Characterization of a new isomer of lipid-linked heptasaccharide formed during in vitro biosynthesis of mammary glycoproteins

Glycosylation of the envelope glycoprotein gp130 of simian immunodeficiency virus from sooty mangabey (Cercocebus atys)

The envelope glycoprotein 130 ('130' referring to an Mr of 130,000) of simian immunodeficiency virus from sooty mangabey (Cercocebus atys) (SIVSM) was isolated from the cell-free supernatant of the SIVSM-infected human T-cell line H9, metabolically labelled with D-[6-3H]glucosamine. After digestion with Staphylococcus aureus V8 proteinase, radiolabelled N-glycans were liberated from resulting glycopeptides by sequential treatment with endo-beta-N-acetylglucosaminidase H and peptide:N-glycosidase F and fractionated by h.p.l.c. and gel filtration. Individual oligosaccharide species were characterized by enzymic microsequencing, chromatographic analyses and, in part, by acetolysis. The oligosaccharide structures thus established include oligomannosidic glycans with five to nine mannose residues as well as fucosylated and partially sialylated bi-, tri- and tetra-antennary N-acetyl-lactosaminic oligosaccharide species, the latter of which carry, in part, additional galactose residues or N-acetyl-lactosamine repeats. In comparison with the corresponding envelope glycoprotein 120 from human immunodeficiency virus type 1 (HIV-1), propagated in the same cell line [Geyer, Holschbach, Hunsmann and Schneider (1988) J. Biol. Chem. 263, 11760-11767], carbohydrates of the simian glycoprotein were found to consist of decreased amounts of oligomannosidic glycans and increased quantities of higher-branched N-acetyl-lactosaminic species.

Study of the N-glycoprotein biosynthesis through dolichol intermediates in the mitochondrial membranes

1. In the mitochondria, the biosynthesis of N-glycoprotein products, through the dolichol intermediates pathway, appears in the outer and in the inner membranes. 2. The biosynthesis of dolichol-pyrophosphoryl-N-acetyl-glucosamine, dolichol-pyrophosphoryl-di-N-acetylchitobiose, dolichol-phosphoryl-glucose and dolichol-phosphoryl-mannose is effective in both membranes. 3. The lipid-linked oligosaccharides biosynthesized in both membranes contain high mannose-type oligosaccharides ranging in size from Man9-GlcNac2 to Man4-GlcNac2. 4. The assembly of the dolichol-pyrophosphoryl-oligosaccharides on the trimannosidic core begins by the elongation of the alpha-1,3 mannose branch in the outer membrane and of the alpha-1,6 mannose branch in the inner membrane.

The effect of mannosamine on the formation of lipid-linked oligosaccharides and glycoproteins in canine kidney cells

Madin-Darby canine kidney (MDCK) cells normally form lipid-linked oligosaccharides having mostly the Glc3Man9GlcNAc2 oligosaccharide. However, when MDCK cells are incubated in 1 to 10 mM mannosamine and labeled with [2-3H]mannose, the major oligosaccharides associated with the dolichol were Man5GlcNAc2 and Man6GlcNAc2 structures. Since both of these oligosaccharides were susceptible to digestion by endo-beta-N-acetylglucosaminidase H, the Man5GlcNAc2 must be different in structure than the Man5GlcNAc2 usually found as a biosynthetic intermediate in the lipid-linked oligosaccharides. Methylation analysis also indicated that this Man5GlcNAc2 contained 1----3 linked mannose residues. Since pulse chase studies indicated that the lesion was in biosynthesis, it appears that mannosamine inhibits the in vivo formation of lipid-linked oligosaccharides perhaps by inhibiting the alpha-1,2-mannosyl transferases. Although the lipid-linked oligosaccharides produced in the presence of mannosamine were smaller in size than those of control cells and did not contain glucose, the oligosaccharides were still transferred in vivo to protein. Furthermore, the oligosaccharide portions of the glycoproteins were still processed as shown by the fact that the glycopeptides were of the complex and hybrid types and were labeled with [3H]mannose or [3H]galactose. In contrast, control cells produced complex and high-mannose structures but no hybrid oligosaccharides were detected. The inhibition by mannosamine could be overcome by adding high concentrations of glucose to the medium.

Structure of the oligosaccharides sensitive to endo-beta-n-acetylglucosaminidase H in the glycoprotein of Friend murine leukemia virus

