Diminished activity of the first N-glycosylation enzyme, dolichol-P-dependent N-acetylglucosamine-1-P transferase (GPT), gives rise to mutant phenotypes in yeast

Protein N-glycosylation in oral cancer: dysregulated cellular networks among DPAGT1, E-cadherin adhesion and canonical Wnt signaling

N-Linked glycosylation (N-glycosylation) of proteins has long been associated with oncogenesis, but not until recently have the molecular mechanisms underlying this relationship begun to be unraveled. Here, we review studies describing how dysregulation of the N-glycosylation-regulating gene, DPAGT1, drives oral cancer. DPAGT1 encodes the first and rate-limiting enzyme in the assembly of the lipid-linked oligosaccharide precursor in the endoplasmic reticulum and thus mediates N-glycosylation of many cancer-related proteins. DPAGT1 controls N-glycosylation of E-cadherin, the major epithelial cell-cell adhesion receptor and a tumor suppressor, thereby affecting intercellular adhesion and cytoskeletal dynamics. DPAGT1 also regulates and is regulated by Wnt/β-catenin signaling, impacting the balance between proliferation and adhesion in homeostatic tissues. Thus, aberrant induction of DPAGT1 promotes a positive feedback network with Wnt/β-catenin that represses E-cadherin-based adhesion and drives tumorigenic phenotypes. Further, modification of receptor tyrosine kinases (RTKs) with N-glycans is known to control their surface presentation via the galectin lattice, and thus increased DPAGT1 expression likely contributes to abnormal activation of RTKs in oral cancer. Collectively, these studies suggest that dysregulation of the DPAGT1/Wnt/E-cadherin network underlies the etiology and pathogenesis of oral cancer.

Response to environmental stresses, cell-wall integrity, and virulence are orchestrated through the calcineurin pathway in Ustilago hordei

In eukaryotes, several biological processes are regulated through calcium signaling. Calcineurin is a calcium-calmodulin-regulated serine/threonine phosphatase consisting of catalytic subunit A and regulatory subunit B. Phosphatase activity resides in the catalytic subunit, which activates by dephosphorylation downstream components such as transcription factor Crz1. The importance of this pathway to respond to environmental stress has been explored in several fungal pathogens. The basidiomycete Ustilago hordei causes covered smut of barley. We addressed the role of the Ca(2+)-calcineurin activated pathway by deleting UhCna1 and UhCnb1. These genes were not essential in U. hordei but the corresponding mutants displayed a variety of phenotypes when applying environmental stress such as sensitivity to pH, temperature, H₂O₂, mono- and divalent cations; and to genotoxic, acid, or oxidative stresses. Cell-wall integrity was compromised and mutants displayed altered cell morphologies. Mating was delayed but not abolished, and combined sensitivities likely explained a severely reduced virulence toward barley plants. Expression analyses revealed that response to salt stress involved the induction of membrane ATPase genes UhEna1 and UhEna2, which were regulated through the calcineurin pathway. Upregulation of UhFKS1, a 1,3-β-d-glucan synthase gene, correlated with the increased amount of 1,3-β-d-glucan in the calcineurin mutants grown under salt stress.

A comparative analysis of DNA barcode microarray feature size

Background

Microarrays are an invaluable tool in many modern genomic studies. It is generally perceived that decreasing the size of microarray features leads to arrays with higher resolution (due to greater feature density), but this increase in resolution can compromise sensitivity.

Results

We demonstrate that barcode microarrays with smaller features are equally capable of detecting variation in DNA barcode intensity when compared to larger feature sizes within a specific microarray platform. The barcodes used in this study are the well-characterized set derived from the Yeast KnockOut (YKO) collection used for screens of pooled yeast (Saccharomyces cerevisiae) deletion mutants. We treated these pools with the glycosylation inhibitor tunicamycin as a test compound. Three generations of barcode microarrays at 30, 8 and 5 μm features sizes independently identified the primary target of tunicamycin to be ALG7.

Conclusion

We show that the data obtained with 5 μm feature size is of comparable quality to the 30 μm size and propose that further shrinking of features could yield barcode microarrays with equal or greater resolving power and, more importantly, higher density.

