CUL7 assembles a non-traditional SCF-like E3 ubiquitin ligase that interacts withp53

Selective protein degradation targeted by cullin-ROC1-based E3 ubiquitin ligases plays critical roles in cell biology. Cullins are scaffold proteins that share a C-terminally located cullin domain, essential for interaction with ROC1, a RING finger protein that recruits E2 ubiquitin-conjugating enzymes to cullin-based ligases. CUL1, the best-characterized cullin, utilizes its N-terminus to bind to the small adapter Skp1, which in turn associates with the F-box motif of numerous F-box proteins. Thus, the CUL1 scaffold assembles multi-subunit SCF (S&barbelow;kp1·C&barbelow;UL1·F&barbelow;-box protein·ROC1) complexes, where interchangeable F-box proteins recruit specific phosphorylated substrates for ubiquitination by the ROC-1-bound E2 conjugating enzyme.; Presented here is the identification of CUL7 as a novel ROC1-interacting protein. CUL7 assembles an SCF-like E3 ligase complex consisting of Skp1, CUL7, the Fbx29 F-box protein, and ROC1. In contrast to CUL1, CUL7 is unable to associate with Skp1 alone but specifically recognizes the Skp1·Fbx29 protein module. Additionally CUL7 selectively interacts with Skp1·Fbx29, but not with Skp1·betaTRCP2 or Skp1·Skp2. Therefore, CUL7 may provide a selective scaffold that enables the specific ubiquitination of Fbx29-bound substrates. Moreover, work by others described mouse CUL7 as a pro-apoptotic, SV-40 large T antigen-interacting protein, with a role in vascular and lung development. Collectively, these data suggest that the CUL7 E3 ligase may regulate apoptosis and differentiation, by targeting a specific set of substrates for degradation.; The CUL7 ligase specifically associates with the p53 tumor suppressor and is able to promote its ubiquitination in vitro and in vivo. The CUL7-p53 complex is stable and surprisingly large, suggesting that CUL7 associates with p53 oligomers or with additional proteins, in the cytoplasm. CUL7 utilizes distinct domains to recruit p53 and the Skp1·Fbx29 complex, suggesting that p53 ubiquitination is independent of Fbx29. Thus, the CUL7 complex may act as an E3 for p53, and play a role in regulating the function of this tumor suppressor. Our results further raise the possibility that CUL7 assembles a unique E3 ligase that combines the properties of the SCF with additional F-box-independent mechanisms of substrate ubiquitination.