Identification of an avian oncovirus polyprotein in uninfected chick cells

Identification of the fragment containing cross-reacting antigenic determinants in the variable surface glycoprotein ofTrypanosoma brucei

125I-labelled, isolated variable surface glycoproteins (VSGs) ofTrypanosoma b. bruceibind both homologous and heterologous anti-VSG sera and binding to heterologous antisera may be blocked by other unlabelled VSGs (Barbet & McGuire, 1978). This paper presents results which suggest that oligosaccharide residues have importance in the antigenic structure of VSG cross-reacting determinants. The ability of VSG to bind heterologous anti-VSG sera was destroyed by periodate oxidation but not by extensive proteolysis. A VSG glycopeptide fragment was isolated from two different VSGs, which blocked by 100 % the binding of VSG to heterologous anti-VSG sera and therefore contained the cross-reacting determinants. The native glycopeptide fragment was resistant to digestion with trypsin, pronase or leucine aminopeptidase and prolidase. We also show that a VSG synthesized in the reticulocyte lysate cell-free system was not immunoprecipitated by heterologous anti-VSG sera in contrast to the same VSG labelled by metabolic incorporation of [35S]methioninein vivo.

Role of methylation in the induced and spontaneous expression of the avian endogenous virus ev-1: DNA structure and gene products

The endogenous avian provirus ev-1 is widespread in white leghorn chickens. Although it has no major structural defects, ev-1 has not been associated with any phenotype and is ordinarily expressed at a very low level. In this report, we describe a chicken embryo (Number 1836) cell culture containing both ev-1 and ev-6 which spontaneously expressed the ev-1 provirus. This culture released a high level of noninfectious virions containing a full complement of virion structural (gag) proteins but devoid of reverse transcriptase activity or antigen. These virions contained 70S RNA closely related to the genome of Rous-associated virus type 0, but identifiable as the ev-1 genome by oligonucleotide mapping. A fraction of the RNA molecules in the 70S complex were unusual in that they were polyadenylated 100 to 200 nucleotides downstream of the usual polyadenylation site. Eight sibling embryo cultures did not share this unusual phenotype with 1836, indicating that it was not inherited. However, an identical phenotype was inducible in the sibling cultures by treatment with 5-azacytidine, an inhibitor of DNA methylation, and the induced expression was stable for more than 10 generations. Analysis of chromatin structure and DNA methylation of the ev-1 provirus in 1836 cells revealed the presence (in a fraction of the proviruses) of both DNase I hypersensitive sites in the long terminal repeats and in gag and a pattern of cleavage sites for methyl-sensitive restriction endonuclease not found in a nonexpressing sibling. These results lend strong support to the role of DNA methylation in the control of gene expression. Additionally, they explain the lack of phenotype associated with ev-1 as due to a combination of its low expression and defectiveness in pol and env.

Role of methylation in the induced and spontaneous expression of the avian endogenous virus ev-1: DNA structure and gene products

The endogenous avian provirus ev-1 is widespread in white leghorn chickens. Although it has no major structural defects, ev-1 has not been associated with any phenotype and is ordinarily expressed at a very low level. In this report, we describe a chicken embryo (Number 1836) cell culture containing both ev-1 and ev-6 which spontaneously expressed the ev-1 provirus. This culture released a high level of noninfectious virions containing a full complement of virion structural (gag) proteins but devoid of reverse transcriptase activity or antigen. These virions contained 70S RNA closely related to the genome of Rous-associated virus type 0, but identifiable as the ev-1 genome by oligonucleotide mapping. A fraction of the RNA molecules in the 70S complex were unusual in that they were polyadenylated 100 to 200 nucleotides downstream of the usual polyadenylation site. Eight sibling embryo cultures did not share this unusual phenotype with 1836, indicating that it was not inherited. However, an identical phenotype was inducible in the sibling cultures by treatment with 5-azacytidine, an inhibitor of DNA methylation, and the induced expression was stable for more than 10 generations. Analysis of chromatin structure and DNA methylation of the ev-1 provirus in 1836 cells revealed the presence (in a fraction of the proviruses) of both DNase I hypersensitive sites in the long terminal repeats and in gag and a pattern of cleavage sites for methyl-sensitive restriction endonuclease not found in a nonexpressing sibling. These results lend strong support to the role of DNA methylation in the control of gene expression. Additionally, they explain the lack of phenotype associated with ev-1 as due to a combination of its low expression and defectiveness in pol and env.

