Accumulation and secretion of exoglucanase activity in yeast secretory mutants

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.

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.

The major exoglucanase from Candida albicans: a non-glycosylated secretory monomer related to its counterpart from Saccharomyces cerevisiae

Exoglucanases secreted by two different strains from Candida albicans have been purified to homogeneity. The purified enzyme from each strain behaved as a non-glycosylated monomer (molecular weight 38,000) that was identical in terms of sodium dodecyl sulphate/polyacrylamide gel electrophoresis comigration, amino acid analysis and amino terminal sequence. The amino acid composition was similar to that of the major exoglucanase from Saccharomyces cerevisiae. In addition, these two enzymes displayed a 50% homology in the first 35 amino acids of the amino terminus. Antibodies against the deglycosylated exoglucanase (treated with Endo H) from S. cerevisiae were reactive with the exoglucanase from C. albicans and vice versa. Immunoblotting proved to be a semiquantitative method to detect C. albicans antigen in culture fluids. The exoglucanase from C. albicans appears to enter the secretory pathway without undergoing N-glycosylation.

Two glycosylation patterns for a single protein (exoglucanase) in Saccharomyces cerevisiae

Exoglucanases (beta-glucosidases) I and II secreted into the culture medium by Saccharomyces cerevisiae were purified from cell cultures harvested at the early exponential phase of growth in order to avoid contamination of the second by a new immunologically-related material. The amino acid composition of the purified enzymes was roughly the same. In addition, both exoglucanases exhibited an identical NH2-terminal sequence (50 residues). These results confirm our previous results about the identity of the protein moieties of both enzymes. Exoglucanase I appears to arise by elongation of one or both short oligosaccharides present in enzyme II.

A similar protein portion for two exoglucanases secreted by Saccharomyces cerevisiae

Exoglucanase (exo-1,3-beta-D-glucan glycohydrolase, EC 3.2.1.56) activity secreted by Saccharomyces cerevisiae into the culture medium was separated by ion exchange chromatography into two glycoprotein isoenzymes which contributed 10% (exoglucanase I) and 90% (exoglucanase II) towards the total activity. Analysis of the "in vitro" deglycosylated products by polyacrylamide gel electrophoresis under native or denaturing conditions indicated that the protein portions of both exoglucanases exhibited identical mobility, each one consisting of two polypeptides with Mr of 47,000 and 48,000. The same profile was shown by the exoglucanase secreted in the presence of tunicamycin. Antibodies raised against the protein portion of exoglucanase II did react with both native exoglucanases and their deglycosylated products with a pattern indicative of immunological identity. Digestion of the "in vitro" deglycosylated products of both exoglucanases with Staphylococcus aureus V-8 protease or trypsin generated the same proteolytic fragments in each case. Only exoglucanase II was secreted by protoplasts. These and previously reported results indicate that the protein portions of both isoenzymes may be the product of the same gene (or a family of related genes), and that exoglucanase I is a product of enzyme II, modified by a process occurring beyond the permeability barrier of the cell.

The major yeast exoglucanase: an extracellular glycoprotein lacking the carbohydrate outer chain

The major exoglucanase (1,3-beta-D-glucan glucanohydrolase, EC 3.2.1.39) secreted by Saccharomyces cerevisiae contains protein, mannose and phosphate in a molar ratio of 1:27:1. When digested with endo-beta-N-acetylglucosaminidase H (EC 3.2.1.96) it sequentially released two asparagine-linked oligosaccharide chains. Oligosaccharides were fractionated into a neutral and acidic component, each one accounting for 50% of the total carbohydrate. The neutral oligosaccharide consisted of a mixture of three homologues ranging from GlcNAc-(Man)12 to GlcNAc-(Man)14. The acidic carbohydrate was, in turn, split into two components. The major one (45% of the initial material) contained a phosphodiester bond and released only mannose when subjected to mild acid hydrolysis. From the filtration pattern, it was shown to be a mixture of oligosaccharides ranging from GlcNAc-(Man)11-P to GlcNAC-(Man)13-P. The minor phosphorylated component, which represented the residual carbohydrate (5%), contained a phosphomonoester bond. It was also heterogeneous in size, the several homologues having one mannose less than their counterparts from the phosphodiester oligosaccharide. These results clearly indicate that the addition of an outer chain of carbohydrate is not a requirement for the externalization of yeast glycoproteins.

Two ionic forms of exoglucanase in yeast secretory mutants

Analysis of exoglucanase activity accumulated by sec mutants from Saccharomyces cerevisiae revealed the presence of two ionic forms of the major exoglucanase (exo II) secreted into the culture medium. From the accumulation pattern of representative sec mutants and the carbohydrate composition it appears that the less acidic form is converted into the more acidic one by addition of one phosphate to one of the oligosaccharide cores as the enzyme progresses through the secretory pathway. Exoglucanase I, the heavier isoenzyme, was not accumulated by the mutants. Accordingly, it should arise from exoglucanase II after the execution point of sec1 mutation.