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Inactivation of the p53-homologue p73 by the mdm2-oncoprotein
M Dobbelstein1, S Wienzek, C König
1Institut für Virologie, Zentrum für Mikrobiologie und Hygiene, Philipps-Universität Marburg, Germany.
Abstract:
The p73beta protein shares structural and functional similarities with the tumor suppressor gene product p53. Both proteins activate transcription from p53-responsive promoters. p53's activity is antagonized by the mdm2 protein (also termed hdm2 in human cells). Complex formation between p53 and mdm2 results in p53's transcriptional inactivation and destabilization. Here we show that overexpression of mdm2 reduces p73beta's ability to activate transcription, too. The mdm2 protein forms a specific complex with p73beta in vitro with an efficiency comparable to p53-binding. Further, both p73beta and p53 relocalize a transport-defective mutant of mdm2 from the cytoplasm to the nucleus, arguing that complex formation occurs in vivo as well. Mutational analysis suggests that the interaction between p73beta and mdm2 follows structural principles analogous to the p53-mdm2-complex. Whereas p53 is destabilized in the presence of mdm2, the amount of intracellular p73beta was not detectably reduced by mdm2. The carboxyterminal RING finger domain of mdm2 was found to be required to reduce the intracellular abundance of p53, but it was dispensable for transcriptionally inactivating either p53 or p73beta. Our results suggest that the autoregulatory feedback loop between p53 and mdm2 also controls p73's activity, but that mdm2-mediated protein degradation is unique to p53.
Insights
The mdm2 protein inhibits both p53 and p73beta transcriptional activity by forming complexes. However, mdm2 only reduces p53 protein levels, not p73beta, suggesting differential regulation in cancer pathways.
Area of Science:
- Molecular Biology
- Cancer Research
- Protein Interactions
Background:
- The p73beta protein is structurally and functionally similar to the tumor suppressor p53.
- The mdm2 protein antagonizes p53 activity through complex formation, leading to transcriptional inactivation and destabilization.
- Understanding p73beta regulation is crucial for cancer research.
Purpose of the Study:
- To investigate the interaction between mdm2 and p73beta.
- To determine if mdm2 affects p73beta's transcriptional activity and stability.
- To compare the regulatory mechanisms of mdm2 on p53 and p73beta.
Main Methods:
- In vitro complex formation assays between mdm2 and p73beta.
- Analysis of p73beta transcriptional activity upon mdm2 overexpression.
- In vivo studies using transport-defective mdm2 mutants.
- Mutational analysis of mdm2 and p73beta interaction domains.
- Western blot analysis to assess protein levels.
Main Results:
- Overexpression of mdm2 reduces p73beta's transcriptional activity.
- mdm2 forms a specific complex with p73beta in vitro and in vivo.
- Mutational analysis indicates analogous interaction principles between p73beta-mdm2 and p53-mdm2 complexes.
- mdm2 does not reduce intracellular p73beta levels, unlike its effect on p53.
- The RING finger domain of mdm2 is required for p53 destabilization but not for transcriptional inactivation of p53 or p73beta.
Conclusions:
- The autoregulatory feedback loop between p53 and mdm2 extends to p73beta.
- mdm2 regulates p73beta activity through transcriptional inactivation, similar to p53.
- mdm2-mediated protein degradation is a mechanism specific to p53 regulation, not p73beta.