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

Structure of the protein encoded by the chicken proto-oncogene c-myb

The retroviral oncogene v-myb arose by transduction of the chicken proto-oncogene c-myb. We isolated and sequenced cDNA that represents the entire coding domain of chicken c-myb. By transcribing the cDNA into mRNA in vitro and then translating the RNA, we were able to document the integrity of the cDNA and to identify the codon responsible for initiation of translation from c-myb. Two different alleles of v-myb are extant, one in the genome of avian myeloblastosis virus (AMV) and the other in the genome of erythroblastosis virus 26 (E26V). The proteins encoded by the AMV and E26V alleles of v-myb differ from the product of c-myb in three ways: at their amino termini, they lack 71 and 80 amino acids respectively; at their carboxy termini, they are deficient in 199 and 278 residues; and 11 substitutions of amino acids are scattered throughout the product of AMV allele, whereas the product of the E26V allele contains only a single substitution. The structural origins of tumorigenicity by v-myb and the biological functions of c-myb remain enigmatic. The findings and molecular clones described here should now permit a systematic exploration of these enigmas.

Nuclear compartmentalization of the v-myb oncogene product

Nuclei obtained from chicken leukemic myeloblasts transformed by avian myeloblastosis virus were fractionated into various subnuclear compartments, which were then analyzed by specific immunoprecipitation for the presence of the leukemogenic product, p48v-myb, of the viral oncogene. In cells labeled for 30 or 60 min with L-[35S]methionine and in unlabeled exponentially dividing leukemic cells analyzed by Western blotting, p48v-myb was detected within the nucleoplasm (29 +/- 9% [standard deviation] of the total), chromatin (7 +/- 4%), and lamina-nuclear matrix (64 +/- 9%). Also, in myeloblasts analyzed by immunofluorescence during mitosis, p48v-myb appeared to be dispersed through the cell like the lamina-nuclear matrix complex. Strong attachment to the nuclear matrix-lamina complex suggests that p48v-myb may be involved in DNA replication or transcription or both.

Transformation-defective mutant of avian myeloblastosis virus that is temperature sensitive for production of transforming protein p45v-myb

We have characterized a mutant of avian myeloblastosis virus (strain GA907/7) that shows a reduced capacity to transform myelomonocytic cells at the nonpermissive temperature. Myeloblasts transformed by this mutant suffer a substantial decrease in the amount of the transforming protein p45v-myb when shifted from the permissive to the nonpermissive temperature. We presume that the 5- to 10-fold decrease in the amount of p45v-myb causes the loss of the transformed phenotype. The decrease is due to a reduction in the level of v-myb mRNA. Mutant GA907/7 thus provides genetic evidence that p45v-myb is the transforming protein of avian myeloblastosis virus and apparently represents an unusual defect in the production or stability of mRNA.

The proto-oncogene c-ets is preferentially expressed in lymphoid cells

The transforming sequences of the avian acute leukemia virus, E26, contain two distinct oncogenes, v-mybE and v-ets, fused together. By using a probe containing v-ets sequences, polyadenylated transcripts of the c-ets proto-oncogene were detected in avian tissues; they included a major 7.0-kilobase and a minor 2.0-kilobase species. These c-ets mRNAs were detected at high levels only in lymphoid organs and in avian T and B lymphoid cell lines. A similar pattern of c-ets transcription was observed in human hematopoietic cell lines, with transcripts detected in lymphoid B and T cells but not in erythroid or myeloid cells. The E26 oncogene was inserted into an inducible expression vector, and a 90-kilodalton protein (bp90) was produced in bacteria. Rabbit antisera raised to purified bp90 precipitated P135gag-mybE-ets, the v-mybE-ets polyprotein expressed in E26-transformed cells, and also reacted with p50v-mybA, the transforming protein of the avian myeloblastosis virus. Antiserum to bp90 was absorbed with a bacterially synthesized v-mybA protein to remove anti-myb activity. The absorbed anti-bp90 serum retained the ability to immunoprecipitate P135gag-mybE-ets from E26-transformed cells and specifically reacted with a 56-kilodalton polypeptide (p56) detected in chicken lymphoid organs and in T and B lymphocytes of both avian and human origin. The data suggest that p56 is a translational product of the c-ets proto-oncogene and imply that p56 may be involved in regulating the growth of lymphoid cells.

Identification of c-myb (chicken), c-myb (mouse) and v-myb (AMV) protein products by immunoprecipitation with antibodies directed against a synthetic peptide

A synthetic nonadecapeptide (IL 19) derived from a sequence of v-myb was covalently bound to haemocyanin and used for immunization. Anti-IL 19 serum immunoprecipitated a 75 kDa protein in the lysate of metabolically labelled chicken and murine thymus cells. Presaturation of the serum with IL 19 abolished this immunoprecipitation, thus indicating that the product of c-myb in both chicken and murine thymuses is the 75 kDa protein (p75c-myb). Anti IL 19 serum also precipitated p48v-myb in the lysate of nonproducer myeloblasts.

