Protein glycosylation defects in the Saccharomyces cerevisiae mnn7 mutant class. Support for the stop signal proposed for regulation of outer chain elongation

The impact of protein glycosylation on Flo11-dependent adherence in Saccharomyces cerevisiae

Fungal cell adhesion molecules are critical for the attachment of cells to each other and to surfaces and in pathogens contribute to virulence. Fungal adhesins are typically heavily glycosylated. The impact of protein glycosylation on the function and regulation of adhesion glycoproteins is not clear. We examined the role of protein glycosylation on the adherence properties of the major adhesion molecule Muc1/Flo11 in the budding yeast Saccharomyces cerevisiae. Using a conditional mutant required for an early step in protein glycosylation, pmi40-101, we show that the glycosylation of Flo11 is required for invasive growth and biofilm/mat formation. Underglycosylated Flo11 was not defective in cell-surface localization or binding to wild-type cells in trans. However, wild-type Flo11 was defective for binding to the surface of cells undergoing a glycosylation stress. Shed Flo11 and other shed glycoproteins (Msb2 and Hkr1) were extremely stable with half-lives on the order of days. The glycosylation of Flo11 contributed to its stability. Moreover, the overall balance between Flo11 production, shedding, and turnover favored accumulation of the shed protein over time. Our findings may be applicable to fungal adhesion molecules in other species including pathogens.

Differential cell wall remodeling of two chitin synthase deletants Δchs3A and Δchs3B in the pathogenic yeast Candida glabrata

It is known that cell wall remodeling and the salvaging pathway act to compensate for an impaired or a damaged cell wall. Lately, it has been indicated that this mechanism is partly required for resistance to the glucan synthesis inhibitor echinocandin. While cell wall remodeling has been described in mutants of glucan or mannan synthesis, it has not yet been reported in a chitin synthesis mutant. Here, we describe a novel cell wall remodeling and salvaging pathway in chitin synthesis mutants, Δchs3A and Δchs3B, of the pathogenic yeast Candida glabrata. Electron microscopic analysis revealed a thickened mannoprotein layer in Δchs3A cells and a thickened chitin-glucan layer of Δchs3B cells, and it indicated the hypothesis that mannan synthase and chitin-glucan synthase indemnify Δchs3A and Δchs3B cells, respectively. The double-mutant CHS3A and MNN10, encoding α-1,6-mannosyltransferase, showed synergistic stress sensitization, and the Δchs3B strain showed supersensitivity to echinocandins. Hence, these findings support the above hypothesis of remodeling. Furthermore, unlike Δchs3A cells, Δchs3B cells showed supersensitivity to calcineurin inhibitor FK506 and Tor1p kinase inhibitor rapamycin, indicating that the Δchs3B strain uses the calcineurin pathway and a Tor1p kinase for cell wall remodeling.

Identification of the MNN3 gene of Saccharomyces cerevisiae

The MNN3 gene of Saccharomyces cerevisiae has been identified as a synonym of VPS74. We have compared phenotype characteristics of the original mnn3 mutant, including low dye binding phenotype, size of external invertase, clump formation, and sodium orthovanadate resistance and found these to be identical to those shown by vps74Δ. Mating of both haploid strains resulted in non-complementation of mutant phenotypes. Finally, a vector containing wild-type VPS74 complemented the defects of both vps74Δ and mnn3. This work completes the identification of the entire collection of genes that are defective in mnn mutants. In addition, we have identified the mnn3 mutation by sequencing the VPS74 gene from the original mnn3 strain. We found a single amino acid change of Arg97 to Cys. This unique alteration seems to be sufficient to account for the phenotype of mnn3.

The REC41 gene of Saccharomyces cerevisiae: isolation and genetic analysis

Recombination-deficient strains have been proven useful for the understanding of the genetic control of homologous recombination. As the genetic screens used to isolate recombination-deficient (rec(-)) yeast mutants have not been saturated, we sought to develop a simple colony color assay to identify mutants with low or elevated rates of recombination. Using this system we isolated a collection of rec(-) mutants. We report the characterization of the REC41 gene identified in this way. REC41 is required for normal levels of interplasmid recombination and gamma-ray induced mitotic interchromosomal recombination. The rec41-1 mutant failed to grow at 37 degrees C. Microscopic analysis of plated cells showed that 45-50% of them did not form visible colonies at permissive temperature. Haploid cells of the rec41 mutant show the same gamma-ray sensitivity as wild type ones. However, the diploid rec41 mutant shows gamma-ray sensitivity which is comparable with heterozygous REC41/rec41-1 diploid cells. This fact indicates semidominance of the rec41-1 mutation. Diploid strains homozygous for the rec41 rad52 mutations had the same gamma-ray sensitivity as single rad52 diploids and exhibited dramatically decreased growth rate. The expression of the HO gene does not lead to inviability of rec41 cells. The rec41 mutation has an effect on meiosis, likely meiotic recombination, even in the heterozygous state. We cloned the REC41 gene. Sequence analysis revealed that the REC41 gene is encoded by ORF YDR245w. Earlier, this ORF was attributed to MNN10, BED1, SLC2, CAX5 genes. Two multicopy plasmids with suppressers of the rec41-1 mutation (pm21 and pm32) were isolated. The deletion analysis showed that only DNA fragments with the CDC43 and HAC1 genes can partially complement the rec41-1 mutation.

