytic subunit DPMI, the DPMIII subunit is also essential to form an active DPM synthase in yeast. Additional expression of the DPMII protein, considered to be a regulatory subunit of DPM synthase, decreased the enzymatic activity. We also characterizedS. cerevisiaestrains expressing thedpm1,2,3ordpm1, 3genes and analyzed the consequences ofdpmexpression on protein O-glycosylationin vivoand on the cell wall composition.

Aberrant protein N-glycosylation impacts upon infection-related growth transitions of the haploid plant-pathogenic fungus Mycosphaerella graminicola

The ascomycete fungus Mycosphaerella graminicola is the causal agent of Septoria Tritici Blotch disease of wheat and can grow as yeast-like cells or as hyphae depending on environmental conditions. Hyphal growth is however essential for successful leaf infection. A T-DNA mutagenesis screen performed on haploid spores identified a mutant, which can undergo yeast-like growth but cannot switch to hyphal growth. For this reason the mutant was non-pathogenic towards wheat leaves. The gene affected, MgAlg2, encoded a homologue of Saccharomyces cerevisiae ScAlg2, an alpha-1,2-mannosyltransferase, which functions in the early stages of asparagine-linked protein (N-) glycosylation. Targeted gene deletion and complementation experiments confirmed that loss of MgAlg2 function prevented the developmental growth switch. MgAlg2 was able to functionally complement the S. cerevisiae ScAlg2-1 temperature sensitive growth phenotype. Spores of ΔMgAlg2 mutants were hypersensitive to the cell wall disrupting agent Calcofluor white and produced abnormally hypo-N-glycosylated proteins. Gene expression, proteome and glycoproteome analysis revealed that ΔMgAlg2 mutant spores show responses typically associated with the accumulation of mis-folded proteins. The data presented highlight key roles for protein N-glycosylation in regulating the switch to hyphal growth, possibly as a consequence of maintaining correct folding and localization of key proteins involved in this process.

Effect of Saccharomyces cerevisiae ret1-1 mutation on glycosylation and localization of the secretome

To study the effect of the ret1-1 mutation on the secretome, the glycosylation patterns and locations of the secretory proteins and glycosyltransferases responsible for glycosylation were investigated. Analyses of secretory proteins and cell wall-associated glycoproteins showed severe impairment of glycosylation in this mutant. Results from 2D-polyacrylamide gel electrophoresis (PAGE) indicated defects in the glycosylation and cellular localization of SDS-soluble cell wall proteins. Localization of RFP-tagged glycosyltransferase proteins in ret1-1 indicated an impairment of Golgi-to retrograde transport at a non-permissive temperature. Thus, impaired glycosylation caused by the mislocalization of ER resident proteins appears to be responsible for the alterations in the secretome and the increased sensitivity to ER stress in ret1-1 mutant cells.

Glycosylation status of the C. albicans cell wall affects the efficiency of neutrophil phagocytosis and killing but not cytokine signaling

The cell wall of the opportunistic human fungal pathogen, Candida albicans is a complex, layered network of rigid structural polysaccharides composed of β-glucans and chitin that is covered with a fibrillar matrix of highly glycosylated mannoproteins. Polymorphonuclear cells (PMNs, neutrophils) are the most prevalent circulating phagocytic leukocyte in peripheral blood and they are pivotal in the clearance of invading fungal cells from tissues. The importance of cell-wall mannans for the recognition and uptake of C. albicans by human PMNs was therefore investigated. N- and O-glycosylation-deficient mutants were attenuated in binding and phagocytosis by PMNs and this was associated with reduced killing of C. albicans yeast cells. No differences were found in the production of the respiratory burst enzyme myeloperoxidase (MPO) and the neutrophil chemokine IL-8 in PMNs exposed to control and glycosylation-deficient C. albicans strains. Thus, the significant decrease in killing of glycan-deficient C. albicans strains by PMNs is a consequence of a marked reduction in phagocytosis rather than changes in the release of inflammatory mediators by PMNs.

Engineering of glycosylation in yeast and other fungi: current state and perspectives

With the increasing demand for recombinant proteins and glycoproteins, research on hosts for producing these proteins is focusing increasingly on more cost-effective expression systems. Yeasts and other fungi are promising alternatives because they provide easy and cheap systems that can perform eukaryotic post-translational modifications. Unfortunately, yeasts and other fungi modify their glycoproteins with heterogeneous high-mannose glycan structures, which is often detrimental to a therapeutic protein's pharmacokinetic behavior and can reduce the efficiency of downstream processing. This problem can be solved by engineering the glycosylation pathways to produce homogeneous and, if so desired, human-like glycan structures. In this review, we provide an overview of the most significant recently reported approaches for engineering the glycosylation pathways in yeasts and fungi.

