Immediate glycosylation of Sindbis virus membrane proteins

Current challenges in the discovery of treatments against Mayaro fever

INTRODUCTION

Mayaro fever is an emerging viral disease that manifests as an acute febrile illness. The disease is self-limiting, however joint pain can persist for months leading to chronic arthralgia. There is no specific treatment available, which ultimately leads to socioeconomic losses in populations at risk as well as strains to the public health systems.

AREAS COVERED

We reviewed the candidate treatments proposed for Mayaro virus (MAYV) infection and disease, including antiviral compounds targeting viral or host mechanisms, and pathways involved in disease development and pathogenicity. We assessed compound screening technologies and experimental infection models used in these studies and indicated the advantages and limitations of available technologies and intended therapeutic strategies.

EXPERT OPINION

Although several compounds have been suggested as candidate treatments against MAYV infection, notably those with antiviral activity, most compounds were assessed only in vitro. Compounds rarely progress toin vivo or preclinical studies, and such difficulty may be associated with limited experimental models. MAYV biology is largely inferred from related alphaviruses and reflected by few studies focusing on target proteins or mechanisms of action for MAYV. Therapeutic strategies targeting pathogenic inflammatory responses have shown potential against MAYV-induced disease in vivo, which might reduce long-term sequelae.

Alphavirus-Induced Membrane Rearrangements during Replication, Assembly, and Budding

Alphaviruses are arthropod-borne viruses mainly transmitted by hematophagous insects that cause moderate to fatal disease in humans and other animals. Currently, there are no approved vaccines or antivirals to mitigate alphavirus infections. In this review, we summarize the current knowledge of alphavirus-induced structures and their functions in infected cells. Throughout their lifecycle, alphaviruses induce several structural modifications, including replication spherules, type I and type II cytopathic vacuoles, and filopodial extensions. Type I cytopathic vacuoles are replication-induced structures containing replication spherules that are sites of RNA replication on the endosomal and lysosomal limiting membrane. Type II cytopathic vacuoles are assembly induced structures that originate from the Golgi apparatus. Filopodial extensions are induced at the plasma membrane and are involved in budding and cell-to-cell transport of virions. This review provides an overview of the viral and host factors involved in the biogenesis and function of these virus-induced structures. Understanding virus-host interactions in infected cells will lead to the identification of new targets for antiviral discovery.

Dissecting the Components of Sindbis Virus from Arthropod and Vertebrate Hosts: Implications for Infectivity Differences

Sindbis virus (SINV) is an enveloped, single-stranded RNA virus, which is transmitted via mosquitos to a wide range of vertebrate hosts. SINV produced by vertebrate, baby hamster kidney (BHK) cells is more than an order of magnitude less infectious than SINV produced from mosquito (C6/36) cells. The cause of this difference is poorly understood. In this study, charge detection mass spectrometry was used to determine the masses of intact SINV particles isolated from BHK and C6/36 cells. The measured masses are substantially different: 52.88 MDa for BHK derived SINV and 50.69 MDa for C6/36 derived. Further analysis using several mass spectrometry-based methods and biophysical approaches indicates that BHK derived SINV has a substantially higher mass than C6/36 derived because in the lipid bilayer, there is a higher portion of lipids containing long chain fatty acids. The difference in lipid composition could influence the organization of the lipid bilayer. As a result, multiple stages of the viral lifecycle may be affected including assembly and budding, particle stability during transmission, and fusion events, all of which could contribute to the differences in infectivity.

A compendium of small molecule direct-acting and host-targeting inhibitors as therapies against alphaviruses

Alphaviruses were amongst the first arboviruses to be isolated, characterized and assigned a taxonomic status. They are globally widespread, infecting a large variety of terrestrial animals, birds, insects and even fish. Moreover, they are capable of surviving and circulating in both sylvatic and urban environments, causing considerable human morbidity and mortality. The re-emergence of Chikungunya virus (CHIKV) in almost every part of the world has caused alarm to many health agencies throughout the world. The mosquito vector for this virus, Aedes, is globally distributed in tropical and temperate regions and capable of thriving in both rural and urban landscapes, giving the opportunity for CHIKV to continue expanding into new geographical regions. Despite the importance of alphaviruses as human pathogens, there is currently no targeted antiviral treatment available for alphavirus infection. This mini-review discusses some of the major features in the replication cycle of alphaviruses, highlighting the key viral targets and host components that participate in alphavirus replication and the molecular functions that were used in drug design. Together with describing the importance of these targets, we review the various direct-acting and host-targeting inhibitors, specifically small molecules that have been discovered and developed as potential therapeutics as well as their reported in vitro and in vivo efficacies.

