Defectiveness of avian erythroblastosis virus: synthesis of a 75K gag-related protein

Envelope gene sequences which encode the gp52 protein of spleen focus-forming virus are required for the induction of erythroid cell proliferation

A series of insertion-deletion mutants was constructed in a molecularly cloned DNA copy of the Friend strain of spleen focus-forming virus (SFFV). The mutants were produced by inserting a synthetic oligonucleotide linker containing the recognition sequence of SalI endonuclease into several different locations of the SFFV DNA. Three classes of mutants were isolated: insertion-deletion mutants in the 5' half of the SFFV genome, in the long terminal repeat of the SFFV genome, and in the env gene of the SFFV genome. The env gene mutant has a deletion of sequences shared in common between the env gene of SFFV and the env genes of mink cell focus-inducing murine leukemia viruses. From analyses of the biological activity of the various mutants and a biologically active subgenomic SFFV DNA fragment described herein, we can deduce that the coding sequence encompassing the env gene of SFFV is required for the biological activity. This region, required for the pathogenic phenotype, cannot be larger than 1.5 kilobase pairs, a size only slightly more than that sufficient to encode the nonglycosylated precursor of the gp52 env gene product.

Proteins specified by avian erythroblastosis virus: coding region localization and identification of a previously undetected erb-B polypeptide

Avian erythroblastosis virus (AEV) induces erythroblastosis and sarcomas in chickens. Two domains within the viral genome, erb-A and erb-B, have been implicated in AEV-mediated oncogenesis. By use of hybridization-arrested translations and hybridization-selections of mRNA, we have mapped on the viral genome the polypeptides specified by the erb domains. The results of hybridization-arrest with DNA representing the spliced 5' leader region of the AEV mRNA suggested that the authentic product of the erb-B domain was a 61,000 molecular weight protein, not a 41,000 molecular weight polypeptide previously identified.

Nuclear location of the putative transforming protein of avian myelocytomatosis virus

The putative transforming protein of avian myelocytomatosis virus MC29 is a 110,000 dalton (P110gag-myc) polyprotein comprised of sequences derived from both the gag region and the MC29-specific myc region. Two approaches have been taken to determine the location of the MC29 gag-related proteins in transformed cells: subcellular fractionation and immunofluorescence. Analysis of subcellular fractions of MC29-transformed cells by immunoprecipitation indicates that the majority of the gag-myc polyprotein is found in the nuclear fractions of Q8 cells (a nonproducer line of MC29-transformed quail embryo fibroblasts) and nonproducer cells derived from a liver tumor of MC20-infected quail. This is in contrast to the distribution of gag-related helper virus proteins lacking myc, which are found only in nonnuclear fractions of superinfected Q8 cells. The purity of unlabeled nuclei was assessed by electron microscopy and enzyme assays, revealing little contaminating material from other subcellular fractions. Immunofluorescence experiments using monospecific anti-gag serum showed specific, intense immunofluorescence in the nuclei of fixed Q8 cells. In contrast, the majority of P75gag-erb, a candidate transforming protein produced by avian erythroblastosis virus (AEV), is absent from the nuclei of nonproducer AEV-transformed chick embryo fibroblasts. The nuclear association of the MC29 transforming protein may be related to some of the unique properties of MC29-transformed cells.

Hormone-dependent terminal differentiation in vitro of chicken erythroleukemia cells transformed by ts mutants of avian erythroblastosis virus

Chicken erythroblast cell strains and a cell line transformed by ts mutants of avian erythroblastosis virus (AEV) terminally differentiate when shifted to the nonpermissive temperature (42 degrees C). The differentiated cells resemble mature erythrocytes with respect to morphology and ultrastructure, expression of differentiation-specific cell-surface antigens, pattern of protein synthesis and hemoglobin content. Terminal differentiation is dependent on conditions favoring the differentiation of normal erythroid progenitor cells, including an erythropoietin-like factor. Colonies of ts AEV cells grown at 42 degrees C semisolid medium resemble erythrocyte colonies derived from normal erythroid progenitor cells. The colonies obtained were comparable in size or slightly larger than the late erythroid precursor (CFU-E) colonies. These results suggest that AEV-transformed cells are blocked at a stage of differentiation that is more advanced than that of the uninfected target cells. ts AEV cells are irreversibly committed to terminal differentiation within 20 to 30 hr after shift to 42 degrees C.