Cu, Zn superoxide dismutase and NADP(H) homeostasis are required for tolerance of endoplasmic reticulum stress in Saccharomyces cerevisiae

Genome-wide screening for sensitivity to chronic endoplasmic reticulum (ER) stress induced by dithiothreitol and tunicamycin (TM) identified mutants deleted for Cu, Zn superoxide dismutase (SOD) function (SOD1, CCS1) or affected in NADPH generation via the pentose phosphate pathway (TKL1, RPE1). TM-induced ER stress led to an increase in cellular superoxide accumulation and an increase in SOD1 expression and Sod1p activity. Prior adaptation of the hac1 mutant deficient in the unfolded protein response (UPR) to the superoxide-generating agent paraquat reduced cell death under ER stress. Overexpression of the ER oxidoreductase Ero1p known to generate hydrogen peroxide in vitro, did not lead to increased superoxide levels in cells subjected to ER stress. The mutants lacking SOD1, TKL1, or RPE1 exhibited decreased UPR induction under ER stress. Sensitivity of the sod1 mutant to ER stress and decreased UPR induction was partially rescued by overexpression of TKL1 encoding transketolase. These data indicate an important role for SOD and cellular NADP(H) in cell survival during ER stress, and it is proposed that accumulation of superoxide affects NADP(H) homeostasis, leading to reduced UPR induction during ER stress.

N-glycosylation affects the molecular organization and stability of E-cadherin junctions

Epithelial cell-cell adhesion is mediated by E-cadherin, an intercellular N-glycoprotein adhesion receptor that functions in the assembly of multiprotein complexes anchored to the actin cytoskeleton named adherens junctions (AJs). E-cadherin ectodomains 4 and 5 contain three potential N-glycan addition sites, although their significance in AJ stability is unclear. Here we show that sparse cells lacking stable AJs produced E-cadherin that was extensively modified with complex N-glycans. In contrast, dense cultures with more stable AJs had scarcely N-glycosylated E-cadherin modified with high mannose/hybrid and limited complex N-glycans. This suggested that variations in AJ stability were accompanied by quantitative and qualitative changes in E-cadherin N-glycosylation. To further examine the role of N-glycans in AJ function, we generated E-cadherin N-glycosylation variants lacking selected N-glycan addition sites. Characterization of these variants in CHO cells, lacking endogenous E-cadherin, revealed that site 1 on ectodomain 4 was modified with a prominent complex N-glycan, site 2 on ectodomain 5 did not have a substantial oligosaccharide, and site 3 on ectodomain 5 was decorated with a high mannose/hybrid N-glycan. Removal of complex N-glycan from ectodomain 4 led to a dramatically increased interaction of E-cadherin-catenin complexes with vinculin and the actin cytoskeleton. The latter effect was further enhanced by the deletion of the high mannose/hybrid N-glycan from site 3. In MDCK cells, which produce E-cadherin, a variant lacking both complex and high mannose/hybrid N-glycans functioned like a dominant positive displaying increased interaction with gamma-catenin and vinculin compared with the endogenous E-cadherin. Collectively, our studies show that N-glycans, and complex oligosaccharides in particular, destabilize AJs by affecting their molecular organization.

Endoplasmic reticulum glucosidase II is required for pathogenicity of Ustilago maydis

We identified a nonpathogenic strain of Ustilago maydis by tagging mutagenesis. The affected gene, glucosidase1 (gas1), displays similarity to catalytic alpha-subunits of endoplasmic reticulum (ER) glucosidase II. We have shown that Gas1 localizes to the ER and complements the temperature-sensitive phenotype of a Saccharomyces cerevisiae mutant lacking ER glucosidase II. gas1 deletion mutants were normal in growth and mating but were more sensitive to calcofluor and tunicamycin. Mutant infection hyphae displayed significant alterations in the distribution of cell wall material and were able to form appressoria and penetrate the plant surface but arrested growth in the epidermal cell layer. Electron microscopy analysis revealed that the plant-fungal interface between mutant hyphae and the plant plasma membrane was altered compared with the interface of penetrating wild-type hyphae. This may indicate that gas1 mutants provoke a plant response.

