Ubiquitination of MEIS1 by MDM2 serves as a switch for p53 stabilization and DNA damage response activation.

Targeting MDM2 by disrupting its interaction with p53 or inhibiting its E3 ligase activity is a promising strategy to restore p53 functionality. However, achieving anticancer efficacy while minimizing dose-limiting toxicities remains a significant challenge. Moreover, MDM2 also ubiquitinates various non-p53 targets, complicating its therapeutic targeting. In this study, we demonstrate that MDM2 directly facilitates K48-linked polyubiquitination of MEIS1 at K178, leading to its proteasomal degradation. Notably, MEIS1 forms a non-competitive ternary complex with MDM2 and p53, effectively promoting ubiquitin transfer to itself and preventing p53 ubiquitination. The MEIS1 K178R mutant, which is deficient in ubiquitination, fails to suppress MDM2-mediated p53 ubiquitination, demonstrating a mechanistic link between MEIS1 self-ubiquitination and p53 stabilization. Furthermore, MDM2-mediated MEIS1 ubiquitination is a prerequisite for p53 activation in the DNA damage response. Importantly, a MEIS1-derived peptide, which mimics the MDM2-mediating ubiquitination motif, enhances both MEIS1 and p53 stability, suppresses cell proliferation and tumor growth. Collectively, our findings identify MEIS1 as a molecular decoy that competes for ubiquitin transfer to protect p53 and highlight that MEIS1 ubiquitination could be a novel therapeutic target for reactivating p53-dependent tumor suppression.