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

Retroviral oncogenes: a historical primer

Retroviruses are the original source of oncogenes. The discovery and characterization of these genes was made possible by the introduction of quantitative cell biological and molecular techniques for the study of tumour viruses. Key features of all retroviral oncogenes were first identified in src, the oncogene of Rous sarcoma virus. These include non-involvement in viral replication, coding for a single protein and cellular origin. The MYC, RAS and ERBB oncogenes quickly followed SRC, and these together with PI3K are now recognized as crucial driving forces in human cancer.

Association of v-ErbA with Smad4 disrupts TGF-beta signaling

Disruption of the transforming growth factor-beta (TGF-beta) pathway is observed in the majority of cancers. To further understand TGF-beta pathway inactivation in cancer, we stably expressed the v-ErbA oncoprotein in TGF-beta responsive cells. v-ErbA participates in erythroleukemic transformation of cells induced by the avian erythroblastosis virus (AEV). Here we demonstrate that expression of v-ErbA was sufficient to antagonize TGF-beta-induced cell growth inhibition and that dysregulation of TGF-beta signaling required that v-ErbA associate with the Smad4 which sequesters Smad4 in the cytoplasm. We also show that AEV-transformed erythroleukemia cells were resistant to TGF-beta-induced growth inhibition and that TGF-beta sensitivity could be recovered by reducing v-ErbA expression. Our results reveal a novel mechanism for oncogenic disruption of TGF-beta signaling and provide a mechanistic explanation of v-ErbA activity in AEV-induced erythroleukemia.

Reflections on the pathogenesis of diseases caused by the acute avian leukosis/sarcoma viruses with special reference to avian erythroblastosis

The various diseases that follow experimental infection with the acute and non-acute avian oncoviruses are discussed with special reference to the pathogenesis of avian erythroblastosis. One view, based on in vitro studies, sees erythroblastosis as the product of a failure in the differentiation of virus-infected stem cells to mature erythrocytes, as a result of cell 'transformation'. The results of some in vivo studies, however, point to a resemblance of the disease to a haemolytic anaemia, where cellular death is an important component. It seems probable that the disease is the result of transformation of cells of the erythroblastic series followed by the death of many of these cells due to influences that have not yet been determined. Determination of the causes of this cellular death may prove to be as important for our understanding of the problem of leukaemia as the work that has already been accomplished in explaining the causes of cell transformation. It is also suggested that the tendency of gs amino acid sequences of the avian leukosis viruses and mouse leukaemia viruses to form fusion proteins with a variety of proto-oncogenes may be part of a wider phenomenon, and that these sequences may fuse with other proteins, altering their properties. More work is required on the possibility that there is an undiscovered immunological component in the progression of the L/S diseases.

Inhibitors of HIV-1 protease

The human immunodeficiency virus (HIV), the etiological agent for the acquired immune deficiency syndrome (AIDS), is a retrovirus which makes use of a virally-encoded aspartic protease to perform specific proteolytic processing of two of its gene products in order to form active enzymes and structural proteins within the mature virion. Accordingly, specific, exogenous inhibition of the HIV-1 protease is thought to be a viable approach for the development of novel therapeutics for the treatment of AIDS. Indeed, this hypothesis has been validated in virally-infected cell culture with synthetic inhibitors of HIV-1 protease. This chapter reviews the current status of the development of inhibitors of this enzyme.

Ectopic expression of the erythrocyte band 3 anion exchange protein, using a new avian retrovirus vector

A retrovirus vector was constructed from the genome of avian erythroblastosis virus ES4. The v-erbA sequences of avian erythroblastosis virus were replaced by those coding for neomycin phosphotransferase, creating a gag-neo fusion protein which provides G418 resistance as a selectable marker. The v-erbB sequences following the splice acceptor were replaced by a cloning linker allowing insertion of foreign genes. The vector has been tested in conjunction with several helper viruses for the transmission of G418 resistance, titer, stability, transcription, and the transduction and expression of foreign genes in both chicken embryo fibroblasts and the QT6 quail cell line. The results show that the vector is capable of producing high titers of Neor virus from stably integrated proviruses. These proviruses express a balanced ratio of genome length to spliced transcripts which are efficiently translated into protein. Using the Escherichia coli beta-galactosidase gene cloned into the vector as a test construct, expression of enzyme activity could be detected in 90 to 95% of transfected target cells and in 80 to 85% of subsequently infected cells. In addition, a cDNA encoding the avian erythrocyte band 3 anion exchange protein has been expressed from the vector in both chicken embryo fibroblasts and QT6 cells and appears to function as an active, plasma membrane-based anion transporter. The ectopic expression of band 3 protein provides a visual marker for vector function in these cells.

