p53-binding protein 1 (53BP1) is a critical regulator of cellular response to DNA double-strand breaks (DSBs) [1]. To accomplish its repair function, 53BP1 must be recruited to the chromatin surrounding DSB sites that carry H4 methylation at Lys20 and H2A ubiquitination at Lys15 [2-5]. The structural basis of this recognition process was recently revealed by the complex structure of 53BP1 bound to a nucleosome core particle (NCP) containing Lys20-dimethylated H4 (H4K20me2) and Lys15-mono-ubiquitinated H2A (H2AK15monoUb) [6]. It is fascinating to note that ubiquitin ligase RNF168 ubiquitinated H2A not only on Lys15, but also on Lys13 without selectivity, and H2A bearing the K15Q mutation was still poly-ubiquitinated at Lys13 in vivo [2-4, 7]. This leads to two questions. First, is 53BP1 also a reader of H2A Lys13 ubiquitin mark? Second, is poly-ubiquitination redundant at the 53BP1 recruitment event?

中文翻译：

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化学合成的组蛋白H2A Lys13双泛素化促进53BP1与核小体的结合

p53结合蛋白1（53BP1）是细胞对DNA双链断裂（DSB）的重要调节因子[1]。为了完成其修复功能，必须将53BP1募集到DSB位点周围的染色质上，该位点在Lys20处进行H4甲基化，在Lys15处进行H2A泛素化[2-5]。最近，这种识别过程的结构基础是由与含有Lys20-二甲基化H4（H4K20me2）和Lys15-单泛素化H2A（H2AK15monoUb）的核小体核心颗粒（NCP）结合的53BP1复杂结构揭示的[6]。令人着迷的是，泛素连接酶RNF168不仅在Lys15上泛素化H2A，而且在Lys13上也泛素化，而没有选择性，并且带有K15Q突变的H2A在体内的Lys13上仍是泛素化的[2-4，7]。这导致两个问题。首先，53BP1还是H2A Lys13泛素标记的阅读者吗？第二，