Molecular cloning and expression of cDNA encoding the rat UDP-N-acetylglucosamine:alpha-6-D-mannoside beta-1,2-N-acetylglucosaminyltransferase II

Efficient production and characterization of soluble active human β-1,2-N-acetylglucosaminyltransferase II in bacteria

In humans, almost all the cell surface and secreted glycoproteins are modified with complex-type N-glycans. Thus, it is essential to obtain complex-type N-glycans to fully understand the biological properties of glycoproteins. Here, human β-1,2-N-acetylglucosaminyltransferase II (hGnT-II), a Golgi-localized enzyme integral to complex-type N-glycan biosynthesis, was cloned as a truncated transmembrane form (GnT-II-ΔTM) and heterologously overexpressed in Escherichia coli. Our results showed that hGnT-II could be overexpressed in its soluble form by fusing the truncated enzyme with a thioredoxin (Trx)-tag in the Rosetta-Gami 2 strain. Using the optimized induction conditions, the expression level of recombinant protein was enhanced to yield approximately 4 mg per liter culture after affinity purification. The enzyme exhibited appropriate glycosyltransferase activity, and the calculated Km value was 52.4 μM, similar to the protein expressed in mammalian cells. Furthermore, the effect of MGAT2-CDG mutations on enzyme activity was also measured. These results suggested that the E. coli expression system was capable of the large-scale production of bioactive hGnT-II, which can be used for functional study and effective synthesis of complex-type N-glycans.

Constructing a human complex type N-linked glycosylation pathway in Kluyveromyces marxianus

Glycosylation can affect various protein properties such as stability, biological activity, and immunogenicity. To produce human therapeutic proteins, a host that can produce glycoproteins with correct glycan structures is required. Microbial expression systems offer economical, rapid and serum-free production and are more amenable to genetic manipulation. In this study, we developed a protocol for CRISPR/Cas9 multiple gene knockouts and knockins in Kluyveromyces marxianus, a probiotic yeast with a rapid growth rate. As hyper-mannosylation is a common problem in yeast, we first knocked out the α-1,3-mannosyltransferase (ALG3) and α-1,6-mannosyltransferase (OCH1) genes to reduce mannosylation. We also knocked out the subunit of the telomeric Ku domain (KU70) to increase the homologous recombination efficiency of K. marxianus. In addition, we knocked in the MdsI (α-1,2-mannosidase) gene to reduce mannosylation and the GnTI (β-1,2-N-acetylglucosaminyltransferase I) and GnTII genes to produce human N-glycan structures. We finally obtained two strains that can produce low amounts of the core N-glycan Man₃GlcNAc₂ and the human complex N-glycan Man₃GlcNAc₄, where Man is mannose and GlcNAc is N-acetylglucosamine. This study lays a cornerstone of glycosylation engineering in K. marxianus toward producing human glycoproteins.

Substrate specificities and intracellular distributions of three N-glycan processing enzymes functioning at a key branch point in the insect N-glycosylation pathway

Man(α1-6)[GlcNAc(β1-2)Man(α1-3)]ManGlcNAc(2) is a key branch point intermediate in the insect N-glycosylation pathway because it can be either trimmed by a processing β-N-acetylglucosaminidase (FDL) to produce paucimannosidic N-glycans or elongated by N-acetylglucosaminyltransferase II (GNT-II) to produce complex N-glycans. N-acetylglucosaminyltransferase I (GNT-I) contributes to branch point intermediate production and can potentially reverse the FDL trimming reaction. However, there has been no concerted effort to evaluate the relationships among these three enzymes in any single insect system. Hence, we extended our previous studies on Spodoptera frugiperda (Sf) FDL to include GNT-I and -II. Sf-GNT-I and -II cDNAs were isolated, the predicted protein sequences were analyzed, and both gene products were expressed and their acceptor substrate specificities and intracellular localizations were determined. Sf-GNT-I transferred N-acetylglucosamine to Man(5)GlcNAc(2), Man(3)GlcNAc(2), and GlcNAc(β1-2)Man(α1-6)[Man(α1-3)]ManGlcNAc(2), demonstrating its role in branch point intermediate production and its ability to reverse FDL trimming. Sf-GNT-II only transferred N-acetylglucosamine to Man(α1-6)[GlcNAc(β1-2)Man(α1-3)]ManGlcNAc(2), demonstrating that it initiates complex N-glycan production, but cannot use Man(3)GlcNAc(2) to produce hybrid or complex structures. Fluorescently tagged Sf-GNT-I and -II co-localized with an endogenous Sf Golgi marker and Sf-FDL co-localized with Sf-GNT-I and -II, indicating that all three enzymes are Golgi resident proteins. Unexpectedly, fluorescently tagged Drosophila melanogaster FDL also co-localized with Sf-GNT-I and an endogenous Drosophila Golgi marker, indicating that it is a Golgi resident enzyme in insect cells. Thus, the substrate specificities and physical juxtapositioning of GNT-I, GNT-II, and FDL support the idea that these enzymes function at the N-glycan processing branch point and are major factors determining the net outcome of the insect cell N-glycosylation pathway.

