Abstract
53BP1 integrates pRB activity with the DNA damage response by binding to methylated K810.
Major finding: 53BP1 integrates pRB activity with the DNA damage response by binding to methylated K810.
Mechanism: 53BP1 binding maintains pRB in a hypophosphorylated state and results in reduced DNA damage.
Impact: 53BP1-mediated reading of methylated K810 modulates the tumor-suppressive function of pRB.
The tumor-suppressive activity of the RB protein (pRB) is negatively regulated by cyclin-dependent kinase (CDK)–mediated phosphorylation, which inhibits the interaction of pRB with E2F transcription factors and facilitates cell-cycle progression. However, other posttranslational modifications have been shown to modulate the function of pRB, including methylation at lysine 810 (K810), which is upregulated following DNA damage and promotes pRB-driven growth suppression by preventing CDK-dependent phosphorylation of serine 807. Carr and colleagues found that tumor protein p53-binding protein 1 (TP53BP1, also known as 53BP1), a tudor domain–containing chromatin reader protein that participates in DNA double-strand break (DSB) repair, specifically bound monomethylated and dimethylated, but not trimethylated or unmethylated, pRB K810. The interaction of 53BP1 with methylated pRB in cancer cell lines was increased in response to DNA damage and was mediated by binding of K810 and the surrounding residues to a pocket composed of the tandem tudor domains of 53BP1 via hydrophobic interactions. Depletion of Rb or expression of a mutant pRB in which K810 was substituted with arginine (K810R) resulted in increased levels of γH2AX, a marker of DNA DSBs, suggesting that methylation of pRB K810 and its interaction with 53BP1 directly contribute to DNA repair. In addition, 53BP1 recruitment to chromatin-bound pRB at E2F-regulated target genes was enhanced in the presence of DNA damage, and maintained pRB in a hypophosphorylated state, indicating that 53BP1 promotes the growth-suppressive function of pRB. Consistent with this idea, the ability of mutant pRB K810R to induce cellular senescence was reduced. These findings provide further insight into the regulation of the tumor-suppressive function of pRB and identify a methylation-dependent role for 53BP1 in integrating pRB activity with DNA damage repair.