Studies of the mechanisms of action of the antiretroviral agents hypericin and pseudohypericin

Administration of the aromatic polycyclic dione compounds hypericin or pseudohypericin to experimental animals provides protection from disease induced by retroviruses that give rise to acute, as well as slowly progressive, diseases. For example, survival from Friend virus-induced leukemia is significantly prolonged by both compounds, with hypericin showing the greater potency. Viremia induced by LP-BM5 murine immunodeficiency virus is markedly suppressed after infrequent dosage of either substance. These compounds affect the retroviral infection and replication cycle at least at two different points: (i) Assembly or processing of intact virions from infected cells was shown to be affected by hypericin. Electron microscopy of hypericin-treated, virus-producing cells revealed the production of particles containing immature or abnormally assembled cores, suggesting the compounds may interfere with processing of gag-encoded precursor polyproteins. The released virions contain no detectable activity of reverse transcriptase. (ii) Hypericin and pseudohypericin also directly inactivate mature and properly assembled retroviruses as determined by assays for reverse transcriptase and infectivity. Accumulating data from our laboratories suggest that these compounds inhibit retroviruses by unconventional mechanisms and that the potential therapeutic value of hypericin and pseudohypericin should be explored in diseases such as AIDS.

Thyroid hormone action and the erbA oncogene family

In this review, we discuss the biological action and biochemical function of the v-erbA oncogene product, and the role of c-erbA proto-oncogene products as thyroid hormone receptors, as related to the molecular structure and function of the nuclear hormone receptors at large.

c-erbA encodes multiple proteins in chicken erythroid cells

To identify and characterize the proteins encoded by the erbA proto-oncogene, we expressed the C-terminal region of v-erbA in a bacterial trpE expression vector system and used the fusion protein to prepare antiserum. The anti-trp-erbA serum recognized the P75gag-erbA protein encoded by avian erythroblastosis virus and specifically precipitated six highly related proteins ranging in size from 27 to 46 kilodaltons from chicken embryonic erythroid cells. In vitro translation of a chicken erbA cDNA produced essentially the same pattern of proteins. Partial proteolytic maps and antigenicity and kinetic analyses of the in vivo and in vitro proteins indicated that they are related and that the multiple bands are likely to arise from internal initiations within c-erbA to generate a nested set of proteins. All of the c-erbA proteins are predominantly associated with chicken erythroblast nuclei. However, Nonidet P-40 treatment resulted in extraction of the three smaller proteins, whereas the larger proteins were retained. During differentiation of erythroid cells in chicken embryos, we found maximal levels of c-erbA protein synthesis at days 7 to 8 of embryogenesis. By contrast, c-erbA mRNA levels remained essentially constant from days 5 to 12. Together, our results indicate that posttranscriptional or translational mechanisms are involved in regulation of c-erbA expression and in the complexity of its protein products.

c-erbA encodes multiple proteins in chicken erythroid cells

To identify and characterize the proteins encoded by the erbA proto-oncogene, we expressed the C-terminal region of v-erbA in a bacterial trpE expression vector system and used the fusion protein to prepare antiserum. The anti-trp-erbA serum recognized the P75gag-erbA protein encoded by avian erythroblastosis virus and specifically precipitated six highly related proteins ranging in size from 27 to 46 kilodaltons from chicken embryonic erythroid cells. In vitro translation of a chicken erbA cDNA produced essentially the same pattern of proteins. Partial proteolytic maps and antigenicity and kinetic analyses of the in vivo and in vitro proteins indicated that they are related and that the multiple bands are likely to arise from internal initiations within c-erbA to generate a nested set of proteins. All of the c-erbA proteins are predominantly associated with chicken erythroblast nuclei. However, Nonidet P-40 treatment resulted in extraction of the three smaller proteins, whereas the larger proteins were retained. During differentiation of erythroid cells in chicken embryos, we found maximal levels of c-erbA protein synthesis at days 7 to 8 of embryogenesis. By contrast, c-erbA mRNA levels remained essentially constant from days 5 to 12. Together, our results indicate that posttranscriptional or translational mechanisms are involved in regulation of c-erbA expression and in the complexity of its protein products.

The avian erythroblastosis virus erbA oncogene encodes a DNA-binding protein exhibiting distinct nuclear and cytoplasmic subcellular localizations

The protein product of the v-erbA oncogene of avian erythroblastosis virus was analyzed by use of site-specific antisera. The v-erbA protein was found to exist in distinct nuclear and cytoplasmic forms. Both nuclear and cytoplasmic species of the v-erbA protein were capable of binding to DNA, a property predicted based on the structural relatedness the v-erbA polypeptide shares with the thyroid and steroid hormone receptors. A mutation within the v-erbA coding region which inhibited DNA binding and nuclear localization also inhibited the ability of the v-erbA protein to potentiate erythroid transformation, consistent with a model of the v-erbA protein as a transcriptional regulator.

Temperature-sensitive v-sea transformed erythroblasts: a model system to study gene expression during erythroid differentiation

The isolation and characterization of a temperature-sensitive mutant (ts1 S13) of the avian erythroblastosis virus, S13, is described. The temperature-sensitive lesion in ts1 S13 was identified as affecting the tyrosine kinase activity but not the plasma membrane localization of the ts1 S13 v-sea gene product. Erythroblasts transformed by ts1 S13 can be induced to synchronously differentiate into erythrocytes in an erythropoietin (EPO)-dependent fashion. Analysis of erythrocyte-specific gene expression in ts1 S13 erythroblasts reveals that the transformed, self-renewing erythroblasts obtained at permissive temperature already express all erythrocyte genes tested for, although at a low level. Upon differentiation induction, expression of erythrocyte-specific genes is not coordinately regulated but rather involves complex regulatory mechanisms that appear to be specific for the individual genes.

