Antiviral activity of tunicamycin on herpes simplex virus

Nelfinavir impairs glycosylation of herpes simplex virus 1 envelope proteins and blocks virus maturation

Nelfinavir (NFV) is an HIV-1 aspartyl protease inhibitor that has numerous effects on human cells, which impart attractive antitumor properties. NFV has also been shown to have in vitro inhibitory activity against human herpesviruses (HHVs). Given the apparent absence of an aspartyl protease encoded by HHVs, we investigated the mechanism of action of NFV herpes simplex virus type 1 (HSV-1) in cultured cells. Selection of HSV-1 resistance to NFV was not achieved despite multiple passages under drug pressure. NFV did not significantly affect the level of expression of late HSV-1 gene products. Normal numbers of viral particles appeared to be produced in NFV-treated cells by electron microscopy but remain within the cytoplasm more often than controls. NFV did not inhibit the activity of the HSV-1 serine protease nor could its antiviral activity be attributed to inhibition of Akt phosphorylation. NFV was found to decrease glycosylation of viral glycoproteins B and C and resulted in aberrant subcellular localization, consistent with induction of endoplasmic reticulum stress and the unfolded protein response by NFV. These results demonstrate that NFV causes alterations in HSV-1 glycoprotein maturation and egress and likely acts on one or more host cell functions that are important for HHV replication.

Bitter-sweet symphony: glycan-lectin interactions in virus biology

Glycans are carbohydrate modifications typically found on proteins or lipids, and can act as ligands for glycan-binding proteins called lectins. Glycans and lectins play crucial roles in the function of cells and organs, and in the immune system of animals and humans. Viral pathogens use glycans and lectins that are encoded by their own or the host genome for their replication and spread. Recent advances in glycobiological research indicate that glycans and lectins mediate key interactions at the virus-host interface, controlling viral spread and/or activation of the immune system. This review reflects on glycan-lectin interactions in the context of viral infection and antiviral immunity. A short introduction illustrates the nature of glycans and lectins, and conveys the basic principles of their interactions. Subsequently, examples are discussed highlighting specific glycan-lectin interactions and how they affect the progress of viral infections, either benefiting the host or the virus. Moreover, glycan and lectin variability and their potential biological consequences are discussed. Finally, the review outlines how recent advances in the glycan-lectin field might be transformed into promising new approaches to antiviral therapy.

Analysis of Rab GTPase-activating proteins indicates that Rab1a/b and Rab43 are important for herpes simplex virus 1 secondary envelopment

Assembly of herpes simplex virus 1 (HSV-1) occurs in the cytoplasm, where the capsid and tegument bud into host cell membranes. It is at this point that the viral glycoproteins are incorporated into the virion, as they are located at the assembly site. We investigated the role of the Rab GTPases in coordinating the assembly process by overexpressing 37 human Rab GTPase-activating proteins (GAPs) and assessing infectious titers. Rab GTPases are key cellular regulators of membrane trafficking events that, by their membrane association and binding of effector proteins, ensure the appropriate fusion of membranes. We identified that TBC1D20 and RN-tre and their partner Rabs, Rab1a/b and Rab43, respectively, are important for virion assembly. In the absence of Rab1a/b, the viral glycoproteins are unable to traffic from the endoplasmic reticulum to the assembly compartment, and thus unenveloped particles build up in the cytoplasm. The defect resulting from Rab43 depletion is somewhat more complex, but it appears that the fragmentation and dispersal of the trans-Golgi network and associated membranes render these compartments unable to support secondary envelopment.

Absence of asparagine-linked oligosaccharides from glycoprotein D of herpes simplex virus type 1 results in a structurally altered but biologically active protein

Glycoprotein D (gD) of herpes simplex virus contains three utilized sites (Asn-X-Ser/Thr) for addition of asparagine-linked carbohydrates (N-CHO). Previously, we used oligonucleotide-directed mutagenesis to alter serine or threonine residues to alanine at each N-CHO addition site. Studies with monoclonal antibodies showed that a mutant protein lacking all three sites (now designated AAA) was structurally altered because of the amino acid change at residue 96 as well as the absence of the N-CHO. In this study, we constructed additional single mutations at site 1 (residues 94 and 96) and found that in most cases, the amino acid change itself adversely affected the conformation of gD. However, changing asparagine 94 to glutamine (Q) at site 1 had the least effect on gD. We constructed a second triple mutant, QAA, which lacked all three N-CHO signals. The antigenic conformation of QAA was similar to that of gD produced in the presence of tunicamycin (TM-gD). However, binding of MAbs to the AAA protein or to single mutants altered at site 1 was reduced compared with TM-gD. Wild-type gD and QAA proteins were equally susceptible to digestion by trypsin or Staphylococcus aureus V8 protease. In contrast, the AAA protein was more sensitive to trypsin but less sensitive to V8, again suggesting conformational alterations of the AAA protein. Despite what appeared to be large changes in structure, each mutant complemented the infectivity of a virus lacking gD (F-gD beta). We conclude that the N-CHO and amino acids at N-CHO site 1 play an important role in forming and/or maintaining gD structure, but none of the N-CHO are required for gD to function in the complementation assay.

Modification of Epstein-Barr virus replication by tunicamycin

The effect of tunicamycin, which inhibits N-linked glycosylation, on the replication of Epstein-Barr virus was examined. Tunicamycin markedly reduced the yield of virus from producing cells. At concentrations of 1 to 2 micrograms of tunicamycin per ml, there was a buildup of intracellular virus in P3HR1-Cl13 cells but not in MCUV5 cells; at a concentration of 5 micrograms of tunicamycin per ml in P3HR1-Cl13 cells, viral DNA synthesis was inhibited as well. Viral glycoproteins lacking N-linked sugars were apparently inserted into the cell membrane, and the small amount of virus made in the presence of drug was able to bind specifically to its receptor on B cells. However, the ability of the virus to induce immunoglobulin secretion by fresh human lymphocytes was impaired. This implies a role for viral glycoproteins in the penetration as well as the attachment of virus.

