NBS1 Recruits RAD18 via a RAD6-like Domain and Regulates Pol η-Dependent Translesion DNA Synthesis

Translesion DNA synthesis, a process orchestrated by monoubiquitinated PCNA, is critical for DNA damage tolerance. While the ubiquitin-conjugating enzyme RAD6 and ubiquitin ligase RAD18 are known to monoubiquitinate PCNA, how they are regulated by DNA damage is not fully understood. We show that NBS1 (mutated in Nijmegen breakage syndrome) binds to RAD18 after UV irradiation and mediates the recruitment of RAD18 to sites of DNA damage. Disruption of NBS1 abolished RAD18-dependent PCNA ubiquitination and Polη focus formation, leading to elevated UV sensitivity and mutation. Unexpectedly, the RAD18-interacting domain of NBS1, which was mapped to its C terminus, shares structural and functional similarity with the RAD18-interacting domain of RAD6. These domains of NBS1 and RAD6 allow the two proteins to interact with RAD18 homodimers simultaneously and are crucial for Polη-dependent UV tolerance. Thus, in addition to chromosomal break repair, NBS1 plays a key role in translesion DNA synthesis. [Display omitted] ► NBS1 binds to E3 ubiquitin ligase RAD18 and accumulates at the UV-lesion sites ► Disruption of NBS1 abolishes PCNA ubiquitination and leads to elevated UV sensitivity ► NBS1 shares structural similarity with RAD18-interacting domain of RAD6 ► NBS1 initiates Polη-dependent UV tolerance by interaction with RAD6/RAD18 complex