Mouse models for congenital disorders of glycosylation

Glycoprotein biosynthesis describes the process of co- and posttranslational attachment of sugar chains to proteins, a process that has been found in nearly all known organisms. Human deficiencies evoked by mutations in the glycosylation pathway of glycoproteins lead to congenital disorders of glycosylation (CDG), a rapidly expanding group of autosomal recessive inherited metabolic diseases with multisystemic phenotypes that are mostly combined with severe neurological impairment. Although investigations on new types of CDG have proceeded rapidly in recent years, the correlation between inaccurate protein glycosylation and pathological loss of functionality of distinct organ systems remains widely unknown, and therapeutics for the patients are mostly not available. Therefore, mouse models provide an outstanding helpful tool for investigations on different aspects of glycosylation deficiencies that cannot be performed in patients or cell culture. This review focuses on existing mouse models generated for the types of CDG that affect the N-glycosylation pathway.

A highly efficient chemoenzymatic approach toward glycoprotein synthesis

[reaction: see text] A highly efficient endoglycosidase-catalyzed synthesis of homogeneous glycoproteins is described. By using ribonuclease B as a model system, it was demonstrated that Endo-A could efficiently attach a preassembled oligosaccharide to a GlcNAc-containing protein in a regio- and stereospecific manner, when the corresponding sugar oxazoline was used as the donor substrate. The method allows the synthesis of both natural and tailor-made N-linked glycoproteins in excellent yield.

Comparing N-glycan processing in mammalian cell lines to native and engineered lepidopteran insect cell lines

In the past decades, a large number of studies in mammalian cells have revealed that processing of glycoproteins is compartmentalized into several subcellular organelles that process N-glycans to generate complex-type oligosaccharides with terminal N -acetlyneuraminic acid. Recent studies also suggested that processing of N-glycans in insect cells appear to follow a similar initial pathway but diverge at subsequent processing steps. N-glycans from insect cell lines are not usually processed to terminally sialylated complex-type structures but are instead modified to paucimannosidic or oligomannose structures. These differences in processing between insect cells and mammalian cells are due to insufficient expression of multiple processing enzymes including glycosyltransferases responsible for generating complex-type structures and metabolic enzymes involved in generating appropriate sugar nucleotides. Recent genomics studies suggest that insects themselves may include many of these complex transferases and metabolic enzymes at certain developmental stages but expression is lost or limited in most lines derived for cell culture. In addition, insect cells include an N -acetylglucosaminidase that removes a terminal N -acetylglucosamine from the N-glycan. The innermost N -acetylglucosamine residue attached to asparagine residue is also modified with alpha(1,3)-linked fucose, a potential allergenic epitope, in some insect cells. In spite of these limitations in N-glycosylation, insect cells have been widely used to express various recombinant proteins with the baculovirus expression vector system, taking advantage of their safety, ease of use, and high productivity. Recently, genetic engineering techniques have been applied successfully to insect cells in order to enable them to produce glycoproteins which include complex-type N-glycans. Modifications to insect N-glycan processing include the expression of missing glycosyltransferases and inclusion of the metabolic enzymes responsible for generating the essential donor sugar nucleotide, CMP- N -acetylneuraminic acid, required for sialylation. Inhibition of N -acetylglucosaminidase has also been applied to alter N-glycan processing in insect cells. This review summarizes current knowledge on N-glycan processing in lepidopteran insect cell lines, and recent progress in glycoengineering lepidopteran insect cells to produce glycoproteins containing complex N-glycans.

Glycomics: a pathway to a class of new and improved therapeutics

Complex glycans that are located at the surface of cells, deposited in the extracellular matrix and attached to soluble signalling molecules have a crucial role in the phenotypic expression of cellular genotypes. However, owing to their structural complexity and some redundancy in terms of structures that elicit a function, the therapeutic potential of complex glycans has not been well exploited, with a few notable exceptions. This review outlines recent advances that promise to increase our ability to use complex glycans as therapeutics. Opportunities for the development of further structure-function relationships for these complex molecules are also discussed.

Evidence for alternative splicing and developmental regulation of the Drosophila melanogaster Mgat2 (N-acetylglucosaminyltransferase II) gene

We present a molecular study of the Drosophila melanogaster Mgat2 gene that codes for the N-acetylglucosaminyltransferase II. Isolation and analysis of two cDNA clones indicated that the gene is alternatively spliced, generating two transcripts of 3.3 and 2.7kb. Developmental specificity was observed between the two alternative transcripts during the major developmental stages of D. melanogaster. The deduced amino acid sequences of the two proteins show significant homology to the equivalent mammalian proteins, especially in the carboxyterminal region. In situ hybridizations in embryos and embryonic imaginal discs showed that the gene is expressed mainly but not exclusively in the brain.

A genetic approach to Mammalian glycan function

The four essential building blocks of cells are proteins, nucleic acids, lipids, and glycans. Also referred to as carbohydrates, glycans are composed of saccharides that are typically linked to lipids and proteins in the secretory pathway. Glycans are highly abundant and diverse biopolymers, yet their functions have remained relatively obscure. This is changing with the advent of genetic reagents and techniques that in the past decade have uncovered many essential roles of specific glycan linkages in living organisms. Glycans appear to modulate biological processes in the development and function of multiple physiologic systems, in part by regulating protein-protein and cell-cell interactions. Moreover, dysregulation of glycan synthesis represents the etiology for a growing number of human genetic diseases. The study of glycans, known as glycobiology, has entered an era of renaissance that coincides with the acquisition of complete genome sequences for multiple organisms and an increased focus upon how posttranslational modifications to protein contribute to the complexity of events mediating normal and disease physiology. Glycan production and modification comprise an estimated 1% of genes in the mammalian genome. Many of these genes encode enzymes termed glycosyltransferases and glycosidases that reside in the Golgi apparatus where they play the major role in constructing the glycan repertoire that is found at the cell surface and among extracellular compartments. We present a review of the recently established functions of glycan structures in the context of mammalian genetic studies focused upon the mouse and human species. Nothing tends so much to the advancement of knowledge as the application of a new instrument. The native intellectual powers of men in different times are not so much the causes of the different success of their labours, as the peculiar nature of the means and artificial resources in their possession. T. Hager: Force of Nature (1)