Members of protein O-mannosyltransferase family in Aspergillus fumigatus differentially affect growth, morphogenesis and viability

O-mannosylation is an essential protein modification in eukaryotes. It is initiated at the endoplasmic reticulum by O-mannosyltransferases (PMT) that are evolutionary conserved from yeast to humans. The PMT family is phylogenetically classified into PMT1, PMT2 and PMT4 subfamilies, which differ in protein substrate specificity and number of genes per subfamily. In this study, we characterized for the first time the whole PMT family of a pathogenic filamentous fungus, Aspergillus fumigatus. Genome analysis showed that only one member of each subfamily is present in A. fumigatus, PMT1, PMT2 and PMT4. Despite the fact that all PMTs are transmembrane proteins with conserved peptide motifs, the phenotype of each PMT deletion mutant was very different in A. fumigatus. If disruption of PMT1 did not reveal any phenotype, deletion of PMT2 was lethal. Disruption of PMT4 resulted in abnormal mycelial growth and highly reduced conidiation associated to significant proteomic changes. The double pmt1pmt4 mutant was lethal. The single pmt4 mutant exhibited an exquisite sensitivity to echinocandins that is associated to major changes in the expression of signal transduction cascade genes. These results indicate that the PMT family members play a major role in growth, morphogenesis and viability of A. fumigatus.

New insights into protein O-mannosylation in actinomycetes

Glycosylation is a common post-translational modification of surface exposed proteins and lipids present in all kingdoms of life. Information derived from bacterial genome sequencing, together with proteomic and genomic analysis has allowed the identification of the enzymatic glycosylation machinery. Among prokaryotes, O-mannosylation of proteins has been found in the actinomycetes and resembles protein O-mannosylation in fungi and higher eukaryotes. In this review we summarize the main features of the biosynthetic pathway of O-mannosylation in prokaryotes with special emphasis on the actinomycetes, as well as the biological role of the glycosylated target proteins.

Protein glycosylation in Candida

Candidiasis is a significant cause of invasive human mycosis with associated mortality rates that are equivalent to, or worse than, those cited for most cases of bacterial septicemia. As a result, considerable efforts are being made to understand how the fungus invades host cells and to identify new targets for fungal chemotherapy. This has led to an increasing interest in Candida glycobiology, with an emphasis on the identification of enzymes essential for glycoprotein and adhesion metabolism, and the role of N- and O-linked glycans in host recognition and virulence. Here, we refer to studies dealing with the identification and characterization of enzymes such as dolichol phosphate mannose synthase, dolichol phosphate glucose synthase and processing glycosidases and synthesis, structure and recognition of mannans and discuss recent findings in the context of Candida albicans pathogenesis.

Heterologous expression of glucose oxidase in the yeast Kluyveromyces marxianus

BACKGROUND

In spite of its advantageous physiological properties for bioprocess applications, the use of the yeast Kluyveromyces marxianus as a host for heterologous protein production has been very limited, in constrast to its close relative Kluyveromyces lactis. In the present work, the model protein glucose oxidase (GOX) from Aspergillus niger was cloned into K. marxianus CBS 6556 and into K. lactis CBS 2359 using three different expression systems. We aimed at verifying how each expression system would affect protein expression, secretion/localization, post-translational modification, and biochemical properties.

RESULTS

The highest GOX expression levels (1552 units of secreted protein per gram dry cell weight) were achieved using an episomal system, in which the INU1 promoter and terminator were used to drive heterologous gene expression, together with the INU1 prepro sequence, which was employed to drive secretion of the enzyme. In all cases, GOX was mainly secreted, remaining either in the periplasmic space or in the culture supernatant. Whereas the use of genetic elements from Saccharomyces cerevisiae to drive heterologous protein expression led to higher expression levels in K. lactis than in K. marxianus, the use of INU1 genetic elements clearly led to the opposite result. The biochemical characterization of GOX confirmed the correct expression of the protein and showed that K. marxianus has a tendency to hyperglycosylate the protein, in a similar way as already observed for other yeasts, although this tendency seems to be smaller than the one of e.g. K. lactis and S. cerevisiae. Hyperglycosylation of GOX does not seem to affect its affinity for the substrate, nor its activity.

CONCLUSIONS

Taken together, our results indicate that K. marxianus is indeed a good host for the expression of heterologous proteins, not only for its physiological properties, but also because it correctly secretes and folds these proteins.

