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

Hakuto Kageyama, Taeko Nishiwaki, Masato Nakajima, Hideo Iwasaki, Tokitaka Oyama, Takao Kondo

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127 Citations (Scopus)


KaiA, KaiB, and KaiC are essential proteins of the circadian clock in the cyanobacterium Synechococcus elongatus PCC 7942. The phosphorylation cycle of KaiC that occurs in vitro after mixing the three proteins and ATP is thought to be the master oscillation governing the circadian system. We analyzed the temporal profile of complexes formed between the three Kai proteins. In the phosphorylation phase, KaiA actively and repeatedly associated with KaiC to promote KaiC phosphorylation. High levels of phosphorylation of KaiC induced the association of the KaiC hexamer with KaiB and inactivate KaiA to begin the dephosphorylation phase, which is closely linked to shuffling of the monomeric KaiC subunits among the hexamer. By reducing KaiC phosphorylation, KaiB dissociated from KaiC, reactivating KaiA. We also confirmed that a similar model can be applied in cyanobacterial cells. The molecular model proposed here provides mechanisms for circadian timing systems.

Original languageEnglish
Pages (from-to)161-171
Number of pages11
JournalMolecular Cell
Issue number2
Publication statusPublished - 2006 Jul 21
Externally publishedYes




ASJC Scopus subject areas

  • Molecular Biology

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