Overlapping RAD18- and DNA-binding interfaces in DNA polymerase η contribute to UV-induced DNA damage tolerance.

DNA polymerase η (Polη) bypasses UV-induced pyrimidine dimers and thereby confers tolerance to UV irradiation. Although the C-terminus of Polη has been reported to interact with ubiquitinated PCNA and RAD18, how Polη engages RAD18 is not fully understood. Here, we show that Polη and RAD18 interact through two distinct modes in human cells: a ubiquitinated-PCNA-dependent mode that requires the Polη C-terminus, and an unexpected PCNA-independent mode mediated by its N-terminal region. We focused our subsequent analyses on this newly identified PCNA-independent mode. Using purified recombinant proteins, we demonstrate direct binding of the N-terminal region of human Polη (PolηΔC) to RAD18 in vitro. Although PolηΔC and RAD18 each bound primer-template DNA, we were unable to detect a ternary PolηΔC-RAD18-DNA complex, and DNA competitively inhibited RAD18 binding to both PolηΔC and full-length Polη. Mutational analyses revealed that the DNA-binding and RAD18-binding domains within Polη overlap. A separation-of-function mutant, PolηΔC(K317A), which retains near-normal DNA-binding and polymerase activities but exhibits reduced RAD18 binding in vitro, displayed a diminished ability to rescue the UV sensitivity of Polη-deficient cells. Notably, the detrimental impact of the K317A persisted in a PCNA-binding-defective background but was attenuated in RAD18-knockout cells. These findings demonstrate that RAD18 binding to the N-terminal domain of Polη contributes to efficient bypass of pyrimidine dimers independently of the Polη-PCNA interaction and provide mechanistic insights into how Polη-RAD18 complexes assemble and dissociate during translesion DNA synthesis.