Schizosaccharomyces pombe och1(+) encodes alpha-1,6-mannosyltransferase that is involved in outer chain elongation of N-linked oligosaccharides

A strategy for interfering with the formation of thick cell walls in Haematococcus pluvialis by down-regulating the mannan synthesis pathway

The challenges associated with effective cell wall disruption remain an important bottleneck that has restricted efforts to extract astaxanthin from Haematococcus pluvialis. Here, available transcriptomic data and an Agrobacterium tumefaciens-mediated transformation system were used to establish an H. pluvialis strain in which the key cell wall formation-related enzyme α-1,6-mannosyltransferase (HpOCH1) was downregulated in an effort to thin cell walls and thereby simplify the astaxanthin extraction process. The cell wall remodeling activity observed in these HpOch1 knockdown H. pluvialis cells resulted in dramatic reductions in the mannan organization and protective ability of the established cell walls. The cell fragmentation rate increased by 58% in HpOch1^- group relative to the control group. Critically, astaxanthin synthesis was not altered in the HpOch1 knockdown cells. Overall, this study highlights a novel technical approach to artificial cell wall thinning, offering a foundation for further efforts to more effectively leverage the astaxanthin resources of H. pluvialis.

Overexpression of cell-wall GPI-anchored proteins restores cell growth of N-glycosylation-defective och1 mutants in Schizosaccharomyces pombe

The glycoproteins of yeast contain a large outer chain on N-linked oligosaccharides; therefore, yeast is not suitable for producing therapeutic glycoproteins for human use. Using a deletion mutant strain of α1,6-mannosyltransferase (och1Δ), we previously produced humanized N-glycans in fission yeast; however, the Schizosaccharomyces pombe och1Δ cells displayed a growth delay even during vegetative growth, resulting in reduced productivity of heterologous proteins. To overcome this problem, here we performed a genome-wide screen for genes that would suppress the growth defect of temperature-sensitive och1Δ cells. Using a genomic library coupled with screening of 18,000 transformants, we identified two genes (pwp1⁺, SPBC1E8.05), both encoding GPI-anchored proteins, that increased the growth rate of och1Δ cells, lacking the outer chain. We further showed that a high copy number of the genes was needed to improve the growth rate. Mutational analysis of Pwp1p revealed that the GPI-anchored region of Pwp1p is important in attenuating the growth defect. Analysis of disruptants of pwp1⁺ and SPBC1E8.05 showed that neither gene was essential for cell viability; however, both mutants were sensitive β-glucanase, suggesting that Pwp1p and the protein encoded by SPBC1E8.05 non-enzymatically support β-glucan on the cell-surface of S. pombe. Collectively, our work not only sheds light on the functional relationships between GPI-anchored proteins and N-linked oligosaccharides of glycoproteins in S. pombe, but also supports the application of S. pombe to the production of human glycoprotein. KEY POINTS: • We screened for genes that suppress the growth defect of fission yeast och1Δ cells. • Appropriate expression of GPI-anchored proteins alleviates the growth delay of och1Δ cells. • The GPI-anchor domain of Pwp1p is important for suppressing the growth defect of och1Δ cells.

The fission yeast gmn2+ gene encodes an ERD1 homologue of Saccharomyces cerevisiae required for protein glycosylation and retention of luminal endoplasmic reticulum proteins

The gmn2 mutant of Schizosaccharomyces pombe has previously been shown to exhibit defects in protein glycosylation of N-linked oligosaccharides (Ballou, L. and Ballou, CE., Proc. Natl. Acad. Sci. USA, 92, 2790-2794 (1995)). Like most glycosylation-defective mutants, the S. pombe gmn2 mutant was found to be sensitive to hygromycin B, an aminoglycoside antibiotic. As a result of complementation analysis, the gmn2⁺ gene was found to be a single open reading frame that encodes a polypeptide of 373 amino acids consisting of multiple membrane-spanning regions. The Gmn2 protein shares sequence similarity with Kluyveromyces lactis and Saccharomyces cerevisiae Erd1 proteins, which are required for retention of luminal endoplasmic reticulum (ER) proteins. Although disruption of the gmn2⁺ gene is not lethal, the secreted glycoprotein showed a significant glycosylation defect with destabilization of the glycosyltransferase responsible for N-glycan elongation. It was also shown that a significant amount of BiP was missorted to the cell surface according to ADEL receptor destabilization. Fluorescent microscopy revealed that the functional Gmn2-EGFP fusion protein is mainly localized in the Golgi membrane. These results indicate that the Gmn2 protein is required for protein glycosylation and for retention of ER-resident proteins in S. pombe cells.

Native and non-native host assessment towards metabolic pathway reconstructions of plant natural products

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Plant metabolic networks are highly complex.

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Engineering the phytochemical pathways fully in heterologous hosts is challenging.

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Single plant cells with amplified multiple fission enable homogeneity.

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Homogeneity and high cell division rate can facilitate stable product scale-up.