A hypomorphic allele of the first N-glycosylation gene, ALG7, causes mitochondrial defects in yeast

The modification of proteins at asparagine residues with oligosaccharides (N-glycans) plays critical roles in diverse cell functions. N-glycans originate from a common lipid-linked oligosaccharide (LLO) precursor whose synthesis is initiated by the Dol-P-dependent GlcNAc-1-P transferase (GPT) encoded by an essential ALG7 gene. To identify cellular processes affected by ALG7 and N-glycosylation, we replaced the genomic copy of ALG7 with its hypomorphic allele in two genetically distinct haploid yeast cells. We show that ALG7 knockdown gave rise to an unexpected phenotype of mitochondrial dysfunction. The alg7 mutants did not grow on glycerol and DNA arrays revealed the absence of mitochondrial genes' expression. Accordingly, the alg7 mutants displayed no detectable mtDNA and respiratory activity. Both mutants exhibited diminished abundance of LLO and under-glycosylation of carboxypeptidase Y (CPY). Moreover, another N-glycosylation mutant with a LLO defect, alg6, was respiratory deficient. Collectively, our studies provide evidence that the dysregulation of N-glycosylation in haploid yeast cells leads to mitochondrial dysfunction.

Unfolded protein response followed by induction of cell death in cultured tobacco cells treated with tunicamycin

When correct folding of protein in the endoplasmic reticulum (ER) is prevented, cells respond to overcome the accumulation of unfolded proteins. This cellular response, which includes the induction of ER chaperones, is called an unfolded protein response (UPR). Although a link between the UPR and apoptosis has been reported in mammalian cells, little is known about this mechanism in plant cells. Asparagine (N)-linked glycosylation of proteins is critical for protein folding in the ER; and tunicamycin, a potent inhibitor of N-linked glycosylation, induces UPR. Growth arrest was observed in cultured tobacco cells treated with tunicamycin. Cell death and induction of Hsr203J, a marker for programmed cell death, were observed in the 24-h period after addition of tunicamycin, following UPR that started within 2 h. These results indicate a strong link between UPR and programmed cell death in plant cells.

Analysis of the interaction between piD261/Bud32, an evolutionarily conserved protein kinase of Saccharomyces cerevisiae, and the Grx4 glutaredoxin

The Saccharomyces cerevisiae piD261/Bud32 protein and its structural homologues, which are present along the Archaea-Eukarya lineage, constitute a novel protein kinase family (the piD261 family) distantly related in sequence to the eukaryotic protein kinase superfamily. It has been demonstrated that the yeast protein displays Ser/Thr phosphotransferase activity in vitro and contains all the invariant residues of the family. This novel protein kinase appears to play an important cellular role as deletion in yeast of the gene encoding piD261/Bud32 results in the alteration of fundamental processes such as cell growth and sporulation. In this work we show that the phosphotransferase activity of Bud32 is relevant to its functionality in vivo, but is not the unique role of the protein, since mutants which have lost catalytic activity but not native conformation can partially complement the disruption of the gene encoding piD261/Bud32. A two-hybrid approach has led to the identification of several proteins interacting with Bud32; in particular a glutaredoxin (Grx4), a putative glycoprotease (Ykr038/Kae1) and proteins of the Imd (inosine monophosphate dehydrogenase) family seem most plausible interactors. We further demonstrate that Grx4 directly interacts with Bud32 and that it is phosphorylated in vitro by Bud32 at Ser-134. The functional significance of the interaction between Bud32 and the putative protease Ykr038/Kae1 is supported by its evolutionary conservation.

Characterisation of the gptA gene, encoding UDP N-acetylglucosamine: dolichol phosphate N-acetylglucosaminylphosphoryl transferase, from the filamentous fungus, Aspergillus niger

