Golgi-resident Glycosyltransferases Research Articles

Biochemical and Genetic Evidence for the Involvement of Yeast Ypt6-GTPase in Protein Retrieval to Different Golgi Compartments

Yeast Ypt6p, the homologue of the mammalian Rab6 GTPase, is not essential for cell viability. Based on previous studies with ypt6 deletion mutants, a regulatory role of the GTPase either in protein retrieval to the trans-Golgi network or in forward transport between the endoplasmic reticulum (ER) and early Golgi compartments was proposed. To assess better the primary role(s) of Ypt6p, temperature-sensitive ypt6 mutants were generated and analyzed biochemically and genetically. Defects in N-glycosylation of proteins passing the Golgi and of Golgi-resident glycosyltransferases as well as protein sorting defects in the trans-Golgi were recorded shortly after functional loss of Ypt6p. ER-to-Golgi transport and protein secretion were delayed but not interrupted. Mis-sorting of the vesicular SNARE Sec22p to the late Golgi was also observed. Combination of the ypt6-2 mutant allele with a number of mutants in forward and retrograde transport between ER, Golgi, and endosomes led to synthetic negative growth defects. The results obtained indicate that Ypt6p acts in endosome-to-Golgi, in intra-Golgi retrograde transport, and possibly also in Golgi-to-ER trafficking.

Molecular Genetics of Non-processive Glycosyltransferases

Although Arabidopsis has become the model for plant genomic and developmental research, it remains behind the rest of the field with respect to biochemical and structural analyses. However, the availability of the genome sequence provides an alternative and probable first approach towards identification, by homology, of potentially interesting genes. Subsequently, such a genetic approach has been useful in identifying a class of enzymes involved in cell wall biosynthesis, the polysaccharide glycosyltransferases. Most enzymes that are involved in glycan biosynthesis are glycosyltransferases. These enzymes act mainly by adding monosaccharides (donor molecules) one at a time to specific positions on acceptor molecules and thus assemble monosaccharides into linear and branched sugar chains. It has been estimated that more than 100 distinct glycosidic linkages are present in the glycoconjugate repertoire of any given multicellular organism. Because each linkage is usually the product of a different glycosyltransferase, it appears that a large number of distinct glycosyltransferases is present in higher eukaryotes. Many, but not all, of these enzymes are found within the ER-Golgi pathway where the synthesis of many glycoconjugates occurs. Glycosyltransferases may act as either processive or non-processive enzymes. Processive glycosyltransferases (often referred to as polysaccharide synthases) add sugar moieties to the growing end of a linear polysaccharide without releasing the acceptor substrate. One classic example is cellulose synthase. Non-processive glycosyltransferases add single monosaccharides to specific positions of an acceptor molecule, then move on to other acceptor sites. These enzymes are usually type II membrane proteins possessing a single hydrophobic segment spanning the Golgi membrane, which functions as a signal-anchor sequence. A short amino-terminal portion faces the cytosol and the carboxy-terminal catalytic domain is positioned within the lumen of the Golgi apparatus (Figure 1). Figure 1. Schematic diagram of most Golgi-resident glycosyltransferases. They are characterized by a short NH2-terminus oriented toward the cytoplasmic side of the Golgi membrane, a single membrane spanning region, and a variable length stem connecting the globular ... Many glycosyltransferases have been identified and extensively studied in animal, fungal, and bacterial systems; these are classified into different families, based on the type of sugar that they transfer and the type of linkage that they establish. For instance, an enzyme transferring a galactose residue from UDP-D-galactose onto the 2-hydroxyl group of another sugar moiety would be considered a (1,2)-galactosyltransferase. The sugar moiety may be added in either alpha or beta configuration adding further complexity to the types of linkages that can be formed. If in the above example the newly attached galactose residue is in the beta configuration, the enzyme would be classified as a *b(1,2)-galactosyltransferase. Since the sugar moiety in UDP-D-galactose exists in the alpha configuration, b-galactosyltransferases act with inversion of configuration whereas a-galactosyltransferases act with retention of configuration. Like other eukaryotic organisms, plants have glycoproteins and glycolipids; however, a major feature distinguishing plants from animals is the presence of a complex wall surrounding every cell. These walls play a crucial role in development, signal transduction, and response to environmental factors, including microbial pathogens and insects. Plant cell walls are mainly composed of cellulose microfibrils and matrix polysaccharides that encompass hemicelluloses and pectins. The synthesis of polysaccharides can be divided into two main stages: synthesis of the backbone, performed by polysaccharide synthases, and addition of side-chain residues mediated by glycosyltransferases. The structural complexity of many of these polysaccharides is attributed to numerous modifications that involve additions of various sugars by glycosyltransferases followed by acetylations, methylations, and addition of phenolics. Although biochemical analyses have demonstrated the existence of many glycosyltransferases in plants (White et al., 1993; Keegstra & Raikhel, 2001 and references therein), to date only three genes encoding non-processive glycosyltransferases in cell wall polysaccharide biosynthesis have been cloned, and several genes that are involved in glycoprotein biosynthesis have been identified as well. Low amino acid sequence similarity between families of glycosyltransferases and the difficulties of obtaining acceptor substrates contribute to the lack of success in identifying plant cell wall glycosyltransferases. In this chapter we will highlights some of the advances made in this area over the past few years.

