The AAA+ protein KaiC is the central pacemaker for cyanobacterial circadian rhythms. Composed of two hexameric rings with tightly coupled activities, KaiC undergoes changes in autophosphorylation on its C-terminal (CII) domain that restrict binding of of clock proteins on its N-terminal (CI) domain to the evening. Here, we use cryo-electron microscopy to investigate how daytime and nighttime states of CII regulate KaiB binding to CI. We find that the CII hexamer is destabilized during the day but takes on a rigidified C₂-symmetric state at night,concomitant with ring-ring compression. Residues at the CI-CII interface are required for phospho-dependent KaiB association, coupling ATPase activity on CI to cooperative KaiB recruitment. Together these studies reveal how daily changes in KaiC phosphorylation regulate cyanobacterial circadian rhythms.

中文翻译：

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昼夜节律起搏器中隐藏的构造区分白天和黑夜

AAA+ 蛋白 KaiC 是蓝藻昼夜节律的中心起搏器。KaiC 由两个具有紧密耦合活性的六聚体环组成，其 C 端 (CII) 结构域上的自磷酸化发生变化，从而将时钟蛋白在其 N 端 (CI) 结构域上的结合限制到晚上。在这里，我们使用冷冻电子显微镜来研究 CII 的白天和夜间状态如何调节 KaiB 与 CI 的结合。我们发现 CII 六聚体在白天不稳定，但呈现出刚性的 C ₂- 夜间对称状态，伴随环环压缩。CI-CII 界面上的残基是磷酸依赖性 KaiB 结合所必需的，将 CI 上的 ATP 酶活性与合作的 KaiB 募集相结合。这些研究共同揭示了 KaiC 磷酸化的日常变化如何调节蓝藻昼夜节律。