Isolation and structural analysis of a biologically active chicken c-fos cDNA: identification of evolutionarily conserved domains in fos protein.

To identify functionally important domains in the fos gene product we have studied the evolutionary divergence between chicken and mammalian fos proteins. A cDNA containing the entire chicken c-fos coding region was isolated and its nucleotide sequence determined. The deduced 367-amino acid sequence was compared to that of the mouse and human proteins. This comparison revealed a highly conserved domain (98% homology between mouse and chicken) in the center of the protein (85 amino acids) that coincides with a region known to be indispensible for transforming activity. This highly charged domain presumably contains contact sites for DNA and other proteins as well as a nuclear location signal sequence. Two other regions, that are dispensable for transformation, are also highly conserved and may thus be important for the physiological function of c-fos. These are the N-terminal 88 amino acids (85% homology) and the C-terminal 62 amino acids (92% homology). The C-terminus not only contains a potential DNA-binding Zn-finger structure but is also the least divergent region in the protein at the nucleotide level (92% conservation between chicken and mouse), supporting the hypothesis that mRNA secondary structures in this region may contribute to post-transcriptional regulatory mechanisms. In contrast, the domains between the terminal sequences and the center region of fos protein show considerable divergence (39% and 45% homology, respectively), indicating a minor role, if any, for these sequences. The significance of these conclusions is emphasized by the observation that the chicken c-fos protein, expressed from the cDNA inserted into a retrovirally-derived expression vector, efficiently induces morphological transformation in rat fibroblasts. The chicken c-fos gene product could be identified by immunoprecipitation and in vitro transcription/translation of the isolated cDNA as a protein of Mr approximately 60 K.