Oligosaccharide structures of the major exoglucanase secreted by Saccharomyces cerevisiae

The Fusarium oxysporum gnt2, encoding a putative N-acetylglucosamine transferase, is involved in cell wall architecture and virulence

With the aim to decipher the molecular dialogue and cross talk between Fusarium oxysporum f.sp. lycopersci and its host during infection and to understand the molecular bases that govern fungal pathogenicity, we analysed genes presumably encoding N-acetylglucosaminyl transferases, involved in glycosylation of glycoproteins, glycolipids, proteoglycans or small molecule acceptors in other microorganisms. In silico analysis revealed the existence of seven putative N-glycosyl transferase encoding genes (named gnt) in F. oxysporum f.sp. lycopersici genome. gnt2 deletion mutants showed a dramatic reduction in virulence on both plant and animal hosts. Δgnt2 mutants had αalterations in cell wall properties related to terminal αor β-linked N-acetyl glucosamine. Mutant conidia and germlings also showed differences in structure and physicochemical surface properties. Conidial and hyphal aggregation differed between the mutant and wild type strains, in a pH independent manner. Transmission electron micrographs of germlings showed strong cell-to-cell adherence and the presence of an extracellular chemical matrix. Δgnt2 cell walls presented a significant reduction in N-linked oligosaccharides, suggesting the involvement of Gnt2 in N-glycosylation of cell wall proteins. Gnt2 was localized in Golgi-like sub-cellular compartments as determined by fluorescence microscopy of GFP::Gnt2 fusion protein after treatment with the antibiotic brefeldin A or by staining with fluorescent sphingolipid BODIPY-TR ceramide. Furthermore, density gradient ultracentrifugation allowed co-localization of GFP::Gnt2 fusion protein and Vps10p in subcellular fractions enriched in Golgi specific enzymatic activities. Our results suggest that N-acetylglucosaminyl transferases are key components for cell wall structure and influence interactions of F. oxysporum with both plant and animal hosts during pathogenicity.

Characterization of the N-linked oligosaccharides from human chorionic gonadotropin expressed in the methylotrophic yeast Pichia pastoris

Human chorionic gonadotropin (hCG) is a heterodimeric, placental glycoprotein hormone involved in the maintenance of the corpus luteum during the first trimester of pregnancy. Biologically active hCG has been successfully expressed in the yeast Pichia pastoris (phCG). In the context of structural studies and therapeutic applications of phCG, detailed information about its glycosylation pattern is a prerequisite. To this end N-glycans were released with peptide-N(4)-(N-acetyl-beta-glucosaminyl)asparagine amidase F and fractionated via anion-exchange chromatography (Resource Q) yielding both neutral (80%) and charged, phosphate-containing (20%) high-mannose-type structures. Subfractionations were carried out via normal phase (Lichrosorb-NH(2)) and high-pH anion-exchange (CarboPac PA-1) chromatography. Structural analyses of the released N-glycans were carried out by using HPLC profiling of fluorescent 2-aminobenzamide derivatives, MALDI-TOF mass spectrometry, and 500-MHz(1)H-NMR spectroscopy. Detailed neutral oligosaccharide structures, in the range of Man(8)GlcNAc(2) to Man(11)GlcNAc(2) including molecular isomers, could be established, and structures up to Man(15)GlcNAc(2) were indicated. Phosphate-containing oligosaccharides ranged from Man(9)PGlcNAc(2) to Man(13)PGlcNAc(2). Mannosyl O-glycans were not detected. Profiling studies carried out on different production batches showed that the oligosaccharide structures are similar, but their relative amounts varied with the culturing media.

