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

Rsc14-controlled expression ofMNN6, MNN4andMNN1regulates mannosylphosphorylation ofSaccharomyces cerevisiaecell wall mannoproteins

ybl006cDelta was selected using an Alcian-blue-based screen aimed to identify nonessential genes involved in the regulation of mannosylphosphorylation. When cells of this deletant were mixed with the cationic dye Alcian blue in a typical assay, they remained white, indicating a low number of mannosylphosphate groups on the cell surface. ybl006cDelta cells did not show any defect in growth rate nor in the glycosylation or secretion rate of the major exoglucanase Exg1. Transcriptome analysis of ybl006cDelta using macroarrays showed at least two-fold changes in the expression of 52 genes (<0.9% of the genome). Three of these have previously been reported to be directly (MNN6 and MNN4) or indirectly (MNN1) implicated in the transfer of mannosylphosphate to the N- and O-oligosaccharides. Alterations in the expression of these genes were confirmed by Northern analysis. YBL006C product was recently identified as a subunit (Rsc14) of the RSC chromatin-remodelling complex of Saccharomyces cerevisiae. It follows that remodelling of chromatin can be an important regulatory mechanism for the maturation of cell wall mannoproteins.

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.

The ER protein folding sensor UDP-glucose glycoprotein-glucosyltransferase modifies substrates distant to local changes in glycoprotein conformation

We present in vitro data that explain the recognition mechanism of misfolded glycoproteins by UDP-glucose glycoprotein-glucosyltransferase (UGGT). The glycoprotein exo-(1,3)-beta-glucanase (beta-Glc) bearing two glycans unfolds in a pH-dependent manner to become a misfolded substrate for UGGT. In the crystal structure of this glycoprotein, the local hydrophobicity surrounding each glycosylation site coincides with the differential recognition of N-linked glycans by UGGT. We introduced a single F280S point mutation, producing a beta-Glc protein with full enzymatic activity that was both recognized as misfolded and monoglucosylated by UGGT. Contrary to current views, these data show that UGGT can modify N-linked glycans positioned at least 40 A from localized regions of disorder and sense subtle conformational changes within structurally compact, enzymatically active glycoprotein substrates.

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.

Yeast 1,3-beta-glucan synthase activity is inhibited by phytosphingosine localized to the endoplasmic reticulum

1,3-beta-D-Glucan, a major filamentous component of the cell wall in the budding yeast Saccharomyces cerevisiae, is synthesized by 1,3-beta-glucan synthase (GS). Although a yeast gene whose product is required for GS activity in vitro, GNS1, was isolated and characterized, its role in GS function has remained unknown. In the current study we show that Deltagns1 cells accumulate a non-competitive and non-proteinous inhibitor(s) in the membrane fraction. Investigations of inhibitory activity on GS revealed that the inhibitor(s) is mainly present in the sphingolipid fraction. It is shown that Deltagns1 cells contain phytosphingosine (PHS), an intermediate in the sphingolipid biosynthesis, 30-fold more than wild-type cells do. The membrane fraction isolated from Deltasur2 cells contains an increased amount of dihydrosphingosine (DHS) and also exhibits reduced GS activity. Among constituents of the sphingolipid fraction, PHS and DHS show striking inhibition in a non-competitive manner. The intracellular level of DHS is much lower than that of PHS in wild-type cells, suggesting that PHS is the primary inhibitor of GS in vivo. The localization of PHS to the endoplasmic reticulum in wild-type cells coincides with that of the inhibitor(s) in Deltagns1 cells. Taken together, our results indicate that PHS is a potent inhibitor of yeast GS in vivo.

N-glycosylation by transfer of GlcNAc2 from dolichol-PP-GlcNAc2 to the protein moiety of the major yeast exoglucanase

Transfer of truncated oligosaccharides to yeast exoglucanase (Exg) in Saccharomyces cerevisiae alg1 has been investigated. When incubated at the non-permissive temperature, alg1 cells secreted into the culture medium, in addition to the exoglucanase glycoforms secreted by wild type, underglycosylated forms as well as material with ionic properties of the non-glycosylated enzyme. As expected, none of the latter had affinity towards concanavalin A, but part of it bound to wheat germ agglutinin (WGA), suggesting that it contained, in addition to non-glycosylated Exg, glycoforms carrying non-reducing terminal GlcNAc. Only the WGA-bound material could be labelled with galactosyltransferase; furthermore, the label could be released by treatment with peptide-N4-N-acetyl-beta-glucosamine asparagine amidase. These results unambiguously demonstrate that GlcNAc2 can be transferred from dolichol-PP-GlcNAc2 to one or both sequons of yeast Exg. Accordingly, they support previous observations suggesting that this early intermediate is able to translocate in vivo in order to make its sugar portion accessible to the oligosaccharyltransferase in the lumen of the endoplasmic reticulum.

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.

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.