Robust and tunable circadian rhythms from differentially sensitive catalytic domains.

Area of Science:

• * Molecular Biology
• * Biochemistry
• * Systems Biology

Background:

• * Circadian clocks are endogenous biological oscillators crucial for synchronizing organismal physiology and behavior with the daily environmental cycle.
• * Maintaining a precise circadian period (near 24 hours) is essential, even when external signals alter clock phase and amplitude.
• * The cyanobacterial circadian clock, involving KaiA, KaiB, and KaiC proteins, serves as a model for understanding fundamental clock mechanisms.

Purpose of the Study:

• * To elucidate the distinct functional roles of the C-terminal and N-terminal domains of the KaiC protein in regulating circadian clock dynamics.
• * To investigate how KaiC integrates environmental input signals while maintaining a robust circadian period.
• * To develop a mathematical model explaining the mechanism of period robustness in the cyanobacterial circadian clock.

Main Methods:

• * Utilized a reconstituted cyanobacterial circadian system for in vitro studies.
• * Investigated the biochemical activities (autokinase, ATPase) of KaiC domains.
• * Developed and analyzed a mathematical model of the KaiC-based clock mechanism.

Main Results:

• * The C-terminal autokinase domain of KaiC integrates input signals via the ATP/ADP ratio.
• * The N-terminal ATPase domain functions as an input-independent timer, crucial for clock periodicity.
• * Phosphorylation in the C-terminal domain, followed by an N-terminal ATPase cycle, is required for forming inhibitory KaiB•KaiC complexes.
• * The ATPase-mediated delay in negative feedback allows for tunable phase and amplitude while ensuring period robustness.

Conclusions:

• * Distinct domains of KaiC possess specialized functions that resolve conflicting requirements for clock regulation.
• * The interplay between input sensing (C-terminal) and timekeeping (N-terminal) in KaiC ensures a stable circadian period.
• * The identified mechanism provides a framework for understanding how biological clocks achieve robustness against varying environmental conditions.