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Physical interaction of p73 with c-Myc and MM1, a c-Myc-binding protein, and modulation of the p73 function
Ken-ichi Watanabe1, Toshinori Ozaki, Takahito Nakagawa
1Division of Biochemistry, Chiba Cancer Center Research Institute, Chiba 260-8717, Japan.
Abstract:
p73 shares high sequence homology with the tumor suppressor p53. Like p53, ectopic overexpression of p73 induces cell cycle arrest and/or apoptosis, and these biological activities are linked to its sequence-specific transactivation function. The COOH-terminal region of p73 is unique and has a function to modulate DNA-binding ability and transactivation activity. To identify and characterize cellular proteins that interact with the COOH-terminal region of p73 alpha and regulate its activity, we employed a yeast-based two-hybrid screen with a human fetal brain cDNA library. We found MM1, a nuclear c-Myc-binding protein, was associated with p73 alpha in both yeast two-hybrid and in vitro pull-down assays. In mammalian cells, MM1 co-immunoprecipitated with p73 alpha, whereas p73 beta and tumor suppressor p53 did not interact with MM1. Overexpression of MM1 in p53-deficient osteosarcoma SAOS-2 cells enhanced the p73 alpha-dependent transcription from the p53/p73-responsive Bax and PG13 promoters, whereas p73 beta- and p53-mediated transcriptional activation was unaffected in the presence of MM1. MM1 also stimulated the p73 alpha-mediated growth suppression in SAOS-2 cells. More importantly, we found that c-Myc was physically associated with p73 alpha and significantly impaired the transcriptional activity of p73 alpha on Bax and p21(waf1) promoters. Expression of MM1 strongly reduced the c-Myc-mediated inhibitory activity on p73 alpha. These results suggest that MM1 may act as a molecular partner for p73 to prevent the c-Myc-mediated inhibitory effect on its activity.
Insights
The protein MM1 interacts with p73 alpha, enhancing its tumor-suppressing activity by preventing inhibition from c-Myc. This discovery offers new insights into p73 regulation and cancer research.
Area of Science:
- Molecular Biology
- Cancer Research
- Protein Interactions
Background:
- The tumor suppressor p73, homologous to p53, regulates cell cycle arrest and apoptosis through transactivation.
- The unique COOH-terminal region of p73 modulates its DNA-binding and transactivation functions.
- Understanding proteins interacting with p73 is crucial for elucidating its regulatory mechanisms.
Purpose of the Study:
- To identify and characterize cellular proteins interacting with the COOH-terminal region of p73 alpha.
- To investigate the regulatory role of these interacting proteins on p73 alpha activity.
- To explore the functional consequences of these interactions in cellular processes.
Main Methods:
- Yeast-based two-hybrid screening using a human fetal brain cDNA library.
- In vitro pull-down assays and co-immunoprecipitation in mammalian cells.
- Reporter gene assays to assess transcriptional activity on p53/p73-responsive promoters (Bax, PG13, p21(waf1)).
Main Results:
- MM1, a nuclear c-Myc-binding protein, was identified as a p73 alpha interacting partner.
- MM1 specifically co-immunoprecipitated with p73 alpha, but not p73 beta or p53.
- MM1 enhanced p73 alpha-dependent transcription and growth suppression in p53-deficient cells.
- c-Myc inhibited p73 alpha transcriptional activity, an effect reduced by MM1 expression.
Conclusions:
- MM1 acts as a molecular partner for p73 alpha, specifically counteracting c-Myc-mediated inhibition.
- This interaction modulates p73 alpha's transcriptional activity and growth-suppressive functions.
- MM1 represents a potential target for therapeutic strategies involving p73 in cancer.