Cyanobacterial Circadian Pacemaker: Kai Protein Complex Dynamics in the KaiC Phosphorylation Cycle In Vitro

The phosphorylation cycle of KaiC is essential for regulating the circadian clock in the cyanobacterium Synechococcus elongatus PCC 7942.

• KaiA promotes the phosphorylation of KaiC by forming complexes with it during the phosphorylation phase.
• High levels of phosphorylated KaiC lead to its association with KaiB, which then inactivates KaiA and triggers the dephosphorylation phase.
• The shuffling of monomeric KaiC subunits among the hexamer is closely linked to the transition between phosphorylation and dephosphorylation phases.
• Reducing the phosphorylation of KaiC results in the dissociation of KaiB, reactivating KaiA.
• The proposed molecular model also appears applicable in cyanobacterial cells.

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