Processing of asparagine-linked oligosaccharides in insect cells: evidence for alpha-mannosidase II

Diversity and functions of protein glycosylation in insects

The majority of proteins is modified with carbohydrate structures. This modification, called glycosylation, was shown to be crucial for protein folding, stability and subcellular location, as well as protein-protein interactions, recognition and signaling. Protein glycosylation is involved in multiple physiological processes, including embryonic development, growth, circadian rhythms, cell attachment as well as maintenance of organ structure, immunity and fertility. Although the general principles of glycosylation are similar among eukaryotic organisms, insects synthesize a distinct repertoire of glycan structures compared to plants and vertebrates. Consequently, a number of unique insect glycans mediate functions specific to this class of invertebrates. For instance, the core α1,3-fucosylation of N-glycans is absent in vertebrates, while in insects this modification is crucial for the development of wings and the nervous system. At present, most of the data on insect glycobiology comes from research in Drosophila. Yet, progressively more information on the glycan structures and the importance of glycosylation in other insects like beetles, caterpillars, aphids and bees is becoming available. This review gives a summary of the current knowledge and recent progress related to glycan diversity and function(s) of protein glycosylation in insects. We focus on N- and O-glycosylation, their synthesis, physiological role(s), as well as the molecular and biochemical basis of these processes.

Cloning, Expression, and Characterization of Capra hircus Golgi α-Mannosidase II

Golgi α-mannosidase II (GMII), a key glycosyl hydrolase in the N-linked glycosylation pathway, has been demonstrated to be closely associated with the genesis and development of cancer. In this study, we cloned cDNA-encoding Capra hircus GMII (chGMII) and expressed it in Pichia pastoris expression system. The chGMII cDNA contains an open reading frame of 3432 bp encoding a polypeptide of 1144 amino acids. The deduced molecular mass and pI of chGMII was 130.5 kDa and 8.04, respectively. The gene expression profile analysis showed GMII was the highest expressed gene in the spleen. The recombinant chGMII showed maximum activity at pH 5.4 and 42 °C and was activated by Fe(2+), Zn(2+), Ca(2+), and Mn(2+) and strongly inhibited by Co(2+), Cu(2+), and EDTA. By homology modeling and molecular docking, we obtained the predicted 3D structure of chGMII and the probable binding modes of chGMII-GnMan5Gn, chGMII-SW. A small cavity containing Tyr355 and zinc ion fixed by residues Asp290, His176, Asp178, and His570 was identified as the active center of chGMII. These results not only provide a clue for clarifying the catalytic mechanism of chGMII but also lay a theoretical foundation for subsequent investigations in the field of anticancer therapy for mammals.

Limited Addition of the 6-Arm β1,2-linked N-Acetylglucosamine (GlcNAc) Residue Facilitates the Formation of the Largest N-Glycan in Plants

The most abundant N-glycan in plants is the paucimannosidic N-glycan with core β1,2-xylose and α1,3-fucose residues (Man3XylFuc(GlcNAc)2). Here, we report a mechanism in Arabidopsis thaliana that efficiently produces the largest N-glycan in plants. Genetic and biochemical evidence indicates that the addition of the 6-arm β1,2-GlcNAc residue by N-acetylglucosaminyltransferase II (GnTII) is less effective than additions of the core β1,2-xylose and α1,3-fucose residues by XylT, FucTA, and FucTB in Arabidopsis. Furthermore, analysis of gnt2 mutant and 35S:GnTII transgenic plants shows that the addition of the 6-arm non-reducing GlcNAc residue to the common N-glycan acceptor GlcNAcMan3(GlcNAc)2 inhibits additions of the core β1,2-xylose and α1,3-fucose residues. Our findings indicate that plants limit the rate of the addition of the 6-arm GlcNAc residue to the common N-glycan acceptor as a mechanism to facilitate formation of the prevalent N-glycans with Man3XylFuc(GlcNAc)2 and (GlcNAc)2Man3XylFuc(GlcNAc)2 structures.

Biochemical characterization of a novel β-N-acetylhexosaminidase from the insect Ostrinia furnacalis

The β-N-acetylhexosaminidase FDL specifically removes the β-1,2-GlcNAc residue conjugated to the α-1,3-mannose residue of the core structure of insect N-glycans, playing significant physiological roles in post-translational modification in the Golgi apparatus. Little is known about its enzymatic properties. We obtained the OfFDL gene from the insect Ostrinia furnacalis by RT-PCR. The full length cDNA of FDL is 2241 bp carrying an opening reading frame of 1923 bp encoding 640 amino acids. The recombinant protein OfFDL in a soluble and active form was obtained with high purity through a two-step purification strategy. The recombinant OfFDL exclusively hydrolyzes the terminal β-1,2-GlcNAc residue from the α-1,3 branch instead of the α-1,6 branch of the substrate GnGn-PA. Several kinetic parameters including kcat/Km values toward four artificial substrates and Ki values of three representative hexosaminidase inhibitors were obtained.

