Structure-function relationship of KaiC around dawn.

KaiC is a multifunctional enzyme functioning as the core of the circadian clock system in cyanobacteria: its N-terminal domain has adenosine triphosphatase (ATPase) activity, and its C-terminal domain has autokinase and autophosphatase activities targeting own S431 and T432. The coordination of these multiple biochemical activities is the molecular basis for robust circadian rhythmicity. Therefore, much effort has been devoted to elucidating the cooperative relationship between the two domains. However, structural and functional relationships between the two domains remain unclear especially with respect to the dawn phase, at which KaiC relieves its nocturnal history through autodephosphorylation. In this study, we attempted to design a double mutation of S431 and T432 that can capture KaiC as a fully dephosphorylated form with minimal impacts on its structure and function, and investigated the cooperative relationship between the two domains in the night to morning phases from many perspectives. The results revealed that both domains cooperate at the dawn phase through salt bridges formed between the domains, thereby non-locally co-activating two events, ATPase de-inhibition and S431 dephosphorylation. Our further analysis using existing crystal structures of KaiC suggests that the states of both domains are not always in one-to-one correspondence at every phase of the circadian cycle, and their coupling is affected by the interactions with KaiA or adjacent subunits within a KaiC hexamer.