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

ABSTRACT

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.