Cloning and characterization of the glucosidase II alpha subunit gene of Trichoderma reesei: a frameshift mutation results in the aberrant glycosylation profile of the hypercellulolytic strain Rut-C30

We describe isolation and characterization of the gene encoding the glucosidase II alpha subunit (GIIalpha) of the industrially important fungus Trichoderma reesei. This subunit is the catalytic part of the glucosidase II heterodimeric enzyme involved in the structural modification within the endoplasmic reticulum (ER) of N-linked oligosaccharides present on glycoproteins. The gene encoding GIIalpha (gls2alpha) in the hypercellulolytic strain Rut-C30 contains a frameshift mutation resulting in a truncated gene product. Based on the peculiar monoglucosylated N-glycan pattern on proteins produced by the strain, we concluded that the truncated protein can still hydrolyze the first alpha-1,3-linked glucose residue but not the innermost alpha-1,3-linked glucose residue from the Glc2Man9GlcNAc2 N-glycan ER structure. Transformation of the Rut-C30 strain with a repaired T. reesei gls2alpha gene changed the glycosylation profile significantly, decreasing the amount of monoglucosylated structures and increasing the amount of high-mannose N-glycans. Full conversion to high-mannose carbohydrates was not obtained, and this was probably due to competition between the endogenous mutant subunit and the introduced wild-type GIIalpha protein. Since glucosidase II is also involved in the ER quality control of nascent polypeptide chains, its transcriptional regulation was studied in a strain producing recombinant tissue plasminogen activator (tPA) and in cultures treated with the stress agents dithiothreitol (DTT) and brefeldin A (BFA), which are known to block protein transport and to induce the unfolded protein response. While the mRNA levels were clearly upregulated upon tPA production or BFA treatment, no such enhancement was observed after DTT addition.

A search for hyperglycosylation signals in yeast glycoproteins

N-oligosaccharides of Saccharomyces cerevisiae glycoproteins are classified as core and mannan types. The former contain 13-14 mannoses whereas mannan-type structures consist of an inner core extended with an outer chain of up to 200-300 mannoses, a process known as hyperglycosylation. The selection of substrates for hyperglycosylation poses a theoretical and practical question. To identify hyperglycosylation determinants, we have analyzed the influence of the second amino acid (Xaa) of the sequon in this process using the major exoglucanase as a model. Our results indicate that negatively charged amino acids inhibit hyperglycosylation, whereas positively charged counterparts promote it. On the basis of the tridimensional structure of Exg1, we propose that Xaa influences the orientation of the inner core making it accessible to mannan polymerase I in the appropriate position for the addition of alpha-1,6-mannoses. The presence of Glu in the Xaa of the second sequon of the native exoglucanase suggests that negative selection may drive evolution of these sites. However, a comparison of invertases secreted by S. cerevisiae and Pichia anomala suggests that hyperglycosylation signals are also subjected to positive selection.

Screening for new yeast mutants affected in mannosylphosphorylation of cell wall mannoproteins

We have carried out a screen of 622 deletion strains generated during the EUROFAN B0 project to identify non-essential genes related to the mannosylphosphate content of the cell wall. By examining the affinity of the deletants for the cationic dye alcian blue and the ion exchanger QAE-Sephadex, we have selected 50 strains. On the basis on their reactivity (blue colour intensity) in the alcian blue assay, mutants with a lower phosphate content than wild-type cells were then arranged in groups defined by previously characterized mutants, as follows: group I (mnn6), group II (between mnn6 and mnn9) and group III (mnn9). Similarly, strains that behaved like mnn1 (i.e. a blue colour deeper than wild-type) were included in group VI. To confirm the association between the phenotype and a specific mutation, strains were complemented with clones or subjected to tetrad analysis. Selected strains were further tested for extracellular invertase and exoglucanase. Within groups I, II and III, we found some genes known to be involved in oligosaccharide biosynthesis (ALG9, ALG12, HOC1), secretion (BRE5, COD4/COG5, VPS53), transcription (YOL072w/THP1, ELP2, STB1, SNF11), cell polarity (SEP7, RDG1), mitochondrial function (YFH1), cell metabolism, as well as orphan genes. Within group VI, we found genes involved in environmentally regulated transduction pathways (PAL2 and RIM20) as well as others with miscellaneous or unknown functions. We conclude that mannosylphosphorylation is severely impaired in some deletants deficient in specific glycosylation/secretion processes, but many other different pathways may also modulate the amount of mannosylphosphate in the cell wall.