Expression of molecular clones of v-myb in avian and mammalian cells independently of transformation

We demonstrated that molecular clones of the v-myb oncogene of avian myeloblastosis virus (AMV) can direct the synthesis of p48v-myb both in avian and mammalian cells which are not targets for transformation by AMV. To accomplish this, we constructed dominantly selectable avian leukosis virus derivatives which efficiently coexpress the protein products of the Tn5 neo gene and the v-myb oncogene. The use of chemically transformed QT6 quail cells for proviral DNA transfection or retroviral infection, followed by G418 selection, allowed the generation of cell lines which continuously produce both undeleted infectious neo-myb viral stocks and p48v-myb. The presence of a simian virus 40 origin of replication in the proviral plasmids also permitted high-level transient expression of p48v-myb in simian COS cells without intervening cycles of potentially mutagenic retroviral replication. These experiments establish that the previously reported DNA sequence of v-myb does in fact encode p48v-myb, the transforming protein of AMV.

Studies of the human c-myb gene and its product in human acute leukemias

The myb gene is the transforming oncogene of the avian myeloblastosis virus (AMV); its normal cellular homolog, c-myb, is conserved across a broad span of evolution. In humans, c-myb is expressed in malignant hematopoietic cell lines and in primary hematopoietic tumors. Partial complementary DNA clones were generated from blast cells of patients with acute myelogenous leukemia. The sequences of the clones were compared to the c-myb of other species, as well as the v-myb of AMV. In addition, the carboxyl terminal region of human c-myb was placed in an expression vector to obtain protein for the generation of antiserum, which was used to identify the human c-myb gene product. Like v-myb, this protein was found within the nucleus of leukemic cells where it was associated with the nuclear matrix. These studies provide further evidence that c-myb might be involved in human leukemia.

Structural organization and nucleotide sequence of mouse c-myb oncogene: activation in ABPL tumors is due to viral integration in an intron which results in the deletion of the 5' coding sequences

Bacteriophage libraries of mouse DNA were screened for sequences homologous to the v-myb oncogene and two overlapping clones containing the v-myb related region were isolated. Restriction enzyme mapping, heteroduplex analysis and nucleotide sequence analysis revealed the presence of nine exons. Six of these exons are homologous to the v-myb region while the other three exons are derived from the 5' region which is deleted in the viral oncogene. The sequences downstream to the sixth v-myb exon are not included in the 17 kbp of DNA sequences analyzed in this study. Comparison of the structure of the normal c-myb clone with its rearranged couterpart present in plasmacytoid lymphosarcomas revealed that the rearrangements occur in this locus as a result of viral integration. Present studies demonstrate that such a viral insertion interrupts the c-myb coding region at a region identical to that observed in the generation of the v-myb gene of avian myeloblastosis virus and results in the synthesis of mRNAs that lack the same 5' coding region.

Transforming potential of the v-myb oncogene from avian myeloblastosis virus: alterations in the oncogene product may reveal a new target specificity

Transfection of brown leghorn chicken embryo fibroblasts by DNA containing v-myb sequences cloned either in a complete AMV proviral DNA or in a retroviral derived vector has led to the isolation of two kinds of transformed cells. A characterization of the proviral sequences retained and expressed in these transformed cells revealed that they contained either new or altered v-myb-related RNA species. The experiments presented in this paper also show that both types of transformants expressed truncated myb-related polypeptides, suggesting that alterations of the v-myb product may result in a new target specificity, leading to the transformation of chicken embryo fibroblasts.

Antibodies to the evolutionarily conserved amino-terminal region of the v-myb-encoded protein detect the c-myb protein in widely divergent metazoan species

Antibodies directed against a bacterial fusion protein that contains the domain encoded by the highly evolutionarily conserved 5' one-third of the v-myb oncogene of avian myeloblastosis virus (AMV) detect the protein products of various members of the myb gene family. Immunoprecipitation or immunoblot analyses with these antibodies yielded the following information. First, the products of the v-myb oncogenes of AMV (p48v-myb) and of E26 virus (p135gag-myb-ets) contain this highly conserved amino acid sequence, as previously hypothesized. Second, p75c-myb, the product of the chicken c-myb protooncogene, also contains this protein domain. Third, these antibodies have identified the products of the human, murine, and Drosophila c-myb genes, which were all found to be nuclear proteins of Mr 75,000-80,000. The human c-myb protein product is present in immature cells of the erythroid, myeloid, and lymphoid lineages.

Coordinate regulation of myelomonocytic phenotype by v-myb and v-myc

Both avian myeloblastosis virus (by the action of v-myb) and avian myelocytomatosis virus MC29 (by the action of v-myc) transform cells of the myelomonocytic lineage. Whereas avian myeloblastosis virus elicits a relatively immature phenotype, cells transformed by MC29 resemble mature macrophages. When cells previously transformed by v-myb were superinfected with MC29, their phenotype was rapidly altered to that of a more mature cell. These superinfected cells expressed both v-myb (at a level similar to that found before superinfection) and v-myc. It therefore appears that the expression of v-myc can elicit certain properties of a more differentiated phenotype. In addition, unlike cells transformed by v-myb alone, the cells expressing both v-myb and v-myc could not be induced by the tumor promoter 12-O-tetradecanoylphorbol-13-acetate to differentiate to fully mature macrophages. Cells with a morphology similar to that of the superinfected cells were elicited by simultaneously infecting yolk sac macrophages with avian myeloblastosis virus and MC29. Such cells expressed both v-myb and v-myc. These results indicate that expression of v-myb and v-myc in infected cells coordinately regulates myelomonocytic phenotype and that the two viral oncogenes vary in their ability to interfere with tumor promoter-induced differentiation. Our findings also sustain previous suggestions that the oncogenes v-myb and v-myc may not transform target cells by simply blocking differentiation.