The surface glycoprotein (mixture of isoglycoproteins with Mr 69 000 and 71 000) was isolated from the particles of Friend murine leukemia virus, and was successively digested with protease and with endo-beta-N-acetylglucosaminidase from Streptomyces griseus. Roughly 20% (w/w) of the carbohydrates in this glycoprotein were thus released, and they were fractionated by high-performance liquid chromatography after reduction with KB3H4/NaBH4. The radioactive oligosaccharitol fractions obtained were analyzed by exoglycosidase digestion, by acetolysis, and, after permethylation, by fast atom-bombardment mass spectrometry, as well as by capillary gas-liquid chromatography/mass spectrometry following hydrolysis, reduction and peracetylation. Around 85% (mol/mol) of the endo-H-sensitive viral glycans were thus found to be oligomannosidic oligosaccharitols of size classes Man5GlcNAcOH, Man6GlcNAcOH, Man8GlcNAcOH, Man7GlcNAcOH, and Man9GlcNAcOH (in order of prevalence), and the major structural isomers of each size class were identified. About another 15% (mol/mol) of the oligosaccharitols were shown to be of the 'mixed type', comprising mainly four species in which the Man(alpha 1----6)-branch of the Man alpha 1----6 (Man alpha 1----3) Man beta 1----4GlcNAcOH core is substituted by one or two additional alpha-mannoses, while the Man(alpha 1----3)-branch carries an N-acetyllactosamine unit substituted by sialic acid, or by Gal(alpha 1----3).

Characterization of Crithidia fasciculata oligosaccharide-lipid and its elongation by bovine mammary microsomes

It was recently shown that a Man7(GlcNAc)2-lipid species serves as the precursor for the biosynthesis of asparagine-linked glycoproteins in the trypanosomatid Crithidia fasciculata. Preliminary results indicated it to be similar to the intermediate in the major pathway for the biosynthesis of lipid-linked Glc3Man9(GlcNAc)2 in animal systems. To explore the potential of this glycolipid as an acceptor for studying the biosynthesis of mammary glycoproteins, we conducted a detailed structural analysis of the labelled Man7(GlcNAc)2-lipid isolated from exponentially growing cells of C. fasciculata. The results showed its structure to be Man alpha 1----2Man alpha 1----2Man alpha 1----3(Man alpha 1----2Man alpha 1----3Man alpha 1----6)Man beta----GlcNAc beta----GlcNAc, identical to the nonasaccharide synthesized by the animal systems. Incubation of the Man7(GlcNAc)2-lipid with bovine mammary microsomes along with GDP-mannose or mannosyl-phosphodolichol elongates it to give Man9(GlcNAc)2-lipid having the same structure as the corresponding intermediate in animal systems. Inhibition of the elongation reaction by EDTA or amphomycin indicates that the intermediary formation of mannosyl-phosphodolichol is required for the incorporation of mannose residues into the nonasaccharide-lipid. Mannosyl.phosphoretinol failed to serve as a mannosyl donor in this reaction.

Structure of the major oligosaccharides in the fusion glycoprotein of Newcastle disease virus

The fusion glycoprotein (F0) was isolated from Newcastle disease virus (NDV) particles metabolically labelled with [2-3H]mannose; it was successively digested with protease and with endo-beta-N-acetylglucosaminidase from Streptomyces griseus. In this manner, the majority of the oligosaccharides in NDV F0 could be liberated. After reduction with NaBH4, they were separated by high-performance liquid chromatography, and were subjected to structural analysis. Using micromethylation/capillary gas chromatography/mass fragmentography, alpha-mannosidase digestion, and acetolysis, it was found that the enzymatically released NDV F0 oligosaccharides are common oligomannosidic glycoprotein glycans of size classes (Man)8GlcNAc, Man)7GlcNAc, (Man)6GlcNAc, (Man)9GlcNAc, and (Man)5GlcNAc (in order of prevalence). The major structural isomers present in the NDV F0 (Man)8GlcNAc to (Man)5GlcNAc fractions were shown to lack mannose residues D2, D1D2 or D2D3, D1D2D3, and CD1D2D3, respectively, of (Man)9GlcNAc.

Solubilization of mannosyltransferase activities for the biosynthesis of mammary glycoproteins. Elongation of tetrasaccharide-lipid to heptasaccharide-lipid by a solubilized enzyme preparation

The microsomal preparation from the lactating bovine mammary tissue was solubilized by treatment with nonionic detergent, NP-40, at a protein/detergent ratio of 1.5:1 and a detergent concentration of 0.5%. Following centrifugation at 147000 X g for 120 min, the supernatant fraction was incubated with labeled sugar nucleotides, GDP-Man and UDP-GlcNAc. It was found to synthesize a series of lipid-linked saccharides up to (Man)5-(GlcNAc)2. The solubilized glycosyltransferases retained up to about 60% of the activity after two weeks of storage at 4 degrees C. The biosynthesis of glycolipids was stimulated by a mixture of lipids obtained by extracting the mammary microsomes with CHCl3/CH3OH (2:1). A labeled lipid-linked tetrasaccharide of the structure Man alpha 1----3 Man beta----GlcNAc beta----GlcNAc was isolated by labeling baby hamster kidney cells with [2-3H]mannose under conditions of glucose starvation followed by extraction of the cells with CHCl3/CH3OH (2:1) and separation of the lipids by high-performance liquid chromatography. When this lipid-linked tetrasaccharide was incubated with the solubilized bovine mammary microsomes and GDP-Man, it was elongated to a lipid-linked heptasaccharide having the structure Man alpha 1----2Man alpha 1----2Man alpha 1----3(Man alpha 1----6)Man beta----GlcNAc beta----GlcNAc. The kinetics of the elongation reaction also revealed the intermediary formation of smaller amounts of lipid-linked pentasaccharide and hexasaccharide. The elongation reaction did not require any divalent metal ion and had a broad pH optimum between 6.8 and 7.6. The lack of inhibition of the elongation reaction by EDTA or amphomycin support earlier studies that GDP-Man rather than mannosylphosphoryldolichol, is the direct donor of mannosyl residues for the biosynthesis of glycolipids up to (Man)5(GlcNAc)2. Mannosylphosphorylretinol was ineffective as mannosyl donor for the elongation reaction.