Low glucose depletes glycan precursors, reduces site occupancy and galactosylation of a monoclonal antibody in CHO cell culture

Controlled feeding of glucose has been employed previously to enhance the productivity of recombinant glycoproteins but there is a concern that low concentrations of glucose could limit the synthesis of precursors of glycosylation. Here we investigate the effect of glucose depletion on the metabolism, productivity and glycosylation of a chimeric human-llama monoclonal antibody secreted by CHO cells. The cells were inoculated into media containing varying concentrations of glucose. Glucose depletion occurred in cultures with an initial glucose ≤5.5 mM and seeded at low density (2.5 × 10(5) cells/mL) or at high cell inoculum (≥2.5 × 10(6) cells/mL) at higher glucose concentration (up to 25 mM). Glucose-depleted cultures produced non-glycosylated Mabs (up to 51%), lower galactosylation index (GI <0.43) and decreased sialylation (by 85%) as measured by mass spectrometry and HPLC. At low glucose a reduced intracellular pool of nucleotides (0.03-0.23 fmoles/cell) was measured as well as a low adenylate energy charge (<0.57). Low glucose also reduced GDP-sugars (by 77%) and UDP-hexosamines (by 90%). The data indicate that under glucose deprivation, low levels of intracellular nucleotides and nucleotide sugars reduced the availability of the immediate precursors of glycosylation. These results are important when applied to the design of fed-batch cultures.

Fundamental difference in the content of high-mannose carbohydrate in the HIV-1 and HIV-2 lineages

The virus-encoded envelope proteins of human immunodeficiency virus (HIV) and simian immunodeficiency virus (SIV) typically contain 26 to 30 sites for N-linked carbohydrate attachment. N-linked carbohydrate can be of three major types: high mannose, complex, or hybrid. The lectin proteins from Galanthus nivalis (GNA) and Hippeastrum hybrid (HHA), which specifically bind high-mannose carbohydrate, were found to potently inhibit the replication of a pathogenic cloned SIV from rhesus macaques, SIVmac239. Passage of SIVmac239 in the presence of escalating concentrations of GNA and HHA yielded a lectin-resistant virus population that uniformly eliminated three sites (of 26 total) for N-linked carbohydrate attachment (Asn-X-Ser or Asn-X-Thr) in the envelope protein. Two of these sites were in the gp120 surface subunit of the envelope protein (Asn244 and Asn460), and one site was in the envelope gp41 transmembrane protein (Asn625). Maximal resistance to GNA and HHA in a spreading infection was conferred to cloned variants that lacked all three sites in combination. Variant SIV gp120s exhibited dramatically decreased capacity for binding GNA compared to SIVmac239 gp120 in an enzyme-linked immunosorbent assay (ELISA). Purified gp120s from six independent HIV type 1 (HIV-1) isolates and two SIV isolates from chimpanzees (SIVcpz) consistently bound GNA in ELISA at 3- to 10-fold-higher levels than gp120s from five SIV isolates from rhesus macaques or sooty mangabeys (SIVmac/sm) and four HIV-2 isolates. Thus, our data indicate that characteristic high-mannose carbohydrate contents have been retained in the cross-species transmission lineages for SIVcpz-HIV-1 (high), SIVsm-SIVmac (low), and SIVsm-HIV-2 (low).

A structural and functional perspective of alphavirus replication and assembly

Alphaviruses are small, spherical, enveloped, positive-sense ssRNA viruses responsible for a considerable number of human and animal diseases. Alphavirus members include Chikungunya virus, Sindbis virus, Semliki Forest virus, the western, eastern and Venezuelan equine encephalitis viruses, and the Ross River virus. Alphaviruses can cause arthritic diseases and encephalitis in humans and animals and continue to be a worldwide threat. The viruses are transmitted by blood-sucking arthropods, and replicate in both arthropod and vertebrate hosts. Alphaviruses form spherical particles (65-70 nm in diameter) with icosahedral symmetry and a triangulation number of four. The icosahedral structures of alphaviruses have been defined to very high resolutions by cryo-electron microscopy and crystallographic studies. In this review, we summarize the major events in alphavirus infection: entry, replication, assembly and budding. We focus on data acquired from structural and functional studies of the alphaviruses. These structural and functional data provide a broader perspective of the virus lifecycle and structure, and allow additional insight into these important viruses.

