Implication of the ubiquitin/proteasome system in Myc-regulated transcription.

The c-Myc oncoprotein is a transcription factor that controls genes involved in cell growth, apoptosis and oncogenesis. We and others recently showed that the F-box protein Skp2 interacts with c-Myc and participates in its ubiquitylation and proteasomal degradation. Surprisingly, Skp2 was also found to act as a positive cofactor for c-Myc-regulated transcription. Further, Skp2, ubiquitylated proteins and subunits of the proteasome were demonstrated to be associated with a c-Myc target promoter in vivo. We show here that c-Myc interacts with Skp2 as part of the SCFSkp2 E3 ubiquitin ligase complex. Further, c-Myc interacts with the Sug1, an AAA ATPase subunit of the 19S regulatory particle of the proteasome. Inhibition of Sug1 expression by siRNA reduced transcription from a Myc target promoter to the same extent as c-Myc or Skp2 siRNA, implicating Sug1in this process. Taken together these findings suggest a role of the ubiquitin/proteasome system in c-Myc-regulated transcription. A hypothetical model discussing the link between ubiquitylation and transcription will be presented.