Molecular cloning of the Golgi apparatus uridine diphosphate-N-acetylglucosamine transporter from Kluyveromyces lactis

Eukaryotic transmembrane solute transport systems

A comprehensive classification system for transmembrane molecular transporters has been proposed. This system is based on (i) mode of transport and energy-coupling mechanism, (ii) protein phylogenetic family, (iii) phylogenetic cluster, and (iv) substrate specificity. The proposed "Transport Commission" (TC) system is superficially similar to that implemented decades ago by the Enzyme Commission for enzymes, but it differs from the latter system in that it uses phylogenetic and functional data for classification purposes. Very few families of transporters include members that do not function exclusively in transport. Analyses reported reveal that channels, primary carriers, secondary carriers (uni-, sym-, and antiporters), and group translocators comprise distinct categories of transporters, and that transport mode and energy coupling are relatively immutable characteristics. By contrast, substrate specificity and polarity of transport are often readily mutable. Thus, with very few exceptions, a unified family of transporters includes members that function by a single transport mode and energy-coupling mechanism although a variety of substrates may be transported with either inwardly or outwardly directed polarity. The TC system allows cross-referencing according to substrates transported and protein sequence database accession numbers. Thus, familial assignments of newly sequenced transport proteins are facilitated. In this article I examine families of transporters that are eukaryotic specific. These families include (i) channel proteins, mostly from animals; (ii) facilitators and secondary active transport carriers; (iii) a few ATP-dependent primary active transporters; and (iv) transporters of unknown mode of action or energy-coupling mechanism. None of the several ATP-independent primary active transport energy-coupling mechanisms found in prokaryotes is represented within the eukaryotic-specific families. The analyses reported provide insight into transporter families that may have arisen in eukaryotes after the separation of eukaryotes from archaea and bacteria. On the basis of the reported analyses, it is suggested that the horizontal transfer of genes encoding transport proteins between eukaryotes and members of the other two domains of life occurred very infrequently during evolutionary history.

Regulation of mouse kidney tubular epithelial cell-specific expression of core 2 GlcNAc transferase

A mouse gene, Gsl5, controls the expression of Galbeta1-4(Fucalpha1-3)GlcNAcbeta1-6(Galbeta1-3)Gb4Cer and its precursor glycolipids in the kidney by regulating transcription of beta-1,6-GlcNAc transferase. Here we report that Gsl5 controls the expression of the core 2 structure [GlcNAcbeta1-6(Galbeta1-3)GalNAcalpha1-Ser/Thr] of glycoproteins as well as the glycolipid, GlcNAcbeta1-6(Galbeta1-3)GalNAcbeta1-3Galalpha1-4Galbeta1-4Glcbeta1-ceramide. Immunohistochemical studies using an anti-(core 2-Lex) monoclonal antibody demonstrated that lysosome-like vesicles of proximal tubule cells were clearly stained in a Gsl5 wild type mouse, but not in a Gsl5 mutant strain of mice. Western blotting of microsomal fractions of kidney tissue with the same antibody confirmed the histological findings. In situ hybridization with an antisense probe to the kidney-specific mRNA demonstrated that the mRNA is localized at proximal tubule-cells in the cortex adjacent to the medulla, but not detected in glomeruli nor in collecting duct cells in the medulla. The results obtained by immunohistological staining and in situ hybridyzation are compatible and lead to the conclusion that the kidney specific mRNA is expressed in a proximal tubular cell specific manner and produces core 2 GlcNAc transferase responsible for the production of glycoproteins localized at vesicles in the proximal tubular cells. Glycosylation regulated by Gsl5 gene may modify functions of membrane glycoproteins in proximal tubular cells.

Nucleotide sugar transporters of the Golgi apparatus

Glycosylation, sulfation and phosphorylation of proteins, proteoglycans and lipids occur in the lumen of the Golgi apparatus. The nucleotide substrates of these reactions must be first transported from the cytosol into the Golgi lumen by specific transporters. The topology and structure of these hydrophobic, multi-transmembrane-spanning proteins are beginning to be understood.

vga Mutants of Kluyveromyces lactis show cell integrity defects

We studied the cell wall alterations that occur in mutants of Kluyveromyces lactis impaired in glycosylation. The mutants belong to four complementation groups named vga1 to vga4 (vanadate glycosylation affected), characterized by sodium orthovanadate resistance and alteration of the glycosylation profile of native invertase. A drastic reduction of the alkali-soluble fraction of the beta-D-glucan was observed in vga1, vga2 and vga3 cells, accompanied by an increase in the chitin content of the cell wall. In vga4 cells, both beta-D-glucan fractions (alkali-soluble and alkali-insoluble) were reduced to about half of the corresponding wild-type value but the chitin content was normal. A protein related to Fks1p, the catalytic subunit of the major 1,3-beta-D-glucan synthase of S. cerevisiae, was detected in K. lactis. The amount of this Fks1p-like protein increased 7-10 times in vga1, vga2 and vga3 mutants as compared to wild-type cells; the same strains released significant amounts of beta-D-glucan in the culture supernatant. These mutations also resulted in abnormally thick cell walls with conspicuous irregularities in the structure, as revealed by electron microscopy and by an altered resistance to Zymolyase. The observed high responsiveness of cell wall phenotypes to alterations of glycosylation make K. lactis an attractive system for studying the interconnections between these processes.