A simple method for immunoaffinity purification of nondenatured avian sarcoma and leukemia virus gag-containing proteins

We have developed a one-step purification procedure for proteins containing the N-terminal portion of the gag protein of avian sarcoma and leukemia viruses. In this procedure, a resin with a covalently attached monoclonal antibody to the gag protein p19 is used to bind gag-containing proteins from crude extracts. After washing of the resin, the bound proteins are eluted with 2 M MgCl2. For the transforming protein kinase encoded by Fujinami sarcoma virus p130gag-fps, this procedure gave an enrichment of several thousand-fold, a yield of over 10%, a final purity of over 20%, and no significant loss of protein kinase activity. Similar purifications were obtained with three other gag-containing proteins. The immunoaffinity purification described may be of general utility as a first step in purification of the several other avian retroviral transforming proteins that are synthesized from fusions of an oncogene with the viral gag gene.

v-myb does not prevent the expression of c-myb in avian erythroblasts

The v-myb oncogene of avian myeloblastosis virus transforms myeloid cells exclusively, both in vivo and in vitro. The c-myb proto-oncogene from which v-myb arose is expressed at relatively high levels in immature hematopoietic cells of the lymphoid, erythroid, and myeloid lineages but not in myeloblasts transformed by v-myb. This finding suggested that the nuclear v-myb gene product p48v-myb might act directly to inhibit the normal expression of the c-myb gene. I have therefore used a selectable avian retroviral vector to express p48v-myb in avian erythroblasts which normally express high levels of the c-myb gene product p75c-myb. The results demonstrate that p48v-myb and p75c-myb can be coexpressed in the nuclei of cloned cells. Therefore, p48v-myb does not invariably prevent the expression of p75c-myb.

The chicken c-erbA proto-oncogene is preferentially expressed in erythrocytic cells during late stages of differentiation

We analyzed the expression of the c-erbA proto-oncogene in different tissues of chicken embryos. c-erbA transcripts were found at low levels in the lung, kidney, liver, and heart and in high amounts in embryonic blood cells. Nuclease mapping assays proved that these transcripts were true c-erbA transcripts. In situ hybridization on fractionated embryonic blood cells showed that c-erbA transcripts were predominantly found in erythroblasts, particularly during the final step of differentiation. Life span analysis of c-erbA mRNAs revealed their relative instability, demonstrating that the high level of c-erbA transcripts in embryonic erythroblasts was not the result of passive accumulation. These results suggest that the c-erbA genes play some role in erythrocyte differentiation.

Nucleotide sequence of the chicken proto-oncogene c-erbA corresponding to domain 1 of v-erbA

The nucleotide sequence of the chicken proto-oncogene c-erbA, the cellular counterpart of the viral oncogene v-erbA domain 1, has been determined. The c-erbA gene has an exon-intron structure characteristic of the eukaryotic split genes. The c-erbA domain 1 is composed of at least eight exons and seven introns. Analysis of the sequence data reveals a long open reading frame of 239 amino acid residues that share highly significant homology with the viral protein. Our results show that the viral erbA oncogene is a truncated form of its cellular homologue. In fact the cellular gene is 42 nucleotides longer at its 5' coding extremity. The presence of a gag-specific (17/20) nucleotide stretch in this region favours the hypothesis of a homologous recombination event between the cellular erbA and the gag gene of a parental retrovirus. We have also sequenced 1400 bp of DNA lying 5' to the long open reading frame in search of the transcription initiation sites for the c-erbA messenger RNAs. Our findings and interpretations are presented here.

The chicken c-erbA proto-oncogene is preferentially expressed in erythrocytic cells during late stages of differentiation

We analyzed the expression of the c-erbA proto-oncogene in different tissues of chicken embryos. c-erbA transcripts were found at low levels in the lung, kidney, liver, and heart and in high amounts in embryonic blood cells. Nuclease mapping assays proved that these transcripts were true c-erbA transcripts. In situ hybridization on fractionated embryonic blood cells showed that c-erbA transcripts were predominantly found in erythroblasts, particularly during the final step of differentiation. Life span analysis of c-erbA mRNAs revealed their relative instability, demonstrating that the high level of c-erbA transcripts in embryonic erythroblasts was not the result of passive accumulation. These results suggest that the c-erbA genes play some role in erythrocyte differentiation.

Expression of the v-erbA oncogene in chicken embryo fibroblasts stimulates their proliferation in vitro and enhances tumor growth in vivo

In contrast to uninfected chicken embryo fibroblasts (CEFs), CEFs infected with a retroviral vector that carries the v-erbA gene of avian erythroblastosis virus displayed new properties. These included limited anchorage-independent growth in soft agar, growth without latency in serum-supplemented medium, ability to overcome quiescence induced by serum deprivation, growth at low cell density, and an extended life span in vitro. Furthermore, when explanted in vivo onto the chorioallantoic membrane of chicken embryo, the transformed CEFs expressing v-erbA in addition to v-erbB exhibited a high proliferative rate, giving rise to fibrosarcoma tumors that were ten times larger than those developed from transformed CEFs expressing v-erbB alone. All these data show that CEFs expressing the v-erbA oncogene display activated growth and suggest that the v-erbA product interferes with the mechanisms regulating the growth and/or differentiation of primary CEFs.