Plant-based biopreparations are reasonably priced and are devoid of viral, prion and endotoxin contaminants. However, synthesizing these natural plant products by chemical methods is quite expensive. The structural complexity of plant-derived natural products poses a challenge for chemical synthesis at a commercial scale. Failure of commercial-scale synthesis is the chief reason why metabolic reconstructions in heterologous hosts are inevitable. This review discusses plant metabolite pathway reconstructions experimented in various heterologous hosts, and the inherent challenges involved. Plants as native hosts possess enhanced post-translational modification ability, along with rigorous gene edits, unlike microbes. To achieve a high yield of metabolites in plants, increased cell division rate is one of the requisites. This improved cell division rate will promote cellular homogeneity. Incorporation and maintenance of plant cell synchrony, in turn, can program stable product scale-up.

Secretory production of N-glycan-deleted glycoprotein in Aspergillus oryzae

The pharmaceutical industry has a high demand for glycoprotein production. The glycoform of glycoproteins is crucial for pharmacological activity. However, in general, cells produce glycoproteins with a heterologous glycoform, which is unfavorable for making uniform, efficacious therapeutic proteins. Here, to produce more glycoproteins with N-glycan uniformity, we applied the GlycoDelete strategy, in which endo-β-N-acetylglucosaminidase (ENGase) from the fungus Hypocrea jecorina (EndoT) is expressed at the Golgi membrane to cleave N-glycan from secretory glycoproteins, to Aspergillus oryzae cells. First, we selected candidate transmembrane domains to target EndoT to the Golgi membrane in A. oryzae cells, generated constructs for expressing the transmembrane-fused EndoT proteins and produced four potential AoGlycoDelete strains. We then confirmed that these strains produced α-amylase with a molecular weight lower than that of native α-amylase without an effect on growth. To test whether the A. oryzae α-amylase proteins had been cleaved by EndoT, we expressed and purified HA-tagged α-amylase AmyB and glucoamylase GlaA proteins from the AoGlycoDelete strain. MS and N-glycan analyses of the intact proteins confirmed neither AmyB-HA nor GlaA-HA produced from the AoGlycoDelete strain contained N-glycan. Lastly, we determined the enzymatic activities of the amylases produced by the AoGlycoDelete strain, which showed that the lack of N-glycan did not affect their activity under the conditions tested. Collectively, our findings demonstrate successful generation of an AoGlycoDelete strain that might be a good candidate for producing pharmaceutical glycoproteins with a uniform N-glycan structure.

The neutral N-linked glycans of the Basidiomycetous yeasts Pseudozyma antarctica and Malassezia furfur (Subphylum Ustilaginomycotina)

Pseudozyma antarctica and Malassezia furfur are basidiomycetous yeasts under the subphylum Ustilaginomycotina. P. antarctica is a commensal organism found in certain plant species, while M. furfur is associated with several skin diseases of animals including humans. N-linked glycans of P. antarctica and M. furfur were prepared, digested with glycosidases, and structurally analyzed using high performance liquid chromatography (HPLC) and mass spectrometry (MS). Analyses revealed the presence of neutral N-linked glycans ranging in length from Man₃GlcNAc₂-PA to Man₉GlcNAc₂-PA. The two species shared the most abundant neutral N-linked glycan: Manα1-2Manα1-6(Manα1-3)Manα1-6(Manα1-2Manα1-2Manα1-3)Manβ1-4GlcNAcβ1-4GlcNAc (M8A). The second and third most abundant neutral N-linked glycans for P. antarctica were Manα1-2Manα1-6(Manα1-2Manα1-3)Manα1-6(Manα1-2Manα1-2Manα1-3)Manβ1-4GlcNAcβ1-4GlcNAc (M9A) and Manα1-6(Manα1-3)Manα1-6(Manα1-3)Manβ1-4GlcNAcβ1-4GlcNAc (M5A), respectively. In the case of M. furfur, Manα1-2Manα1-6(Manα1-3)Manα1-6(Manα1-2Manα1-3)Manβ1-4GlcNAcβ1-4GlcNAc (M7A) was the second most abundant, while both M8A and M9A were tied for the third most abundant. The presence of putative galactose residues in the hypermannosylated neutral N-linked glycans is also discussed. This report is the first to analyze the neutral N-linked glycans of P. antarctica and M. furfur.

