Aurora B twists on histones for activation
Aurora B twists on histones for activation
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DOI：
10.1080/15384101.2016.1224758
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发表时间：
2016-08
期刊：
影响因子：
4.3
通讯作者：
M. Shimada;M. Nakanishi
中科院分区：
文献类型：
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作者：
M. Shimada;M. Nakanishi
在有丝分裂过程中，染色体成功分离对于遗传信息向子细胞的传递是必不可少的。染色体分离由位于染色体上的蛋白质的时空协调磷酸化介导，并且这种分离中的缺陷导致染色体不稳定性和非整倍性。在有丝分裂过程中，染色体乘客复合体（CPC）是协调各种事件的核心参与者，例如染色体对齐、纺锤体组装检查点的激活和胞质分裂。该复合物由Aurora B激酶和其他3个调节亚基如内着丝粒蛋白（INCENP）、Survivin和Borealin组成。Aurora B激活和定位的微调调节确保了参与有丝分裂多个关键步骤的底物的及时和空间受限的磷酸化。虽然最大激活极光B在着丝粒需要在T232在其激活环的自磷酸化，在这个残基在早期有丝分裂突然磷酸化的机制尚未完全理解。我们最近的研究揭示了H2 AX的一个新作用，H2 AX是一种组蛋白H2 A变体，在Aurora B的自激活中发挥作用。丝氨酸121处的H2 AX磷酸化（H2 AX-pS121）积累并增强着丝粒处的Aurora B的活化。HeLa细胞中H2 AX的缺失，以及来自H2 AX//小鼠的小鼠胚胎成纤维细胞显示出由于有丝分裂异常（如染色体错配和落后染色体）导致的严重生长缺陷。这些有丝分裂表型与通过受损的H2 AX依赖性S期检查点发生的DNA复制错误无关。H2 AX的耗尽严重损害Aurora B活化，表明T232及其下游靶点如着丝粒组蛋白CENP-A处Aurora B的磷酸化减少。H2 AX具有独特的C-末端区域，其具有包含共价修饰的各种残基。例如，在S139（gH 2AX）处的磷酸化是将各种蛋白质募集到DNA损伤位点所必需的。有趣的是，有丝分裂磷酸化数据库表明，在S121的H2 AX在体内磷酸化，这种磷酸化在有丝分裂早期达到最大值。该丝氨酸残基在典型H2 A和其他H2 AX变体中不保守。H2 AX中围绕该丝氨酸残基的区域与Aurora B共有磷酸化基序匹配良好。使用磷酸化特异性抗体H2 AX-pS121，我们成功地证明，H2 AX-pS121主要定位于有丝分裂着丝粒，特别是附近的动粒。鉴于这种磷酸化在Aurora B耗尽的细胞中严重受损，表明Aurora B是负责H2 AX-pS121的激酶。在有丝分裂过程中，各种组蛋白被磷酸化，显示出独特的时空模式。CPC在有丝分裂早期通过两种组蛋白磷酸化作用积累在内部着丝粒，如Haspin介导的H3-T3磷酸化（H3-pT 3）和Bub 1介导的H2 A-T120磷酸化（H2 A-pT 120）。H2 AX的缺失减少了染色质上Aurora B依赖性Haspin磷酸化，随后损害H3-pT 3，但不影响Bub 1介导的H2 A-pT 120。相反，Bub 1对于H2 AX-pS121是关键的。总的来说，这些结果表明Aurora B介导的H2 AX-pS121在着丝粒处促进Aurora B自激活回路，作用于Haspin-H3-pT 3的上游和Bub 1-H2 A-pT 120的下游（图1）。支持这一模型，在S121处的H2 AX的拟磷酸化突变体与野生型对应物相比在体外更强地结合活化的极光B。许多有丝分裂调节因子如Aurora B和Bub 1的缺失导致小鼠胚胎死亡。奇怪的是，尽管H2 AX或Haspin敲除细胞系中存在严重的有丝分裂异常，但H2 AX或Haspin敲除小鼠是存活的。这表明，H2 AX-pS121和Haspin-H3-pT 3在着丝粒激活Aurora B的过程中可能是冗余的。因此，观察小鼠中H2 AX和Haspin的双敲除是否导致早期胚胎发育缺陷是非常有趣的。有趣的是，注意到几种DNA损伤响应蛋白，如ATM和MDC 1，在有丝分裂期间被募集到动粒或着丝粒，即使在没有DNA损伤的情况下。ATM被Aurora B以M期依赖性方式磷酸化和激活。这种磷酸化对于
Successful chromosome segregation during mitosis is essential for transmission of genetic information to daughter cells. Chromosome segregation is mediated by spatio-temporally coordinated phosphorylation of proteins located on chromosomes and a defect in this segregation result in chromosomal instability and aneuploidy. During mitosis, chromosomal passenger complex (CPC) is a central player that orchestrates various events, such as chromosome alignment, activation of the spindle assembly checkpoint and cytokinesis. This complex comprises of Aurora B kinase and 3 other regulatory subunits such as inner centromere protein (INCENP), Survivin and Borealin. Fine-tuned regulation of Aurora B activation and localization ensures the timely and spatially restricted phosphorylation of substrates that are involved in multiple key steps of mitosis. Although maximal activation of Aurora B at the centromere requires autophosphorylation at T232 within its activation loop, the mechanisms underlying the abrupt phosphorylation at this residue at early mitosis has not yet been fully understood. Our recent study uncovers a novel role of H2AX, a histone H2A variant in autoactivation of Aurora B. H2AX phosphorylation at