Murine myb protooncogene mRNA: cDNA sequence and evidence for 5' heterogeneity

We have sequenced two overlapping cDNA clones from a murine pro-B cell library to generate a composite sequence that includes 3413 bases of the murine c-myb mRNA. There is a single long open reading frame, beginning at the first base of this sequence, and continuing from the first methionine codon at nucleotide 265 to a TGA termination codon at nucleotide 2173. The predicted murine translation product contains 636 amino acid residues and is about 71 kDa long, which is in good agreement with the 75-kDa molecular size determined for the avian c-myb protein. The murine c-myb protein shows a striking 82% amino acid homology in the region (amino acids 71-444) where it can be compared to the published avian c-myb gene sequence. S1 nuclease protection analysis indicates extreme heterogeneity at the 5' end of steady-state murine c-myb mRNA.

A cryptic transcription promoter in the myb oncogene of avian myeloblastosis virus

The potential regulatory signals contained in the v-myb oncogene of avian myeloblastosis virus have been inserted upstream to the herpes simplex type 1 thymidine kinase gene in order to test their promoter activity. The isolation of TK+ transformants after transfection of clone 1D(TK-) mouse cells with the resulting recombinant DNAs indicated that the expression of the TK gene was made possible by the myb-derived sequences. Analysis of the TK specific RNA expressed in different TK+ transformants revealed that the regulatory signals contained in v-myb correspond to a weak functional promoter.

Identification and preferential expression in thymic and bursal lymphocytes of a c-ets oncogene-encoded Mr 54,000 cytoplasmic protein

The avian retrovirus E26 is unique among acute leukemia viruses in its ability to induce transformation of cells belonging to either the myeloid or erythroid lineage. The genome of E26 carries two oncogenes, v-myb and v-ets, that are derived from distinct cellular loci, c-myb and c-ets. We have constructed a plasmid vector that allows expression of part of the coding region of v-ets in a bacterial host. Antisera to the bacterially synthesized ets protein specifically precipitated the E26-encoded P135gag-myb-ets transforming protein. These antisera permitted us to identify a chicken c-ets-encoded protein of Mr 54,000 (P54c-ets) that shares 7 out of 10 of its major [35S]methionine-containing tryptic peptides with the v-ets-encoded domain of P135gag-myb-ets. Unlike P135gag-myb-ets and the Mr 75,000 translation product of c-myb (P75c-myb), which are nuclear proteins, P54c-ets was found to be predominantly cytoplasmic. P54c-ets is expressed at low levels in most cell lines and tissues tested, including bone marrow cells and circulating lymphocytes. P54c-ets, together with a minor but closely related Mr 56,000 protein, was found to be expressed at high levels in chicken thymocytes and bursal lymphocytes.

Nucleotide sequence of chicken c-myb complementary DNA and implications for myb oncogene activation

Avian myeloblastosis virus (AMV), like other acute transforming viruses, arose by recombination between its helper virus and host cellular sequences. The latter sequences, termed v-myb, are responsible for the oncogenic properties of the virus. AMV causes acute myeloblastic leukaemia in chickens and transforms a specific class of haematopoietic cells in vitro, but does not induce morphological transformation of cultured fibroblasts, suggesting that only a restricted target-cell population is responsive to its transforming gene product. The normal cellular counterpart of v-myb, c-myb, is highly conserved and is present in all vertebrate and some invertebrate species examined. DNA rearrangements and altered expression of the myb oncogene have been reported in mouse lymphoid tumours and human myeloid and colon tumours. The mechanism of activation of the cellular proto-oncogenes is thought to involve the structural alteration of the coding regions that result in either the synthesis of an altered gene product or the enhanced expression of a proto-oncogene caused by alterations in its regulatory elements. To distinguish between these two mechanisms, we have cloned and sequenced the chicken c-myb complementary DNA and compared it with that of v-myb sequences. We demonstrate that during the transduction of the cellular sequences and/or viral passage a substantial portion of the coding region of the c-myb gene has been lost from both the 5' and 3' ends, resulting in the generation of a truncated gene product that mediates the transforming function of the virus.

Two modes of c-myb activation in virus-induced mouse myeloid tumors

Two modes of disruption of the protooncogene c-myb by viral insertional mutagenesis in mouse myeloid tumor cells are described. The first mode was found in six tumors in which a Moloney murine leukemia virus component had inserted in the same transcriptional orientation upstream of the 5'-most exon with v-myb homology (vE1). cDNA sequence data indicate the presence of a truncated c-myb mRNA that is initiated in the upstream 5' long terminal repeat of the integrated provirus and processed via a cryptic splice donor sequence in the gag region to the splice acceptor site in vE1 of the c-myb gene, thus removing the remaining downstream viral and myb intronic sequences. Unlike most gag-onc transcripts, the gag and myb sequences in the hybrid transcript were not in the same reading frame. It is presumed that the gag sequence provides a cryptic translation initiation site for the novel amino-truncated c-myb protein. The second mode of disruption was by downstream virus insertion at the 3' side of the c-myb, which results in the synthesis of a small (approximately 2 kilobase) myb transcript. The 5' long terminal repeat of the inserted provirus provides a TGA termination codon that results in the elimination of 240 normal c-myb amino acid residues from the carboxyl terminus of the tumor-specific myb protein. These results suggest that truncated myb proteins play a role in neoplastic transformation of myeloid cells.

