Viral glycoprotein synthesis studies in an established line of Japanese quail embryo cells infected with the Bryan high-titer strain of Rous sarcoma virus

The effects of 2-deoxyglucose and tunicamycin on the biosynthesis of the murine mammary tumor virus proteins, and on the assembly and release of the virus

The role of glycosylation in the biosynthesis, processing, and shedding of the murine mammary tumor virus (MuMTV) glycoproteins and in virus production was investigated in a clonal mammary tumor cell line, GR-3A, using two inhibitors of protein glycosylation, 2-deoxyglucose (2-DG) and tunicamycin (TM). It was found that both 2-DG and TM completely inhibited the synthesis of the MuMTV envelope precursor polyprotein, Pr70env, and, as a consequence, the synthesis of the viral glycoproteins gp52 and gp36. By contrast, the synthesis of Pr73gag, the polyprotein precursor of the internal structural proteins of the virus, was only inhibited by 10-15% by 2-DG and TM. Although 2-DG and TM blocked the synthesis of Pr70env, a new polypeptide, related to gp52 and gp36, with a mol wt of 60,000 (P60env) was found to be synthesized in the treated cells. The P60env molecules appeared to be degraded intracellularly since they were not found to (1) undergo site-specific cleavage; (2) accumulate inside the cell or on the cell surface; (3) be secreted into the culture medium; and (4) be incorporated into the virions produced during the drug treatment. In spite of the lack of gp52 and gp36 synthesis in the presence of TM and 2-DG, mature MuMTV particles containing the characteristic surface projections known to be composed of gp52 and gp36 continued to be assembled and released at a reduced rate for at least 30 hr. In addition, the buoyant density and the polypeptide composition of the particles were found to be identical to virions produced by untreated cells. Thus, the virions assembled and released during 2-DG and TM treatment were not defective. Our investigations into the origin of gp52 and gp36 in these particles revealed that both molecules were synthesized prior to 2-DG and TM treatment and continued to be incorporated, along with the newly synthesized viral core proteins, into budding virions during the drug treatment. Furthermore, we found that gp52 and P75env (an aberrant form of Pr70env) that were not incorporated into virions continued to be shed normally from the cell during drug treatment. In conclusion, our results suggest that MuMTV assembly is not dependent on the synchronized synthesis of the viral core and envelope polypeptides, and that the assembled virions contain the correct ratio of these polypeptides, even when their ratio in the cell varies.

Complete env gene deletions of three replication-defective strains of Rous sarcoma virus and a model for the origin of their genetic structures

Replication-defective deletion mutants of Rous sarcoma virus (RSV) have been described which transform cells in culture and elaborate envelope (-) defective particles. The env deletions of two clonal variants of the Bryan strain of RSV, RSV(-)3, and RSV(-)16, and of a replication-defective variant of Schmidt-Ruppin RSV (SRN8) were analyzed by fingerprinting oligonucleotides hybridized by a molecularly cloned env DNA probe that spans from near the 3' end of pol to the 3' end of env. It was observed that all three replication-defective RSV strains are essentially complete env deletions but retain the 3' end of pol. Based on a common pol-src junction oligonucleotide that may reflect a homologous sequence repeated at both ends of env in nondefective RSV, the env deletions of RSV(-)3 and 16 appear to be isogenic. The original deletion may have involved recombination between these sequences. The absence of this oligonucleotide in SRN8 indicates that the env deletion of SRN8 has different borders and represents an independent env deletion of nondefective RSV. All three defective RSVs have the genetic structure gag-pol-src. This genetic structure is consistent with the need for a complete gag to make a particle and with the assumption that an independent src gene rather than a gag- or gag-pol-src hybrid gene functions in transformation. It is suggested that a complete pol is not necessary for, but may assist, virus particle formation.

Rous sarcoma virus p19 and gp35 can be chemically crosslinked to high molecular weight complexes. An insight into virus assembly

We have used the method of chemical crosslinking in order to determine the spatial interactions between components of Rous sarcoma virus. A high molecular weight complex formed by crosslinking has been isolated by ultracentrifugation on sucrose density gradients containing 0.1% (w/v) sodium dodecyl sulphate. This complex is composed of the two viral glycoproteins gp85 and gp35, the gag protein p19, and the viral RNA. Two types of bonding are important for the formation and stability of the complex: first, native disulphide bonds between gp85 and gp35 and between individual p19 molecules; and second, hetero-crosslinking between gp35 and p19 as well as homo-crosslinking between p19. Although viral RNA is quantitatively present in the complex, experiments with RNase treatment show that it is not essential for its formation or stability. A small amount of lipid is present in the complex and appears to be crosslinked to p19. In vitro-labelling of purified virus with the lipophilic photoactivatable reagent [125I] iodonaphthylazide resulted in the labelling of gp35 and p19/23. In vivo-labelling of virus with [3H]palmitate resulted in only gp35 becoming labelled. These results substantiate the membrane association of these proteins. The significance of the interactions in the high molecular weight complex for the stability of the virus and, by implication, the role which they may play in viral assembly are discussed.