N-glycan maturation is crucial for cytokinin-mediated development and cellulose synthesis in Oryza sativa

To explore the physiological significance of N-glycan maturation in the plant Golgi apparatus, gnt1, a mutant with loss of N-acetylglucosaminyltransferase I (GnTI) function, was isolated in Oryza sativa. gnt1 exhibited complete inhibition of N-glycan maturation and accumulated high-mannose N-glycans. Phenotypic analyses revealed that gnt1 shows defective post-seedling development and incomplete cell wall biosynthesis, leading to symptoms such as failure in tiller formation, brittle leaves, reduced cell wall thickness, and decreased cellulose content. The developmental defects of gnt1 ultimately resulted in early lethality without transition to the reproductive stage. However, callus induced from gnt1 seeds could be maintained for periods, although it exhibited a low proliferation rate, small size, and hypersensitivity to salt stress. Shoot regeneration and dark-induced leaf senescence assays indicated that the loss of GnTI function results in reduced sensitivity to cytokinin in rice. Reduced expression of A-type O. sativa response regulators that are rapidly induced by cytokinins in gnt1 confirmed that cytokinin signaling is impaired in the mutant. These results strongly support the proposed involvement of N-glycan maturation in transport as well as in the function of membrane proteins that are synthesized via the endomembrane system.

Golgi alpha-mannosidase II cleaves two sugars sequentially in the same catalytic site

Golgi alpha-mannosidase II (GMII) is a key glycosyl hydrolase in the N-linked glycosylation pathway. It catalyzes the removal of two different mannosyl linkages of GlcNAcMan(5)GlcNAc(2), which is the committed step in complex N-glycan synthesis. Inhibition of this enzyme has shown promise in certain cancers in both laboratory and clinical settings. Here we present the high-resolution crystal structure of a nucleophile mutant of Drosophila melanogaster GMII (dGMII) bound to its natural oligosaccharide substrate and an oligosaccharide precursor as well as the structure of the unliganded mutant. These structures allow us to identify three sugar-binding subsites within the larger active site cleft. Our results allow for the formulation of the complete catalytic process of dGMII, which involves a specific order of bond cleavage, and a major substrate rearrangement in the active site. This process is likely conserved for all GMII enzymes-but not in the structurally related lysosomal mannosidase-and will form the basis for the design of specific inhibitors against GMII.

Protein N-glycosylation in the baculovirus-insect cell system

One of the major advantages of the baculovirus-insect cell system is that it is a eukaryotic system that can provide posttranslational modifications, such as protein N-glycosylation. However, this is a vastly oversimplified view, which reflects a poor understanding of insect glycobiology. In general, insect protein glycosylation pathways are far simpler than the corresponding pathways of higher eukaryotes. Paradoxically, it is increasingly clear that various insects encode and can express more elaborate protein glycosylation functions in restricted fashion. Thus, the information gathered in a wide variety of studies on insect protein N-glycosylation during the past 25 years has provided what now appears to be a reasonably detailed, comprehensive, and accurate understanding of the protein N-glycosylation capabilities of the baculovirus-insect cell system. In this chapter, we discuss the models of insect protein N-glycosylation that have emerged from these studies and how this impacts the use of baculovirus-insect cell systems for recombinant glycoprotein production. We also discuss the use of these models as baselines for metabolic engineering efforts leading to the development of new baculovirus-insect cell systems with humanized protein N-glycosylation pathways, which can be used to produce more authentic recombinant N-glycoproteins for drug development and other biomedical applications.

Biosynthesis of truncated N-linked oligosaccharides results from non-orthologous hexosaminidase-mediated mechanisms in nematodes, plants, and insects

In many invertebrates and plants, the N-glycosylation profile is dominated by truncated paucimannosidic N-glycans, i.e. glycans consisting of a simple trimannosylchitobiosyl core often modified by core fucose residues. Even though they lack antennal N-acetylglucosamine residues, the biosynthesis of these glycans requires the sequential action of GlcNAc transferase I, Golgi mannosidase II, and, finally, beta-N-acetylglucosaminidases. In Drosophila, the recently characterized enzyme encoded by the fused lobes (fdl) gene specifically removes the non-reducing N-acetylglucosamine residue from the alpha1,3-antenna of N-glycans. In the present study, we examined the products of five beta-N-acetylhexosaminidase genes from Caenorhabditis elegans (hex-1 to hex-5, corresponding to reading frames T14F9.3, C14C11.3, Y39A1C.4, Y51F10.5, and Y70D2A.2) in addition to three from Arabidopsis thaliana (AtHEX1, AtHEX2, and AtHEX3, corresponding to reading frames At1g65590, At3g55260, and At1g05590). Based on homology, the Caenorhabditis HEX-1 and all three Arabidopsis enzymes are members of the same sub-family as the aforementioned Drosophila fused lobes enzyme but either act as chitotriosidases or non-specifically remove N-acetylglucosamine from both N-glycan antennae. The other four Caenorhabditis enzymes are members of a distinct sub-family; nevertheless, two of these enzymes displayed the same alpha1,3-antennal specificity as the fused lobes enzyme. Furthermore, a deletion of part of the Caenorhabditis hex-2 gene drastically reduces the native N-glycan-specific hexosaminidase activity in mutant worm extracts and results in a shift in the N-glycan profile, which is a demonstration of its in vivo enzymatic relevance. Based on these data, it is hypothesized that the genetic origin of paucimannosidic glycans in nematodes, plants, and insects involves highly divergent members of the same hexosaminidase gene family.

