Docking of a specialized PIP Box onto chromatin-bound PCNA creates a degron for the ubiquitin ligase CRL4Cdt2

Mol Cell. 2009 Jul 10;35(1):93-104. doi: 10.1016/j.molcel.2009.05.012.


Substrates of the E3 ubiquitin ligase CRL4(Cdt2), including Cdt1 and p21, contain a PCNA-binding motif called a PIP box. Upon binding of the PIP box to PCNA on chromatin, CRL4(Cdt2) is recruited and the substrate is ubiquitylated. Importantly, a PIP box cannot be sufficient for destruction, as most PIP box proteins are stable. Using Xenopus egg extracts, we identify two sequence elements in CRL4(Cdt2) substrates that promote their proteolysis: a specialized PIP box that confers exceptionally efficient PCNA binding and a basic amino acid 4 residues downstream of the PIP box, which recruits CRL4(Cdt2) to the substrate-PCNA complex. We also identify two mechanisms that couple CRL4(Cdt2)-dependent proteolysis to the chromatin-bound form of PCNA, ensuring that this proteolysis pathway is active only in S phase or after DNA damage. Thus, CRL4(Cdt2) recognizes an unusual degron, which is assembled specifically on chromatin via the binding of a specialized PIP box to PCNA.

Publication types

  • Research Support, N.I.H., Extramural

MeSH terms

  • Amino Acid Sequence
  • Animals
  • Binding Sites / genetics
  • Cell Cycle Proteins / genetics
  • Cell Cycle Proteins / metabolism
  • Chromatin / metabolism*
  • Cyclin-Dependent Kinase Inhibitor p21 / chemistry
  • Cyclin-Dependent Kinase Inhibitor p21 / genetics
  • Cyclin-Dependent Kinase Inhibitor p21 / metabolism
  • DNA-Binding Proteins / genetics
  • DNA-Binding Proteins / metabolism
  • Flap Endonucleases / genetics
  • Flap Endonucleases / metabolism
  • Humans
  • Immunoprecipitation
  • Models, Molecular
  • Molecular Sequence Data
  • Multiprotein Complexes / genetics
  • Multiprotein Complexes / metabolism
  • Mutation
  • Proliferating Cell Nuclear Antigen / chemistry
  • Proliferating Cell Nuclear Antigen / genetics
  • Proliferating Cell Nuclear Antigen / metabolism*
  • Protein Binding
  • Protein Structure, Tertiary
  • Sequence Homology, Amino Acid
  • Ubiquitin-Protein Ligases / genetics
  • Ubiquitin-Protein Ligases / metabolism*
  • Xenopus
  • Xenopus Proteins / genetics
  • Xenopus Proteins / metabolism*


  • Cdt1 protein, Xenopus
  • Cell Cycle Proteins
  • Chromatin
  • Cyclin-Dependent Kinase Inhibitor p21
  • DNA-Binding Proteins
  • Multiprotein Complexes
  • Proliferating Cell Nuclear Antigen
  • Xenopus Proteins
  • Ubiquitin-Protein Ligases
  • Flap Endonucleases