Target cells infected by avian erythroblastosis virus differentiate and become transformed

Transformation in vitro of bone marrow cells by avian erythroblastosis virus (AEV) gives rise to rapidly growing cells of erythroid nature. Target cells of neoplastic transformation by AEV are recruited among the early progenitors of the erythroid lineage, the burst-forming units-erythroid (BFU-E). They express a brain-related antigen at a high level and an immature antigen at a low level. We show that AEV-transformed cells express low levels of the brain antigen and high levels of the immature antigen. Their response to specific factors regulating the erythroid differentiation indicates that they are very sensitive to erythropoietin. Furthermore, cells transformed by a temperature-sensitive mutant of AEV differentiate into hemoglobin-synthesizing cells 4 days after being shifted to the nonpermissive temperature. All these properties are similar to those of late progenitors of the erythroid lineage, the colony-forming units-erythroid (CFU-E). These results indicate that the AEV-transformed cells are blocked in their differentiation at the CFU-E stage.

Isolation and biochemical characterization of partially transformation-defective mutants of avian myelocytomatosis virus strain MC29: localization of the mutation to the myc domain of the 110,000-dalton gag-myc polyprotein

Recently, we isolated three mutants of MC29 virus which, although able to transform fibroblasts with the same efficiency as wild-type MC29, were 100-fold less efficient at transforming macrophages. In this study we found that MC29-transformed quail producer cell line Q10 was able to generate these partially transformation defective mutants at a high frequency. Using tryptic peptide mapping, we determined that the smaller gag-myc polyproteins encoded by the transformation-defective viruses had lost myc-specific tryptic peptides. This suggested that the mutations which resulted in the transformation-defective viruses being inefficient at transforming macrophages were located in the v-myc sequence and thus directly implicated v-myc and the gag-myc polyprotein in transformation by MC29.

Biochemical characterization of transformation-specific proteins of acute avian leukemia and sarcoma viruses

The biological and biochemical properties of the transformation-specific proteins of three avian oncornaviruses with different oncogenic potentials were compared, namely the gag-myc protein of the avian myelocytomatosis virus MC29, the gag-erb A protein of the avian erythroblastosis virus AEV, and the gag-fps protein of Fujinami sarcoma virus FSV. These oncogenes were analyzed in transformed fibroblasts that expressed only the transforming proteins but showed no virus replication. Monoclonal antibodies against the viral structural protein p19, which is the N-terminus of the proteins, were used for indirect immunofluorescence, for immunoprecipitation of the proteins from subcellular fractions, and for immunoaffinity column chromatography. With this last method a 3000-fold purification of the proteins was obtained. By indirect immunofluorescence it was shown that the gag-myc protein was located in the nucleus, and bound to DNA after purification. The gag-erb A protein was not nuclear but probably located in the cytoplasm and did not bind to DNA after purification. Neither of the two proteins exhibited protein kinase activity. In contrast, the gag-fps protein did not bind to DNA but showed protein kinase activity after purification. It was not located in the nucleus either.