Teaching dolichol-linked oligosaccharides more tricks with alternatives to metabolic radiolabeling

The dolichol cycle involves synthesis of the lipid-linked oligosaccharide (LLO) Glc(3)Man(9)GlcNAc(2)-P-P-dolichol (G(3)M(9)Gn(2)-P-P-Dol), transfer of G(3)M(9)Gn(2) to asparaginyl residues of nascent endoplasmic reticulum (ER) polypeptides by oligosaccharyltransferase (OT), and recycling of the resultant Dol-P-P to Dol-P for new rounds of LLO synthesis. The importance of the dolichol cycle in secretory and membrane protein biosynthesis, ER function, and human genetic disease is now widely accepted. Elucidation of the fundamental properties of the dolichol cycle in intact cells was achieved through the use of radioactive sugar precursors, typically [(3)H]-labeled or [(14)C]-labeled d-mannose, d-galactose, or d-glucosamine. However, difficulties were encountered with cells or tissues not amenable to metabolic labeling, or in experiments influenced by isotope dilution, variable rates of LLO turnover, or special culture conditions required for the use of radioactive sugars. This article will review recently developed alternatives for LLO analysis that do not rely upon metabolic labeling with radioactive precursors, and thereby circumvent these problems. New information revealed by these methods with regard to regulation, genetic disorders, and evolution of the dolichol cycle, as well as caveats of radiolabeling techniques, will be discussed.

Locations of carbohydrate sites on alphavirus glycoproteins show that E1 forms an icosahedral scaffold

There are 80 spikes on the surface of Sindbis virus arranged as an icosahedral surface lattice. Each spike consists of three copies of each of the glycoproteins E1 and E2. There are two glycosylation sites on E1 and two on E2. These four sites have been located by removal of the glycosylation recognition motifs using site-specific mutagenesis, followed by cryoelectron microscopy. The positions of these sites have demonstrated that E2 forms the protruding spikes and that E1 must be long and narrow, lying flat on the viral surface, forming an icosahedral scaffold analogous to the arrangement of the E glycoprotein in flaviviruses. This arrangement of E1 leads to both dimeric and trimeric intermolecular contacts, consistent with the observed structural changes that occur on fusion with host cell membranes, suggesting a similar fusion mechanism for alpha- and flaviviruses.

The alphaviruses: gene expression, replication, and evolution

The alphaviruses are a genus of 26 enveloped viruses that cause disease in humans and domestic animals. Mosquitoes or other hematophagous arthropods serve as vectors for these viruses. The complete sequences of the +/- 11.7-kb plus-strand RNA genomes of eight alphaviruses have been determined, and partial sequences are known for several others; this has made possible evolutionary comparisons between different alphaviruses as well as comparisons of this group of viruses with other animal and plant viruses. Full-length cDNA clones from which infectious RNA can be recovered have been constructed for four alphaviruses; these clones have facilitated many molecular genetic studies as well as the development of these viruses as expression vectors. From these and studies involving biochemical approaches, many details of the replication cycle of the alphaviruses are known. The interactions of the viruses with host cells and host organisms have been exclusively studied, and the molecular basis of virulence and recovery from viral infection have been addressed in a large number of recent papers. The structure of the viruses has been determined to about 2.5 nm, making them the best-characterized enveloped virus to date. Because of the wealth of data that has appeared, these viruses represent a well-characterized system that tell us much about the evolution of RNA viruses, their replication, and their interactions with their hosts. This review summarizes our current knowledge of this group of viruses.

Asparagine-linked oligosaccharides of Semliki Forest virus grown in mosquito cells

The structure of the N-linked oligosaccharides of Semliki Forest viral glycoproteins produced in infected mosquito cells (C6/36) was investigated by biosynthetic labeling, enzymic deglycosylation using endo-beta-N-acetylglucosaminidases H, D, F/glycopeptidase F, exoglycosidase and analysis of the sugars on Concanavalin A-Sepharose columns and by gel filtration chromatography. The results demonstrated that the glycoproteins decorating the virus shed from infected cells have N-linked glycans with a trimannosyl core similar to the core glycans produced by vertebrate and yeast cells. However, the E1 glycoprotein produced by infected C6/36 cells exhibited both a trimannosyl core and a modified trimannosyl core most probably with terminal N-acetylglucosamine. The carbohydrate side chains of Semliki Forest envelope proteins displayed two types of structural heterogeneities existing either at different N-glycosylation sites as in the case of E2, or at the same N-glycosylation site as in the case of E1. In the presence of 1-deoxymannojirimycin, no structural heterogeneities in the glycan chains were found. This strongly suggests that the glycosylation events that lead to the observed sugar heterogeneities occur in the Golgi membranes.