Effect of N-glycosylation on turnover and subcellular distribution of N-acetylgalactosaminyltransferase I and sialyltransferase II in neuroblastoma cells.

Gangliosides are sialylated glycosphingolipids whose biosynthesis is catalyzed by a series of endoplasmic reticulum (ER)- and Golgi-resident glycosyltransferases. Protein expression, processing, and subcellular localization of the key regulatory enzymes for ganglioside biosynthesis, sialyltransferase II (ST-II) and N-acetylgalactosaminyltransferase I (GalNAcT), were analyzed upon transient expression of the two enzymes in the neuroblastoma cell lines NG108-15 and F-11. The enzymes were endowed with a C-terminal epitope tag peptide (FLAG) for immunostaining and immunoaffinity purification using a FLAG-specific antibody. Mature ST-II-FLAG and GalNAcT-FLAG were expressed as N-glycoproteins with noncomplex oligosaccharides. ST-II-FLAG was distributed to the Golgi apparatus, whereas GalNAcT-FLAG was found in the ER and Golgi. Inhibition of early N-glycoprotein processing with castanospermine resulted in a distribution of ST-II-FLAG to the ER, whereas that of GalNAcT-FLAG remained unaltered. In contrast to GalNAcT, the activity of ST-II and the amount of immunostained enzyme were reduced concomitantly by 75% upon incubation with castanospermine. This was due to a fourfold increased turnover of ST-II-FLAG, which was not found with GalNAcT-FLAG. The ER retention and increased turnover of ST-II-FLAG were most likely due to its inability to bind to calnexin upon inhibition of early N-glycoprotein processing. Calnexin binding was not observed for GalNAcT-FLAG, indicating a differential effect of N-glycosylation on the turnover and subcellular localization of the two glycosyltransferases.

Mouse β 1,3‐galactosyltransferase (GA1/GM1/GD1b synthase): Protein characterization, tissue expression, and developmental regulation in neural retina

The composition of cell surface gangliosides is largely dependent on the relative activities of Golgi resident glycosyltransferases. In the brain of birds and mammals, complex gangliosides (GM2, GM1, GD1a, GD1b, GT1b) abound at late stages of development and in the adult, due to the relatively high activities of the UDP-GalNAc:LacCer/GM3/GD3 β 1,4-N-acetylgalactosaminyltransferase (GalNAc-T) and the UDP-Gal: GA2/GM2/GD2 β 1,3-galactosyltransferase (Gal-T2) relative to that of CMP-NeuAc:GM3 α 2,8-sialyltransferase (Sial-T2). Unlike brain, the mature mammalian neural retina abundantly expresses the simple ganglioside GD3, in relation to complex gangliosides, due to the low activity of GalNAc-T and Gal-T2 relative to Sial-T2. Here we describe the isolation and characterization of a mouse Gal-T2 cDNA that drives the synthesis of an epitope-tagged protein of molecular mass 43 kDa, which was enzymatically active and localized to the Golgi complex in transfected cell lines. Using this cDNA as a probe, it was found that Gal-T2 is coded by a single gene located in chromosome 17, and that the coding sequence is contained in a single exon. The expression of the specific Gal-T2 mRNA (∼1.8 kb) was highest in testis, which also showed elevated Gal-T2 activity. In the postnatal neural retina, Gal-T2 mRNA increased after day 3, maintained high levels of expression by days 4–7, and then decreased to initial values by day 10. The developmental pattern of mRNA expression was temporally coincident with that of Gal-T2 activity expression, indicating that this enzyme is under transcriptional control in the neural retina. J. Neurosci. Res. 58:318–327, 1999. © 1999 Wiley-Liss, Inc.