The α-1,6-mannosyltransferase VdOCH1 plays a major role in microsclerotium formation and virulence in the soil-borne pathogen Verticillium dahliae

Sunflower yellow wilt is a widespread and destructive disease caused by the soil-borne pathogen Verticillium dahliae (V. dahliae). To better understand the pathogenesis mechanism of V. dahliae in sunflower, T-DNA insertion library was generated via Agrobacterium tumefaciens mediated transformation system (ATMT). Eight hundred positive transformants were obtained. Transformants varied in colony morphology, growth rate, conidia production and pathogenicity in sunflower compared to the wild type strain. A mutant, named VdGn3-L2, was chosen for further analysis based on its deprivation on microsclerotia formation. The flanking sequence of T-DNA insertion site of VdGn3-L2 was identified via hiTAIL-PCR, and the interrupted gene encoded an initiation-specific α-1, 6-mannosyltransferase, named as VdOCH1. The deletion mutant ΔVdOCH1 was impaired in certain characteristics such as fungal growth, conidia production, and microsclerotia formation. Also, ΔVdOCH1 mutants were more sensitive to the cell wall perturbing reagents, such as SDS and Congo red, lost their penetration ability through cellophane membrane, and exhibited dramatically decreased pathogenicity to sunflower. The impaired phenotypes could be restored to the wild type level by complementation of the deletion mutant with full-length VdOCH1 gene. In conclusion, VdOCH1, encoded α-1,6-mannosyltransferase, manipulating the biological characteristics, microsclerotia formation and pathogenic ability of V. dahliae in sunflower.

Production of 3-hydroxypropionic acid via the malonyl-CoA pathway using recombinant fission yeast strains

3-Hydroxypropionic acid (3-HP) can be converted into derivatives such as acrylic acid, a source for producing super absorbent polymers. Although Escherichia coli has often been used for 3-HP production, it exhibits low tolerance to 3-HP. To circumvent this problem, we selected the fission yeast Schizosaccharomyces pombe as this microorganism has higher tolerance to 3-HP than E. coli. Therefore, we constructed S. pombe transformants overexpressing two genes, one encoding the S. pombe acetyl-CoA carboxylase (Cut6p) and the other encoding the malonyl-CoA reductase derived from Chloroflexus aurantiacus (CaMCR). To prevent the degradation of these expressed proteins, we employed an S. pombe protease-deficient strain. Moreover, to increase the cytosolic concentration of acetyl-CoA, we supplemented acetate to the medium, which improved 3-HP production. To further produce 3-HP by overexpressing Cut6p and CaMCR, we exploited the highly expressing S. pombe hsp9 promoter. Finally, culturing in high-density reached 3-HP production to 7.6 g/L at 31 h.

More Than Just Oligomannose: An N-glycomic Comparison of Penicillium Species

N-glycosylation is an essential set of post-translational modifications of proteins; in the case of filamentous fungi, N-glycans are present on a range of secreted and cell wall proteins. In this study, we have compared the glycans released by peptide/N-glycosidase F from proteolysed cell pellets of three Penicillium species (P. dierckxii, P. nordicum and P. verrucosum that all belong to the Eurotiomycetes). Although the major structures are all within the range Hex(5-11)HexNAc(2) as shown by mass spectrometry, variations in reversed-phase chromatograms and MS/MS fragmentation patterns are indicative of differences in the actual structure. Hydrofluoric acid and mannosidase treatments revealed that the oligomannosidic glycans were not only in part modified with phosphoethanolamine residues and outer chain och1-dependent mannosylation, but that bisecting galactofuranose was present in a species-dependent manner. These data are the first to specifically show the modification of N-glycans in fungi with zwitterionic moieties. Furthermore, our results indicate that mere mass spectrometric screening is insufficient to reveal the subtly complex nature of N-glycosylation even within a single fungal genus.

Functional characterization of the gene FoOCH1 encoding a putative α-1,6-mannosyltransferase in Fusarium oxysporum f. sp. cubense

Fusarium oxysporum f. sp. cubense (FOC) is the causal agent of banana Fusarium wilt and has become one of the most destructive pathogens threatening the banana production worldwide. However, few genes related to morphogenesis and pathogenicity of this fungal pathogen have been functionally characterized. In this study, we identified and characterized the disrupted gene in a T-DNA insertional mutant (L953) of FOC with significantly reduced virulence on banana plants. The gene disrupted by T-DNA insertion in L953 harbors an open reading frame, which encodes a protein with homology to α-1,6-mannosyltransferase (OCH1) in fungi. The deletion mutants (ΔFoOCH1) of the OCH1 orthologue (FoOCH1) in FOC were impaired in fungal growth, exhibited brighter staining with fluorescein isothiocyanate (FITC)-Concanavalin A, had less cell wall proteins and secreted more proteins into liquid media than the wild type. Furthermore, the mutation or deletion of FoOCH1 led to loss of ability to penetrate cellophane membrane and decline in hyphal attachment and colonization as well as virulence to the banana host. The mutant phenotypes were fully restored by complementation with the wild type FoOCH1 gene. Our data provide a first evidence for the critical role of FoOCH1 in maintenance of cell wall integrity and virulence of F. oxysporum f. sp. cubense.

Ght2⁺ is required for UDP-galactose synthesis from extracellular galactose by Schizosaccharomyces pombe

Schizosaccharomyces pombe has eight hexose transporter genes, ght1 (+) to ght8 (+). Here we report that ght2 (+), which is highly expressed in the presence of glucose, is essential for UDP-galactose synthesis from extracellular galactose when cells grow on glucose. The galactosylation defect of a uge1Δ mutant defective in synthesis of UDP-galactose from glucose was suppressed in galactose-containing medium, but disruption of ght2 (+) in the uge1Δ mutant reversed suppression of the galactosylation defect. Expression of Saccharomyces cerevisiae GAL2 in uge1Δght2Δ cells suppressed the defective galactosylation phenotype in galactose-containing medium. These results indicate that galactose is transported from the medium to the cytosol in a Ght2-dependent manner, and is then converted into UDP-galactose.

