The oligosaccharide units of a human type L immunoglobulin M (Macroglobulin)

Membrane Glycoproteins of Enveloped Viruses

This chapter focuses on the recent information of the glycoprotein components of enveloped viruses and points out specific findings on viral envelopes. Although enveloped viruses of different major groups vary in size and shape, as well as in the molecular weight of their structural polypeptides, there are general similarities in the types of polypeptide components present in virions. The types of structural components found in viral membranes are summarized briefly in the chapter. All the enveloped viruses studied to date possess one or more glycoprotein species and lipid as a major structural component. The presence of carbohydrate covalently linked to proteins is demonstrated by the incorporation of a radioactive precursor, such as glucosamine or fucose, into viral polypeptides, which is resolved by sodium dodecyl sulfate (SDS) polyacrylamide gel electrophoresis. Enveloped viruses share many common features in the organization of their structural components, as indicated by several approaches, including electron microscopy, surface-labeling, and proteolytic digestion experiments, and the isolation of subviral components. The chapter summarizes the detailed structure of the glycoproteins of four virus groups: (1) influenza virus glycoproteins, (2) rhabdovirus G protein, (3) togavirus glycoprotein, and (4) paramyxovirus glycoproteins The information obtained includes the size and shape of viral glycoproteins, the number of polypeptide chains in the complete glycoprotein structure, and compositional data on the polypeptide and oligosaccharide portions of the molecules.

A membrane glycoprotein that accumulates intracellularly: cellular processing of the large glycoprotein of LaCrosse virus

The intracellular transport and certain posttranslational modifications of the large glycoprotein (G1) of LaCrosse virus (LAC) in BHK cells have been studied. G1 from released LAC virus was characterized by complex oligosaccharides (endo H-resistant) and covalently attached fatty acid. Only a small fraction of total cellular G1 was present on the baby hamster kidney cell surface. Cell-surface G1 contained complex oligosaccharides, while total G1 in infected cells contained largely unprocessed (endo H-sensitive) oligosaccharides. In addition, cell G1 contained significantly less fatty acid than virion-associated G1. Pulse-chase experiments showed that the oligosaccharides of G1 were processed to the complex from much more slowly than the oligosaccharides of the vesicular stomatitis virus (VSV) glycoprotein (G). In addition, transit of LAC G1 to the cell surface and into extracellular virions was two to three fold slower than the transit of VSV G. Thus LAC G1 accumulates intracellularly and is only slowly processed by intracellular processing enzymes. Treatment with monensin caused accumulation in the cell of a form of G1 with partial sensitivity toward endo H, suggesting that monensin may act to inhibit the glycosylation process directly.

Localization and overall structure of a mannose-rich glycopeptide from a pathologic immunoglobulin

The structure of a mannose-rich glycopeptide from a human pathological IgM has been investigated. It belongs to the group I (simple) glycopeptides and contains only mannose and N-acetylglucosamine residues in a molar ratio of 10:2. The structures of its oligosaccharide moiety and peptide chain have been determined: its molecular localization is specified and the relation between its biosynthesis and the oligosaccharide structure determine is discussed. Based on the alpha- and beta-mannosidase digestions and permethylation studies for the oligosaccharide moiety, and on the results obtained after sequential analysis of the peptide chain, the following structure is proposed for the mannose-rich IgM Du glycopeptide: (Formula: see text). The recovery of one molecule of this glycopeptide per molecule of heavy chain and the determination of the amino acid sequence have led us to locate this glycopeptide on asparagine 402 of the Fc portion of the heavy chain mu of IgM Du.

Carbohydrate components of influenza C virions

The carbohydrate components of influenza C virions grown in chicken kidney (CK) cells were analyzed by gel filtration following exhaustive digestion with Pronase. The [(3)H]glucosamine-labeled glycopeptides were larger and more heterogeneous than those of influenza A/WSN virions; three major size classes (G(1), G(2), and G(3)) were resolved. Treatment with Vibrio cholerae neuraminidase caused a decrease in size of G(1) and G(2), along with release of about 16% of the (3)H label. The released sugar components were identified as N-acetylneuraminic acid by thin-layer chromatography. Peak G(3) was highly labeled with [(3)H]mannose, whereas G(1) and G(2) contained lower levels of mannose. The three major viral glycoproteins gp88, gp65, and gp30 were isolated from sodium dodecyl sulfate-polyacrylamide gels, and their glycopeptide components were analyzed after Pronase digestion. The three size classes of glycopeptides were obtained from any of the three glycoproteins; however, the relative amounts of the three components varied among the glycoproteins. Host cell-derived components, which appear to be mucopolysaccharides and glycoproteins, were found associated with influenza C virions grown in CK cells. These components contained glycopeptides that were mainly of sizes similar to peak G(2) from influenza C virions. Previous studies have shown that influenza A/WSN virus grown in several cell types contained only two size classes of glycopeptides. Two size classes comparable to peaks G(2) and G(3) from influenza C virions were also observed in influenza A/WSN grown in CK cells. Thus the large G(1) glycopeptides appear to be characteristic of influenza C virions.

