Proteasomal turnover of p21(Cip1) does not require p21(Cip1) ubiquitination

The Cdk inhibitor p21(Cip1) is an unstable protein. Pharmacologic inhibitio n 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 degradat ion of p21, we constructed mutant versions of p21 that were not ubiquitinat ed in vivo. Remarkably, these mutants remained unstable and increased in ab undance upon proteasome inhibition, indicating that direct ubiquitination o f p21 is not necessary for its turnover by the proteasome. The frequently o bserved correlation between protein ubiquitination and proteasomal degradat ion is insufficient to conclude that ubiquitination is a prerequisite for d egradation.