Identification of novel α1,3-galactosyltransferase and elimination of α-galactose-containing glycans by disruption of multiple α-galactosyltransferase genes in Schizosaccharomyces pombe

The oligosaccharides from fission yeast Schizosaccharomyces pombe contain large amounts of D-galactose (Gal) in addition to D-mannose (Man), in contrast to the budding yeast Saccharomyces cerevisiae. Detailed structural analysis has revealed that the Gal residues are attached to the N- and O-linked oligosaccharides via α1,2- or α1,3-linkages. Previously we constructed and characterized a septuple α-galactosyltransferase disruptant (7GalTΔ) anticipating a complete lack of α-Gal residues. However, the 7GalTΔ strain still contained oligosaccharides consisting of α1,3-linked Gal residues, indicating the presence of at least one more additional unidentified α1,3-galactosyltransferase. In this study we searched for unidentified putative glycosyltransferases in the S. pombe genome sequence and identified three novel genes, named otg1(+)-otg3(+) (α one, three-galactosyltransferase), that belong to glycosyltransferase gene family 8 in the Carbohydrate Active EnZymes (CAZY) database. Gal-recognizing lectin blotting and HPLC analyses of pyridylaminated oligosaccharides after deletion of these three additional genes from 7GalTΔ strain demonstrated that the resultant disruptant missing 10 α-galactosyltransferase genes, 10GalTΔ, exhibited a complete loss of galactosylation. In an in vitro galactosylation assay, the otg2(+) gene product had Gal transfer activity toward a pyridylaminated Man(9)GlcNAc(2) oligosaccharide and pyridylaminated Manα1,2-Manα1,2-Man oligosaccharide. In addition, the otg3(+) gene product exhibited Gal transfer activity toward the pyridylaminated Man(9)GlcNAc(2) oligosaccharide. Generation of an α1,3-linkage was confirmed by HPLC analysis, α-galactosidase digestion analysis, (1)H NMR spectroscopy, and LC-MS/MS analysis. These results indicate that Otg2p and Otg3p are involved in α1,3-galactosylation of S. pombe oligosaccharides.

Engineering the yeast Yarrowia lipolytica for the production of therapeutic proteins homogeneously glycosylated with Man₈GlcNAc₂ and Man₅GlcNAc₂

Background

Protein-based therapeutics represent the fastest growing class of compounds in the pharmaceutical industry. This has created an increasing demand for powerful expression systems. Yeast systems are widely used, convenient and cost-effective. Yarrowia lipolytica is a suitable host that is generally regarded as safe (GRAS). Yeasts, however, modify their glycoproteins with heterogeneous glycans containing mainly mannoses, which complicates downstream processing and often interferes with protein function in man. Our aim was to glyco-engineer Y. lipolytica to abolish the heterogeneous, yeast-specific glycosylation and to obtain homogeneous human high-mannose type glycosylation.

Results

We engineered Y. lipolytica to produce homogeneous human-type terminal-mannose glycosylated proteins, i.e. glycosylated with Man₈GlcNAc₂ or Man₅GlcNAc₂. First, we inactivated the yeast-specific Golgi α-1,6-mannosyltransferases YlOch1p and YlMnn9p; the former inactivation yielded a strain producing homogeneous Man₈GlcNAc₂ glycoproteins. We tested this strain by expressing glucocerebrosidase and found that the hypermannosylation-related heterogeneity was eliminated. Furthermore, detailed analysis of N-glycans showed that YlOch1p and YlMnn9p, despite some initial uncertainty about their function, are most likely the α-1,6-mannosyltransferases responsible for the addition of the first and second mannose residue, respectively, to the glycan backbone. Second, introduction of an ER-retained α-1,2-mannosidase yielded a strain producing proteins homogeneously glycosylated with Man₅GlcNAc₂. The use of the endogenous LIP2pre signal sequence and codon optimization greatly improved the efficiency of this enzyme.

Conclusions

We generated a Y. lipolytica expression platform for the production of heterologous glycoproteins that are homogenously glycosylated with either Man₈GlcNAc₂ or Man₅GlcNAc₂ N-glycans. This platform expands the utility of Y. lipolytica as a heterologous expression host and makes it possible to produce glycoproteins with homogeneously glycosylated N-glycans of the human high-mannose-type, which greatly broadens the application scope of these glycoproteins.

