Target sizes of galactosyltransferase, sialyltransferase, and uridine diphosphatase in Golgi apparatus of rat liver

Nucleotide sugar transporters of the Golgi apparatus

The Golgi apparatus is the major site of protein, lipid and proteoglycan glycosylation. The glycosylation enzymes, as well as kinases and sulfatases that catalyze phosphorylation and sulfation, are localized within the Golgi cisternae in characteristic distributions that frequently reflect their order in a particular pathway (Kornfeld and Kornfeld 1985; Colley 1997). The glycosyl-transferases, sulfotransferases and kinases are “transferases” that require activated donor molecules for the reactions they catalyze. For eukaryotic, fungal and protozoan glycosyltransferases these are the nucleotide sugars UDP-N-acetylglucosamine (UDP-GlcNAc), UDP-galactose (UDP-Gal), GDP-fucose (GDP-Fuc), CMP-sialicacid (CMP-Sia), UDP-glucuronicacid (UDP-GlcA), GDP-mannose (GDP-Man), and UDP-xylose (UDP-Xyl) (Hirschberg et al. 1998). For the kinases, ATP functions as the donor, while for the sulfotransferases, adenosine 3′-phosphate 5′-phosphate (PAPS) acts as the donor (Hirschberg et al. 1998). The active sites of all these enzymes are oriented towards the lumen of the Golgi cisternae. This necessitates the translocation of their donors from the cytosol into the lumenal Golgi compartments. In this chapter we will focus on the structure, function and localization of the Golgi nucleotide sugar transporters (NSTs), and highlight the diseases and developmental defects associated with defective transporters. We direct the reader to several excellent reviews on Golgi transporters for additional details and references (Hirschberg et al. 1998; Berninsone and Hirschberg 2000; Gerardy-Schahn et al. 2001; Handford et al. 2006; Caffaro and Hirschberg 2006).

Human TPST1 Transmembrane Domain Triggers Enzyme Dimerisation and Localisation to the Golgi Compartment

TPST1 is a human tyrosylprotein sulfotransferase that uses 3'phosphoadenosine-5'phosphosulfate (PAPS) to transfer the sulfate moiety to proteins predominantly designated for secretion. To achieve a general understanding of the cellular role of human tyrosine-directed sulfotransferases, we investigated targeting, structure and posttranslational modification of TPST1. Golgi localisation of the enzyme in COS-7 and HeLa cells was visualised by fluorescence imaging techniques. PNGase treatment and mutational studies determined that TPST1 bears N-linked glycosyl residues exclusively at position Asn60 and Asn262. By alanine mutation of these asparagine residues, we could determine that the N-linked oligosaccharides do not have an influence on Golgi retention of TPST1. In concert with N and C-terminal flanking residues, the transmembrane domain of TPST1 was determined to act in targeting and retention of the enzyme to the trans-Golgi compartment. This domain exhibits a pronounced secondary structure in a lipid environment. Further in vivo FRET studies using the transmembrane domain suggest that the human tyrosylprotein sulfotransferase may be functional as homodimer/oligomer in the trans-Golgi compartment.

Highly conserved cysteines of mouse core 2 beta1,6-N-acetylglucosaminyltransferase I form a network of disulfide bonds and include a thiol that affects enzyme activity

Core 2 beta1,6-N-acetylglucosaminyltransferase I (C2GnT-I) plays a pivotal role in the biosynthesis of mucin-type O-glycans that serve as ligands in cell adhesion. To elucidate the three-dimensional structure of the enzyme for use in computer-aided design of therapeutically relevant enzyme inhibitors, we investigated the participation of cysteine residues in disulfide linkages in a purified murine recombinant enzyme. The pattern of free and disulfide-bonded Cys residues was determined by liquid chromatography/electrospray ionization tandem mass spectrometry in the absence and presence of dithiothreitol. Of nine highly conserved Cys residues, under both conditions, one (Cys217) is a free thiol, and eight are engaged in disulfide bonds, with pairs formed between Cys59-Cys413, Cys100-Cys172, Cys151-Cys199, and Cys372-Cys381. The only non-conserved residue within the beta1,6-N-acetylglucosaminyltransferase family, Cys235, is also a free thiol in the presence of dithiothreitol; however, in the absence of reductant, Cys235 forms an intermolecular disulfide linkage. Biochemical studies performed with thiolreactive agents demonstrated that at least one free cysteine affects enzyme activity and is proximal to the UDP-GlcNAc binding site. A Cys217 --> Ser mutant enzyme was insensitive to thiol reactants and displayed kinetic properties virtually identical to those of the wild-type enzyme, thereby showing that Cys217, although not required for activity per se, represents the only thiol that causes enzyme inactivation when modified. Based on the pattern of free and disulfide-linked Cys residues, and a method of fold recognition/threading and homology modeling, we have computed a three-dimensional model for this enzyme that was refined using the T4 bacteriophage beta-glucosyltransferase fold.

