小柯机器人

美国圣裘德儿童研究医院Mario Halic研究小组宣布他们发现DNA断裂桥接通过激活PARP2-HPF1通路修饰染色质。该研究于2020年9月16日在线发表于国际学术期刊《自然》杂志。

为了揭示PARP酶识别染色质内DNA断裂的分子机制，研究人员解析了与核小体结合的人PARP2-HPF1的冷冻电镜结构。结构表明PARP2-HPF1桥接两个核小体，断裂的DNA排列在合适的连接位置，这揭示了双链DNA断裂修复的第一步。桥接导致PARP2发生结构变化，从而导致催化结构域识别DNA断裂信号，并结合HPF1和激活PARP2。

该研究数据表明，活跃的PARP2循环经过不同的构象状态，以交换NAD⁺和底物，这可能使PARP酶在与染色质结合的过程中具有持续性作用。该研究揭示的PARP活化过程和PARP催化循环可以解释对PARP抑制剂拮抗的机制，并将有助于研发更好的抑制剂来治疗癌症。

据介绍，DNA链断裂会招募PARP1蛋白及其旁系蛋白PARP2，通过添加单和聚（ADP-核糖）（PAR）来修饰组蛋白和其他底物。在DNA损伤过程中，这种翻译后修饰主要发生在丝氨酸残基上，并且需要HPF1，辅助因子HPF1可将PARP1和PARP2的氨基酸特异性从天冬氨酸或谷氨酸转变为丝氨酸。聚（ADP）核糖基化（PARylation）对于随后的染色质解旋非常重要，并为下游信号和修复因子结合到DNA断裂位点提供了锚定位点。

附：英文原文

Title: Bridging of DNA breaks activates PARP2–HPF1 to modify chromatin

Author: Silvija Bilokapic, Marcin J. Suskiewicz, Ivan Ahel, Mario Halic

Issue&Volume: 2020-09-16

Abstract: 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)1,2,3,4,5. In the DNA damage responses, this post-translational modification occurs predominantly on serine residues6,7,8 and requires HPF1, an accessory factor that switches the amino acid specificity of PARP1 and PARP2 from aspartate or glutamate to serine9,10. 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 breaks2,11. 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 treatments12,13,14,15,16.

DOI: 10.1038/s41586-020-2725-7

Source: https://www.nature.com/articles/s41586-020-2725-7

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