Orchestration of Proteins in cyanobacterial Circadian Clock System 1

ABSTRACT Circadian rhythm by Cyanobacteria is one of the simplest biological clocks: the clock consists of only three proteins, KaiA, KaiB, and KaiC. Their oligomers, KaiA dimer (A 2 ), KaiB tetramer (B 4 ), and KaiC hexamer (C 6 ) oscillate an association–disassociation cycle with 24-hr period. In a widely accepted model, the oscillation process is as follows. From the viewpoint of a base unit (C 6 ), C 6 homo-oligomer → A 2 C 6 complex → B 6 C 6 complex → A n B 6 C 6 complex ( n ≤12) →C 6 homo-oligomer. In this study, Small-Angle X-ray Scattering, Contrast Matching-Small-Angle Neutron Scattering, Analytical Ultracentrifuge, and phosphorylation-analysis PAGE measurements were performed to reveal the kinetics not only of KaiC hexamer but also of all components in a working Kai clock. The complementary analysis disclosed that the oscillation is not the single process as the widely accepted model but composed with synchronized multiple association-dissociation reactions between components. Namely, there are various reactions between components, which proceed simultaneously, in a working Kai-clock.