Reduced expression of the O-mannosyltransferase 2 (AfPmt2) leads to deficient cell wall and abnormal polarity in Aspergillus fumigatus

Protein O-mannosyltransferases (PMTs) initiate O-mannosylation of secretory proteins, which are of fundamental importance in eukaryotes. The human fungal pathogen Aspergillus fumigatus possesses three genes encoding for PMTs, namely, Afpmt1, Afpmt2 and Afpmt4. We have previously shown that lack of AfPmt1 leads to a temperature-sensitive phenotype featured with severe defects in hyphal growth, conidiation, cell wall integrity and morphology at elevated temperatures. In this study, a conditional mutant P2 was constructed by replacing the native promoter of the Afpmt2 with the Aspergillus nidulans alcA promoter. Reduced expression of the Afpmt2 gene led to a lagged germination, retarded hyphal growth, reduced conidiation and defect in cell wall integrity; however, no temperature-sensitive growth was observed. Further analysis revealed that reduced expression of the Afpmt2 caused a failure of the actin re-arrangement. Our results suggest that Afpmt2 gene was required for growth and played a role distinct from that of the Afpmt1 in A. fumigatus.

Identification and characterization of endonuclein binding proteins: evidence of modulatory effects on signal transduction and chaperone activity

Background

We have previously identified endonuclein as a cell cycle regulated WD-repeat protein that is up-regulated in adenocarcinoma of the pancreas. Now, we aim to investigate its biomedical functions.

Results

Using the cDNA encoding human endonuclein, we have expressed and purified the recombinant protein from Escherichia coli using metal affinity chromatography. The recombinant protein was immobilized to a column and by affinity chromatography several interacting proteins were purified from several litres of placenta tissue extract. After chromatography the eluted proteins were further separated by two-dimensional gel electrophoresis and identified by tandem mass spectrometry. The interacting proteins were identified as; Tax interaction protein 1 (TIP-1), Aα fibrinogen transcription factor (P16/SSBP1), immunoglobulin heavy chain binding protein (BiP), human ER-associated DNAJ (HEDJ/DNAJB11), endonuclein interaction protein 8 (EIP-8), and pregnancy specific β-1 glycoproteins (PSGs). Surface plasmon resonance analysis and confocal fluorescence microscopy were used to further characterize the interactions.

Conclusions

Our results demonstrate that endonuclein interacts with several proteins indicating a broad function including signal transduction and chaperone activity.

Role of N-glycans in maintaining the activity of protein O-mannosyltransferases POMT1 and POMT2

The complex of protein O-mannosyltransferase 1 (POMT1) and POMT2 catalyzes the initial step of O-mannosyl glycan biosynthesis. The mutations in either POMT1 or POMT2 can lead to Walker-Warburg syndrome, a congenital muscular dystrophy with abnormal neuronal migration. Here, we used three algorithms for predicting transmembrane helices to construct the secondary structural models of human POMT1 and POMT2. In these models, POMT1 and POMT2 have seven- and nine-transmembrane helices and contain four and five potential N-glycosylation sites, respectively. To determine whether these sites are actually glycosylated, we prepared mutant proteins that were defective in each site by site-directed mutagenesis. Three of the POMT1 sites and all of the POMT2 sites were found to be N-glycosylated, suggesting that these sites face the luminal side of the endoplasmic reticulum. Mutation of any single site did not significantly affect POMT activity, but mutations of all N-glycosylation sites of either POMT1 or POMT2 caused a loss of POMT activity. The loss of activity appeared to be due to the decreased hydrophilicity. These results suggest that the N-glycosylation of POMT1 and POMT2 is required for maintaining the conformation as well as the activity of the POMT1-POMT2 complex.

The O-mannosyltransferase PMT4 is essential for normal appressorium formation and penetration in Ustilago maydis

In Saccharomyces cerevisiae, the PMT, KRE2/MNT1, and MNN1 mannosyltransferase protein families catalyze the steps of the O-mannosylation pathway, sequentially adding mannoses to target proteins. We have identified members of all three families and analyzed their roles in pathogenesis of the maize smut fungus Ustilago maydis. Furthermore, we have shown that PMT4, one of the three PMT family members in U. maydis, is essential for tumor formation in Zea mays. Significantly, PMT4 seems to be required only for pathogenesis and is dispensable for other aspects of the U. maydis life cycle. We subsequently show that the deletion of pmt4 results in a strong reduction in the frequency of appressorium formation, with the few appressoria that do form lacking the capacity to penetrate the plant cuticle. Our findings suggest that the O-mannosylation pathway plays a key role in the posttranslational modification of proteins involved in the pathogenic development of U. maydis. The fact that PMT homologs are not found in plants may open new avenues for the development of fungal control strategies. Moreover, the discovery of a highly specific requirement for a single O-mannosyltransferase should aid in the identification of the proteins directly involved in fungal plant penetration, thus leading to a better understanding of plant-fungi interactions.

