Abstract
NBS characterized by high sensitivity to IR and UV, chromosome instability, and predisposition to lymphoid cancer. NBS1 functions to the cell cycle check point and the HR repair after IR. However, the function in the UV damage response mechanism is unknown. We show that NBS1 is recruited to UV-induced damage and DSBs by distinct mechanisms.
NBS cells were high sensitivity to UV and NBS1 accumulated in the stalled replication fork. Moreover NBS1-deficient cells showed reduced RAD18 and Pol eta focus formation, and severely reduced mono-ubiquitination of PCNA. NBS-deficient cells were high mutation frequency in similar Pol eta-deficient cells. We show that NBS1 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 Pol eta focus formation, leading to elevated UV sensitivity and mutation.
Unexpectedly, the RAD18-interacting motif of NBS1, which was mapped to its C terminus, shares structural and functional similarity with the RAD18-interacting motif of RAD6. These motifs of NBS1 and RAD6 allow the two proteins to interact with RAD18 homodimers simultaneously, and are crucial for UV tolerance. Thus, in addition to chromosomal break repair, NBS1 plays a key role in translesion DNA synthesis.