Distinct protein domains of the yeast Golgi GDP-mannose transporter mediate oligomer assembly and export from the endoplasmic reticulum

The substrates for glycan synthesis in the lumen of the Golgi are nucleotide sugars that must be transported from the cytosol by specific membrane-bound transporters. The principal nucleotide sugar used for glycosylation in the Golgi of the yeast Saccharomyces cerevisiae is GDP-mannose, whose lumenal transport is mediated by the VRG4 gene product. As the sole provider of lumenal mannose, the Vrg4 protein functions as a key regulator of glycosylation in the yeast Golgi. We have undertaken a functional analysis of Vrg4p as a model for understanding nucleotide sugar transport in the Golgi. Here, we analyzed epitope-tagged alleles of VRG4. Gel filtration chromatography and co-immunoprecipitation experiments demonstrate that the Vrg4 protein forms homodimers with specificity and high affinity. Deletion analyses identified two regions essential for Vrg4p function. Mutant Vrg4 proteins lacking the predicted C-terminal membrane-spanning domain fail to assemble into oligomers (Abe, M., Hashimoto, H., and Yoda, K. (1999) FEBS Lett. 458, 309-312) and are unstable, while proteins lacking the N-terminal cytosolic tail are stable and multimerize efficiently, but are mislocalized to the endoplasmic reticulum (ER). Fusion of the N terminus of Vrg4p to related ER membrane proteins promote their transport to the Golgi, suggesting that sequences in the N terminus supply information for ER export. The dominant negative phenotype resulting from overexpression of truncated Vrg4-DeltaN proteins provides strong genetic evidence for homodimer formation in vivo. These studies are consistent with a model in which Vrg4p oligomerizes in the ER and is subsequently transported to the Golgi via a mechanism that involves positive sorting rather than passive default.

Characterization of Yeast Yea4p, a uridine diphosphate-N-acetylglucosamine transporter localized in the endoplasmic reticulum and required for chitin synthesis

Chitin is an essential cell wall component, synthesis of which is regulated throughout the cell cycle in the yeast Saccharomyces cerevisiae. We cloned an S. cerevisiae gene, YEA4, whose product is homologous to the Kluyveromyces lactis uridine diphosphate-N-acetylglucosamine (UDP-GlcNAc) transporter. An epitope-tagged Yea4p localized mainly in the 10,000 x g pellet (P2), suggesting endoplasmic reticulum (ER) localization. Membrane vesicles from the P2 fraction showed an 8-fold higher UDP-GlcNAc transport activity in cells harboring a multicopy YEA4 plasmid than in cells harboring vector alone. The activity distribution is identical with the protein distribution in P2, whether the gene is overexpressed or not, suggesting its native localization in P2. Immunolocalization of epitope-tagged Yea4p further revealed ER localization. The increase in transport activity due to the YEA4 overexpression is specific for UDP-GlcNAc, but not for UDP-galactose and GDP-mannose. Deltayea4-disrupted cells showed a reduced rate of UDP-GlcNAc transport, contained less chitin, and were larger and rounder in shape than the wild type cells. Our results indicate that YEA4 encodes an ER-localized UDP-GlcNAc transporter that is required for cell wall chitin synthesis in S. cerevisiae.

Reconstitution, identification, and purification of the rat liver golgi membrane GDP-fucose transporter

Glycosylation of glycoproteins, proteoglycans, and glycolipids occurring in the Golgi apparatus requires the translocation of nucleotide sugars from the cytosol into the lumen of the Golgi. Translocation is mediated by specific nucleotide sugar transporters, integral Golgi membrane proteins that regulate the above glycosylation reactions. A defect in GDP-fucose transport into the lumen of the Golgi apparatus has been recently identified in a patient affected by leukocyte adhesion deficiency type II syndrome (Lubke, T., Marquardt, T., von Figura, K., and Korner, C. (1999) J. Biol. Chem. 274, 25986-25989). We have now identified and purified the rat liver Golgi membrane GDP-fucose transporter, a protein with an apparent molecular mass of 39 kDa, by a combination of column chromatography, native functional size determination on a glycerol gradient, and photoaffinity labeling with 8-azidoguanosine-5'-[alpha-(32)P] triphosphate, an analog of GDP-fucose. The purified transporter appears to exist as a homodimer within the Golgi membrane. When reconstituted into phosphatidylcholine liposomes, it was active in GDP-fucose transport and was specifically photolabeled with 8-azidoguanosine-5'-[alpha-(32)P]triphosphate. Transport was also stimulated 2-3-fold after preloading proteoliposomes with GMP, the putative antiporter.

Cloning and characterization of a putative mouse acetyl-CoA transporter cDNA

A mouse acetyl-CoA transporter (Acatn) cDNA was isolated by PCR cloning. Mouse Acatn exhibited 92% homology with human sequence on the basis of amino-acid sequence. The predicted gene product of Acatn is a 61 kDa hydrophobic protein with six to 10 transmembrane domains. Transfection of mouse Acatn cDNA into HeLa/GT3+ cells resulted in significant increase in the amount of 9-O-acetylated gangliosides, suggesting that Acatn does play an important role in the acetylation of gangliosides. Northern blot analysis of Acatn mRNA suggested that transcript of Acatn is widely distributed in various adult tissues. Expression of Acatn was found to be developmentally regulated, with high expression levels during early embryonic stages, and then there was a subsequent decrease in expression levels in the later embryonic stages.

