The reversible function switching of the circadian clock protein KaiA is encoded in its structure

Background: Circadian rhythms are important to the evolution of organisms and human health, and recent studies proved that post-translational circadian clocks widely exist in all phyla. The circadian clock of cyanobacteria is an important model system as the first verified circadian oscillator independent of transcriptional-/translational-level regulations. This circadian oscillator consists of three proteins, KaiA, KaiB, and KaiC, in which KaiA stimulates KaiC's phosphorylation but KaiB antagonizes KaiA. Despite of intense research on the molecular mechanism of this oscillator in the last decades, the regulation mechanism of KaiA's function remains unclear. Methods: In this study, we combined computational tools and experimental assays to study the function switching of KaiA. We adopted different strategies to re-design KaiA protein to elucidate its function switch during the circadian oscillation. Results: We showed that KaiA's function switch is determined by its structural dynamics, and KaiB antagonizes KaiA by switching it from an active state to an inactive state with the help of KaiC. Conclusions: The reversible function switching of KaiA is key to the KaiABC oscillator, and the switching could be regulated by the 3-D domain swapped homo-dimer conformation of KaiA, which provides the necessary structural flexibility.