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A potent transrepression domain in the retinoblastoma protein induces a cell cycle arrest when bound to E2F sites
W R Sellers1, J W Rodgers, W G Kaelin
1Dana-Farber Cancer Institute, Boston, MA, USA.
Abstract:
An intact T/E1A-binding domain (the pocket) is necessary, but not sufficient, for the retinoblastoma protein (RB) to bind to DNA-protein complexes containing E2F and for RB to induce a G1/S block. Indirect evidence suggests that the binding of RB to E2F may, in addition to inhibiting E2F transactivation function, generate a complex capable of functioning as a transrepressor. Here we show that a chimera in which the E2F1 transactivation domain was replaced with the RB pocket could, in a DNA-binding and pocket-dependent manner, mimic the ability of RB to repress transcription and induce a cell cycle arrest. In contrast, a transdominant negative E2F1 mutant that is capable of blocking E2F-dependent transactivation did not. Fusion of the RB pocket to a heterologous DNA-binding domain unrelated to E2F likewise generated a transrepressor protein when scored against a suitable reporter. These results suggest that growth suppression by RB is due, at least in part, to transrepression mediated by the pocket domain bound to certain promoters via E2F.
Insights
The retinoblastoma protein (RB) pocket domain is crucial for repressing transcription and cell cycle arrest. This pocket domain, when bound to promoters via E2F, acts as a transrepressor, contributing to RB
Area of Science:
- Molecular Biology
- Cell Cycle Regulation
- Cancer Biology
Background:
- The retinoblastoma protein (RB) is a key regulator of the cell cycle, primarily known for its role in preventing uncontrolled cell proliferation.
- RB binds to E2F transcription factors, inhibiting their activity and causing a G1/S cell cycle block.
- Previous studies suggested RB might also function as a transrepressor, but the precise mechanism was unclear.
Purpose of the Study:
- To investigate the role of the RB pocket domain in transcriptional repression and cell cycle control.
- To determine if RB's growth-suppressive function is mediated by transrepression via its pocket domain.
Main Methods:
- Constructed chimeric proteins replacing the E2F1 transactivation domain with the RB pocket domain.
- Assessed the ability of these chimeras to repress transcription and induce cell cycle arrest.
- Utilized a transdominant negative E2F1 mutant and fused the RB pocket to a heterologous DNA-binding domain.
Main Results:
- A chimera with the RB pocket domain mimicked RB's ability to repress transcription and induce G1/S arrest in a DNA-binding and pocket-dependent manner.
- A transdominant negative E2F1 mutant that blocked E2F-dependent transactivation did not induce repression or arrest.
- Fusion of the RB pocket to an unrelated DNA-binding domain also created a transrepressor.
Conclusions:
- The RB pocket domain is sufficient to mediate transcriptional repression and cell cycle arrest.
- RB-mediated growth suppression is, at least partly, due to transrepression by the pocket domain binding to specific promoters via E2F.
- This provides a mechanistic insight into RB's tumor suppressor function.