The production of asparagine (N)-linked oligosaccharides is of vital importance in the formation of glycosylated proteins in eukaryotes and is mediated by the dolichol pathway. As part of studies to allow manipulation of this pathway, the gene coding for the production of the enzyme UDP N-acetylglucosamine: dolichol phosphate N-acetylglucosaminylphosphoryl transferase (GPT), catalysing the first step in the assembly of dolichol-linked oligosaccharides, was cloned from the filamentous fungus Aspergillus niger. Degenerate-PCR was used to amplify a 470-bp fragment of the gene, which was labelled as a probe to obtain a full-length clone from a genomic library of A. niger. This contained a 1557-bp open reading frame encoding a highly hydrophobic protein of 468 amino acids with a predicted molecular weight of 51.4 kDa. The gene contained two intron sequences and putative dolichol recognition sites (PDRSs) were present in the deduced amino acid sequence. Comparison with other eukaryotic GPTs revealed the A. niger GPT to share 45-47% identity with yeasts (Saccharomyces cerevisiae and Schizosaccharomyces pombe) and 41-42% identity with mammals (mouse, hamster, human). Nested-PCR of a cDNA library was used to confirm the position of an intron. A complete cDNA clone of A. niger gpt was obtained by employing a recombinant PCR approach. This was used to rescue a conditional lethal mutant of S. cerevisiae carrying a dysfunctional gpt gene by heterologous expression, confirming that the gpt genes from A. niger and S. cerevisiae are functionally equivalent.

Parallel phenotypic analysis of sporulation and postgermination growth in Saccharomyces cerevisiae

We have quantitatively monitored the sporulation and germination efficiencies of approximately 4,200 yeast deletion strains in parallel by using a molecular bar coding strategy. In a single study, we doubled the number of genes functionally implicated in sporulation to approximately 400, identifying both positive and negative regulators. Our set of 261 sporulation-deficient genes illustrates the importance of autophagy, carbon utilization, and transcriptional machinery during sporulation. These general cellular factors are more likely to exhibit fitness defects when deleted and less likely to be transcriptionally regulated than sporulation-specific genes. Our postgermination screening assay identified recombinationchromosome segregation genes, aneuploid strains, and possible germination-specific factors. Finally, our results facilitate a genome-wide comparison of expression pattern and mutant phenotype for a developmental process and suggest that 16% of genes differentially expressed during sporulation confer altered efficiency of spore production or defective postgermination growth when disrupted.

A comprehensive analysis of gene expression profiles in a yeast N-glycosylation mutant

Although protein N-glycosylation is critical to many cell functions, its downstream targets remain largely unknown. In all eukaryotes, N-glycosylation utilizes the lipid-linked oligosaccharide (LLO) precursor, whose synthesis is initiated by the ALG7 gene. To elucidate the key signaling and metabolic events affected by N-glycosylation, we performed genomewide expression profiling of yeast cells carrying a hypomorphic allele of ALG7. DNA microarrays showed that of more than 97% of known or predicted yeast genes, 29 displayed increased expression while 23 were repressed in alg7 mutants. Changes in transcript abundance were observed for a and alpha mating-type genes, for genes functioning in several mitogen-activated protein kinase (MAPK) cascades, as well as in phosphate, amino acid, carbohydrate, mitochondrial and ATP metabolism. Therefore, DNA microarrays have revealed direct and indirect targets, including internal feedback loops, through which N-glycosylation affects signaling and metabolic activities and is functionally linked with cellular regulatory circuitry.

Defects in protein glycosylation cause SHO1-dependent activation of a STE12 signaling pathway in yeast

In haploid Saccharomyces cerevisiae, mating occurs by activation of the pheromone response pathway. A genetic selection for mutants that activate this pathway uncovered a class of mutants defective in cell wall integrity. Partial loss-of-function alleles of PGI1, PMI40, PSA1, DPM1, ALG1, MNN10, SPT14, and OCH1, genes required for mannose utilization and protein glycosylation, activated a pheromone-response-pathway-dependent reporter (FUS1) in cells lacking a basal signal (ste4). Pathway activation was suppressed by the addition of mannose to hexose isomerase mutants pgi1-101 and pmi40-101, which bypassed the requirement for mannose biosynthesis in these mutants. Pathway activation was also suppressed in dpm1-101 mutants by plasmids that contained RER2 or PSA1, which produce the substrates for Dpm1. Activation of FUS1 transcription in the mannose utilization/protein glycosylation mutants required some but not all proteins from three different signaling pathways: the pheromone response, invasive growth, and HOG pathways. We specifically suggest that a Sho1 --> Ste20/Ste50 --> Ste11 --> Ste7 --> Kss1 --> Ste12 pathway is responsible for activation of FUS1 transcription in these mutants. Because loss of pheromone response pathway components leads to a synthetic growth defect in mannose utilization/protein glycosylation mutants, we suggest that the Sho1 --> Ste12 pathway contributes to maintenance of cell wall integrity in vegetative cells.

