The effect of glycosylation inhibitors on the maturation and intracellular polypeptide synthesis induced by snowshoe hare bunyavirus

Analysis of glycoproteins of viruses in the family Bunyaviridae

The membrane glycoproteins (Gn and Gc) of viruses in the family Bunyaviridae form projections on the virion envelope and are involved in virus entry and eliciting protective immunity. The glycoproteins are modified by N-linked glycosylation and accumulate in the Golgi complex where virions mature and bud. In this chapter, we describe the methods that have been used in our laboratory for the study of the glycoproteins of Bunyamwera virus, the prototype of the family. The protocols cover the expression of viral glycoproteins, examination of intracellular localization by immnunofluorescent confocal microscopy, radiolabeling, immunoprecipitation, and SDS-PAGE analysis of the proteins, and the improved reverse genetic system to rescue recombinant viruses that contain mutations at N-linked glycosylation sites.

Key Golgi factors for structural and functional maturation of bunyamwera virus

Several complex enveloped viruses assemble in the membranes of the secretory pathway, such as the Golgi apparatus. Among them, bunyaviruses form immature viral particles that change their structure in a trans-Golgi-dependent manner. To identify key Golgi factors for viral structural maturation, we have purified and characterized the three viral forms assembled in infected cells, two intracellular intermediates and the extracellular mature virion. The first viral form is a pleomorphic structure with fully endo-beta-N-acetylglucosaminidase H (Endo-H)-sensitive, nonsialylated glycoproteins. The second viral intermediate is a structure with hexagonal and pentagonal contours and partially Endo-H-resistant glycoproteins. Sialic acid is incorporated into the small glycoprotein of this second viral form. Growing the virus in glycosylation-deficient cells confirmed that acquisition of Endo-H resistance but not sialylation is critical for the trans-Golgi-dependent structural maturation and release of mature viruses. Conformational changes in viral glycoproteins triggered by changes in sugar composition would then induce the assembly of a compact viral particle of angular contours. These structures would be competent for the second maturation step, taking place during exit from cells, that originates fully infectious virions.

The functions of oligosaccharide chains associated with influenza C viral glycoproteins. I. The formation of influenza C virus particles in the absence of glycosylation

The effect of a glycosylation inhibitor, tunicamycin (TM) on the replication of influenza C virus was investigated. Incorporation of [3H]-glucosamine into the gp88 glycoproteins of this virus was completely inhibited by TM at the concentrations higher than 0.25 microgram/ml. Under these conditions, the synthesis of internal proteins NP and M was shown in TM-treated cells but the synthesis of gp88 was not. The disappearance of gp88 was however accompanied with the appearance of two new polypeptides with molecular weights of 80,000 (T80) and 76,000 (T76). While T80 was identified by peptide mapping as a host cell protein whose synthesis was enhanced by TM, T76 was shown to correspond to a nonglycosylated form of gp88. Pulse-chase experiments revealed that there was no significant difference in the intracellular stability of T76 and gp88. Although TM depressed the production of infectious progeny virus greater than 100-fold, only a five-fold decrease was observed in the release of noninfectious physical particles, suggesting that glycosylation is not essential for the formation of influenza C virus particles. However, the virions from TM-treated cells had a lower buoyant density in isopycnic sucrose gradients and lacked surface proteins in either glycosylated or nonglycosylated form.

Ambisense RNA genomes of arenaviruses and phleboviruses

This chapter reviews the evidence that shows that arenaviruses and members of one genus of the Bunyaviridae (phleboviruses) have some proteins coded in subgenomic, viral-sense mRNA species and other proteins coded in subgenomic, viral-complementary mRNA sequences. This unique feature is discussed in relation to the implications it has on the intracellular infection process and how such a coding arrangement may have evolved. The chapter presents a list of the known members of the arenaviridae, their origins, and the vertebrate hosts from which isolates have been reported. It discusses the structural components, the infection cycle, and genetic attributes of arenaviruses. In order to determine how arenaviruses code for gene products, the S RNA species of Pichinde virus and that of a viscerotropic strain of LCM virus (LCM-WE) have been cloned into DNA and sequenced. The arenavirus S RNA is described as having an ambisense strategy, to denote the fact that both viral and viral-complementary sequences are used to make gene products. The chapter discusses the infection cycle, the structural and genetic properties of bunyaviridae member.