Mouse ? 1,3-galactosyltransferase (GA1/GM1/GD1b synthase): Protein characterization, tissue expression, and developmental regulation in neural retina

The composition of cell surface gangliosides is largely dependent on the relative activities of Golgi resident glycosyltransferases. In the brain of birds and mammals, complex gangliosides (GM2, GM1, GD1a, GD1b, GT1b) abound at late stages of development and in the adult, due to the relatively high activities of the UDP-GalNAc:LacCer/GM3/GD3 beta 1,4-N-acetylgalactosaminyltransferase (GalNAc-T) and the UDP-Gal: GA2/GM2/GD2 beta 1, 3-galactosyltransferase (Gal-T2) relative to that of CMP-NeuAc:GM3 alpha 2,8-sialyltransferase (Sial-T2). Unlike brain, the mature mammalian neural retina abundantly expresses the simple ganglioside GD3, in relation to complex gangliosides, due to the low activity of GalNAc-T and Gal-T2 relative to Sial-T2. Here we describe the isolation and characterization of a mouse Gal-T2 cDNA that drives the synthesis of an epitope-tagged protein of molecular mass 43 kDa, which was enzymatically active and localized to the Golgi complex in transfected cell lines. Using this cDNA as a probe, it was found that Gal-T2 is coded by a single gene located in chromosome 17, and that the coding sequence is contained in a single exon. The expression of the specific Gal-T2 mRNA (approximately 1.8 kb) was highest in testis, which also showed elevated Gal-T2 activity. In the postnatal neural retina, Gal-T2 mRNA increased after day 3, maintained high levels of expression by days 4-7, and then decreased to initial values by day 10. The developmental pattern of mRNA expression was temporally coincident with that of Gal-T2 activity expression, indicating that this enzyme is under transcriptional control in the neural retina.

Recycling of golgi-resident glycosyltransferases through the ER reveals a novel pathway and provides an explanation for nocodazole-induced Golgi scattering.

During microtubule depolymerization, the central, juxtanuclear Golgi apparatus scatters to multiple peripheral sites. We have tested here whether such scattering is due to a fragmentation process and subsequent outward tracking of Golgi units or if peripheral Golgi elements reform through a novel recycling pathway. To mark the Golgi in HeLa cells, we stably expressed the Golgi stack enzyme N-acetylgalactosaminyltransferase-2 (GalNAc-T2) fused to the green fluorescent protein (GFP) or to an 11-amino acid epitope, VSV-G (VSV), and the trans/TGN enzyme beta1,4-galactosyltransferase (GalT) fused to GFP. After nocodazole addition, time-lapse microscopy of GalNAc-T2-GFP and GalT-GFP revealed that scattered Golgi elements appeared abruptly and that no Golgi fragments tracked outward from the compact, juxtanuclear Golgi complex. Once formed, the scattered structures were relatively stable in fluorescence intensity for tens of minutes. During the entire process of dispersal, immunogold labeling for GalNAc-T2-VSV and GalT showed that these were continuously concentrated over stacked Golgi cisternae and tubulovesicular Golgi structures similar to untreated cells, suggesting that polarized Golgi stacks reform rapidly at scattered sites. In fluorescence recovery after photobleaching over a narrow (FRAP) or wide area (FRAP-W) experiments, peripheral Golgi stacks continuously exchanged resident proteins with each other through what appeared to be an ER intermediate. That Golgi enzymes cycle through the ER was confirmed by microinjecting the dominant-negative mutant of Sar1 (Sar1pdn) blocking ER export. Sar1pdn was either microinjected into untreated or nocodazole-treated cells in the presence of protein synthesis inhibitors. In both cases, this caused a gradual accumulation of GalNAc-T2-VSV in the ER. Few to no peripheral Golgi elements were seen in the nocodazole-treated cells microinjected with Sar1pdn. In conclusion, we have shown that Golgi-resident glycosylation enzymes recycle through the ER and that this novel pathway is the likely explanation for the nocodazole-induced Golgi scattering observed in interphase cells.