A deletion in the golgi alpha-mannosidase II gene of Caenorhabditis elegans results in unexpected non-wild-type N-glycan structures

The processing of N-linked oligosaccharides by alpha-mannosidases in the endoplasmic reticulum and Golgi is a process conserved in plants and animals. After the transfer of a GlcNAc residue to Asn-bound Man(5)GlcNAc(2) by N-acetylglucosaminyltransferase I, an alpha-mannosidase (EC 3.2.1.114) removes one alpha1,3-linked and one alpha1,6-linked mannose residue. In this study, we have identified the relevant alpha-mannosidase II gene (aman-2; F58H1.1) from Caenorhabditis elegans and have detected its activity in both native and recombinant forms. For comparative studies, the two other cDNAs encoding class II mannosidases aman-1 (F55D10.1) and aman-3 (F48C1.1) were cloned; the corresponding enzymes are, respectively, a putative lysosomal alpha-mannosidase and a Co(II)-activated alpha-mannosidase. The analysis of the N-glycan structures of an aman-2 mutant strain demonstrates that the absence of alpha-mannosidase II activity results in a shift to structures not seen in wild-type worms (e.g. N-glycans with the composition Hex(5-7)HexNAc(2-3)Fuc(2)Me) and an accumulation of hybrid oligosaccharides. Paucimannosidic glycans are almost absent from aman-2 worms, indicative also of a general lack of alpha-mannosidase III activity. We hypothesize that there is a tremendous flexibility in the glycosylation pathway of C. elegans that does not impinge, under standard laboratory conditions, on the viability of worms with glycotypes very unlike the wild-type pattern.

Null Mutations in Drosophila N-Acetylglucosaminyltransferase I Produce Defects in Locomotion and a Reduced Life Span

UDP-GlcNAc:alpha3-D-mannoside beta1,2-N-acetylglucosaminyltransferase I (encoded by Mgat1) controls the synthesis of hybrid, complex, and paucimannose N-glycans. Mice make hybrid and complex N-glycans but little or no paucimannose N-glycans. In contrast, Drosophila melanogaster and Caenorhabditis elegans make paucimannose N-glycans but little or no hybrid or complex N-glycans. To determine the functional requirement for beta1,2-N-acetylglucosaminyltransferase I in Drosophila, we generated null mutations by imprecise excision of a nearby transposable element. Extracts from Mgat1(1)/Mgat1(1) null mutants showed no beta1,2-N-acetylglucosaminyltransferase I enzyme activity. Moreover, mass spectrometric analysis of these extracts showed dramatic changes in N-glycans compatible with lack of beta1,2-N-acetylglucosaminyltransferase I activity. Interestingly, Mgat1(1)/Mgat1(1) null mutants are viable but exhibit pronounced defects in adult locomotory activity when compared with Mgat1(1)/CyO-GFP heterozygotes or wild type flies. In addition, in null mutants males are sterile and have a severely reduced mean and maximum life span. Microscopic examination of mutant adult fly brains showed the presence of fused beta lobes. The removal of both maternal and zygotic Mgat1 also gave rise to embryos that no longer express the horseradish peroxidase antigen within the central nervous system. Taken together, the data indicate that beta1,2-N-acetylglucosaminyltransferase I-dependent N-glycans are required for locomotory activity, life span, and brain development in Drosophila.

Molecular cloning and characterization of Arabidopsis thaliana Golgi alpha-mannosidase II, a key enzyme in the formation of complex N-glycans in plants

N-glycosylation is one of the major post-translational modifications of proteins in eukaryotes; however, the processing reactions of oligomannosidic N-glycan precursors leading to hybrid-type and finally complex-type N-glycans are not fully understood in plants. To investigate the role of Golgi alpha-mannosidase II (GMII) in the formation of complex N-glycans in plants, we identified a putative GMII from Arabidopsis thaliana (AtGMII; EC 3.2.1.114) and characterized the enzyme at a molecular level. The putative AtGMII cDNA was cloned, and its deduced amino acid sequence revealed a typical type II membrane protein of 1173 amino acids. A soluble recombinant form of the enzyme produced in insect cells was capable of processing different physiologically relevant hybrid N-glycans. Furthermore, a detailed N-glycan analysis of two AtGMII knockout mutants revealed the predominant presence of unprocessed hybrid N-glycans. These results provide evidence that AtGMII plays a central role in the formation of complex N-glycans in plants. Furthermore, conclusive evidence was obtained that alternative routes in the conversion of hybrid N-glycans to complex N-glycans exist in plants. Transient expression of N-terminal AtGMII fragments fused to a GFP reporter molecule demonstrated that the transmembrane domain and 10 amino acids from the cytoplasmic tail are sufficient to retain a reporter molecule in the Golgi apparatus and that lumenal sequences are not involved in the retention mechanism. A GFP fusion construct containing only the transmembrane domain was predominantly retained in the ER, a result that indicates the presence of a motif promoting ER export within the last 10 amino acids of the cytoplasmic tail of AtGMII.