A single amino acid substitution in v-erbB confers a thermolabile phenotype to ts167 avian erythroblastosis virus-transformed erythroid cells

A library of recombinant bacteriophage was prepared from ts167 avian erythroblastosis virus-transformed erythroid precursor cells (HD6), and integrated proviruses from three distinct genomic loci were isolated. A subclone of one of these proviruses (pAEV1) was shown to confer temperature-sensitive release from transformation of erythroid precursor cells in vitro. The predicted amino acid sequence of the v-erbB polypeptide from the mutant had a single amino acid change when compared with the wild-type parental virus. When the wild-type amino acid was introduced into the temperature-sensitive avian erythroblastosis virus provirus in pAEV1, all erythroid clones produced in vitro were phenotypically wild type. The mutation is a change from a histidine to an aspartic acid in the temperature-sensitive v-erbB polypeptide. It is located in the center of the tyrosine-specific protein kinase domain and corresponds to amino acid position 826 of the human epidermal growth factor receptor sequence.

Immunoblotting with polyclonal and monoclonal antibody to avian myeloblastosis protein p27: studies of liver proteins in chickens with erythroblastosis

An antigen detected by complement fixation with polyclonal antibody to avian myeloblastosis virus (AMV) antigen p27, appears in the livers of chickens inoculated with avian erythroblastosis virus (AEV). It can be demonstrated at the 30,000 dalton (30K) molecular weight level by Western immunoblotting of electropherograms of AEV infected liver extracts. The 30K protein reacted strongly with this polyclonal antibody but only weakly with a monoclonal antibody to the same viral antigen and possible explanations for this have been suggested. Both antibodies also appeared to react with other than viral components in the preparations of AMV used. As this apparent non-specific attachment of highly specific antibody may have as its explanation the failure of the gelatin to prevent nonimmunologically determined binding of the immunoglobulin; other blocking agents should be investigated.

Control of erythroid differentiation: asynchronous expression of the anion transporter and the peripheral components of the membrane skeleton in AEV- and S13-transformed cells

Chicken erythroblasts transformed with avian erythroblastosis virus or S13 virus provide suitable model systems with which to analyze the maturation of immature erythroblasts into erythrocytes. The transformed cells are blocked in differentiation at around the colony-forming unit-erythroid stage of development but can be induced to differentiate in vitro. Analysis of the expression and assembly of components of the membrane skeleton indicates that these cells simultaneously synthesize alpha-spectrin, beta-spectrin, ankyrin, and protein 4.1 at levels that are comparable to those of mature erythroblasts. However, they do not express any detectable amounts of anion transporter. The peripheral membrane skeleton components assemble transiently and are subsequently rapidly catabolized, resulting in 20-40-fold lower steady-state levels than are found in maturing erythrocytes. Upon spontaneous or chemically induced terminal differentiation of these cells expression of the anion transporter is initiated with a concommitant increase in the steady-state levels of the peripheral membrane-skeletal components. These results suggest that during erythropoiesis, expression of the peripheral components of the membrane skeleton is initiated earlier than that of the anion transporter. Furthermore, they point a key role for the anion transporter in conferring long-term stability to the assembled erythroid membrane skeleton during terminal differentiation.

Structure and expression of the chicken epidermal growth factor receptor gene locus

Similarity between the carboxyl-terminal portion of the human epidermal growth factor (EGF) receptor and the deduced protein sequence of the chicken-derived oncogene v-erbB, of avian erythroblastosis virus strain H, has suggested that the chicken cellular erbB locus, c-erbB, might be part of a longer EGF-receptor gene in the chicken, whose entire coding capacity remained to be defined. The c-erbB locus spans more than 20 X 10(3) base pairs (20 kbp) of DNA and contains at least 1.8 kbp homologous to the v-erbB oncogene. We show here that human EGF receptor cDNA and chicken genomic DNA share homology not only within the c-erbB locus but also within a 25.1-kbp DNA region situated 5' to this locus. The 3' region of the EGF receptor overlaps, in sequence homology, the c-erbB locus. The EGF receptor/c-erbB locus in chicken generates six related but distinctly different mRNAs of sizes 12, 9, 5, 3.6, 3.2 and 2.6 kb. The transcripts of 12, 9, and 3.6 kb contain sequences coding for both the extracellular EGF-binding domain of the receptor and the intracellular tyrosine kinase domain. The 12-kb and 9-kb transcripts, which have already been shown to contain the sequences coding for the v-erbB, were found to possess, in addition, sequences that encode the entire chicken EGF receptor. The 3.2-kb and 2.6-kb mRNAs are homologous only to the 5' portion of the EGF receptor gene. These results therefore indicate that the c-erbB locus, initially defined by homology to the viral transforming gene, corresponds to the 3' region of the EGF receptor gene in the chicken genome. The multiple, related, chicken EGF receptor RNA transcripts reported here are reminiscent of the various human EGF receptor RNA transcripts observed in normal and transformed cells.

