RNA-splicing factor SART3 regulates translesion DNA synthesis

Translesion DNA synthesis (TLS) is one mode of DNA damage tolerance that uses specialized DNA polymerases to replicate damaged DNA. DNA polymerase eta (Pol eta) is well known to facilitate TLS across ultraviolet (UV) irradiation and mutations in POLH are implicated in skin carcinogenesis. However, the basis for recruitment of Pol eta to stalled replication forks is not completely understood. In this study, we used an affinity purification approach to isolate a Pol eta-containing complex and have identified SART3, a pre-mRNA splicing factor, as a critical regulator to modulate the recruitment of Pol eta and its partner RAD18 after UV exposure. We show that SART3 interacts with Pol eta and RAD18 via its C-terminus. Moreover, SART3 can form homodimers to promote the Pol eta/RAD18 interaction and PCNA monoubiquitination, a key event in TLS. Depletion of SART3 also impairs UV-induced single-stranded DNA (ssDNA) generation and RPA focus formation, resulting in an impaired Pol eta recruitment and a higher mutation frequency and hypersensitivity after UV treatment. Notably, we found that several SART3 missense mutations in cancer samples lessen its stimulatory effect on PCNA monoubiquitination. Collectively, our findings establish SART3 as a novel Pol eta/RAD18 association regulator that protects cells from UV-induced DNA damage, which functions in a RNA binding-independent fashion.