Detailed glycan analysis of serum glycoproteins of patients with congenital disorders of glycosylation indicates the specific defective glycan processing step and provides an insight into pathogenesis

The fundamental importance of correct protein glycosylation is abundantly clear in a group of diseases known as congenital disorders of glycosylation (CDGs). In these diseases, many biological functions are compromised, giving rise to a wide range of severe clinical conditions. By performing detailed analyses of the total serum glycoproteins as well as isolated transferrin and IgG, we have directly correlated aberrant glycosylation with a faulty glycosylation processing step. In one patient the complete absence of complex type sugars was consistent with ablation of GlcNAcTase II activity. In another CDG type II patient, the identification of specific hybrid sugars suggested that the defective processing step was cell type-specific and involved the mannosidase III pathway. In each case, complementary serum proteome analyses revealed significant changes in some 31 glycoproteins, including components of the complement system. This biochemical approach to charting diseases that involve alterations in glycan processing provides a rapid indicator of the nature, severity, and cell type specificity of the suboptimal glycan processing steps; allows links to genetic mutations; indicates the expression levels of proteins; and gives insight into the pathways affected in the disease process.

Mice with a homozygous deletion of the Mgat2 gene encoding UDP-N-acetylglucosamine:alpha-6-D-mannoside beta1,2-N-acetylglucosaminyltransferase II: a model for congenital disorder of glycosylation type IIa

Mice homozygous for a deletion of the Mgat2 gene encoding UDP-N-acetylglucosamine:alpha-6-D-mannoside beta1,2-N-acetylglucosaminyltransferase II (GlcNAcT-II, EC 2.4.1.143) have been reported. GlcNAcT-II is essential for the synthesis of complex N-glycans. The Mgat2-null mice were studied in a comparison with the symptoms of congenital disorder of glycosylation type IIa (CDG-IIa) in humans. Mutant mouse tissues were shown to be deficient in GlcNAcT-II enzyme activity and complex N-glycan synthesis, resulting in severe gastrointestinal, hematologic and osteogenic abnormalities. All mutant mice died in early post-natal development. However, crossing the Mgat2 mutation into a distinct genetic background resulted in a low frequency of survivors exhibiting additional and novel disease signs of CDG-IIa. Analysis of N-glycan structures in the kidneys of Mgat2-null mice showed a novel bisected hybrid N-glycan structure in which the bisecting GlcNAc residue was substituted with a beta1,4-linked galactose or the Lewis(x) structure. These studies suggest that some of the functions of complex N-glycan branches are conserved in mammals and that human disease due to aberrant protein N-glycosylation may be modeled in the mouse, with the expectation in this case of gaining insights into CDG-IIa disease pathogenesis. Further analyses of the Mgat2-deficient phenotype in the mouse have been accomplished involving cells in which the Mgat2 gene is dispensable, as well as other cell lineages in which a severe defect is present. Pre-natal defects appear in a significant number of embryos, and likely reflect a limited window of time in which a future therapeutic approach might effectively operate.

UDP-N-acetylglucosamine:alpha-3-D-mannoside beta-1,2-N-acetylglucosaminyltransferase I and UDP-N-acetylglucosamine:alpha-6-D-mannoside beta-1,2-N-acetylglucosaminyltransferase II in Caenorhabditis elegans

UDP-N-acetylglucosamine:alpha-3-D-mannoside beta-1,2-N-acetylglucosaminyltransferase I (GnT I) and UDP-N-acetylglucosamine:alpha-6-D-mannoside beta-1,2-N-acetylglucosaminyltransferase II (GnT II) are key enzymes in the synthesis of Asn-linked hybrid and complex glycans. We have cloned cDNAs from Caenorhabditis elegans for three genes homologous to mammalian GnT I (designated gly-12, gly-13 and gly-14) and one gene homologous to mammalian GnT II. All four cDNAs encode proteins which have the domain structure typical of previously cloned Golgi-type glycosyltransferases and show enzymatic activity (GnT I and GnT II, respectively) on expression in transgenic worms. We have isolated worm mutants lacking the three GnT I genes by the method of ultraviolet irradiation in the presence of trimethylpsoralen (TMP); null mutants for GnT II have not yet been obtained. The gly-12 and gly-14 mutants as well as the gly-14;gly-12 double mutant displayed wild-type phenotypes indicating that neither gly-12 nor gly-14 is necessary for worm development under standard laboratory conditions. This finding and other data indicate that the GLY-13 protein is the major functional GnT I in C. elegans. The mutation lacking the gly-13 gene is partially lethal and the few survivors display severe morphological and behavioral defects. We have shown that the observed phenotype co-segregates with the gly-13 deletion in genetic mapping experiments although a second mutation near the gly-13 gene cannot as yet be ruled out. Our data indicate that complex and hybrid N-glycans may play critical roles in the morphogenesis of C. elegans, as they have been shown to do in mice and men.