Genome organisation of the FBR-osteosarcoma virus complex: identification of a subgenomic fos-specific message

The FBR murine virus complex together with the FBJ murine virus complex are known to be bone tumor inducers in newborn mice. Both transforming viruses have transduced c-proto-fos-derived sequences in their genome. FBR-MuSV was molecularly cloned as a biologically active 10-kbp EcoRI fragment from non-productively transformed rat embryo fibroblasts into Charon phage 4A (lambda MOL503) and subsequently subcloned in plasmid pBR322 (pMOL503). Its natural associated helper FBR-MuLV, excized as an internal 8.2-kbp PstI proviral DNA fragment from chronically infected NIH/3T3 cells, was cloned into the unique PstI site of pBR322. Comparative analysis of the restriction maps of FBR-MuSV and FBR-MuLV together with the electron microscopic analysis of heteroduplex DNA molecules formed between both molecular clones suggested that FBR-MuLV is the parental virus of FBR-MuSV. fos- and fox-specific DNA hybridisation probes identified a genomic sized 3.3-kb mRNA and a subgenomic 2.2-kb messenger RNA. Using a 5'-gag hybridisation probe, only the genomic 3.3-kb RNA molecule was detected, demonstrating that a donor splice site is present upstream of the gag sequences and used to generate the fos-specific 2.2-kb subgenomic mRNA.

A single amino acid substitution in v-erbB confers a thermolabile phenotype to ts167 avian erythroblastosis virus-transformed erythroid cells

A library of recombinant bacteriophage was prepared from ts167 avian erythroblastosis virus-transformed erythroid precursor cells (HD6), and integrated proviruses from three distinct genomic loci were isolated. A subclone of one of these proviruses (pAEV1) was shown to confer temperature-sensitive release from transformation of erythroid precursor cells in vitro. The predicted amino acid sequence of the v-erbB polypeptide from the mutant had a single amino acid change when compared with the wild-type parental virus. When the wild-type amino acid was introduced into the temperature-sensitive avian erythroblastosis virus provirus in pAEV1, all erythroid clones produced in vitro were phenotypically wild type. The mutation is a change from a histidine to an aspartic acid in the temperature-sensitive v-erbB polypeptide. It is located in the center of the tyrosine-specific protein kinase domain and corresponds to amino acid position 826 of the human epidermal growth factor receptor sequence.

The putative transforming protein of S13 avian erythroblastosis virus is a transmembrane glycoprotein with an associated protein kinase activity

S13 is an avian retrovirus that transforms both fibroblasts and erythroblasts. The gene product responsible for the oncogenic effects of S13 is the env-related glycoprotein gp155. In this report we show that gp155 is a transmembrane protein with a 55-kDa cytoplasmic domain. Pulse-chase analysis shows that gp155 was cleaved posttranslationally into two glycosylated proteins, gp85 and gp70. In addition, we show that a tyrosine protein kinase activity is associated only with the gp70 protein in microsomes and in immune complexes.

Mutagenesis of avian carcinoma virus MH2: only one of two potential transforming genes (delta gag-myc) transforms fibroblasts

Avian carcinoma virus MH2 contains two potential transforming genes, delta gag-mht and delta gag-myc. Thus, MH2 may be a model for two-gene carcinogenesis in which transformation depends on two synergistic genes. Most other directly oncogenic viruses contain single, autonomous transforming (onc) genes and are models for single-gene carcinogenesis. To determine which role each potential onc gene of MH2 plays in oncogenesis, we have prepared deletion and frameshift mutants of each of the two MH2 genes by in vitro mutagenesis of cloned proviral DNA and have tested transforming function and virus production in cultured primary quail cells. We have found that mht deletion mutants and wild-type virus transform primary cells and that myc deletion and frameshift mutants do not. The morphologies of cells transformed by the mht deletion mutants and by wild-type MH2 are similar yet vary considerably. Nevertheless, typical mutant transformed cells can often be distinguished from cells transformed by wild-type MH2. We conclude that the delta gag-myc gene transforms primary cells by itself, without the second potential onc gene. This myc-related gene is the smallest that has direct transforming function. delta gag-mht is without detectable transforming function but may affect transformation by delta gag-myc. Thus, MH2 behaves like a virus with a single onc gene, although it expresses two potential onc genes, and it appears not to be a model for two-gene carcinogenesis. Further work is necessary to determine whether the delta gag-mht gene possibly enhances oncogenic function of delta gag-myc or has independent oncogenic function in animals.