The Drosophila fused lobes Gene Encodes an N-Acetylglucosaminidase Involved in N-Glycan Processing

Most processed, e.g. fucosylated, N-glycans on insect glycoproteins terminate in mannose, yet the relevant modifying enzymes require the prior action of N-acetylglucosaminyltransferase I. This led to the hypothesis that a hexosaminidase acts during the course of N-glycan maturation. To determine whether the Drosophila melanogaster genome indeed encodes such an enzyme, a cDNA corresponding to fused lobes (fdl), a putative beta-N-acetylglucosaminidase with a potential transmembrane domain, was cloned. When expressed in Pichia pastoris, the enzyme exhibited a substrate specificity similar to that previously described for a hexosaminidase activity from Sf-9 cells, i.e. it hydrolyzed exclusively the GlcNAc residue attached to the alpha1,3-linked mannose of the core pentasaccharide of N-glycans. It also hydrolyzed p-nitrophenyl-N-acetyl-beta-glucosaminide, but not chitooligosaccharides; in contrast, Drosophila HEXO1 and HEXO2 expressed in Pichia cleaved both these substrates but not N-glycans. The localization of recombinant FDL tagged with green fluorescent protein in Drosophila S2 cells by immunoelectron microscopy showed that this enzyme transits through the Golgi, is present on the plasma membrane and in multivesicular bodies, and is secreted. Finally, the N-glycans of two lines of fdl mutant flies were analyzed by mass spectrometry and reversed-phase high-performance liquid chromatography. The ratio of structures with terminal GlcNAc over those without (i.e. paucimannosidic N-glycans) was drastically increased in the fdl-deficient flies. Therefore, we conclude that the fdl gene encodes a novel hexosaminidase responsible for the occurrence of paucimannosidic N-glycans in Drosophila.

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.

Molecular cloning and functional characterization of a Lepidopteran insect beta4-N-acetylgalactosaminyltransferase with broad substrate specificity, a functional role in glycoprotein biosynthesis, and a potential functional role in glycolipid biosynthesis

A degenerate PCR approach was used to isolate a lepidopteran insect cDNA encoding a beta4-galactosyl-transferase family member. The isolation and initial identification of this cDNA was based on bioinformatics, but its identification as a beta4-galactosyltransferase family member was experimentally confirmed. The newly identified beta4-galactosyltransferase family member had unusually broad donor and acceptor substrate specificities in vitro, as transferred galactose, N-acetylglucosamine, and N-acetylgalactosamine to carbohydrate, glycoprotein, and glycolipid acceptors. However, the enzyme preferentially utilized N-acetylgalactosamine as the donor for all three acceptors, and its derived amino acid sequence was closely related to a known N-acetylgalactosaminyltransferase. These data suggested that the newly isolated cDNA encodes a beta4-N-acetylgalactosaminyltransferase that functions in insect cell glycoprotein biosynthesis, glycolipid biosynthesis, or both. The remainder of this study focused on the role of this enzyme in N-glycoprotein biosynthesis. The results showed that the purified enzyme transferred N-acetylgalactosamine, but no detectable galactose or N-acetylglucosamine, to a synthetic N-glycan in vitro. The structure of the reaction product was confirmed by chromatographic, mass spectroscopic, and nuclear magnetic resonance analyses. Co-expression of the new cDNA product in insect cells with an N-glycoprotein reporter showed that it transferred N-acetylgalactosamine, but no detectable galactose or N-acetylglucosamine, to this N-glycoprotein in vivo. Confocal microscopy showed that a GFP-tagged version of the enzyme was localized in the insect cell Golgi apparatus. In summary, this study demonstrated that lepidopteran insect cells encode and express a beta4-N-acetylgalactosaminyltransferase that functions in N-glycoprotein biosynthesis and perhaps in glycolipid biosynthesis, as well. The isolation and characterization of this gene and its product contribute to our basic understanding of insect protein N-glycosylation pathways and to the growing body of evidence that insects can produce glycoproteins with complex N-glycans.

Unaltered complex N-glycan profiles in Nicotiana benthamiana despite drastic reduction of beta1,2- N -acetylglucosaminyltransferase I activity

UDP-GlcNAc:alpha3-D-mannoside beta1,2- N -acetylglucosaminyltransferase I (GnTI; EC 2.4.1.101) is a Golgi-resident glycosyltransferase that is essential for the processing of oligomannose to hybrid and complex N-glycans in higher eukaryotes. The cDNA of Nicotiana tabacum GnTI has been cloned and characterised previously. To assess the influence of GnTI expression levels on the formation of complex N-glycans we used posttranscriptional gene silencing to knock down the expression of GnTI in the tobacco related species Nicotiana benthamiana. 143 independent transgenic plants containing GnTI constructs in either sense or antisense orientation were generated. 23 lines were selected for measurement of GnTI activity and 10 lines thereof showed a reduction of more than 85% in in vitro assays as compared to wildtype plants. GnTI reduction was stably inherited and did not interfere with the viability of the transformants. Noteworthy one line, 34S/2, exhibited a residual GnTI activity below the detection limit. beta1,2- N -acetylglucosaminyltransferase II (GnTII), an enzyme which acts further downstream in the N-glycosylation pathway, as well as other control enzymes (alpha-mannosidase, beta- N -acetylglucosaminidase) were not affected indicating the specific downregulation of GnTI. Remarkably, immunoblots and mass spectrometric N-glycan profiling revealed no significant changes of the total N-glycan comparable to wildtype plants.