The major exoglucanase secreted by Saccharomyces cerevisiae as a model to study protein glycosylation

The major yeast exoglucanase (ExgIb) consists of a 408 amino acid polypeptide carrying two short N-linked oligosaccharides attached to asparagines 165 (Asn(165)) and 325 (Asn(325)). These oligosaccharides are very similar, in both length and composition, to those present in the vacuolar protease carboxypeptidase Y. Minor glycoforms of exoglucanase arise by underglycosylation of the protein precursor (Exg(165) and Exg(325)) or by elongation of the second oligosaccharide (ExgIa). The fact that these glycoforms can be readily separated and identified by HPLC and/or Western blots converts ExgI in an excellent model to study the role of the several components or branches of the precursor oligosaccharide in the efficiency and selectivity of the oligosaccharidyl transferase in vivo. We have found that the presence of a single glucose attached to Dol-PP-GlcNAc(2)-Man(9) increases the efficiency of transfer of that oligosaccharide to the protein acceptor. Also, the glucotriose unit appears to be involved in the selection of the sequons to be occupied, in such a way that its absence results in a bias towards the glycosylation of a particular sequon. Finally, we have shown the transfer of GlcNAc(2) from Dol-PP-GlcNAc(2) to exoglucanase, an indication that this intermediate is able to translocate from the cytoplasmic to the lumenal face of the endoplasmic reticulum membrane.

The mnn2 mutant of Saccharomyces cerevisiae is affected in phosphorylation of N-linked oligosaccharides

We studied the phosphorylation of the inner core region of N-linked oligosaccharides in the mannan defective mutant Saccharomyces cerevisiae mnn2 which was described as unable to synthesize branches on the outer chain. We performed structural studies of the N-oligosaccharides synthesized by the strains mnn2, mnn1mnn2mnn9 and mnn1mnn9ldb8, and the results are compared with previously published structural data of mnn1mnn2mnn10 and mnn1mnn9 [Hernández, L.M., Ballou, L., Alvarado, E., Tsai, P.-K. and Ballou, C.E. (1989) J. Biol. Chem. 264, 13648-13659]. We conclude that the mnn2/ldb8 mutation is responsible for the inhibition of incorporation of phosphate to mannose A(3) (see below), a particular phosphorylation site of the inner core, while phosphorylation at the other possible site (mannose C(1)) is allowed, although it is also reduced. *Phosphorylation sites in mnn1mnn9. (see structure below)

Biochemical characterization of MI-ENG1, a family 5 endoglucanase secreted by the root-knot nematode Meloidogyne incognita

A beta-1,4-endoglucanase named MI-ENG1, homologous to the family 5 glycoside hydrolases, was previously isolated from the plant parasitic root-knot nematode Meloidogyne incognita. We describe here the detection of the enzyme in the nematode homogenate and secretion and its complete biochemical characterization. This study is the first comparison of the enzymatic properties of an animal glycoside hydrolase with plant and microbial enzymes. MI-ENG1 shares many enzymatic properties with known endoglucanases from plants, free-living or rumen-associated microorganisms and phytopathogens. In spite of the presence of a cellulose-binding domain at the C-terminus, the ability of MI-ENG1 to bind cellulose could not be demonstrated, whatever the experimental conditions used. The biochemical characterization of the enzyme is a first step towards the understanding of the molecular events taking place during the plant-nematode interaction.

Filamentous fungi as production organisms for glycoproteins of bio-medical interest

Filamentous fungi are commonly used in the fermentation industry for large scale production of glycoproteins. Several of these proteins can be produced in concentrations up to 20-40 g per litre. The production of heterologous glycoproteins is at least one or two orders of magnitude lower but research is in progress to increase the production levels. In the past years the structure of protein-linked carbohydrates of a number of fungal proteins has been elucidated, showing the presence of oligo-mannosidic and high-mannose chains, sometimes with typical fungal modifications. A start has been made to engineer the glycosylation pathway in filamentous fungi to obtain strains that show a more mammalian-like type of glycosylation. This mini review aims to cover the current knowledge of glycosylation in filamentous fungi, and to show the possibilities to produce glycoproteins with these organisms with a more mammalian-like type of glycosylation for research purposes or pharmaceutical applications.

