OVERPRODUCTION OF V-MYC IN THE NUCLEUS AND ITS EXCESS OVER MAX ARE NOT REQUIRED FOR AVIAN FIBROBLAST TRANSFORMATION

The cellular proto-oncogene c-myc can acquire transforming potential b y a number of different means, including retroviral transduction. The transduced allele generally contains point mutations relative to c-myc and is overexpressed in infected cells, usually as a v-Gag-Myc fusion protein. Upon synthesis, v-Gag-Myc enters the nucleus, forms complexe s with its heterodimeric partner Max, and in this complex binds to DNA in a sequence-specific manner. To delineate the role for each of thes e events in fibroblast transformation, we introduced several mutations into the myc gene of the avian retrovirus MC29. We observed that Gag- Myc with a mutated nuclear localization signal is confined predominant ly in the cytoplasm and only about 5% of the protein could be detected in the nucleus (less than the amount of endogenous c-Myc). Consequent ly, only a small fraction of Max is associated with Myc. However, cell s infected with this mutant exhibit a completely transformed phenotype in vitro, suggesting that production of enough v-Gag-Myc to tie up al l cellular Max is not needed for transformation. While the nuclear loc alization signal is dispensable for transformation, minimal changes in the v-Gag-Myc DNA-binding domain completely abolish its transforming potential, consistent with a role of Myc as a transcriptional regulato r. One of its potential targets might be the endogenous c-myc, which i s repressed in wild-type MC29-infected cells. Our experiments with MC2 9 mutants demonstrate that c-myc down-regulation depends on the integr ity of the v-Myc DNA-binding domain and occurs at the RNA level. Hence , it is conceivable that v-Gag-Myc, either directly or circuitously, r egulates c-myc transcription.