Recent developments in the cell biology and biochemistry of glycosylphosphatidylinositol lipids (review)

Glycosylphosphatidylinositols (GPIs) represent an abundant and ubiquitous class of eukaryotic glycolipids. Although these structures were originally discovered in the form of GPI-anchored cell surface glycoproteins, it is becoming increasingly clear that a significant proportion of the GPI synthetic output of a cell is not directed to protein anchoring. Indeed, pools of non-protein-linked GPIs can approach 10(7) molecules per cell in some cell types, especially the protozoa, with a large proportion of these molecules being displayed at the cell surface. Recent studies which form the subject of this review indicate that there is (a) considerable diversity in the range of structural modifications found on GPI glycolipids within and between species and cell types, (b) complexity in the topological arrangement of the GPI biosynthetic pathway in the endoplasmic reticulum, and (c) spatial restriction of the biosynthetic pathway within the endoplasmic reticulum. Furthermore, consistent with additional functional roles for these lipids beyond serving as protein anchor precursors, products of the GPI biosynthetic pathway appear to be widely distributed in the cellular endomembrane system. These studies indicate that there is still much to learn about the organization of glycolipid biosynthetic pathways in eukaryotic cells, the nature and subcellular distribution of the lipid products of these pathways, and the function of these lipids within cells.

Oligomerization and topology of the Golgi membrane protein glucosylceramide synthase

Glucosylceramide synthase (GCS) catalyzes the transfer of glucose from UDP-glucose to ceramide to form glucosylceramide, the precursor of most higher order glycosphingolipids. Recently, we characterized GCS activity in highly enriched fractions from rat liver Golgi membranes (Paul, P., Kamisaka, Y., Marks, D. L., and Pagano, R. E. (1996) J. Biol. Chem. 271, 2287-2293), and human GCS was cloned by others (Ichikawa, S., Sakiyama, H., Suzuki, G., Hidari, K. I.-P. J., and Hirabayashi, Y. (1996) Proc. Natl. Acad. Sci. U. S. A. 93, 4638-4643). However, the polypeptide responsible for GCS activity has never been identified or characterized. In this study, we made polyclonal antibodies against peptides based on the predicted amino acid sequence of human GCS and used these antibodies to characterize the GCS polypeptide in rat liver Golgi membranes. Western blotting of rat liver Golgi membranes, human cells, and recombinant rat GCS expressed in bacteria showed that GCS migrates as an approximately 38-kDa protein on SDS-polyacrylamide gels. Trypsinization and immunoprecipitation studies with Golgi membranes showed that both the C terminus and a hydrophilic loop near the N terminus of GCS are accessible from the cytosolic face of the Golgi membrane. Treatment of Golgi membranes with N-hydroxysuccinimide ester-based cross-linking reagents yielded an approximately 50-kDa polypeptide recognized by anti-GCS antibodies; however, treatment of approximately 10,000-fold purified Golgi GCS with the same reagents did not yield cross-linked GCS forms. These results suggest that GCS forms a dimer or oligomer with another protein in the Golgi membrane. The migration of solubilized Golgi GCS in glycerol gradients was also consistent with a predominantly oligomeric organization of GCS.

