Separation and characterization of two alpha 1,2-mannosyltransferase activities from Saccharomyces cerevisiae

Architecture and biosynthesis of the Saccharomyces cerevisiae cell wall

The wall gives a Saccharomyces cerevisiae cell its osmotic integrity; defines cell shape during budding growth, mating, sporulation, and pseudohypha formation; and presents adhesive glycoproteins to other yeast cells. The wall consists of β1,3- and β1,6-glucans, a small amount of chitin, and many different proteins that may bear N- and O-linked glycans and a glycolipid anchor. These components become cross-linked in various ways to form higher-order complexes. Wall composition and degree of cross-linking vary during growth and development and change in response to cell wall stress. This article reviews wall biogenesis in vegetative cells, covering the structure of wall components and how they are cross-linked; the biosynthesis of N- and O-linked glycans, glycosylphosphatidylinositol membrane anchors, β1,3- and β1,6-linked glucans, and chitin; the reactions that cross-link wall components; and the possible functions of enzymatic and nonenzymatic cell wall proteins.

The components of the Saccharomyces cerevisiae mannosyltransferase complex M-Pol I have distinct functions in mannan synthesis

The yeast Saccharomyces cerevisiae processes N-linked glycans in the Golgi apparatus in two different ways. Whereas most of the proteins of internal membranes receive a simple core-type structure, a long branched polymer termed mannan is attached to the glycans of many of the proteins destined for the cell wall. The first step in mannan synthesis is the initiation and extension of an alpha-1,6-linked polymannose backbone. This requires the sequential action of two enzyme complexes, mannan polymerases (M-Pol) I and II. M-Pol I contains the proteins Mnn9p and Van1p, although the stoichiometry and individual contributions to enzyme action are unclear. We report here that the two proteins are each present as a single copy in the complex. Both proteins contain a DXD motif found in the active site of many glycosyltransferases, and mutations in this motif in Mnn9p or Van1p reveal that both proteins contribute to mannose polymerization. However, the effects of these mutations on both the in vivo and in vitro activity are distinct, suggesting that the two proteins may have different roles in the complex. Finally, we show that a simple glycoprotein based on hen egg lysozyme can be used as a substrate for modification by purified M-Pol I in vitro.

Mutants defective in secretory/vacuolar pathways in the EUROFAN collection of yeast disruptants

We have screened the EUROFAN (European Functional Analysis Network) deletion strain collection for yeast mutants defective in secretory/vacuolar pathways and/or associated biochemical modifications. We used systematic Western immunoblotting to analyse the electrophoretic pattern of several markers of the secretory/vacuolar pathways, the soluble alpha-factor, the periplasmic glycoprotein invertase, the plasma membrane GPI-anchored protein Gas1p, and two vacuolar proteins, the soluble carboxypeptidase Y and the membrane-bound alkaline phosphatase, which are targeted to the vacuole by different pathways. We also used colony immunoblotting to monitor the secretion of carboxypeptidase Y into the medium, to identify disruptants impaired in vacuolar targeting. We identified 25 mutants among the 631 deletion strains. Nine of these mutants were disrupted in genes identified in recent years on the basis of their involvement in trafficking (VPS53, VAC7, VAM6, APM3, SYS1), or glycosylation (ALG12, ALG9, OST4, ROT2). Three of these genes were identified on the basis of trafficking defects by ourselves and others within the EUROFAN project (TLG2, RCY1, MON2). The deletion of ERV29, which encodes a COPII vesicle protein, impaired carboxypeptidase Y trafficking from the endoplasmic reticulum to the Golgi apparatus. We also identified eight unknown ORFs, the deletion of which reduced Golgi glycosylation or impaired the Golgi to vacuole trafficking of carboxypeptidase Y. YJR044c, which we identified as a new VPS gene, encodes a protein with numerous homologues of unknown function in sequence databases.