Aspergillus nidulans protein O-mannosyltransferases play roles in cell wall integrity and developmental patterning

Protein O-mannosyltransferases (Pmts) initiate O-mannosyl glycan biosynthesis from Ser and Thr residues of target proteins. Fungal Pmts are divided into three subfamilies, Pmt1, -2, and -4. Aspergillus nidulans possesses a single representative of each Pmt subfamily, pmtA (subfamily 2), pmtB (subfamily 1), and pmtC (subfamily 4). In this work, we show that single Deltapmt mutants are viable and have unique phenotypes and that the DeltapmtA DeltapmtB double mutant is the only viable double mutant. This makes A. nidulans the first fungus in which all members of individual Pmt subfamilies can be deleted without loss of viability. At elevated temperatures, all A. nidulans Deltapmt mutants show cell wall-associated defects and increased sensitivity to cell wall-perturbing agents. The Deltapmt mutants also show defects in developmental patterning. Germ tube emergence is early in DeltapmtA and more frequent in DeltapmtC mutants than in the wild type. In DeltapmtB mutants, intrahyphal hyphae develop. All Deltapmt mutants show distinct conidiophore defects. The DeltapmtA strain has swollen vesicles and conidiogenous cells, the DeltapmtB strain has swollen conidiophore stalks, and the DeltapmtC strain has dramatically elongated conidiophore stalks. We also show that AN5660, an ortholog of Saccharomyces cerevisiae Wsc1p, is modified by PmtA and PmtC. The Deltapmt phenotypes at elevated temperatures, increased sensitivity to cell wall-perturbing agents and restoration to wild-type growth with osmoticum suggest that A. nidulans Pmts modify proteins in the cell wall integrity pathway. The altered developmental patterns in Deltapmt mutants suggest that A. nidulans Pmts modify proteins that serve as spatial cues.

HPLC and mass spectrometry analysis of dolichol-phosphates at the cell culture scale

Dolichols (Dol) are polyprenol lipids that are essential structural components of eukaryotic membranes. In addition, the phosphorylated derivatives of Dol function as lipid anchors of mono- and oligosaccharide precursors involved in protein glycosylation. The biological importance of Dol phosphates (Dol-P) is illustrated by the severe outcome of human disorders linked to Dol biosynthetic defects, such as Dol-kinase deficiency. For characterization of inherited human diseases and evaluation of therapeutic trials, cultured cells often serve as a sole possible source for experimentation. Limited amounts of cell culture material render the quantitative analysis of Dol a challenging task. Here, we present HPLC- and mass spectrometry-based approaches to analyze and quantitate Dol-P from cultured human cells. The composition of naturally occurring Dol-P and the saturation state of the alpha-isoprene units was identified by negative-ion electrospray ionization mass spectrometry. Furthermore, fluorescently labeled Dol-P were separated by HPLC and quantified by comparison to known amounts of the internal standard polyprenol-P. The effect of pravastatin, a 3-hydroxy-3-methyl-glutaryl coenzyme-A reductase inhibitor, on the formation of Dol-P in HeLa cells was investigated. As expected, this treatment led to a decrease of Dol-P down to 35% of normal levels.

Protein O-mannosyltransferases B and C support hyphal development and differentiation in Aspergillus nidulans

Aspergillus nidulans possesses three pmt genes encoding protein O-d-mannosyltransferases (Pmt). Previously, we reported that PmtA, a member of the PMT2 subfamily, is involved in the proper maintenance of fungal morphology and formation of conidia (T. Oka, T. Hamaguchi, Y. Sameshima, M. Goto, and K. Furukawa, Microbiology 150:1973-1982, 2004). In the present paper, we describe the characterization of the pmtA paralogues pmtB and pmtC. PmtB and PmtC were classified as members of the PMT1 and PMT4 subfamilies, respectively. A pmtB disruptant showed wild-type (wt) colony formation at 30 degrees C but slightly repressed growth at 42 degrees C. Conidiation of the pmtB disruptant was reduced to approximately 50% of that of the wt strain; in addition, hyperbranching of hyphae indicated that PmtB is involved in polarity maintenance. A pmtA and pmtB double disruptant was viable but very slow growing, with morphological characteristics that were cumulative with respect to either single disruptant. Of the three single pmt mutants, the pmtC disruptant showed the highest growth repression; the hyphae were swollen and frequently branched, and the ability to form conidia under normal growth conditions was lost. Recovery from the aberrant hyphal structures occurred in the presence of osmotic stabilizer, implying that PmtC is responsible for the maintenance of cell wall integrity. Osmotic stabilization at 42 degrees C further enabled the pmtC disruptant to form conidiophores and conidia, but they were abnormal and much fewer than those of the wt strain. Apart from the different, abnormal phenotypes, the three pmt disruptants exhibited differences in their sensitivities to antifungal reagents, mannosylation activities, and glycoprotein profiles, indicating that PmtA, PmtB, and PmtC perform unique functions during cell growth.

Characterization of the PMT gene family in Cryptococcus neoformans

Background

Protein-O-mannosyltransferases (Pmt's) catalyze the initial step of protein-O-glycosylation, the addition of mannose residues to serine or threonine residues of target proteins.