Abnormal glycosylation of red cell membrane band 3 in the congenital disorder of glycosylation Ig

A description is provided of the clinical presentation in an infant of the recently described congenital disorder of glycosylation type Ig, and the changes affecting glycosylation of red cell membrane band 3, the anion exchanger. It has been shown that the condition stems from a homozygous mutation within the human ortholog of yeast ALG12 gene, which encodes a dolichol-P-mannose:Man7GlcNAc2-PP-dolichol alpha,1-6 mannosyltransferase of the endoplasmic reticulum. The clinical phenotype included prominent central and peripheral manifestations in the CNS. Although the infant studied had no anemia, band 3 abnormally separated into two fractions upon electrophoresis. The chemical composition of the glycans of both fractions was analyzed in detail. The fraction with low electrophoretic mobility was moderately hypoglycosylated (by 27%) and its mannose content was normal. The fraction with high electrophoretic mobility was deeply carbohydrate deficient (by 64%) and had 1 mol mannose in excess but only three residues of N-acetylglucosamine. Glycophorin A was hypoglycosylated with respect to O-linked glycans. Glycosphingolipids of red cells were normal. We suggest that the incomplete biosynthesis of the N-linked glycan of band 3 was largely caused by the persistence of the 3-linked mannose residue on the 6-mannose arm of the trimannosyl moiety of the glycoprotein. It is remarkable that the changes recorded in band 3 have no clinical consequences. Band 3 alteration might serve as an additional indicator (some serum N-glycoproteins of hepatic origin are also indicative) of the congenital disorder of glycosylation type Ig.

Synthesis of paucimannose N-glycans by Caenorhabditis elegans requires prior actions of UDP-N-acetyl-D-glucosamine:alpha-3-D-mannoside beta1,2-N-acetylglucosaminyltransferase I, alpha3,6-mannosidase II and a specific membrane-bound beta-N-acetylglucosaminidase

We have previously reported three Caenorhabditis elegans genes ( gly-12, gly-13 and gly-14 ) encoding UDP- N -acetyl-D-glucosamine:alpha-3-D-mannoside beta1,2- N -acetylglucosaminyltransferase I (GnT I), an enzyme essential for hybrid and complex N-glycan synthesis. GLY-13 was shown to be the major GnT I in worms and to be the only GnT I cloned to date which can act on [Manalpha1,6(Manalpha1,3)Manalpha1,6](Manalpha1,3)Manbeta1, 4GlcNAcbeta1,4GlcNAc-R, but not on Manalpha1,6(Manalpha1,3)Manbeta1- O -R substrates. We now report the kinetic constants, bivalent-metal-ion requirements, and optimal pH, temperature and Mn(2+) concentration for this unusual enzyme. C. elegans glycoproteins are rich in oligomannose (Man(6-9)GlcNAc(2)) and 'paucimannose' Man(3-5)GlcNAc(2)(+/-Fuc) N-glycans, but contain only small amounts of complex and hybrid N-glycans. We show that the synthesis of paucimannose Man(3)GlcNAc(2) requires the prior actions of GnT I, alpha3,6-mannosidase II and a membrane-bound beta- N -acetylglucosaminidase similar to an enzyme previously reported in insects. The beta- N -acetylglucosaminidase removes terminal N -acetyl-D-glucosamine from the GlcNAcbeta1, 2Manalpha1,3Manbeta- arm of Manalpha1,6(GlcNAcbeta1,2Manalpha1,3) Manbeta1,4GlcNAcbeta1,4GlcNAc-R to produce paucimannose Man(3)GlcNAc(2) N-glycan. N -acetyl-D-glucosamine removal was inhibited by two N -acetylglucosaminidase inhibitors. Terminal GlcNAc was not released from [Manalpha1,6(Manalpha1,3)Manalpha 1,6] (GlcNAcbeta1,2Manalpha1,3)Manbeta1,4GlcNAcbeta1,4GlcNAc-R nor from the GlcNAcbeta1,2Manalpha1,6Manbeta- arm. These findings indicate that GLY-13 plays an important role in the synthesis of N-glycans by C. elegans and that therefore the worm should prove to be a suitable model for the study of the role of GnT I in nematode development.

Structure of Golgi alpha-mannosidase II: a target for inhibition of growth and metastasis of cancer cells

Golgi alpha-mannosidase II, a key enzyme in N-glycan processing, is a target in the development of anti- cancer therapies. The crystal structure of Drosophila Golgi alpha-mannosidase II in the absence and presence of the anti-cancer agent swainsonine and the inhibitor deoxymannojirimycin reveals a novel protein fold with an active site zinc intricately involved both in the substrate specificity of the enzyme and directly in the catalytic mechanism. Identification of a putative GlcNAc binding pocket in the vicinity of the active site cavity provides a model for the binding of the GlcNAcMan(5)GlcNAc(2) substrate and the consecutive hydrolysis of the alpha1,6- and alpha1,3-linked mannose residues. The enzyme-inhibitor interactions observed provide insight into the catalytic mechanism, opening the door to the design of novel inhibitors of alpha-mannosidase II.

