1. Department of Biophysics, Graduate School of Science and
2. Department of Gene Mechanisms, Graduate School of Biostudies, Kyoto University, Yoshida-Honmachi, Sakyo-ku, Kyoto 606-8501, Japan
3. Initial Research Project, Okinawa Institute of Science and Technology, Okinawa 904-2234, Japan

Correspondence to: Mitsuhiro Yanagida^1,2,3 Email: yanagida@kozo.lif.kyoto-u.ac.jp

Sister chromatids are held together by cohesins¹. At anaphase, separase is activated by degradation of its inhibitory partner, securin^{2, 3}. Separase then cleaves cohesins^{4, 5, 6}, thus allowing sister chromatid separation. Fission yeast securin (Cut2) has destruction boxes and a separase (Cut1) interaction site in the amino and carboxyl terminus, respectively^{7, 8}. Here we show that securin is essential for separase stability and also for proper repair of DNA damaged by ultraviolet, X-ray and -ray irradiation. The cut2^EA2 mutant is defective in the repair of ultraviolet damage lesions, although the DNA damage checkpoint is activated normally. In double mutant analysis of ultraviolet sensitivity, checkpoint kinase chk1 (ref. 9) and excision repair rad13 (ref. 10) mutants were additive with cut2^EA2, whereas recombination repair rhp51 (ref. 11) and cohesin subunit rad21 (ref. 12) mutants were not. Cohesin was hyper-modified on ultraviolet irradiation in a Rad3 kinase-dependent way¹³. Experiments using either mutant cohesin that cannot be cleaved by separase or a protease-dead separase provide evidence that this DNA repair function of securin–separase acts through the cleavage of cohesin. We propose that the securin–separase complex might aid DNA repair by removing local cohesin in interphase cells.

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