Proteasomal turnover of p21Cip1 does not require p21Cip1 ubiquitination

Mol Cell. 2000 Feb;5(2):403-10. doi: 10.1016/s1097-2765(00)80435-9.


The Cdk inhibitor p21Cip1 is an unstable protein. Pharmacologic inhibition of the proteasome increases the half-life of p21 from less than 30 min to more than 2 hr and results in the accumulation of p21-ubiquitin conjugates. To determine whether ubiquitination was required for proteasomal degradation of p21, we constructed mutant versions of p21 that were not ubiquitinated in vivo. Remarkably, these mutants remained unstable and increased in abundance upon proteasome inhibition, indicating that direct ubiquitination of p21 is not necessary for its turnover by the proteasome. The frequently observed correlation between protein ubiquitination and proteasomal degradation is insufficient to conclude that ubiquitination is a prerequisite for degradation.

Publication types

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

MeSH terms

  • 3T3 Cells
  • Animals
  • Cyclin-Dependent Kinase Inhibitor p21
  • Cyclin-Dependent Kinases / antagonists & inhibitors*
  • Cyclins / metabolism*
  • Cysteine Endopeptidases / metabolism*
  • Humans
  • Mice
  • Multienzyme Complexes / metabolism*
  • Proteasome Endopeptidase Complex
  • Protein Processing, Post-Translational*
  • Ubiquitins / metabolism*


  • CDKN1A protein, human
  • Cdkn1a protein, mouse
  • Cyclin-Dependent Kinase Inhibitor p21
  • Cyclins
  • Multienzyme Complexes
  • Ubiquitins
  • Cyclin-Dependent Kinases
  • Cysteine Endopeptidases
  • Proteasome Endopeptidase Complex