A new type of carbohydrate-deficient glycoprotein syndrome due to a decreased import of GDP-fucose into the golgi

The fucosylation of glycoproteins was found to be deficient in a patient with a clinical phenotype resembling that of leukocyte adhesion deficiency type II (LAD II). While in LAD II hypofucosylation of glycoconjugates is secondary to an impaired synthesis of GDP-fucose due to a deficiency of the GDP-D-mannose-4, 6-dehydratase, synthesis of GDP-fucose was normal in our patient (Körner, C., Linnebank, M., Koch, H., Harms, E., von Figura, K., and Marquardt, T. (1999) J. Leukoc. Biol., in press). Import of GDP-fucose into Golgi-enriched vesicles was composed of a saturable, high affinity and a nonsaturable component. In our patient the saturable high affinity import of GDP-fucose was deficient, while import of UDP-galactose and the activity of GDPase, which generates the nucleoside phosphate required for antiport of GDP-fucose, were normal. Addition of L-fucose to the medium of fibroblasts restored the fucosylation of glycoproteins. We propose that this new form of carbohydrate-deficient glycoprotein syndrome is caused by impaired import of GDP-fucose into the Golgi.

In vivo synthesis of complex N-glycans by expression of human N-acetylglucosaminyltransferase I in the filamentous fungus Trichoderma reesei

The human N-acetylglucosaminyltransferase I gene was introduced in the genome of Trichoderma reesei strain VTT-D-80133. Expression was studied after induction from the cellobiohydrolase I promoter. Successful in vivo transfer of GlcNAc was demonstrated by analyzing the neutral N-glycans which were synthesized on cellobiohydrolase I. Final proof of the formation of GlcNAcMan5GlcNAc2 was obtained by NMR analysis.

Schizosaccharomyces pombe UDP-galactose transporter: identification of its functional form through cDNA cloning and expression in mammalian cells

The Schizosaccharomyces pombe UDP-galactose transporter cDNA (SpUGT cDNA), encoding the product of the gms1+ gene which consists of two exon sequences separated by a 173-bp intron, was cloned by RT-PCR. Its product, a hydrophobic protein of 353 amino acid residues resembling its human counterpart, was expressed in the Golgi membranes of UDP-galactose transporter-deficient Lec8 cells, and complemented the genetic defect of the mutant cells. This indicated that SpUGT cDNA encodes the functional S. pombe UDP-galactose transporter. The product of an ORF found in the second exon, which was previously assumed to be the S. pombe UDP-galactose transporter, thus represents an inactive, truncated form of the SpUGT protein.

The KlPMR1 gene of Kluyveromyces lactis encodes for a P-type Ca(2+)-ATPase

A novel P-type Ca(2+)-ATPase gene has been cloned and sequenced in the yeast Kluyveromyces lactis. The gene has been named KlPMR1 and is localized on chromosome I. The putative gene product contains 936 residues and has a calculated molecular weight of 102,437 Da. Analysis of deduced amino acid sequence (KlPmr1p) indicated that the encoded protein retains all the highly conserved domains characterizing the P-type ATPases. KlPmr1p shares 71% amino acid identity with Pmr1p of S. cerevisiae, 62% with HpPmr1p of Hansenula polymorpha, 56% with Y1Pmr1p of Yarrowia lipolytica and 52% with the Ca(2+)-ATPase encoded for by the SPCA1 gene of Rattus norvegicus; these similarities place KlPmr1p in the SPCA group (secretory pathway Ca(2+)-ATPase) of the P-type ATPases. The K. lactis strain harbouring the Klpmr1 disrupted gene is not able to grow in presence of low calcium concentrations and shows hypersensitivity to high concentrations of EGTA in the medium. These defects are relieved by PMR1 of S. cerevisiae on a centromeric plasmid, demonstrating that KlPMR1 encodes for a functional Pmr1p homologue.

Uridine diphosphate-glucose transport into the endoplasmic reticulum of Saccharomyces cerevisiae: in vivo and in vitro evidence

It has been proposed that synthesis of beta-1,6-glucan, one of Saccharomyces cerevisiae cell wall components, is initiated by a uridine diphosphate (UDP)-glucose-dependent reaction in the lumen of the endoplasmic reticulum (ER). Because this sugar nucleotide is not synthesized in the lumen of the ER, we have examined whether or not UDP-glucose can be transported across the ER membrane. We have detected transport of this sugar nucleotide into the ER in vivo and into ER-containing microsomes in vitro. Experiments with ER-containing microsomes showed that transport of UDP-glucose was temperature dependent and saturable with an apparent Km of 46 microM and a Vmax of 200 pmol/mg protein/3 min. Transport was substrate specific because UDP-N-acetylglucosamine did not enter these vesicles. Demonstration of UDP-glucose transport into the ER lumen in vivo was accomplished by functional expression of Schizosaccharomyces pombe UDP-glucose:glycoprotein glucosyltransferase (GT) in S. cerevisiae, which is devoid of this activity. Monoglucosylated protein-linked oligosaccharides were detected in alg6 or alg5 mutant cells, which transfer Man9GlcNAc2 to protein; glucosylation was dependent on the inhibition of glucosidase II or the disruption of the gene encoding this enzyme. Although S. cerevisiae lacks GT, it contains Kre5p, a protein with significant homology and the same size and subcellular location as GT. Deletion mutants, kre5Delta, lack cell wall beta-1,6 glucan and grow very slowly. Expression of S. pombe GT in kre5Delta mutants did not complement the slow-growth phenotype, indicating that both proteins have different functions in spite of their similarities.

Comparison of secretion of a hepatitis C virus glycoprotein in Saccharomyces cerevisiae and Kluyveromyces lactis

A C-terminally truncated form of the hepatitis C virus (HCV) putative envelope glycoprotein E2 was expressed in two yeast species, Saccharomyces cerevisiae and Kluyveromyces lactis, using a yeast signal peptide sequence to direct the viral glycoprotein to the endoplasmic reticulum (ER) pathway of secretion. Characterization of secreted E2 showed that the protein is endoglycosidase-H-sensitive in both yeasts. Moreover, in vivo inhibition of glycosylation with tunicamycin prevented secretion of E2 and showed that, of its 11 putative N-linked glycosylation sites, at least eight were core-glycosylated. Analysis of the heterologous glycoprotein by SDS-PAGE under nonreducing conditions and by gel filtration demonstrated the formation of multiple disulphides, which resulted in secretion of heterogeneous aggregates with an average molecular mass of 770-1000 kDa in both yeasts. However, variations were observed in the binding of the glycoprotein secreted by the two yeasts to a mannose-specific lectin, and also in its reactivity with anti-E2-specific antibodies. This denotes differences between the two yeasts in folding and/or modification of the E2 glycoprotein.

