Characterization of O-mannosyltransferase family in Schizosaccharomyces pombe

O-mannosyltransferase MaPmt2 contributes to stress tolerance, cell wall integrity and virulence in Metarhizium acridum

As a conserved post-translational modification, O-mannosyltransferase families play important roles in many cellular processes. Three subfamilies (MaPmt1, MaPmt2 and MaPmt4) are grouped in Metarhizium acridum according to sequence homology. The functions of MaPmt1 and MaPmt4 have been characterized in M. acridum previously. In this study, the functions of another member belonging to the Pmt2 subfamily, MaPmt2, were identified through RNAi strategy. The three MaPmt2 knockdown mutants showed dramatically decreased expression of MaPmt2. Phenotypic analyses showed that the mutants exhibited decreased tolerances to wet-heat, UV-B irradiation and cell wall perturbing chemicals. Further studies revealed that the mutants presented thinner cell walls observed by transmission electron microscope combined with changed cell wall components. Besides, knockdown of MaPmt2 decelerated conidial germination and decreased conidial yield. Compared with the wild-type strain, the MaPmt2 knockdown mutants caused impaired virulence only by topical inoculation. Results illustrated that the decreased virulence by inoculation could result from the delayed conidial germination on locust wings, reduced appressorium formation, as well as reduced turgor pressure in MaPmt2 knockdown mutants.

Deletion of AS87_03730 gene changed the bacterial virulence and gene expression of Riemerella anatipestifer

Riemerella anatipestifer is an important pathogen of waterfowl, which causes septicemia anserum exsudativa in ducks. In this study, an AS87_03730 gene deletion R. anatipestifer mutant Yb2ΔAS87_03730 was constructed to investigate the role of AS87_03730 on R. anatipestifer virulence and gene regulation. By deleting a 708-bp fragment from AS87_03730, the mutant Yb2ΔAS87_03730 showed a significant decreased growth rate in TSB and invasion capacity in Vero cells, compared to wild-type strain Yb2. Moreover, the median lethal dose (LD₅₀) of Yb2ΔAS87_03730 was 1.24 × 10⁷ colony forming units (CFU), which is about 80-fold attenuated than that of Yb2 (LD₅₀ = 1.53 × 10⁵ CFU). Furthermore, RNA-Seq analysis and Real-time PCR indicated 19 up-regulated and two down-regulated genes in Yb2ΔAS87_03730. Functional analysis revealed that 12 up-regulated genes were related to “Translation, ribosomal structure and biogenesis”, two were classified into “Cell envelope biogenesis, outer membrane”, one was involved in “Amino acid transport and metabolism”, and the other four had unknown functions. Polymerase chain reaction and sequence analysis indicated that the AS87_03730 gene is highly conserved among R. anatipestifer strains, as the percent sequence identity was over 93.5%. This study presents evidence that AS87_03730 gene is involved in bacterial virulence and gene regulation of R. anatipestifer.

Fungal chitinases: function, regulation, and potential roles in plant/pathogen interactions

In the past decades our knowledge about fungal cell wall architecture increased tremendously and led to the identification of many enzymes involved in polysaccharide synthesis and remodeling, which are also of biotechnological interest. Fungal cell walls play an important role in conferring mechanic stability during cell division and polar growth. Additionally, in phytopathogenic fungi the cell wall is the first structure that gets into intimate contact with the host plant. A major constituent of fungal cell walls is chitin, a homopolymer of N-acetylglucosamine units. To ensure plasticity, polymeric chitin needs continuous remodeling which is maintained by chitinolytic enzymes, including lytic polysaccharide monooxygenases N-acetylglucosaminidases, and chitinases. Depending on the species and lifestyle of fungi, there is great variation in the number of encoded chitinases and their function. Chitinases can have housekeeping function in plasticizing the cell wall or can act more specifically during cell separation, nutritional chitin acquisition, or competitive interaction with other fungi. Although chitinase research made huge progress in the last decades, our knowledge about their role in phytopathogenic fungi is still scarce. Recent findings in the dimorphic basidiomycete Ustilago maydis show that chitinases play different physiological functions throughout the life cycle and raise questions about their role during plant-fungus interactions. In this work we summarize these functions, mechanisms of chitinase regulation and their putative role during pathogen/host interactions.

Functional characterization of extracellular chitinase encoded by the YlCTS1 gene in a dimorphic yeast Yarrowia lipolytica

The hemiascomycetes yeast Yarrowia lipolytica is a dimorphic yeast with alternating yeast and mycelia forms. Bioinformatic analysis revealed the presence of three putative chitinase genes, YlCTS1, YlCTS2, and YlCTS3, in the Y. lipolytica genome. Here, we demonstrated that the protein of YlCTS1 (YlCts1p), which contains an N-terminal secretion signal peptide, a long C-terminal Ser/Thr-rich domain, and a chitin-binding domain, is a homologue to Saccharomyces cerevisiae chitinase 1 (ScCts1p). Deletion of YlCTS1 remarkably reduced extracellular endochitinase activity in the culture supernatant of Y. lipolytica and enhanced cell aggregation, suggesting a role of YlCts1p in cell separation as ScCts1p does in S. cerevisiae. However, loss of YlCts1p function did not affect hyphal formation induced by fetal bovine serum addition. The mass of YlCts1p was dramatically decreased by jack bean α-mannosidase digestion but not by PNGase F treatment, indicating that YlCts1p is modified only by O-mannosylation without N-glycosylation. Moreover, the O-glycan profile of YlCts1p was identical to that of total cell wall mannoproteins, supporting the notion that YlCts1p can be used as a good model for studying O-glycosylation in this dimorphic yeast.