The Saccharomyces cerevisiae protein Mnn10p/Bed1p is a subunit of a Golgi mannosyltransferase complex

In the yeast Saccharomyces cerevisiae many of the N-linked glycans on cell wall and periplasmic proteins are modified by the addition of mannan, a large mannose-containing polysaccharide. Mannan comprises a backbone of approximately 50 alpha-1,6-linked mannoses to which are attached many branches consisting of alpha-1,2-linked and alpha-1,3-linked mannoses. The initiation and subsequent elongation of the mannan backbone is performed by two complexes of proteins in the cis Golgi. In this study we show that the product of the MNN10/BED1 gene is a component of one of these complexes, that which elongates the backbone. Analysis of interactions between the proteins in this complex shows that Mnn10p, and four previously characterized proteins (Anp1p, Mnn9p, Mnn11p, and Hoc1p) are indeed all components of the same large structure. Deletion of either Mnn10p, or its homologue Mnn11p, results in defects in mannan synthesis in vivo, and analysis of the enzymatic activity of the complexes isolated from mutant strains suggests that Mnn10p and Mnn11p are responsible for the majority of the alpha-1, 6-polymerizing activity of the complex.

Characterization of beta-1,2-mannosyltransferase in Candida guilliermondii and its utilization in the synthesis of novel oligosaccharides

A particulate insoluble enzyme fraction containing mannosyltransferases from Candida guilliermondii IFO 10279 strain cells was obtained as the residue after extracting a 105,000 x g pellet of cell homogenate with 1% Triton X-100. Incubation of this fraction with a mannopentaose, Manalpha1-->3(Manalpha1-->6)Manalpha1-->2Manalpha1+ ++-->2Man, in the presence of GDP-mannose and Mn2+ ion at pH 6.0 gave a third type of beta-1,2 linkage-containing mannohexaose, Manbeta1-->2Manalpha1-->3(Manalpha1-->6)Manalpha1++ +-->2Manalpha1-->2Man , the structure of which was identified by means of a sequential NMR assignment. The results of a substrate specificity study indicated that the beta-1,2-mannosyltransferase requires a mannobiosyl unit, Manalpha1--> 3Manalpha1-->, at the nonreducing terminal site. We synthesized novel oligosaccharides using substrates possessing a nonreducing terminal alpha-1,3-linked mannose unit prepared from various yeast mannans. Further incubation of the enzymatically synthesized oligosaccharide with the enzyme fraction gave the following structure, Manbeta1-->2Manbeta1-->2Manalpha1-->3(Manalpha1- ->6)Manalpha1--> 2Manalpha1-->2Man, which has been found to correspond to antigenic factor 9. Incubation of Candida albicans serotype B mannan with the enzyme fraction gave significantly transformed mannan, which contains the third type of beta-1,2-linked mannose units.

Characterization of alpha-1,6-mannosyltransferase responsible for the synthesis of branched side chains in Candida albicans mannan

A particulate insoluble fraction from Candida albicans NIH B-792 (serotype B) strain cells was obtained as the residue after extracting a 105000 x g pellet of cell homogenate with 1% Triton X-100. Incubation of this fraction with a mannopentaose, Man alpha 1-->3Man alpha 1-->2Man alpha 1-->Man alpha 1-->2Man, in the presence of GDP-mannose and Mn2+ at pH 6.0 gave a branched mannohexaose, [sequence: see text] 6 the structure of which was identified by means of sequential off assignment. However, the enzyme fraction obtained from Candida parapsilosis gave Man alpha 1-->2Man alpha 1-->3Man alpha 1-->2Man alpha 1-->2 Man alpha 1-->2Man under the same conditions. These results demonstrate the finding that the structural difference in the mannans of these two species is due to the presence of alpha-1.6-linked branching mannose units in the C. albicans mannan [Shibata, N., Ikuta, K., Imai, T., Satoh, Y., Satoh, R., Suzuki, A., Kojima, C., Kobayashi, H., Hisamichi, K. & Suzuki, S. (1995) J. Biol. Chem. 270, 1113-1122]. The substrate-specificity study of the enzyme indicated that the structural requirement of the alpha-1,6-mannosyltransferase is Man alpha 1-->3Man alpha 1-->. The alpha-1,6-mannosyltransferase also transferred the alpha-1,6-linked branching mannose unit to the mannan of Saccharomyces cerevisiae. The transformation of the mannan was detected by the appearance of antigenic factor 4 using an enzyme-linked immunosorbent assay and two-dimensional homonuclear Hartmann-Hahn spectroscopy.