Unraveling unique structure and biosynthesis pathway of N-linked glycans in human fungal pathogen Cryptococcus neoformans by glycomics analysis

The encapsulated fungal pathogen Cryptococcus neoformans causes cryptococcosis in immunocompromised individuals. Although cell surface mannoproteins have been implicated in C. neoformans pathogenicity, the structure of N-linked glycans assembled on mannoproteins has not yet been elucidated. By analyzing oligosaccharide profiles combined with exoglycosidase treatment, we report here that C. neoformans has serotype-specific high mannose-type N-glycans with or without a β1,2-xylose residue, which is attached to the trimannosyl core of N-glycans. Interestingly, the neutral N-glycans of serotypes A and D were shown to contain a xylose residue, whereas those of serotype B appeared to be much shorter and devoid of a xylose residue. Moreover, analysis of the C. neoformans uxs1Δ mutant demonstrated that UDP-xylose is utilized as a donor sugar in N-glycan biosynthesis. We also constructed and analyzed a set of C. neoformans mutant strains lacking genes putatively assigned to the reconstructed N-glycan biosynthesis pathway. It was shown that the outer chain of N-glycan is initiated by CnOch1p with addition of an α1,6-mannose residue and then subsequently extended by CnMnn2p with multiple additions of α1,2-mannose residues. Finally, comparative analysis of acidic N-glycans from wild-type, Cnoch1Δ, Cnmnn2Δ, and Cnuxs1Δ strains strongly indicated the presence of xylose phosphate attached to mannose residues in the core and outer region of N-glycans. Our data present the first report on the unique structure and biosynthesis pathway of N-glycans in C. neoformans.

α-1,6-Mannosylation of N-linked oligosaccharide present on cell wall proteins is required for their incorporation into the cell wall in the filamentous fungus Neurospora crassa

The enzyme α-1,6-mannosyltransferase (OCH-1) is required for the synthesis of galactomannans attached to the N-linked oligosaccharides of Neurospora crassa cell wall proteins. The Neurospora crassa och-1 mutant has a tight colonial phenotype and a defective cell wall. A carbohydrate analysis of the och-1 mutant cell wall revealed a 10-fold reduction in the levels of mannose and galactose and a total lack of 1,6-linked mannose residues. Analysis of the integral cell wall protein from wild-type and och-1 mutant cells showed that the mutant cell wall had reduced protein content. The och-1 mutant was found to secrete 18-fold more protein than wild-type cells. Proteomic analysis of the proteins released by the mutant into the growth medium identified seven of the major cell wall proteins. Western blot analysis of ACW-1 and GEL-1 (two glycosylphosphatidylinositol [GPI]-anchored proteins that are covalently integrated into the wild-type cell wall) showed that high levels of these proteins were being released into the medium by the och-1 mutant. High levels of ACW-1 and GEL-1 were also released from the och-1 mutant cell wall by subjecting the wall to boiling in a 1% SDS solution, indicating that these proteins are not being covalently integrated into the mutant cell wall. From these results, we conclude that N-linked mannosylation of cell wall proteins by OCH-1 is required for their efficient covalent incorporation into the cell wall.

Characterization of two different types of UDP-glucose/-galactose 4-epimerase involved in galactosylation in fission yeast

Schizosaccharomyces species are currently the only known organisms with two types of genes encoding UDP-glucose/-galactose 4-epimerase, uge1(+) and gal10(+). A strain deleted for uge1(+) exhibited a severe galactosylation defect and a decrease in activity and in UDP-galactose content when grown in glucose-rich medium (2 % glucose), indicating that Uge1p is a major UDP-glucose/-galactose 4-epimerase under these growth conditions. In contrast, gal10(+) was efficiently expressed and involved in galactosylation of cell-surface proteins in low-glucose medium (0.1 % glucose and 2 % glycerol), but not in galactose-containing medium. In a uge1Deltagal10Delta strain, the galactosylation defect was suppressed and UDP-galactose content restored to wild-type levels in galactose-containing medium. Disruption of gal7(+), encoding galactose-1-phosphate uridylyltransferase, in the uge1Deltagal10Delta strain reversed suppression of the galactosylation defect and reduced levels of UDP-galactose, indicating that galactose is transported from the medium to the cytosol and is converted into UDP-galactose via galactose 1-phosphate by Gal7p in Sch. pombe.

Disruption of the OCH1 and MNN1 genes decrease N-glycosylation on glycoprotein expressed in Kluyveromyces lactis