Regulation of the biosynthesis of N-acetylglucosaminylpyrophosphoryldolichol, feedback and product inhibition

The assembly of the core oligosaccharide region of asparagine-linked glycoproteins proceeds by means of the dolichol pathway. The first step of this pathway, the reaction of dolichol phosphate with UDP-GlcNAc to form N-acetylglucosaminylpyrophosphoryldolichol (GlcNAc-P-P-dolichol), is under investigation as a possible site of metabolic regulation. This report describes feedback inhibition of this reaction by the second intermediate of the pathway, N-acetylglucosaminyl-N-acetylglucosaminylpyrophosphoryldolichol (GlcNAc-GlcNAc-P-P-dolichol), and product inhibition by GlcNAc-P-P-dolichol itself. These influences were revealed when the reactions were carried out in the presence of showdomycin, a nucleoside antibiotic, present at concentrations that block the de novo formation of GlcNAc-GlcNAc-P-P-dolichol but not that of GlcNAc-P-P-dolichol. The apparent K(i) values for GlcNAc-P-P-dolichol and GlcNAc-GlcNAc-P-P-dolichol under basal conditions were 4.4 and 2.8 microM, respectively. Inhibition was also observed under conditions where mannosyl-P-dolichol (Man-P-dol) stimulated the biosynthesis of GlcNAc-P-P-dolichol; the apparent K(i) values for GlcNAc-P-P-dolichol and GlcNAc-GlcNAc-P-P-dolichol were 2.2 and 11 microM, respectively. Kinetic analysis of the types of inhibition indicated competitive inhibition by GlcNAc-P-P-dolichol toward the substrate UDP-GlcNAc and non-competitive inhibition toward dolichol phosphate. Inhibition by GlcNAc-GlcNAc-P-P-dolichol was uncompetitive toward UDP-GlcNAc and competitive toward dolichol phosphate. A model is presented for the kinetic mechanism of the synthesis of GlcNAc-P-P-dolichol. GlcNAc-P-P-dolichol also exerts a stimulatory effect on the biosynthesis of Man-P-dol, i.e. a reciprocal relationship to that previously observed between these two intermediates of the dolichol pathway. This network of inhibitory and stimulatory influences may be aspects of metabolic control of the pathway and thus of glycoprotein biosynthesis in general.

Overexpression of a gene that encodes the first enzyme in the biosynthesis of asparagine-linked glycans makes plants resistant to tunicamycin and obviates the tunicamycin-induced unfolded protein response

The cytotoxic drug tunicamycin kills cells because it is a specific inhibitor of UDP-N-acetylglucosamine:dolichol phosphate N-acetylglucosamine-1-P transferase (GPT), an enzyme that catalyzes the initial step of the biosynthesis of dolichol-linked oligosaccharides. In the presence of tunicamycin, asparagine-linked glycoproteins made in the endoplasmic reticulum are not glycosylated with N-linked glycans, and therefore may not fold correctly. Such proteins may be targeted for breakdown. Cells that are treated with tunicamycin normally experience an unfolded protein response and induce genes that encode endoplasmic reticulum chaperones such as the binding protein (BiP). We isolated a cDNA clone for Arabidopsis GPT and overexpressed it in Arabidopsis. The transgenic plants have a 10-fold higher level of GPT activity and are resistant to 1 microg/mL tunicamycin, a concentration that kills control plants. Transgenic plants grown in the presence of tunicamycin have N-glycosylated proteins and the drug does not induce BiP mRNA levels as it does in control plants. BiP mRNA levels are highly induced in both control and GPT-expressing plants by azetidine-2-carboxylate. These observations suggest that excess GPT activity obviates the normal unfolded protein response that cells experience when exposed to tunicamycin.