Functional characterization of the YUR1, KTR1, and KTR2 genes as members of the yeast KRE2/MNT1 mannosyltransferase gene family

Eukaryotic glycan structures are progressively elaborated in the secretory pathway. Following the addition of a core N-linked carbohydrate in the endoplasmic reticulum, glycoproteins move to the Golgi complex where the elongation of O-linked sugar chains and processing of complex N-linked oligosaccharide structures take place. In order to better define how such post-translational modifications occur, we have been studying a yeast gene family in which at least one member, KRE2/MNT1, is involved in protein glycosylation. The family currently contains five other members: YUR1, KTR1, KTR2, KTR3 and KTR4 (Mallet, L., Bussereau, F., and Jacquet, M. (1994) Yeast 10, 819-831). All encode putative type II membrane proteins with a short cytoplasmic N terminus, a membrane-spanning region, and a highly conserved catalytic lumenal domain. Kre2p/Mnt1p is a alpha 1,2-mannosyltransferase involved in O- and N-linked glycosylation (Häusler, A., Ballou, L., Ballou, C.E., and Robbins, P.W. (1992) Proc. Natl. Acad. Sci. U.S.A. 89, 6846-6850); however, the role of the other proteins has not yet been established. We have carried out a functional analysis of Ktr1p, Ktr2p, and Yur1p. By in vitro assays, Ktr1p, Ktr2p, and Yur1p have been shown to be mannosyltransferase but, in vivo, do not appear to be involved in O-glycosylation. Examination of the electrophoretic mobility of the N-linked modified protein invertase in null mutant strains indicates that Ktr1p, Ktr2p, and Yur1p are involved in N-linked glycosylation, possibly as redundant enzymes. As found with Kre2p (Hill, K., Boone, C., Goebl, M., Puccia, R., Sdicu, A.-M., and Bussey, H. (1992) Genetics 130, 273-283), Ktr1p, Ktr2p, and Yur1p also seem to be implicated in the glycosylation of cell wall mannoproteins, since yeast cells containing different gene disruptions become K1 killer toxin-resistant. Immunofluorescence microscopy reveals that like Kre2p; Ktr1p, Ktr2p and Yur1p are localized in the Golgi complex.

The yeast VRG4 gene is required for normal Golgi functions and defines a new family of related genes

Sodium vanadate is an effective agent for the enrichment of yeast mutants with defects in glycosylation steps that occur in the Golgi complex (Ballou, L., Hitzeman, R. A., Lewis, M. S., and Ballou, C. E. (1991) Proc. Natl. Acad. Sci. U. S. A. 88, 3209-3212). We isolated and screened vanadate-resistant glycosylation mutants in the budding yeast, Saccharomyces cerevisiae, to identify any that may be defective in the secretory pathway, since changes in normal glycosylation may reflect defects within the secretory pathway. We identified one such mutant, allelic to vrg4/van2, that is defective in processes that occur specifically in the Golgi complex. Protein secreted from vrg4 mutants lacks the outer chain glycosylation that is normally extended during passage through the Golgi. This mutant fails to retrieve soluble endoplasmic reticulum proteins from the Golgi and accumulates the Golgi-specific biosynthetic intermediate of the vacuolar protein, carboxypeptidase Y. Analyses of intracellular membranes by staining with the fluorescent lipophilic dye, DiOC6, and by electron microscopy reveals a dramatic alteration in the membrane morphology of vrg4 mutant cells. The VRG4 gene encodes a 36.9-kDa membrane protein that is essential for cell viability. A sequence homology search has identified five related genes, establishing that VRG4 is a founding member of a family of structurally similar genes. Taken together, these results suggest that the VRG4 gene plays an important role in regulating Golgi functions and in maintaining the normal organization of intracellular membranes.

