Abrogation of
ubiquitin/proteasome-dependent turnover of p53 is critical for its
activation. UbL-UBA proteins, including human homolog of Rad23 (hHR23) proteins, may regulate proteasomal degradation of
substrates such as p53, due to their ability to interact with both ubiquitinated
substrates and the
proteasome. siRNA-mediated depletion of hHR23A or hHR23B in
human cell lines accelerated p53 degradation. In contrast,
overexpression of hHR23 proteins
led to the accumulation of ubiquitinated p53, and purified hHR23 proteins also blocked p53
proteasome degradation in vitro. An hHR23-MDM2 complex was identified, suggesting that
MDM2 and hHR23 cooperate in the regulation of p53
proteasome degradation. Consistent with this hypothesis, an
MDM2 mutant that demonstrated increased binding in vivo to hHR23A was able to ubiquitinate, but not degrade p53. Moreover, the defective
phenotype of this
MDM2 mutant was rescued by siRNA
knockdown of hHR23A. Our data indicate that
MDM2 acts at a step in the p53 degradation pathway after
ubiquitination, to counteract hHR23 inhibition of p53 turnover. Moreover, our data suggest the possibility that
ubiquitin ligase/UbL-UBA
protein complexes, as exemplified by the
MDM2/hHR23 complex, may serve a general role in regulating
substrate degradation by the
proteasome.
