Nucleolar stress induces ubiquitination-independent proteasomal degradation of PICT1 [Protein Synthesis and Degradation]

June 12th, 2014 by Maehama, T., Kawahara, K., Nishio, M., Suzuki, A., Hanada, K.

A nucleolar protein, PICT1, regulates the tumor suppressor p53 by tethering ribosomal protein L11 within the nucleolus to repress the binding of L11 to the E3 ligase MDM2. PICT1 depletion results in the release of L11 to the nucleoplasm to inhibit MDM2, leading to p53 activation. Here, we demonstrate that nucleolar stress induces proteasome-mediated degradation of PICT1 in a ubiquitin-independent manner. Treatment of H1299 cells with nucleolar stress inducers, such as actinomycin D, 5-fluorouridine, or doxorubicin, induced the degradation of PICT1 protein. The proteasome inhibitors MG132, lactacystin, and epoxomicin blocked PICT1 degradation, while the inhibition of E1 ubiquitin-activating enzyme by a specific inhibitor and genetic inactivation fails to repress PICT1 degradation. In addition, the 20S proteasome was able to degrade purified PICT1 protein in vitro. We also found a PICT1 mutant showing nucleoplasmic localization did not undergo nucleolar stress-induced degradation, while the same mutant underwent in vitro degradation by the 20S proteasome, suggesting that nucleolar localization is indispensable for the stress-induced PICT1 degradation. These results suggest that PICT1 employs atypical proteasome-mediated degradation machinery to sense nucleolar stress within the nucleolus.