Characterization of lipid-linked octa-, nona-, and decasaccharides formed during in vitro synthesis of mammary glycoproteins

The lipid-linked octa-, nona-, and decasaccharides, isolated from incubations of a membrane preparation from the lactating bovine mammary tissue with GDP-[14C]mannose and UDP-N-acetylglucosamine were subjected to mild acid hydrolysis and purified extensively by repeated gel filtration and paper chromatography. Structural characterization of the oligosaccharides containing six to eight mannose residues linked to an N,N'-diacetylchitobiose unit utilizing digestions with alpha-mannosidase, beta-mannosidase, endo-beta-N-acetylglucosaminidase, D, H, and L, acetolysis, and methylation analysis revealed the presence of several isomers within each size species. Supplementation of the incubations with 0.1 mM dolichol phosphate reduces the number of isomers within these oligosaccharides; the predominant isomers of saccharides from these incubations appear to be similar to the saccharides isolated from in vivo preparations of Chinese hamster ovary cells.

Characterization of lipid-linked octa- through undecasaccharides implicated in the biosynthesis of Saccharomyces cerevisiae mannoproteins

The lipid-linked oligosaccharide Glc3-Man9(GlcNAc)2 (Glc, glucose; Man, mannose; GlcNAc, N-acetylglucosamine) serves as a precursor for the biosynthesis of the inner core portion of the asparagine-linked polysaccharide of Saccharomyces cerevisiae mannoproteins. It has been shown previously that incubation of a microsomal preparation from this organism with UDP-N-acetylglucosamine and GDP-[14C]mannose gives rise to a series of lipid-linked oligosaccharides of the general structure Mann(GlcNAc)2, with n from 1 to 9. A structural characterization of Man1- to Man5(GlcNAc)2 oligosaccharides indicated that the major structures among these were identical to the intermediates proposed for the biosynthesis of animal glycoproteins (C. Prakash and I. K. Vijay, Biochemistry 21:4810-4818, 1982). In the present study, the structural characterization of the Man6- through Man9(GlcNAc)2 species was conducted. The Man6- through Man8(GlcNAc)2 species have two isomers, whereas Man9(GlcNAc)2 is monoisomeric. One isomer each of Man6- through Man8(GlcNAc)2 and the monoisomeric Man9(GlcNAc)2 are identical to the intermediates for the biosynthesis of asparagine-linked glycoproteins in animal systems. It is proposed that the steps of the lipid-linked assembly of the carbohydrate precursor for S. cerevisiae mannoproteins are identical to those of the major pathway in animal systems. A lack of acceptor substrate specificity by the mannosyltransferases, as observed with in vitro studies with animal systems, also might be responsible for the biosynthesis of multiple isomers reported here.

Biosynthesis of mammary glycoproteins. Structural characterization of lipid-linked glucosyloligosaccharides

The lipid-linked glucosyloligosaccharide intermediates for the biosynthesis of asparagine-linked glycoproteins in the lactating bovine mammary tissue were isolated from incubations of microsomes of this tissue with labeled sugar-nucleotides. These were shown to have the general structure (Glc)1 -3(Man)9(GlcNAc)2. A structural characterization indicated that glucose residues in these oligosaccharides are linked as Glc alpha 1 leads to 3, Glc alpha 1 leads to 3Glc alpha 1 leads to 3, and Glc1 leads to 2Glc alpha 1 leads to 3Glc alpha 1 leads to 3, to the tetramannosyl branch of (Man)9(GlcNAc)2 portion previously characterized as Man alpha 1 leads to 2Man alpha 1 leads to 2Man alpha 1 leads to 3(Man alpha 1 leads to 2Man alpha 1 leads to 6[Man alpha 1 leads to 2Man alpha 1 leads to 3]-Man alpha 1 leads to 6)Man beta 1 leads to 4(3)GlcNAc beta 1 leads to 4(3)GlcNAc [Vijay, I. K., Perdew, G. H., and Lewis, D. E. (1980) J. Biol. Chem. 255, 11210-11220]. The lipid-linked (Man)9(GlcNAc)2 appears to be the initial acceptor of the first glucosyl residue. Even though incubations of the bovine mammary tissue in vitro synthesize multiple isomers of a number of mannosyloligosaccharides, the glucosyloligosaccharides are all monoisomeric.