Inhibition of the MDM2-p53 feedback loop is critical for p53 activation in response to cellular stresses. The ribosomal proteins L5, L11, and L23 can block this loop by inhibiting MDM2-mediated p53 ubiquitination and degradation in response to ribosomal stress. Here, we show that L11, but not L5 and L23, leads to a drastic accumulation of ubiquitinated and native MDM2. This effect is dependent on the ubiquitin ligase activity of MDM2, but not p53, and requires the central MDM2 binding domain (residues 51-108) of L11. We further show that L11 inhibited 26 S proteasomemediated degradation of ubiquitinated MDM2 in vitro and consistently prolonged the half-life of MDM2 in cells. These results suggest that L11, unlike L5 and L23, differentially regulates the levels of ubiquitinated p53 and MDM2 and inhibits the turnover and activity of MDM2 through a postubiquitination mechanism.The tumor suppressor protein p53 is a transcription factor activated in response to stress to induce expression of its target genes. The proteins encoded by these genes then mediate multiple cellular responses, such as cell cycle arrest, apoptosis, differentiation, cell senescence, or DNA repair (1). Also, p53 can directly trigger mitochondria-mediated apoptosis in response to DNA damage (2-4). The tumor suppression function of p53 is well reflected in the fact that more than half of human tumors harbor mutations in the p53 gene, and many others retain impaired function of the p53 pathway (5,6). Because of its inhibitory effect on cell growth, p53 is maintained at a low steady-state level and in an inert form in physiological conditions. This duty is mainly fulfilled by the E3 2 ubiquitin ligase MDM2 that mediates p53 constant degradation through a ubiquitin-dependent proteasome pathway (7,8). The mdm2 gene itself is a downstream target of p53, thus forming a tight autoregulatory feedback loop (9-11). Consistent with this notion, gene amplification and overexpression of MDM2 have also been shown in a variety of tumors, particularly in soft tissue sarcomas, lymphomas, and breast and lung cancers (12)(13)(14)(15)(16). Interfering with the MDM2-p53 feedback loop leads to p53 activation, ultimately preventing neoplasia. One example of this regulation is alternative reading frame (ARF) (p14 ARF in human, p19 ARF in mouse)-mediated inhibition of this loop in response to * This work was supported in part by NCI, National Institutes of Health Grants CA93614, CA095441, and CA079721 (to H. L.) and CA107532 (to S. R. G.). 1 To whom correspondence should be addressed: Dept. of Biochemistry and Molecular Biology, OR Health and Science University, 3181 SW Sam Jackson Park Rd., Portland,; E-mail: luh@ohsu.edu.. 2 The abbreviations used are: E3, ubiquitin-protein isopeptide ligase; ARF, alternative reading frame; E1, ubiquitin activating enzyme (UBA); E2, ubiquitin carrier protein; HA, hemagglutinin; Ni-NTA, nickel nitrilotriacetic acid; GFP, green fluorescent protein; MEF, murine embryonic fibroblast cell. 18). Also, in response to DNAdamaging agen...