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CIR, a corepressor linking the DNA binding factor CBF1 to the histone deacetylase complex
1Molecular Virology Laboratories, Department of Pharmacology and Molecular Sciences and Department of Oncology, Johns Hopkins School of Medicine, 725 North Wolfe Street, Baltimore, MD 21205, USA.
Abstract:
CBF1 is a member of the CSL family of DNA binding factors, which mediate either transcriptional repression or transcriptional activation. CSL proteins play a central role in Notch signaling and in Epstein-Barr virus-induced immortalization. Notch is a transmembrane protein involved in cell-fate decisions, and the cytoplasmic domain of Notch (NotchIC) targets CBF1. The Epstein-Barr virus-immortalizing protein EBNA2 activates both cellular and viral gene expression by targeting CBF1 and mimicking NotchIC. We have examined the mechanism of CBF1-mediated repression and show that CBF1 binds to a unique corepressor, CBF1 interacting corepressor (CIR). A CIR homolog is encoded by Caenorhabditis elegans, indicating that CIR is evolutionarily conserved. Two CBF1 mutants that were unable to bind CIR did not function as repressors, suggesting that targeting of CIR to CBF1 is an important component of repression. When expressed as a Gal4 fusion protein, CIR repressed reporter gene expression. CIR binds to histone deacetylase and to SAP30 and serves as a linker between CBF1 and the histone deacetylase complex.
Insights
CBF1 DNA binding factors mediate gene expression. A novel corepressor, CBF1 interacting corepressor (CIR), links CBF1 to histone deacetylase, revealing a key mechanism in transcriptional repression.
Area of Science:
- Molecular Biology
- Epigenetics
- Cell Signaling
Background:
- CBF1 is a CSL family DNA binding factor involved in Notch signaling and Epstein-Barr virus (EBV) functions.
- Notch signaling regulates cell-fate decisions, with its intracellular domain (NotchIC) interacting with CBF1.
- EBV protein EBNA2 mimics NotchIC to activate gene expression by targeting CBF1.
Purpose of the Study:
- To elucidate the mechanism of CBF1-mediated transcriptional repression.
- To identify and characterize novel proteins interacting with CBF1 during repression.
Main Methods:
- Investigated CBF1 interactions using biochemical assays.
- Utilized mutant CBF1 proteins to assess the functional role of interactions.
- Employed Gal4 fusion protein assays to evaluate the repressive activity of identified interactors.
Main Results:
- Identified a novel corepressor, CBF1 interacting corepressor (CIR), that specifically binds to CBF1.
- Demonstrated that CIR is evolutionarily conserved, with a homolog in Caenorhabditis elegans.
- Showed that CBF1 mutants unable to bind CIR lose their repressive function.
- CIR directly binds to histone deacetylase (HDAC) and SAP30, acting as a molecular bridge.
Conclusions:
- CIR is essential for CBF1-mediated transcriptional repression.
- CIR functions as a linker, connecting CBF1 to the histone deacetylase complex.
- This interaction highlights a conserved mechanism for epigenetic gene regulation involving CBF1.