A rapid PCR-based test for the endogenous viral element ev3 of chickens

A short fragment of chicken genomic DNA encompassing the insertion site of the endogenous avian leucosis viral element ev3 was isolated using the inverse polymerase chain reaction (inverse PCR) technique. The nucleotide sequence of the unoccupied site was used to design PCR primers that can be used to unambiguously determine the genetic status of any chicken, with respect to ev3. Screening of a small number of individuals from exotic breeds of chickens suggested that the frequency of ev3 is highly variable. The ev3 integration site shows a high degree of sequence homology with the macrophage-specific tyrosine kinase gene, bmk, in mice.

ELISA for bovine herpesvirus 1 (BHV1) antibody with HIRT supernatant

Detailed studies were made of the ability of HIRT supernatant (HS) from bovine herpesvirus 1 (BHV1) infected cultures to sensitize plates for enzyme-linked immunosorbent assay (ELISA). The ultracentrifuged pellet of HS had less sensitizing activity than the supernatant, but the antigen was removed completely by 0.22 micron filters and to some extent by 0.45 micron filters; it was minimally affected by sonication; it was destroyed by the action of Pronase but not by DNAase I when a kallikrein inactivator was added and the mixture incubated; incubation with DNAase II had no effect. Thus the presence of DNA was not required for the sensitizing activity of HS and the antigens recognized by antibodies in HS in ELISA were directed to its protein component. Strong reactions were given in immunoblotting of HS from BHV1 infected tissue cultures with anti-BHV1 glycoprotein monoclonal IgG, but HS from uninfected tissue cultures did not react with the same monoclonal IgG.

Truncated gag-related proteins are produced by large deletion mutants of Rous sarcoma virus and form virus particles

Large deletion (LD) mutants of Prague strain Rous sarcoma virus subgroup B (PrB), derived by serial undiluted passage through chicken (C/E) cells, contain two deletions relative to wild-type virus. One of these joins gag sequences in the p12 coding region to env sequences in region encoding gp37; the other deletion spans the src region. Analysis of the viral proteins of QT6 cell clones containing only LD proviruses by sodium dodecyl sulfate-polyacrylamide gel electrophoresis revealed a major truncated gag-related phosphoprotein of 60,000 to 66,000 daltons (P63LD). P63LD was stable, but could be cleaved in vitro to the predicted products by p15gag. A second gag-related LD protein of about 68,000 to 74,000 molecular weight (P70LD) was also found which often reacted with an anti-gp37 serum. P70LD was unstable and may represent a short-lived gag-gp37 fusion protein. Finally, immunoprecipitation indicated that particles containing P63LD were shed from QT6-LD clones. Thin section preparations of these clones viewed in an electron microscope showed enveloped budding particles of "immature" morphology. Thus, the synthesis and release of particles from infected cells does not require cleavage of the gag precursor, nor does it require the presence of p15 or (most of) p12.

Immunoglobulin heavy chain class switch from IgM to IgG in a hybridoma

A subclone of an IgM-producing hybridoma has been identified which has switched to producing an IgG1 antibody. The parent hybridoma, PC-140, produces an antibody which binds phosphorylcholine and reacts with monoclonal antibodies that recognize myelomas of the T-15 idiotype. The IgG1 antibody binds phosphorylcholine with the same affinity as the parental IgM and also reacts with the anti-T-15 monoclonal antibodies. While the IgM-producing parent hybridoma does not express detectable surface IgM, the IgG1-producing subclone produces both membrane and secreted IgG1.

