The AAA⁺ family member KaiC is the central pacemaker for circadian rhythms in the cyanobacterium Synechococcus elongatus. Composed of two hexameric rings of adenosine triphosphatase (ATPase) domains with tightly coupled activities, KaiC undergoes a cycle of autophosphorylation and autodephosphorylation on its C-terminal (CII) domain that restricts binding of clock proteins on its N-terminal (CI) domain to the evening. Here, we use cryogenic-electron microscopy to investigate how daytime and nighttime states of CII regulate KaiB binding on 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 clarify a key step in the regulation of cyanobacterial circadian rhythms by KaiC phosphorylation.

AAA ^{+家族成员KaiC是蓝藻细长}聚球藻昼夜节律的中心起搏器。由具有紧密耦合活性的三磷酸腺苷 (ATPase) 结构域的两个六聚环组成，KaiC 在其 C 末端 (CII) 结构域上经历一个自磷酸化和自去磷酸化循环，从而限制时钟蛋白在其 N 末端 (CI) 结构域上的结合晚上。在这里，我们使用低温电子显微镜来研究 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，伴随着环-环压缩。CI-CII 界面处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 募集结合起来。总之，这些研究阐明了通过 KaiC 磷酸化调节蓝藻昼夜节律的关键步骤。