K48-linked ubiquitination and protein degradation regulate 53BP1 recruitment at DNA damage sites

Abstract

Efficient DNA damage sensing and repair is crucial to preserve genomic integrity and failure to detect or repair DNA breaks can cause mutations, contributing to the formation of tumors. One key protein required for mediating DNA repair is the tumor suppressor 53BP1. Recent studies now demonstrate the crucial role of K48-linked ubiquitination and protein degradation for 53BP1 recruitment at sites of DNA damage.The linking of ubiquitin chains to proteins is a multistep process involving three different types of enzymes. First, the ubiquitin-activating enzyme E1 catalyzes the transfer of ubiquitin to the ubiquitin-conjugating enzyme E2, which finally conjugates the ubiquitin moeities to target proteins with E3 ubiquitin ligases. Ubiquitin possesses seven lysine residues (K6, K11, K27, K29, K33, K48 and K63) and an N-terminal methionine (M1) that may be utilized to form poly-ubiquitin chains. The various types of linkage are usually associated with different cellular functions, as K48-linked polyubiquitin chains are involved in proteasomal degradation while K63-linked ubiquitination is a docking site for mediating proteinprotein interactions or conformational changes. While the role of K63-linked ubiquitination in the DNA damage response has been demonstrated [1], the role of K48-polyubiquitin chains and degradation remained unclear until recently.In the budding yeast Saccharomyces cerevisiae, components of the 19S and 20S proteasome are recruited to DNA breaks induced by the HO endonuclease [2] and mutant strains lacking components of the proteasome displayed enhanced radio-sensitivity. In mammalian cells, inhibition of the proteasome causes defects in the recruitment of multiple players of the DNA damage response pathway including phosphorylated ATM, 53BP1, NBS1, BRCA1, FANCD2 and RAD51 [3]. Furthermore, depletion of the proteasomal subunits PSMB3 and PSMD4 inhibits the formation of FANCD2 foci following ionizing radiation [3]. In addition, K48-linked polyubiquitin chains accumulate at sites of DNA damage, suggesting a role for protein degradation during the DNA damage response [4]. However, the molecular mechanisms controlling protein ubiquitination and degradation following DNA damage are still unclear. The recent discovery of the ubiquitination cascade triggered by the RING finger E3 ubiquitin ligase RNF8 uncovers a novel pathway responsible for the recruitment of DNA damage effectors. In the presence of DNA breaks, the PI3-like protein kinase ATM phosphorylates the histone variant H2AX on serine 139, creating a docking station for the mediator protein MDC1, which in turn recruits the ubiquitin ligase RNF8 [5][6][7]. RNF8 then stimulates the K63-linked ubiquitination of histones H2A and H2AX neighboring the DNA break. The RNF168 and HERC2 ubiquitin ligases also facilitate the linking of ubiquitin to H2A at damaged sites [8][9][10]. The ubiquitination cascade mediated by RNF8, RNF168 and HERC2 is responsible for the efficient recruitment of RAP80/BRCA1 and 53BP1 to DNA damage foci. The formation of RAP80 foci is...

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