Human cell surface proteins selectively assembled into vesicular stomatitis virus virions

Vesicular stomatitis virus (VSV) selectively assembled proteins from human cells into progeny virions. These proteins can be surface labeled before infection with 125I, and when purified virus was examined by sodium dodecyl sulfate (SDS)-polyacrylamide gel electrophoresis, only two or three bands of proteins (Mr around 100K) were seen. Antisera to these proteins were produced, using as immunizing antigen VSV tsO45 mutant, defective in assembly of G protein, which had been made at the nonpermissive temperature in the three human tumor cell lines, HeLa (cervical carcinoma), T47D (breast carcinoma), and HMB2 (melanoma). After absorption with wild-type VSV, each of the antisera displayed a different pattern of reactivity; at least three antigenic specificities were detected. Two of them, corresponding to antigens selected by VSV from HeLa and T47D, were to some extent related and they showed an association mainly with epithelial cell-derived gynecological tumors, but they were absent in carcinomas of lung or of digestive tract. These (or related) antigens were expressed in a lower level in some normal tissues, mainly in ovaries. Antigen(s) assembled by VSV from the melanoma cell line was entirely different and appeared to be associated with cell growth. The grounds for selective assembly of these specific proteins by VSV are not clear; they either share with viral surface glycoproteins some physical or structural properties, which are critical for incorporation into the viral envelope, or conceivably they even may represent uncleaved precursor proteins coded by env genes of incomplete genomes of endogenous human retroviruses.

In vitro synthesis of thymosin beta 4 encoded by rat spleen mRNA

Thymosin beta 4, containing 43 amino acids and acetylated at the NH2 terminus, is synthesized in vitro in a rabbit reticulocyte lysate or in a yeast protein-synthesis system in the presence of mRNA from rat spleen. The product formed was identified as beta 4 by immunoprecipitation by a specific anti-beta 4 antiserum, comigration with authentic beta 4 in NaDodSO4/polyacrylamide gel electrophoresis and in HPLC, and identity of peptide fragments. The immunoprecipitable product generated in the wheat germ protein-synthesizing system emerged slightly ahead of beta 4 in HPLC and appeared to lack the NH2-terminal acetyl group. There was no evidence for formation of a larger polypeptide precursor of beta 4 in any of the three systems used. In sucrose density gradient centrifugation, the mRNA coding for beta 4 was recovered in the 7-8S mRNA fraction.

Generation of transforming viruses in cultures of chicken fibroblasts infected with an avian leukosis virus

During serial passages of an avian leukosis virus (the transformation-defective, src deletion mutant of Bratislava 77 avian sarcoma virus, designated tdB77) in chicken embryo fibroblasts, viruses which transformed chicken embryo fibroblasts in vitro emerged. Chicken embryo fibroblasts infected with these viruses (SK770 and Sk780) had a distinctive morphology, formed foci in monolayer cultures, and grew independent of anchorage in semisolid agar. Bone marrow cells were not transformed by these viruses. Another virus (SK790) with similar properties emerged during serial subcultures of chicken embryo fibroblasts after a single infection with tdB77. The 50S to RNAs isolated from these viruses contained a tdB77-sized genome (7.6 kilobases), 8.7- and 5.7-kilobase RNAs, and either a 4.1-kilobase RNA or a 4.6-kilobase RNA. These RNAs did not hybridize with cDNA's representing the src, erb, mac, and myb genes of avian acute transforming viruses. Cells transformed by any one of the Sk viruses (SK770, SK780, or SK790) synthesized two novel gag-related polyproteins having molecular weights of 110,000 (p110) and 125,000 (p125). We investigated the compositions of these proteins with monospecific antiviral protein sera. We found that p110 was a gag-pol fusion protein which contained antigenic determinants, leaving 49,000 daltons which was antigenically unrelated to the structural and replicative proteins of avian leukosis viruses. An analysis of the SK viral RNAs with specific DNA probes indicated that the 5.7-kilobase RNA contained gag sequences but lacked pol sequences and, therefore, probably encoded p125. The transforming ability, the deleted genome, and the induced polyproteins of the SK viruses were reminiscent of the properties of several replication-defective acute transforming viruses.