ALG gene expression and cell cycle progression

The evolutionarily conserved ALG genes function in the dolichol pathway in the synthesis of the lipid-linked oligosaccharide precursor for protein N-glycosylation. Increasing evidence suggests a role for these genes in the cell cycle. In Saccharomyces cerevisiae, coordinate regulation of the ALG genes makes up the primary genomic response to growth stimulation; several features of the ALG genes' expression resemble mammalian early growth response genes. However, only the first gene in the pathway, ALG7, is downregulated in response to an antimitogenic signal that leads to cell cycle arrest and differentiation, suggesting that selective inhibition of the first gene may be sufficient to regulate the dolichol pathway for the withdrawal from the cell cycle. The availability of mutants in the early essential ALG genes has established functional relationships between these genes' expression and G1/S transition, budding, progression through G2 and withdrawal from the cell cycle. Analysis of the regulation of ALG7 has provided insights into how this gene's expression is controlled at the molecular level. Recent studies have also begun to reveal how ALG7 expression is linked to cell cycle arrest in response to antimitogenic cues and have identified G1 cyclins as some of its downstream targets. Since the functions of the ALG genes appear to be as conserved among eukaryotes as the cell cycle machinery, it is likely that these genes play a similar role in mammalian cell proliferation and differentiation.

Confocal imaging of gene expression during hamster submandibular gland biogenesis

The data presented here provide evidence that the abundance of the ALG7 protein product, GPT, correlates with high proliferative activity during the postnatal development of the hamster SMG development, and that it becomes downregulated with differentiation. Based on our previous studies with yeast, changes in the level of ALG7 expression may be necessary for the events directing salivary cell polarization, migration, differentiation, and apoptosis at distinct developmental stages.

Molecular dissection of the genetic targets of ALG7 in the serpentine receptor-mediated signal transduction pathway in yeast

These initial studies show that deregulated expression of ALG7 affects diverse cellular functions crucial to development, including proliferation, differentiation, and morphogenesis. Furthermore, the data suggest multiple genetic targets for ALG7 and provide the basis for future dissection of these developmentally relevant pathways.

Characterization of multiple transcripts of the hamster dolichol-P-dependent N-acetylglucosamine-1-P transferase suggests functionally complex expression

The evolutionarily conserved dolichol-P-dependent N-acetylglucosamine-1-P transferase gene, ALG7, functions by initiating the dolichol pathway of protein N-glycosylation. In yeast, ALG7 has a complex expression pattern and plays a critical role in diverse cellular functions, including proliferation and morphological response. In Chinese hamster ovary cells (CHO), ALG7 gives rise to three mRNAs of 1.5, 1.9 and 2.2 kb. We report results of RNA blotting assays, ribonuclease protection, PCR-amplification and sequencing of the CHO ALG7 transcripts 5' and 3' ends which suggest that the 1.5 and 1.9 kb transcripts are produced as a consequence of initiation at 2 distinct start sites, 350-379 bp apart. The transcriptional start site for the 1.5 kb mRNA is positioned between the first two in frame ATGs, while that of the 1.9 kb species is located upstream of these two in-frame ATGs. In order to test the translational competence of the 1.5 and 1.9 kb mRNAs, we constructed DNA templates specifying these transcripts and used them for in vitro transcription/translation. Our data show that the 1.9 kb mRNA served in the synthesis of 36 and 24 kDa species, as well as a low-abundance 32 kDa protein. The 1.5 kb transcript gave rise to a translation product of 32 kDa. The latter is synthesized in CHO cells and hamster submandibular glands. These results suggest the possibility that the 1.5 and 1.9 kb transcripts give rise to related protein isoforms with different lengths of their NH2-terminal regions.

