CDK inhibitor p21 is degraded by a proliferating cell nuclear antigen-coupled Cul4-DDB1Cdt2 pathway during S phase and after UV irradiation

J Biol Chem. 2008 Oct 24;283(43):29045-52. doi: 10.1074/jbc.M806045200. Epub 2008 Aug 14.


Previous reports showed that chromatin-associated PCNA couples DNA replication with Cul4-DDB1(Cdt2)-dependent proteolysis of the licensing factor Cdt1. The CDK inhibitor p21, another PCNA-binding protein, is also degraded both in S phase and after UV irradiation. Here we show that p21 is degraded by the same ubiquitin-proteasome pathway as Cdt1 in HeLa cells. When PCNA or components of Cul4-DDB1(Cdt2) were silenced or when the PCNA binding site on p21 was mutated, degradation of p21 was prevented both in S phase and after UV irradiation. p21 was co-immunoprecipitated with Cul4A and DDB1 proteins when expressed in cells. The purified Cul4A-DDB1(Cdt2) complex ubiquitinated p21 in vitro. Consistently, p21 protein levels are low during S phase and increase around G(2) phase. Mutational analysis suggested that in addition to the PCNA binding domain, its flanking regions are also important for recognition by Cul4-DDB1(Cdt2). Our findings provide a new aspect of proteolytic control of p21 during the cell cycle.

Publication types

  • Research Support, Non-U.S. Gov't

MeSH terms

  • Binding Sites
  • Cullin Proteins / metabolism*
  • Cyclin-Dependent Kinase Inhibitor p21 / metabolism
  • Cyclin-Dependent Kinase Inhibitor p21 / physiology*
  • DNA Mutational Analysis
  • DNA-Binding Proteins / metabolism*
  • HeLa Cells
  • Humans
  • Immunoprecipitation
  • Models, Biological
  • Mutation
  • Nuclear Proteins / metabolism*
  • Proliferating Cell Nuclear Antigen / metabolism*
  • S Phase
  • Ubiquitin / chemistry
  • Ubiquitin-Protein Ligases
  • Ultraviolet Rays


  • CUL4A protein, human
  • Cullin Proteins
  • Cyclin-Dependent Kinase Inhibitor p21
  • DDB1 protein, human
  • DNA-Binding Proteins
  • DTL protein, human
  • Nuclear Proteins
  • Proliferating Cell Nuclear Antigen
  • Ubiquitin
  • Ubiquitin-Protein Ligases