c-Fos proto-oncoprotein is degraded by the proteasome independently of its own ubiquitinylation in vivo.
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.
D002455:Cell Division; D003546:Cysteine Endopeptidases / Q000378:metabolism*; D016193:G1 Phase; D006367:HeLa Cells; D006801:Humans; D015151:Immunoblotting; D008954:Models, Biological; D009097:Multienzyme Complexes / Q000378:metabolism*; D016297:Mutagenesis, Site-Directed; D010641:Phenotype; D010957:Plasmids / Q000378:metabolism; D046988:Proteasome Endopeptidase Complex; D011485:Protein Binding; D017434:Protein Structure, Tertiary; D016760:Proto-Oncogene Proteins c-fos / Q000378:metabolism*; D016192:Resting Phase, Cell Cycle; D013696:Temperature; D013997:Time Factors; D014158:Transcription, Genetic; D014162:Transfection; D025801:Ubiquitin / Q000378:metabolism*
D009097:Multienzyme Complexes; D016760:Proto-Oncogene Proteins c-fos; D025801:Ubiquitin; D003546:Cysteine Endopeptidases; D046988:Proteasome Endopeptidase Complex