N-Glycan analysis by matrix-assisted laser desorption/ionization mass spectrometry of electrophoretically separated nonmammalian proteins: application to peanut allergen Ara h 1 and olive pollen allergen Ole e 1

A method has been developed which allows the analysis of glycoproteins separated by SDS-PAGE. The procedure, though applicable to N-glycosylated glycoproteins of any origin, is particularly devised for glycoproteins potentially containing fucose in alpha1,3-linkage to the reducing GlcNAc as may be found in plants and invertebrates, e.g., insects and parasitic helminths. Starting with an established procedure for mass spectrometric peptide mapping, the analysis of N-glycans by matrix-assisted laser desorption/ionization mass spectrometry involved the use of peptide:N-glycosidase A, a triphasic microcolumn for sample cleanup, and a new matrix mixture consisting of 2,5-dihyhydroxybenzoic acid, 1-hydroxyisoquinoline, and arabinosazone. The method was tested on proteins with N-glycans of known structure, i.e., as horseradish peroxidase, zucchini ascorbate oxidase, soybean agglutinin, honeybee venom hyaluronidase, bovine ribonuclease B, and bovine fetuin. An electrophoretic band corresponding to 4 microg of glycoprotein was generally sufficient to allow detection of the major N-glycan species. As an additional benefit, a peptide mass map is generated which serves to identify the analyzed protein. The method was applied to glycoprotein allergens whose glycan structures were unknown. Ara h 1 and Ole e 1, major allergens from peanut and olive pollen, respectively, contained mainly xylosylated N-glycans with the composition Man(3(-4))XylGlcNAc(2) in the case of Ara h 1 and GlcNAc(1-2)Man(3)XylGlcNAc(2) in the case of Ole e 1 where also some GlcNAc(0-2)Man(3)XylFucGlcNAc(2) was found.

Constraints on the transport and glycosylation of recombinant IFN-gamma in Chinese hamster ovary and insect cells

In this study we compare intracellular transport and processing of a recombinant glycoprotein in mammalian and insect cells. Detailed analysis of the N-glycosylation of recombinant human IFN-gamma by matrix-assisted laser-desorption mass spectrometry showed that the protein secreted by Chinese hamster ovary and baculovirus-infected insect Sf9 cells was associated with complex sialylated or truncated tri-mannosyl core glycans, respectively. However, the intracellular proteins were predominantly associated with high-mannose type oligosaccharides (Man-6 to Man-9) in both cases, indicating that endoplasmic reticulum to cis-Golgi transport is a predominant rate-limiting step in both expression systems. In CHO cells, although there was a minor intracellular subpopulation of sialylated IFN-gamma glycoforms identical to the secreted product (therefore associated with late-Golgi compartments or secretory vesicles), no other intermediates were evident. Therefore, anterograde transport processes in the Golgi stack do not limit secretion. In Sf9 insect cells, there was no direct evidence of post-ER glycan-processing events other than core fucosylation and de-mannosylation, both of which were glycosylation site-specific. To investigate the influence of nucleotide-sugar availability on cell-specific glycosylation, the cellular content of nucleotide-sugar substrates in both mammalian and insect cells was quantitatively determined by anion-exchange HPLC. In both host cell types, UDP-hexose and UDP-N-acetylhexosamine were in greater abundance relative to other substrates. However, unlike CHO cells, sialyltransferase activity and CMP-NeuAc substrate were not present in uninfected or baculovirus-infected Sf9 cells. Similar data were obtained for other insect cell hosts, Sf21 and Ea4. We conclude that although the limitations on intracellular transport and secretion of recombinant proteins in mammalian and insect cells are similar, N-glycan processing in Sf insect cells is limited, and that genetic modification of N-glycan processing in these insect cell lines will be constrained by substrate availability to terminal galactosylation.

Insect cells as hosts for the expression of recombinant glycoproteins

Baculovirus-mediated expression in insect cells has become well-established for the production of recombinant glycoproteins. Its frequent use arises from the relative ease and speed with which a heterologous protein can be expressed on the laboratory scale and the high chance of obtaining a biologically active protein. In addition to Spodoptera frugiperda Sf9 cells, which are probably the most widely used insect cell line, other mainly lepidopteran cell lines are exploited for protein expression. Recombinant baculovirus is the usual vector for the expression of foreign genes but stable transfection of - especially dipteran - insect cells presents an interesting alternative. Insect cells can be grown on serum free media which is an advantage in terms of costs as well as of biosafety. For large scale culture, conditions have been developed which meet the special requirements of insect cells. With regard to protein folding and post-translational processing, insect cells are second only to mammalian cell lines. Evidence is presented that many processing events known in mammalian systems do also occur in insects. In this review, emphasis is laid, however, on protein glycosylation, particularly N-glycosylation, which in insects differs in many respects from that in mammals. For instance, truncated oligosaccharides containing just three or even only two mannose residues and sometimes fucose have been found on expressed proteins. These small structures can be explained by post-synthetic trimming reactions. Indeed, cell lines having a low level of N-acetyl-beta-glucosaminidase, e.g. Estigmene acrea cells, produce N- glycans with non-reducing terminal N-acetylglucosamine residues. The Trichoplusia ni cell line TN-5B1-4 was even found to produce small amounts of galactose terminated N-glycans. However, there appears to be no significant sialylation of N-glycans in insect cells. Insect cells expressed glycoproteins may, though, be alpha1,3-fucosylated on the reducing-terminal GlcNAc residue. This type of fucosylation renders the N-glycans on one hand resistant to hydrolysis with PNGase F and on the other immunogenic. Even in the absence of alpha1,3-fucosylation, the truncated N-glycans of glycoproteins produced in insect cells constitute a barrier to their use as therapeutics. Attempts and strategies to "mammalianise" the N-glycosylation capacity of insect cells are discussed.