The effect on IgG glycosylation of altering beta1, 4-galactosyltransferase-1 activity in B cells

An absence of galactose on the N-linked oligosaccharides of immunoglobulin G (IgG) has been shown to affect the functional activity of the antibody molecule. In patients with rheumatoid arthritis there is an increased proportion of IgG which lacks galactose and correspondingly lower levels of beta1, 4-galactosyltransferase (beta4Gal-T) activity. The recent demonstration of several expressed beta4Gal-T genes in man raises the possibility that the enzyme responsible for the decreased IgG galactose is not the "classical" beta4Gal-T (beta4Gal-T1). To directly address the question of whether reduced beta4Gal-T1 would lead to reduced IgG galactose, the level of beta4Gal-T1 in a human IgG-secreting B cell line was specifically altered using stable transfection with sense (SpcDNA3-Gal-T1) or antisense (ASpcDNA3-Gal-T1) human beta4Gal-T1 cDNA. SpcDNA3-Gal-T1 B cell transfectants expressed up to a 2.5-fold higher level of beta4Gal-T enzyme activity for the exogenous neoglycoconjugate acceptor GlcNAc-pITC-BSA than did ASpcDNA3-Gal-T1 transfectants. Flow cytometric analysis with Ricinus communis agglutinin I (RCAI) revealed an overall greater number of Galbeta1,4GlcNAc structures in the fixed and permeabilized SpcDNA3-Gal-T1 B cell transfectants compared with the ASpcDNA3-Gal-T1 transfectants. Moreover, there was increased galactosylation of IgG secreted from the SpcDNA3-Gal-T1 transfectants relative to the ASpcDNA3-Gal-T1 B cell transfectants. Alteration of the level of the "classical" beta4Gal-T (beta4Gal-T1) in B cells therefore affects IgG glycosylation.

Activity of the yeast MNN1 alpha-1,3-mannosyltransferase requires a motif conserved in many other families of glycosyltransferases

A wide diversity of biological molecules are modified by the addition of sugar residues, and a large number of glycosyltransferases have been identified that are responsible for these reactions. Despite catalyzing closely related reactions, many of these transferases show little apparent sequence homology. By comparing two apparently unrelated families of yeast Golgi mannosyltransferases, a short motif containing two aspartate residues was observed that was conserved in both groups of proteins. Mutagenesis of one of the members of these families, the alpha-1, 3-mannosyltransferase Mnn1p, showed that altering either of these aspartates eliminates all enzymatic activity. These changes do not appear to affect the overall folding and assembly of Mnn1p. A similar aspartate-containing sequence was found to be conserved in a diverse range of other glycosyltransferase families, much more frequently than would be expected by chance, suggesting that it is a feature of the catalytic site, or an element of a structural fold, shared by many glycosyltransferases.

Golgi localization and functional expression of human uridine diphosphatase

A full-length E(ecto)-ATPase (Plesner, L. (1995) Int. Rev. Cytol. 158, 141-214) cDNA was cloned from a human brain cDNA library; it encodes a 610-amino acid protein that contains two putative transmembrane domains. Heterologous expression of this protein in COS-7 cells caused a significant increase in intracellular membrane-bound nucleoside phosphatase activity. The activity was highest with UDP as substrate and was stimulated by divalent cations in the following order: Ca2+ >> Mg2+ > Mn2+. The results of immunofluorescence staining indicate that this protein is located in the Golgi apparatus. UDP hydrolysis was increased in the presence of Triton X-100 or alamethicin, an ionophore that facilitates movement of UDP across the membrane, suggesting that the active site of this UDPase is on the luminal side of the Golgi apparatus. This is the first identification of a mammalian Golgi luminal UDPase gene. Computer-aided sequence analysis of the EATPase superfamily indicates that the human UDPase is highly similar to two hypothetical proteins of the nematode Caenorhabditis elegans and to an unidentified 71.9-kDa yeast protein and is less related to the previously identified yeast Golgi GDPase.