Biosynthesis of the oligosaccharides of influenza viral glycoproteins

Glycosylation of influenza viral glycoproteins was investigated by pulse-labeling of infected BHK21-F cells with radioactive sugar precursors and by cell fractionation and analysis of Pronase-digested viral glycopeptides by gel filtration. The results with short pulses of [3H]mannose suggested that the initial event in glycosylation is the en bloc transfer of oligomannosyl cores to viral glycoproteins associated with rough membranes. The molecular weight of the glycopeptides which represent the cores was estimated to be approximately 1600-2200. Some mannose residues appear to be subsequently removed from oligosaccharide cores. [3H]mannose-labeled glycopeptides obtained either from cells pulsed for brief periods or from rough membranes, which contain predominantly oligosaccharide cores, were sensitive to digestion by endo-p-N-acetylglucosaminidase H (endo-H). On the other hand, glycopeptides larger than oligosaccharide cores, which appeared during chases or after migration of viral glycoproteins from rough to smooth membranes, were resistant to endo-H treatment. The branched sugars (glucosamine, galactose, and fucose), which are contained only in the complex (type I) oligosaccharide chains of virions, appear to be added in a stepwise manner to the trimmed oligosaccharide cores primarily on smooth membranes. Mannoserich glycopeptides of virions (type II) are similar in size to oligosaccharide cores detected in infected cells and are totally sensitive to endo-H, suggesting that type II glycopeptides may represent oligomannosyl cores which escape trimming as well as addition of branched sugars. Comparison of glycopeptides of infected and uninfected BHK21-F cells suggests that influenza viral glycoproteins contain oligosaccharide chains similar in size to those of host cells except for the absence of sialic acid in viral glycoproteins. Further, we observed that intracytoplasmic membranes from infected cells contain much less sialic acid than those from uninfected cells, indicating that viral neuraminidase present in the interior of infected cells possesses enzymatic activity.

Carbohydrates of influenza virus. I. Glycopeptides derived from viral glycoproteins after labeling with radioactive sugars

The carbohydrate moiety of the influenza glycoproteins NA, HA(1), and HA(2) were analyzed by labeling with radioactive sugars. Analysis of glycopeptides obtained after digestion with Pronase indicated that there are at least two different types of carbohydrate side chains. The side chain of type I is composed of glucosamine, mannose, galactose, and fucose. It is found on NA, HA(1), and HA(2). The side chain of type II contains a high amount of mannose and is found only on NA and HA(2). The molecular weights of the corresponding glycopeptides obtained from virus grown in chicken embryo cells are 2,600 for type I and 2,000 for type II. The glycoproteins of virus grown in MDBK cells have a higher molecular weight than those of virus grown in chicken embryo cells, and there is a corresponding difference in the molecular weights of the glycopeptides. Under conditions of partial inhibition of glycosylation, virus particles were isolated that contained hemagglutinin with reduced carbohydrate content. Glycopeptide analysis indicated that this reduction is due to the lack of whole carbohydrate side chains and not to the incorporation of incomplete ones. This observation suggests that glycosylation of the viral glycoproteins involves en bloc transfer of the core sugars to the polypeptide chains.

Oligosaccharides of the glycoprotein of rabies virus

The number of oligosaccharide side chains on rabies virus glycoprotein (G-protein) was investigated. Analysis of glycopeptides obtained by protease digestion of desialated G-protein revealed three discrete glycopeptides. Comparison of the protease digestion products from desialated and from untreated G-protein indicated a heterogeneity among the glycopeptides in the sialic acid content. Two major tryptic glycopeptides were isolated from desialated rabies virus G-protein and analyzed after protease digestion; one contained two oligosaccharide side chains and the other contained a single oligosaccharide side chain.

Immediate glycosylation of Sindbis virus membrane proteins

The mechanism by which the membrane proteins of Sindbis virus are initially glycosylated during growth of the virus in chick cells was studied. The experiments suggest strongly that the two viral glycoproteins are glycosylated before release from the polysome, and that this glycosylation involves transfer of a large 1800 dalton oligosaccharide to the polypeptide chains. The donor of the oligosaccharide is most probably a lipid.