Functional and comparative bioinformatic analysis of expressed genes from wheat spikes infected with Fusarium graminearum

Fusarium head blight, caused by the fungus Fusarium graminearum, is a major disease on wheat (Triticum aestivum L.). Expressed sequence tags (ESTs) were used to identify genes expressed during the wheat-F. graminearum interaction. We generated 4,838 ESTs from a cDNA library prepared from spikes of the partially resistant cultivar Sumai 3 infected with F. graminearum. These ESTs were composed of 2,831 singlet (single-copy transcripts) and 715 contigs (multiple-copy transcripts) for a total of 3,546 non-redundant sequences. Four sets of nonredundant sequences were identified. One set contains numerous, common biotic and abiotic stress-related genes. Many of these stress-related genes were represented by multiple ESTs, indicating that they are abundantly expressed. A second set comprised 16 nonredundant sequences from F. graminearum that may be required for pathogenicity. A subset of these fungal genes encodes proteins associated with plant cell wall degradation. A third set of 326 nonredundant sequences had no DNA or amino acid sequence similarity to almost 1 million plant and over 7 million animal sequences in dbEST (as of 22 June 2001). Thus, these 326 nonredundant sequences have only been found in our F. graminearum-infected 'Sumai 3' cDNA library. A fourth set of 29 nonredundant sequences was found in our F. graminearum-infected wheat and another plant-pathogen interaction cDNA library. Some of these sequences encode proteins that may act in establishing various plant-fungal interactions.

Two closely related forms of UDP-GlcNAc: alpha6-D-mannoside beta1,2-N-acetylglucosaminyltransferase II occur in the clawed frog Xenopus laevis

UDP-GlcNAc:alpha6-D-mannoside beta1,2-N-acetylglucosaminyltransferase II (GnT II; EC 2.4.1.143) is a medial-Golgi resident enzyme that catalyses an essential step in the biosynthetic pathway leading from high mannose to complex N-linked oligosaccharides. Screening a cDNA library from Xenopus laevis ovary with a human GnT II DNA probe resulted in the isolation of two cDNA clones encoding two closely related GnT II isoenzymes, GnT II-A and GnT II-B. Analysis of the corresponding genomic DNAs revealed that the open reading frame of both X. laevis GnT II genes resides within a single exon. The GnT II-A gene was found to be transcriptionally active in all X. laevis tissues tested. In contrast, expression of the GnT II-B gene was detected only in a limited number of tissues. Both GnT II-A and GnT II-B exhibit a type II transmembrane protein topology with a putative N-terminal cytoplasmic tail of 9 amino acids followed by a transmembrane domain of 18 residues, and a C-terminal luminal domain of 405 residues. The two proteins differ at 28 amino acid positions within their luminal regions. Heterologous expression of soluble forms of the enzymes in insect cells showed that GnT II-A and GnT II-B are both catalytically active and exhibit similar specific activities. Both recombinant proteins are modified with N-linked oligosaccharides. N-terminal deletion studies demonstrated that the first 49 amino acid residues are not essential for proper folding and enzymatic activity of X. laevis GnT II.

Molecular cloning and expression of cDNA encoding chicken UDP-N-acetyl-D-glucosamine (GlcNAc): GlcNAcbeta 1-6(GlcNAcbeta 1-2)- manalpha 1-R[GlcNAc to man]beta 1,4N-acetylglucosaminyltransferase VI

A cDNA that encodes UDP-N-acetyl-d-glucosamine (GlcNAc):GlcNAcbeta1-6(GlcNAcbeta1-2)Manalpha1-R[GlcNA c to Man]beta1, 4N-acetylglucosaminyltransferase VI (GnT VI), which is responsible for the formation of pentaantennary asparagine-linked oligosaccharides (N-glycans), has been cloned from a hen oviduct cDNA library based on the partial amino acid sequences of the purified enzyme. The isolated cDNA clone contained an open reading frame encoding 464 amino acids, including all of the peptides that were sequenced. The deduced amino acid sequence predicts a type II transmembrane topology and contains two potential N-glycosylation sites. The primary structure was found to be significantly similar to human GnT IV-homologue, the gene for which was cloned from the deleted region in pancreatic cancer, and to human and bovine GnT IVs. Chicken GnT VI-transfected COS-1 cells showed a high GnT VI activity (26.8 pmol/h/mg protein), whereas nontransfected, mock-transfected, or human GnT IV-homologue-transfected COS-1 cells had no activity. Northern blot analysis using poly(A)(+) RNA from hen oviduct indicated that the size of GnT VI mRNA is 2.1 kilobases. Reverse transcription-polymerase chain reaction analysis showed that GnT VI mRNA was relatively highly expressed in oviduct, spleen, lung, and colon.