Membrane topology of the mammalian CMP-sialic acid transporter

Nucleotide sugar transporters form a family of distantly related membrane proteins of the Golgi apparatus and the endoplasmic reticulum. The first transporter sequences have been identified within the last 2 years. However, information about the secondary and tertiary structure for these molecules has been limited to theoretical considerations. In the present study, an epitope-insertion approach was used to investigate the membrane topology of the CMP-sialic acid transporter. Immunofluorescence studies were carried out to analyze the orientation of the introduced epitopes in semipermeabilized cells. Both an amino-terminally introduced FLAG sequence and a carboxyl-terminal hemagglutinin tag were found to be oriented toward the cytosol. Results obtained with CMP-sialic acid transporter variants that contained the hemagglutinin epitope in potential intermembrane loop structures were in good correlation with the presence of 10 transmembrane regions. This building concept seems to be preserved also in other mammalian and nonmammalian nucleotide sugar transporters. Moreover, the functional analysis of the generated mutants demonstrated that insertions in or very close to membrane-spanning regions inactivate the transport process, whereas those in hydrophilic loop structures have no detectable effect on the activity. This study points the way toward understanding structure-function relationships of nucleotide sugar transporters.

The genes for the Golgi apparatus N-acetylglucosaminyltransferase and the UDP-N-acetylglucosamine transporter are contiguous in Kluyveromyces lactis

The mannan chains of Kluyveromyces lactis mannoproteins are similar to those of Saccharomyces cerevisiae except that they lack mannose phosphate and have terminal alpha(1-->2)-linked N-acetylglucosamine. Previously, Smith et al. (Smith, W. L. Nakajima, T., and Ballou, C. E. (1975) J. Biol. Chem. 250, 3426-3435) characterized two mutants, mnn2-1 and mnn2-2, which lacked terminal N-acetylglucosamine in their mannoproteins. The former mutant lacks the Golgi N-acetylglucosaminyltransferase activity, whereas the latter one was recently found to be deficient in the Golgi UDP-GlcNAc transporter activity. Analysis of extensive crossings between the two mutants led Ballou and co-workers (reference cited above) to conclude that these genes were allelic or tightly linked. We have now cloned the gene encoding the K. lactis Golgi membrane N-acetylglucosaminyltransferase by complementation of the mnn2-1 mutation and named it GNT1. The mnn2-1 mutant was transformed with a 9.5-kilobase (kb) genomic fragment previously shown to contain the gene encoding the UDP-GlcNAc transporter; transformants were isolated, and phenotypic correction was monitored after cell surface labeling with fluorescein isothiocyanate-conjugated Griffonia simplicifolia II lectin, which binds terminal N-acetylglucosamine, and a fluorescence-activated cell sorter. The above 9.5-kb DNA fragment restored the wild-type lectin binding phenotype of the transferase mutant; further subcloning of this fragment yielded a smaller one containing an opening reading frame of 1,383 bases encoding a protein of 460 amino acids with an estimated molecular mass of 53 kDa, which also restored the wild-type phenotype. Transformants had also regained the ability to transfer N-acetylglucosamine to 3-0-alpha-D-mannopyranosyl-D-mannopyranoside. The gene encoding the above transferase was found to be approximately 1 kb upstream from the previously characterized MNN2 gene encoding the UDP-GlcNAc Golgi transporter. Each gene can be transcribed independently by their own promoter. To our knowledge this is the first demonstration of two Golgi apparatus functionally related genes being contiguous in a genome.

Identification and purification of the rat liver Golgi membrane UDP-N-acetylgalactosamine transporter

Glycosylation of glycoproteins, proteoglycans, and glycosphingolipids occurs mainly in the lumen of the endoplasmic reticulum and the Golgi apparatus. Nucleotide sugars, donors of all the sugars involved in Golgi glycosylation reactions, are synthesized in the cytoplasm and require specialized transporters to be translocated into the lumen of the Golgi apparatus. By controlling the supply of sugar nucleotides in the lumen of the Golgi apparatus, these transporters directly regulate the glycosylation of macromolecules transiting the Golgi. We have identified and purified the rat liver Golgi membrane UDP-N-acetylgalactosamine transporter. The transporter was purified to apparent homogeneity by a combination of conventional and dye color chromatography. An approximately 63,000-fold purification (6% yield) was achieved starting from crude rat liver Golgi membranes and resulting in a protein with an apparent molecular mass of 43 kDa. The transporter was active when reconstituted into phosphatidylcholine vesicles and could be specifically photolabeled with P3-(4-azidoanilido)-uridine-5'-[P1-32P]triphosphate, an analog of UDP-N-acetylgalactosamine. Native functional size determination on a glycerol gradient suggested that the transporter exists as a homodimer within the Golgi membrane.