Intracellular transport and processing of Sindbis virus glycoproteins

The intracellular transport and processing of Sindbis virus envelope glycoproteins were studied in cells infected with Sindbis virus using the mannose-specific enzyme, endoglycosidase H (endo H). In pulse/chase labeling experiments of hamster cells with [35S]methionine, Sindbis glycoproteins PE2 and E1 became endo H resistant in two steps at 12.5 and 20.0 min after a 5-min pulse, suggesting that the glycoproteins required this period of time to be transported to the Golgi compartments containing the enzymes which process the high mannose side chains acquired in the endoplasmic reticulum. E2 could be detected at the end of a 5-min pulse and the E2 produced early was found to be endo H sensitive. The rate at which PE2 was converted to E2, relative to the acquisition of endo H resistance, suggests the independence of this proteolytic event from cellular protein transport and raises the possibility that the proteolytic function is of viral origin.

The proteolytic cleavage of PE2 to envelope glycoprotein E2 is not strictly required for the maturation of Sindbis virus

The ionophore monensin has been shown previously to block the maturation of Sindbis virus as well as prevent the cleavage of pE2 to E2 when applied to cells in high concentration. We found that a moderate dose of monensin reduced virus titer and inhibited the cleavage of pE2 to E2. Under these conditions, pE2 appeared on the cell surface in a form susceptible to lactoperoxidase-mediated iodination. This pE2 was incorporated into virions, replacing E2. PE2-containing virions had a normal PFU-to-particle ratio, cosedimented with normal virus, and retained a normal morphology when negatively stained preparations were examined by electron microscopy. We conclude that the cleavage of pE2 to form E2 is not an absolute prerequisite for virus maturation. Recently, Russell et al. have reached a similar conclusion (D. L. Russell, J. M. Dalrymple, and R. E. Johnston, J. Virol. 63:1619-1629, 1989).

Molecular pathogenesis of Sindbis virus encephalitis in experimental animals

In general, the analysis of a number of strains of Sindbis virus has revealed amino acid differences of potential importance for virulence at relatively few positions in the E2-glycoprotein. Only 10 amino acid changes are potentially implicated, and 9 of these 10 lie in the N-terminal half of the protein (Fig. 1.). Currently, there is strong evidence to implicate 3 of these positions (E2-55, -114, and -172) in virulence (Table V). As more recombinant viruses are prepared and analyzed, the evidence for or against the relevance of other changes should become apparent. As is generally true in alphaviruses, the E1 gene is more invariant than E2 and analysis of several strains has revealed amino acid changes at only four positions (Fig. 2). Two, 72, and 75, are just N-terminal to the hydrophobic segment postulated to be the site of fusion activity, suggesting the possibility that virus entry into the host cell could be affected by amino acid differences at these locations. The other two changes (at 237 and 313) are distant from the fusion site on the linear molecule, but changes at 313 do affect the pH fusion suggesting participation of this site in providing stability to the glycoprotein trimers. The mechanism of altered virulence associated with any amino acid change in the E1- or E2-glycoproteins has yet to be determined. The change at E2-114 associated with reduced virulence in mice shows reduced latency and increased virulence in BHK-21 cells in vitro. This suggests that some changes result in enhanced replication that is host cell-specific. There are several points in the replication cycle of Sindbis virus where the glycoproteins and their ability to undergo conformational changes play an important role in efficiency of replication. These include attachment, fusion, transport through the Golgi, assembly, and budding from the cell surface. Some steps in replication involve host cell proteins (Baric et al., 1983), so that there may be unique, unexplored interactions with neurons or ependymal cells leading to increased neurovirulence for mice that are not represented in the typical BHK, Vero, or chick embryo fibroblast cell culture system. The task now will be to determine why specific amino changes in the proteins of Sindbis virus cause such dramatic changes in the biological properties of the virus.

