Decreased DNA-binding ability of purified transformation-specific proteins from deletion mutants of the acute avian leukemia virus MC29.

Avian myelocytomatosis virus MC29 is a highly oncogenic replication-defective retrovirus that predominantly affects hematopoietic cells and causes acute leukemia in vivo and that transforms hematopoietic cells as well as fibroblasts in vitro. The transformation-specific sequence, v-myc, is expressed as part of a fusion protein that contains the viral structural protein p19. By use of monoclonal antibodies against p19 we showed that the v-myc-encoded protein is located in the nucleus of MC29-transformed fibroblasts and that after purification over an immunoaffinity column the protein binds to double-stranded DNA. In this report we describe the analysis of the v-myc gene product from MC29-transformed bone marrow cells. The immunoaffinity column-purified protein from these cells also bound to DNA and was indistinguishable from the purified protein from MC29-transformed fibroblasts. In addition, the v-myc gene products from fibroblasts transformed by three nonconditional mutants of MC29--which transform hematopoietic cells with a markedly decreased efficiency in vivo and in vitro but still transform fibroblasts in vitro, expressing deleted v-myc proteins--were analyzed. In contrast to the wild-type protein, the purified mutant proteins had decreased DNA-binding abilities. Furthermore, a preferential binding of the wild-type protein to poly(dG) . poly(dC) duplexes was observed. Such a specificity was lost with a mutant protein. These results provide evidence that the interaction of the v-myc protein with DNA may be directly involved in transformation of the hematopoietic target cells. Further, the transformation-specific fusion proteins purified from cells transformed by avian erythroblastosis virus, which belongs to a different class of acute leukemia viruses, and by Fujinami sarcoma virus were found not to be DNA-binding proteins, suggesting the existence of different transformation mechanisms.