Mechanistic insights into Schizosaccharomyces pombe GT-A family protein Pvg3 in the biosynthesis of pyruvylated ß1,3-galactose of N-linked oligosaccharides.

N-linked oligosaccharides in the fission yeast Schizosaccharomyces pombe contain large amounts of d-galactose (Gal), which mainly comprises α1,2- and α1,3-linked Gal except for pyruvylated ß1,3-linked Gal (PvGalß) at the non-reducing end. The PvGalß unit of N-glycans is important for regulating nonsexual flocculation and invasive growth, but the mechanistic basis for ß-galactosylation in fission yeast is poorly understood. To gain insight into this mechanism, we have characterized three genes previously identified to be involved in PvGalß biosynthesis (pvg2, pvg3, and pvg5), with a focus on pvg3, which is predicted to contain a domain conserved in galactosyltransferase family 31 (GT31) proteins. Fluorescent microscopy revealed that Pvg3 is stably localized at the Golgi membrane, regardless of the presence of pvg2+ or pvg5+, suggesting that Pvg2 and Pvg5 are essential for the function of Pvg3 as a ß1,3-galactosyltransferase, and not for its localization to the Golgi. Mutation of the GT31 family DXD motif and GT-A fold in Pvg3 resulted in loss of catalytic activity in vivo, supporting the idea that Pvg3 is a GT-A type ß1,3-galactosyltransferase. Docking simulations further indicated that Pvg3 can recognize donor and acceptor substrates suitable for ß-(1→3) bond formation. Yeast two-hybrid assay showed that Pvg5 physically interacts with Pvg3 and the pyruvyltransferase Pvg1. Collectively, these results provide insight into ß-galactosylation catalyzed by Pvg3 and the supporting role of Pvg5 in PvGalß biosynthesis.

Galactosylation of cell-surface glycoprotein required for hyphal growth and cell wall integrity in Schizosaccharomyces japonicus.

Schizosaccharomyces japonicus is a dimorphic yeast, transiting between unicellular and hyphal growth. The glycoproteins of fission yeast contain, in addition to mannose (Man), a large number of galactose (Gal) residues. Previously, we reported that the cell-surface O-glycans of S. japonicus comprise mainly tri-saccharides (Gal-Man-Man) as a main component, in contrast to the tetra-saccharides observed in other Schizosaccharomyces species. Here we have investigated the function of cell-surface Gal residues in S. japonicus. Because disruption of gms1+, encoding the UDP-Gal transporter required for galactomannan synthesis, abolishes cell-surface galactosylation in Schizosaccharomyces pombe, we constructed a deletion mutant of the homologous gene in S. japonicus gms1Δ [gms1 (S.j)] and determined the N- and O-linked oligosaccharide structures present on the cell surface. Disruption of gms1 (S.j) resulted in a complete lack of Gal on the cell surface, indicating that Gms1 plays an essential role in supplying UDP-Gal from the cytoplasm to the Golgi lumen. Analytical microscopy of gms1Δ demonstrated that the lack of cell-surface Gal did not affect cell growth or morphology during vegetative growth. However, hyphal development was blocked in gms1Δ, even in the presence of the topoisomerase I inhibitor camptothecin, which is known to induce hyphal differentiation in wild-type S. japonicus. Collectively, these findings show that Gal-containing oligosaccharides are required for cell wall integrity during filamentous growth in S. japonicus.

In vivo imaging of fluorescent albumin modified with pyruvylated-human-type complex oligosaccharide reveals sialylation-like biodistribution and kinetics.

Both pyruvylation and sialylation onto the terminus of oligosaccharides of N-glycoproteins seem to be structurally and functionally similar with a property of conferring negative charge. However, detailed molecular characteristics of pyruvylation and sialylation in vivo were elusive. Here, to investigate an effect of terminal pyruvylation to N-glycan on in vivo biodistribution and kinetics, we prepared human serum albumin (HSA) modified with pyruvylated N-glycan (PvG), conjugated with HiLyte Fluor 750 (FL750-PvGHSA). In vivo imaging by using FL750-PvGHSA revealed that terminally pyruvylated N-glycoalbumin was excreted like sialylated N-glycoalbumin, suggesting that pyruvylation mimics sialylation in in vivo biodistribution and kinetics of N-glycoproteins. Terminal pyruvylation onto N-glycans can be a potential tool for a novel glycoengineering strategy.

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 ß-glucanase, suggesting that Pwp1p and the protein encoded by SPBC1E8.05 non-enzymatically support ß-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.

Substrate specificities of α1,2- and α1,3-galactosyltransferases and characterization of Gmh1p and Otg1p in Schizosaccharomyces pombe.