S13, a rapidly oncogenic replication-defective avian retrovirus

The avian leukemia sarcoma virus S13 transforms chicken and Japanese quail embryo fibroblasts and chicken erythroid cells in tissue culture. S13-induced erythroid transformation requires culture conditions suitable for the growth of normal erythroid precursors (H. Beug and M. J. Hayman (1984), Cell 36, 963-972). S13-transformed erythroid colonies contain a high percentage of cells that differentiate in absence of erythropoietin. S13 is defective in pol and env functions but can code for a complete set of gag proteins. Nonproducer cell clones transformed by S13 release a noninfectious viral particle containing gag but no functional env or pol proteins. They also synthesize a transformation-specific protein of 155,000 molecular weight. This protein reacts with antibody to viral envelope glycoproteins and appears to represent onc as well as env sequences. The 155,000-molecular weight env-linked protein does not cross react immunologically with an antiserum against the v-erb A and v-erb B gene products.

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.

Protein phosphorylation at tyrosine is induced by the v-erbB gene product in vivo and in vitro

The v-erbB gene product of avian erythroblastosis virus (AEV) has extensive homology with the receptor for epidermal growth factor (EGF). We report here that chicken embryo fibroblasts (CEF) transformed by AEV show enhanced tyrosine phosphorylation of a number of cellular polypeptides, including the 36 kd protein, which is phosphorylated in avian sarcoma virus-transformed fibroblasts, and the 42 kd protein, which is phosphorylated in mitogen-stimulated cells. CEF infected by AEV mutants with deletions in v-erbA showed enhanced tyrosine phosphorylation, whereas CEF infected by mutants with deletions in v-erbB did not. When membranes from AEV-transformed cells were incubated with gamma-32P-ATP, both the v-erbB gene product and the 36 kd cellular protein became phosphorylated at tyrosine. These results indicate that the v-erbB protein induces tyrosine phosphorylation in vivo and in vitro, and suggest that, like the EGF receptor, it possesses tyrosine-specific protein kinase activity.

The four C-terminal amino acids of the v-erbA polypeptide are encoded by an intronic sequence of the v-erbB oncogene

The genome of avian erythroblastosis virus (AEV), a defective acute leukemia retrovirus, carries two distinct cell-derived oncogenes in the structure 5' delta gag-erbA-erbB-delta env3'. The nucleotide sequence of the v-erbA gene was recently reported. In order to determine the boundary between the two adjacent oncogenes, the sequence of the v-erbA/v-erbB junction of AEV was compared to that of a recombinant lambda phage containing a chicken cellular sequence representing the 5' part of c-erbB. The four C-terminal amino acids of v-erbA are in fact encoded by a c-erbB intron-derived sequence thus demonstrating that the virus acquired a truncated c-erbA gene. Furthermore the 7 to 10 amino acid residues upstream from the 4 C-terminal amino acids mentioned above appeared to be derived from env-related sequences. The splice acceptor site at the beginning of the only open reading frame for v-erbB is also present and functional in c-erbB when expressed to generate a truncated EGF (epidermal growth factor) receptor. Thus AEV joins a truncated erbA gene to a truncated erbB gene through env-derived sequences and intronic sequences from c-erbB.

Autocrine growth induced by src-related oncogenes in transformed chicken myeloid cells

Chicken myeloid cells transformed by the v-myb-or v-myc-containing leukemia viruses, E26 and OK 10, respectively, require chicken myelomonocytic growth factor (cMGF) for proliferation in vitro. Upon superinfection with retroviruses carrying oncogenes of the src gene family, these myeloid cells acquire the ability to grow in the absence of exogenous cMGF. Conditioned medium prepared from superinfected E26 cells contains a growth-stimulating activity similar in biological and immunological properties to cMGF. This activity is reduced by more than 80% following absorption of conditioned media with antiserum against cMGF. Incubation of superinfected E26 cells with an immunoglobulin fraction of antiserum against cMGF inhibits their proliferation, indicating that the cells are dependent on the secreted factor. We conclude that viral oncogenes of the src family can induce chicken myeloid cells to produce a cMGF-like factor(s) that stimulates proliferation of these cells in an autocrine fashion.

Sequencing the erbA gene of avian erythroblastosis virus reveals a new type of oncogene

Avian erythroblastosis virus (AEV) contains two distinct oncogenes, erbA and erbB . The erbB oncogene, which is homologous to a portion of the epidermal growth factor receptor, is related to the src family of oncogenes and efficiently transforms erythroblasts, whereas erbA potentiates the effects of erbB by blocking the differentiation of erythroblasts at an immature stage. This "potentiator" was sequenced; the amino acid sequence deduced from it was clearly different from the sequences of other known oncogene products and was related to carbonic anhydrases. These enzymes participate in the transport of carbon dioxide by erythrocytes, the precursors of which are main targets of avian erythroblastosis virus. A src-related oncogene such as erbB in synergy with an activated specific cell-derived gene such as erbA can profoundly affect early erythroid differentiation.

Subcellular localization of the v-erb-B protein, the product of a transforming gene of avian erythroblastosis virus

Avian erythroblastosis virus (AEV) is an oncogenic retrovirus capable of transforming both fibroblasts and immature erythroid cells. The v-erb-B locus within the AEV genome encodes a glycosylated protein, expression of which is required for oncogenic transformation of either cell type. Subcellular localization of the v-erb-B glycoprotein in AEV-transformed cells is reported here. Results indicate that the v-erb-B protein is synthesized on dense membrane fractions and appears to possess the properties of an integral membrane protein. The bulk of the v-erb-B protein remains with dense membranes after synthesis, although a small quantity may slowly become associated with the plasma membrane. The biogenesis and subcellular location of the v-erb-B protein are thus quite different from those of the transforming proteins that display protein kinase activity. These differences are especially provocative because the amino acid sequences of the v-erb-B protein and the protein kinases are closely related to one another.