Effect of actinomycin D and cycloheximide on replication of Sindbis virus in Aedes albopictus (mosquito) cells

Production of Sindbis virus in the presence of transcription and translation inhibitors was examined in three Aedes albopictus cell lines. Addition of cycloheximide to heat-resistant Sindbis virus (SVHR)-infected mosquito cells arrested viral RNA synthesis completely, in contrast to the effects of this drug on virus-infected vertebrate cells. Production of mature virus by both SVHR (a variant commonly used as a wild-type virus) and SBamr (a mutant which is resistant to the effects of 18 h of pretreatment of vertebrate cells with actinomycin D) in mosquito u4.4, C6-36, and C7-10 cells was inhibited by 2 h of pretreatment with actinomycin D. Pretreatment with this drug for 2 h slightly enhances virus production in vertebrate cells. Treatment of mosquito cells with actinomycin D resulted in shutoff of SVHR RNA synthesis. The mutant SBamr was able to overcome the effects of actinomycin D on viral RNA synthesis and produced both 26S and 49S RNAs, even though no viral structural proteins or mature particles were produced in the presence of the drug. This result suggests that, in the presence of actinomycin, the nonstructural genes of SBamr are translated sufficiently to allow for RNA synthesis but that 26S RNA may not be translated to an extent that allows significant virus production. These data demonstrate that host components are involved in at least two distinct steps in the production of Sindbis virus in mosquito cells: (i) production of viral RNA and (ii) synthesis of viral structural polypeptides.

Monoclonal antibody to the amino-terminal L sequence of murine leukemia virus glycosylated gag polyproteins demonstrates their unusual orientation in the cell membrane

To analyze cell surface murine leukemia virus gag protein expression, we have prepared monoclonal antibodies against the spontaneous AKR T lymphoma KKT-2. One of these antibodies, 43-13, detects an AKR-specific viral p12 determinant. A second monoclonal antibody, 43-17, detects a novel murine leukemia virus-related antigen found on glycosylated gag polyproteins (gp95gag, gp85gag, and gp55gag) on the surface of cells infected with and producing ecotropic endogenous viruses, but does not detect antigens within these virions. The 43-17 antibody immunoprecipitates the precursor of the cell surface gag protein whether in its glycosylated or unglycosylated state, but does not detect the cytoplasmic precursor of the virion gag proteins (Pr65gag). Based on these findings, we have localized the 43-17 determinant to the unique amino-terminal part of the glycosylated gag polyprotein (the L domain). We have determined that gp95gag contains L-p15-p12-p30-p10 determinants, whereas gp85gag lacks the carboxyterminal p10 determinant, and gp55gag lacks both p30 and p10 carboxy terminal determinants. Analysis of cell surface gag expression with the 43-17 antibody leads us to propose that the L domain plays a crucial role in (i) the insertion and orientation of murine leukemia virus gag polyproteins in the cell membrane and (ii) the relative abundance of expression of AKR leukemia virus versus Moloney murine leukemia virus glycosylated gag polyproteins in infected cells.

Maturation of measles virus hemagglutinin glycoprotein

The processing of measles virus hemagglutinin glycoprotein (H) in infected cells was studied by pulse-chase method and two-dimensional isoelectric focusing and SDS-polyacrylamide slab gel electrophoresis. H glycoprotein was synthesized initially as polypeptides smaller than H glycoprotein present in the virions. They were then processed into a cohort of polypeptides of larger molecular size and with reduced charge. The change was associated with the expression of H glycoprotein on the cell surface. The removal of sialic acid from carbohydrate chain of H glycoprotein resulted in the shift of isoelectric point to a more basic range. The entire process of maturation of H glycoprotein required approximately 5 hours. Carbohydrate content in H was determined to be approximately 12 per cent by weight. Mannose, galactose, fucose, N-acetylglucosamine, and N-acetylneuraminic acid were the constituent monosaccharides.

Sequence analysis of three Sindbis virus mutants temperature-sensitive in the capsid protein autoprotease