Protein product of proto-oncogene c-mil

Using antipeptide antibodies with specificity for the carboxyl termini of v-raf and v-mil protein products, two proteins with apparent molecular weights of approximately 71,000/73,000 and 215,000 were detected in immunoprecipitates from normal uninfected chicken cells. The 71,000/73,000-molecular-weight protein was identified as the product of the c-mil proto-oncogene by the close structural relationship of its 42,000-molecular-weight carboxyl-terminal domain to the v-mil-encoded domain of the hybrid protein p100gag-mil specified by the avian retrovirus MH2. The amino-terminal domain of the cellular protein is encoded by 5' c-mil sequences that have not been transduced into the genome of MH2. The c-mil protein (p71/73c-mil) was found to be phosphorylated in vivo, and homologous proteins were detected at variable levels in a variety of vertebrate cells, including human cells.

Peptide antibodies: new tools for cell biology

The process of preparing antibodies against small peptide subsets of larger proteins is now a very routine and effective tool for cell biological investigations. Now that the identification of genes is commonplace, it is imperative to be able to identify, purify, and characterize the products of these genes. Antibodies against synthetic peptides will aid in discovering the elusive functions of these proteins. Over the past 5 years, peptide antibodies have contributed, and they will doubtless continue to contribute, to the identification of functional domains of proteins. Peptide antibodies provide a means for identifying functional domains conserved during the evolution of families of proteins, and for inhibiting specific functions of multifunctional proteins. Domain-specific antibodies have already increased the molecular resolution with which cell biologists can immunologically examine the function of cellular proteins. Finally, many proteins are now known to exist in subtly different forms, either as the products of separate genes or as the result of posttranslational modifications. Peptide antibodies allow molecular cell biologists, for the first time, to design antibodies for the specific assay of altered forms of a protein. Because they are amenable to specific immunolocalization of highly similar species, peptide antibodies can be considered to be subcellular probes of gene expression and posttranslational modification.

Subnuclear localization of proteins encoded by the oncogene v-myb and its cellular homolog c-myb

The retroviral transforming gene v-myb encodes a 45,000-Mr nuclear transforming protein (p45v-myb). p45v-myb is a truncated and mutated version of a 75,000-Mr protein encoded by the chicken c-myb gene (p75c-myb). Like its viral counterpart, p75c-myb is located in the cell nucleus. As a first step in identifying nuclear targets involved in cellular transformation by v-myb and in c-myb function, we determined the subnuclear locations of p45v-myb and p75c-myb. Approximately 80 to 90% of the total p45v-myb and p75c-myb present in nuclei was released from nuclei at low salt concentrations, exhibited DNA-binding activity, and was attached to nucleoprotein particles when released from the nuclei after digestion with nuclease. A minor portion of approximately 10 to 20% of the total p45v-myb and p75c-myb remained tightly associated with the nuclei even in the presence of 2 M NaCl. These observations suggest that both proteins are associated with two nuclear substructures tentatively identified as the chromatin and the nuclear matrix. The function of myb proteins may therefore depend on interactions with several nuclear targets.

The proto-oncogene c-ets is preferentially expressed in lymphoid cells

The transforming sequences of the avian acute leukemia virus, E26, contain two distinct oncogenes, v-mybE and v-ets, fused together. By using a probe containing v-ets sequences, polyadenylated transcripts of the c-ets proto-oncogene were detected in avian tissues; they included a major 7.0-kilobase and a minor 2.0-kilobase species. These c-ets mRNAs were detected at high levels only in lymphoid organs and in avian T and B lymphoid cell lines. A similar pattern of c-ets transcription was observed in human hematopoietic cell lines, with transcripts detected in lymphoid B and T cells but not in erythroid or myeloid cells. The E26 oncogene was inserted into an inducible expression vector, and a 90-kilodalton protein (bp90) was produced in bacteria. Rabbit antisera raised to purified bp90 precipitated P135gag-mybE-ets, the v-mybE-ets polyprotein expressed in E26-transformed cells, and also reacted with p50v-mybA, the transforming protein of the avian myeloblastosis virus. Antiserum to bp90 was absorbed with a bacterially synthesized v-mybA protein to remove anti-myb activity. The absorbed anti-bp90 serum retained the ability to immunoprecipitate P135gag-mybE-ets from E26-transformed cells and specifically reacted with a 56-kilodalton polypeptide (p56) detected in chicken lymphoid organs and in T and B lymphocytes of both avian and human origin. The data suggest that p56 is a translational product of the c-ets proto-oncogene and imply that p56 may be involved in regulating the growth of lymphoid cells.

Nuclear compartmentalization of the v-myb oncogene product

Nuclei obtained from chicken leukemic myeloblasts transformed by avian myeloblastosis virus were fractionated into various subnuclear compartments, which were then analyzed by specific immunoprecipitation for the presence of the leukemogenic product, p48v-myb, of the viral oncogene. In cells labeled for 30 or 60 min with L-[35S]methionine and in unlabeled exponentially dividing leukemic cells analyzed by Western blotting, p48v-myb was detected within the nucleoplasm (29 +/- 9% [standard deviation] of the total), chromatin (7 +/- 4%), and lamina-nuclear matrix (64 +/- 9%). Also, in myeloblasts analyzed by immunofluorescence during mitosis, p48v-myb appeared to be dispersed through the cell like the lamina-nuclear matrix complex. Strong attachment to the nuclear matrix-lamina complex suggests that p48v-myb may be involved in DNA replication or transcription or both.