Identification of a form of the avian erythroblastosis virus erb-B gene product at the cell surface

Avian erythroblastosis virus (AEV) induces both erythroblastosis and fibrosarcoma in chickens. The viral oncogene responsible for these diseases, erb, is divided into two regions, erb-A and erb-B, although recent evidence suggests that it is primarily the erb-B gene product that is responsible for the transforming activity. The erb-B gene product has been reported previously to be a membrane glycoprotein of 68,000 molecular weight (MW), gp68erb -B. However, we show here that gp68erb -B is an intracellular precursor which is modified further to a 74,000 MW protein, gp74erb -B. By the criteria of resistance to digestion with endoglycosidase H, subcellular fractionation and inhibition of biosynthesis by the ionophore monensin, gp74erb -B appears to be located at the cell surface. Recently, a comparison of the erb-B sequence with that of the epidermal growth factor (EGF) receptor has shown that these two genes are highly homologous, and that erb-B appears to represent a truncated form of this growth factor. In light of these data the identification of gp74erb -B at the plasma membrane suggests that this may be the functionally important form of the erb-B gene product.

Temperature-sensitive mutants of avian erythroblastosis virus: surface expression of the erbB product correlates with transformation

The v-erbB gene of avian erythroblastosis virus (AEV) codes for an integral plasma membrane glycoprotein, gp74erbB. Expression of gp74erbB and its intracellular precursors, gp66erbB and gp68erbB, has been studied in cells transformed by two temperature-sensitive mutants of AEV. After shift to 42 degrees C, the processing of gp68erbB is blocked in tsAEV-transformed, but not in wtAEV-transformed, erythroblasts and fibroblasts. In addition, gp74erbB disappears from the surface of tsAEV cells within 12 hr after shift. Thus tsAEV mutants probably bear a lesion in v-erbB that affects the maturation and subcellular localization of gp74erbB. The tsAEV erythroblasts, when "committed" to differentiation by a pulse-shift to 42 degrees C, reexpress gp74erbB during terminal differentiation at 36 degrees C. This suggests that tsAEV erythroblasts become insensitive to the transforming functions of gp74erbB at a certain stage of differentiation.

Site-specific mutagenesis of avian erythroblastosis virus: erb-B is required for oncogenicity

Avian erythroblastosis virus (AEV) induces both erythroblastosis and fibrosarcomas in susceptible birds. Two domains within its replication-defective genome, erb-A and erb-B, have been implicated in AEV-mediated oncogenesis. An efficient transfection system for generating infectious, transforming virus from molecular clones of AEV and RAV-1 (helper virus) was combined with the techniques of site-specific mutagenesis to investigate the contribution of erb-B to the two forms of oncogenesis induced by AEV. Deletion and frameshift mutations were constructed in the erb-B locus of cloned AEV DNA in vitro. Infectious retroviruses harboring these mutations were recovered and their ability to transform fibroblasts in vitro or induce erythroleukemia in vivo was assessed. The presence of mutant viral genomes in chick embryo fibroblasts or erythroblasts of infected birds was confirmed by suitable biochemical analyses. Expression of viral genes in cells infected with AEV mutants was examined by immunoprecipitation with antisera to erb-A and erb-B proteins. It was found that the product of erb-B is necessary for transformation of fibroblasts and induction of erythroblastosis by AEV, although a small portion of this protein at the carboxy terminus is dispensable.

Site-specific mutagenesis of avian erythroblastosis virus: v-erb-A is not required for transformation of fibroblasts

Avian erythroblastosis virus (AEV) is an acutely transforming retrovirus whose putative oncogenes (v-erb-A and v-erb-B) encode the proteins P74gag-erb-A and P61-68erb-B. The existence of these two gene products has prompted the question of whether one or both proteins are required in the transformation of erythroblasts and fibroblasts by AEV. In the accompanying manuscript, we describe the use of site-specific mutagenesis to generate mutants of AEV unable to synthesize P61-68erb-B. Here we present our analysis of the oncogenic potential of an AEV mutant unable to synthesize P74gag-erb-A due to a large deletion encompassing both gag and v-erb-A sequences. The erb-A-mutant retrovirus propagated quite poorly on fibroblasts in culture; however, fibroblasts harboring the erb-A mutant genome were transformed in the absence of P74gag-erb-A expression. The mutant virus failed to induce erythroleukemias in chickens, but the validity of this finding is compromised by the poor replicative capacity of the mutant. The results presented in this and the preceding manuscript indicate that P61-68erb-B is both necessary and sufficient for neoplastic transformation of fibroblasts by AEV; by contrast, a role for p74gag-erb-A in leukemogenesis by AEV has not yet been rigorously excluded.

