Fold‐Switching Sets the Stage for Cooperativity and Competition in the Cyanobacterial Circadian Clock

Abstract

Circadian rhythms are an ancient evolutionary adaptation found across the domains of life that synchronize behavior and physiology to the daily solar cycle for optimal fitness. In cyanobacteria, timekeeping is generated by three Kai proteins (KaiA, B and C) that form distinct protein complexes throughout the day, comprising a molecular clock that measures time in ~24‐hour increments. We recently showed that KaiB undergoes a rare transition from a tetrameric ground state to a monomeric, fold‐switched signaling state that binds to KaiC and plays a central organizing role in the evening phase of the clock. Using a version of KaiB locked into its active, fold‐switched state, we determined the structures of several complexes that illustrate how KaiB docks on the KaiC hexamer to recruit KaiA and the output protein CikA. A new structure of the output protein SasA bound to KaiC suggested that structural mimicry of the KaiB fold‐switched state is utilized to transition through distinct phases of the clock. Biochemical studies demonstrate that structural mimicry is used at low concentrations of SasA to cooperatively recruit KaiB and enhance oscillator function, while it leads to competition for KaiC at high concentrations. Collectively, this work reveals how mutually exclusive binding interfaces are used to regulate day/night transitions as well as both input and output signaling in the cyanobacterial circadian clock.Support or Funding InformationNational Institutes of Health R01 grants GM121507 (to C.L.P.) and GM107521 (to A.L.).