Developmental regulation of c-myb in normal myeloid progenitor cells

Hematopoietic tissues and some leukemic cell lines express elevated levels of c-myb transcripts. We have separated a subpopulation of chicken embryo yolk sac cells that represents about 5% of the yolk sac hematopoietic cells and appears to contain all of the detectable c-myb transcripts. The level of myb expression in this cell population is higher than previously reported for any normal cell population and is in the range of that found in cells transformed by avian myeloblastosis virus and E26 virus. Since the myb gene probe used also detects full-length viral transcripts as well as the v-myb mRNA, it appears that the level of expression of c-myb in this normal population may exceed that found in some transformed cell populations that depend on v-myb to maintain the transformed phenotype. This c-myb-expressing cell population has been identified as primarily M-CFC, the committed progenitor for the macrophage lineage. As cells differentiate to the promonocyte stage there is an abrupt decrease in c-myb expression of greater than 100 fold. These studies thus describe a normal cell population that expresses c-myb at levels similar to the level of v-myb in cells that depend on v-myb for the maintenance of their transformed phenotype. Furthermore, these studies provide direct evidence for the developmental regulation of c-myb during the process of normal macrophage differentiation.

Sites of recombination between the transforming gene of avian myeloblastosis virus and its helper virus

The sites of recombination between the transforming gene of avian myeloblastosis virus (AMV) and its natural helper myeloblastosis-associated virus (MAV) have been determined. In AMV, the cellular sequence substituting for the viral envelope (env) gene gives rise to a different carboxyl terminus of the DNA polymerase. The 5'-recombination site coincides with the RNA splice acceptor site for the production of env mRNA in MAV-infected cells. The 3'-recombination site reveals that the last 11 amino acids including the termination codon are shared by the env protein and AMV transforming protein. The RNA splice acceptor site for the generation of subgenomic v-myb mRNA is located 84 nucleotides downstream from the 5'-recombination site. The AMV transforming protein consists of helper virus-related sequences at both of its amino and carboxyl termini, and all but 84 nucleotides of the cell-derived v-myb sequence. The comparison of MAV gp85 amino acid sequence with those of subgroups B, C, and E indicates that the MAV present in clone lambda 10A2-1 belongs to subgroup B. The high degree of homology among different avian retroviruses of the same subgroup indicates that the amino acid sequence of gp85 is important in determining the conformation of the envelope glycoprotein.

Transformation of Brown Leghorn chicken embryo fibroblasts by avian myeloblastosis virus proviral DNA

Brown Leghorn chicken embryo fibroblasts were transfected with a mixture of avian myeloblastosis virus (AMV) and myeloblastosis-associated virus type 1 (MAV1) proviral DNA purified from lambda-Charon 4A recombinant clones. A transformed cell line (T1AM) able to grow without anchorage in semisolid medium was obtained. The presence of both proviral AMV and MAV sequences was detected in T1AM DNA by hybridization with v-myb- and MAV1-specific probes. Altered AMV and MAV1 proviral genomes were found in T1AM genome. Characterization of the RNA species expressed in transformed cells showed that in addition to a 2.5-kilobase (kb) putative subgenomic v-myb-specific RNA, three other myb-containing RNAs (9.4, 8.4, and 7.0 kb) were present in T1AM cells. No AMV genomic RNA was detected. Also, a new 5.0-kb MAV1-specific RNA species was expressed in transformed cells in addition to MAV1 genomic RNA species (7.8 kb). No infectious AMV virions are released by T1AM cells. Chicken embryo fibroblasts infected by T1AM-released virions contained and expressed all MAV1 sequences detected in T1AM transformed cells but did not express any transformation parameter. These results indicated that the presence of AMV proviral sequences in T1AM cells is responsible for their transformed phenotype.

Studies of the putative transforming protein of the type I human T-cell leukemia virus

The putative transforming protein of the type I human T-cell leukemia virus (HTLV-1) is a 40-kilodalton protein encoded by the X region and is termed p40XI. On the basis of both subcellular fractionation techniques and immunocytochemical analysis, it is now shown that p40XI is a nuclear protein with a relatively short half-life (120 minutes). It is synthesized de novo in considerable quantities in a human T-cell line infected with and transformed by the virus in vitro, and it is not packaged in detectable amounts in the extracellular virus.

Nucleotide sequence analysis of the chicken gene c-mil, the progenitor of the retroviral oncogene v-mil

The nucleotide sequence of the chicken gene c-mil was determined within and around all regions homologous to the oncogene v-mil of avian retrovirus MH2. The regions of homology to the previously determined v-mil sequence, ranging in size from 28 to 177 base pairs (bp), are distributed over 14 kilobase pairs (kbp) of the chicken genome and are organized in 11 exons. All exon-intron boundaries of c-mil, except the 5' boundary of exon 1 and the 3' boundary of exon 11, were unambiguously defined by the identification of consensus splice donor and acceptor sites precisely at positions where homology to v-mil ceases or resumes. The homology to v-mil starts within the coding sequence of exon 1 and ends within the 3' untranslated region of exon 11, 12 nucleotides downstream from the nonsense codon terminating the large open reading frame shared between c-mil and v-mil. The c-mil and v-mil sequences differ at only 7 out of 1153 nucleotide positions, and the predicted sequences of v-mil and c-mil proteins differ by one conservative and four nonconservative substitutions among 379 amino acid residues. Hence, the carboxy-terminal domains of the MH2 gag-mil hybrid protein and of the putative c-mil protein are very similar. However, the amino-terminal domain of the cellular protein is possibly encoded by additional 5' c-mil sequences not present in the transduced v-mil oncogene, while that of the MH2 hybrid protein is encoded by viral gag sequences. The sequence analysis also revealed that c-mil and c-myc derived sequences are immediately adjacent on the MH2 genome carrying both the v-mil and the v-myc oncogene. Hence, transduction of c-mil into MH2 involved recombination, at the 3' site, with either the c-myc locus or a previously transduced v-myc gene, and, at the 5' site, with gag sequences of the transducing virus. At both sites, no significant homologies were found between the sequence elements involved in the recombination.