Characterization of reticuloendotheliosis virus strain T DNA and isolation of a novel variant of reticuloendotheliosis virus strain T by molecular cloning

Reticuloendotheliosis virus strain T (REV-T) is a highly oncogenic avian retrovirus which causes a rapid neoplastic disease of the lymphoreticular system. Upon infection, this virus gives rise to two species of unintegrated linear viral DNA, which are 8.3 and 5.5 kilobase pairs long and represent the helper virus (REV-A) and the oncogenic component (REV-T), respectively. Restriction endonuclease cleavage maps of these two DNA components indicate that REV-T DNA has a large portion of the genome deleted with respect to REV-A DNA and a substitution about 0.8 to 1.5 kilobase pairs long that is unrelated to REV-A DNA. These additional sequences comprise the putative transforming region of REV-T (rel). A chicken spleen cell line transformed by REV-T produced virus which upon infection gives rise to three species of unintegrated linear viral DNA (8.3, 5.5, and 3,3 kilobase pairs). We isolated the proviruses of the 8.3- and 3.3-kilobase pair species from this cell line by cloning in the phage vector Charon 4A. Restriction enzyme mapping showed that the two proviral clones are proviruses of REV-A and a variant of REV-T, respectively. A subclone of the variant REV-T provirus specific for the rel sequences of REV-T was used as a hybridization probe to demonstrate that the rel sequences are different from the putative transforming sequences of Schmidt-Ruppin Rous sarcoma virus strain A, avain myelocytomatosis virus, avian myeloblastosis virus, avian erythroblastosis virus, Abelson murine leukemia virus, and Friend erythroleukemia virus. In addition, the rel-specific hybridization probe was used to identify a specific set of sequences which are present in uninfected avian DNAs digested with several restriction enzymes. The corresponding cell sequences are not arranged like rel in REV-T.

Revertants of rats cells transformed by avian erythroblastosis virus

Morphological revertants of the avian erythroblastosis virus (AEV)-transformed rat cell line ATla were isolated and characterised. The revertants are similar to the uninfected parental rat cell line in that they have regained an organized cytoskeleton and they are no longer capable of anchorage-independent growth. The pattern of integrated viral DNA in the revertants is indistinguishable from that of the transformed parent. However, the revertants do not express the integrated viral genome at either the mRNA or protein level. Phenotypic reversion thus is probably .due to reduced transcription of the AEV-transforming gene below a threshold necessary to induce morphological transformation.

Proposal for naming host cell-derived inserts in retrovirus genomes

We propose a system for naming inserted sequences in transforming retroviruses (i.e., onc genes), based on using trivial names derived from a prototype strain of virus.

Three distinct genes in human DNA related to the transforming genes of mammalian sarcoma retroviruses

Southern blot hybridization was used to identify human and other vertebrate DNA sequences that were homologous to cloned DNA fragments containing the oncogenic nucleic acid sequences of three different type C mammalian retroviruses (simian sarcoma virus, the Snyder-Theilen strain of feline sarcoma virus, and the Harvey strain of murine sarcoma virus). Each onc gene counterpart has a single genetic locus, which probably contains non-onc intervening sequences. The human DNA sequences may represent genes important to cell growth or cell differentiation, or both. Their identification and isolation may allow elucidation of their role in these processes and in neoplasias.

Isolation of monoclonal antibodies against avian oncornaviral protein p19

For the production of monoclonal antibodies against pp60src and the gag precursor protein Pr76gag, the spleens of mice bearing tumors that had been induced by avian sarcoma virus Schmidt-Ruppin D-transformed cells were used. One hybridoma culture produced antibodies that were directed against the p19 portion of the gag precursor. However, no antibodies directed against pp60src could be detected in any of the hybridoma supernatants. The anti-p19-producing hybridoma culture was cloned twice in soft agar, and a stable clone was used for the production of high-titer ascites fluid in mice. The monoclonal antibodies belonged to the immunoglobulin G subclass 2b. The antibodies precipitated Pr76gag and the processed virion-associated p19, as well as the 75,000-molecular-weight gag fusion protein from avian erythroblastosis virus-transformed bone marrow cells. Also, viral ribonucleoprotein complexes were specifically precipitable, indicating that they contain p19 molecules.

