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Transcriptional repression and growth suppression by the p107 pocket protein
P Starostik1, K N Chow, D C Dean
1Department of Medicine, Washington University School of Medicine, St. Louis, Missouri 63110, USA.
Abstract:
p107 is a member of the pocket family of proteins that includes the retinoblastoma tumor suppressor. Overexpression of p107 arrests cells in G1, suggesting that it is important for cell cycle control. This growth suppression is mediated at least in part through the interaction of p107 with a member of the E2F family of cell cycle transcription factors, and this interaction can be disrupted by oncoproteins from DNA tumor viruses such as adenovirus E1a that bind p107. Not only does the binding of p107 to E2F inactivate E2F, but also we show that when p107 is tethered to the promoter through binding to E2F it functions as a general transcriptional repressor. This general repressor activity was also evident when p107 was fused to the DNA binding domain of Gal4 so that it could be directly targeted to the promoter in an E2F-independent fashion. Using p107 mutants, we compared the regions of the protein required for transcriptional repression and cell growth suppression. We found that the pocket domain is sufficient for inactivation of E2F, general repressor activity, and most of the growth suppressor activity. Binding of conserved region 1 from Ela to p107 blocked interaction with E2F, but it did not affect general repressor activity, demonstrating that binding and inactivation of E2F and general repressor activity are distinguishable properties of p107. Within the pocket, two conserved domains, A and B, were sufficient for growth suppression and transcriptional repressor activity. Surprisingly, we found that these two domains were fully functional when they were coexpressed as separate proteins, and we present results suggesting that the domains may interact at the promoter to form an active pocket.
Insights
p107 protein, a cell cycle regulator, suppresses growth by interacting with E2F transcription factors. Its pocket domain mediates this repression and E2F inactivation, with domains A and B being crucial for its function.
Area of Science:
- Molecular Biology
- Cell Biology
- Cancer Research
Background:
- p107 is a pocket protein family member, similar to the retinoblastoma tumor suppressor.
- Overexpression of p107 leads to G1 cell cycle arrest, indicating its role in cell cycle control.
Purpose of the Study:
- To investigate the mechanisms by which p107 mediates cell growth suppression and transcriptional repression.
- To identify the specific regions of p107 responsible for these functions and their interaction with E2F transcription factors.
Main Methods:
- Utilized p107 mutants to compare regions required for transcriptional repression and cell growth suppression.
- Assessed p107's repressor activity independently of E2F by fusing it to the Gal4 DNA-binding domain.
- Investigated the effect of adenovirus E1a binding on p107's interaction with E2F and its repressor activity.
Main Results:
- The p107 pocket domain is sufficient for E2F inactivation, general transcriptional repression, and most growth suppression.
- Adenovirus E1a binding to p107 inhibits E2F interaction but not general repressor activity, distinguishing these functions.
- Domains A and B within the pocket are sufficient for growth suppression and transcriptional repression, and can function when co-expressed separately.
Conclusions:
- p107 functions as a general transcriptional repressor when targeted to promoters, independent of E2F.
- The pocket domain, particularly domains A and B, is critical for p107's growth suppressor and transcriptional repressor activities.
- These domains may interact at the promoter to form an active complex, suggesting a novel mechanism for p107 function.