Identification of a small region of the v-fos gene product that is sufficient for transforming potential and growth-stimulating activity.

To analyze the structure-function relationship for the v-fos protein, we constructed in-frame insertion and deletion mutants of the v-fos gene carried by FBJ-MuSV, and expressed them in chicken primary cells using retrovirus vectors. We assessed the effects of these mutations on the ability of the v-fos protein to transform chicken embryo fibroblasts and to stimulate cellular proliferation of chicken neuroretinal cells. The mutant which retains only the central region of the v-fos protein (Met111-Ile206) have both activities, but the mutants which have deletions in this region, and one of the mutants that has a four amino acid insertion in it, lost both activities. The central region that is sufficient for these activities includes the evolutionarily highly conserved region among human, mouse and chicken c-fos proteins. Additionally, this sequence shares some homology with the DNA binding domain of GCN4 and c-jun protein. The truncated fos protein that contains only part of the central region is not phosphorylated in chicken embryo fibroblasts, indicating that phosphorylation of the fos protein is not necessary for the transforming activity.