The cyanobacterial clock proteins KaiA and KaiB are proposed as regulators of the circadian rhythm in cyanobacteria. The overall fold of the KaiA monomer is that of a four-helix bundle, which forms a dimer in the known structure (PMID:15071498).
KaiA is a 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, KaiA 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 antagonising the interaction between kaiA and kaiC. The activity of KaiA activates kaiBC expression, while KaiC represses it. The overall fold of the KaiA monomer is that of a four-helix bundle, which forms a dimer in the known structure [ (PUBMED:15071498) ]. KaiA functions as a homodimer. Each monomer is composed of three functional domains: the N-terminal amplitude-amplifier domain, the central period-adjuster domain and the C-termianl clock-oscillator domain. The N-terminal domain of KaiA, from cyanobacteria, acts as a psuedo-receiver domain, but lacks the conserved aspartyl residue required for phosphotransfer in response regulators [ (PUBMED:12438647) ]. The C-terminal domain is responsible for dimer formation, binding to KaiC, enhancing KaiC phosphorylation and generating the circadian oscillations [ (PUBMED:15170179) ]. The KaiA protein from Anabaena sp. (strain PCC 7120) lacks the N-terminal CheY-like domain.
Anabaena circadian clock proteins KaiA and KaiB reveal a potential common bindingsite to their partner KaiC.
EMBO J. 2004; 23: 1688-98
Display abstract
The cyanobacterial clock proteins KaiA and KaiB are proposed as regulators of thecircadian rhythm in cyanobacteria. Mutations in both proteins have been reported to alter or abolish circadian rhythmicity. Here, we present molecular models ofboth KaiA and KaiB from the cyanobacteria Anabaena sp PCC7120 deduced by crystal structure analysis, and we discuss how clock-changing or abolishing mutations maycause their resulting circadian phenotype. The overall fold of the KaiA monomeris that of a four-helix bundle. KaiB, on the other hand, adopts an alpha-betameander motif. Both proteins purify and crystallize as dimers. While the folds ofthe two proteins are clearly different, their size and some surface features ofthe physiologically relevant dimers are very similar. Notably, the functionallyrelevant residues Arg 69 of KaiA and Arg 23 of KaiB align well in space. Theapparent structural similarities suggest that KaiA and KaiB may compete for apotential common binding site on KaiC.