Nucleic acid sequence database VI: Retroviral oncogenes and cellular proto-oncogenes

The databases of the Protein Identification Resource at the National Biomedical Research Foundation (NBRF) contain nucleic acid and protein sequences from 18 retroviral oncogenes (v-onc) and 8 cellular proto-oncogenes (c-onc). Comparison of the sequences between the v-onc and c-onc genes reveals: (i) The c-src, c-abl, c-mos, c-fos, c-ras, c-myb, c-myc, and c-sis genes contain coding regions that are highly conserved in the respective v-onc genes with a small number of base changes. (ii) There are more transitions than transversions. (iii) Some of these base changes are silent mutations and others generate amino acid substitutions in the viral proteins. The causes of these base changes in the coding sequences and the significance to oncogenic transformation of the amino acid substitutions in the viral proteins remain to be determined.

DNA-binding activity is associated with purified myb proteins from AMV and E26 viruses and is temperature-sensitive for E26 ts mutants

Oncogene protein products from avian myeloblastosis virus, p48v-myb, and from avian leukemia virus E26, p135gag-myb-ets, are located predominantly in the nucleus of nonproducer bone marrow cell clones, as revealed by indirect immunofluorescence. Both oncogene proteins were purified by immunoaffinity chromatography using monoclonal antibodies against p19 and immunoglobulins specific for myb, which was expressed in bacteria for antibody production. The purified proteins bind to DNA in vitro. In contrast, purified p135gag-myb-ets proteins from several mutants of E26 virus, temperature-sensitive for myeloblast transformation, either lost their abilities to bind to DNA or exhibited highly thermolabile DNA-protein interactions in vitro. DNA binding of AMV and E26 oncogene proteins is inhibited by myb-specific immunoglobulins. Our results suggest that lesions in the myb oncogene affect transformation as well as DNA binding of myb proteins in vitro.

Characterization of the protein apparently responsible for the elevated tyrosine protein kinase activity in LSTRA cells

The LSTRA murine thymoma cell line contains an elevated level of tyrosine protein kinase activity. When a microsomal preparation from these cells is incubated in vitro with ATP, the principal tyrosine protein kinase substrate is a 56,000-dalton protein, p56. We have found that an activity phosphorylating p56 on tyrosine can also be detected at low levels in microsomes from most, but not all, T lymphoma cell lines and from normal thymic tissue. Only 1 of 30 other lymphoma cell lines was found to contain an elevated level of such a tyrosine protein kinase. An activity that phosphorylated p56 in vitro was not detectable in the cells of other hematopoietic lineages. Anti-peptide antibodies reactive with the site of in vitro tyrosine phosphorylation of p56 allowed us to determine that the apparent abundance of the p56 polypeptide parallels closely the level of the tyrosine protein kinase activity in the cell lines examined. This suggests that p56 is the protein kinase responsible for the elevated tyrosine protein kinase activity in LSTRA cells and that the phosphorylation of p56 observed in vitro results from autophosphorylation. Two-dimensional tryptic peptide mapping revealed that p56 is distinct from the proteins encoded by the cellular genes which are the progenitors of retroviral tyrosine protein kinases, src, yes, fgr, abl, fes, and ros. Additionally, none of these proto-oncogenes was found to be transcribed at elevated levels in LSTRA or Thy19 cells. Like the catalytic subunit of the cyclic AMP-dependent protein kinase, the cellular and viral forms of p60src, and the protein phosphatase calcineurin B, p56 contains covalently bound fatty acid.

Induced differentiation of avian myeloblastosis virus-transformed myeloblasts: phenotypic alteration without altered expression of the viral oncogene

Cells of a clone of avian myeloblastosis virus-transformed myeloblasts were induced to differentiate to adherent myelomonocytic cells by treatment with lipopolysaccharide. These adherent cells were subcultured and maintained as a line for more than 6 months with lipopolysaccharide present. Cells of this line were induced to differentiate to nondividing macrophage-like cells by the addition of the tumor promoter 12-O-tetradecanoylphorbol-13-acetate. In this way, the following homogeneous cell populations representing three distinct stages of myeloid differentiation were obtained: I, actively dividing myeloblasts that grew in suspension: II, actively dividing adherent cells; and III, fully differentiated nondividing cells resembling macrophages. When the expression of v-myb (the oncogene of avian myeloblastosis virus) was examined in cells of these three differentiation stages, it was found that the protein encoded by v-myb (p45v-myb) continued to be synthesized in similar quantities and showed no obvious alteration (assessed by partial proteolytic digestion and two-dimensional gel electrophoresis) during differentiation. These results show that cells transformed by v-myb can be induced to differentiate without affecting the expression of v-myb and imply that, during differentiation, the effect of v-myb is suppressed by a mechanism other than altered expression of the oncogene.

