In vitro translation of avian erythroblastosis virus RNA: identification of two major polypeptides

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.

AEV-transformed erythroleukemia cell induced differentiation: expression of specific cell membrane antigenic molecules

A simultaneous decay of the expression of Im 140 kDa, Im 150 kDa and Im 160 kDa high MW membrane antigens, concomitant with the cell proliferation arrest, was observed during erythropoietin induced differentiation of ts 34 AEV-transformed erythroid cells cultivated at the restrictive temperature. Expression of embryo-immature antigens was maintained during induced differentiation of erythroleukemia cells, but their MW shifted from 50 to 48 kDa, which corresponds to the MW of embryo-immature antigens detected on normal erythroid cells. In the absence of erythropoietin at the restrictive temperature, conditions under which the ts 34 AEV-transformed erythroid cells fail to differentiate and maintain their capacity to proliferate, the expression of high MW antigens as well as the expression of embryo-immature antigens remained unaffected. Therefore, it is shown that the expression of specific membrane antigens is modulated under conditions rendering the erythroleukemia cell differentiation process possible.

A single species of pX mRNA of human T-cell leukemia virus type I encodes trans-activator p40x and two other phosphoproteins

Human T-cell leukemia virus type I (HTLV-I) contains the pX sequence which codes for the trans-activator of the long terminal repeat (LTR) and is thus postulated to be associated with leukemogenesis in adult T-cell leukemia. Overlapping open reading frames (ORF) in the pX sequence were recently found to code for p27x-III and p21x-III by ORF III, in addition to p40x coded for by ORF IV. The mechanism of expression of these newly identified proteins and their possible association with trans-activation were studied. On transfection of an expression plasmid that contains a cDNA sequence of the pX mRNA, products from both ORFs III and IV were detected in the cells. The RNA was synthesized in vitro from the cDNA clone by SP6 RNA polymerase and translated in a rabbit reticulocyte lysate. As translation products, two proteins, p27x-III and p21x-III, were detected in addition to p40x. Elimination of the first and second ATG codons in ORF III resulted in loss of the ability to code for p27x-III and p21x-III, respectively, which indicated that the translations from these two ATG codons were independent. A mutant that lacked both ATG codons was fully active in trans-activation of chloramphenicol acetyltransferase gene expression directed by the LTR. These results indicate that a 2.1-kilobase pX mRNA of HTLV-I independently encodes three proteins, p40x, p27x-III, and p21x-III, by different ORFs and that the last two proteins are not involved in trans-activation of the unintegrated LTR.

Mutagenesis of the avian erythroblastosis virus erbB coding region: an intact extracellular domain is not required for oncogenic transformation

Avian erythroblastosis virus (AEV) is an oncogenic retrovirus of birds. The AEV-encoded erbB polypeptide, a transmembrane glycoprotein bearing an N-terminal domain exposed on the surface of virally transformed cells, plays a crucial role in AEV-mediated oncogenesis. We report here a characterization of a mutated form of the AEV erbB protein which lacks over two-thirds of the extracellular region of this oncogenic protein. This mutant v-erbB protein, although lacking the three possible extracellular sites of N-linked protein glycosylation, appears unimpaired in the ability to transform cells to an oncogenic phenotype.

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.

The erbB gene of avian erythroblastosis virus is a member of the src gene family

The erbB gene of an avian erythroblastosis virus, AEV-H, was determined to be 1812 nucleotides long and was predicted to code for a protein of 67,638 daltons. Unexpectedly, a sequence of 285 amino acids in the middle of the protein showed a significant homology (38%) with the sequence in the carboxy terminus of p60src. The nucleotide sequence of a mutant of AEV-H, td-130, which induces sarcomas but not erythroblastosis in chicken, was also analyzed. A deletion of 169 nucleotides was identified in the 3' half of the erbB gene, indicating that the gene codes for a truncated protein with the predicted molecular weight of 46,667. These findings suggest that the homologous domain of erbB protein with its N-terminal portion is sufficient for the transformation of fibroblasts and that one-third of the carboxy-terminal domain has a key role for the transformation of erythroid cells.

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 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.

On the possible presence of a beta 2-microglobulin-like protein in extracts of livers from normal chickens and chickens with erythroblastosis--I. Recognition of a small-molecular weight (mol. wt 11,000) protein

1. An extract from the livers of both normal chickens (N) and chickens infected with avian erythroblastosis virus (Eb) contains a small molecular weight protein (SMWP, mol. wt 11,000). 2. Double immunodiffusion studies with rabbit antiserum against fowl serum proteins shows a precipitin arc for SMWP in N and Eb extracts, which is continuous with one from one of the normal chicken serum proteins. 3. When treated with 60% saturated ammonium sulphate the SMWP in the liver extracts divides between the precipitate and the supernatant although the specific serological activity of Eb extracts (gag--or COFAL--determined antigenic activity) is restricted to the precipitated SMWP fraction. 4. The COFAL activity of Eb liver extracts could be associated with SMWP by its attachment to this protein, or this phenomenon of "association" could represent the result of changes in synthesis of SMWP or post-synthetic changes.

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.

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.

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 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.

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.

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.