PAXX is a novel protein involved in NHEJ pathway. Non-homologous end joining (NHEJ) is a major pathway for the detection and repair of DSBs in mammalian cells. We perform systematic interactome analyses of human NHEJ factors and identify a novel XRCC4-like protein named PAXX. The crystal structure study reveals that the N-terminus of PAXX forms a homodimer that is structurally similar to XRCC4 and XLF. In contrast to XRCC4 and XLF, PAXX directly and strongly interacts with the Ku complex. Interestingly, PAXX and XLF cooperate to promote the ligation of non-cohesive but not blunt ends in vitro. Importantly, the epistasis analysis suggests that PAXX is upstream of XRCC4 and Lig4 in DSB repair; while PAXX and XLF display both epistatic and non-epistatic interactions that are dependent on the complexity of the DSB ends. Thus, our data reveal that PAXX is a new core NHEJ factor that plays a key role in the organization of NHEJ complexes for the repair of perse types of DSB ends.

Rif1 participates in DSB repair pathway choice. DNA double-strand breaks (DSBs) are highly deleterious lesions. To avoid adverse outcomes, cells must choose from a group of mutually exclusive DSB repair processes based on cell cycle position and the nature of the DNA end. DSBs are repaired primarily by nonhomologous end joining (NHEJ) and homologous recombination (HR). We identify RIF1 as the critical effector of 53BP1 during DSB repair. Remarkably, RIF1 accumulation at DSB sites is strongly antagonized by BRCA1 and its interacting partner CtIP. Lastly, we show that depletion of RIF1 is able to restore end resection and RAD51 loading in BRCA1-depleted cells. This work therefore identifies a cell cycle-regulated circuit, underpinned by RIF1 and BRCA1, that governs DSB repair pathway choice to ensure that NHEJ dominates in G1 and HR is favored in S-G2 phase.

Top3β is the first RNA topoisomerase. Topoisomerases are crucial for solving DNA topological problems, but they have not been linked to RNA metabolism. Here we show that human topoisomerase 3β (Top3β) is an RNA topoisomerase that biochemically and genetically interacts with FMRP, a protein that is deficient in fragile X syndrome. Notably, the FMRP-Top3β interaction is abolished by a disease-associated mutation of FMRP, suggesting that Top3β may contribute to the pathogenesis of mental disorders. Synapse formation is defective in Top3β mutant flies and mice, as well as in FMRP mutant flies and mice. Our findings suggest that Top3β acts as an RNA topoisomerase and works with FMRP to promote the expression of mRNAs that are crucial for neurodevelopment and mental health.

Rong Guo, Jingfeng Shang, Zuer Lu, Yichen Bai, Xuejun Ma, Kaiwen Liu, Jiahui Wu, Zihan Lin, Yifan Liu, Yishan Xie