Amino acid sequence of p15 from avian myeloblastosis virus complex

The complete amino acid sequence of the p15 gag protein from avian myeloblastosis virus (AMV) complex has been determined by sequential Edman degradation of the intact molecule and of peptide fragments generated by limited tryptic cleavage, cleavage with staphylococcal protease, and cyanogen bromide cleavage. AMV p15 is a single-chain protein containing 124 amino acids. The charged amino acids tend to be clustered in the primary structure. p15 contains a single cysteine at position 113 which may be essential for the p15 associated proteolytic activity. However, p15 shows no appreciable sequence homology with papain or other classical thiol proteases.

Genome structure of avian sarcoma virus Y73 and unique sequence coding for polyprotein p90

The genome structure of a newly isolated sarcoma virus, Y73, was studied. Y73 is a defective, potent sarcomagenic virus and contains 4.8-kilobase (kb) RNA as its genome; in contrast, helper virus associated with Y73 had 8.5-kb RNA, similar to other avian leukemia viruses. Fingerprinting analysis these RNAs demonstrated that the 4.8-kb RNA contains a specific RNA sequence of 2.5 kb, which represents the transforming gene (yas) of Y73. This specific sequence was mapped in the middle of the genome and had at both ends 1- to 1.5-kb sequences in common with Y73-associated virus RNA. This structure is very similar to those of avian and mammalian leukemia viruses. In vitro translation of the 4.8-kb RNA and the immunospecificity of the products directly demonstrated that polyprotein p90, containing p19, is a product translated from capped 4.8-kb RNA and that the specific peptide portion is coded by the yas sequence. Protein 90, which was also found in cells transformed with Y73, was suggested to be a transforming protein.

Characterization of the oncogene (erb) of avian erythroblastosis virus and its cellular progenitor

Avian erythroblastosis virus (AEV) induces primarily erythroblastosis when injected intravenously into susceptible chickens. In vitro, the hematopoietic target cells for transformation are the erythroblasts. Occasional sarcomas are also induced by intramuscular injection, and chicken or quail fibroblasts can be transformed in vitro. The transforming capacity of AEV was shown to be associated with the presence of a unique nucleotide sequence denoted erb in its genomic RNA. Using a simplified procedure, we prepared radioactive complementary DNA (cDNAaev) representative of the erb sequence at a high yield. Using a cDNAaev excess liquid hybridization technique adapted to defective retroviruses, we determined the complexity of the erb sequence to be 3,700 +/- 370 nucleotides. AEV-transformed erythroblasts, as well as fibroblasts, contained two polyadenylated viral mRNA species of 30 and 23S in similar high abundance (50 to 500 copies per cell). Both species were efficiently packaged into the virions. AEV-transformed erythroblasts contained additional high-molecular-weight mRNA species hybridizing with cDNAaev and cDNA5' but not with cDNA made to the helper leukosis virus used (cDNArep). The nature and the role, if any, of these bands remain unclear. The erb sequence had its counterpart in normal cellular DNA of all higher vertebrate species tested, including humans and fish (1 to 2 copies per haploid genome in the nonrepetitive fraction of the DNA). These cellular sequences (c-erb) were transcribed at low levels (1 to 2 RNA copies per cell) in chicken and quail fibroblasts, in which the two alleged domains of AEV-specific sequences corresponding to the 75,000- and 40,000-molecular-weight proteins seemed to be conserved phylogenetically and transcribed at similar low rates.

Avian reticuloendotheliosis virus: characterization of genome structure by heteroduplex mapping

The genome structure of defective, oncogenic avian reticuloendotheliosis virus (REV) was studied by heteroduplex mapping between the full-length complementary DNA of the helper virus REV-T1 and the 30S REV RNA. The REV genome (5.5 kilobases) had a deletion of 3.69 kilobases in the gag-pol region, confirming the genetic defectiveness of REV. In addition, REV lacked the sequences corresponding to the env gene but contained, instead, a contiguous stretch (1.6 to 1.9 kilobases) of the specific sequences presumably related to viral oncogenicity. Unlike those of other avian acute leukemia viruses, the transformation-specific sequences of REV were not contiguous with the gag-pol deletion. Thus, REV has a genome structure similar to that of a defective mink cell focus-inducing virus or a defective murine sarcoma virus. An additional class of heteroduplex molecules containing the gag-pol deletion and two other smaller deletion loops was observed. These molecules probably represented recombinants between the oncogenic REV and its helper virus.

