c-Fos Proto-Oncoprotein Is Degraded by the Proteasome Independently of Its Own Ubiquitinylation In Vivo

Abstract

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 G₀/G₁ phase transition upon stimulation by mitogens and constitutive expression during asynchronous growth. Moreover, c-Fos destruction during the G₀/G₁ 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.

ACKNOWLEDGMENTS

We are indebted to E. Andermarcher, O. Coux, R. Hipskind, and M. Scheffner for critical reading of the manuscript.

This work was supported by grants from the CNRS, the ARC, the Ligue Nationale contre le Cancer, and the European Commission contract QLG1-CT-2001-02026.