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)

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.

Asparagine-linked glycosylation in the yeast Golgi

The Golgi complex is the site where the terminal carbohydrate modification of proteins and lipids occurs. These carbohydrates play a variety of biological roles, ranging from the stabilization of glycoprotein structure to the provision of ligands for cell-cell interactions to the regulation of cell surface properties. Progress in our understanding of the biosynthesis and regulation of glycoconjugates has been accelerating at a rapid pace. Recent advances in the field of yeast glycobiology have been particularly impressive. This review focuses on glycosylation of proteins in the Golgi of the yeast Saccharomyces cerevisiae, with emphasis on the candidate mannosyltransferases that participate in the synthesis of N-linked oligosaccharides. Current views on how these enzymes may be regulated and how glycosylation relates on other cellular processes are also discussed.

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.

Glycoprotein biosynthesis in Saccharomyces cerevisiae: ngd29, an N-glycosylation mutant allelic to och1 having a defect in the initiation of outer chain formation

Outer chain glycosylation in Saccharomyces cerevisiae leads to heterogeneous and immunogenic asparagine-linked saccharide chains containing more than 50 mannose residues on secreted glycoproteins. Using a [3H]mannose suicide selection procedure a collection of N-glycosylation defective mutants (designated ngd) was isolated. One mutant, ngd29, was found to have a defect in the initiation of the outer chain and displayed a temperature growth sensitivity at 37 degrees C allowing the isolation of the corresponding gene by complementation. Cloning, sequencing and disruption of NGD29 showed that it is a non lethal gene and identical to OCH1. It complemented both the glycosylation and growth defect. Membranes isolated from an ngd29 disruptant or an ngd29mnn1 double mutant were no longer able, in contrast to membranes from wild type cells, to transfer mannose from GDPmannose to Man8GlcNAc2, the in vivo acceptor for building up the outer chain. Heterologous expression of glucose oxidase from Aspergillus niger in an ngd29mnn1 double mutant produced a secreted uniform glycoprotein with exclusively Man8GlcNAc2 structure that in wild type yeast is heavily hyperglycosylated. The data indicate that this mutant strain is a suitable host for the expression of recombinant glycoproteins from different origin in S. cerevisiae to obtain mammalian oligomannosidic type N-linked carbohydrate chains.

Existence of branched side chains in the cell wall mannan of pathogenic yeast, Candida albicans. Structure-antigenicity relationship between the cell wall mannans of Candida albicans and Candida parapsilosis

Isolation of side chain oligosaccharides from mannans of Candida albicans NIH B-792 (serotype B) and Candida parapsilosis IFO 1396 strains has been conducted by acetolysis under mild conditions. Structural study of these oligosaccharides by 1H and 13C NMR and methylation analyses indicated the presence of novel branched side chains with the following structures in C. albicans mannan. [sequence: see text] It was observed that the H-1 proton chemical shifts of the second and the third mannose units from the reducing terminus in each oligosaccharide are shifted upfield by substitution with an alpha-linked mannose unit at position 6 of the 3-O-substituted mannose unit. An agglutination inhibition assay between factor 4 serum and cells of Candida stellatoidea IFO 1397 lacking the beta-1,2-linked mannose unit, with oligosaccharides obtained from these mannans, indicated that only the branched oligosaccharides were active. This finding suggests that the branched oligosaccharides correspond to the epitope of antigenic factor 4. The presence of the branched structure in other mannans was detected by the characteristic H-1-H-2-correlated cross-peak of the alpha-1,2-linked mannose unit connected with the 3,6-di-O-substituted one by two-dimensional homonuclear Hartmann-Hahn spectroscopy.