serine 121 (H2AX-pS121) accumulates and enhances activation of Aurora B at centromeres. Depletion of H2AX in HeLa cells, as well as mouse embryonic fibroblasts from H2AX¡/¡ mice shows a severe growth defect due to mitotic abnormalities, such as chromosome misalignment and lagging chromosomes. These mitotic phenotypes are independent of DNA replication errors that occur through impaired H2AXdependent S phase checkpoint. Depletion of H2AX severely compromises Aurora B activation showing that reduced phosphorylations of Aurora B at T232 and its downstream targets such as centromeric histone CENP-A. H2AX possesses a unique C-terminal region with various residues harboring covalent modifications. For example, phosphorylation at S139 (gH2AX) is required for recruitment of various proteins to DNA damage sites. Interestingly, a mitotic phosphorylation database demonstrates that H2AX at S121 is phosphorylated in vivo and this phosphorylation reaches a maximum at early mitosis. This serine residue is not conserved in canonical H2A and other H2AX variants. The region surrounding this serine residue in H2AX matches well with the Aurora B consensus phosphorylation motif. With the use of phospho-specific antibodies to H2AX-pS121, we successfully demonstrate that H2AX-pS121 predominantly localizes at mitotic centromeres, especially near the kinetochores. Given that this phosphorylation is severely compromised in Aurora B depleted cells, suggesting that Aurora B is a kinase responsible for H2AX-pS121. During mitosis, various histones are phosphorylated showing unique spatio-temporal patterns. CPC accumulates at the inner centromere during early mitosis through 2 histone phosphorylations such as Haspin-mediated H3-T3 phosphorylation (H3-pT3), and Bub1-mediated H2A-T120 phosphorylation (H2A-pT120). Loss of H2AX reduces Aurora B dependent Haspin phosphorylation on chromatin, and subsequently compromises H3-pT3, but does not affect Bub1-mediated H2A-pT120. In contrast, Bub1 is critical for H2AX-pS121. Collectively, these results indicate that Aurora B-mediated H2AX-pS121 facilitates Aurora B autoactivation circuitry at centromeres, acting upstream of Haspin-H3-pT3 and downstream of Bub1-H2A-pT120 (Fig. 1). Supporting this model, a phosphomimic mutant of H2AX at S121 more strongly binds activated Aurora B in vitro when compared with the wild-type counterpart. Loss of many mitotic regulators, such as Aurora B and Bub1, leads to embryonic lethality in mice. Paradoxically, despite severe mitotic abnormalities in H2AX or Haspin knockdown cell lines, the knockout mice for H2AX or Haspin are viable. This suggests that, H2AX-pS121 and Haspin-H3-pT3 may act redundantly during the activation of Aurora B at centromeres. Thus, it is of great interest to see whether double-knockout of H2AX and Haspin in mice results in a defect in early embryonic development. It is interesting to note that several DNA damage responsive proteins, such as ATM and MDC1, are recruited to kinetochore or centromere during mitosis, even in the absence of DNA damage. ATM is phosphorylated and activated by Aurora B in an M phase-dependent manner. This phosphorylation is essential for