Evidence for three classes of avian sarcoma viruses: comparison of the transformation-specific proteins of PRCII, Y73, and Fujinami viruses

The gag-linked transformation-specific proteins (polyproteins( of PRCII, Fujinami, and Y73 avian sarcoma viruses have been compared by tryptic peptide mapping. In addition to shared gag peptides, PRCII polyprotein p105 and FSV polyprotein p140 were found to have seven methionine-containing and five cysteine-containing tryptic peptides in common. These represent the majority of the non-gag peptides for each virus. In contrast, no overlap was detected with the non-gag peptides of the Y73 polyprotein p90. Examination of the tryptic phosphopeptides of p105, p140, and p90 labeled by their associated protein kinases gave similar results. Although the major phosphopeptides of p105 and p140 comigrated, they were distinct from the phosphopeptide of p90. Three classes of transformation-specific proteins can now be identified among known avian sarcoma viruses. After the pp60src of Rous sarcoma virus and B77 virus, the proteins of PRCII and Fujinami virus form a second class and Y73--currently the only representative--characterizes the third. Despite their structural differences, these viruses may share a common mechanism of transformation, effected by their associated protein kinases.

Biological activity of the spleen focus-forming virus is encoded by a molecularly cloned subgenomic fragment of spleen focus-forming virus DNA

A biologically active subgenomic DNA fragment of a polycythemia-inducing strain of the replication-defective spleen focus-forming virus (SFFV) has been molecularly cloned. The SFFV DNA fragment includes 2.0 kilobase pairs (kbp) from the 3' end of SFFV, the long terminal repeat sequences of SFFV, and 0.4 kbp from the 5' end of SFFV. The fragment contains the previously described env-related gene of SFFV. All the properties associated with SFFV can be assigned to this SFFV DNA fragment by using a two-stage DNA transfection assay with infectious helper virus DNA. The virus recovered from the transfection assays can induce erythroblastosis, splenic foci, and polycythemia in infected mice. Fibroblast cultures transfected with the SFFV DNA fragment synthesize gp52, the known intracellular product of the env-related gene of SFFV. gp52 can also be detected in spleens from diseased mice infected with the virus recovered in the two-stage transfection. The results are consistent with the hypothesis that the env-related gene sequences of SFFV and their product gp52 are required for the initiation of SFFV-induced disease.

Nucleotide sequence and formation of the transforming gene of a mouse sarcoma virus

The complete nucleotide sequence of the transforming gene of a mouse sarcoma virus has been determined. It codes for a protein of 374 amino acids. The nucleotide sequence of the junctions between a murine leukaemia virus and cellular sequences leading to the formation of the viral transforming gene have also been elucidated. The viral transforming sequence and its cellular homologue share an uninterrupted stretch of 1,159 nucleotides, with few base substitutions. The predicted amino acid sequence of the mouse sarcoma virus transforming gene was found to share considerable homology with the proposed amino acid sequence of the avian sarcoma virus oncogene (src) product.