We have cloned and sequenced the cDNA made to the region of RNA encoding the structural proteins of three complementation group C mutants of Sindbis virus, ts2, ts5, and ts13, and of their revertants. These mutants possess defects in the posttranslational processing of their structural proteins at the nonpermissive temperature. Comparison of the deduced amino acid sequences of the mutants with those of the revertants and with the parental HR strain of virus showed all three mutants to have single amino acid substitutions in the highly conserved COOH-terminal half of the capsid protein that give rise to temperature sensitivity. ts2 and ts5 were found to have the same lesion and thus represent independent isolations of the same mutant, whereas ts13 possessed a different change. Reversion to temperature insensitivity in all three mutants occurred by reversion of the mutated nucleotide to the parental nucleotide, restoring the original amino acid. It has been previously postulated that the capsid protein possesses an autoproteolytic activity that cleaves the capsid protein from the nascent polyprotein during translation. Comparison of the amino acid sequence of the capsid protein with that of serine proteases leads us to hypothesize that histidine-141, aspartate-147, and serine-215 of the Sindbis capsid protein form the catalytic triad of a serine protease. This hypothesis is supported by the finding that all three temperature-sensitive lesions mapped occur near these residues: ts2 and ts5 change proline-218 to serine and in ts13 lysine-138 has been replaced by isoleucine.

Effect of tunicamycin on the development of the cytopathic effect in Sindbis virus-infected avian fibroblasts

In Sindbis virus-infected avian cells the development of the cytopathic effect is correlated with the disruption of plasma membrane function. Sindbis virus inhibits the activity of the Na+K+ATPase, a membrane-associated enzyme complex which regulates intracellular monovalent cation levels. Tunicamycin, which blocks envelope protein glycosylation, prevents inhibition of Na+K+ATPase activity and the development of morphological changes in Sindbis virus-infected cells. Although inhibition of Na+K+ATPase activity is not essential for the termination of host protein synthesis, membrane-mediated events may favor the selective translation of viral proteins. The termination of host protein synthesis does not contribute to the development of these cytopathic changes in the time frame examined. In tunicamycin-treated, Sindbis virus-infected cells, unglycosylated E1 is inserted into the plasma membrane but virus release is prevented. In productively infected cells, therefore, the inhibition of Na+K+ATPase activity and the development of the cytopathic effect may result from terminal events in virus assembly and/or virus release.

Pattern of glycosylation of Sindbis virus envelope proteins synthesized in hamster and chicken cells

The tryptic glycopeptides of the Sindbis virus envelope glycoproteins E1 and E2 grown in BHK and chick cells were purified by gel filtration followed by high-pressure liquid chromatography. Each of the purified glycopeptides was analyzed by N-terminal sequencing to identify from which of the potential glycosylation sites it was derived. The type of oligosaccharide chain attached to each glycopeptide was determined from gel filtration analysis of the pronase-digested glycopeptides, and the relative incorporation of radiolabeled galactose, mannose, and glucosamine into each glycopeptide was used to confirm these determinations. The glycosylation patterns for the two proteins were essentially identical in the two host cells. The E2 glycosylation sites at Asn196 and Asn318 contained exclusively complex-type and simple-type oligosaccharide chains, respectively. In E1, the glycosylation site at Asn139 contained only complex-type chains, but the site at Asn245 contained a mixture of simple (75-85%) and complex (15-25%) type chains. These results are discussed in relation to previously reported results and a prediction as to the relative importance of the different glycosylation sites to the function of the proteins is made.

Biochemical studies of the maturation of the small Sindbis virus glycoprotein E3

A small glycoprotein (E3) was purified from the culture fluid of Sindbis virus-infected primary chick embryo fibroblasts. Tryptic peptide mapping and pulse-chase studies verified that this protein was produced as a by-product of the cleavage of the precursor protein PE2 to produce the envelope glycoprotein E2. A 2600-fold purification was achieved via a procedure which used differential ethanol precipitation, gel filtration, ion-exchange chromatography, and affinity chromatography on a lentil lectin column. Amino acid composition analysis, N-terminal microsequencing, and labeling studies yielded information about the fine structure of E3 and its relationship to E2 and virion maturation. The N-terminal sequence of E3 is identical to that of PE2, including the result that 90% of the molecules appear to be blocked. The first 19 amino acids are uncharged and presumably serve as the signal sequence for the insertion of PE2 into the membrane of the endoplasmic reticulum, but this sequence is unusual in that it is not immediately cleaved from PE2 and is glycosylated at the asparagine at position 14. The two residues at the C-terminus of E3, Lys-Arg, are removed during or shortly after cleavage from PE2. Labeling studies imply that, although the PE2----E2 + E3 cleavage is necessary for virion budding, these two events are not closely coupled. E3 is cleaved and released into the culture fluid under conditions where virions do not bud, and the kinetics of the appearance of E3 in the culture fluid and E2 in virions are quite dissimilar. The maturation of E3 is discussed as it relates to the processing of cellular membrane and secretory glycoproteins.