Methodology/Principal Findings

Based on protein similarities, this highly conserved protein family can be divided into three subfamilies: the Pmt1 sub-family, the Pmt2 sub-family and the Pmt4 sub-family. In contrast to Saccharomyces cerevisiae and Candida albicans, but similar to filamentous fungi, three putative PMT genes (PMT1, PMT2, and PMT4) were identified in the genome of the human fungal pathogen Cryptococcus neoformans. Similar to Schizosaccharomyces pombe and C. albicans, C. neoformans PMT2 is an essential gene. In contrast, the pmt1 and pmt4 single mutants are viable; however, the pmt1/pmt4 deletions are synthetically lethal. Mutation of PMT1 and PMT4 resulted in distinct defects in cell morphology and cell integrity. The pmt1 mutant was more susceptible to SDS medium than wild-type strains and the mutant cells were enlarged. The pmt4 mutant grew poorly on high salt medium and demonstrated abnormal septum formation and defects in cell separation. Interestingly, the pmt1 and pmt4 mutants demonstrated variety-specific differences in the levels of susceptibility to osmotic and cell wall stress. Delayed melanin production in the pmt4 mutant was the only alteration of classical virulence-associated phenotypes. However, the pmt1 and pmt4 mutants showed attenuated virulence in a murine inhalation model of cryptococcosis.

Conclusion/Significance

These findings suggest that C. neoformans protein-O-mannosyltransferases play a crucial role in maintaining cell morphology, and that reduced protein-O-glycosylation leads to alterations in stress resistance, cell wall composition, cell integrity, and survival within the host.

Production of heterologous proteins using the fission-yeast (Schizosaccharomyces pombe) expression system

The fission yeast Schizosaccharomyces pombe is a particularly useful model for studying the function and regulation of genes from higher eukaryotes. The genome of Sc. pombe has been sequenced, and DNA microarray, proteome and transcriptome analyses have been carried out. Among the well-characterized yeast species, Sc. pombe is considered an attractive host for the production of heterologous proteins. Expression vectors for high-level expression in Sc. pombe have been developed and many foreign proteins have been successfully expressed. However, further improvements in the protein-expressing host systems are still required for the production of heterologous proteins involved in post-translational modification, metabolism and intracellular trafficking. This minireview focuses on recent advances in heterologous protein production by use of engineered fission-yeast strains.

Protein O-mannosylation: conserved from bacteria to humans

Protein O-mannosylation is an essential modification in fungi and animals. Different from most other types of O-glycosylation, protein O-mannosylation is initiated in the endoplasmic reticulum by the transfer of mannose from dolichol monophosphate-activated mannose to serine and threonine residues of secretory proteins. In recent years, it has emerged that even bacteria are capable of O-mannosylation and that the biosynthetic pathway of O-mannosyl glycans is conserved between pro- and eukaryotes. In this review, we summarize the observations that have opened up the field and highlight characteristics of O-mannosylation in the different domains/kingdoms of life.

A tandem mass spectrometric approach to the identification of O-glycosylated glargine glycoforms in active pharmaceutical ingredient expressed in Pichia pastoris

Glycoforms of glargine expressed in Pichia pastoris were isolated by high-performance liquid chromatography and analyzed by a series of chemical and mass spectrometric methods for the identification of various glycoforms, glycosylation position, nature and structure of glycans. Reduction and alkylation, peptide mapping techniques were used to decipher the amino acid site at which glycosylation had taken place. Chemical methods were coupled with mass spectrometry techniques such as electrospray ionization and matrix-assisted laser desorption/ionization for identification of the glycosylation site.

Glycosylation of the phosphate binding protein, PstS, in Streptomyces coelicolor by a pathway that resembles protein O-mannosylation in eukaryotes

Previously mutations in a putative protein O-mannosyltransferase (SCO3154, Pmt) and a polyprenol phosphate mannose synthase (SCO1423, Ppm1) were found to cause resistance to phage, phiC31, in the antibiotic producing bacteria Streptomyces coelicolor A3(2). It was proposed that these two enzymes were part of a protein O-glycosylation pathway that was necessary for synthesis of the phage receptor. Here we provide the evidence that Pmt and Ppm1 are indeed both required for protein O-glycosylation. The phosphate binding protein PstS was found to be glycosylated with a trihexose in the S. coelicolor parent strain, J1929, but not in the pmt(-) derivative, DT1025. Ppm1 was necessary for the transfer of mannose to endogenous polyprenol phosphate in membrane preparations of S. coelicolor. A mutation in ppm1 that conferred an E218V substitution in Ppm1 abolished mannose transfer and glycosylation of PstS. Mass spectrometry analysis of extracted lipids showed the presence of a glycosylated polyprenol phosphate (PP) containing nine repeated isoprenyl units (C(45)-PP). S. coelicolor membranes were also able to catalyse the transfer of mannose to peptides derived from PstS, indicating that these could be targets for Pmt in vivo.