Proteolytic processing of a secreted glycoprotein and O-glycosylation of mannoproteins are affected in the N-glycosylation mutant Saccharomyces cerevisiae ldb1

In a previous work [P.I. Mañas, I. Olivero, M. Avalos, L.M. Hernández, Glycobiology, 7 (1997) 487-497], we described the isolation and characterization of the Saccharomyces cerevisiae ldb1 mutant which is affected in several steps of the N-glycosylation of mannoproteins probably due to a malfunction of the Golgi apparatus. Here, we found that two further functions assigned to the Golgi cisternae are also affected in the mutant: proteolytic processing of a secreted protein and O-glycosylation. We found that around 70% of the exoglucanase activity that is secreted into the culture medium by ldb1 bears an extra tetrapeptide in its NH2-terminus due to incomplete proteolytic processing. The O-linked oligosaccharides from ldb1 mnn1 were indistinguishable from those synthesized by the parental strain mnn1. However, when the O-oligosaccharides from the wild type and ldb1 were compared, we found a significant decrease in the tetrasaccharide in the latter, as well as a concomitant increase in the disaccharide, suggesting a defect in the Kre2p/Mnt1p involved in the transfer of the third mannose of these residues.

Structural characterization of N-linked oligosaccharides from cellobiohydrolase I secreted by the filamentous fungus Trichoderma reesei RUTC 30

We have characterized the primary structures of the predominant N-linked oligosaccharides on cellobiohydrolase I from the filamentous fungus Trichoderma reesei RUTC30. Different enzymatic and chromatographic techniques were used to analyze six oligosaccharides. The combined data showed that the fungal carbohydrates have a core structure that is identical to the mammalian N-linked core. In the bulk of the N-glycans, the alpha-1,3 arm is extended with two mannoses and a glucose, suggesting incomplete processing of the oligosaccharides in the endoplasmic reticulum. The alpha-1,6 arm shows a remarkable heterogeneity: in addition to alpha-1,2-Man and alpha-1,6-Man, the presence of a terminal mannose alpha-1,6-phosphodiester was observed. This latter substituent has not been characterized before on mannosidase-processed N-glycan and its function and synthesis pathway are entirely unknown. The predominant N-glycans on cellobiohydrolase I can be represented as follows: GlcMan8GlcNAc2, GlcMan7GlcNAc2, Man7GlcNAc2, ManPGlcMan7GlcNAc2, GlcMan5GlcNAc2 and Man5GlcNAc2.

Preferential transfer to truncated oligosaccharides to the first sequon of yeast exoglucanase in Saccharomyces cerevisiae alg3 cells

In addition to the exoglucanases (Exg) secreted into the culture medium by wild type cells, ExgIa and ExgIb, which have oligosaccharides attached to both potential N-glycosylation sites, Saccharomyces cerevisiae alg3 mutant secreted substantial amounts (35--44%) of underglycosylated and unglycosylated forms. Quantification of these forms indicated that no more than 78% of the available N-sites were occupied. About 50% of the transferred oligosaccharides were endo H sensitive, indicating that the lipid-linked precursor had completed its synthesis to Glc3-Man9-GlcNAc2. The other 50% remained endo H-resistant and, accordingly, it should be derived from the precursor oligosaccharide Man5-GlcNAc2 synthesized by this mutant. A closer analysis of forms that have received two oligosaccharides (ExgIb) showed that the first sequon was enriched in truncated residues, whereas the second one was enriched in regular counterparts. Similarly, analysis of the individual underglycosylated glycoforms indicated that 38% of the oligosaccharides attached to the second site were regular. This percentage dropped to 20% for glycoforms carrying the oligosaccharide in the first sequon. The preferential transfer of truncated oligosaccharides to the first glycosylation site seems to be a consequence of (1) the low percentage of truncated lipid linked oligosaccharides that receives the glucotriose unit, and (2) the effect of the glucotriose unit on the selection of N-sites to be glycosylated.