Glycoproteins secreted by the yeast Kluyveromyces lactis are usually modified by the addition at asparagines-linked glycosylation sites of heterogeneous mannan residues. The secreted glycoproteins in K. lactis that become hypermannosylated will bear a non-human glycosylation pattern and can adversely affect the half-life, tissue distribution and immunogenicity of a therapeutic protein. Here, we describe engineering a K. lactis strain to produce non-hypermannosylated glycoprotein, decreasing the outer-chain mannose residues of N-linked oligosaccharides. We investigated and developed the method of two-step homologous recombination to knockout the OCH1 gene, encoding alpha1,6-mannosyltransferase and MNN1 gene, which is homologue of Saccharomyces cerevisiae MNN1, encoding a putative alpha1,3-mannosyltransferase. We found the Kloch1 mutant strain has a defect in hyperglycosylation, inability in adding mannose to the core oligosaccharide. The N-linked oligosaccharides assembled on a secretory glycoprotein, HSA/GM-CSF in Kloch1 mutant, contained oligosaccharide Man(13-14)GlcNAc(2), and in Kloch1 mnn1 mutant, contained oligosaccharide Man(9-11)GlcNAc(2), whereas those in the wild-type strain, consisted of oligosaccharides with heterogeneous sizes, Man(>30)GlcNAc(2). Taken together, these results indicated that KlOch1p plays a key role in the outer-chain mannosylation of N-linked oligosaccharides in K. lactis. The KlMnn1p, was proved to be certain contribution to the outer hypermannosylation, most possibly encodes alpha1,3-mannosyltransferase. Therefore, the Kloch1 and Kloch1 mnn1 mutants can be used as a foundational host to produce glycoproteins lacking the outer-chain hypermannoses and further maybe applicable to be a promising system for yeast therapeutic protein production.

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.

Development of valuable yeast strains using a novel mutagenesis technique for the effective production of therapeutic glycoproteins

Yeast cells producing mammalian-type N-linked oligosaccharide show severe growth defects and the decreased protein productivity because of the disruption of yeast-specific glycosyltransferases. This decreased protein productivity in engineered yeast strains is an obstacle to the development of efficient glycoprotein production in yeast. For economic and effective synthesis of such therapeutic glycoproteins in yeast, the development of appropriate strains is highly desirable. We applied a novel mutagenesis technique that utilized the proofreading-deficient DNA polymerase delta variant encoded by the pol3-01 gene of Saccharomyces cerevisiae or the cdc6-1 gene of Schizosaccharomyces pombe to the engineered S. cerevisiae TIY20 strain and S. pombe KT97 strain, respectively. TIY20, which is deficient in the outer chain of mannan due to the disruption of three genes (och1Delta, mnn1 Delta, mnn4 Delta), and KT97, which is an och1 disruptant, are impractical as hosts for the production of therapeutic glycoproteins since they show a temperature-sensitive (ts) phenotype, a growth defect phenotype, and decreased protein productivity. We successfully isolated YAB mutants that alleviated the growth defect of the TIY20 strain. Surprisingly, these mutants generally secreted foreign proteins better than the wild-type strain. Furthermore, we successfully isolated YPAB mutants that alleviated the growth defect of the KT97 strain, too. The development of these new mutants by the combination of genetic engineering of yeast and this mutagenesis technique are major breakthroughs for the production of therapeutic glycoproteins in engineered yeast cells.

Engineering of the yeast Yarrowia lipolytica for the production of glycoproteins lacking the outer-chain mannose residues of N-glycans

In an attempt to engineer a Yarrowia lipolytica strain to produce glycoproteins lacking the outer-chain mannose residues of N-linked oligosaccharides, we investigated the functions of the OCH1 gene encoding a putative alpha-1,6-mannosyltransferase in Y. lipolytica. The complementation of the Saccharomyces cerevisiae och1 mutation by the expression of YlOCH1 and the lack of in vitro alpha-1,6-mannosyltransferase activity in the Yloch1 null mutant indicated that YlOCH1 is a functional ortholog of S. cerevisiae OCH1. The oligosaccharides assembled on two secretory glycoproteins, the Trichoderma reesei endoglucanase I and the endogenous Y. lipolytica lipase, from the Yloch1 null mutant contained a single predominant species, the core oligosaccharide Man8GlcNAc2, whereas those from the wild-type strain consisted of oligosaccharides with heterogeneous sizes, Man8GlcNAc2 to Man12GlcNAc2. Digestion with alpha-1,2- and alpha-1,6-mannosidase of the oligosaccharides from the wild-type and Yloch1 mutant strains strongly supported the possibility that the Yloch1 mutant strain has a defect in adding the first alpha-1,6-linked mannose to the core oligosaccharide. Taken together, these results indicate that YlOCH1 plays a key role in the outer-chain mannosylation of N-linked oligosaccharides in Y. lipolytica. Therefore, the Yloch1 mutant strain can be used as a host to produce glycoproteins lacking the outer-chain mannoses and further developed for the production of therapeutic glycoproteins containing human-compatible oligosaccharides.