Characterization of the protein apparently responsible for the elevated tyrosine protein kinase activity in LSTRA cells

The LSTRA murine thymoma cell line contains an elevated level of tyrosine protein kinase activity. When a microsomal preparation from these cells is incubated in vitro with ATP, the principal tyrosine protein kinase substrate is a 56,000-dalton protein, p56. We have found that an activity phosphorylating p56 on tyrosine can also be detected at low levels in microsomes from most, but not all, T lymphoma cell lines and from normal thymic tissue. Only 1 of 30 other lymphoma cell lines was found to contain an elevated level of such a tyrosine protein kinase. An activity that phosphorylated p56 in vitro was not detectable in the cells of other hematopoietic lineages. Anti-peptide antibodies reactive with the site of in vitro tyrosine phosphorylation of p56 allowed us to determine that the apparent abundance of the p56 polypeptide parallels closely the level of the tyrosine protein kinase activity in the cell lines examined. This suggests that p56 is the protein kinase responsible for the elevated tyrosine protein kinase activity in LSTRA cells and that the phosphorylation of p56 observed in vitro results from autophosphorylation. Two-dimensional tryptic peptide mapping revealed that p56 is distinct from the proteins encoded by the cellular genes which are the progenitors of retroviral tyrosine protein kinases, src, yes, fgr, abl, fes, and ros. Additionally, none of these proto-oncogenes was found to be transcribed at elevated levels in LSTRA or Thy19 cells. Like the catalytic subunit of the cyclic AMP-dependent protein kinase, the cellular and viral forms of p60src, and the protein phosphatase calcineurin B, p56 contains covalently bound fatty acid.

Isolation of monoclonal antibodies specific for products of avian oncogene myb

We isolated a series of monoclonal antibodies which were raised against a bacterially expressed protein, bp37v-myb, and coded for by part of the avian v-myb gene. These monoclonal antibodies recognized a range of antigenic specificities on bp37v-myb, and this was reflected in their differing specificities for the gene products of the v-myb, c-myb, and E26 viral oncogenes. One monoclonal antibody recognized, in addition to the v-myb and c-myb gene products, a conserved nuclear protein found in all tested cells. We describe the characterization of these monoclonal antibodies.

Mouse prepro-epidermal growth factor synthesis by the kidney and other tissues

Epidermal growth factor (EGF), a protein comprising 53 amino acids, is derived from a precursor of 1,217 amino acids that includes at least seven EGF-like sequences. EGF has diverse biological activities: it is a potent mitogen for many tissue culture cells, inhibits gastric acid secretion from the intestinal mucosa and promotes healing of the corneal epithelium. EGF given to fetal animals accelerates several developmental processes including palate formation, incisor eruption, eyelid opening and lung maturation. However, the physiological roles of EGF in vivo are unknown. The presence of high-affinity receptors in many fetal and adult tissues suggests that EGF is involved in normal cellular functions. Immunocytochemical studies have revealed the presence of EGF in mouse and human submaxillary glands, rat brain and human intestine. The low levels of EGF in extracts from many tissues may reflect sequestration rather than synthesis of the polypeptide. We show here that several mouse tissues contain preproEGF mRNA and that it is synthesized mainly in the distal tubules of the kidney. PreproEGF does not seem to be processed to EGF or other peptides in this tissue.

New human transforming genes detected by a tumorigenicity assay

We have developed a sensitive bioassay for transforming genes based on the tumorigenicity of cotransfected NIH3T3 cells in nude mice. The assay differs substantially from the NIH3T3 focus assay. Using it, we have detected the transfer of three transforming genes from the DNA of MCF-7, a human mammary carcinoma cell line. One of these is N-ras, which is amplified in MCF-7 DNA. The other two, which we have called mcf2 and mcf3, do not appear to be related to known oncogenes. We cannot detect their transfer by using the NIH3T3 focus assay. We do not yet know whether either mcf2 or mcf3 is associated with genetic abnormalities in MCF-7 cells.

Isolation of monoclonal antibodies specific for products of avian oncogene myb

We isolated a series of monoclonal antibodies which were raised against a bacterially expressed protein, bp37v-myb, and coded for by part of the avian v-myb gene. These monoclonal antibodies recognized a range of antigenic specificities on bp37v-myb, and this was reflected in their differing specificities for the gene products of the v-myb, c-myb, and E26 viral oncogenes. One monoclonal antibody recognized, in addition to the v-myb and c-myb gene products, a conserved nuclear protein found in all tested cells. We describe the characterization of these monoclonal antibodies.