Engineering N-glycosylation pathways in the baculovirus-insect cell system

The inability to produce eukaryotic glycoproteins with complex N-linked glycans is a major limitation of the baculovirus-insect cell expression system. Recent studies have demonstrated that metabolic engineering can be used to extend the glycoprotein processing capabilities of lepidopteran insect cells. This approach is being used to develop new baculovirus-insect cell expression systems that can produce more authentic recombinant glycoproteins and obtain new information on insect N-glycosylation pathways.

Strict order of (Fuc to Asn-linked GlcNAc) fucosyltransferases forming core-difucosylated structures

In insect cells fucose can be either alpha1,6- or alpha1,3-linked to the asparagine-bound GlcNAc residue of N-glycans. Difucosylated glycans have also been found. Kinetic studies and acceptor competition experiments demonstrate that two different enzymes are responsible for this alpha1,6- and alpha1,3-linkage of fucose. Using dansylated acceptor substrates a strict order of these enzymes can be established for the formation of difucosylated structures. First, the alpha1,6-fucosyltransferase catalyses the transfer of fucose into alpha1,6-linkage to the non-fucosylated acceptor and then the alpha1,3-fucosyltransferase completes the difucosylation.

Purification and properties of alpha-mannosidase II from Golgi-like membranes of baculovirus-infected Spodoptera frugiperda (IPLB-SF-21AE) cells

An alpha-mannosidase II-like activity was identified in baculovirus-infected Spodoptera frugiperda (IPLB-SF21-AE) cells. The enzyme responsible was purified from Golgi-type membranes to apparent homogeneity by using a combination of steps including DEAE-cellulose, hydroxyapatite, concanavalin A-Sepharose and gel filtration chromatography. The molecular mass of this purified protein was approx. 120 kDa by SDS/PAGE under reducing conditions and approx. 240 kDa under non-reducing conditions, indicating that the enzyme is a disulphide-linked dimer. Substrates demonstrated to undergo hydrolysis with this enzyme were GlcNAc-Man5-GlcNAc-GlcNAc (non-reduced and reduced) and p-nitrophenyl alpha-d-mannopyranoside. The oligosaccharide substrate was converted into GlcNAc-Man3-GlcNAc-GlcNAc through an intermediate GlcNAc-Man4-GlcNAc-GlcNAc. Treatment of the isolated intermediate oligosaccharide with endoglycosidase H resulted in its conversion into GlcNAc-Man4-GlcNAc. This indicated that it contained the alpha-1,3-linked mannose residue on the alpha-1,6-linked mannose arm and showed that the alpha-1,6-linked mannose residue on the alpha-1,6-linked mannose arm had been preferentially hydrolysed by the mannosidase. The oligosaccharide lacking the beta-1,2-linked GlcNAc residue on the alpha-1,3-linked mannose arm (Man5-GlcNAc-GlcNAc) was not hydrolysed in the presence of the enzyme. Metal ions were not required for enzymic activity on any of the substrates, but Cu2+ was strongly inhibitory. The activity of the enzyme was inhibited at low concentrations of swainsonine, but much higher concentrations of 1-deoxymannojirimycin were required to achieve inhibition. All of these properties are characteristic of mannosidase II enzymes from other eukaryotic tissues. The presence of mannosidase II in lepidopteran insect cells would allow entry of N-linked glycoproteins into the complex processing reaction pathway or into the terminal Man3-GlcNAc-GlcNAc pathway.

Differential N-glycan patterns of secreted and intracellular IgG produced in Trichoplusia ni cells

Structures of the N-linked oligosaccharide attached to the heavy chain of a heterologous murine IgG2a produced from Trichoplusia ni (TN-5B1-4, High Five) insect cells were characterized. Coexpression of the chaperone immunoglobulin heavy chain-binding protein (BiP) in the baculovirus-infected insect cells increased the soluble intracellular and secreted IgG level. This facilitated the detailed analysis of N-glycans from both intracellular and secreted IgG. Following purification of the immunoglobulins using Protein A-Sepharose, glycopeptides, prepared by trypsin-chymotrypsin digestion, were further digested with glycoamidase from sweet almond emulsin to obtain the oligosaccharide moieties. The resulting oligosaccharides were then reductively aminated with 2-aminopyridine and the structures identified by two-dimensional high performance liquid chromatography mapping (Tomiya, N., Awaya, J., Kurono, M., Endo, S., Arata, Y., and Takahashi, N. (1988) Anal. Biochem. 171, 73-90). The N-glycans obtained from the secreted IgG contain 35% complex type, some with terminal galactose residues at either alpha1, 3-Man or alpha1,6-Man branches of the Man3GlcNAc2 core. The remaining oligosaccharides detected in the secreted IgG were principally hybrid (30%) and paucimannosidic (35%) type N-glycans. Most (84%) of these secreted glycoforms contained fucose alpha1, 6-linked to the innermost GlcNAc residue and the presence of a potentially allergenic fucose alpha1,3-linked to the innermost GlcNAc residue was also detected. In contrast, the intracellular immunoglobulins included 50% high mannose-type N-glycans with lower levels of complex, hybrid, and paucimannosidic-type structures. Reverse phase one-dimensional high performance liquid chromatography analysis of the IgG N-glycans in the absence of heterologous BiP exhibited a similar distribution of intracellular and secreted glycoforms. These studies indicate that Trichoplusia ni TN-5B1-4 cells are capable of terminal galactosylation. However, the processing pathways in these cell lines appear to diverge from mammalian cells in the formation of paucimannosidic structures, in the presence of alpha1,3-fucose linkages, and in the absence of sialylation.

