Susceptibility of p53 unstructured N terminus to 20 S proteasomal degradation programs the stress response.

The N-terminal transcription activation domain of p53 is intrinsically unstructured. We show in vitro and in vivo that this domain initiates p53 degradation by the 20 S proteasome in a ubiquitin-independent fashion. The decay of metabolically labeled p53 follows biphasic kinetics with an immediate fast phase that is ubiquitin-independent and a second slower phase that is ubiquitin-dependent. The 20 S proteasome executes the first phase by default, whereas the second phase requires the 26 S proteasome. p53 N-terminal binding proteins, such as Hdmx, can selectively block the first phase of degradation. Remarkably, gamma-irradiation inhibits both p53 decay phases, whereas UV selectively negates the second phase, giving rise to discrete levels of p53 accumulation. Our data of a single protein experiencing double mode degradation mechanisms each with unique kinetics provide the mechanistic basis for programmable protein homeostasis (proteostasis).