Coexpression of alpha1,2 galactosyltransferase and UDP-galactose transporter efficiently galactosylates N- and O-glycans in Saccharomyces cerevisiae

We have studied in vivo neo-galactosylation in Saccharomyces cerevisiae and analyzed the critical factors involved in this system. Two heterologous genes, gma12(+) encoding alpha1, 2-galactosyltransferase (alpha1,2 GalT) from Schizosaccharomyces pombe and UGT2 encoding UDP-galactose (UDP-Gal) transporter from human, were functionally expressed to examine the intracellular conditions required for galactosylation. Detection by fluorescence labeled alpha-galactose specific lectin revealed that 50% of the cells incorporated galactose to cell surface mannoproteins only when the gma12(+) and hUGT2 genes were coexpressed in galactose media. Integration of both genes in the Delta mnn1 background cells increased galactosylation to 80% of the cells. Correlation between cell surface galactosylation and UDP-galactose transport activity indicated that an exogenous supply of UDP-Gal transporter rather than alpha1,2 GalT played a key role for efficient galactosylation in S.cerevisiae. In addition, this heterologous system enabled us to study the in vivo function of S. pombe alpha1,2 GalT to prove that it transfers galactose to both N - and O -linked oligosaccharides. Structural analysis indicated that this enzyme transfers galactose to O -mannosyl residue attached to polypeptides and produces Galalpha1,2-Man1-O-Ser/Thr structure. Thus, we have successfully generated a system for efficient galactose incorporation which is originally absent in S. cerevisiae, suggesting further possibilities for in vivo glycan remodeling toward therapeutically useful galactose containing heterologous proteins in S. cerevisiae.

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.

Overview of N- and O-linked oligosaccharide structures found in various yeast species

Yeast and most higher eukaryotes utilize an evolutionarily conserved N-linked oligosaccharide biosynthetic pathway that involves the formation of a Glc3Man9GlcNAc2-PP-dolichol lipid-linked precursor, the glycan portion of which is co-translationally transferred in the endoplasmic reticulum (ER) to suitable Asn residues on nascent polypeptides. Subsequently, ER processing glycohydrolases remove the three glucoses and, with the exception of Schizosaccharomyces pombe, a single, specific mannose residue. Processing sugar transferases in the Golgi lead to the formation of core-sized structures (Hex<15GlcNac2) as well as cores with an extended poly-alpha1,6-Man 'backbone' that is derivatized with various carbohydrate side chains in a species-specific manner (Hex50-200GlnNAc2). In some cases these are short alpha1,2-linked Man chains with (Saccharomyces cerevisiae) or without (Pichia pastoris) alpha1,3-Man caps, while in other yeast (S. pombe), the side chains are alpha1,2-linked Gal, some of which are capped with beta-1,3-linked pyruvylated Gal residues. Charged groups are also found in S. cerevisiae and P. pastoris N-glycans in the form of mannose phosphate diesters. Some pathogenic yeast (Candida albicans) add poly-beta1,2-Man extension through a phosphate diester to their N-glycans, which appears involved in virulence. O-Linked glycan synthesis in yeast, unlike in animal cells where it is initiated in the Golgi using nucleotide sugars, begins in the ER by addition of a single mannose from Man-P-dolichol to selected Ser/Thr residues in newly made proteins. Once transported to the Golgi, sugar transferases add one (C. albicans) or more (P. pastoris) alpha1,2-linked mannose that may be capped with one or two alpha1,3-linked mannoses (S. cerevisiae). S. pombe is somewhat unique in that it synthesizes a family of mixed O-glycans with additional alpha1,2-linked Man and alpha1,2- and 1, 3-linked Gal residues.

Mutants of Kluyveromyces lactis with altered protein glycosylation are affected in cell wall morphogenesis

We isolated spontaneous mutants resistant to sodium orthovanadate in the biotechnologically significant yeast Kluyveromyces lactis. Resistance behaved as a recessive character in all mutants analyzed. Four genes were defined by complementation analysis, from vga1 to vga4. These mutants showed defects in N-linked as well as O-linked glycosylation processes. In addition, the mutants exhibited sensitivity to the aminoglycoside hygromycin B and to calcofluor white, with the exception of vga4; this mutant grew in the presence of the antibiotic as well as the parental wild type and was resistant to calcofluor. The mutations were accompanied by alterations in the cell wall structure, as revealed by the delocalization of chitin, changes in cell shape and size and by the clumpy aspect of the cultures. The mutants isolated provide basic tools for molecular and cellular analysis of glycosylation processes in K. lactis.

Transporters of nucleotide sugars, ATP, and nucleotide sulfate in the endoplasmic reticulum and Golgi apparatus

The lumens of the endoplasmic reticulum and Golgi apparatus are the subcellular sites where glycosylation, sulfation, and phosphorylation of secretory and membrane-bound proteins, proteoglycans, and lipids occur. Nucleotide sugars, nucleotide sulfate, and ATP are substrates for these reactions. ATP is also used as an energy source in the lumen of the endoplasmic reticulum during protein folding and degradation. The above nucleotide derivatives and ATP must first be translocated across the membrane of the endoplasmic reticulum and/or Golgi apparatus before they can serve as substrates in the above lumenal reactions. Translocation of the above solutes is mediated for highly specific transporters, which are antiporters with the corresponding nucleoside monophosphates as shown by biochemical and genetic approaches. Mutants in mammals, yeast, and protozoa showed that a defect in a specific translocator activity results in selective impairments of the above posttranslational modifications, including loss of virulence of pathogenic protozoa. Several of these transporters have been purified and cloned. Experiments with yeast and mammalian cells demonstrate that these transporters play a regulatory role in the above reactions. Future studies will address the structure of the above proteins, how they are targeted to different organelles, their potential as drug targets, their role during development, and the possible occurrence of specific diseases.