On the role of oligosaccharide trimming in the maturation of Sindbis and influenza virus

The alpha-glucosidase inhibitor bromoconduritol inhibits the formation of the N-linked, complex-type oligosaccharides of the glycoproteins from influenza viruses (fowl plague virus, influenza virus PR-8) and from sindbis virus. Viral glycoproteins produced in bromoconduritol-treated chicken-embryo and baby-hamster kidney cells are fully glycosylated, but accumulate N-linked, high-mannose oligosaccharides of the composition Glc1Manx (GlcNAc)2 (x = 7, 8, and 9). Other alpha-glucosidase inhibitors (nojirimycin, deoxynojirimycin, acarbose) were not specific inhibitors of oligosaccharide processing under the conditions used in the present investigation. In bromoconduritol-treated, sindbis virus-infected chicken-embryo and baby-hamster kidney cells, the sindbis glycoproteins are metabolically stable. Specific proteolytic cleavage of the polyprotein precursors to form E2 and E1 occurs in bromoconduritol-treated chicken-embryo cells, but cleavage of PE2 to E2 is prevented in the infected baby-hamster kidney cells. Yet, release of infectious sindbis virus particles is inhibited in both cell types indicating that the formation of complex oligosaccharides is required for a late step in virus formation. The release of virus particles from influenza virus PR-8-infected bromoconduritol-treated chicken-embryo cells is not inhibited, and virus with only high-mannose oligosaccharides is formed. In contrast, when chicken-embryo cells were infected with the influenza virus fowl plague virus, release of infectious particles was inhibited. The fowl plague virus hemagglutinin is cleaved in chicken-embryo cells, in contrast to the hemagglutinin of the PR-8 virus. However, the cleavage products HA1 and HA2 do not reach the cell surface. In addition, or as a consequence, HA1 and HA2 are proteolytically broken down, whereas uncleaved hemagglutinin of PR-8 appeared metabolically stable. These results may explain the decrease in formation of fowl plague virus particles and the lack of effect on PR-8 virus in bromoconduritol-treated cells. This work thus shows different biological roles for oligosaccharide processing.

Inhibitors of the biosynthesis and processing of N-linked oligosaccharides

A number of glycoproteins have oligosaccharides linked to protein in a GlcNAc----asparagine bond. These oligosaccharides may be either of the complex, the high-mannose or the hybrid structure. Each type of oligosaccharides is initially biosynthesized via lipid-linked oligosaccharides to form a Glc3Man9GlcNAc2-pyrophosphoryl-dolichol and transfer of this oligosaccharide to protein. The oligosaccharide portion is then processed, first of all by removal of all three glucose residues to give a Man9GlcNAc2-protein. This structure may be the immediate precursor to the high-mannose structure or it may be further processed by the removal of a number of mannose residues. Initially four alpha 1,2-linked mannoses are removed to give a Man5 - GlcNAc2 -protein which is then lengthened by the addition of a GlcNAc residue. This new structure, the GlcNAc- Man5 - GlcNAc2 -protein, is the substrate for mannosidase II which removes the alpha 1,3- and alpha 1,6-linked mannoses . Then the other sugars, GlcNAc, galactose, and sialic acid, are added sequentially to give the complex types of glycoproteins. A number of inhibitors have been identified that interfere with glycoprotein biosynthesis, processing, or transport. Some of these inhibitors have been valuable tools to study the reaction pathways while others have been extremely useful for examining the role of carbohydrate in glycoprotein function. For example, tunicamycin and its analogs prevent protein glycosylation by inhibiting the first step in the lipid-linked pathway, i.e., the formation of Glc NAc-pyrophosphoryl-dolichol. These antibiotics have been widely used in a number of functional studies. Another antibiotic that inhibits the lipid-linked saccharide pathway is amphomycin, which blocks the formation of dolichyl-phosphoryl-mannose. In vitro, this antibiotic gives rise to a Man5GlcNAc2 -pyrophosphoryl-dolichol from GDP-[14C]mannose, indicating that the first five mannose residues come directly from GDP-mannose rather than from dolichyl-phosphoryl-mannose. Other antibodies that have been shown to act at the lipid-level are diumycin , tsushimycin , tridecaptin, and flavomycin. In addition to these types of compounds, a number of sugar analogs such as 2-deoxyglucose, fluoroglucose , glucosamine, etc. have been utilized in some interesting experiments. Several compounds have been shown to inhibit glycoprotein processing. One of these, the alkaloid swainsonine , inhibits mannosidase II that removes alpha-1,3 and alpha-1,6 mannose residues from the GlcNAc- Man5GlcNAc2 -peptide. Thus, in cultured cells or in enveloped viruses, swainsonine causes the formation of a hybrid structure.(ABSTRACT TRUNCATED AT 400 WORDS)

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.