Botrytis cinerea protein O-mannosyltransferases play critical roles in morphogenesis, growth, and virulence

Protein O-glycosylation is crucial in determining the structure and function of numerous secreted and membrane-bound proteins. In fungi, this process begins with the addition of a mannose residue by protein O-mannosyltransferases (PMTs) in the lumen side of the ER membrane. We have generated mutants of the three Botrytis cinerea pmt genes to study their role in the virulence of this wide-range plant pathogen. B. cinerea PMTs, especially PMT2, are critical for the stability of the cell wall and are necessary for sporulation and for the generation of the extracellular matrix. PMTs are also individually required for full virulence in a variety of hosts, with a special role in the penetration of intact plant leaves. The most significant case is that of grapevine leaves, whose penetration requires the three functional PMTs. Furthermore, PMT2 also contributes significantly to fungal adherence on grapevine and tobacco leaves. Analysis of extracellular and membrane proteins showed significant changes in the pattern of protein secretion and glycosylation by the pmt mutants, and allowed the identification of new protein substrates putatively glycosylated by specific PMTs. Since plants do no possess these enzymes, PMTs constitute a promising target in the development of novel control strategies against B. cinerea.

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.

Mannosylinositol phosphorylceramide is a major sphingolipid component and is required for proper localization of plasma-membrane proteins in Schizosaccharomyces pombe

In Saccharomyces cerevisiae, three classes of sphingolipids contain myo-inositol--inositol phosphorylceramide (IPC), mannosylinositol phosphorylceramide (MIPC) and mannosyldiinositol phosphorylceramide [M(IP)(2)C]. No fission yeast equivalent of Ipt1p, the inositolphosphotransferase that synthesizes M(IP)(2)C from MIPC, has been found in the Schizosaccharomyces pombe genome. Analysis of the sphingolipid composition of wild-type cells confirmed that MIPC is the terminal and most abundant complex sphingolipid in S. pombe. Three proteins (Sur1p, Csg2p and Csh1p) have been shown to be involved in the synthesis of MIPC from IPC in S. cerevisiae. The S. pombe genome has three genes (SPAC2F3.01, SPCC4F11.04c and SPAC17G8.11c) that are homologues of SUR1, termed imt1(+), imt2(+) and imt3(+), respectively. To determine whether these genes function in MIPC synthesis in S. pombe, single and multiple gene disruptants were constructed. Single imt disruptants were found to be viable. MIPC was not detected and IPC levels were increased in the triple disruptant, indicating that the three SUR1 homologues are involved in the synthesis of MIPC. GFP-tagged Imt1p, Imt2p and Imt3p localized to Golgi apparatus membranes. The MIPC-deficient mutant exhibited pleiotropic phenotypes, including defects in cellular and vacuolar morphology, and in localization of ergosterols. MIPC seemed to be required for endocytosis of a plasma-membrane-localized amino acid transporter, because sorting of the transporter from the plasma membrane to the vacuole was severely impaired in the MIPC-deficient mutant grown under nitrogen-limiting conditions. These results suggest that MIPC has multiple functions not only in the maintenance of cell and vacuole morphology but also in vesicular trafficking in fission yeast.

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.

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.

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.

Protein O-glycosylation in fungi: diverse structures and multiple functions

Protein glycosylation is essential for eukaryotic cells from yeasts to humans. When compared to N-glycosylation, O-glycosylation is variable in sugar components and the mode of linkages connecting the sugars. In fungi, secretory proteins are commonly mannosylated by protein O-mannosyltransferase (PMT) in the endoplasmic reticulum, and subsequently glycosylated by several glycosyltransferases in the Golgi apparatus to form glycoproteins with diverse O-glycan structures. Protein O-glycosylation has roles in modulating the function of secretory proteins by enhancing the stability and solubility of the proteins, by affording protection from protease degradation, and by acting as a sorting determinant in yeasts. In filamentous fungi, protein O-glycosylation contributes to proper maintenance of fungal morphology, hyphal development, and differentiation. This review describes recent studies of the structure and function of protein O-glycosylation in industrially and medically important fungi.

Repression of ergosterol level during oxidative stress by fission yeast F-box protein Pof14 independently of SCF

We describe a new member of the F-box family, Pof14, which forms a canonical, F-box dependent SCF (Skp1, Cullin, F-box protein) ubiquitin ligase complex. The Pof14 protein has intrinsic instability that is abolished by inactivation of its Skp1 interaction motif (the F-box), Skp1 or the proteasome, indicating that Pof14 stability is controlled by an autocatalytic mechanism. Pof14 interacts with the squalene synthase Erg9, a key enzyme in ergosterol metabolism, in a membrane-bound complex that does not contain the core SCF components. pof14 transcription is induced by hydrogen peroxide and requires the Pap1 transcription factor and the Sty1 MAP kinase. Pof14 binds to and decreases Erg9 activity in vitro and a pof14 deletion strain quickly loses viability in the presence of hydrogen peroxide due to its inability to repress ergosterol synthesis. A pof14 mutant lacking the F-box and an skp1-3 ts mutant behave as wild type in the presence of oxidant showing that Pof14 function is independent of SCF. This indicates that modulation of ergosterol level plays a key role in adaptation to oxidative stress.