Cyclin F - EXO1 axis controls cell cycle dependent execution of double strand break repair

Ubiquitination is a crucial post-translational modification required for the proper repair of DNA double-strand breaks (DSBs) induced by ionising radiation (IR). DSBs are mainly repaired through homologous recombination (HR) when template DNA is present and non-homologous end joining (NHEJ) in its absence. Additionally, microhomology-mediated end joining (MMEJ) and single strand annealing (SSA) provide back-up DSBs repair pathways. However, the mechanisms controlling their use remain poorly understood. By employing a high-resolution CRISPR screen of the ubiquitin system after IR, we systematically uncover genes required for cell survival and elucidate a critical role of the E3 ubiquitin ligase SCFcyclin F in cell cycle dependent DSB repair. We show that SCFcyclin F -mediated EXO1 degradation prevents DNA end resection in mitosis, allowing MMEJ to take place. Moreover, we identify a conserved cyclin F recognition motif, distinct from the one used by other cyclins, with broad implications in cyclin specificity for cell cycle control.

泛素化（Ubiquitination）是电离辐射（ionising radiation, IR）诱导的DNA双链断裂（DNA double-strand breaks, DSBs）正确修复所必需的关键翻译后修饰。DNA双链断裂主要在存在模板DNA时通过同源重组（homologous recombination, HR）完成修复，在无模板DNA时则通过非同源末端连接（non-homologous end joining, NHEJ）进行修复；此外，微同源介导的末端连接（microhomology-mediated end joining, MMEJ）与单链退火（single strand annealing, SSA）可作为DNA双链断裂修复的备用通路。然而，调控这些修复通路选择的分子机制仍不甚明晰。本研究通过对电离辐射处理后泛素系统开展高分辨率CRISPR筛选，系统性地鉴定出与细胞存活相关的核心基因，并阐明了E3泛素连接酶（E3 ubiquitin ligase）SCFcyclin F在细胞周期依赖性DNA双链断裂修复中的关键作用。研究结果显示，SCFcyclin F介导的EXO1降解可抑制有丝分裂期的DNA末端切除过程，从而允许微同源介导的末端连接通路发挥功能；此外，本研究还鉴定出一种保守的cyclin F识别基序，该基序与其他周期蛋白所识别的基序存在显著差异，这一发现对于解析细胞周期调控中的周期蛋白特异性具有广泛的参考价值。