Phosphorylation- and Skp1-independent in vitro ubiquitination of E2F1 by multiple ROC-cullin ligases

Ubiquitin-dependent proteolysis plays a critical role in the control of many cellular processes and is mediated by a cascade of enzymes involving ubiquitin activating (El), conjugating (E2), and ligating (E3) activities. Cullin 1/CDC53 functions as an E3 ligase by interacting with RING finger protein ROC1 and recruiting phosphorylated substrate. We report here that E2F1 transcription factor can be ubiquitinated in vitro and in vivo by multiple ROC-cullin ligases. In vitro, E2F1 can be ubiquitinated by E2/Ubc5 but not by E2/CDC34, is dependent on catalytically active ROC1, and is protected by the Rb protein. In contrast to substrates of the SKP1-Cullin 1-F box (SCF) complexes, in vitro ubiquitination of E2F1 by CUL1-ROC1 ligase does not require E2F1 phosphorylation, is not stimulated by overexpression of F box protein SKP2, and is not affected by immunodepletion of SKP1 or mutations in CUL1 disrupting SKPI binding. These results suggest a novel, SKP1-independent mechanism for targeting E2F1 ubiquitination.