Purification and characterization of UDP-GlcNAc: GlcNAcbeta 1-6(GlcNAcbeta 1-2)Manalpha 1-R [GlcNAc to Man]-beta 1, 4-N-acetylglucosaminyltransferase VI from hen oviduct

A new beta1,4-N-acetylglucosaminyltransferase (GnT) responsible for the formation of branched N-linked complex-type sugar chains has been purified 64,000-fold in 16% yield from a homogenate of hen oviduct by column chromatography procedures using Q-Sepharose FF, Ni(2+)-chelating Sepharose FF, and UDP-hexanolamine-agarose. This enzyme catalyzes the transfer of GlcNAc from UDP-GlcNAc to tetraantennary oligosaccharide and produces pentaantennary oligosaccharide with the beta1-4-linked GlcNAc residue on the Manalpha1-6 arm. It requires a divalent cation such as Mn(2+) and has an apparent molecular weight of 72,000 under nonreducing conditions. The enzyme does not act on biantennary oligosaccharide (GnT I and II product), and beta1,6-N-acetylglucosaminylation of the Manalpha1-6 arm (GnT V product) is essential for its activity. This clearly distinguishes it from GnT IV, which is known to generate a beta1-4-linked GlcNAc residue only on the Manalpha1-3 arm. Based on these findings, we conclude that this enzyme is UDP-GlcNAc:GlcNAcbeta1-6(GlcNAcbeta1-2)Manalpha1-R [GlcNAc to Man]-beta1,4-N-acetylglucosaminyltransferase VI. This is the only known enzyme that has not been previously purified among GnTs responsible for antenna formation on the cores of N-linked complex-type sugar chains.

The joys of HexNAc. The synthesis and function of N- and O-glycan branches

This review covers discoveries made over the past 30-35 years that were important to our understanding of the synthetic pathway required for initiation of the antennae or branches on complex N-glycans and O-glycans. The review deals primarily with the author's contributions but the relevant work of other laboratories is also discussed. The focus of the review is almost entirely on the glycosyltransferases involved in the process. The following topics are discussed. (1) The localization of the synthesis of complex N-glycan antennae to the Golgi apparatus. (2) The "evolutionary boundary" at the stage in N-glycan processing where there is a change from oligomannose to complex N-glycans; this switch correlates with the appearance of multicellular organisms. (3) The discovery of the three enzymes which play a key role in this switch, N-acetylglucosaminyltransferases I and II and mannosidase II. (4) The "yellow brick road" which leads from oligomannose to highly branched complex N-glycans with emphasis on the enzymes involved in the process and the factors which control the routes of synthesis. (5) A short discussion of the characteristics of the enzymes involved and of the genes that encode them. (6) The role of complex N-glycans in mammalian and Caenorhabditis elegans development. (7) The crystal structure of N-acetylglucosaminyltransferase I. (8) The discovery of the enzymes which synthesize O-glycan cores 1, 2, 3 and 4 and their elongation.

Regulation of expression of the human beta-1,2-N-acetylglucosaminyltransferase II gene (MGAT2) by Ets transcription factors

Oncogenic transformation of fibroblasts by the src oncogene has long been known to cause an increase in the size of cell-surface protein-bound oligosaccharides, owing primarily to increased N-glycan branching mediated by increased beta-1,6-N-acetylglucosaminyltransferase V (GnT V) activity. The src-responsive element of the GnT V promoter was localized to Ets-binding sites and the promoter was transcriptionally stimulated by both ets-1 and ets-2 expression [Buckhaults, Chen, Fregien and Pierce (1997) J. Biol. Chem. 272, 19575-19581; Kang, Saito, Ihara, Miyoshi, Koyama, Sheng and Taniguchi (1996) J. Biol. Chem. 271, 26706-26712]. Because GnT V action requires the prior action of beta-1,2-N-acetylglucosaminyltransferase II (GnT II) and the human GnT II promoter contains four putative Ets-binding sites [Chen, Zhou, Tan and Schachter (1998) Glycoconj. J. 15, 301-308], GnT II might also be under oncogenic control via Ets transcription factors. We now report that co-transfection into HepG2 or COS-1 cells of either ets-1 or ets-2 expression plasmids together with chimaeric GnT II promoter-chloramphenicol acetyltransferase plasmids results in a 2-4-fold stimulation of promoter activity. Mobility-shift assays and South-Western blots localized the functional Ets-binding site to one of the four putative sites on the GnT II promoter. The GnT II promoter, unlike the GnT V promoter, is not activated by either src or neu. Therefore although both promoters are stimulated by a member of the Ets family of transcription factors, the functional role of this Ets transcriptional control seems to be different for the two genes.

Molecular cloning of cDNA encoding N-acetylglucosaminyltransferase II from Arabidopsis thaliana

N-acetylglucosaminyltransferase II (GnTII, EC 2.4.1.143) is a Golgi enzyme involved in the biosynthesis of glycoprotein-bound N-linked oligosaccharides, catalysing an essential step in the conversion of oligomannose-type to complex N-glycans. GnTII activity has been detected in both animals and plants. However, while cDNAs encoding the enzyme have already been cloned from several mammalian sources no GnTII homologue has been cloned from plants so far. Here we report the molecular cloning of an Arabidopsis thaliana GnTII cDNA with striking homology to its animal counterparts. The predicted domain structure of A. thaliana GnTII indicates a type II transmembrane protein topology as it has been established for the mammalian variants of the enzyme. Upon expression of A. thaliana GnTII cDNA in the baculovirus/insect cell system, a recombinant protein was produced that exhibited GnTII activity.