The genome and the intracellular RNAs of avian myeloblastosis virus

Avian myeloblastosis virus (AMV) is an acute leukemia virus which causes a myeloblastic leukemia in birds and transforms myeloid hematopoietic cells in vitro. We have analyzed RNA from AMV virions and from AMV-transformed producer and nonproducer cells by gel electrophoresis followed by transfer to chemically activated paper and hybridization to several complementary DNA (cDNA) probes. Using a cDNA probe specific for AMV, we identified two RNA species of 7.2 and 2.3 kb, which were present in all AMV-transformed cells and in all AMV virion preparations examined. The 7.2 kb species, which is presumably the genome of AMV, appears to contain the entire retroviral gag gene and at least part of the pol gene, but lacks much (or all) of the env gene. Thus AMV differs from other acute leukemia viruses described to date, since the latter have genomes of 5.5 to 5.6 kb, have only part of the gag gene and lack pol sequences. The smaller RNA does not contain gag-, pol- or env-specific nucleotide sequences but does carry nucleotide sequences from both the 5' and 3' termini of the genome, suggesting that it may be a subgenomic mRNA. Both the 7.2 and 2.3 kb species were associated with the 70S RNA complex in virions. These results suggest that AMV, unlike other acute leukemia viruses, does not express its transforming gene via a gag-related "fusion" protein but rather as a (so far unidentified) protein translated from a subgenomic mRNA.

OK10, an avian acute leukemia virus of the MC 29 subgroup with a unique genetic structure

The RNA of defective avian acute leukemia virus OK10 was isolated from a defective virus particle, released by OK10-transformed nonproducer avian fibroblasts, as a 60S complex consisting of 8.6-kilobase subunits. Oligonucleotide fingerprinting and RNA.cDNA hybridization identified two sets of sequences in OK10 RNA: group-specific sequences, which are related to all nondefective members of the avian tumor virus group, and a sequence closely related to the subgroup-specific sequences (mcv) of the myelocytomatosis virus (MC29) subgroup of avian acute leukemia viruses. Hence, OK10 is classified as a member of the MC29 subgroup of avian tumor viruses, in agreement with classification based on its oncogenic spectrum. The group-specific sequences of OK10 RNA include partial (Delta) pol and env genes, a c-region, and, unlike those of all other members of the MC29 subgroup, a complete gag gene. Oligonucleotide mapping revealed 5'-gag-Deltapol-mcv-Deltaenv-c-3' as the order of the subgroup-specific and group-specific elements of OK10 RNA. The genetic unit gag-Deltapol-mcv, measuring approximately 6.4 kilobases, codes for the nonstructural, presumably transforming, 200,000-dalton OK10-specific protein and also includes the gag gene coding for the internal virion proteins. Because gag is the only intact virion gene shared in addition to regulatory RNA sequences between OK10 and nondefective avian tumor viruses, it is concluded that the gag gene is sufficient for the formation of a defective virus particle. Comparisons among the RNAs and gene products of different viruses of the MC29 subgroup show that they share 5'-terminal gag-related and internal mcv sequences but differ from each other in intervening gag-, pol-, and mcv-related sequences. It follows that the probable transforming genes and their protein products have two essential domains, one consisting of conserved 5' gag-related and the other of 3' mcv-related sequence elements. In the light of this and previous knowledge we can now distinguish two designs among five different transforming onc genes of avian tumor viruses: onc genes with coding sequences unrelated to virion genes, like those of Rous sarcoma virus and avian myeloblastosis virus, and onc genes with coding sequences that are hybrids of virion genes and specific sequences, like those of the MC29 subgroup viruses, of avian erythroblastosis virus, and of Fujinami sarcoma virus.

Avian erythroblastosis virus produces two mRNA's

We analyzed the viral mRNA's present in fibroblast nonproducer clones transformed by avian erythroblastosis virus. Two size classes of mRNA (28 to 30S and 22 to 24S) were identified by solution hybridization with both complementary DNA strong stop and complementary DNA made against the unique sequences of avian erythroblastosis virus. Based upon the kinetics of hybridization with complementary DNA made against the unique sequences of avian erythroblastosis virus, we estimated that there were 400 to 500 copies of the 28 to 30S RNA per cell and 200 to 250 copies of the 22 to 24S RNA per cell. Both RNA species were packaged in the virion. In vitro translation of the 28 to 30S virion RNA yielded a 75,000-dalton protein which was the 75,000-dalton gag-related polyprotein found in avian erythroblastosis virus-transformed cells. In vitro translation of the 22 to 24S virion RNA yielded two proteins (46,000 and 48,000 daltons). This indicates that there may be two genes in avian erythroblastosis virus, one coding for the 75,000-dalton gag-related polyprotein and the second coding for the 46,000- or 48,000-dalton protein or both.