Chromatin structure of endogenous retroviral genes and activation by an inhibitor of DNA methylation

The transcriptionally active ev-3 and inactive ev-1 endogenous retrovirus loci in chick cells differ in that ev-3 is undermethylated, preferentially sensitive to DNase I digestion, and contains nuclease hypersensitive sites in each of its two long terminal repeats. Transient exposure of cells to 5-azacytidine, a cytosine analogue which cannot be methylated at the 5 position, results in the hypomethylation and transcriptional activation of ev-1, as well as the acquisition of at least one nuclease-hypersensitive site within the chromosomal domain of ev-1.

Partial phosphorylation in vivo of the avian retrovirus pp32 DNA endonuclease

Avian retrovirus pp32, a DNA endonuclease which is structurally related to the avian retrovirus DNA polymerase beta polypeptide, has been demonstrated to be partially phosphorylated in vivo. Unlabeled or [35S]methionine-labeled pp32 from avian sarcoma virus or avian myeloblastosis virus migrated as an electrophoretic doublet on discontinuous sodium dodecyl sulfate-polyacrylamide slab gels. However, pp32 immunoprecipitated from avian sarcoma virus labeled in vivo with [32P]orthophosphoric acid migrated as a single band, which co-electrophoresed with the slower-moving band of the doublet represented by unlabeled or 35S-labeled pp32. The presence of a slower-migrating phosphorylated band in pp32 suggests that the observed electrophoretic heterogeneity of purified pp32 is due to partial phosphorylation. Tryptic peptide analysis of 32P-labeled avian sarcoma virus beta and pp32 demonstrated that all the three labeled peptides in the beta polypeptide were also present in pp32. However, pp32 had one tryptic peptide which was preferentially labeled in comparison to the comigrating peptide found in beta digests, suggesting that phosphorylation may play a role in the processing of pp32 from beta or in the regulation of its associated DNA endonuclease activity.

Synthesis and processing of viral glycoproteins in two nonconditional mutants of Rous sarcoma virus

We have studied the pattern of glycoprotein synthesis in two nonconditional mutants of Rous sarcoma virus. One mutant, SE33, produces no viral particles but synthesizes Pr92env, which is cleaved intracellularly to mature glycoproteins. The second mutant, SE521, encodes a gPr92env which is not cleaved to gp85 or gp37 and therefore produces virions with the phenotype of Bryan RSV(-) or NY8. Neither of these mutants have detectable genomic deletions. The study of these mutants has led to the following conclusions. (i) In the absence of particle production or p15 synthesis, gPr92env can be cleaved to the mature glycoprotein which is found on the cell surface. (ii) Noncleaved gPr92env is not packaged into virions but is found on the cell surface. (iii) gPr92env alone can account for subgroup specific viral interference. (iv) gPr92env is probably transported to the cell surface before additional glycosylation or cleavage to mature virion glycoprotein. The nonprocessed precursor of SE521 appears to be glycosylated normally, and thus far we have been unable to determine the basis for the defect in this mutant.

Synthesis and processing of polymerase proteins of wild-type and mutant avian retroviruses