Structural study of a cell-wall mannan of Saccharomyces kluyveri IFO 1685 strain. Presence of a branched side chain and beta-1,2-linkage

Acetolysis of the cell-wall mannan of Saccharomyces kluyveri under mild conditions, gave fragments with 1-6 mannose residues. The structures of mannopentaose and mannohexaose were determined to be [Formula; see text] respectively, by two-dimensional homonuclear Hartmann-Hahn spectroscopy and a sequential NMR assignment method that combines 1H-13C correlated spectroscopy, relayed coherence transfer spectroscopy, 1H-detected heteronuclear multiple-bond connectivity and methylation analysis. The H1 proton chemical shift of a neighboring alpha-1,2-linked mannose unit of the 3-O-substituted structure was shifted upfield by the addition of a mannose unit to the adjacent 3-O-substituted unit by an alpha-1,6 linkage. The characteristic H1--H2-correlated cross-peak of the alpha-1,3-linked mannose unit substituted by a beta-1,2 linkage, beta 1-->2Man alpha 1-->3, in the mannan of S. kluyveri, as also found by two-dimensional homonuclear Hartmann-Hahn spectroscopy in the mannan of Candida guilliermondii, a pathogenic yeast in man.

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.

Cytological immunodetection of yeast glycoprotein secretion

Expression of antigenic epitopes shared by secreted yeast glycoproteins was studied using specific immunological probes. Application of cytological and ultrastructural methods of immunodetection, employing monoclonal antibodies, permitted us to localize these glycoproteins in the cytoplasm, through the cell wall and at the yeast cell surface. Importance of glycosylation-secretion relationships were evaluated in the secretion process of these molecules. The cell wall crossing and the cell surface distribution of antigenic glycoproteins was described in immunoelectron microscopy and immunofluorescence. Some preferential secretion "ways" were suspected through the yeast cell wall leading to an heterogenous distribution of cell surface glycoproteins destined to be excreted into the medium. Antigenic variability of cell wall glycoproteins expression was discussed in relation with the glycoprotein secretion.

Identification and characterization of a gene and protein required for glycosylation in the yeast Golgi

The MNN2 gene of Saccharomyces cerevisiae has been cloned by complementation of the mnn2 mutant phenotype scored by a change in cell surface carbohydrate structure resulting from a lack of alpha 1----2-mannose branching in the outer chain. The gene was subcloned as a 3 kb DNA fragment that integrated at the MNN2 locus, and a gene disruption yielded the mnn2 phenotype. A lacZ-MNN2 gene fusion protein, produced in Escherichia coli, was used to raise a specific antiserum that recognized a 65 kD wild-type yeast protein. This MNN2 gene product lacks N-linked carbohydrate but appears to be an integral membrane protein. Overproduction of MNN2p does not enhance the alpha 1----2-mannosyltransferase activity of yeast cells. The results suggest that MNN2p is a Golgi-associated protein that is involved in mannoprotein sorting rather than glycosylation.

Separation of yeast asparagine-linked oligosaccharides by high-performance anion-exchange chromatography

Oligosaccharides obtained from Saccharomyces cerevisiae mannoproteins by digestion with endo-N-acetyl-beta-D-glucosaminidase H were fractionated by anion-exchange chromatography, by elution with 50-100mM NaOH without or with a sodium-acetate gradient, and detected with a pulsed amperometric detector (PAD). The elution times of homologous oligosaccharides fell on a straight line having a slope characteristic of the structural type. The response of the PAD detector per mole of oligosaccharide increased about 2-fold going from Man3GlcNAc to Man13GlcNAc, and appeared to depend primarily on the oxidation of the reducing-end N-acetylglucosamine unit common to all the oligosaccharides. The digestion of a Man10GlcNAc with jack-bean alpha-mannosidase was monitored by injecting portions of the crude reaction mixture, and the intermediates were characterized by their elution positions and n.m.r. spectra in the anomeric proton region. One commercial jack-bean alpha-mannosidase preparation contained a novel endolytic activity that released N-acetylglucosamine from the reducing ends of the oligosaccharides and was shown to convert P----6 alpha Man----6 alpha Man----6 beta Man----4 alpha beta GlcNAc to P----6 alpha Man----6 alpha Man----6 alpha beta Man plus free N-acetylglucosamine. Another commercial jack-bean alpha-mannosidase converted the Man10GlcNAc to a Man3GlcNAc having the structure alpha Man----6 beta Man----4 alpha beta GlcNAc, [formula: see text] whereas the Oerskovia sp. alpha-mannosidase converted the same oligosaccharide to a Man4GlcNAc having the structure alpha Man----6 alpha Man----6 beta Man----4 alpha beta GlcNAc. [formula: see text]