The role of glucosidase I (Cwh41p) in the biosynthesis of cell wall beta-1,6-glucan is indirect

CWH41, a gene involved in the assembly of cell wall beta-1,6-glucan, has recently been shown to be the structural gene for Saccharomyces cerevisiae glucosidase I that is responsible for initiating the trimming of terminal alpha-1,2-glucose residue in the N-glycan processing pathway. To distinguish between a direct or indirect role of Cwh41p in the biosynthesis of beta-1,6-glucan, we constructed a double mutant, alg5Delta (lacking dolichol-P-glucose synthase) cwh41Delta, and found that it has the same phenotype as the alg5Delta single mutant. It contains wild-type levels of cell wall beta-1,6-glucan, shows moderate underglycosylation of N-linked glycoproteins, and grows at concentrations of Calcofluor White (which interferes with cell wall assembly) that are lethal to cwh41Delta single mutant. The strong genetic interactions of CWH41 with KRE6 and KRE1, two other genes involved in the beta-1,6-glucan biosynthetic pathway, disappear in the absence of dolichol-P-glucose synthase (alg5Delta). The triple mutant alg5Deltacwh41Deltakre6Delta is viable, whereas the double mutant cwh41Deltakre6Delta in the same genetic background is not. The severe slow growth phenotype and 75% reduction in cell wall beta-1,6-glucan, characteristic of the cwh41Deltakre1Delta double mutant, are not observed in the triple mutant alg5Deltacwh41Deltakre1Delta. Kre6p, a putative Golgi glucan synthase, is unstable in cwh41Delta strains, and its overexpression renders these cells Calcofluor White resistant. These results demonstrate that the role of glucosidase I (Cwh41p) in the biosynthesis of cell wall beta-1,6-glucan is indirect and that dolichol-P-glucose is not an intermediate in this pathway.

Mutants of the CMP-sialic acid transporter causing the Lec2 phenotype

Chinese hamster ovary (CHO) mutants belonging to the Lec2 complementation group are unable to translocate CMP-sialic acid to the lumen of the Golgi apparatus. Complementation cloning in these cells has recently been used to isolate cDNAs encoding the CMP-sialic acid transporter from mouse and hamster. The present study was carried out to determine the molecular defects leading to the inactivation of CMP-sialic acid transport. To this end, CMP-sialic acid transporter cDNAs derived from five independent clones of the Lec2 complementation group, were analyzed. Deletions in the coding region were observed for three clones, and single mutants were found to contain an insertion and a point mutation. Epitope-tagged variants of the wild-type transporter protein and of the mutants were used to investigate the effect of the structural changes on the expression and subcellular targeting of the transporter proteins. Mutants derived from deletions showed reduced protein expression and in immunofluorescence showed a diffuse staining throughout the cytoplasm in transiently transfected cells, while the translation product derived from the point-mutated cDNA (G189E) was expressed at the level of the wild-type transporter and co-localized with the Golgi marker alpha-mannosidase II. This mutation therefore seems to directly affect the transport activity. Site-directed mutagenesis was used to change glycine 189 into alanine, glutamine, and isoleucine, respectively. While the G189A mutant was able to complement CMP-sialic acid transport-deficient Chinese hamster ovary mutants, the exchange of glycine 189 into glutamine or isoleucine dramatically affected the transport activity of the CMP-sialic acid transporter.

Mammalian Golgi apparatus UDP-N-acetylglucosamine transporter: molecular cloning by phenotypic correction of a yeast mutant

Transporters in the Golgi apparatus membrane translocate nucleotide sugars from the cytosol into the Golgi lumen before these can be substrates for the glycosylation of proteins, lipids, and proteoglycans. We have cloned the mammalian Golgi membrane transporter for uridine diphosphate-N-acetylglucosamine by phenotypic correction with cDNA from MDCK cells of a recently characterized Kluyveromyces lactis mutant deficient in Golgi transport of the above nucleotide sugar. Phenotypically corrected transformants were separated from mutants in a fluorescent-activated cell sorter after labeling of K. lactis cells with fluorescein isothiocyanate (FITC) conjugated to Griffonia simplicifolia II lectin, which binds terminal N-acetylglucosamine. A 2-kb DNA fragment was found to restore the wild-type cell lectin binding phenotype, which reverted to the mutant one upon loss of the plasmid. The DNA fragment contained an ORF encoding a hydrophobic, multitransmembrane spanning protein of 326 aa that had only 22% amino acid sequence identity with the corresponding transporter from K. lactis but showed 53% amino acid sequence identity to the mammalian UDP-galactose transporters and 40% to the CMP-sialic acid transporter. Golgi vesicles from the transformant regained their ability to transport UDP-GlcNAc in an assay in vitro. The above results demonstrate that the mammalian Golgi UDP-GlcNAc transporter gene has all of the necessary information for the protein to be expressed and targeted functionally to the Golgi apparatus of yeast and that two proteins with very different amino acid sequences may transport the same solute within the same Golgi membrane.

Metabolism of uridine 5'-diphosphate-glucose in golgi vesicles from pea stems

Uridine 5'-diphosphate-glucose (UDP-Glc) is transported into the lumen of the Golgi cisternae, where is used for polysaccharide biosynthesis. When Golgi vesicles were incubated with UDP-[3H]Glc, [3H]Glc was rapidly transferred to endogenous acceptors and UDP-Glc was undetectable in Golgi vesicles. This result indicated that a uridine-containing nucleotide was rapidly formed in the Golgi vesicles. Since little is known about the fate of the nucleotide derived from UDP-Glc, we analyzed the metabolism of the nucleotide moiety of UDP-Glc by incubating Golgi vesicles with [alpha-32P]UDP-Glc, [beta-32P]UDP-Glc, and [3H]UDP-Glc and identifying the resulting products. After incubation of Golgi vesicles with these radiolabeled substrates we could detect only uridine 5'-monophosphate (UMP) and inorganic phosphate (Pi). UDP could not be detected, suggesting a rapid hydrolysis of UDP by the Golgi UDPase. The by-products of UDP hydrolysis, UMP and Pi, did not accumulate in the lumen, indicating that they were able to exit the Golgi lumen. The exit of UMP was stimulated by UDP-Glc, suggesting the presence of a putative UDP-Glc/UMP antiporter in the Golgi membrane. However, the exit of Pi was not stimulated by UDP-Glc, suggesting that the exit of Pi occurs via an independent membrane transporter.