A new avian erythroblastosis virus, AEV-H, carries erbB gene responsible for the induction of both erythroblastosis and sarcomas

The genome structure of a newly isolated avian erythroblastosis virus, AEV-H, was analyzed. Using DNA probes specific for the LTR sequence of SR-RSV-A, and for the erbA gene and the erbB gene of the ES4 strain of AEV, we have shown that the genome of AEV-H is 35S in size and carries the erbB gene but not the erbA gene. Comparison of the restriction sites of molecularly cloned AEV-H DNA with that of cloned DNA of the associated virus revealed that the env gene of the associated virus was replaced with the erbB gene to generate AEV-H. The genome structure of AEV-H is, therefore, determined to be 5'-gag-pol-erbB-3'. Moreover, we have isolated a mutant of AEV-H, td-130, that can induce sarcomas but not erythroblastosis in chickens. The restriction analysis of proviral DNA of the td-130 showed that it carries a deletion of about 150 to 200 nucleotides in the erbB gene. These data indicate that the erbB protein is responsible for both erythroblastosis and sarcomas.

The product of the retroviral transforming gene v-myb is a truncated version of the protein encoded by the cellular oncogene c-myb

Avian myeloblastosis virus (AMV) is an oncogenic retrovirus that rapidly causes myeloblastic leukemia in chickens and transforms myeloid cells in culture. AMV carries an oncogene, v-myb, that is derived from a cellular gene, c-myb, found in the genomes of vertebrate species. We constructed a plasmid vector that allows expression of a portion of the coding region for v-myb in a procaryotic host. We then used the myb-encoded protein produced in bacteria to immunize rabbits. The antisera obtained permitted identification of the proteins encoded by both v-myb and chicken c-myb. The molecular weights of the products of v-myb and c-myb (45,000 and 75,000 respectively) indicate that the v-myb protein is an appreciably truncated version of the c-myb protein.

The product of the avian erythroblastosis virus erbB locus is a glycoprotein

Avian erythroblastosis virus (AEV) induces both erythroblastosis and fibrosarcomas in susceptible birds. A locus, v-erbB, within the viral genome has been implicated in AEV-mediated oncogenesis. We report here the detection and partial characterization of the protein product of the v-erbB oncogene in AEV-transformed cells. We obtained the antisera necessary for our analysis by expressing a portion of the molecularly cloned v-erbB locus in Escherichia coli and immunizing rabbits with the resulting bacterial erbB polypeptide. Antisera directed against the bacterial polypeptide reacted with v-erbB proteins obtained from virus-infected avian cells. By three criteria--tunicamycin inhibition, lectin binding and metabolic labeling with radioactive sugar precursors--the product of the v-erbB gene appears to be a glycoprotein.

Identification and characterization of the avian erythroblastosis virus erbB gene product as a membrane glycoprotein

Avian erythroblastosis virus causes erythroid leukemia and sarcomas in chickens. The viral oncogene responsible for these diseases, erb, is divided into two regions known as erbA and erbB, and recent evidence suggests that it is the erbB gene that is responsible for the transforming activity. From rats bearing avian erythroblastosis virus-induced sarcomas, we have obtained antisera which are specific for the erb gene products. Using such antisera, we have been able to characterize the erbB gene product as a 68,000 molecular weight protein. Pulse-chase and cell-free in vitro translation experiments show that the initial product is a 62,500 dalton protein which is initially modified to a 66,000 dalton protein, and then further modified to a 68,000 dalton form. These modifications could be shown to be associated with glycosylation and phosphorylation. Cell fractionation experiments revealed that the 66,000 and 68,000 dalton proteins were located in cell membrane fractions, and immunofluorescence results showed the erbB gene product to be expressed on the cell surface.

Detection of avian hematopoietic cell surface antigens with monoclonal antibodies to myeloid cells. Their distribution on normal and leukemic cells of various lineages

The isolation and characterization of monoclonal antibodies reacting with cell surface antigenic determinants of normal and leukemic avian hematopoietic cells is described. The antibodies were produced by immunizing mice with normal macrophages, as well as with myeloid cells transformed with the avian acute leukemia viruses MC29, AMV and E26. Eleven antibodies were characterized for their reactivity with a variety of normal and leukemic cells of the myeloid, B- and T-lymphoid and of the erythroid cell lineage. Using several methods, they could be subdivided into five distinct types: I. Four antibodies were specific for the myeloid lineage, predominantly reacting with immature myeloid cells. II. One antibody reacted with mature and immature myeloid cells as well as with T-lymphoid cells. III. Four antibodies reacted with myeloid, erythroid and T-lymphoid cells. IV. One antibody reacted with myeloid as well as with T- and B-lymphoid cells. V. One antibody reacted with all kinds of chicken hematopoietic cells except erythrocytes. The first type of antibodies detected glycoproteins with MWs of 170 and 130 kD. The pattern of antigens precipitated varied with the different monoclonal antibodies of this group. The antibody of the fourth type precipitated a 30 kD polypeptide from extracts of myeloid and lymphoid cells. None of the other antibodies precipitated any detectable proteins.