Protein N-glycosylation: molecular genetics and functional significance

Protein N-glycosylation is a metabolic process that has been highly conserved in evolution. In all eukaryotes, N-glycosylation is obligatory for viability. It functions by modifying appropriate asparagine residues of proteins with oligosaccharide structures, thus influencing their properties and bioactivities. N-glycoprotein biosynthesis involves a multitude of enzymes, glycosyltransferases, and glycosidases, encoded by distinct genes. The majority of these enzymes are transmembrane proteins that function in the endoplasmic reticulum and Golgi apparatus in an ordered and well-orchestrated manner. The complexity of N-glycosylation is augmented by the fact that different asparagine residues within the same polypeptide may be modified with different oligosaccharide structures, and various proteins are distinguished from one another by the characteristics of their carbohydrate moieties. Furthermore, biological consequences of derivatization of proteins with N-glycans range from subtle to significant. In the past, all these features of N-glycosylation have posed a formidable challenge to an elucidation of the physiological role for this modification. Recent advances in molecular genetics, combined with the availability of diverse in vivo experimental systems ranging from yeast to transgenic mice, have expedited the identification, isolation, and characterization of N-glycosylation genes. As a result, rather unexpected information regarding relationships between N-glycosylation and other cellular functions--including secretion, cytoskeletal organization, proliferation, and apoptosis--has emerged. Concurrently, increased understanding of molecular details of N-glycosylation has facilitated the alignment between N-glycosylation deficiencies and human diseases, and has highlighted the possibility of using N-glycan expression on cells as potential determinants of disease and its progression. Recent studies suggest correlations between N-glycosylation capacities of cells and drug sensitivities, as well as susceptibility to infection. Therefore, knowledge of the regulatory features of N-glycosylation may prove useful in the design of novel therapeutics. While facing the demanding task of defining properties, functions, and regulation of the numerous, as yet uncharacterized, N-glycosylation genes, glycobiologists of the 21st century offer exciting possibilities for new approaches to disease diagnosis, prevention, and cure.

Deregulation of the first N-glycosylation gene, ALG7, perturbs the expression of G1 cyclins and cell cycle arrest in Saccharomyces cerevisiae

The evolutionarily conserved ALG7 gene encodes the dolichol-P-dependent N-acetylglucosamine-1-P transferase (GPT) and functions by initiating the dolichol pathway of protein N-glycosylation. In Saccharomyces cerevisiae, ALG7 has been shown to play a role in cell proliferation. The yeast alpha-factor-induced cell cycle arrest in G1 occurs, in part, by downregulation of CLN1 and CLN2. The function of ALG7 in G1 arrest was examined in alg7 mutants containing diminished GPT activity. In wild type, CLN1 and CLN2 mRNAs were rapidly downregulated, while in alg7 mutants, these transcripts were only transiently repressed before becoming greatly augmented. Analyses of DNA contents and budding indices showed that alg7 mutants resumed cycling when wild type cells remained arrested. Thus, deregulation of ALG7 interferes with cell cycle arrest by preventing a sustained downregulation of CLN1 and CLN2 mRNAs. These results provide a molecular insight into the role of ALG7, and protein N-glycosylation in general, in proliferation.

The essential OST2 gene encodes the 16-kD subunit of the yeast oligosaccharyltransferase, a highly conserved protein expressed in diverse eukaryotic organisms

Oligosaccharyltransferase catalyzes the transfer of a preassembled high mannose oligosaccharide from a dolichol-oligosaccharide donor to consensus glycosylation acceptor sites in newly synthesized proteins in the lumen of the rough endoplasmic reticulum. The Saccharomyces cerevisiae oligosaccharyltransferase is an oligomeric complex composed of six non-identical subunits (alpha-zeta). The alpha, beta, gamma, and delta subunits of the oligosaccharyltransferase are encoded by the OST1, WBP1, OST3, and SWP1 genes, respectively. Here we describe the functional characterization of the OST2 gene that encodes the epsilon-subunit of the oligosaccharyltransferase. Genomic disruption of the OST2 locus was lethal in haploid yeast showing that expression of the Ost2 protein is essential for viability. Overexpression of the Ost2 protein suppresses the temperature-sensitive phenotype of the wbp1-2 allele and increases in vivo and in vitro oligosaccharyltransferase activity in a wbp1-2 strain. An analysis of a series of conditional ost2 mutants demonstrated that defects in the Ost2 protein cause pleiotropic underglycosylation of soluble and membrane-bound glycoproteins. Microsomal membranes isolated from ost2 mutant yeast show marked reductions in the in vitro transfer of high mannose oligosaccharide from exogenous lipid-linked oligosaccharide to a glycosylation site acceptor tripeptide. Surprisingly, the Ost2 protein was found to be 40% identical to the DAD1 protein (defender against apoptotic cell death), a highly conserved protein initially identified in vertebrate organisms. The protein sequence of ost2 mutant alleles revealed mutations at highly conserved residues in the Ost2p/DAD1 protein sequence.