Nucleotide sugars, nucleotide sulfate, and ATP transporters of the endoplasmic reticulum and Golgi apparatus

The lumina of the endoplasmic reticulum and Golgi apparatus are the subcellular sites where glycosylation, sulfation, and phosphorylation of secretory and membrane-bound proteins, proteoglycans, and lipids occur. Nucleotide sugars, nucleotide sulfate, and ATP are substrates in the above reactions and must first be translocated from the cytosol into the lumen of these organelles. Translocation of these nucleotide derivatives is mediated by highly specific transporters, which are antiporters with the corresponding nucleoside monophosphate, as shown by genetic and biochemical approaches in mammals and yeast. Studies with mammalian, yeast, and protozoa mutants have shown that a defect in a specific translocator results in selective impairments of glycosylation of proteins, lipids and proteoglycans in vivo. Several of these transporters have been purified, cloned, and found to encode very hydrophobic proteins with multitransmembrane domains. Experiments with yeast and mammalian cells demonstrate that these transporters play a regulatory role in posttranslational modifications.

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.

The VRG4 gene is required for GDP-mannose transport into the lumen of the Golgi in the yeast, Saccharomyces cerevisiae

In the yeast Saccharomyces cerevisiae, glycoproteins and sphingolipids are modified in the Golgi by the addition of mannose residues. The critical mannosyl donor for these reactions is the nucleotide sugar, GDP-mannose, whose transport into the Golgi from the cytoplasm is required for mannosylation. This transport reaction has been well characterized, but the nucleotide sugar transporter has yet to be identified in yeast. VRG4 is an essential gene whose product is required for a number of Golgi-specific functions, including glycosylation and the organization of the endomembrane system. Here, data are presented that demonstrate that the primary role of Vrg4p is in the transport of GDP-mannose into the Golgi. The vrg4 mutation causes a general impairment in mannosylation, affecting N-linked and O-linked glycoprotein modifications as well as the mannosylation of sphingolipids. By using an in vitro assay, vrg4 mutants were shown to be specifically defective in the transport of GDP-mannose into Golgi vesicles. The Vrg4 protein localizes to the Golgi complex in a pattern that suggests a wide distribution throughout the Golgi. Vrg4p displays homology to other putative nucleotide sugar transporters, suggesting that the VRG4 gene encodes a Golgi GDP-mannose transporter. As Vrg4p is essential, these results suggest that a complete lack of mannosylation of glycoproteins in the Golgi leads to inviability. Alternatively, the essential function of Vrg4p in yeast involves its effect on sphingolipids, which would imply a critical role for mannosylinositol phosphorylceramides or mannosyl diphosphoinositol ceramides on growth and viability.

Tissue-specific regulation of mouse core 2 beta-1,6-N-acetylglucosaminyltransferase

Mouse kidney beta-1,6-GlcNAc-transferase (GNT) is the key enzyme for the synthesis of a glycosphingolipid (Galbeta1-4(Fucalpha1-3)GlcNAcbeta1-6(Galbeta1 -3)GalNAcbeta1-3Galalph a1-4Galbeta1-4Glcbeta1-ceramide) that contains the LeX trisaccharide epitope at its nonreducing terminus. The expression of this glycolipid in the kidney is polymorphic; it is expressed in BALB/c but not DBA/2 mice; and a single autosomal gene (Gsl5) is responsible for this polymorphism. We report here the cDNA sequence that encodes the kidney GNT of BALB/c mice, which possess a wild-type Gsl5 gene. The deduced amino acid sequence exhibits 84% identity to that of human core 2 beta-1,6-GlcNAc-transferase, which suggests that kidney GNT is a mouse homologue of human core 2 beta-1, 6-GlcNAc-transferase. The GNT mRNA is expressed abundantly in the kidney, but was not detected in other BALB/c organs or in the kidneys of DBA/2 mice by Northern blot analysis. In addition, we were able to clone and sequence another homologous cDNA from the submandibular gland. The two sequences differ only in their 5'-untranslated region. The submandibular gland type of cDNA was detected in various organs of DBA/2 mice by reverse transcription-polymerase chain reaction, which indicates that the submandibular gland type is ubiquitous and that its expression is not regulated by the Gsl5 gene. Results obtained using the long accurate polymerase chain reaction method indicate that the GNT gene is approximately 45 kilobases long, and the order of the exons from the 5'-end is exon 1 of the kidney type, exon 1 of the ubiquitous type, exon 2, and exon 3. Exons 2 and 3 are present in both transcripts, and the translated region is in exon 3. These data suggest that the expression of GNT is regulated by an alternative splicing mechanism and also probably by tissue-specific enhancers and that Gsl5 regulates the expression of GNT only in the kidney.