Carbohydrate-deficient glycoprotein syndrome type II

The carbohydrate-deficient glycoprotein syndromes (CDGS) are a group of autosomal recessive multisystemic diseases characterized by defective glycosylation of N-glycans. This review describes recent findings on two patients with CDGS type II. In contrast to CDGS type I, the type II patients show a more severe psychomotor retardation, no peripheral neuropathy and a normal cerebellum. The CDGS type II serum transferrin isoelectric focusing pattern shows a large amount (95%) of disialotransferrin in which each of the two glycosylation sites is occupied by a truncated monosialo-monoantennary N-glycan. Fine structure analysis of this glycan suggested a defect in the Golgi enzyme UDP-GlcNAc:alpha-6-D-mannoside beta-1,2-N-acetylglucosaminyltransferase II (GnT II; EC 2.4.1.143) which catalyzes an essential step in the biosynthetic pathway leading from hybrid to complex N-glycans. GnT II activity is reduced by over 98% in fibroblast and mononuclear cell extracts from the CDGS type II patients. Direct sequencing of the GnT II coding region from the two patients identified two point mutations in the catalytic domain of GnT II, S290F (TCC to TTC) and H262R (CAC to CGC). Either of these mutations inactivates the enzyme and probably also causes reduced expression. The CDG syndromes and other congenital defects in glycan synthesis as well as studies of null mutations in the mouse provide strong evidence that the glycan moieties of glycoproteins play essential roles in the normal development and physiology of mammals and probably of all multicellular organisms.

Expression cloning of a human alpha1, 4-N-acetylglucosaminyltransferase that forms GlcNAcalpha1-->4Galbeta-->R, a glycan specifically expressed in the gastric gland mucous cell-type mucin

Among mucus-secreting cells, the gastric gland mucous cells, Brunner's glands, accessory glands of pancreaticobiliary tract, and pancreatic ducts exhibiting gastric metaplasia are unique in that they express class III mucin identified by paradoxical Con A staining composed of periodate oxidation, sodium borohydride reduction, Con A, and horseradish peroxidase reaction. Recently it was shown that these mucous cells secrete glycoproteins having GlcNAcalpha1-->4Galbeta-->R at nonreducing terminals of the carbohydrate moieties. Herein we describe the expression cloning of a cDNA encoding a human alpha1,4-N-acetylglucosaminyltransferase (alpha4GnT), a key enzyme for the formation of GlcNAcalpha1-->4Galbeta1-->R. COS-1 cells were thus cotransfected with a stomach cDNA library and a leukosialin cDNA. Transfected COS-1 cells were screened by using monoclonal antibodies specific for GlcNAcalpha1-->4Galbeta-->R and enriched by fluorescence-activated cell sorting. Sibling selection of recovered plasmids resulted in a cDNA clone that directs the expression of GlcNAcalpha1-->4Galbeta-->R. The deduced amino acid sequence predicts a type II membrane protein with 340 amino acids, showing no significant similarity with any other proteins. The alpha4GnT gene is located at chromosome 3p14.3, and its transcripts are expressed in the stomach and pancreas. An in vitro GlcNAc transferase assay by using a soluble alpha4GnT revealed that alpha1,4-linked GlcNAc residues are transferred most efficiently to core 2 branched O-glycans (Galbeta1-->4GlcNAcbeta1-->6(Galbeta1-->3)GalNAc), forming GlcNAcalpha1-->4Galbeta1-->4GlcNAcbeta1-->6(GlcNAca lpha1-->4Galbeta1- ->3)GalNAc. Transfection of alpha4GnT cDNA into gastric adenocarcinoma AGS cells produced class III mucin, indicating that alpha4GnT is responsible for the formation of class III Con A reactivity. These results indicate that the alpha4GnT is a glycosyltransferase that forms alpha1,4-linked GlcNAc residues, preferentially in O-glycans.

Molecular cloning and characterization of cDNA coding for beta1, 2N-acetylglucosaminyltransferase I (GlcNAc-TI) from Nicotiana tabacum

In plants as well as in animals beta1, 2N-acetylglucosaminyltransferase I (GlcNAc-TI) is a Golgi resident enzyme that catalyzes an essential step in the biosynthetic pathway leading from oligomannosidic N-glycans to complex or hybrid type N-linked oligosaccharides. Employing degenerated primers deduced from known GlcNAc-TI genes from animals, we were able to identify the cDNA coding for GlcNAc-TI from a Nicotiana tabacum cDNA library. The complete nucleotide sequence revealed a 1338 base pair open reading frame that codes for a polypeptide of 446 amino acids. Comparison of the deduced amino acid sequence with that of already known GlcNAc-TI polypeptides revealed no similarity of the tobacco clone within the putative cytoplasmatic, transmembrane, and stem regions. However, 40% sequence similarity was found within the putative C-terminal catalytic domain containing conserved single amino acids and peptide motifs. The predicted domain structure of the tobacco polypeptide is typical for type II transmembrane proteins and comparable to known GlcNAc-TI from animal species. In order to confirm enzyme activity a truncated form of the protein containing the putative catalytic domain was expressed using a baculovirus/insect cell system. Using pyridylaminated Man(5)- or Man(3)GlcNAc(2)as acceptor substrates and HPLC analysis of the products GlcNAc-TI activity was shown. This demonstrates that the C-terminal region of the protein comprises the catalytic domain. Expression of GlcNAc-TI mRNA in tobacco leaves was detected using RT-PCR. Southern blot analysis gave two hybridization signals of the gene in the amphidiploid genomes of the two investigated species N. tabacum and N.benthamiana.