Membrane topology and post-translational modification of the Saccharomyces cerevisiae essential protein Rot1

ROT1 is an essential gene that has been related to cell wall biosynthesis, the actin cytoskeleton and protein folding. In order to help to understand its molecular function, we carried out a characterization of the Rot1 protein. It is primarily located at the endoplasmic reticulum-nuclear membrane facing the lumen. Rot1 migrates more slowly than expected, which might suggest post-translational modification. Our results indicate that Rot1 is a protein that is neither GPI-anchored nor O-glycosylated. In contrast, it is N-glycosylated. By a directed mutagenesis of several Asn residues, we identified that the protein is simultaneously glycosylated at N103, N107 and N139. Although the mutation of these three N sites is not lethal, cellular growth is impaired. Sequence analysis predicts a transmembrane domain at the C-terminus. This fragment affects neither the targeting of the Rot1 protein to the ER nor its N-glycosylation, although it is important for the anchoring of the protein to the membrane and for its functionality. The existence of a signal sequence at the N-terminus has been suggested. However, deletion of this fragment impedes neither translocation to the ER nor N-glycosylation, but it is required for cell viability. Finally, we found that Rot1 is translocated to the ER by an SRP-independent post-translational mechanism which depends on Sec62.

The presence of an ER exit signal determines the protein sorting upon ER exit in yeast

In yeast, there are at least two vesicle populations upon ER (endoplasmic reticulum) exit, one containing Gap1p (general aminoacid permease) and a glycosylated α-factor, gpαF (glycosylated proα-factor), and the other containing GPI (glycosylphosphatidylinositol)-anchored proteins, Gas1p (glycophospholipid-anchored surface protein) and Yps1p. We attempted to identify sorting determinants for this protein sorting event in the ER. We found that mutant Gas1 proteins that lack a GPI anchor and/or S/T region (serine- and threonine-rich region), two common characteristic features conserved among yeast GPI-anchored proteins, were still sorted away from Gap1p-containing vesicles. Furthermore, a mutant glycosylated α-factor, gpαGPI, which contains both the GPI anchor and S/T region from Gas1p, still entered Gap1p-containing vesicles, demonstrating that these conserved characteristics do not prevent proteins from entering Gap1p-containing vesicles. gpαF showed severely reduced budding efficiency in the absence of its ER exit receptor Erv29p, and this residual budding product no longer entered Gap1p-containing vesicles. These results suggest that the interaction of gpαF with Erv29p is essential for sorting into Gap1p-containing vesicles. We compared the detergent solubility of Gas1p and the gpαGPI in the ER with that in ER-derived vesicles. Both GPI-anchored proteins similarly partitioned into the DRM (detergent-resistant membrane) in the ER. Based on the fact that they entered different ER-derived vesicles, we conclude that DRM partitioning of GPI-anchored proteins is not the dominant determinant of protein sorting upon ER exit. Interestingly, upon incorporation into the ER-derived vesicles, gpαGPI was no longer detergent-insoluble, in contrast with the persistent detergent insolubility of Gas1p in the ER-derived vesicles. We present different explanations for the different behaviours of GPI-anchored proteins in distinct ER-derived vesicle populations.

Protein glycosylation pathways in filamentous fungi

Glycosylation of proteins is important for protein stability, secretion, and localization. In this study, we have investigated the glycan synthesis pathways of 12 filamentous fungi including those of medical/agricultural/industrial importance for which genomes have been recently sequenced. We have adopted a systems biology approach to combine the results from comparative genomics techniques with high confidence information on the enzymes and fungal glycan structures, reported in the literature. From this, we have developed a composite representation of the glycan synthesis pathways in filamentous fungi (both N- and O-linked). The N-glycosylation pathway in the cytoplasm and endoplasmic reticulum was found to be highly conserved evolutionarily across all the filamentous fungi considered in the study. In the final stages of N-glycan synthesis in the Golgi, filamentous fungi follow the high mannose pathway as in Saccharomyces cerevisiae, but the level of glycan mannosylation is reduced. Highly specialized N-glycan structures with galactofuranose residues, phosphodiesters, and other insufficiently trimmed structures have also been identified in the filamentous fungi. O-Linked glycosylation in filamentous fungi was seen to be highly conserved with many mannosyltransferases that are similar to those in S. cerevisiae. However, highly variable and diverse O-linked glycans also exist. We have developed a web resource for presenting the compiled data with user-friendly query options, which can be accessed at www.fungalglycans.org. This resource can assist attempts to remodel glycosylation of recombinant proteins expressed in filamentous fungal hosts.