Production of Man5GlcNAc2-type sugar chain by the methylotrophic yeastOgataea minuta

The methylotrophic yeast Ogataea minuta IFO 10746 was selected as a suitable strain for producing human-compatible glycoproteins by means of analyses of its cell-wall mannoproteins. First, the OmURA3 gene encoding an orotidine-5'-phosphate decarboxylase was cloned and disrupted to generate a host strain with a uracil auxotrophic marker. Second, both the promoters and the terminators from the OmAOX1 gene encoding an alcohol oxidase for an inducible promoter, or those from the OmTDH1 gene encoding a glyceraldehyde-3-phosphate dehydrogenase for a constitutive promoter, were isolated to construct an expression vector system for heterologous genes. Next, the OmOCH1 gene encoding a starting enzyme with alpha-1,6-mannosyltransferase activity to form a backbone of the N-linked outer sugar chain peculiar to yeast was disrupted, and an alpha-1,2-mannosidase gene from Aspergillus saitoi with an endoplasmic reticulum retention signal (HDEL) under the control of the OmAOX1 promoter was introduced to convert the sugar chain to Man5GlcNAc2 in O. minuta. As a result, we succeeded in breeding a new methylotrophic yeast, O. minuta, producing a Man5GlcNAc2-high-mannose-type sugar chain as a prototype of a human-compatible sugar chain. We also elucidate here the usefulness of the strategy for producing human-compatible sugar chains in yeast.

Saccharomyces cerevisiae alpha1,6-mannosyltransferase has a catalytic potential to transfer a second mannose molecule

In yeast, the N-linked oligosaccharide modification in the Golgi apparatus is initiated by alpha1,6-mannosyltransferase (encoded by the OCH1 gene) with the addition of mannose to the Man(8)GlcNAc(2) or Man(9)GlcNAc(2) endoplasmic reticulum intermediates. In order to characterize its enzymatic properties, the soluble form of the recombinant Och1p was expressed in the methylotrophic yeast Pichia pastoris as a secreted protein, after truncation of its transmembrane region and fusion with myc and histidine tags at the C-terminus, and purified using a metal chelating column. The enzymatic reaction was performed using various kinds of pyridylaminated (PA) sugar chains as acceptor, and the products were separated by high performance liquid chromatography. The recombinant Och1p efficiently transferred a mannose to Man(8)GlcNAc(2)-PA and Man(9)GlcNAc(2)-PA acceptors, while Man(5)GlcNAc(2)-PA, which completely lacks alpha1,2-linked mannose residues, was not used as an acceptor. At high enzyme concentrations, a novel product was detected by HPLC. Analysis of the product revealed that a second mannose was attached at the 6-O-position of alpha1,3-linked mannose branching from the alpha1,6-linked mannose that is attached to beta1,4-linked mannose of Man(10)GlcNAc(2)-PA produced by the original activity of Och1p. Our results indicate that Och1p has the potential to transfer two mannoses from GDP-mannose, and strictly recognizes the overall structure of high mannose type oligosaccharide.

Functional characterization of the Hansenula polymorpha HOC1, OCH1, and OCR1 genes as members of the yeast OCH1 mannosyltransferase family involved in protein glycosylation

The alpha-1,6-mannosyltransferase encoded by Saccharomyces cerevisiae OCH1 (ScOCH1) is responsible for the outer chain initiation of N-linked oligosaccharides. To identify the genes involved in the first step of outer chain biosynthesis in the methylotrophic yeast Hansenula polymorpha, we undertook the functional analysis of three H. polymorpha genes, HpHOC1, HpOCH1, and HpOCR1, that belong to the OCH1 family containing seven members with significant sequence identities to ScOCH1. The deletions of these H. polymorpha genes individually resulted in several phenotypes suggestive of cell wall defects. Whereas the deletion of HpHOC1 (Hphoc1Delta) did not generate any detectable changes in N-glycosylation, the null mutant strains of HpOCH1 (Hpoch1Delta) and HpOCR1 (Hpocr1Delta) displayed a remarkable reduction in hypermannosylation. Although the apparent phenotypes of Hpocr1Delta were most similar to those of S. cerevisiae och1 mutants, the detailed structural analysis of N-glycans revealed that the major defect of Hpocr1Delta is not in the initiation step but rather in the subsequent step of outer chain elongation by alpha-1,2-mannose addition. Most interestingly, Hpocr1Delta showed a severe defect in the O-linked glycosylation of extracellular chitinase, representing HpOCR1 as a novel member of the OCH1 family implicated in both N- and O-linked glycosylation. In contrast, addition of the first alpha-1,6-mannose residue onto the core oligosaccharide Man8GlcNAc2 was completely blocked in Hpoch1Delta despite the comparable growth of its wild type under normal growth conditions. The complementation of the S. cerevisiae och1 null mutation by the expression of HpOCH1 and the lack of in vitro alpha-1,6-mannosyltransferase activity in Hpoch1Delta provided supportive evidence that HpOCH1 is the functional orthologue of ScOCH1. The engineered Hpoch1Delta strain with the targeted expression of Aspergillus saitoi alpha-1,2-mannosidase in the endoplasmic reticulum was shown to produce human-compatible high mannose-type Man5GlcNAc2 oligosaccharide as a major N-glycan.

