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

Functions of alphavirus nonstructural proteins in RNA replication

Alphaviruses are enveloped positive-strand RNA viruses transmitted to vertebrate hosts by mosquitoes. Several alphaviruses are pathogenic to humans or domestic animals, causing serious central nervous system infections or milder infections, for example, arthritis, rash, and fever. The structure and replication of Semliki Forest virus (SFV) and Sindbis virus (SIN) have been studied extensively during the past 30 years. Alphaviruses have been important probes in cell biology to study the translation, glycosylation, folding, and transport of membrane glycoproteins, as well as endocytosis and membrane fusion mechanisms. A new organelle, the intermediate compartment, operating between the endoplasmic retieulum and the Golgi complex has been found by the aid of SFV. During the past 10 years, alphavirus replicons have been increasingly used as expression vectors for basic research, for the generation of vaccines, and for the production of recombinant proteins in industrial scale. The main approaches of laboratories in the recent years have been twofold. On one hand, they have discovered and characterized the enzymatic activities of the individual replicase proteins and on the other hand, they have studied the localization, membrane association, and other cell biological aspects of the replication complex.

Efficient transport of Semliki Forest virus glycoproteins through a Golgi complex morphologically altered by Uukuniemi virus glycoproteins

In infected BHK21 cells, the glycoproteins G1 and G2 of a temperature-sensitive mutant (ts12) of Uukuniemi virus (UUK) accumulate at 39 degrees C in the Golgi complex (GC) causing an expansion and vacuolization of this organelle. We have studied whether such an altered Golgi complex can carry out the glycosylation and transport to the plasma membrane (PM) of the Semliki Forest virus (SFV) glycoproteins in double-infected cells. Double-immunofluorescence staining showed that approximately 90% of the cells became infected with both viruses. Almost the same final yield of infectious SFV was obtained from double-infected cells as from cells infected with SFV alone. The rate of transport from the endoplasmic reticulum (ER) via the GC to the plasma membrane of the SFV glycoproteins was analysed by immunofluorescence, surface radioimmunoassay and pulse-chase labeling followed by immunoprecipitation, endoglycosidase H digestion and SDS-PAGE. The results showed that: the SFV glycoproteins were readily transported to the cell surface in double-infected cells, whereas the UUK glycoproteins were retained in the GC; the transport to the PM was retarded by approximately 20 min, due to a delay between the ER and the central Golgi; E1 of SFV appeared at the PM in a sialylated form. These results indicate that the morphologically altered GC had retained its functional integrity to glycosylate and transport plasma membrane glycoproteins.

Major carbohydrate structures at five glycosylation sites on murine IgM determined by high resolution 1H-NMR spectroscopy

Mouse myeloma immunoglobulin IgM heavy chains were cleaved with cyanogen bromide into nine peptide fragments, four of which contain asparagine-linked glycosylation. Three glycopeptides contain a single site, including Asn 171, 402, and 563 in the intact heavy chain. Another glycopeptide contains two sites at Asn 332 and 364. The carbohydrate containing fragments were treated with Pronase and fractionated by elution through Bio-Gel P-6. The major glycopeptides from each site were analyzed by 500 MHz 1H-NMR and the carbohydrate compositions determined by gas-liquid chromatography. The oligosaccharide located at Asn 171 is a biantennary complex and is highly sialylated. The amount of sialic acid varies, and some oligosaccharides contain alpha 1,3-galactose linked to the terminal beta 1,4-galactose. The oligosaccharides at Asn 332, Asn 364, an Asn 402 are all triantennary and are nearly completely sialylated on two branches and partially sialylated on the triantennary branch linked beta 1,4 to the core mannose. The latter is sialylated about 40% of the time for all three glycosylation sites. The major oligosaccharide located at Asn 563 is of the high mannose type. The 1H-NMR determination of structures at Asn 563 suggests that the high mannose oligosaccharide contains only three mannose residues.