In vivo processing of the precursor of the major exoglucanase by KEX2 endoprotease in the Saccharomyces cerevisiae secretory pathway

We have established the main post-translational modification of the major exoglucanase of Saccharomyces cerevisiae as the enzyme progresses through the secretory pathway. The protein portion of the enzyme accumulated by sec18 cells was about 2 kDa larger than that of the secreted enzyme. This precursor (form A) was stable when maintained in the endoplasmic reticulum but was processed to the mature form (form B) before the block imposed by the sec7 mutation. Sec7 cells, when incubated at 37 degrees C, accumulated form B first, but upon prolonged incubation, form A was preferentially accumulated. When the supply of newly synthesized exoglucanase was prevented by the addition of cycloheximide, the accumulated A was transformed into B in the presence of altered Sec7p that still prevented secretion. Conversion of A into B was prevented in the double mutant sec7 kex2-1, indicating that Kex2p is central to the in vivo processing. Consistent with this, a KEX2 deletion mutant secreted form A exclusively. Conversion of A into B was also prevented in sec7 cells by the presence of dinitrophenol, a poison that depletes ATP levels, indicating that processing is dependent upon intracellular transport which involves ER --> Golgi and/or, at least, one intra-Golgi step(s). It follows that this transport step(s) is independent of functional Sec7p.

Molecular biology of yeast exoglucanases

Three exoglucanase (Exg) genes have been reported in Saccharomyces cerevisiae. Gene EXG1 encodes the major isoenzyme (ExgI). Differential glycosylation of the primary translation product throughout the secretory pathway results in the secretion of several glycoforms. The major glycoform (ExgIb) contains two short carboxypeptidase Y-like oligosaccharides attached to both potential glycosylation sites present in the molecule. A minor glycoform (ExgIa) arises from the former by elongation of the second oligosaccharide. The protein portion is processed in the secretory pathway by the Kex2 protease. Gene EXG2 encodes a 63 kDa polypeptide with 12 potential glycosylation sites. The predicted protein, ExgII, carries a signal peptide at the amino terminus and a glycosyl-phosphatidyl inositol anchoring motif at the carboxyl end. The latter appears responsible for the particulate nature of this isoenzyme, since its elimination results in the secretion of this activity into the culture medium. Gene SSG1 encodes a 52 kDa polypeptide which is specifically synthesized during sporulation of diploids. SSG1 expression is under control of both sexual (a1-alpha 2 element) and nutritional control. Although homozygous ssg1/ssg1 diploid strains are still able to complete sporulation, they exhibited a delay in the appearance of mature asci. Single or double disruption of EXG1 and EXG2 did not result in any relevant phenotype and the triple mutant behaved as ssg1/ssg1. A ExgI-related enzyme is secreted by Candida albicans. All these four enzymes share 8 highly conserved regions in the same relative positions, indicating that they derived from a common ancestor. However, no clear function has so far been demonstrated for them.

Selective elongation of the oligosaccharide attached to the second potential glycosylation site of yeast exoglucanase: effects on the activity and properties of the enzyme

Three exoglucanases (Exgs), ExgIa, ExgIb and Exg325, are secreted by Saccharomyces cerevisiae cells. They share a common protein portion with two potential glycosylation sites (sequons) but differ in the amount of N-linked carbohydrate [Basco, R.D., Muñoz, M.D., Hernández, L.M., Váquez de Aldana, C. and Larriba, G. (1993) Yeast 9, 221-234]. ExgIb contains two short oligosaccharides attached to asparagines (Asn) 165 and 325 of the primary translation product [Hernández, L.M., Olivero, I., Alvarado, E. and Larriba, G. (1992) Biochemistry 31, 9823-9831]. Exg325 carries a single, short oligosaccharide bound to Asn325 whereas ExgIa has at least one large oligosaccharide, since it has not been produced by mutant mnn9. To address the question of the origin of ExgIa, both sequons were individually mutated by substituting Gln for Asn. An ExgIa-like isoenzyme was still secreted by mutant Exg165 but not by mutant Exg325. Additional studies on sequential deglycosylation of ExgIa with endo-beta-N-acetylglucosaminidase H (endo H), the susceptibility of both oligosaccharides to the endoglycosidase, and analysis of the presence of GlcNAc at both asparagine residues after total deglycosylation with endo H, indicated that ExgIa contained two oligosaccharides, a short one bound to Asn165 and a large one bound to Asn325, and, accordingly, originated from ExgIb. The elongation of the second oligosaccharide did not result in a higher stability towards thermal inactivation or unfolding, or in an increased resistance to proteases as compared with ExgIb; however, the affinity of the enzyme towards laminarin decreased by 50%. This site-specific elongation occurred in the oligosaccharide that was less susceptible to endo H, indicating that these properties are determined by different conformational constraints.

