c-Fos Protein Transport into the Nucleus
The transport of c-Fos protein into the nucleus is regulated by various extracellular signals. Since c-Fos protein transport into the nucleus is regulated, it is also possible that both nuclear localization signals and motifs for cytoplasmic retention are colocalized within the protein, the latter being dominant over the former when fused to the pyruvate kinase sequence. The idea that c-Fos protein transport into the nucleus undergoes regulation by extracellular signals comes from the observation that c-Fos immunostaining concomitantly disappears from the nucleus and increases in the cytoplasm of mouse fibroblasts either transfected or microinjected with a constitutive c-fos gene. Unpublished experiments by the group have also shown that serum factors are not the only agents able to trigger c-Fos protein translocation into the nucleus in vitro. Mutations in Finkel-Biskis-Jenkins and Finkel-Biskis-Reilly retroviruses allow viral proteins to evade the nuclear transport control and are valuable for identifying the c-Fos protein domains responsible for cytoplasmic retention.