Roles of Phosphorylation of KaiC in the Cyanobacterial Circadian Clock

Phosphorylation of circadian clock proteins is ubiquitously found in many organisms. The cyanobacterial circadian clock can be reconstituted in vitro by mixing three clock proteins, KaiA, KaiB, KaiC, and ATP, in which the phosphorylation state of KaiC exhibits robust circadian rhythm. In contrast to eukaryotic clock proteins, sometimes containing tens of phosphorylation sites, KaiC has only two phosphorylation sites, serine 431 and threonine 432, which enables the full understanding of the functions of clock protein phosphorylation in the circadian oscillation. Unusual properties of KaiC have been revealed with the use of the reconstituted circadian clock. KaiC has extremely slow but stable ATPase activity, which is essential for generating circadian oscillation. KaiC can phosphorylate and dephosphorylate itself by the phosphotransfer reaction between KaiC-bound adenine nucleotide and the hydroxyl group on Ser and Thr residues. KaiC phosphorylation has critical roles in sustaining robust oscillation of protein-based circadian clock, whose period is defined by ATPase activity of KaiC, by regulating the interaction among Kai proteins. KaiC phosphorylation also serves as one of the major output pathways connecting the protein-based oscillator and genome-wide transcription.