In the fission yeast Schizosaccharomyces pombe, α1,2- and α1,3-linked D-galactose (Gal) residues are transferred to N- and O-linked oligosaccharides of glycoproteins by galactosyltransferases. Although the galactomannans are important for cell-cell communication in S. pombe (e.g., in nonsexual aggregation), the mechanisms underlying galactosylation in cells remain unclear. Schizosaccharomyces pombe has 10 galactosyltransferase-related genes: seven belonging to glycosyltransferase (GT) family 34 and three belonging GT family 8. Disruption of all 10 α-galactosyltransferases (strain Δ10GalT) has been shown to result in a complete lack of α-Gal residues. Here, we have investigated the function and substrate specificities of galactosyltransferases in S pombe by using strains expressing single α-galactosyltransferases in the Δ10GalT background. High-performance liquid chromatography (HPLC) analysis of pyridylaminated O-linked oligosaccharides showed that two GT family 34 α1,2-galactosyltransferases (Gma12p and Gmh6p) and two GT family 8 α1,3-galactosyltransferases (Otg2p and Otg3p) are involved in galactosylation of O-linked oligosaccharide. Moreover, 1H-NMR of N-glycans revealed that three GT family 34 α1,2-galactosyltransferases (Gmh1p, Gmh2p and Gmh3p) are required for the galactosylation of N-linked oligosaccharides. Furthermore, HPLC and lectin-blot analysis revealed that Otg1p showed α1,3-galactosyltransferase activity under conditions of co-expression with Gmh6p, indicating that α-1,2-linked galactose is required for the galactosylation activity of Otg1p in S. pombe. In conclusion, eight galactosyltransferases have been shown to have activity in S. pombe with different substrate specificities. These findings will be useful for genetically tailoring the galactosylation of both N- and O-glycans in fission yeast.

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.

Golgi localization of glycosyltransferases requires Gpp74p in Schizosaccharomyces pombe.

The majority of Golgi glycosyltransferases are type II membrane proteins with a small cytosolic tail at their N-terminus. Several mechanisms for localizing these glycosyltransferases to the Golgi have been proposed. In Saccharomyces cerevisiae, the phosphatidylinositol-4-phosphate-binding protein ScVps74p interacts with the cytosolic tail of a Golgi glycosyltransferase and contributes to its localization. In this study, we investigated whether a similar mechanism functions in the fission yeast Schizosaccharomyces pombe. First, we identified gpp74+ (GPP34 domain-containing Vps74 homolog protein), a gene encoding the S. pombe homolog of S. cerevisiae Vps74p. Deletion of the gpp74+ gene resulted in the missorting of three Golgi glycosyltransferases, SpOch1p, SpMnn9p, and SpOmh1p, to vacuoles, but not SpAnp1p, indicating Gpp74p is required for targeting some glycosyltransferases to the Golgi apparatus. Gpp74p with an N-terminal GFP-tag localized to both the Golgi apparatus and the cytosol. Golgi localization of Gpp74p was dependent on the phosphatidylinositol 4-kinase SpPik1p. Site-directed mutagenesis of hydrophobic and basic amino acids in the cytosolic tails of SpOch1p and SpMnn9p resulted in their missorting to vacuoles, indicating these cytosolic N-terminal residues are important for localization in the Golgi. Unexpectedly, no prominent alternations in protein glycosylation were observed in S. pombe gpp74Δ cells, probably due to the residual Golgi localization of some SpOch1p and SpMnn9p in these cells. Collectively, these results demonstrate that both Gpp74p-dependent and Gpp74p-independent mechanisms are responsible for the Golgi localization of glycosyltransferases to the Golgi in S. pombe. KEY POINTS: • Gpp74p is involved in the localization of glycosyltransferases to the Golgi. • The cytosolic tails of glycosyltransferases are important for Golgi localization. • Gpp74p localizes to the Golgi in a SpPik1p-dependent manner.

Characterization of N- and O-linked galactosylated oligosaccharides from fission yeast species.

The N- and O-linked oligosaccharides from fission yeast Schizosaccharomyces pombe not only contain large amounts of d-mannose (Man) but also contain large amounts of d-galactose (Gal). Although the galactomannans of S. pombe are mainly composed of α1,2- or α1,3-linked Gals, some of the terminal α1,2-linked Gals are found to be linked to pyruvylated ß1,3-linked galactose (PvGal). We have determined the structural characteristics of the N-glycans and O-glycans in three Schizosaccharomyces species (S. japonicus, S. octosporus, and S. cryophilus) using lectin blot, 1H NMR spectroscopy, and size-fractionation high performance liquid chromatography (HPLC), and found that the galactosylation of oligosaccharides was a common feature in fission yeasts. In addition, each of the terminal Galα1,2-, Galß1,3- and non-substituted Man residues exhibited distinct characteristics. A BLAST search of gene databases in Schizosaccharomyces identified genes homologous to pvg1 encoding pyruvyltransferase of S. pombe. These genes, when expressed in an S. pombe pvg1Δ strains, led to the pyruvylation of non-reducing terminal ß-linked Gal, suggesting the biosynthetic pathway of PvGal-containing oligosaccharides is highly conserved in fission yeasts.