Early precursors in the erythroid lineage are the specific target cells of avian erythroblastosis virus in vitro

In chickens the erythroid differentiation proceeds from stem cells to erythrocytes through several intermediate steps which have been identified in vivo and in vitro. To determine whether Avian erythroblastosis virus (AEV) is able to transform in vitro either one or several types of these precursors, bone marrow cells were separated by physical and immunological methods. It was found that the target cells which could be transformed in vitro by AEV were cells of light density (1.060-1.065 g/cm3), having a modal sedimentation velocity at unit gravity between 4.0 and 6.0 mm/hr, expressing an immature antigen at a low level and a brain-related antigen at a high level. These results indicated that the target cells of neoplastic transformation by AEV were early erythroid precursors, since these precursors shared the same physical and immunological properties with AEV target cells.

Phosphorylation of the nonstructural proteins encoded by three avian acute leukemia viruses and by avian fujinami sarcoma virus

The gag gene-related, nonstructural proteins of three avian acute leukemia viruses (namely, myelocytomatosis viruses MC29 and CMII and avian erythroblastosis virus) and of avian Fujinami sarcoma virus (FSV) isolated by immunoprecipitation from cellular lysates with anti-gag serum were shown to be phosphoproteins in vivo. The specific 32P radioactivity of the nonstructural proteins of MC29, CMII, and FSV was significantly higher than that of helper viral, intracellular gag proteins. Two of these proteins, i.e., the 140,000-dalton FSV and the 110,000-dalton MC29 proteins, were also phosphorylated in vitro by a kinase activity associated with immunocomplexes. This kinase activity is either separated from these proteins or inactivated by incubation of cellular lysates with normal serum followed by adsorption to staphylococcal protein A or sedimentation at 100,000 x g or both. It remains to be resolved whether the 110,000-dalton MC29 and 140,000-dalton FV proteins, in addition to being substrates for phosphorylation, also have intrinsic kinase activity.

Molecular cloning of the avian erythroblastosis virus genome and recovery of oncogenic virus by transfection of chicken cells

Avian erythroblastosis virus (AEV) causes erythroblastosis and sarcomas in birds and transforms both erythroblasts and fibroblasts to neoplastic phenotypes in culture. The viral genetic locus required for oncogenesis by AEV is at present poorly defined; moreover, we know very little of the mechanism of tumorigenesis by the virus. To facilitate further analysis of these problems, we used molecular cloning to isolate the genome of AEV as recombinant DNA in a procaryotic vector. The identity of the isolated DNA was verified by mapping with restriction endonucleases and by tests for biological activity. The circular form of unintegrated AEV DNA was purified from synchronously infected quail cells and cloned into the EcoRI site of lambda gtWES x B. A restriction endonuclease cleavage map was established. By hybridization with complementary DNA probes representing specific parts of avian retrovirus genomes, the restriction map of the cloned AEV DNAs was correlated with a genetic map. These data show that nucleotide sequences unique to AEV comprise at least 50% of the genome and are located approximately in the middle of the AEV genome. Our data confirm and extend previous descriptions of the AEV genome obtained by other procedures. We studied in detail two recombinant clones containing AEV DNA: the topography of the viral DNA in the two clones was virtually identical, except that one clone apparently contained two copies of the terminal redundancy that occurs in linear viral DNA isolated from infected cells; the other clone probably contained only one copy of the redundant sequence. To recover infectious virus from the cloned DNA, we developed a procedure for transfection that compensated for the defectiveness of AEV in replication. We accomplished this by ligating cloned AEV DNA to the cloned DNA of a retrovirus (Rous-associated virus type 1) whose genome could complement the deficiencies of AEV. Ligation of the two viral DNAs was facilitated by using a neutral fragment of DNA as linker between otherwise noncompatible termini. Cloned AEV DNA gave rise to infectious AEV capable of transforming fibroblasts and bone marrow cells in culture and of inducing both sarcomas and erythroleukemia in chickens. We conclude that the cloned DNAs represent the authentic genome of AEV undisturbed by the cloning procedure. Molecular cloning offers a powerful approach to the identification and characterization of retrovirus genomes.