Serum glycoprotein-type sequence of monosaccharides in membrane glycoproteins of Semliki Forest virus

Semliki Forest virus was grown in BHK-21 cells and labelled in vivo with radioactive monosaccharides. The virus was disrupted with sodium dodecyl sulphate and the polypeptides were hydrolyzed with pronase. A mixture of type A glycopeptides (for nomenclature, see Johnson and Clamp (1971) Biochem. J. 123, 739-745) of the membrane glycoproteins E1 and E3 was isolated by gel filtration and subjected to sequential degradation with exo-glycosidases. The reduction in the apparent molecular weight and the cleavage of radioactive monosaccharides were monitored with gel filtration. The results suggest that the type A oligosaccharides have similar average structures and contain at the non-reducing terminus 3.4 mol of alpha-D-sialic acid and 0.7 mol of alpha-L-focose, folloled by 3.1 mol of beta-D-galactose, 4.2 mol of N-acetyl-beta-D-glucosamine, 0.7-1.5 mol of alpha-D-mannose, 0.5 mol of beta-D-mannose and 0.6-2.2 mol of N-acetyl-beta-D-glucosamine attached to 1.0 mol of N-acetylglucosamine resistant to N-acetyl-beta-D-glucosaminidase. This innermost monosaccharide unit, therefore, appears to be attached to the peptide. The peptides attached to this N-acetyl-glucosamine had an apparent molecular weight of 720+/-100. We propose the following average structure, compatible with most of our data, for the type A glycopeptides of Semliki Forest virus:.

Purification and composition of the proteins from Sindbis virus grown in chick and BHK cells

Procedures are described for the purification of the Sindbis virus structural proteins. The amino acid and carbohydrate compositions of the purified proteins are presented for virus grown in BHK-21/13 and chicken embryo cells. Glycoprotein E1 from virus grown in BHK cells is deficient in a mannose-rich glycopeptide found on that glycoprotein when virus is grown in chicken embryo cells. The complex glactose-containing glycopeptides appear similar for virus grown in both hosts. However, when virus is grown in BHK cells, both glycoproteins are enriched in those glycopeptides containing more sialic acid. Since the two viral glycoproteins are difficult to separate cleanly during purification, it is suggested that there may be strong, but noncovalent, interactions between glycoproteins E1 and E2. It is also suggested that there may be an interaction between glycoprotein E2 and a component of the nucleocapsid.

Protein-bound oligosaccharides of Semliki Forest virus

Semliki Forest virus was grown in BHK cells and labeled in vivo with radioactive monosaccharides. Pronase digests of the virus chromatographed on Bio-Gel P6 revealed glycopeptides of A-type and B-type. (For the nomenclature see Johnson, J. and Clamp, J.R. (1971) Biochem. J. 123, 739-745.) The former was labeled with [3H]fucose, [3H]galactose, [3H]mannose and [14C]glucosamine, the latter only with [3H]mannose and [14C]glucosamine. The three envelope glycoproteins E1, E2 and E3 were isolated by sodium dodecyl sulfate-polyacrylamide gel electrophoresis and subjected to pronase digestion. The glycoproteins E1 and E3 revealed glycopeptides of A-type. E2 revealed glycopeptides of B-type. E2 yielded additionally a glycopeptide (Mr3100) which was heavily labeled from [3H]galactose, but only marginally from [14C]glucosamine, [3H]fucose and [3H]mannose. Whether this glycopeptide belongs to the A-type or not remains uncertain. The apparent molecular weights of the A-type units measured by gel filtration were 3400 in E1 and 4000 in E3; the B-type unit of E2 had an apparent molecular weight of 2000. Combined with the findings of our earlier chemical analysis these data suggest that E1 and E3 contain on the average one A-type unit; E2 probably contains one 3100 dalton unit plus one or two B-type units.

Virus-dependent glycosylation

The oligosaccharides of the membrane glycoproteins of Sindbis virus, vesicular stomatitis virus, and Rous sarcoma virus were compared on the basis of apparent size and sugar composition. It appears that each virus acquires a different set of oligosaccharides during growth in a single type of cell.

Sindbis virus glycoproteins: effect of the host cell on the oligosaccharides

Sindbis virus was grown in four different host cells and the carbohydrate portions of the glycoproteins were analyzed. Sindbis virus grown in BHK-21 cells has more sialic acid and galactose than Sindbis virus grown in chicken embryo cells. In other respects the carbohydrates from virus grown in these two hosts are very similar. Sindbis virus grown either in chick cells transformed by Rous sarcoma virus or in BHK cells transformed by polyoma virus was also examined. In comparisons of virus from normal and transformed cells, differences in the amount of sialic acid were observed; but otherwise the carbohydrate structures appeared basically similar. The growth conditions used for the host cell also affected the degree of completion of the carbohydrate chains of the viral glycoproteins.

Glycoproteins of Sindbis virus: priliminary characterization of the oligosaccharides

The carbohydrate content of Sindbis virus was determined by gas chromatographic analysis. The two viral glycoproteins were found to be approximately 8% carbohydrate by weight. Mannose is the sugar present in the largest amount. Smaller amounts of glucosamine, galactose, sialic acid, and fucose were also detected. Each of the two viral glycoproteins appears to contain two structurally unrelated oligosaccharides. Two of the three Sindbis-specific glycoproteins found in infected chick cells were shown to contain short, unfinished oligosaccharides.