We have studied the biosynthesis of avian retrovirus proteins related to reverse transcriptase in permissive avian embryonic cells. Analysis of immune precipitates from avian sarcoma virus (ASV)-infected cells demonstrated the presence of the 180,000-dalton gag-pol "read-through" protein (Pr180gag-pol) and a 130,000-dalton polypeptide (Pr130gag-pol). Pr130gag-pol was found, in serological and peptide mapping studies, to consist primarily of sequences related to reverse transcriptase and the gag-encoded protein p15. Pr180gag-pol was found to be phosphorylated, whereas Pr130gag-pol was not. In addition, only Pr180gag-pol but not Pr130gag-pol was susceptible to cleavage with the virion protease p15. Although the structure of Pr130gag-pol would suggest that it is generated by removal of a portion of the gag region from Pr180gag-pol, an analysis of labeling kinetics has failed to demonstrate unequivocally whether Pr130gag-pol is a cleavage product of Pr180gag-pol or a primary translation product. We were repeatedly unable to detect either Pr180gag-pol or Pr130gag-pol in virus particles released from the cell, whereas both beta and alpha subunits were readily observed. Several presumed intermediates between Pr130gag-pol and the beta subunit of reverse transcriptase were also observed in virions. These studies indicate cleavage of polyemrase precursors at the time of virus budding. On the basis of these data, we present a processing scheme for the generation of reverse transcriptase subunits. We have also examined reverse transcriptase biosynthesis in cells producing two mutants that fail to package the enzyme. Previous work showed that integrated proviruses of both mutants are missing DNA sequences in pol: one mutant, PH9 (Mason et al., J. Virol. 30:132-140, 1979), contains a deletion near the 3' end of pol, whereas the other, SE52d (linial et al., Virology 87:130-141, 1978), may have inserted a host cell sequence near the 5' end of pol. Neither mutant synthesized Pr180gag-pol or Pr130gag-pol, but instead produced novel proteins comprised of sequences shared with gag proteins plus a region antigenically related to reverse transcriptase. Both proteins were defective as precursors to reverse transcriptase. Whereas Pr180gag-pol and Pr130gag-pol were precipitated by an antiserum raised against p32 (a virion protein derived from the portion of the beta subunit removed during processing of beta to alpha [Schiff and Grandgenett, J. Virol. 28:279-291, 1978]), the novel protein synthesized by PH9 ws not precipitated. This suggets that the alpha subunit is generated by a COOH-terminal cleavage of the beta subunit.

Avian myeloblastosis virus proteins in leukemic chicken myeloblasts

We have analyzed the avian myeloblastosis virus proteins in two types of leukemic myeloblasts: established myeloblastic cell lines (DU 1765 and DU 11157) and leukemic myeloblasts obtained from the peripheral blood of a leukemic C/E Spafas chicken (no. 21957). Using monospecific antisera for immunoprecipitation and polyacrylamide gel electrophoresis, we have detected gag gene-related proteins in the myeloblasts. The DU 1765 and DU 11157 cells contained a p100 protein which possessed antigenic determinants of the viral proteins p27, p19, p15, and p12. The p100 was not found in leukemic myeloblasts from Spafas chickens, and pulse-chase experiments showed that the p100 was not a precursor for the viral proteins. However, the p100 is present in uninfected line 15 chicken embryos. A pr76-like protein was identified in DU 1765 cells but migrated slightly further into gels than the pr76 of Spafas-derived leukemic myeloblasts. The Spafas-derived myeloblasts produced a pr60, whereas the DU 1765 cells contained instead a related protein of 62,000 daltons. Using anti-avian myeloblastosis virus gp85 sera, a glycoprotein of 120,000 daltons (gp120) was detected in all the tested leukemic myeloblasts. The gp120 was also present, in low amounts, in uninfected embyonic spleen and yolk sac cells. The anti-gp85 sera also precipitated a 27,000-dalton protein (h27) in these same cells. Both the gp120 and h27 could not be detected in either uninfected or myeloblastosis-associated virus-infected fibroblasts. Limited peptide hydrolysis revealed that h27 is different from the viral structural protein p27. In conclusion, monospecific antisera for gag and env gene products of avian myeloblastosis virus did not precipitate any unique or aberrant avian myeloblastosis virus protein from leukemic myeloblasts.