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.

Glycosylation of herpes simplex virus type 1 gC in the presence of tunicamycin

The presence of O-glycosidic linkages on herpes simplex virus type 1 (HSV-1) glycoproteins was indicated by the synthesis and glycosylation of HSV-1 glycoproteins in the presence of tunicamycin. Monospecific antiserum to HSV-1 gC immunoprecipitated a 92,000-molecular-weight protein synthesized in the presence of tunicamycin and isotopically labeled with glucosamine or galactose. Anti-gAB did not immunoprecipitate a carbohydrate-labeled HSV-1 protein synthesized in the presence of tunicamycin. The purified glucosamine-labeled 92,000-molecular-weight protein synthesized in the presence of tunicamycin and the fully glycosylated forms of gAB and gC were tested for their sensitivity to mild alkaline hydrolysis. Purified gAB was resistant to mild alkaline hydrolysis, whereas gC and the 92,000-molecular-weight protein were both sensitive to mild alkaline hydrolysis. These results suggest that O-glycosidic linkages are associated with the HSV-1 gC glycoprotein.

Populations of herpes simplex virus glycoprotein gC with and without affinity for the N-acetyl-galactosamine specific lectin of Helix pomatia

Two fractions of herpes simplex virus glycoprotein gC were isolated and characterized by means of immunosorbent-purification with monoclonal antibodies against gC and Helix pomatia lectin (HPA) affinity chromatography. About 25 per cent of the glycoprotein gC population demonstrated affinity for the lectin, compatible with presence of N-acetylgalactosamine as terminal sugar of the oligosaccharide. The HPA-binding populations of gC appeared as two electrophoretic bands with lower molecular weights than the non-binding gC. The gC subfraction without affinity for the HPA was subjected to treatments aiming to desialylize the carbohydrate moiety. Only 5 per cent of the initially non-reactive fraction of gC became reactive to HPA after the treatments, suggesting that masking of penultimate N-acetylgalactosamine by sialic acid was not a main reason for lack of HPA affinity. Results of treatment with alkaline Na BH4 demonstrated presence of oligosaccharide-peptide linkages sensitive to beta-elimination suggesting O-glycosidic type of linkage. The subfraction of gC demonstrating affinity for HPA as well as gC devoid of HPA binding capacity both revealed affinity for Con A. Therefore N-glycosidically as well as O-glycosidically linked oligosaccharides seemed to be present on the one and same glycoprotein. On the basis of the results presented we assume that the glycosylation of HSV glycoprotein gC may lead to, at least, two populations of the glycoprotein gC, one with terminal N-acetylgalactosamine residues of oligosaccharides O-glycosidically linked to the polypeptide and the other without affinity for HPA. However, both populations of gC contain similar proportions of oligosaccharides of the high mannose or complex types with N-glycosidic carbohydrate-peptide linkages as indicated by their affinity for Con A.

Characterization of an herpes simplex virus type 2 mutant, which is resistant to acycloguanosine and causes fusion of BSC1 cells

A mutant of herpes simplex virus type 2, which induces low levels of thymidine-kinase activity in infected BSC1 cells and consequently able to grow in the presence of acycloguanosine, was isolated. This mutant has also been shown to cause fusion of BSC1 cells. In BSC1 cells, co-infected with the wild-type strain and the mutant, the yield of each of the two viruses was normal but the rounding and aggregation of cells observed, resembled that found in wild-type infected cultures. When the mixed infection was performed in the presence of acycloguanosine (100 micrometers), the growth of the two virus strains was inhibited, as well as the cytopathic effect in the cultures. It is suggested that under these conditions, the thymidine-kinase which was induced in the infected cells by the wild-type strain, phosphorylated acycloguanosine and the activated drug formed, inhibited the growth of the two viruses by interference in their DNA syntheses.

Herpesvirus glycoprotein synthesis and insertion into plasma membranes

In the presence of the antibiotic tunicamycin (TM), glycosylation of herpes simplex virus glycoproteins is inhibited and non-glycosylated polypeptides analogous to the glycoproteins are synthesized (Pizer et al., J. Virol. 34:142-153, 1980). The synthesis of viral proteins and DNA occurs in TM-treated cells. By electron microscopy, nucleocapsids can be observed both in the nucleus and the cytoplasm of TM-treated cells; a small number of enveloped virions were observed on the cell surface. Analyses of the proteins in partially purified virus readily detects viral glycoproteins in the control cells, but neither glycoproteins nor nonglycosylated polypeptide analogs were observed in the virus prepared from TM-treated cells. By labeling the surface of infected cells with 125I, viral glycoproteins were detected as soon as 90 min after infection even when protein synthesis was inhibited with cycloheximide and glycosylation was blocked with TM. Labeling the proteins synthesized in infected cells with [35S]methionine showed that the surface glycoproteins detected in the cycloheximide- and TM-treated cells were not synthesized de novo after infection, but were placed on the cell surface by the infecting virus. Studies with metabolic inhibitors and a temperature-sensitive mutant blocked early in the infectious cycle showed that glycoproteins gA/gB and gD were synthesized soon after infection, but that the synthesis of gC was delayed. Under conditions of infection, in which gC and its precursor pgC are not produced, we have been able to observe the relationships between the glycosylated polypeptides that correspond to pgA/pgB and the nonglycosylated analog made in the presence of TM.