時計タンパク質KaiCの構造にみる概日リズムの起源

The circadian clock, an internal timekeeping system created and utilized by organisms adapting to the Earth’s Rotation, is driven by the rhythmic chemical reaction cycles of biomolecules such as clock proteins and clock genes. The cyanobacterial clock system is composed of three clock proteins, KaiA, KaiB, and KaiC, which concert the circadian rhythms even in the test tube in the presence of adenosine-tri-phosphate（ATP）. KaiC orchestrates the rhythm through an ATP hydrolysis（ATPase cycle）and auto-phosphorylation/dephosphorylation（Phospho-cycle）. The N-terminal ring of functional KaiC hexamer determines the clock speed during the ATPase cycle, while another C-terminal ring changes phosphorylation status like time stamps during Phospho-cycle. Structural biology has been investigating the origin of the circadian rhythm in the KaiC double-ring structure for a quarter century. We conducted the comprehensive structural analyses on KaiC and finally identified structural factors that ensure the smooth and tight communication between those distant two catalytic sites, which is critical for the rhythmicity. We also revealed that the time information is propagated to the entire cell through the rearrangement of the KaiC double ring and the assembly/disassembly of KaiA and KaiB.