c-Myc is a predominately nuclear transcription factor that is a substrate for rapid turnover by the proteasome system. Cancer-related mutations in c-Myc lead to defects in its degradation and thereby contribute to the increase in its cellular level that is associated with the disease. Little is known about the mechanisms that target c-Myc to the proteasomes. By using a GFP fusion protein and live analysis we show that c-Myc shuttles between the nucleus and cytoplasm and thus it could be degraded in either compartment. Strikingly, at elevated levels of expression c-Myc accumulates at nucleoli in some cells, consistent with saturation of a nucleolus-associated degradation system in these cells. This idea is further supported by the observation that proteasome inhibitor treatment causes accumulation of c-Myc at the nucleoli of essentially all cells. Under these conditions c-Myc is relatively stably associated with the nucleolus, as would be expected if the nucleolus functions as a sequestration/degradation site for excess c-Myc. Furthermore, during elevated c-Myc expression or proteasome inhibition, nucleoli that are associated with c-Myc also accumulate proteasomes. c-Myc and proteasomes co-localise in intranucleolar regions distinct from the dense fibrillar component of the nucleolus. Based on these results we propose a model for c-Myc downregulation where c-Myc is sequestered at the nucleoli. Sequestration of c-Myc is accompanied by recruitment of proteasomes and may lead to subsequent degradation.

The c-Myc oncoprotein regulates transcription of genes that are associated with cell growth, proliferation and apoptosis(1). c-Myc levels are modulated by ubiquitin/proteasome-mediated degradation(1). Proteasome inhibition leads to c-Myc accumulation within nucleoli(2), indicating that c-Myc might have a nucleolar function. Here we show that the proteins c-Myc and Max interact in nucleoli and are associated with ribosomal DNA. This association is increased upon activation of quiescent cells and is followed by recruitment of the Myc cofactor TRRAP, enhanced histone acetylation, recruitment of RNA polymerase I (Pol I), and activation of rDNA transcription. Using small interfering RNAs (siRNAs) against c-Myc and an inhibitor of Myc - Max interactions, we demonstrate that c-Myc is required for activating rDNA transcription in response to mitogenic signals. Furthermore, using the ligand-activated MycER ( ER, oestrogen receptor) system, we show that c-Myc can activate Pol I transcription in the absence of Pol II transcription. These results suggest that c-Myc coordinates the activity of all three nuclear RNA polymerases, and thereby plays a key role in regulating ribosome biogenesis and cell growth.