p27(Kip1) ubiquitination and degradation is regulated by the SCF(Skp2) complex through phosphorylated Thr187 in p27

Curr Biol. 1999 Jun 17;9(12):661-4. doi: 10.1016/s0960-9822(99)80290-5.


Many tumorigenic processes affect cell-cycle progression by their effects on the levels of the cyclin-dependent kinase inhibitor p27(Kip1) [1,2]. The phosphorylation- and ubiquitination-dependent proteolysis of p27 is implicated in control of the G1-S transition in the cell cycle [3-6]. To determine the factors that control p27 stability, we established a cell-free extract assay that recapitulates the degradation of p27. Phosphorylation of p27 at Thr187 was essential for its degradation. Degradation was also dependent on SCF(Skp2), a protein complex implicated in targeting phosphorylated proteins for ubiquitination [7-10]. Immunodepletion of components of the complex - Cul-1, Skp1, or Skp2 - from the extract abolished p27 degradation, while addition of purified SCF(Skp2) to Skp2- depleted extract restored the capacity to degrade p27. A specific association was observed between Skp2 and a p27 carboxy-terminal peptide containing phosphorylated Thr187, but not between Skp2 and the non-phosphorylated peptide. Skp2-dependent associations between Skp1 or Cul-1 and the p27 phosphopeptide were also detected. Isolated SCF(Skp2) contained an E3 ubiquitin ligase activity towards p27. Our data thus suggest that SCF(Skp2) specifically targets p27 for degradation during cell-cycle progression.

Publication types

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

MeSH terms

  • Cell Cycle
  • Cell Cycle Proteins*
  • Cell-Free System
  • Cyclin-Dependent Kinase Inhibitor p27
  • HeLa Cells
  • Humans
  • Microtubule-Associated Proteins / chemistry
  • Microtubule-Associated Proteins / metabolism*
  • Peptide Synthases / genetics
  • Peptide Synthases / metabolism*
  • Phosphorylation
  • Recombination, Genetic
  • SKP Cullin F-Box Protein Ligases
  • Threonine / chemistry
  • Threonine / metabolism
  • Tumor Suppressor Proteins*
  • Ubiquitins / metabolism*


  • Cell Cycle Proteins
  • Microtubule-Associated Proteins
  • Tumor Suppressor Proteins
  • Ubiquitins
  • Cyclin-Dependent Kinase Inhibitor p27
  • Threonine
  • SKP Cullin F-Box Protein Ligases
  • Peptide Synthases