Semliki Forest virus: a probe for membrane traffic in the animal cell

The traffic among the cellular compartments is thought to be mediated by membrane vesicles, which bud from one compartment and fuse with the next. Despite the continuous exchange of membrane components among them, the organelles maintain their characteristic protein and lipid compositions such that the traffic remains selective, thus, avoiding intermixing of components. This membrane traffic recycles components from the cell surface to the interior of the cell and back to the cell surface again. The membrane traffic between the ER and the cell surface involves a major sorting problem. Little is known of how the animal cell has solved this problem in molecular terms. One experimental tool in this direction is provided by some enveloped animal viruses, which mature at the cell surface of infected cells. Such viruses include influenza virus, Semliki Forest virus (SFV), Sindbis virus, and vesicular stomatitis virus (VSV). They are extremely simple in makeup and hence are very well characterized. The purpose of this article is to illustrate the use of the enveloped viruses as tools in the study of membrane traffic in the animal cell. This is done in the context of the life cycle of the virus in the host cell. The article will be concerned mainly with Semliki Forest virus (SFV), which is the virus that has been worked upon in the chapter. SFV belongs to the alphaviruses, a genus of the togavirus family.

Correlation of glycosylation forms with position in amino acid sequence

We surveyed published reports on about 50 glycoproteins whose amino acid sequence, glycosylation sites, and type of glycosylation at a particular site have been established. We note that high-mannose substances were rarely found at the N-terminal side of a previously glycosylated complex site. There was a very definite distribution of complex sites about the N-terminal region. Furthermore, secreted glycoproteins usually contained only complex oligosaccharides whereas membrane proteins contained both types. We suggest that the position of the glycosylation site with respect to the N-terminus affects the extent of oligosaccharide processing and subsequent presentation of complex or high-mannose structures in the mature glycoprotein. This review relates glycosylation type to its position in the known sequence of given proteins and discusses these observations in light of known glycosylation processing reactions.

Viral membrane proteins acquire galactose in trans Golgi cisternae during intracellular transport

Frozen, thin sections of baby hamster kidney (BHK) cells were incubated with either concanavalin A (Con A) or Ricinus communis agglutinin I (RCA) to localize specific oligosaccharide moieties in endoplasmic reticulum (ER) and Golgi membranes. These lectins were then visualized using an anti-lectin antibody followed by protein A conjugated to colloidal gold. All Golgi cisternae and all ER membranes were uniformly labeled by Con A. In contrast, RCA gave a uniform labeling of only half to three-quarters of those cisternae on the trans side of the Golgi stack; one or two cis Golgi cisternae and all ER membranes were essentially unlabeled. This pattern of lectin labeling was not affected by infection of the cells with Semliki Forest virus (SFV). Infected cells transport only viral spike glycoproteins from their site of synthesis in the ER to the cell surface via the stacks of Golgi cisternae where many of the simple oligosaccharids on the spike proteins are converted to complex ones (Green, J., G. Griffiths, D. Louvard, P. Quinn, and G. Warren. 1981. J. Mol. Biol. 152:663-698). It is these complex oligosaccharides that were shown, by immunoblotting experiments, to be specifically recognized by RCA. Loss of spike proteins from Golgi cisternae after cycloheximide treatment (Green et al.) was accompanied by a 50% decrease in the level of RCA binding. Hence, about half of the RCA bound to Golgi membranes in thin sections was bound to spike proteins bearing complex oligosaccharides and these were restricted to the trans part of the Golgi stack. Our results strongly suggest that complex oligosaccharides are constructed in trans Golgi cisternae and that the overall movement of spike proteins is from the cis to the trans side of the Golgi stack.

The biosynthetic pathway of the asparagine-linked oligosaccharides of glycoproteins

This review deals with the structure and addition of the different types of oligosaccharides to asparagine residues in proteins. This process occurs in several steps, first an oligosaccharide which contains N-acetylglucosamine mannose and glucose is built up joined to dolichyl diphosphate. The oligosaccharide is then transferred to a polypeptide chain, loses its glucose, and is modified by removal of some monosaccharides and addition of others giving rise to a variety of saccharides.

