Prior ubiquitinylation of the unstable c-Fos proto-oncoprotein is thought to be required for recognition and degradation by the proteasome. Contradicting this view, we report that, although c-Fos can form conjugates with ubiquitin in vivo, nonubiquitinylatable c-Fos mutants show regulated degradation identical to that of the wild-type protein in living cells under two classical conditions of study: transient c-fos gene expression during the G(0)/G(1) phase transition upon stimulation by mitogens and constitutive expression during asynchronous growth. Moreover, c-Fos destruction during the G(0)/G(1) phase transition is unusual because it depends on two distinct but cumulative mechanisms. We report here that one mechanism involves a C-terminal destabilizer which does not need an active ubiquitin cycle, whereas the other involves an N-terminal destabilizer dependent on ubiquitinylation of an upstream c-Fos breakdown effector. In addition to providing new insights into the mechanisms of c-Fos protein destruction, an important consequence of our work is that ubiquitinylation-dependent proteasomal degradation claimed for a number of proteins should be reassessed on a new experimental basis.
c-Fos proto-oncoprotein is degraded by the proteasome independently of its own ubiquitinylation in vivo
Bossis, G.; Ferrara, P.; Acquaviva, C.; Jariel-Encontre, I.; Piechaczyk, M.
Mol Cell Biol
2003-10 / vol 23 / pages 7425-36
Humans; Models, Biological; Protein Binding; Phenotype; Time Factors; Immunoblotting; Transcription, Genetic; Proto-Oncogene Proteins c-fos/*metabolism; Multienzyme Complexes/*metabolism; Protein Structure, Tertiary; Temperature; Mutagenesis, Site-Directed; Transfection; Hela Cells; Cell Division; Plasmids/metabolism; Cysteine Endopeptidases/*metabolism; G0 Phase; Proteasome Endopeptidase Complex; Ubiquitin/*metabolism; G1 Phase