Genetic variation and host markers in the src gene of recovered avian sarcoma viruses

The src genes of three recovered avian sarcoma viruses were compared by RNase T1 oligonucleotide fingerprinting and tryptic peptide analysis. In all three recovered avian sarcoma viruses the oligonucleotide composition of src was different and also distinct from that of the parental Schmidt-Ruppin strain of Rous sarcoma virus. This evidence for genetic variation src was strengthened by two dimensional peptide maps of the src gene products pp60src, translated in a reticulocyte lysate system in vitro. Numerous differences between the peptide patterns of the pp60src proteins produced by the parental and the recovered viruses were detected. No two src proteins were identical, while the tryptic peptide maps of the internal gag proteins synthesized by these viruses were indistinguishable. The src proteins of recovered avian sarcoma viruses also contained peptides that were absent from the src protein of parental Schmidt-Ruppin D virus but were found in the endogenous src protein of normal cells. We conclude that there is considerable genetic variation in the src gene of recovered avian sarcoma viruses and that these recovered src genes contain host cell-derived markers.

Mutant of avian erythroblastosis virus defective for erythroblast transformation: deletion in the erb portion of p75 suggests function of the protein in leukemogenesis

Previous studies have shown that td359 AEV, a mutant of avian erythroblastosis virus (AEV), is unable to transform erythroblasts in vitro or in vivo but is capable of transforming fibroblasts in vitro and of causing sarcomas in chicks. In this paper we show that the mutant synthesizes a gag-gene related protein (delta p75) which is about 1000 daltons smaller than the protein, p75, induced by wild-type AEV. The mutant protein lacks 3 of the approximately 53 lysine-arginine tryptic peptides resolved in p75 and also contains an additional peptide. By cleavage of delta p75 with p15 protease and analysis of the fragments for size and peptide composition, the deletion in delta p75 could be located in the non-gag region of the molecule. In contrast, with p40 AEV, a second AEV-specific protein synthesized in in vitro translation experiments, there is no change in size of translation products obtained from td359 AEV RNA. Our data provide direct evidence that p75 is required for erythroblast transformation.

Characterization of Y73, an avian sarcoma virus: a unique transforming gene and its product, a phosphopolyprotein with protein kinase activity

The Y73 strain of avian sarcoma virus recently isolated in Japan is defective in replication and is associated with subgroup A leukosis virus (YAV). The virus caused sarcoma but not acute leukosis when inoculated into chickens. Studies on the viral RNA showed that a 26S RNA, etimated to be 4.8 kilobases long, was Y73 viral RNA carrying a transforming gene. The 26S RNA has sequences in common with the RNA of an avian leukosis virus but no homology with the src gene sequence of avian sarcoma virus (ASV). Thus, Y73 has a unique sarcoma-inducing gene. A phosphorylated polyprotein of 90,000 daltons (p90) was immunoprecipitated from extracts of Y73-transformed chicken embryo cells by a variety of antisera reacting with gag gene products. When a bacteria-bound immunocomplex containing the p90 protein was incubated with [gamma-32P]ATP, the Y73-specific p90 and the IgG heavy chain were phosphorylated by a p90-associated protein kinase. The amino acid phosphorylated in vitro was exclusively tyrosine in both cases, whereas p90 phosphorylated in vivo contained phosphoserine as a major phospho amino acid with traces of phosphotyrosine and phosphothreoine.

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.