Structural basis for DNA damage-induced phosphoregulation of MDM2 RING domain

Area of Science:

• Molecular biology
• Biochemistry
• Cellular signaling

Background:

• DNA damage response pathways are crucial for maintaining genomic stability.
• MDM2 (mouse double minute 2 homolog) is a key E3 ubiquitin ligase that regulates the tumor suppressor p53.
• ATM (ataxia-telangiectasia mutated) kinase plays a central role in the DNA damage response.

Purpose of the Study:

• To elucidate the molecular mechanism by which ATM-mediated phosphorylation of MDM2 leads to p53 activation.
• To investigate the role of Ser429 phosphorylation in regulating MDM2 ubiquitin ligase activity and stability.

Main Methods:

• X-ray crystallography to determine the structure of phospho-Ser429 MDM2.
• In vitro biochemical assays to measure ubiquitin ligase activity.
• Cellular experiments to assess MDM2 autoubiquitination and degradation in response to DNA damage.

Main Results:

• Ser429 phosphorylation selectively enhances the activity of MDM2 homodimers, but not MDM2-MDMX heterodimers.
• Structural analysis revealed that pS429 stabilizes a closed E2-ubiquitin conformation, boosting ubiquitin transfer.
• In cells, Ser429 phosphorylation promotes MDM2 autoubiquitination and degradation upon DNA damage, while S429A substitution confers resistance to degradation.

Conclusions:

• Ser429 phosphorylation acts as a molecular switch to enhance MDM2 homodimer activity and promote its self-degradation.
• This mechanism facilitates rapid p53 stabilization following DNA damage, resolving controversies regarding MDM2 auto-degradation.