Distinct N-glycan fucosylation potentials of three lepidopteran cell lines

HPLC method for the determination of Fuc to Asn-linked GlcNAc fucosyltransferases

Mammalian cells often contain an enzyme which transfers fucose onto the reducing terminal GlcNAc (GlcNAc-1) of N-glycans with an alpha1,6-linkage. In plants, on the other hand, the fucose is transferred to GlcNAc-1 with an alpha1,3-inkage. Insect cells can exhibit both enzymatic activities. Hitherto, the activity of these fucosyltransferases has been determined by the incorporation of radioactively labelled fucose into an acceptor glycopeptide. This assay, however, cannot discriminate these two activities. Here we report on the use of dansylated glycoasparagine for the specific determination of 1,3- and 1,6-fucosyltransferases. The two possible products and the substrate are separated on a reversed phase column and detected by fluorescence.

Structural characterization of the N-linked oligosaccharides derived from HIVgp120 expressed in lepidopteran cells

The oligosaccharides of recombinant HIV gp120 expressed in lepidopteran Sf9 cells were analysed after hydrazine release by gel permeation and high pH anion exchange chromatography. N-Linked glycans were exclusively of the oligomannose series and no evidence for charged complex or hybrid type glycans was found. However a glycosylation reaction similar to those found in vertebrates was evident. The major glycoform of gp120, that comprised 30% of all the species analysed, was structurally identified by exoglycosidase digestion and found to be a core fucosylated structure, Manalpha1,6(Manalpha1,3)Manbeta1,4GlcNAc(Fucalpha1+ ++,6)GlcNAc. Further confirmation of the ability of lepidopteran cells to fucosylate N-linked glycans was provided by an in vitro analysis of this reaction using authentic oligosaccharide substrates.

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.

Identification and characterization of an eight-cysteine repeat of the latent transforming growth factor-beta binding protein-1 that mediates bonding to the latent transforming growth factor-beta1

Most cultured cell types secrete small latent transforming growth factor-beta (TGF-beta) as a disulfide-bonded complex with a member of the latent TGF-beta binding protein (LTBP) family. Using the baculovirus expression system, we have mapped the domain of LTBP-1 mediating covalent association with small latent TGF-beta1. Coexpression in Sf9 cells of small latent TGF-beta1 with deletion mutants of LTBP-1 showed that the third eight-cysteine repeat of LTBP-1 is necessary and sufficient for covalent interaction with small latent TGF-beta1. Analysis by mass spectrometry of this eight-cysteine repeat, produced as a recombinant peptide in Sf9 cells, confirmed that it was N-glycosylated, as expected from the primary sequence. No other post-translational modifications of this domain were detected. Alkylation of the recombinant peptide with vinyl pyridine failed to reveal any free cysteines, indicating that, in the absence of small latent TGF-beta, the eight cysteines of this domain are engaged in intramolecular bonds. These data demonstrate that the third LTBP-1 eight-cysteine repeat recognizes and associates covalently with small latent TGF-beta1 through a mechanism that does not require any specific post-translational modification of this domain. They also suggest that this domain adopts different conformations depending on whether it is free or bound to small latent TGF-beta.

Functional purification and characterization of a GDP-fucose: beta-N-acetylglucosamine (Fuc to Asn linked GlcNAc) alpha 1,3-fucosyltransferase from mung beans

An alpha 1,3-fucosyltransferase was purified 3000-fold from mung bean seedlings by chromatography on DE 52 cellulose and Affigel Blue, by chromatofocusing, gelfiltration and affinity chromatography resulting in an apparently homogenous protein of about 65 kDa on SDS-PAGE. The enzyme transferred fucose from GDP-fucose to the Asn-linked N-acetylglucosaminyl residue of an N-glycan, forming an alpha 1,3-linkage. The enzyme acted upon N-glycopeptides and related oligosaccharides with the glycan structure GlcNAc2Man3 GlcNAc2. Fucose in alpha 1,6-linkage to the asparagine-linked GlcNAc had no effect on the activity. No transfer to N-glycans was observed when the terminal GlcNAc residues were either absent or substituted with galactose. N-acetyllactosamine, lacto-N-biose and N-acetylchito-oligosaccharides did not function as acceptors for the alpha 1,3-fucosyltransferase. The transferase exhibited maximal activity at pH 7.0 and a strict requirement for Mn2+ or Zn2+ ions. The enzyme's activity was moderately increased in the presence of Triton X-100. It was not affected by N-ethylmaleimide.

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.

Expression of human interferon omega 1 in Sf9 cells. No evidence for complex-type N-linked glycosylation or sialylation

Human interferon omega 1 (IFN-omega 1) was expressed in Spodoptera frugiperda Sf9 insect cells using the baculovirus expression system. Half of the protein purified by immunoaffinity chromatography was shown to be N-glycosylated at the same site as the natural IFN-omega 1. The degree of glycosylation was independent of the expression rate. While natural IFN-omega 1 was shown to carry complex-type oligosaccharides [Adolf, G. R., Maurer-Fogy, I., Kalsner, I. & Cantell, K. (1990) J. Biol. Chem. 265, 9290-9295], the insect cell produced protein which was demonstrated by lectin blot, mass spectroscopy and HPLC analysis to contain only the core oligosaccharide. Two different structures, (Man)2(GlcNAc)2[Fuc] and (Man)3(GlcNAc)2[Fuc] were identified. The fucosylation was identified to be (alpha 1-6)-linked to the core saccharide. Sialic acid residues were clearly absent. IFN-omega 1 expressed in S. frugiperda cells was shown to be partially truncated at the C-terminus by nine residues; its antiviral activity when glycosylated was significantly lower than the activity of IFN-omega 1 produced by Sendai-virus-stimulated leukocytes. Circular dichroism and fluorescence spectroscopy did not reveal any structural differences between glycosylated and nonglycosylated IFN-omega 1. This implies the importance of a complex-type glycosylation for the maximal biological activity of human IFN-omega 1.

Primary structures of the N-linked carbohydrate chains from honeybee venom phospholipase A2

The N-linked carbohydrate chains of phospholipase A2 from honeybee (Apis mellifera) were released from glycopeptides with peptide-N-glycanase A and reductively aminated with 2-aminopyridine. The fluorescent derivatives were separated by size-fractionation and reverse-phase HPLC, yielding 14 fractions. Structural analysis was accomplished by compositional and methylation analyses, by comparison of the HPLC elution patterns with reference oligosaccharides, by stepwise exoglycosidase digestions which were monitored by HPLC, and, where necessary, by 500-MHz 1H-NMR spectroscopy. Ten oligosaccharides consisted of mannose, N-acetylglucosamine and fucose alpha 1-6 and/or alpha 1-3 linked to the innermost N-acetylglucosamine. Four compounds, which comprised 10% of the oligosaccharide pool from phospholipase A2, contained a rarely found terminal element with N-acetylgalactosamine. The structures of the 14 N-glycans from honeybee phospholipase A2 can be arranged into the following three series: [formula: see text]

Carbohydrate analysis

Carbohydrate analysis is an active field that is expanding rapidly. Hundreds of new structures are reported each year and methods for screening glycopolymers for known structures are now becoming accessible to the nonspecialist. Detailed structure analysis of recombinant glycoproteins has become relatively routine in specialist laboratories. Rapid advances are being made in the understanding of structure and function of biologically active carbohydrates that are of interest to the pharmaceutical industry.