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Sci Rep. 2016 Apr 26;6:25129. doi: 10.1038/srep25129.

A dynamic interaction process between KaiA and KaiC is critical to the cyanobacterial circadian oscillator.

Dong P1,2, Fan Y2, Sun J3, Lv M2, Yi M4, Tan X1,2, Liu S1,2.

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Hubei Key Laboratory of Tumor Microenvironment and Immunotherapy, China Three Gorges University, Yichang 443002, China.
College of Medical Science, China Three Gorges University, Yichang 443002, China.
School of Statistics, Shandong Institute of Business and Technology, Yantai, 264005, China.
Department of Physics, College of Sciences, Huazhong Agricultural University, Wuhan 430070, China.


The core circadian oscillator of cyanobacteria consists of three proteins, KaiA, KaiB, and KaiC. This circadian oscillator could be functionally reconstituted in vitro with these three proteins, and therefore has been a very important model in circadian rhythm research. KaiA can bind to KaiC and then stimulate its phosphorylation, but their interaction mechanism remains elusive. In this study, we followed the "second-site suppressor" strategy to investigate the interaction mechanism of KaiA and KaiC. Using protein sequence analyses, we showed that there exist co-varying residues in the binding interface of KaiA and KaiC. The followed mutagenesis study verified that these residues are important to the functions of KaiA and KaiC, but their roles could not be fully explained by the reported complex structures of KaiA and KaiC derived peptides. Combining our data with previous reports, we suggested a dynamic interaction mechanism in KaiA-KaiC interaction, in which both KaiA and the intrinsically disordered tail of KaiC undergo significant structural changes through conformational selection and induced fit during the binding process. At last, we presented a mathematic model to support this hypothesis and explained the importance of this interaction mechanism for the KaiABC circadian oscillator.

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