Regulation of MDMX nuclear import and degradation by Chk2 and 14-3-3

The MDM2 homolog MDMX is an important regulator of p53 during mouse embryonic development. DNA damage promotes MDMX phosphorylation, nuclear translocation, and degradation by MDM2. Here we show that MDMX copurifies with 14-3-3, and DNA damage stimulates MDMX binding to 14-3-3. Chk2-mediated phosphorylation of MDMX on S367 is important for stimulating 14-3-3 binding, MDMX nuclear import by a cryptic NLS, and degradation by MDM2. Mutation of MDMX S367 inhibits ubiquitination and degradation by MDM2, and prevents MDMX nuclear import. Expression of 14-3-3 stimulates the degradation of phosphorylated MDMX. Chk2 and 14-3-3 cooperatively stimulate MDMX ubiquitination and overcome the inhibition of p53 by MDMX. These results suggest that MDMX-14-3-3 interaction plays a role in p53 response to DNA damage by regulating MDMX localization and stability. We also show the identification of a cryptic nuclear localization sequence within the C-terminus RING finger domain MDMX. Mutation of MDMX on one lysine residue at position 468 to glutamic acid completely abrogates the nuclear import after DNA damage. This mutation had no effect on MDM2-mediated nuclear import of MDMX in cotransfection assays, suggesting that it is specifically required for the MDM2-independent nuclear import. Interestingly, the MDMX- K468E mutant induces the expression of p21 more efficiently than the wild-type MDMX after ionizing radiation (IR). Furthermore, the K468E mutant induction of p21 is associated with enhanced G1 arrest after DNA damage. These results indicate an important function of MDMX nuclear import in regulating p53 activity after DNA damage