Ubiquitination and proteolysis of cancer-derived Smad4 mutants by SCFSkp2

Mol Cell Biol. 2004 Sep;24(17):7524-37. doi: 10.1128/MCB.24.17.7524-7537.2004.


Smad4/DPC4, a common signal transducer in transforming growth factor beta (TGF-beta) signaling, is frequently inactivated in human cancer. Although the ubiquitin-proteasome pathway has been established as one mechanism of inactivating Smad4 in cancer, the specific ubiquitin E3 ligase for ubiquitination-mediated proteolysis of Smad4 cancer mutants remains unclear. In this report, we identified the SCFSkp2 complex as candidate Smad4-interacting proteins in an antibody array-based screen and further elucidated the functions of SCFSkp2 in mediating the metabolic instability of cancer-derived Smad4 mutants. We found that Skp2, the F-box component of SCFSkp2, physically interacted with Smad4 at the physiological levels. Several cancer-derived unstable mutants exhibited significantly increased binding to Skp2, which led to their increased ubiquitination and accelerated proteolysis. These results suggest an important role for the SCFSkp2 complex in switching cancer mutants of Smad4 to undergo polyubiquitination-dependent degradation.

Publication types

  • Research Support, Non-U.S. Gov't
  • Research Support, U.S. Gov't, P.H.S.

MeSH terms

  • Amino Acid Motifs
  • Animals
  • Cell Line
  • Cysteine Endopeptidases / metabolism
  • DNA-Binding Proteins / genetics*
  • DNA-Binding Proteins / metabolism*
  • Humans
  • JNK Mitogen-Activated Protein Kinases
  • Mitogen-Activated Protein Kinases / metabolism
  • Multienzyme Complexes / metabolism
  • Neoplasms / genetics
  • Neoplasms / metabolism*
  • Proteasome Endopeptidase Complex
  • Protein Binding
  • Protein Processing, Post-Translational
  • RNA Interference
  • Recombinant Fusion Proteins / genetics
  • Recombinant Fusion Proteins / metabolism
  • S-Phase Kinase-Associated Proteins / genetics
  • S-Phase Kinase-Associated Proteins / metabolism*
  • Signal Transduction / physiology
  • Smad4 Protein
  • Trans-Activators / genetics*
  • Trans-Activators / metabolism*
  • Ubiquitin / metabolism*
  • Ubiquitin-Protein Ligases / genetics
  • Ubiquitin-Protein Ligases / metabolism*
  • p38 Mitogen-Activated Protein Kinases


  • DNA-Binding Proteins
  • Multienzyme Complexes
  • Recombinant Fusion Proteins
  • S-Phase Kinase-Associated Proteins
  • SMAD4 protein, human
  • Smad4 Protein
  • Trans-Activators
  • Ubiquitin
  • Ubiquitin-Protein Ligases
  • JNK Mitogen-Activated Protein Kinases
  • Mitogen-Activated Protein Kinases
  • p38 Mitogen-Activated Protein Kinases
  • Cysteine Endopeptidases
  • Proteasome Endopeptidase Complex