Evidence for a Gp85-related glycoprotein in cells transformed by the Bryan high titer strain of Rous sarcoma virus

The Bryan High Titer strain of Rous Sarcoma Virus (BH-RSV) is a deletion mutant in the env-gene coding for the viral envelope glycoproteins gp35 and gp85. In this report experimental evidence is described that cells, transformed by BH-RSV, express a glycoprotein immunologically related to gp85. Animals bearing BH-RSV induced tumors produce antibodies reacting with gp85 of nondefective RSV. Lysates of a BH-RSV transformed quail cell line, R(-):Q, inhibit the immunoprecipitation of gp85 by antibodies against the group-specific determinant of gp85. In R(-):Q cell lysates and in the culture supernatant a glycoprotein of an apparent molecular weight 40,000 (gp40) is found that reacts with monospecific antisera against gp85 of nondefective RSV. In newly synthesized BH-RSV a gp40 associated with the virion is detectable but is easily lost during purification of the virus. Further, a 95k glycoprotein and a 95k phosphoprotein are specifically precipitated from R(-):Q cells by an antiserum against gp85. From these results we conclude that the deletion of the env-gene is incomplete such that part of gp85, bearing group-specific antigenic determinants, is expressed in BH-RSV transformed cells. Analysis of BH-RSV particles freshly harvested from R(-):Q cells reveals that they contain almost exclusively the gag-precursor pr76 and little or no processed gag-proteins. Therefore the R(-):Q cell line seems to be suitable for the study of virus maturation occurring after the budding process.

Envelope assembly mutant of rous sarcoma virus

The properties of a novel nonconditional envelope mutant of Rous sarcoma virus, rdPN3/2-SR-D, defective in the assembly of viral glycoproteins into mature virions, are described.

Detection of natural killer cells in Japanese quails

Spleen cells of normal quails exhibited cytotoxicity on normal embryo cells as well as on virus- and chemically transformed cells. Spleen cells responsible for the destruction of target cells were shown to be non-adherent lymphoid cells which lack surface immunoglobulin and are resistant to the treatment with anti-thymocyte serum and complement. Spleen cells of quails which were injected with chicken amniotic fluid (ChAmF) showed no cytotoxic activity. Furthermore, the addition of ChAmF-treated spleen cells abolished the cytotoxic activity of untreated spleen cells. These results indicated the presence of a spleen-cell population in normal quails compatible with mammalian natural killer (NK) cells and the induction of suppressor cells directed to the NK cells by administration of ChAmF.

Polyethylene glycol-mediated infection with avian sarcoma viruses

In this communication we report that fusion of viral and cellular membranes by polyethylene glycol is a convenient approach to overcoming genetically determined resistance to infection. Using this method, avian sarcoma virus-transformed mammalian cells have been produced which serve as useful model systems for the study of Rous sarcoma virus-specific tumor antigens.

Avian retrovirus-induced surface antigens and their cross-reactivity with chemically-transformed cells and primary embryonic cells of Japanese quails

By testing spleen cells from avian leukosis (ALV) and avian sarcoma virus (ASV)-injected Japanese quails in a microcytotoxicity assay against various target cells, we have demonstrated the existence of several target antigens. With non-transformed ALV-infected Japanese quail cells used as target cells, an avian retrovirus subgroup-specific destruction was obtained when spleen cells from animals infected with either avaian sarcoma or leukosis virus of the same subgroup were employed. This reaction is probably due to the virus envelope glycoproteins (Ve-gp) expressed on the cell surface. Apart from this subgroup-specific reaction, avian retrovirus group-specific destruction of ASV-transformed cells was demonstrated by means of effector cells immunized with ASV of a different subgroup. This reaction is restricted to transformed cells and not due to the virus envelope glycoprotein because the same effector cells are not cytotoxic to ALV-infected non-transformed cells but cytotoxic to sarcoma-virus-transformed cells which lack Ve-gp. Quail methylcholanthrene-tumor cells which show a transformation phenotype similar to that of ASV-transformed cells but which are free of detectable endogenous and exogenous retrovirus were also destroyed by the spleen cells from ASV tumorbearing animals. The same effector cells also exerted a weak cytotoxic effect on uninfected primary embryo cells but not to embryo cells after several passages.

Analysis of precursors to the envelope glycoproteins of avian RNA tumor viruses in chicken and quail cells

Immune precipitation with monospecific antiserum was employed to study the intracellular synthesis of viral glycoproteins gp85 and gp37. Labeled gp85 and gp37 were detected from lysates of cells transformed with Rous sacroma virus, strain B77, after long-term labeling with radioactive glucosamine or phenylalanine. Immune precipitates prepared from lysates of cells pulse-labeled for a short time resulted in a glycoprotein of 92,000 molecular weight (gp92). This precursor was stable in B77-transformed Japanese quail cells for several hours, whereas in chicken cells it could be chased within a few hours into virion glycoproteins gp85 and gp37. Similarly, the precursor for the structural viral proteins, pr76, persisted in quail cells much longer than in chicken cells. During very short pulses or in the presence of a glucosamine block (25 mM glucosamine), the antiserum against the viral envelope glycoproteins detected a precursor of higher electrophoretic mobility of approximately 70,000 molecular weight, "p70." Fucose label entered gp92 and gp85 as well as "p70." Proteolytic treatment of virion-bound gp85 in vitro generated two discrete glycoproteins of 62,000 and 45,000 molecular weight, but did not result in an increase in the amount of gp37.

Suppression of glycoprotein formation of Semliki Forest, influenza, and avian sarcoma virus by tunicamycin

Tunicamycin, a new antibiotic, halts the formation of physical particles of Semliki forest and fowl plague virus, whereas avian oncornavirus particles which show a reduction in infectivity and do not contain detectable labeled glycoprotein are released in the presence of the drug. In Semliki forest virus-infected cells only the protein moieties of the glycoproteins could be labeled. In cells infected with fowl plague and avian sarcoma virus neither intact glycoproteins nor their protein moieties could be detected. By using a protease inhibitor (N-alpha-p-tosyl-L-lysin chloromethyl ketone, TLCK) it could be shown, however, that the carbohydrate-free hemagglutinin precursor of influenza virus is synthesized but is presumably degraded by intracellular proteases in the absence of TLCK as a consequence of the lack of carbohydrate.