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PACT: cloning and characterization of a cellular p53 binding protein that interacts with Rb
A Simons1, C Melamed-Bessudo, R Wolkowicz
1Department of Genetics, The Hebrew University of Jerusalem, Israel.
Abstract:
Cellular functions of tumor suppressor proteins can be mediated by protein-protein interactions. Using p53 as a probe to screen an expression library, a cDNA encoding a 250 kDa protein was isolated. Recombinant forms of this protein, designated PACT, bind to wild type p53 while two different mutations abolish this interaction. PACT protein can also interfere with p53 specific DNA binding. PACT contains a serine/arginine (SR) rich region and a C' terminal lysine rich domain. The 250 kDa PACT protein can be precipitated from cell lysates by a method specific for SR proteins. snRNPs can be co-immunoprecipitated from cells with anti-PACT antibodies. These antibodies stain cell nuclei in a speckled pattern reminiscent of the distribution of known splicing factors. Recently, RBQ1, a truncated human homologue of PACT was identified by virtue of Rb binding. We show that RBQ1 is truncated as a result of a possible mutational event. PACT can interact with both cellular Rb and p53.
Insights
Researchers identified PACT, a 250 kDa protein that interacts with tumor suppressor p53 and affects its DNA binding. PACT also associates with splicing factors and the Rb protein, suggesting a role in cellular regulation.
Area of Science:
- Molecular Biology
- Cell Biology
- Cancer Research
Background:
- Tumor suppressor protein functions, like those of p53, rely on protein-protein interactions.
- Identifying novel interacting partners is crucial for understanding cellular regulation and disease mechanisms.
Purpose of the Study:
- To identify and characterize novel proteins interacting with the tumor suppressor p53.
- To investigate the functional implications of these interactions in cellular processes.
Main Methods:
- p53-based yeast two-hybrid screening to identify interacting proteins.
- Recombinant protein expression and binding assays.
- Co-immunoprecipitation and Western blotting.
- Immunofluorescence microscopy.
Main Results:
- A 250 kDa protein, designated PACT, was isolated and shown to bind wild-type p53.
- PACT binding to p53 was abolished by specific mutations, and PACT interfered with p53 DNA binding.
- PACT contains a serine/arginine-rich region, associates with SR protein-specific precipitation, and co-immunoprecipitates with snRNPs.
- PACT interacts with both p53 and the retinoblastoma tumor suppressor (Rb), and its homologue RBQ1 is a truncated form.
Conclusions:
- PACT is a novel p53-binding protein that modulates p53's DNA binding activity.
- PACT exhibits characteristics of splicing factors and interacts with the Rb protein.
- PACT may play a role in integrating p53 and Rb signaling pathways, potentially through its involvement in RNA splicing.