Saccharomyces cerevisiae Grx6 and Grx7 are monothiol glutaredoxins associated with the early secretory pathway

Saccharomyces cerevisiae Grx6 and Grx7 are two monothiol glutaredoxins whose active-site sequences (CSYS and CPYS, respectively) are reminiscent of the CPYC active-site sequence of classical dithiol glutaredoxins. Both proteins contain an N-terminal transmembrane domain which is responsible for their association to membranes of the early secretory pathway vesicles, facing the luminal side. Thus, Grx6 localizes at the endoplasmic reticulum and Golgi compartments, while Grx7 is mostly at the Golgi. Expression of GRX6 is modestly upregulated by several stresses (calcium, sodium, and peroxides) in a manner dependent on the Crz1-calcineurin pathway. Some of these stresses also upregulate GRX7 expression under the control of the Msn2/4 transcription factor. The N glycosylation inhibitor tunicamycin induces the expression of both genes along with protein accumulation. Mutants lacking both glutaredoxins display reduced sensitivity to tunicamycin, although the drug is still able to manifest its inhibitory effect on a reporter glycoprotein. Grx6 and Grx7 have measurable oxidoreductase activity in vivo, which is increased in the presence of tunicamycin. Both glutaredoxins could be responsible for the regulation of the sulfhydryl oxidative state at the oxidant conditions of the early secretory pathway vesicles. However, the differences in location and expression responses against stresses suggest that their functions are not totally overlapping.

Use of Giardia, which appears to have a single nucleotide-sugar transporter for UDP-GlcNAc, to identify the UDP-Glc transporter of Entamoeba

Nucleotide-sugar transporters (NSTs) transport activated sugars (e.g. UDP-GlcNAc) from the cytosol to the lumen of the endoplasmic reticulum or Golgi apparatus where they are used to make glycoproteins and glycolipids. UDP-Glc is an important component of the N-glycan-dependent quality control (QC) system for protein folding. Because Entamoeba has this QC system while Giardia does not, we hypothesized that transfected Giardia might be used to identify the UDP-Glc transporter of Entamoeba. Here we show Giardia membranes transport UDP-GlcNAc and have apyrases, which hydrolyze nucleoside-diphosphates to make the antiporter nucleoside-monophosphate. The only NST of Giardia (GlNst), which we could identify, transports UDP-GlcNAc in transfected Saccharomyces and is present in perinuclear and peripheral vesicles and increases in expression during encystation. Entamoeba membranes transport three nucleotide-sugars (UDP-Gal, UDP-GlcNAc, and UDP-Glc), and Entamoeba has three NSTs, one of which has been shown previously to transport UDP-Gal (EhNst1). Here we show recombinant EhNst2 transports UDP-Glc in transfected Giardia, while recombinant EhNst3 transports UDP-GlcNAc in transfected Saccharomyces. In summary, all three NSTs of Entamoeba and the single NST of Giardia have been molecularly characterized, and transfected Giardia provides a new system for testing heterologous UDP-Glc transporters.

Analysis of O-glycosylation

Secreted as well as membrane-associated eukaryotic proteins are most commonly glycosylated. Saccharides are attached to proteins mainly through N-and O-glycosydic bonds or as part of the glycosylphosphatidyinositol-membrane anchor. In contrast to N-glycosylation, which involves the co-translational transfer in the endoplasmic reticulum (ER) of the glycan portion of Glc3Man9GlcNAc2-PP-dolichol to suitable Asn residues on nascent polypeptides, O-glycosylation begins with the addition of a single monosaccharide. Contrary to N-glycosylation, which involves an asparagine residue in the sequon Asn-Xaa-Thr/Ser (Xaa can be any amino acid except Pro, and it is rarely Cys), no particular sequence motif has been described for O-glycosylation. This may reflect the fact that: (1) the specificity of the UDP-GalNAc:polypeptide N-acetylgalactosaminyltransferase is presently unknown; and (2) seems to be modulated by sequence context, secondary structure, and surface accessibility (1). An internet server, accessible at http://www.cbs.dtu.dk/netOglyc/cbsnetOglyc.html , produces neural network predictions of mucin-type GalNAc O-glycosylation sites in mammalian proteins based on 299 known and verified mucin-type O-glycosylation sites. The sequence context of glycosylated threonines was found to differ from that of serine, and the sites were found to cluster. Nonclustered sites had a sequence context different from that of clustered sites, and charged residues were disfavored at position -1 and +3.

