THE CIRCADIAN CLOCK FOUND IN CYANOBACTERIA CONTAINS A CORE PROTEIN CIRCUIT
Source: O'Shea Lab
The circadian clock found in cyanobacteria contains a core protein circuit that can sustain robust 24 hour oscillations by post-translational interactions alone. This core oscillator can be reconstituted in a test tube using three recombinantly expressed proteins: KaiA, KaiB and KaiC. KaiC is an autokinase and autophosphatase that can be phosphorylated at two adjacent residues: S431 and T432. To study the role of multisite phosphorylation in this oscillator system, the abundance of each possible phosphorylated form of KaiC was quantified using mass spectrometry with isotopically labeled peptide standards during a 24 cycle of the oscillator. The plot shows the modification state of the pool of KaiC molecules in an oscillating reaction during three full cycles: KaiC phosphorylated only on T432 (green curve), KaiC phosphorylated only on S431 (red curve), KaiC phosphorylated on both sites (blue curve), and the total fraction of phosphorylated KaiC (black curve). Each form of KaiC peaks at a distinct phase of the reaction, indicating that circadian time is encoded by the phosphorylation state of KaiC.