Component of the KaiABC clock protein complex, which constitutes the main circadian regulator in cyanobacteria. The KaiABC complex may act as a promoter-nonspecific transcription regulator that represses transcription, possibly by acting on the state of chromosome compaction. In the complex, it enhances the phosphorylation status of KaiC. In contrast, the presence of KaiB in the complex decreases the phosphorylation status of KaiC, suggesting that KaiB acts by antagonizing the interaction between KaiA and KaiC. A KaiA dimer is sufficient to enhance KaiC hexamer phosphorylation. Homodimer. Component of the KaiABC complex, at least composed of a KaiC homohexamer, a KaiB dimer and two KaiA dimers. The KaiABC complex also interacts with SasA. Expressed constantly throughout the circadian cycle. The N-terminal part (1-102), which is also called pseudo-receiver domain, does not enhance KaiC autophosphorylation. It may have an amplitude-amplifier function and may be involved in transducing input signals to the KaiABC complex. This domain is not conserved in other species. The KaiA domain mediates the interaction with KaiC, the homodimerization, and is responsible for the clock oscillation function. 'Kai' means 'cycle' in Japanese. PubMed:15007067 structure is derived from this sequence and not from the one of S.elongatus.