Transcriptional and physiological adaptation to defective protein-O-mannosylation in Candida albicans

Five Pmt isoforms O-mannosylate secretory proteins in Candida albicans. Comparisons of genome-wide transcript patterns of each pmt mutant revealed commonly downregulated genes involved in glycolysis and glycerol production. Increased phosphorylation of the Cek1p- but not the Mkc1p-MAP kinase, as well as increased transcript levels for some stress-related genes were detected in the pmt1 strain but not in the other pmt mutants. The transcriptomal pattern after short-term inhibition of Pmt1p activity confirmed stress responses, but did not indicate an alteration of glycolytic flow. Short- but not long-term adaptation to Pmt1p inhibition required signalling components Cek1p, Mkc1p, Efg1p and Tpk1p. Cna1p (calcineurin) but not its downstream effectors Crz1p and Crz2p was generally essential to allow growth during Pmt1p inhibition; accordingly, cyclosporin A strongly inhibited growth of the pmt1 mutant. The lack of Pmt isoforms influenced transcript levels for the remaining isoforms both positively and negatively, suggesting complex cross-regulation among PMT genes. These results confirm individual functions of Pmt isoforms but suggest a common biphasic adaptation response to Pmt deficiency. While known signalling pathways modulate adaptation for a short-term, long-term adaptation requires calcineurin, adjustments of remaining Pmt activities and of glycolytic flow.

Rsc14-controlled expression ofMNN6, MNN4andMNN1regulates mannosylphosphorylation ofSaccharomyces cerevisiaecell wall mannoproteins

ybl006cDelta was selected using an Alcian-blue-based screen aimed to identify nonessential genes involved in the regulation of mannosylphosphorylation. When cells of this deletant were mixed with the cationic dye Alcian blue in a typical assay, they remained white, indicating a low number of mannosylphosphate groups on the cell surface. ybl006cDelta cells did not show any defect in growth rate nor in the glycosylation or secretion rate of the major exoglucanase Exg1. Transcriptome analysis of ybl006cDelta using macroarrays showed at least two-fold changes in the expression of 52 genes (<0.9% of the genome). Three of these have previously been reported to be directly (MNN6 and MNN4) or indirectly (MNN1) implicated in the transfer of mannosylphosphate to the N- and O-oligosaccharides. Alterations in the expression of these genes were confirmed by Northern analysis. YBL006C product was recently identified as a subunit (Rsc14) of the RSC chromatin-remodelling complex of Saccharomyces cerevisiae. It follows that remodelling of chromatin can be an important regulatory mechanism for the maturation of cell wall mannoproteins.

Efficient antibody production upon suppression of O mannosylation in the yeast Ogataea minuta

When antibodies were expressed in the methylotrophic yeast Ogataea minuta, we found that abnormal O mannosylation occurred in the secreted antibody. Yeast-specific O mannosylation is initiated by the addition of mannose at serine (Ser) or threonine (Thr) residues in the endoplasmic reticulum via protein O mannosyltransferase (Pmt) activity. To suppress the addition of O-linked sugar chains on antibodies, we examined the possibility of inhibiting Pmt activity by the addition of a Pmt inhibitor during cultivation. The Pmt inhibitor was found to partially suppress the O mannosylation on the antibodies. Surprisingly, the suppression of O mannosylation was associated with an increased amount of assembled antibody (H2L2) and enhanced the antigen-binding activity of the secreted antibody. In this study, we demonstrated the expression of human antibody in O. minuta and elucidated the relationship between O mannosylation and antibody production in yeast.

Anti-Saccharomyces cerevisiae mannan antibodies (ASCA) of Crohn's patients crossreact with mannan from other yeast strains, and murine ASCA IgM can be experimentally induced with Candida albicans

BACKGROUND

Anti-Saccharomyces cerevisiae antibodies (ASCA) present in a subgroup of Crohn's disease (CD) patients indicate loss of tolerance against commensal antigens. ASCA can be induced in Candida albicans-infected rabbits, suggesting their potential crossreactive nature. The present study aimed to determine crossreactivities of ASCA with cell wall mannans from other yeasts, including the opportunistic pathogen C. albicans, and to define the requirements for (crossreactive) ASCA in experimental mice.

METHODS

ASCA were determined by enzyme-linked immunosorbent assay (ELISA). ASCA were neutralized by preincubating sera with purified mannans. Binding of ASCA was visualized by Western blot. Mice were immunized with live yeasts and experimental colitis was induced with dextran sodium sulfate (DSS).

RESULTS

Seroreactivity of ASCA-positive CD patients against S. cerevisiae mannan significantly correlates with that against mannans from 5 other yeast species, including C. albicans. This correlation is due to crossreactive IgG, demonstrated by the loss of reactivity after preincubation of sera with mannans from the other yeasts. Immunization of mice with S. cerevisiae or C. albicans fails to induce (crossreactive) ASCA IgM or IgG antibodies. Subsequent chronic experimental colitis concomitant with feeding live yeasts promotes ASCA IgM but not IgG generation, while titers remain modest compared to those in ASCA-positive CD patients.

CONCLUSIONS

Correlations of ASCA reactivities against mannans from different yeasts are due to crossreactive IgGs. The inability of mice to readily generate ASCA is in line with the current opinion that genetic predisposition is a prerequisite for the development of this and other unusual immune reactivities in CD.