Detection of beta-1,2-mannosyltransferase in Candida albicans cells

A particulate insoluble fraction from Candida albicans J-1012 (serotype A) strain cells was obtained as the residue after extracting a 105,000 x g pellet of cell homogenate with 1% Triton X-100. Incubation of this fraction with a mannopentaose, Man beta 1-->2Man alpha 1-->(2Man alpha 1-->)(2)2Man (alpha beta Man5), in the presence of GDP-mannose followed by high performance liquid chromatography showed the formation of a mannohexaose. Analysis of the product by 1H NMR indicates that alpha beta Man5 was changed to Man beta 1-->2Man beta 1-->2Man alpha 1-->(2Man alpha 1-->)2 2Man (alpha beta Man6). This beta-1,2-mannosyltransferase (ManTase) II activity was completely inhibited by Zn2+ and was not restored by the addition of EDTA. The corresponding enzyme fraction from C. albicans NIH B-792 (serotype B) strain cells, the mannan of which does not possess both the alpha beta Man5 and alpha beta Man6 side chains, also exhibited the same beta-1,2-ManTase II activity.

Does an animal peptide: N-glycanase have the dual role as an enzyme and a carbohydrate-binding protein?

Recently, we have reported purification and characterization of a de-N-glycosylating enzyme, peptide: N-glycanase (PNGase) found in C3H mouse fibroblast L-929 cells, and designated L-929 PNGase [Suzuki T, Seko A, Kitajima K, Inoue Y, Inoue S (1994) J Biol Chem 269, 17611-18]. The unique properties of L-929 PNGase are that the enzyme had a high affinity to the substrate glycopeptide (e.g. Km = 114 microM for fetuin derived glycopentapeptide) and that the PNGase-catalysed reaction is strongly inhibited by the released free oligosaccharides but not by the free peptides formed, suggesting that L-929 PNGase is able to bind to a certain type of carbohydrate chain. In this study, we report the new findings of the mannan-binding property of L-929 PNGase: the de-N-glycosylating enzyme activity of L-929 PNGase was inhibited by yeast mannan and triomannose, Man alpha 1-->3(Man alpha 1-->6)Man, but not by mannose and alpha-methyl-D-mannoside. Furthermore, L-929 PNGase was revealed to bind to the glycan moiety of yeast mannan by using mannan-conjugated Sepharose 4B gel as a ligand, suggesting that L-929 PNGase could serve not only as an enzyme but also as a carbohydrate recognition protein in vivo. Such 'dual' properties found for animal-derived L-929 PNGase are unique and are not shared with other previously characterized plant- and bacterial-origin PNGases--PNGase A and PNGase F, respectively.

Yeast flocculation: receptor definition by mnn mutants and concanavalin A

Yeast flocculation involves the binding of surface lectins on flocculent yeasts, to carbohydrate receptors present as constituents of yeast cell walls. Receptors were investigated by coflocculation of flocculent strains of Saccharomyces cerevisiae, of both Flo 1 and NewFlo phenotypes, to known mnn mutants which vary in the wall mannan structure. Strong coflocculation was found with mnn1, mnn4, mnn9 and control strains, while very little coflocculation was found with mnn2 and mnn5 strains. In contrast, aggregation of these mutants by concanavalin A, a lectin with similar sugar inhibition to NewFlo phenotype flocculation, showed strong aggregation of mnn1, mnn4 and mnn5 strains and poor aggregation of mnn2 and mnn9 strains. The mmn mutant data suggested that flocculation receptors were the outer-chain mannan side-branches, two or three mannose residues in length, confirming an earlier theory based on sugar inhibition data. The similarities and differences between flocculation and concanavalin A aggregation are discussed.