Breaks in DNA strands recruit the protein PARP1 and its paralogue PARP2 to modify histones and other substrates through the addition of mono- and poly(ADP-ribose) (PAR) 1 – 5 . In the DNA damage responses, this post-translational modification occurs predominantly on serine residues 6 – 8 and requires HPF1, an accessory factor that switches the amino acid specificity of PARP1 and PARP2 from aspartate or glutamate to serine 9 , 10 . Poly(ADP) ribosylation (PARylation) is important for subsequent chromatin decompaction and provides an anchor for the recruitment of downstream signalling and repair factors to the sites of DNA breaks 2 , 11 . Here, to understand the molecular mechanism by which PARP enzymes recognize DNA breaks within chromatin, we determined the cryo-electron-microscopic structure of human PARP2–HPF1 bound to a nucleosome. This showed that PARP2–HPF1 bridges two nucleosomes, with the broken DNA aligned in a position suitable for ligation, revealing the initial step in the repair of double-strand DNA breaks. The bridging induces structural changes in PARP2 that signal the recognition of a DNA break to the catalytic domain, which licenses HPF1 binding and PARP2 activation. Our data suggest that active PARP2 cycles through different conformational states to exchange NAD + and substrate, which may enable PARP enzymes to act processively while bound to chromatin. The processes of PARP activation and the PARP catalytic cycle we describe can explain mechanisms of resistance to PARP inhibitors and will aid the development of better inhibitors as cancer treatments 12 – 16 . The PARP2–HPF1 histone-modifying complex bridges two nucleosomes to align broken DNA ends for ligation, initiating conformational changes that activate PARP2 and enable DNA damage repair.

中文翻译：

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DNA 断裂的桥接激活 PARP2-HPF1 以修饰染色质

DNA 链的断裂会募集蛋白质 PARP1 及其旁系同源物 PARP2，通过添加单聚（ADP-核糖）（PAR） 1 – 5 来修饰组蛋白和其他底物。在 DNA 损伤反应中，这种翻译后修饰主要发生在丝氨酸残基 6 – 8 上，并且需要 HPF1，这是一种辅助因子，可将 PARP1 和 PARP2 的氨基酸特异性从天冬氨酸或谷氨酸转换为丝氨酸 9、10。多聚 (ADP) 核糖基化 (PARylation) 对随后的染色质分解很重要，并为下游信号和修复因子募集到 DNA 断裂 2、11 位点提供了锚点。在这里，为了了解 PARP 酶识别染色质内 DNA 断裂的分子机制，我们确定了与核小体结合的人 PARP2-HPF1 的低温电子显微镜结构。这表明 PARP2-HPF1 桥接了两个核小体，断裂的 DNA 排列在适合连接的位置，揭示了修复双链 DNA 断裂的第一步。桥接诱导 PARP2 的结构变化，这表明催化结构域识别 DNA 断裂，从而允许 HPF1 结合和 PARP2 激活。我们的数据表明，活跃的 PARP2 循环通过不同的构象状态来交换 NAD + 和底物，这可能使 PARP 酶在与染色质结合的同时发挥作用。我们描述的 PARP 活化过程和 PARP 催化循环可以解释对 PARP 抑制剂的耐药机制，并将有助于开发更好的抑制剂作为癌症治疗 12 - 16。