Modifying the insect cell N-glycosylation pathway with immediate early baculovirus expression vectors

The baculovirus-insect cell expression system is well-suited for recombinant glycoprotein production because baculovirus vectors can provide high levels of expression and insect cells can modify newly synthesized proteins in eucaryotic fashion. However, the N-glycosylation pathway of baculovirus-infected insect cells differs from the pathway found in higher eucaryotes, as indicated by the fact that glycoproteins produced in the baculovirus system typically lack complex biantennary N-linked oligosaccharide side chains containing penultimate galactose and terminal sialic acid residues. We recently developed a new type of baculovirus vector that can express foreign genes immediately after infection under the control of the viral ie1 promoter. These immediate early baculovirus expression vectors can be used to modify the insect cell N-glycosylation pathway and produce a foreign glycoprotein with more extensively processed N-linked oligosaccharides. These vectors can also be used to study the influence of the late steps in N-linked oligosaccharide processing on glycoprotein function. Further development could lead to baculovirus-insect cell expression systems that can produce recombinant glycoproteins with complex biantennary N-linked oligosaccharides structurally identical to those produced by higher eucaryotes.

N-acetyl-beta-glucosaminidase accounts for differences in glycosylation of influenza virus hemagglutinin expressed in insect cells from a baculovirus vector

The hemagglutinin of fowl plague virus has been expressed in Spodoptera frugiperda (Sf9) cells and in Estigmene acrea cells by using a baculovirus vector. Structural analysis revealed that the endo-H-resistant N-glycans of HA from Sf9 cells were predominantly trimannosyl core oligosaccharides, whereas in E. acrea cells most of these cores were elongated by at least one terminal N-acetylglucosamine residue. To understand the difference in carbohydrate structures, enzymes involved in N-glycan processing have been analyzed. The results revealed that the different glycosylation patterns observed are due to an N-acetyl-beta-glucosaminidase activity that was found in Sf9 cells but not in E. acrea cells. This enzyme specifically used the GlcNAcMan(3)GlcNAc(2) oligosaccharide as a substrate. When N-acetyl-beta-glucosaminidase or alpha-mannosidase II was inhibited by specific inhibitors, the amount of terminal N-acetylglucosamine in hemagglutinin from Sf9 cells was significantly enhanced. These results demonstrate that N glycosylation in both cell lines follows the classical pathway up to the stage of GlcNAcMan(3)GlcNAc(2) oligosaccharide side chains. Whereas these structures are the end product in E. acrea cells, they are degraded in Sf9 cells to Man(3)GlcNAc(2) cores by N-acetyl-beta-glucosaminidase.

Insect cells contain an unusual, membrane-bound beta-N-acetylglucosaminidase probably involved in the processing of protein N-glycans

The beta-N-acetylglucosaminidase activity in the lepidopteran insect cell line Sf21 has been studied using pyridylaminated oligosaccharides and chromogenic synthetic glycosides as substrates. Ultracentrifugation experiments indicated that the insect cell beta-N-acetylglucosminidase exists in a soluble and a membrane-bound form. This latter form accounted for two-thirds of the total activity and was associated with vesicles of the same density as those containing GlcNAc-transferase I. Partial membrane association of the enzyme was observed with all substrates tested, i.e. 4-nitrophenyl beta-N-acetylglucosaminide, tri-N-acetylchitotriose, and an N-linked biantennary agalactooligosaccharide. Inhibition studies indicted a single enzyme to be responsible for the hydrolysis of all these substrates. With the biantennary substrate, the beta-N-acetylglucosaminidase exclusively removed beta-N-acetylglucosamine from the alpha 1,3-antenna. GlcNAcMan5GlcNAc2, the primary product of GlcNAc-transferase I, was not perceptibly hydrolyzed. beta-N-Acetylglucosaminidases with the same branch specificity were also found in the lepidopteran cell lines Bm-N and Mb-0503. In contrast, beta-N-acetylglucosaminidase activities from rat or frog (Xenopus laevis) liver and from mung bean seedlings were not membrane-bound, and they did not exhibit a strict branch specificity. An involvement of this unusual beta-N-acetylglucosaminidase in the processing of asparagine-linked oligosaccharides in insects is suggested.