Induction of neoplasms by subgroup E recombinants of exogenous and endogenous avian retroviruses (Rous-associated virus type 60)

Chickens susceptible to infection with subgroup E viruses were inoculated with four independent isolates of Rous-associated virus type 60 (RAV-60) that are subgroup e recombinants of endogenous and exogenous virus. Neoplasms developed in each inoculated group. Therefore, nontransforming viruses of subgroup E can induce lymphoid leukosis at a moderate rate compared with RAV-0, a subgroup E endogenous virus, suggesting that oncogenicity is not a viral envelope (env)-related characteristic. Since the common (c) regions of the RAV-60s examined were of exogenous origin, we suggest that the c region rather than env is important for a high rate of induction of lymphoid leukosis and related neoplasms.

Effect of p15-associated protease from an avian RNA tumor virus on avian virus-specific polyprotein precursors

A proteolytic activity is associated with structural protein p15 in avian RNA tumor viruses. Its effect on the known intracellular viral polyprotein precursors obtained by immunoprecipitation was investigated. Cleavage of Pr76gag resulted in the sequential appearance of p15, p27, and p19. The intracellular precursor Pr180gag-pol was also cleaved by p15, whereas the intracellular glycoprotein precursors of avian RNA tumor viruses, Pr92env, remained unaffected by p15 under all conditions tested. The specificities of the antibodies used to precipitate the precursors influenced the pattern of intermediates and cleavage products obtained by p15 treatment. If virus harvested from the the Prague strain of Rous sarcoma virus, subgroup C-transformed cells at 15-min intervals was incubated at 37 degrees C for further maturation, RNA-dependent DNA polymerase activity showed an optimum of DNA synthesis with 70S viral RNA or synthetic template-primers after short incubation periods. The presence of additional p15 during incubation resulted in a shift of the enzyme activity peak toward earlier time points. Virus harvested at 3-h intervals contained significant amounts of Pr180gag-pol and Pr76gag. The addition of p15 resulted in the cleavage of Pr180gag-pol and Pr76gag, but only a few distinct low-molecular-weight polypeptides appeared. Treatment of purified RNA-dependent DNA polymerase with p15 in vitro resulted in a disappearance of the beta subunit and an enrichment of the alpha subunit. In addition, a polypeptide of 32 x 10(3) molecular weight was generated. The cleavage pattern observed differed from the one obtained by trypsin treatment.

Regulation of expression and chromosomal subunit conformation of avian retrovirus genomes

We have investigated the copy number, chromosomal subunit conformation and regulation of expression of integrated avian retrovirus genomes. Our results indicate that there are approximately two copies of the endogenous viral genomes (RAV-O) per haploid cell genome in uninfected chick embryo fibroblasts (CEF) and red blood cells (RBC). The copy number and subunit conformation (as measured by DNAasel sensitivity) of the RAV-O genomes are independent of the level of expression of these viral DNA sequences. In cells isolated from embryos of the V+, gs-chf- and gs+chf+ phenotypes, approximately one of the two viral genomes is in a DNAase l-sensitive conformation. Upon infection with an exogenous Rous sarcoma virus (PR-RSV-C), one new viral genome is integrated per haploid CEF genome. The newly integrated RSV genome is completely sensitive to DNAase l, and the subunit conformation of the endogenous viral genomes is not altered by the integration of additional exogenous proviruses. Both the endogenous and newly integrated exogenous viral genomes are present in "nu-body" structures, and the selective sensitivity of these proviral DNA sequences to DNAase l is maintained in isolated nucleosomes. Our experiments revealing the DNAase l sensitivity of one of the two RAV-O genomes in gs-chf-CEF led us to reexamine the level of viral specific RNA in CEF of various GS genotypes. We find that GS/GS CEF contain approximately 100 copies of viral RNA per cell, gs/gs CEF contain no detectable viral RNA, and the heterozygote GS/gs CEF contain approximately 50 copies of viral specific RNA per cell. These results suggest that the GS gene controls production of RAV-O RNA sequences in CEF in a "cis" fashion. In RBCs, however, the expression of the RAV-O genome is independent of the GS gene, with both GS/GS and gs/gs RBCs containing roughly equivalent amounts of viral specific RNA. Our results suggest that the chromosomal structure of the endogenous viral genes is independent of the GS gene, and that the GS gene is cis-acting and tissue-specific.