Monosaccharide sequence of protein-bound glycans of Uukuniemi virus

Uukuniemi virus, a member of the Bunyaviridae family, was grown in BHK-21 cells in the presence of [(3)H]mannose. The purified virions were disrupted with sodium dodecyl sulfate and digested with pronase. The [(3)H]mannose-labeled glycopeptides of the mixture of the two envelope glycoproteins G1 and G2 were characterized by degrading the glycans with specific exo-and endoglycosidases, by chemical methods, and by analyzing the products with lectin affinity and gel chromatography. The glycopeptides of Uukuniemi virus fell into three categories: complex, high-mannose type, and intermediate. The complex glycopeptides probably contained mainly two NeuNAc-Gal-GlcNAc branches attached to a core (Man)(3)(GlcNAc)(2) peptide. The high-mannose-type glycans were estimated to contain at least five mannose units attached to two N-acetylglucosamine residues. Both glycan species appeared to be similar to the asparagine-linked oligosaccharides found in many soluble and membrane glycoproteins. The results suggested that the intermediate glycopeptides contained a mannosyl core. In about half of the molecules, one branch appeared to be terminated in mannose, and one appeared to be terminated in N-acetylglucosamine. Such glycans are a novel finding in viral membrane proteins. They may represent intermediate species in the biosynthetic pathway from high-mannose-type to complex glycans. Their accumulation could be connected with the site of maturation of the members of the Bunyaviridae family. Electron microscopic data suggest that the virions bud into smooth-surfaced cisternae in the Golgi region. The relative amounts of [(3)H]mannose in the complex, high-mannose-type, and intermediate glycans were 25, 62, and 13%, respectively, which corresponded to the approximate relative number of oligosaccharide chains of 2:2.8:1, respectively, in the roughly equimolar mixture of G1 and G2. Endoglycosidase H digestion of isolated [(35)S]methionine-labeled G1 and G2 proteins suggested that most of the complex and intermediate chains were attached to G1 and that most of the high-mannose-type chains were attached to G2.

Inhibition of Sindbis virus maturation after treatment of infected cells with trypsin

Brief treatment of Sindbis virus-infected BHK-21 or Vero cells with low concentrations of trypsin irreversibly blocked further production of progeny virions after removal of the enzyme. The inhibitory effects of the trypsin treatment could only be demonstrated in cells in which virus infection was established; optimal inhibition occurred at ca. 3 h postinfection. Production of virus structural proteins PE2, E1, and C occurred at normal levels in inhibited cells. PE2 and E1 were also transported to the cell plasma membrane during inhibition; however, PE2 was not cleaved to E2, and little capsid protein became membrane associated relative to control cells. Although trypsin treatment had no effect on Sindbis protein synthesis, the production of both 26S and 42S RNA was greatly reduced. Similar trypsin treatment of BHK cells infected with vesicular stomatitis virus had no detectable effect on the course of virus infection.

Acylation of viral spike glycoproteins: a feature of enveloped RNA viruses

The covalent attachment of fatty acids to the glycoproteins of orthomyxo-, paramyxo, alpha-, and coronavirus was studied. All enveloped viruses analyzed afford covalently bound fatty acid in at least one species of their spike glycoproteins. No internal components of the viruses studied including the hydrophobic M proteins of myxo- and rhabdoviruses contained fatty acid. Analysis of myxovirus particles devoid of the exposed portions of their spikes revealed that fatty acids are linked to the hydrophobic tail fragment of the glycoprotein which is associated with the viral lipid bilayer. With influenza virus hemagglutinin the fatty acid attachment site could be located at the cyanogen bromide peptide of the small subunit (HA₂) which contains the membrane-embedded region of the polypeptide. The binding of fatty acids to viral glycoproteins occurs in a wide range of host cells including mammalian, avian, and insect cells.

Evidence for a separate signal sequence for the carboxy-terminal envelope glycoprotein E1 of Semliki forest virus

When Semliki Forest virus temperature-sensitive mutant ts-3 was grown at the restrictive temperature an aberrant nascent cleavage of the 130,000-dalton structural polyprotein took place relatively frequently. This cleavage yielded an abnormal 86,000-dalton fusion protein (p86) consisting of the amino-terminal capsid protein linked to the amino acid sequences of envelope protein p62 (a precursor of E3 and E2). The other cleavage product was the carboxy-terminal envelope protein E1. p86 was not glycosylated and was sensitive to the action of protease in the microsomal fraction, whereas E1 was glycosylated and protected from proteases, indicating that it had been segregated into the cysternal side of the microsomal vesicles. All attempts to show the E1 protein at the cell surface have failed so far, suggesting that it remains associated with intracellular membranes. When ts-3-infected cells labeled at the restrictive temperature were shifted to the permissive temperature the only labeled protein released with the virus particles was E1, indicating that E1, synthesized at the restrictive temperature, was competent to participate in the virus assembly. These results suggest strongly that there are two separate signal sequences for the envelope proteins of Semliki Forest virus. One follows the capsid protein as shown previously, and the other is for the carboxy-terminal E1. Even if the insertion of the amino-terminal envelope protein (p62) fails due to a cleavage defect, the other signal sequence can operate independently to guide the E1 through the endoplasmic reticulum membrane.