Sequence analysis of two mutants of Sindbis virus defective in the intracellular transport of their glycoproteins

We have sequenced the complementary DNA corresponding to the genes encoding the viral glycoproteins of ts10 and ts23, mutants of Sindbis virus defective in the intracellular transport of their glycoproteins, and of revertants of these mutants. These studies have been augmented by direct amino acid sequencing of the amino-terminal regions of the glycoproteins of several virus strains. By comparing the deduced amino acid sequence with that of Sindbis HR virus, the parental strain of these mutants, and with the sequence of the revertants, we found ts23 to have a double mutation in glycoprotein E1, while ts10 was a single mutant in the same glycoprotein. In each case reversion to temperature insensitivity occurred by changes at the same site as the mutation, in two cases restoring the original amino acid and in the third case substituting an homologous amino acid (arginine in place of lysine). The three mutations were far apart from each other in the protein, suggesting that the three-dimensional conformation is very important for the correct migration of the glycoproteins from the rough endoplasmic reticulum to the plasma membrane. The sequence data also reveal that a number of other changes have occurred in the various virus strains during mutagenesis or passage.

Compartmentation of asparagine-linked oligosaccharide processing in the Golgi apparatus

Golgi-associated processing of complex-type oligosaccharides linked to asparagine involves the sequential action of at least six enzymes. By equilibrium sucrose density gradient centrifugation of membranes from Chinese hamster ovary cells, we have partially resolved the set of four initial enzymes in the pathway (Mannosidase I, N-acetylglucosamine (GlcNAc) Transferase I, Mannosidase II, and GlcNAc Transferase II) from two later-acting activities (galactosyltransferase and sialyltransferase). In view of the recent demonstration that galactosyltransferase is restricted to the trans face of the Golgi complex in HeLa cells (Roth, J., and E.G. Berger, 1982, J. Cell Biol., 93:223-229), our results suggest that removal of mannose and attachment of peripheral N-acetylglucosamine may occur in some or all of the remaining cisternae on the cis side of the Golgi stack.

Coronavirus proteins: biogenesis of avian infectious bronchitis virus virion proteins

We examined the synthesis of viral structural proteins in cultured cells infected with the avian coronavirus infectious bronchitis virus. Tryptic peptide mapping was used to determine the structural relationships of the intracellular proteins to the virion polypeptides. Pulse-chase experiments were performed to identify precursors to the virus-specific proteins. We found that the nucleocapsid protein, P51, and the small viral membrane proteins GP31, GP28, and P23 do not undergo post-translational proteolytic processing. In contrast, GP90 and GP84, the two large virion membrane proteins, were found to be produced by cleavage of a single precursor, GP155. This demonstrated that at least one coronavirus mRNA specifies two virion proteins.

Endo-beta-N-acetylglucosaminidase H sensitivity of precursors to herpes simplex virus type 1 glycoproteins gB and gC

The endoglycosidase endo-beta-N-acetylglucominidase H (endo H) was used to examine the nature of the oligosaccharides associated with the herpes simplex virus type 1 glycoproteins gA, gB, and gC. Immunoprecipitates from detergent extracts of infected cells, using monospecific antisera to gAB and gC, were treated with endo H. The low-molecular-weight precursor to gC, pgC(105), was found to be sensitive to endo H. Removal of the endo H-sensitive oligosaccharide chains from pgC(105) resulted in a protein with an apparent molecular weight of 75,000. In contrast, the fully glycosylated gC was not sensitive to endo H treatment. These results suggested that the oligosaccharide chains of pgC(105) were primarily of the simple high-mannose type. Both gA and gB were sensitive to endo H treatment; however, gB appeared to be only partially susceptible, whereas [3H]mannose-labeled gA was not detectable after endo H treatment. These results that gB contained both complex- and simple-type oligosaccharides, and gA contained only simple-type oligosaccharides. An accumulation of the high-mannose glycoproteins pgC(105) and gA was observed in monensin-treated infected cells with a concomitant inhibition of gB and gC. Glycoproteins gA and pgC(105) synthesized in the presence of monensin were also sensitive to endo H treatment.

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.