Molecular cloning of the hamster CMP-sialic acid transporter

Chinese hamster ovary (CHO) glycosylation mutants of the Lec2 complementation group are unable to express sialylated glycoproteins and glycolipids due to a defect in the Golgi CMP-sialic acid transporter (CMP-Sia-Tr). Using an expression cloning strategy, we isolated a cDNA encoding the hamster CMP-Sia-Tr which complements the Lec2 phenotype. The deduced amino acid sequence of the cloned cDNA shows 95% identity to the recently cloned murine CMP-Sia-Tr. The expression of a hamster CMP-Sia-Tr fusion protein with an N-terminal MDYKDDDDK (FLAG) sequence revealed Golgi localisation of the transporter. Amino acid sequence comparison revealed strong similarity (44.6% identity and 19.3% similarity) of CMP-Sia-Tr to the recently cloned human UDP-galactose transporter (UDP-Gal-Tr). In contrast, sequence similarities to the yeast UDP-N-acetylglucosamine transporter (UDP-GlcNAc-Tr) and the GDP-mannose transporter (GDP-Man-Tr) of Leishmania donovani are restricted to a region encoding the two most C-terminally located transmembrane helices. A computer-based structural analysis of CMP-Sia-Tr proposes an eight transmembrane helix model with the N- and C-termini located on the cytosolic side of the Golgi membrane.

The molecular genetics of hexose transport in yeasts

Transport across the plasma membrane is the first, obligatory step of hexose utilization. In yeast cells the uptake of hexoses is mediated by a large family of related transporter proteins. In baker's yeast Saccharomyces cerevisiae the genes of 20 different hexose transporter-related proteins have been identified. Six of these transmembrane proteins mediate the metabolically relevant uptake of glucose, fructose and mannose for growth, two others catalyze the transport of only small amounts of these sugars, one protein is a galactose transporter but also able to transport glucose, two transporters act as glucose sensors, two others are involved in the pleiotropic drug resistance process, and the functions of the remaining hexose transporter-related proteins are not yet known. The catabolic hexose transporters exhibit different affinities for their substrates, and expression of their corresponding genes is controlled by the glucose sensors according to the availability of carbon sources. In contrast, milk yeast Kluyveromyces lactis contains only a few different hexose transporters. Genes of other monosaccharide transporter-related proteins have been found in fission yeast Schizosaccharomyces pombe and in the xylose-fermenting yeast Pichia stipitis. However, the molecular genetics of hexose transport in many other yeasts remains to be established. The further characterization of this multigene family of hexose transporters should help to elucidate the role of transport in yeast sugar metabolism.

Functional expression of the murine Golgi CMP-sialic acid transporter in saccharomyces cerevisiae

We have functionally expressed the murine Golgi putative CMP-sialic acid transporter in Saccharomyces cerevisiae. Using a galactose-inducible expression system, S. cerevisiae vesicles were able to transport CMP-sialic acid. Transport was dependent on galactose induction and was temperature-dependent and saturable with an apparent Km of 2.9 microM. Transport was inhibited by CMP, and upon vesicle disruption with Triton X-100 parameters were very similar to the previously described CMP-sialic acid transport characteristics observed with mammalian Golgi vesicles. CMP-sialic acid transport induction was specific as no transport of UDP-galactose was observed even though the latter putative transporter has a high degree of amino acid sequence identity with the CMP-sialic acid transporter. Together, the above results demonstrate that the previously described cDNA encoding the putative CMP-sialic acid transporter encodes the transporter protein per se and suggests that this heterologous expression system may be used for further structural and functional studies of other Golgi membrane transporter proteins.

Expression cloning and characterization of a cDNA encoding a novel membrane protein required for the formation of O-acetylated ganglioside: a putative acetyl-CoA transporter

By expression cloning using COS-1 cells stably transfected with GD3-synthase (COS-1/GD3+) as a recipient cell line, we have isolated a cDNA, termed AT-1, encoding a novel protein required for the formation of O-acetylated (Ac) gangliosides. The cDNA encodes a protein with multitransmembrane spanning domains with a leucine zipper motif. It consists of 549 amino acids and has a molecular mass of 60.9 kDa. Although both O-Ac-GD3 and O-Ac-GT3 were barely detectable in recipient cells or cells transfected with the vector alone, their amount increased significantly in transfectants containing AT-1. When semi-intact cells prepared by treatment with streptolysin O were incubated with [Ac-14C]-Ac-CoA, increased incorporation of radioactivity was found in those cells transfected with AT-1 when compared with the mock transfectants. Northern blot analysis showed two major transcripts of 3.3 and 4.3 kb in all tissues examined. Immunohistochemical study with an antibody specific to the AT-1 protein suggested that it is most probably expressed in the endoplasmic reticulum membrane. Based on these results, the protein encoded by AT-1 is suggested to be an Ac-CoA transporter that is involved in the process of O-acetylation.

The Schizosaccharomyces pombe gms1+ gene encodes an UDP-galactose transporter homologue required for protein galactosylation

In a previous study, we isolated a Schizosaccharomyces pombe mutant defective in protein galactosylation (Takegawa, K., Tanaka, N., Tabuchi, M. and Iwahara, S. (1996) Biosci. Biochem. Biotech. 60, 1156-1159). From an S. pombe genomic library, we cloned the gms1+ gene which restored the galactosylation of cell wall glycoproteins. Gms1 protein shares significant sequence similarity with human UDP-galactose and murine CMP-sialic acid transporters. The fission yeast strains deleted for the gms1+ gene lacked galactose residues in sell surface glycoproteins and were significantly decreased in UDP-galactose transport activity. These results showed that the gms1+ encodes an UDP-galactose transporter, and this protein appears to be an essential role for the incorporation of UDP-galactose into the lumen of Golgi in s. pombe.

Cell biology: alternatives to baker's yeast

Saccharomyces cerevisiae is an excellent model organism for addressing questions in cell biology, but other yeast systems are also providing new insights into several fundamental cellular processes.