Glycosylation of yeast exoglucanase sequons in alg mutants deficient in the glucosylation steps of the lipid-linked oligosaccharide. Presence of glucotriose unit in Dol-PP-GlcNAc2Man9Glc3 influences both glycosylation efficiency and selection of N-linked sites

The major exoglucanase (Exg) from Saccharomyces cerevisiae has a short N-linked oligosaccharide attached to each of the potential glycosylation sites present in the primary translation product. We have studied the Exg glycoforms secreted by alg mutants deficient in the final steps of the assembly of dolichol-P-P-GlcNAc2-Man9-Glc3. These mutants synthesize and transfer to nascent proteins truncated oligosaccharides lacking two (alg8) or three (alg5 and alg6) glucoses. In addition to the enzyme carrying both sugar chains (ExgII), all three mutants secreted underglycosylated forms containing one oligosaccharide attached to either the first (ExgII'1/2) or the second (ExgII1/2) potential glycosylation site, and nonglycosylated enzyme (ExgTuni). As compared with alg5 and alg6, alg8 secreted a higher proportion of ExgII, which was paralleled by a significant drop in the proportion of ExgTuni and, to a lesser extent, of ExgII1/2. The presence of a single glucose attached to Dol-P-P-GlcNAc2-Man9 therefore increases the efficiency of transfer of the that oligosaccharide to the protein acceptor in vivo. Moreover, whereas ExgII'1/2 was never secreted by wild type cells, it was the most abundant underglycosylated form secreted by all three mutants. These mutants are affected in the efficiency at which the individual sequons that are glycosylated, and this suggests a role for the glucotriose unit in the selection of the sequons are to be occupied in glycoproteins synthesized by wild type.

Reduced efficiency in the glycosylation of the first sequon of Saccharomyces cerevisiae exoglucanase leads to the synthesis and secretion of a new glycoform of the molecule

In addition to exoglucanases (EXGs) I and II, old cultures of Saccharomyces cerevisiae secreted into the culture medium a new immunologically-related material that exhibited exoglucanase activity. The new exoglucanase (EXGII1/2) was purified from stationary-phase cultures. It turned out to be a glycoprotein whose protein portion was identical to that of the other two isoenzymes in terms of ionic properties, size, amino acid composition and NH2-terminal sequence (25 residues). Disruption of the structural gene encoding EXGs I and II resulted in a strain unable to secrete all three isoenzymes. EXGII1/2 was indistinguishable in terms of molecular weight from the single intermediate detected during the deglycosylation (mediated by endo H) of EXGII by sodium dodecyl sulphate-polyacrylamide gel electrophoresis. Thus, the new isoenzyme contains only one of the two slightly elongated mannan inner cores present in enzyme II. Two intermediates were, however, detected when the deglycosylation of EXGII was monitored by ion-exchange chromatography (high-pressure liquid chromatography). Site-directed mutagenesis indicated that the major intermediate, which eluted at about the same position as enzyme II1/2, corresponded to protein molecules carrying the oligosaccharide attached to the Asn of the second sequon, whereas the minor one carried the oligosaccharide in the first potential glycosylation site. Several lines of evidence indicate that EXGII1/2 is a biosynthetic product resulting from an imbalance between the rate of protein synthesis and the glycosylation capabilities of the glycosylation machinery.