Tissue specific expression and chromosomal mapping of a human UDP-N-acetylglucosamine: alpha1,3-d-mannoside beta1, 4-N-acetylglucosaminyltransferase

A human cDNA for UDP- N -acetylglucosamine:alpha1,3-d-mannoside beta1,4- N- acetylglucosaminyltransferase (GnT-IV) was isolated from a liver cDNA library using a probe based on a partial cDNA sequence of the bovine GnT-IV. The cDNA encoded a complete sequence of a type II membrane protein of 535 amino acids which is 96% identical to the bovine GnT-IV. Transient expression of the human cDNA in COS7 cells increased total cellular GnT-IV activity 25-fold, demonstrating that this cDNA encodes a functional human GnT-IV. Northern blot analysis of normal tissues indicated that at least five different sizes of mRNA (9.7, 7.6, 5.1, 3.8, and 2.4 kb) forGnT-IV are expressed in vivo. Furthermore, these mRNAs are expressed at different levels between tissues. Large amounts of mRNA were detected in tissues harboring T lineage cells. Also, the promyelocytic leukemia cell line HL-60 and the lymphoblastic leukemia cell line MOLT-4 revealed abundant mRNA. Lastly, the gene was mapped at the locus on human chromosome 2, band q12 by fluorescent in situ hybridization.

A novel second isoenzyme of the human UDP-N-acetylglucosamine:alpha1,3-D-mannoside beta1,4-N-acetylglucosaminyltransferase family: cDNA cloning, expression, and chromosomal assignment

We isolated a novel cDNA encoding a second isoenzyme of UDP-N-acetylglucosamine:alpha1,3-D-mannoside beta1,4-N-acetylglucosaminyltransferase (GnT-IV; EC 2.4.1.145). The nucleotide and deduced amino acid sequences of the cDNA were homologous to those of the previously cloned human GnT-IV cDNA (63% and 62% identity, respectively). The new cDNA is also confirmed to express GnT-IV activity, suggesting that two isoenzymes of human GnT-IV exist. Although genomic Southern analysis suggested that both genes exist in many mammalian species and the chicken, northern analysis revealed that both genes are expressed in different ways in human tissues. This is the first report concerning the gene family of an N-acetylglucosaminyltransferase in mammals.

cDNA cloning and expression of bovine UDP-N-acetylglucosamine: alpha1, 3-D-mannoside beta1,4-N-acetylglucosaminyltransferase IV

UDP-N-acetylglucosamine:alpha1,3-D-mannoside beta1, 4-N-acetylglucosaminyltransferase (GnT-IV) is one of the essential enzymes in the production of tri- and tetra-antennary Asn-linked sugar chains. Recently, we have successfully purified GnT-IV from bovine small intestine. Based on the partial amino acid sequence of the purified bovine GnT-IV enzyme, its cDNA has been cloned from bovine small intestine. The open reading frame is 1,605 base pairs long, and this sequence produced GnT-IV activity on transient expression in COS-7 cells. Although the deduced amino acid sequence does not have any significant homology with other known N-acetylglucosaminyltransferases (GnTs), the hydrophobicity profile showed a typical type II transmembrane protein structure, which is common to many glycosyltransferases. N-terminal amino acid sequencing of the purified GnT-IV revealed that 92 amino acids, including a transmembrane region, were truncated during purification. Of the three potential N-glycosylation sites Asn-458 was actually glycosylated in the purified enzyme, although this N-glycosylation site could be abolished without any reduction in GnT-IV activity. Serial deletions at both the N and C termini proved that the catalytic domain of GnT-IV is located in the central region of the enzyme. The GnT-IV mRNA level correlated with enzymatic activity in the various bovine tissues tested.

Transcriptional regulation of the human UDP-GlcNAc:alpha-6-D-mannoside beta-1-2-N-acetylglucosaminyltransferase II gene (MGAT2) which controls complex N-glycan synthesis

UDP-GlcNAc:alpha-6-D-mannoside beta-1,2-N-acetylglucosaminyltransferase II (GnT II; EC 2.4.1.143) is essential for the normal assembly of complex Asn-linked glycans. Northern analysis showed a major transcript at 2.0 kb and a minor band at approximately 2.9 kb in five different human cell lines. The gene (MGAT2) has three AATAAA polyadenylation sites at 68, 688 and 846 bp downstream of the translation stop codon. 3'-RACE (rapid amplification of cDNA ends) using RNA from the human cell line LS-180 indicated that all three sites were utilized for transcription termination. 5'-RACE and RNase protection analyses showed multiple transcription initiation sites at -440 to -489 bp relative to the ATG translation start codon (+1). The data show that the entire GnT II gene is on a single exon. The gene has a CCAAT box at -587 bp but lacks a TATA box and the 5'-untranslated region is GC-rich and contains consensus sequences suggestive of multiple binding sites for Sp1; these properties are typical for housekeeping genes. A series of chimeric constructs containing different lengths of the 5'-untranslated region fused to the chloramphenicol acetyltransferase (CAT) reporter gene were tested in transient transfection experiments using HeLa cells. The CAT activity of the construct containing the longest insert (-1076 bp relative to the ATG start codon) showed a approximately 38-fold increase as compared to that of the control. Removal of the region between -636 and -553 bp caused a dramatic decrease in CAT activity indicating this to be the main promoter region of the gene.

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.

