Figure 3.

Models of the KaiABC oscillator. (a) Diagram representing the mathematical model for the KaiC phosphorylation cycle. The double circle dumbbell shapes in the center represent KaiC monomers. The KaiC hexamer can associate with and dissociate from KaiA and KaiB. KaiC hexamers are shown in light blue and dark blue, representing two conformational states (approximately equivalent to kinase versus phosphatase forms of KaiC). Red dots are phosphates at KaiC phosphorylation sites (residues S431 and T432). Monomer exchange between KaiC hexamers is depicted with the double-headed arrows in the center. The rates of monomer exchange vary among KaiC states, with a solid line indicating a high rate and a dashed line indicating a low rate (from Reference 63). (b) Diagram showing the formation of KaiA·KaiB·KaiC complexes. Starting from the leftmost molecular representation and proceeding clockwise: during the phosphorylation phase of the cycling reaction, KaiA (red dimers) repeatedly and rapidly interacts with KaiC's C-terminal tentacles. (KaiC molecules are the green double-donut hexamers.) When KaiC becomes hyperphosphorylated (phosphates on T432 and S431 are depicted as red dots), it first binds KaiB (green diamonds) stably. Then, the KaiB·KaiC complex binds KaiA, sequestering it from further interaction with KaiC's tentacles. At that point, KaiC initiates dephosphorylation. When KaiC is hypophosphorylated, it releases KaiB and KaiA, thereby launching a new cycle (from Reference 81).