Role of budding yeast Rad18 in repair of HO-induced double-strand breaks

Post-translational modifications of proliferating cell nuclear antigen: A key signal integrator for DNA damage response (Review)

Previous studies have shown that the post-translational modifications of proliferating cell nuclear antigen (PCNA) may be crucial in influencing the cellular choice between different pathways, such as the cell cycle checkpoint, DNA repair or apoptosis pathways, in order to maintain genomic stability. DNA damage leads to replication stress and the subsequent induction of PCNA modification by small ubiquitin (Ub)-related modifiers and Ub, which has been identified to affect multiple biological processes of genomic DNA. Thus far, much has been learned concerning the behavior of modified PCNA as a key signal integrator in response to DNA damage. In humans and yeast, modified PCNA activates DNA damage bypass via an error-prone or error-free pathway to prevent the breakage of DNA replication forks, which may potentially induce double-strand breaks and subsequent chromosomal rearrangements. However, the exact mechanisms by which these pathways work and by what means the modified PCNA is involved in these processes remain elusive. Thus, the improved understanding of PCNA modification and its implications for DNA damage response may provide us with more insight into the mechanisms by which human cells regulate aberrant recombination events, and cancer initiation and development. The present review focuses on the post-translational modifications of PCNA and its important functions in mediating mammalian cellular response to different types of DNA damage.

Protein phosphatase 2A-dependent dephosphorylation of replication protein A is required for the repair of DNA breaks induced by replication stress

Eukaryotic genomic integrity is safeguarded by cell cycle checkpoints and DNA repair pathways, collectively known as the DNA damage response, wherein replication protein A (RPA) is a key regulator playing multiple critical roles. The genotoxic insult-induced phosphorylation of the 32-kDa subunit of human RPA (RPA32), most notably the ATM/ATR-dependent phosphorylation at T21 and S33, acts to suppress DNA replication and recruit other checkpoint/repair proteins to the DNA lesions. It is not clear, however, how the DNA damage-responsive function of phosphorylated RPA is attenuated and how the replication-associated activity of the unphosphorylated form of RPA is restored when cells start to resume the normal cell cycle. We report here that in cells recovering from hydroxyurea (HU)-induced genotoxic stress, RPA32 is dephosphorylated by the serine/threonine protein phosphatase 2A (PP2A). Interference with PP2A catalytic activity causes persistent RPA32 phosphorylation and increased HU sensitivity. The PP2A catalytic subunit binds to RPA following DNA damage and can dephosphorylate RPA32 in vitro. Cells expressing a RPA32 persistent phosphorylation mimetic exhibit normal checkpoint activation and reenter the cell cycle normally after recovery but display a pronounced defect in the repair of DNA breaks. These data indicate that PP2A-mediated RPA32 dephosphorylation is required for the efficient DNA damage repair.

Evaluation of the roles of Pol zeta and NHEJ in starvation-associated spontaneous mutagenesis in the yeast Saccharomyces cerevisiae

The vast majority of microorganisms live under starvation-associated stress conditions that cause mutagenesis despite the limitation of DNA replication and cell division. In this study, we compared the roles of polymerase zeta (Pol zeta) and non-homologous DNA-end joining (NHEJ) in starvation-associated spontaneous base substitutions and frameshifts, using yeast mutants carrying deletions of REV3 (encoding the catalytic subunit of Pol zeta), YKU80 (encoding a protein involved in the initiation of NHEJ), or both genes. We found that approximately 50% of starvation-associated spontaneous frameshifts and 40% of base substitutions required NHEJ to occur. The role of Pol zeta was only slightly less pronounced, with 30-40% of frameshifts and 35-45% of base substitutions being dependent on Rev3. In comparison with the single mutants, the rev3 yku80 double mutant showed an additive decrease in the level of both base substitutions and frameshifts, indicating that Pol zeta and NHEJ function independently in starvation-associated mutagenesis. Our results also imply that about 30% of starvation-associated base substitutions and frameshifts arise by some unknown mechanism that does not involve Pol zeta or NHEJ.