The Functions of Serine 687 Phosphorylation of Human DNA Polymerase in UV Damage Tolerance

DNA polymerase (pol) is a Y-family translesion synthesis polymerase that plays a key role in the cellular tolerance toward UV irradiation-induced DNA damage. Here, we identified, for the first time, the phosphorylation of serine 687 (Ser(687)), which is located in the highly conserved nuclear localization signal (NLS) region of human pol and is mediated by cyclin-dependent kinase 2 (CDK2). We also showed that this phosphorylation is stimulated in human cells upon UV light exposure and results in diminished interaction of pol with proliferating cell nuclear antigen (PCNA). Furthermore, we demonstrated that the phosphorylation of Ser(687) in pol confers cellular protection from UV irradiation and increases the efficiency in replication across a site-specifically incorporated cyclobutane pyrimidine dimer in human cells. Based on these results, we proposed a mechanistic model where Ser(687) phosphorylation functions in the reverse polymerase switching step of translesion synthesis: The phosphorylation brings negative charges to the NLS of pol, which facilitates its departure from PCNA, thereby resetting the replication fork for highly accurate and processive DNA replication. Thus, our study, together with previous findings, supported that the posttranslational modifications of NLS of pol played a dual role in polymerase switching, where Lys(682) deubiquitination promotes the recruitment of pol to PCNA immediately prior to lesion bypass and Ser(687) phosphorylation stimulates its departure from the replication fork immediately after lesion bypass.