The complete sequence of the M RNA of snowshoe hare bunyavirus reveals the presence of internal hydrophobic domains in the viral glycoprotein

The complete sequence of the viral M RNA of snowshoe hare (SSH) bunyavirus has been determined. The RNA is 4527 nucleotides long (mol wt: 1.5 X 10(6), base composition: 27.5% A, 33.5% U, 17.7% G, 21.3% C), and has 3' and 5' terminal sequences that, depending on how they are arranged, are complementary for some 44 residues. The viral RNA codes in its viral-complementary sequence, for a single primary gene product (the viral glycoprotein) that is comprised of 1441 amino acids (162,391 Da), and is rich in cysteine residues but poor in potential asparagine-linked glycosylation sites. Like the SSH S RNA, the M viral-complementary 5' noncoding region is shorter than the 3' noncoding sequence (61 as opposed to 142 nucleotides). The different functions of the M RNA are discussed in relation to those of the S RNA of SSH virus. No other large open reading frames have been identified in either the viral, or viral-complementary, M RNA sequences. Examination of the sequence of the M gene product reveals the presence of an 18 residue amino terminal hydrophobic sequence (putative signal) and a much longer 32 amino acid carboxy proximal hydrophobic region that is followed by a terminal sequence rich in charged amino acids (12 out of 20 residues). The size and constitution of the carboxy end regions are consistent with a transmembranal and anchor function for the glycoprotein in the viral envelope. In addition to these terminal hydrophobic sequences, a localized internal region of the gene product contains several hydrophobic sequences, 15 to 29 amino acids in length. Their possible role in the morphogenesis of bunyaviruses that occurs in the Golgi cisternae of infected cells is discussed.

Protein synthesis in Rift Valley fever virus-infected cells

Rift Valley fever virus-induced protein synthesis was examined by polyacrylamide gel electrophoresis and fluorography. Five virus-induced polypeptides were detected, the nucleocapsid protein N, the nucleus-associated nonstructural protein NS1, the glycoproteins G1 and G2, and a protein of molecular weight 80K. The N, G1, G2, and 80K proteins were present in virion preparations. Sequential studies showed that NS1 accumulated in the nucleus as soon as it was formed and readily associated with nuclei partitioned from noninfected cells. The G1 and G2 proteins labelled with [3H]glucosamine and [3H]mannose. NS1 was shown to be the only virus-induced protein which was phosphorylated.

Effects of tunicamycin on rotavirus morphogenesis and infectivity

The functions of the two rotavirus glycoproteins were investigated by using tunicamycin and a variant of SA11 rotavirus having nonglycosylated VP7. Results showed that glycosylation of VP7 is not required for normal viral morphogenesis and infectivity and suggested that the nonstructural glycoprotein is involved in assembly of the outer capsid.

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.

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.

Tunicamycin resistant glycosylation of coronavirus glycoprotein: demonstration of a novel type of viral glycoprotein

Tunicamycin has different effects on the glycosylation of the two envelope glycoproteins of mouse hepatitis virus (MHV), a coronavirus. Unlike envelope glycoproteins of other viruses, the transmembrane glycoprotein El is glycosylated normally in the presence of tunicamycin. This suggests that glycosylation of El does not involve transfer of core oligosaccharides from dolichol pyrophosphate intermediates to asparagine residues, but may occur by 0-linked glycosylation of serine or threonine residues. Synthesis of the peplomeric glycoprotein E2 is not readily detectable in the presence of tunicamycin. Inhibition of N-linked glycosylation of E2 by tunicamycin either prevents synthesis or facilitates degradation of the protein moiety of E2. Radiolabeling with carbohydrate precursors and borate gel electrophoresis of glycopeptides show that different oligcsaccharide side chains are attached to El and E2. The two coronavirus envelope glycoproteins thus appear to be glycosylated by different mechanisms. In tunicamycin-treated cells, noninfectious virions lacking peplomers are formed at intracytoplasmic membranes and released from the cells. These virions contain normal amounts of nucleocapsid protein and glycosylated El, but lack E2. Thus the transmembrane glycoprotein El is the only viral glycoprotein required for the formation of the viral envelope or for virus maturation and release. The peplomeric glycoprotein E2 appears to be required for attachment to virus receptors on the plasma membrane. The coronavirus envelope envelope glycoprotein E1 appears to be a novel type of viral glycoprotein which is post-translationally glycosylated by a tunicamycin-resistant process that yields oligosaccharide side chains different from those of N-linked glycoproteins. These findings suggest that El may be particularly useful as a model for studying the biosynthesis, glycosylation, and intracellular transport of 0-linked glycoproteins.