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Published on: January 21, 2012
Multiple mechanisms for E2F binding inhibition by phosphorylation of the retinoblastoma protein C-terminal domain
Jason R Burke1, Tyler J Liban1, Tamara Restrepo1
1Department of Chemistry and Biochemistry, University of California Santa Cruz, CA 95064, USA.
Abstract:
The retinoblastoma protein C-terminal domain (RbC) is necessary for the tumor suppressor protein's activities in growth suppression and E2F transcription factor inhibition. Cyclin-dependent kinase phosphorylation of RbC contributes to Rb inactivation and weakens the Rb-E2F inhibitory complex. Here we demonstrate two mechanisms for how RbC phosphorylation inhibits E2F binding. We find that phosphorylation of S788 and S795 weakens the direct association between the N-terminal portion of RbC (RbC(N)) and the marked-box domains of E2F and its heterodimerization partner DP. Phosphorylation of these sites and S807/S811 also induces an intramolecular association between RbC and the pocket domain, which overlaps with the site of E2F transactivation domain binding. A reduction in E2F binding affinity occurs with S788/S795 phosphorylation that is additive with the effects of phosphorylation at other sites, and we propose a structural mechanism that explains this additivity. We find that different Rb phosphorylation events have distinct effects on activating E2F family members, which suggests a novel mechanism for how Rb may differentially regulate E2F activities.
Insights
Retinoblastoma protein (Rb) phosphorylation inhibits E2F transcription factors by weakening Rb-E2F binding through two distinct mechanisms. These findings reveal how Rb differentially regulates E2F activities, impacting cell growth and tumor suppression.
Area of Science:
- Molecular Biology
- Cell Biology
- Biochemistry
Background:
- The retinoblastoma protein (Rb) is a crucial tumor suppressor.
- Rb inhibits cell growth by binding to E2F transcription factors.
- Cyclin-dependent kinase (CDK) phosphorylation of Rb inactivates its tumor suppressor functions.
Purpose of the Study:
- To elucidate the mechanisms by which RbC phosphorylation inhibits E2F binding.
- To understand how specific phosphorylation sites on RbC affect E2F interaction.
- To explore the differential regulation of E2F family members by Rb phosphorylation.
Main Methods:
- In vitro binding assays to measure Rb-E2F interaction.
- Site-directed mutagenesis to investigate phosphorylation site effects.
- Structural analysis to understand binding mechanisms.
Main Results:
- Phosphorylation of S788 and S795 on RbC weakens the direct association between RbC(N) and E2F/DP.
- Phosphorylation at S788/S795, S807/S811 induces intramolecular RbC association, blocking E2F binding.
- S788/S795 phosphorylation effects are additive with other sites, suggesting a structural basis for additivity.
- Different Rb phosphorylation patterns differentially affect E2F activation.
Conclusions:
- RbC phosphorylation inhibits E2F binding through direct disruption of the RbC-E2F interface and allosteric conformational changes.
- RbC phosphorylation provides a novel mechanism for differential regulation of E2F family members.
- Understanding these mechanisms is key to comprehending cell cycle control and developing cancer therapies.
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