PROTEIN-PROTEIN INTERACTIONS IN THE CYANOBACTERIAL KAIABC CIRCADIAN CLOCK

The discovery that the central oscillator of the cyanobacterial KaiABC circadian clock can be reconstituted in vitro by the protein components KaiA, KaiB and KaiC renders this biological timer a unique target for biochemical and structural studies. The oscillator can be monitored through changes in the KaiC phosphorylation status that is modulated by KaiA and KaiB. As the 24-h period of the recombinant clock remains unaltered as a result of modest variation of temperature, interactions between the three Kai proteins not only form the basis for rhythmic control of levels of KaiC phosphorylation but also provide temperature compensation. A profound understanding of how this biological timer works requires a dissection of the functions of, and interactions between, the three proteins. Three-dimensional structures of the individual Kai proteins have been determined, and the KaiA-KaiC complex has been studied using hybrid structural methods. This chapter provides an overview of progress in the characterization of the cyanobacterial circadian clock with an emphasis on structural aspects of individual Kai proteins and the binary KaiA-KaiC complex