Expression cloning of the Golgi CMP-sialic acid transporter

Translocation of nucleotide sugars across the membrane of the Golgi apparatus is a prerequisite for the synthesis of complex carbohydrate structures. While specific transport systems for different nucleotide sugars have been identified biochemically in isolated microsomes and Golgi vesicles, none of these transport proteins has been characterized at the molecular level. Chinese hamster ovary (CHO) mutants of the complementation group Lec2 exhibit a strong reduction in sialylation of glycoproteins and glycolipids due to a defect in the CMP-sialic acid transport system. By complementation cloning in the mutant 6B2, belonging to the Lec2 complementation group, we were able to isolate a cDNA encoding the putative murine Golgi CMP-sialic acid transporter. The cloned cDNA encodes a highly hydrophobic, multiple membrane spanning protein of 36.4 kDa, with structural similarity to the recently cloned ammonium transporters. Transfection of a hemagglutinin-tagged fusion protein into the mutant 6B2 led to Golgi localization of the hemagglutinin epitope. Our results, together with the observation that the cloned gene shares structural similarities to other recently cloned transporter proteins, strongly suggest that the isolated cDNA encodes the CMP-sialic acid transporter.

A disulfide-bonded dimer of the Golgi beta-galactoside alpha2,6-sialyltransferase is catalytically inactive yet still retains the ability to bind galactose

The alpha2,6-sialyltransferase is a terminal glycosyltransferase localized in the trans Golgi and trans Golgi network. Here we show that 30% of the total rat liver Golgi alpha2,6-sialyltransferase forms a disulfide-bonded 100-kDa species that can be converted to the 50-kDa monomer form of the enzyme upon reduction. Limited proteolysis of both enzyme forms demonstrates that the 100-kDa species is a disulfide-bonded homodimer of the alpha2,6-sialyltransferase. The alpha2,6-sialyltransferase disulfide-bonded dimer is found in bovine liver Golgi membranes and in Golgi membranes prepared and solubilized in the presence of 100 mM iodoacetamide, suggesting that it is not unique to rat liver or formed aberrantly upon membrane lysis. The dimer form of the enzyme possesses no significant catalytic activity and has a much lower affinity for CDP-hexanolamine-agarose compared with the monomer form. In contrast, both the alpha2,6-sialyltransferase monomer and the disulfide-bonded dimer bind strongly to galactose and galactose-terminated substrates. These results suggest that the alpha2,6-sialyltransferase disulfide-bonded dimer lacks catalytic activity due to a weak affinity for its sugar nucleotide donor, CMP-NeuAc, and that this catalytically inactive form of the enzyme may act as a galactose-specific lectin in the Golgi.

Regulation of yeast Golgi glycosylation. Guanosine diphosphatase functions as a homodimer in the membrane

The Golgi lumenal GDPase plays an important role in the mannosylation of proteins and lipids of Saccharomyces cerevisiae by regulating the amount of GDP-mannose available in the Golgi lumen. The enzyme makes available GMP as an antiporter to be coupled with entry of GDP-mannose into the Golgi lumen from the cytosol. Using radiation inactivation and target analysis, we have now determined the functional molecular mass of the GDPase within the Golgi membrane and whether or not the enzyme has functional associations with other Golgi membrane proteins, including mannosyltransferases and the GDP-mannose transporter. The functional size of the GDPase was found to be approximately twice the estimated structural target size of the protein; this strongly suggests that the GDPase protein in situ functions as homodimer and does not require association with other membrane proteins for its function.

Purification of the Golgi adenosine 3'-phosphate 5'-phosphosulfate transporter, a homodimer within the membrane

Sulfation of proteoglycans, secretory and membrane proteins, and glycolipids occurs in the lumen of the Golgi apparatus. Adenosine 3'-phosphate 5'-phosphosulfate (PAPS), the sulfate donor in these reactions, must be transported from the cytosol, its site of synthesis, into the lumen of the Golgi apparatus. We have identified and purified to apparent homogeneity the rat liver Golgi membrane PAPS transporter by a combination of conventional and affinity chromatography as well as photoaffinity radiolabeling with adenosine 3',5'-bisphosphate, a competitive inhibitor of PAPS transport. The transporter, a 75-kDa protein, was purified 70,000-fold over homogenate (6% yield) and transported PAPS into phosphatidylcholine liposomes selectively and in a saturable manner (apparent Km of 1.7 microM). Radiation target-inactivation analyses of the transport activity in rat liver Golgi vesicles, together with the above described biochemical approaches, demonstrate that the PAPS transporter within the Golgi membrane is a homodimer.

Kin recognition. A model for the retention of Golgi enzymes

The surprising result that the spanning domain causes retention of proteins in the Golgi stack poses the question as to the actual mechanism. Here we present a simple model that might have general applicability.