Induced differentiation of avian myeloblastosis virus-transformed myeloblasts: phenotypic alteration without altered expression of the viral oncogene

Cells of a clone of avian myeloblastosis virus-transformed myeloblasts were induced to differentiate to adherent myelomonocytic cells by treatment with lipopolysaccharide. These adherent cells were subcultured and maintained as a line for more than 6 months with lipopolysaccharide present. Cells of this line were induced to differentiate to nondividing macrophage-like cells by the addition of the tumor promoter 12-O-tetradecanoylphorbol-13-acetate. In this way, the following homogeneous cell populations representing three distinct stages of myeloid differentiation were obtained: I, actively dividing myeloblasts that grew in suspension: II, actively dividing adherent cells; and III, fully differentiated nondividing cells resembling macrophages. When the expression of v-myb (the oncogene of avian myeloblastosis virus) was examined in cells of these three differentiation stages, it was found that the protein encoded by v-myb (p45v-myb) continued to be synthesized in similar quantities and showed no obvious alteration (assessed by partial proteolytic digestion and two-dimensional gel electrophoresis) during differentiation. These results show that cells transformed by v-myb can be induced to differentiate without affecting the expression of v-myb and imply that, during differentiation, the effect of v-myb is suppressed by a mechanism other than altered expression of the oncogene.

Avian erythroblastosis virus E26: nucleotide sequence of the tripartite onc gene and of the LTR, and analysis of the cellular prototype of the viral ets sequence

An intact 5.7-kb provirus of the avian erythroblastosis virus E26 has been molecularly cloned for comparisons with avian myeloblastosis virus (AMV) and other avian tumor viruses. E26 and AMV transform hemopoietic cells exclusively. Both cause myeloblastosis, but E26 also causes erythroblastosis. Sequence analysis of the proviral DNA showed that: The tripartite transforming gene of E26 forms a contiguous reading frame of 1046 codons, including 272 gag, 283 mybE, and 491 ets codons. No subgenomic ets-specific mRNA was detected in E26-infected cells. By contrast, the onc gene of AMV consists almost entirely of a mybA sequence expressed via subgenomic mRNA that extends over the 5' and 3' ends of mybE. mybE is only slightly diverged from the mybA homolog of AMV and even less from the cellular proto-myb sequence with no characteristic mutation that sets apart the two viruses from proto-myb. The U5 region of the long terminal repeat (LTR) of E26 and AMV are colinear and differ only in scattered point mutations. The U3 region of the E26 LTR is different from that of AMV but is colinear and closely related with that of avian carcinoma virus MH2 and also with that of Prague Rous sarcoma virus (RSV), except for an unexpected 16-nucleotide substitution of 22 RSV nucleotides. Upstream of the 3' LTR, the c region of E26 appears to be the same as that of RSV for 70 nucleotides and very similar to those of AMV and MH2 for about 20 to 30 nucleotides. Since the U3s of E26, MH2 and RSV are very closely related and neither MH2 nor RSV show a particular erythroblast tropism, it is possible that the U3 does not play a critical role in the erythroblast tropism of E26. Electrophoretic size analyses of chicken DNA digested with restriction enzymes indicate that DNA fragments totaling over 50 kb hybridize with viral ets DNA.

A simple and rapid solid phase enzyme-linked immunoadsorbence assay for screening monoclonal antibodies to poorly soluble proteins

An ELISA is described in which protein antigens are fixed to microtitre wells. This assay is of particular use when antigens are soluble only in solutions containing SDS or other detergents, or else have been purified by preparative SDS polyacrylamide gel electrophoresis. Because the protein antigens are fixed to the microtitre wells, it is also possible to re-use antigen derivatised plates several times.

New human transforming genes detected by a tumorigenicity assay

We have developed a sensitive bioassay for transforming genes based on the tumorigenicity of cotransfected NIH3T3 cells in nude mice. The assay differs substantially from the NIH3T3 focus assay. Using it, we have detected the transfer of three transforming genes from the DNA of MCF-7, a human mammary carcinoma cell line. One of these is N-ras, which is amplified in MCF-7 DNA. The other two, which we have called mcf2 and mcf3, do not appear to be related to known oncogenes. We cannot detect their transfer by using the NIH3T3 focus assay. We do not yet know whether either mcf2 or mcf3 is associated with genetic abnormalities in MCF-7 cells.

Avian myeloblastosis virus and E26 virus oncogene products are nuclear proteins

The defective acute leukemia viruses avian myeloblastosis virus (AMV) and E26 virus each contain an inserted cellular sequence related to the same highly conserved cellular gene, proto-amv. The oncogenes of these two retroviruses differ from this cellular proto-oncogene in gene structure, transcript structure, and gene product. The product of the AMV oncogene (myb) is a 48,000 Mr protein, p48myb, encoded by a transduced segment (amv) of proto-amv flanked by short helper-virus-derived terminal sequences. The E26 virus oncogene product is a 135,000 Mr protein, p135gag-amve-ets, encoded by significant portions of a viral structural gene (gag), sequences related to proto-amv (amve), and additional E26-specific sequences (ets) transduced from cellular proto-ets. Both p48myb and p135gag-amve-ets transforming proteins are located in the nucleus of cells transformed by these viruses. A protein of 110,000 Mr which is specifically immunoprecipitated by antisera to amv peptides and may be the product of the normal cellular gene (proto-amv) has been located in the cytoplasm of cells that express proto-amv mRNA.

Neoplastic transformation by E26 leukemia virus is mediated by a single protein containing domains of gag and myb genes

The genome of avian leukemia virus E26 shares homology with v-myb, the oncogene of avian myeloblastosis virus, and encodes a protein with an Mr of 135,000. Analyses of tryptic oligopeptides show that this protein is related to the proteins encoded by gag (Pr76gag) as well as v-myb (p45v-myb[AMV] ) and c-myb (p75c-myb). We found no evidence for the existence of additional myb-related proteins or subgenomic species of myb-related RNA in myeloblasts transformed by strain E26.