A molecular approach to the structure, polymorphism and function of blood groups

Biochemical and molecular genetic studies have contributed to our molecular knowledge of blood group-associated molecules in the past few years. Among the 23 blood group systems presently identified, almost all have a molecular basis and present investigations are oriented towards the analysis of genetic polymorphisms, tissue-specific expression and structure-function relationships. Antigens defined by carbohydrate structures, among which ABO, Hh, Lewis and Secretor are the main representative species, are indirect gene products. They are synthesized by Golgi-resident glycosyltransferases, which are the direct products of the blood group genes. Many of these enzymes have been cloned and the molecular basis of the silent phenotypes, for instance 0, Bombay/paraBombay, Le(a-b-) and non-secretor, has been elucidated. However, the glycosyltransferases involved in the biosynthesis of Pk, P and P1 antigens are not yet characterized. A large number of blood group antigens carried by red cell polypeptides expressed at the cell surface are not related to a carbohydrate structure, and these proteins are direct blood group gene products. Most have been cloned and characterized recently, for instance MN antigens (glycophorin A), Ss antigens (glycophorin B), Gerbich antigens (glycophorins C and D) and antigens encoded by the RH, LW, KEL, FY, JK, XG, LU and XK loci. Other antigens have been located on proteins already identified, for instance the Cromer antigens on DAF, Knops antigens on CR1, Indian and AnWj antigens on CD44, Yt antigens on AChE, Diego, Wr, Rga and Warr on Band 3, Colton antigens on AQP-1 (water channel). The SC (Scianna) et DO (Dombrock) systems, however, still resist to molecular cloning. On the basis of this information, a tentative classification of blood group antigens into five functional categories is emerging: - Transporters and channels, - Receptors and ligands, - Adhesion molecules, - Enzymes, - Structural proteins. This review will focus on these recent findings and will illustrate how these studies may bring new information for analysis of normal and abnormal phenotypes and for understanding both the mechanisms of tissue specific expression and the potential function of these antigens, particularly those expressed in non-erythroid lineage. In addition, since our knowledge of the molecular basis of blood group polymorphisms has significantly increased, new genotyping techniques potentially useful in clinical applications will become available.

Kex2-dependent invertase secretion as a tool to study the targeting of transmembrane proteins which are involved in ER-->Golgi transport in yeast.

Mutants were isolated that are defective in the retention of a transmembrane protein in the early secretory compartments in yeast. A series of hybrid proteins was tested for their use in the selection of such mutants. Each of these hybrid proteins consisted of a type II transmembrane protein (Nin/Cout) and invertase (Suc2) as a reporter separated by a peptide linker containing a cleavage site for the Golgi protease Kex2. The integral membrane proteins which were used--Sec12p, Sec22/Sly2p or Bet1/Sly12p--are all known to be required for ER-->Golgi transport in yeast. Invertase was readily cleaved from the fusions containing Sec22/Sly2p or Bet1/Sly12p as the membrane anchoring part. In contrast, Sec12--invertase expressing transformants required mutations in either of two different genes for Kex2-dependent invertase secretion. The mutant showing the stronger retention defect (rer1) was used to clone the corresponding gene. RER1 represents the first reading frame left of the centromere of chromosome III. Cells carrying a disruption of the RER1 gene are viable and show the same mislocalizing phenotype as the original mutants. The Rer1 protein, as deduced from the nucleotide sequence, contains four transmembrane domains. It has been suggested before that Sec12p cycles between the ER and the cis-Golgi compartment. Some results obtained by using Sec12-invertase and the rer1 mutants resemble observations on the retention of Golgi-resident glycosyltransferases and viral proteins in mammalian cells. For instance, retention of Sec12-invertase is non-saturable and the membrane-spanning domain of Sec12p seems to constitute an important targeting signal.

Retrograde transport from the Golgi region to the endoplasmic reticulum is sensitive to GTP gamma S.

The involvement of GTP-binding proteins in the intracellular transport of the secretory glycoprotein alpha 1-antitrypsin was investigated in streptolysin O-permeabilized HepG2 cells. This permeabilization procedure allows ready access to the intracellular milieu of the membrane-impermeant, nonhydrolyzable GTP analog GTP gamma S. In streptolysin O-permeabilized HepG2 cells, the constitutive secretory pathway remains functional and is sensitive to GTP gamma S. Exposure of HepG2 cells to brefeldin A resulted in redistribution of Golgi-resident glycosyltransferases (including both alpha 2----3 and alpha 2----6 sialyltransferases) to the ER. This redistribution was sensitive to GTP gamma S. Our results suggest that GTP-binding proteins are involved in the regulation not only of the anterograde, but also of the retrograde, pathway.