Single-molecular and Ensemble-level Oscillations of Cyanobacterial Circadian Clock

TL;DR

This paper presents two theoretical models that connect microscopic ATP hydrolysis events in individual KaiC molecules to the macroscopic oscillations observed in cyanobacterial circadian clocks, revealing the role of ATPase activity in synchronization.

Contribution

The paper introduces the many-molecule and single-molecule models to explain how ATP hydrolysis drives and synchronizes circadian oscillations in cyanobacteria.

Findings

ATP hydrolysis drives individual KaiC oscillations.

ATP hydrolysis is essential for synchronizing multiple KaiC molecules.

Oscillation period remains temperature insensitive due to ADP lifetime sensitivity.

Abstract

When three cyanobacterial proteins, KaiA, KaiB, and KaiC, are incubated with ATP in vitro, the phosphorylation level of KaiC hexamers shows stable oscillation with approximately 24 h period. In order to understand this KaiABC clockwork, we need to analyze both the macroscopic synchronization of a large number of KaiC hexamers and the microscopic reactions and structural changes in individual KaiC molecules. In the present paper, we explain two coarse-grained theoretical models, the many-molecule (MM) model and the single-molecule (SM) model, to bridge the gap between macroscopic and microscopic understandings. In the simulation results with these models, ATP hydrolysis drives oscillation of individual KaiC hexamers and ATP hydrolysis is necessary for synchronizing oscillations of a large number of KaiC hexamers. Sensitive temperature dependence of the lifetime of the ADP bound state in the CI domain of KaiC hexamers makes the oscillation period temperature insensitive. ATPase activity is correlated to the frequency of phosphorylation oscillation in the single molecule of KaiC hexamer, which should be the origin of the observed ensemble-level correlation between the ATPase activity and the frequency of phosphorylation oscillation. Thus, the simulation results with the MM and SM models suggest that ATP hydrolysis randomly occurring in each CI domain of individual KaiC hexamers is a key process for oscillatory behaviors of the ensemble of many KaiC hexamers.