FIGURE 1: Model of the regulation of the KaiC phosphorylation cycle within KaiC hexamer.

(A) In vitro oscillation of KaiC phosphorylation. KaiC were incubated with KaiA and KaiB. Aliquots of the reaction mixtures were collected, and then subjected to SDS-PAGE and Coomassie Brilliant Blue staining. From most to least mobile, bands represent the S/T, pS/T, S/pT, and pS/pT forms of KaiC. The ratio of the amount of each form of KaiC to the total amount of KaiC is plotted against the incubation time. The phosphorylation proceeds in an ordered pattern, as follows: S/T→S/pT→pS/pT→pS/T.

(B) Schematic model of regulation at subunit interfaces. Phosphorylation takes place at subunit interfaces in KaiC hexamers. The phosphorylation states at two phosphorylation sites regulate the nucleotide-binding state at the subunit interface. ADP at the active site promotes dephosphorylation, and ATP at the active site promotes phosphorylation.

(C) Schematic model of regulation by intersubunit communications. KaiC in the phosphorylation phase enhances the kinase activity of other protomers in the same hexamer; inversely, KaiC in the dephosphorylation phase decreases kinase activity. This intersubunit communication allows KaiC molecules that were phosphorylated in the wrong order to turn back, while allowing molecules in the appropriate phosphorylation state to move forward through the cycle. As a result, phase coherence among the ensemble of phosphorylating KaiC hexamers is achieved.