Outer chain N-glycans are required for cell wall integrity and virulence of Candida albicans

The outer layer of the Candida albicans cell wall is enriched in highly glycosylated mannoproteins that are the immediate point of contact with the host and strongly influence the host-fungal interaction. N-Glycans are the major form of mannoprotein modification and consist of a core structure, common to all eukaryotes, that is further elaborated in the Golgi to form the highly branched outer chain that is characteristic of fungi. In yeasts, outer chain branching is initiated by the action of the alpha1,6-mannosyltransferase Och1p; therefore, we disrupted the C. albicans OCH1 homolog to determine the importance of outer chain N-glycans on the host-fungal interaction. Loss of CaOCH1 resulted in a temperature-sensitive growth defect and cellular aggregation. Outer chain elongation of N-glycans was absent in the null mutant, demonstrated by the lack of the alpha1,6-linked polymannose backbone and the underglycosylation of N-acetylglucosaminidase. A null mutant lacking OCH1 was hypersensitive to a range of cell wall perturbing agents and had a constitutively activated cell wall integrity pathway. These mutants had near normal growth rates in vitro but were attenuated in virulence in a murine model of systemic infection. However, tissue burdens for the Caoch1delta null mutant were similar to control strains with normal N-glycosylation, suggesting the host-fungal interaction was altered such that high burdens were tolerated. This demonstrates the importance of N-glycan outer chain epitopes to the host-fungal interaction and virulence.

Characterization of O-mannosyltransferase family in Schizosaccharomyces pombe

Protein O-glycosylation is an essential protein modification in eukaryotic cells. In Saccharomyces cerevisiae, O-mannosylation is initiated in the lumen of the endoplasmic reticulum by O-mannosyltransferase gene products (Pmt1p-7p). A search of the Schizosaccharomyces pombe genome database revealed a total of three O-glycoside mannosyltransferase homologs (ogm1+, ogm2+, and ogm4+), closely related to Saccharomyces cerevisiae PMT1, PMT2, and PMT4. Although individual ogm genes were not found to be essential, ogm1Delta and ogm4Delta mutants exhibited aberrant morphology and failed to agglutinate during mating. The phenotypes of the ogm4Delta mutant were not complemented by overexpression of ogm1+ or ogm2+, suggesting that each of the Ogm proteins does not have overlapping functions. Heterologous expression of a chitinase from S. cerevisiae in the ogm mutants revealed that O-glycosylation of chitinase had decreased in ogm1Delta cells. A GFP-tagged Fus1p from S. cerevisiae was specifically not glycosylated and accumulated in the Golgi in ogm4Delta cells. These results indicate that O-glycosylation initiated by Ogm proteins plays crucial physiological roles and can serve as a sorting determinant for protein transport of membrane glycoproteins in S. pombe.

Five Genes Involved in Biosynthesis of the Pyruvylated Galβ1,3-Epitope in Schizosaccharomyces pombe N-Linked Glycans

The N-linked galactomannans of Schizosaccharomyces pombe have pyruvylated Galbeta1,3-(PvGal) caps on a portion of the Galalpha1,2-residues in their outer chains (Gemmill, T. R., and Trimble, R. B. (1998) Glycobiology 8, 1087-1095). PvGal biosynthesis was investigated by ethyl methanesulfonate mutagenesis of S. pombe, followed by the isolation of cells devoid of negatively charged N-glycans by Q-Sepharose exclusion and failure to bind human serum amyloid P component, which acts as a lectin for terminal PvGal residues. Mutant glycans were characterized by lectin binding, saccharide composition, exoglycosidase sensitivity, and NMR spectroscopy. Restoration of the cell surface negative charge by complementation with an S. pombe genomic library led to the identification of five genes involved in PvGal biosynthesis, which we designated pvg1-pvg5. Pvg1p may be a pyruvyltransferase, since NMR of pvg1(-) mutant N-glycans revealed the absence of only the pyruvyl moiety. Pvg2p-Pvg5p are crucial for attachment of the Galbeta1,3-residue that becomes pyruvylated. Pvg3p is predicted to be a member of the beta1,3-galactosyltransferase family, and Pvg3p-green fluorescent protein labeling was consistent with Golgi localization. Predicted Pvg1p and Pvg3p functions imply that Galbeta1,3-is added to the galactomannans and is then pyruvylated in situ, rather than by an en bloc addition of PvGalbeta1,3-caps to the outer chain. Pvg4p-green fluorescent protein targeted to the nucleus, and its sequence contains a MADS-box DNA-binding and dimerization domain; however, it does not appear to solely control transcription of the other identified genes. Pvg2p and/or Pvg5p may contribute to an enzyme complex. Whereas a functional role for the PvGal epitope in S. pombe remains unclear, it is nonessential for either cell growth or mating under laboratory conditions.

Salt stress induces the expression of Schizosaccharomyces pombe och1+, which encodes an initiation-specific alpha-1,6-mannosyltransferase for N-linked outer chain synthesis of cell wall mannoproteins

The Schizosaccharomyces pombe Och1p is required for the initiation of outer chain elongation of N-linked oligosaccharides. In this report, we investigated the transcriptional control of the S. pombe och1+ gene and found that the expression of the och1+ gene was not regulated during the cell cycle, but was induced by NaCl and KCl through a transcription factor, Atf1p.