Degradation of p27(Kip1) at the G(0)-G(1) transition mediated by a Skp2-independent ubiquitination pathway

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T Hara1, T Kamura, K Nakayama

  • 1Department of Molecular and Cellular Biology, Medical Institute of Bioregulation, Kyushu University, 3-1-1 Maidashi, Higashi-ku, Fukuoka 812-8582, Japan.

The Journal of Biological Chemistry
|October 30, 2001
PubMed

Insights

Skp2 is not solely responsible for p27(Kip1) degradation. Cytoplasmic pathways also contribute to p27(Kip1) proteolysis, ensuring normal cell cycle progression, especially from G(0) to G(1).

Area of Science:

  • Cell Biology
  • Molecular Biology
  • Biochemistry

Background:

  • p27(Kip1) is a cyclin-dependent kinase inhibitor crucial for cell cycle regulation.
  • Skp2, an F-box protein, was previously thought to be the primary mediator of p27(Kip1) proteolysis via the SCF ubiquitin ligase complex.

Purpose of the Study:

  • To investigate the role of Skp2 in p27(Kip1) degradation at different cell cycle phases.
  • To identify alternative pathways involved in p27(Kip1) proteolysis.

Main Methods:

  • Analysis of Skp2-deficient (Skp2(-/-)) lymphocytes.
  • Use of lactacystin, a proteasome inhibitor.
  • Assessment of p27(Kip1) polyubiquitination in nucleus and cytoplasm.
  • Employing a threonine 187 to alanine mutant of p27(Kip1).

Main Results:

  • p27(Kip1) degradation at G(0)-G(1) transition occurs normally in Skp2(-/-) cells, but is impaired during S-G(2) phases.
  • Proteasome inhibition blocks G(0)-G(1) degradation, indicating ubiquitin-proteasome pathway involvement.
  • Skp2-independent cytoplasmic polyubiquitination of p27(Kip1) contributes to early degradation.
  • Skp2(-/-) lymphocytes show normal first cell cycle but inhibited second cycle due to p27(Kip1) accumulation.

Conclusions:

  • Skp2 is essential for p27(Kip1) degradation during S-G(2) phases, but not the G(0)-G(1) transition.
  • A Skp2-independent cytoplasmic pathway facilitates p27(Kip1) degradation at the G(0)-G(1) transition, promoting cell cycle progression.
  • This finding reveals a more complex regulation of p27(Kip1) proteolysis than previously understood.

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