Unconjugated
The DNA replication machinery stalls at damaged sites on templates, but normally restarts by switching to a specialized DNA polymerase(s) that carries out translesion DNA synthesis (TLS). In human cells, DNA polymerase eta (poleta) accumulates at stalling sites as nuclear foci, and is involved in ultraviolet (UV)-induced TLS. Here we show that poleta does not form nuclear foci in RAD18(-/-) cells after UV irradiation. Both Rad18 and Rad6 are required for poleta focus formation. In wild-type cells, UV irradiation induces relocalization of Rad18 in the nucleus, thereby stimulating colocalization with proliferating cell nuclear antigen (PCNA), and Rad18/Rad6-dependent PCNA monoubiquitination. Purified Rad18 and Rad6B monoubiquitinate PCNA in vitro. Rad18 associates with poleta constitutively through domains on their C-terminal regions, and this complex accumulates at the foci after UV irradiation. Furthermore, poleta interacts preferentially with monoubiquitinated PCNA, but poldelta does not. These results suggest that Rad18 is crucial for recruitment of poleta to the damaged site through protein-protein interaction and PCNA monoubiquitination.
In lower eukaryotes, Rad18 plays a crucial role in postreplication repair. Previously, we isolated a human homologue of RAD18 (hRAD18) and showed that human cells overexpressing hRad18 protein with a mutation in the RING finger motif are defective in postreplication repair. Here, we report the construction of RAD18-knockout mouse embryonic stem cells by gene targeting. These cells had almost the same growth rate as wild-type cells and manifested phenotypes similar to those of human cells expressing mutant Rad18 protein: hypersensitivity to multiple DNA damaging agents and a defect in postreplication repair. Mutation was not induced in the knockout cells with any higher frequencies than in wild-type cells, as shown by ouabain resistance. In the knockout cells, spontaneous sister chromatid exchange (SCE) occurred with twice the frequency observed in normal cells. After mild DNA damage, SCE was threefold higher in the knockout cells, while no increase was observed in normal cells. Stable transformation efficiencies were approximately 20-fold higher in knockout cells, and gene targeting occurred with approximately 40-fold-higher frequency than in wild-type cells at the Oct3/4 locus. These results indicate that dysfunction of Rad18 greatly increases both the frequency of homologous as well as illegitimate recombination, and that RAD18 contributes to maintenance of genomic stability through postreplication repair.