Nucleotide sequence of cdna coding for Semliki Forest virus membrane glycoproteins

The genes coding for the three membrane polypeptides of Semliki Forest virus have been sequenced and the primary structures of the proteins deduced. The amino acid sequence gives further insight into how the transmembrane structure of the three-chain virus membrane glycoprotein is generated in the infected cell.

Semliki Forest virus glycans analyzed by affinity chromatography, hydrazinolysis and paper chromatography

N-acetyl-lactosamine type glycopeptides of Semliki Forest virus were fractionated on concanavalin A-Sepharose, and their oligosaccharides were liberated by hydrazinolysis and analyzed by paper chromatography. Comparison with labeled glycans from reference glycopeptides suggests that the viral N-acetyl-lactosamine type glycans are bi-, tri- and tetra-antennary.

Core tetrasaccharide liberated by endo-beta-D-N-acetylglucosaminidase D from lactosamine-type oligosaccharides of Semliki Forest virus membrane proteins

[3H]Mannose- and [3H]glucosamine-labeled lactosamine-type glycopeptides of Semliki Forest virus membrane proteins were stripped of their fucose, sialic acid, galactose and distal N-acetylglucosamine residues and subsequently digested with endo-beta-D-N-acetylglucosaminidase D from Diplococcus pneumoniae. Two products were obtained, a neutral tetrasaccharide and a residual glycopeptide fraction. The tetrasaccharide appeared to consist of two alpha-mannose residues, one beta-mannose residue and one N-acetylglucosamine residue located at the reducing terminus of the molecule. Results of Smith degradation, beta-elimination and acetolysis were compatible with four structures; (1) Man alpha-1-3[Man alpha 1-6]Man beta 1-4GlcNAc; (2) Man alpha 1-3Man beta 1-4[Man alpha 1-6] GlcNAc; (3) Man alpha 1-3Man alpha 1-4[Man beta 1-6]GlcNAc, or (4) Man alpha 1-6Man alpha 1-3Man beta-1-4GlcNAc. The reactivity of the viral glycopeptides with endo-beta-D-N-acetylglucosaminidase D and the chromatographic properties of the liberated core tetrasaccharide suggest that its most likely structure was Man alpha 1-3[Man alpha-1-6]Man beta 1-4GlcNAc. The core tetrasaccharide of glycans of membrane protein E3, one of the viral membrane proteins obtained from infected cell, was similar to that of the virion glycans.

Separation of the integral membrane glycoproteins E1 and E2 of Semliki Forest virus by affinity chromatography on concanavalin A-Sepharose

The membrane glycoproteins E1 and E2 of Semliki Forest virus are of about equal size but can be separated from each other by affinity chromatography on a concanavalin A-Sepharose column in the presence of sodium dodecyl sulfate. The E1 protein eluted like glycopeptides containing two peripheral sugar branches composed of N-acetylglucosamine, mannose, galactose and sialic acid. The E2 eluted like glycopeptides containing only N-acetylglucosamine and mannose.

The molecular size of glycans liberated by hydrazinolysis from Semliki Forest virus proteins

The glycans of well characterized, [6-3H]galactose-labelled glycopeptides, GC-4 from bovine IgG1 as well as GP-V-2 and GP-V-5 from alpha1-acid glycoprotein, were liberated by hydrazinolysis. Molecular weights close to the expected values were observed by gel filtration. Desialated glycans of Semliki Forest virus proteins were likewise liberated by hydrazinolysis and subjected to gel filtration. Metabolically labelled [1-3H]galactose-oligosaccharides of the mixed viral proteins revealed an apparent molecular weight of 1800. The bi-antennary glycan liberated from the reference glycopiptide GC-4 was of 1750 daltons. A mixture of [2-3H]mannose-labelled E1- and E2-proteins of the virus contained L-type glycans of 1800 daltons (formerly called A-type), and M-type glycans of 1200 daltons (formerly called B-type). A fraction of the E3-glycans isolated by affinity chromatography on Concanavlin A-Sepharose showed an average molecular weight of 2150, a value intermediate between the three- and four-antennary glycans liberated from the reference glycopeptides GP-V-5 and GP-V-2. The rest of the E3-glycans were of 1850 daltons, a value close to the bi-antennary GC-4 glycan. We suggest that the comparatively large size of the E3-glycans and the exposed position of E3-proteins on the viral surface may be interrelated.