How do Three Proteins Generate Circadian Rhythms? The Detailed Timing Mechanism of the Cyanobacterial Circadian Oscillator

Abstract

Virtually all organisms contain a circadian clock that regulates gene expression, metabolic pathway and cell division. These regulations enhance the fitness of organisms by anticipating daily changes. To generate a clock, a stable oscillation must be maintained. Thus, we focus on understanding how molecular components form an oscillator which drives a biological clock.To study clock mechanism in detail, we use the cyanobacterial oscillator because it is the only system that can be functionally reconstituted in a test tube. Mixing three clock proteins, KaiA, KaiB, and KaiC, and ATP produces a robust 24-hour rhythm of KaiC phosphorylation/dephosphorylation. All other oscillators are difficult to study at the mechanistic level because they cannot be isolated from the complex milieu of living cells.This oscillator has two distinct phases: 1) in Phosphorylation Phase, KaiA binds to the C-terminal residues of KaiC known as the A-loop and stimulates KaiC phosphorylation; 2) in Dephosphorylation Phase, KaiA is inhibited from binding to A-loop by KaiB, then KaiC dephosphorylates.Our central hypothesis is that the KaiA binding site on KaiC (i.e. A-loop) experiences a range of dynamic changes that drive preferential KaiA-KaiC interaction which is critical to the rhythm. Here we will present our latest findings of this unique oscillator to gain a deep mechanistic insight into how these three proteins form transient and time-dependent interactions that are central to the function of this oscillator.