Molecular cloning of the porcine beta-1,2-N-acetylglucosaminyltransferase II gene and assignment to chromosome 1q23-q27

Glycosyltransferases play an important role in the synthesis of glycoproteins. Here we report the isolation of a brain cDNA coding for 89% of the porcine UDP-N-acetylglucosamine:alpha-6-D-mannoside-beta-1,2-N-acetylglucosaminy ltransferase II (EC 2.4.1.143) (GnTII). The cDNA was used for screening a genomic liver DNA library and isolation of a recombinant lambda FIX II phage containing the complete porcine GnTII gene and upstream and downstream sequences. The beta-1,2-N-acetylglucosaminyltransferase II gene harbours a single exon with an open reading frame of 1338 bp coding for a 446 amino acid protein with a calculated molecular mass of 51.1 kDa. The promoter of the GnTII gene is lacking a TATA-box and shows variable transcription start sites. In the 3'-untranslated region a polymorphic polyadenosine stretch was detected. The porcine GnTII gene contains four polyadenylation sites. PCR analysis of a porcine-rodent hybrid cell panel revealed the chromosomal location of the GnTII gene on SSC 1q23-q27. The mapping data of the cell panel were confirmed by fluorescence in situ hybridization (FISH) on metaphase chromosomes.

Purification and characterization of UDP-N-acetylglucosamine: alpha1,3-D-mannoside beta1,4-N-acetylglucosaminyltransferase (N-acetylglucosaminyltransferase-IV) from bovine small intestine

A new beta1,4-N-acetylglucosaminyltransferase (GnT) which involves in branch formation of Asn-linked complex-type sugar chains has been purified 224,000-fold from bovine small intestine. This enzyme requires divalent cations, such as Mn2+, and catalyzes the transfer of GlcNAc from UDP-GlcNAc to biantennary oligosaccharide and produces triantennary oligosaccharide with the beta1-4-linked GlcNAc residue on the Manalpha1-3 arm. The purified enzyme shows a single band of Mr 58,000 and behaves as a monomer. The substrate specificity demonstrated that the beta1-2-linked GlcNAc residue on the Manalpha1-3 arm (GnT-I product) is essential for the enzyme activity. beta1-4-Galactosylaion to this essential beta1-2-linked GlcNAc residue or N-acetylglucosaminylation to the beta-linked Man residue (bisecting GlcNAc, GnT-III product) blocks the enzyme action, while beta1-6-N-acetylglucosaminylation to the Manalpha1-6 arm (GnT-V product) increases the transfer. Based on these findings, we conclude that the purified enzyme is UDP-N-acetylglucosamine:alpha-3-D-mannoside beta-1,4-N-acetylglucosaminyltransferase IV (GnT-IV), that has been a missing link on biosynthesis of complex-type sugar chains.

Activity of UDP-GlcNAc:GlcNAc beta 1-->6(GlcNAc beta 1-->2) Man alpha 1-->R[GlcNAc to Man] beta 1-->4N-acetylglucosaminyltransferase VI (GnT VI) from the ovaries of Oryzias latipes (Medaka fish)

UDP-GlcNAc:GlcNAc beta 1-->(GlcNAc beta 1-->2)Man alpha 1-R[GlcNAc to Man] beta 1-->4N-acetylglucosaminyltransferase VI (GnT VI) activity was shown to be present in crude homogenates of Medaka fish (Oryzias latipes) ovaries using UDP-[14C]GlcNAc and synthetic GlcNAc beta 1-->6 (GlcNAc beta 1-->2)Man alpha 1-->6Glc beta 1-->octyl as substrates. Characterization of this activity showed a pH optimum at about pH 7.0 and an absolute requirement for divalent cations. The optimum concentration of Mn2+ was at about 25 mM. This finding is the first report on GnT VI activity in fish; the enzyme has previously been described only in avian tissues.

Bovine UDP-N-acetylglucosamine:lysosomal-enzyme N-acetylglucosamine-1-phosphotransferase. I. Purification and subunit structure

UDP-N-acetylglucosamine:lysosomal-enzyme N-acetylglucosamine-1-phosphotransferase (GlcNAc-phosphotransferase) catalyzes the initial step in the synthesis of the mannose 6-phosphate determinant required for efficient intracellular targeting of newly synthesized lysosomal hydrolases to the lysosome. The enzyme was partially purified approximately 30,000-fold by chromatography of solubilized membrane proteins from lactating bovine mammary glands on DEAE-Sepharose, reactive green 19-agarose, and Superose 6. The partially purified enzyme was used to generate a panel of murine monoclonal antibodies. The anti-GlcNAc-phosphotransferase monoclonal antibody PT18 was coupled to a solid support and used to immunopurify the enzyme approximately 480,000-fold to apparent homogeneity with an overall yield of 29%. The purified enzyme has a specific activity of 10-12 micromol of GlcNAc phosphate transferred per h/mg using 100 mM alpha-methylmannoside as acceptor. The subunit structure of the enzyme was determined using a combination of analytical gel filtration chromatography, sodium dodecyl sulfate-polyacrylamide gel electrophoresis, and amino-terminal sequencing. The data indicate that bovine GlcNAc-phosphotransferase is a 540,000-Da complex composed of disulfide-linked homodimers of 166,000- and 51,000-Da subunits and two identical, noncovalently associated 56,000-Da subunits.

Recent progress in the molecular biology of the cloned N-acetylglucosaminyltransferases

Several genes which code for the N-acetylglucosaminyltransferases have been cloned and characterized. Physiological and pathophysiological roles of the genes still remain to be elucidated but accumulated evidence suggests that the N-acetylglucosaminyltransferase genes are implicated in differentiation, morphogenesis and cancer metastasis.