Expression of the CDK inhibitor p21Cip1 is tightly regulated by signals that control cell division. p21 is an unstable protein that is degraded by the proteasome; however, the pathway that leads to proteasomal degradation of p21 has proven to be enigmatic. An important issue is whether proteasomal degradation of p21 occurs independently of ubiquitylation or, alternatively, whether ubiquitylation on its N terminus is crucial. We resolve this uncertainty by showing that endogenous cellular p21 is completely acetylated at its amino terminus and is therefore not a substrate for N-ubiquitylation. We further show that inactivation of essential components of the ubiquitylation machinery does not directly impact endogenous p21 degradation. Our results underscore the importance of N-acetylation in restricting N-ubiquitylation and show, in particular, that ubiquitylation of endogenous p21 either at internal lysines or on the N terminus is unlikely to control its degradation by the proteasome.
