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.     

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.     

DNA-binding activity associated with the v-myb oncogene product is not sufficient for transformation.

The product of the v-myb oncogene of avian myeloblastosis virus is a nuclear protein with an associated DNA-binding activity. We demonstrated that the highly conserved amino-terminal domain of p48v-myb is required for its associated DNA-binding activity. This activity is not required for the nuclear localization of p48v-myb. Furthermore, the associated DNA-binding activity and nuclear localization of p48v-myb together are not sufficient for transformation.     

Structural and functional domains of the myb oncogene: requirements for nuclear transport, myeloid transformation, and colony formation.

The v-myb oncogene of avian myeloblastosis virus causes acute myelomonocytic leukemia in vivo and transforms only myeloid cells in vitro. Its product, p48v-myb, is a nuclear protein of unknown function. To determine structure-function relationships for this protein, we constructed a series of deletion mutants of v-myb, expressed them in retroviral vectors, and studied their biochemical and biological properties. We used these mutants to identify two separate domains of p48v-myb which had distinct roles in its accumulation in the cell nucleus. We showed that the viral sequences which normally encode both termini of p48v-myb were dispensible for transformation. In contrast, both copies of the highly conserved v-myb amino-terminal repeat were required for transformation. We also identified a carboxyl-terminal domain of p48v-myb which was required for the growth of v-myb-transformed myeloblasts in soft agar but not for morphological transformation.     

Transformation by v-myb correlates with trans-activation of gene expression.

The v-myb oncogene of avian myeloblastosis virus causes acute myelomonocytic leukemia in chickens and transforms avian myeloid cells in vitro. Its protein product p48v-myb is a nuclear, sequence-specific, DNA-binding protein which activates gene expression in transient DNA transfection studies. To investigate the relationship between transformation and trans-activation by v-myb, we constructed 15 in-frame linker insertion mutants. The 12 mutants which transformed myeloid cells also trans-activated gene expression, whereas the 3 mutants which did not transform also did not trans-activate. This implies that trans-activation is required for transformation by v-myb. One of the transformation-defective mutants localized to the cell nucleus but failed to bind DNA. The other two transformation-defective mutants localized to the cell nucleus and bound DNA but nevertheless failed to trans-activate. These latter mutants define two distinct domains of p48v-myb which control trans-activation by DNA-bound protein, one within the amino-terminal DNA-binding domain itself and one in a carboxyl-terminal domain which is not required for DNA binding.     

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 on the interaction of the human c-myc protein with cell nuclei: p62c-myc as a member of a discrete subset of nuclear proteins

We have analyzed the localization of the human c-myc product (p62c-myc) at steady state in cells by immunoprecipitation and immunoblotting. We show that p62c-myc is extracted from nuclei by mild salt concentrations (below 200 mM), without affecting gross nuclear structure or causing extraction of major chromatin components. We observe no association between p62c-myc and the nuclear matrix. We also demonstrate that p62c-myc is a member of a discrete subset of nuclear proteins that are all rendered irreversibly insoluble in situ by exposure of isolated nuclei to physiological temperatures (37 degrees C). p62c-myc is sequestered into a similar insoluble complex in cells that have been subjected to heat shock. Finally, we show that avian v-myc and v-myb proteins in isolated nuclei also become insoluble after exposure to temperatures above 37 degrees C. We discuss the possible implications of these results.     

Specific amino acid substitutions are not required for transformation by v-myb of avian myeloblastosis virus.

The protein product of the v-myb oncogene of avian myeloblastosis virus, p48v-myb, differs structurally in several ways from its normal cellular homolog, p75c-myb. We demonstrated that the 11 specific amino acid substitutions found in two independent molecular clones of this virus were not required for the transformation of myeloblasts by v-myb.     

env-encoded residues are not required for transformation by p48v-myb.

The v-myb oncogene of avian myeloblastosis virus induces acute myeloblastic leukemia in chickens and transforms avian myeloid cells in vitro. The protein product of this oncogene, p48v-myb, is partially encoded by the retroviral gag and env genes. We demonstrated that the env-encoded carboxyl terminus of p48v-myb is not required for transformation. Our results showed, in addition, that a coding region of c-myb which is not essential for transformation was transduced by avian myeloblastosis virus.     

The highly conserved amino-terminal region of the protein encoded by the v-myb oncogene functions as a DNA-binding domain.

The retroviral oncogene v-myb encodes a 45,000 Mr nuclear protein (p45v-myb) that is predominantly associated with the chromatin of transformed cells. It has previously been shown that p45v-myb, when released from chromatin by salt-treatment, binds to DNA. To analyse the biochemical properties of p45v-myb in more detail we have expressed the v-myb coding region in Escherichia coli. Our results demonstrate that bacterially expressed myb protein has an intrinsic DNA-binding activity. Using two alternative strategies, (i) inhibition of DNA-binding by monoclonal antibodies and (ii) analysis of DNA-binding activities of partially deleted forms of the bacterial myb protein, we show that the DNA-binding domain is located in the amino-terminal region of the v-myb protein. This region has been highly conserved between myb genes of different species. Our results are therefore consistent with the hypothesis that DNA-binding is an important aspect of myb protein function.     

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.     

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.     

Estrogen-dependent alterations in differentiation state of myeloid cells caused by a v-myb/estrogen receptor fusion protein.

The oncogene v-myb and its cellular progenitor c-myb encode nuclear, DNA binding phosphoproteins that are thought to regulate the expression of myb-responsive genes during myeloid differentiation. To identify such myb-regulated genes, and to explore the mechanisms by which v-myb affects their expression, we have established a conditional expression system for v-myb. We have converted the v-myb protein to an estrogen-inducible transactivator by fusing the protein to the hormone binding domain of the human estrogen receptor. Expression of the chimeric protein in a chicken macrophage cell-line causes estrogen-dependent, reversible changes in the differentiation state as well as alterations in the gene expression program of the cells. We have used this estrogen-dependent v-myb expression system to identify a novel v-myb regulated gene.     

Transformation of chicken myelomonocytic cells by a retrovirus expressing the v-myb oncogene from the long terminal repeats of avian myeloblastosis virus but not Rous sarcoma virus.

To test the effect of long terminal repeat (LTR) regulatory sequences on the transforming capability of the v-myb oncogene from avian myeloblastosis virus (AMV), we have constructed replication-competent avian retroviral vectors with nearly identical structural genes that express v-myb from either AMV or Rous sarcoma virus (RSV) LTRs. After transfection into chicken embryo fibroblasts, virus-containing cell supernatants were used to infect chicken myelomonocytic target cells from preparations of 16-day-old embryonic spleen cells. Both wild-type AMV and the virus expressing v-myb from AMV LTRs (RCAMV-v-myb) were able to transform the splenocyte cultures into a population of immature myelomonocytic cells. The transformed cells expressed the p48v-Myb oncoprotein and formed compact foci when grown in soft agar. In contrast, the virus expressing v-myb from RSV LTRs (RCAS-v-myb) was repeatedly unable to transform the same splenocyte cells, despite being able to infect fibroblasts with high efficiency. This difference in the transforming activities of v-myb-expressing viruses with different LTRs most likely results from the presence of a factor (or factors) within the appropriate myelomonocytic target cell that promotes specific expression from the AMV but not from the RSV LTR.     

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.     

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.