Recombinant Synechococcus elongatus Circadian clock oscillator protein KaiB (kaiB)

Function

A metamorphic protein which reversibly switches between an inactive tetrameric fold and a rare, thioredoxin-like monomeric fold (KaiB(fs)). KaiB(fs) binds phospho-KaiC, KaiA and CikA. KaiA and CikA compete for binding to KaiB(fs), and KaiB(fs) and SasA compete for binding to KaiC, thus the clock oscillator and output signal pathway are tightly coupled

Subcellular Location

Cytoplasm; Cell membrane (Peripheral membrane protein)

Subunit

Undergoes a major conformational rearrangment; in the free state forms homotetramers with 2 dimers (PubMed:23796516). When bound to the CI domain of KaiC switches to a monomeric thioredoxin-fold (KaiB(fs)) (PubMed:26113641, PubMed:28302851, PubMed:28302852). Monomers, homodimers and homotetramers are detected in solution; at low concentrations only monomers are seen. In vitro forms KaiC(6):KaiB(1) and KaiC(6):KaiB(6) complexes (PubMed:24474762). Only associates with 'Ser-431'-phosphorylated KaiC (and not with doubly phosphorylated KaiC) (PubMed:17717528, PubMed:29892030). Complex formation between KaiB and KaiC is regulated by the phosphorylation state of KaiC and by an ATP hydrolysis-driven conformation change in the CI ring of KaiC; complex formation is slow. Slow complex formation is crucial for the timing of the circadian period (PubMed:29892030). In low resolution cryo-EM forms a KaiC(6):KaiB(6) complex (PubMed:23796516). The KaiABC complex composition changes during the circadian cycle to control KaiC phosphorylation. Complexes KaiC(6), KaiA(2-4):KaiC(6), KaiB(6):KaiC(6) and KaiC(6):KaiB(6):KaiA(12) are among the most important forms, many form cooperatively (PubMed:28302852, PubMed:34618577). The KaiB:KaiC complex is more prevalent at 16 hours (in the dark) than at 4 hours (in the light) in the circadian cycle (PubMed:10786837). The KaiA:KaiB complex is only found at 20-24 hours in the circadian cycle (subjective night) (PubMed:15347812). Binds to the CI domain of KaiC; SasA and KaiB compete to bind to the CI domain (PubMed:26113641, PubMed:28302851, PubMed:28302852)

• Ishiura M. et al. Expression of a gene cluster kaiABC as a circadian feedback process in cyanobacteria. — Science (1998)
• Holtman C.K. et al. Synechococcus elongatus PCC7942 cosmid 7G3. (JUN-)
• Copeland A. et al. Complete sequence of chromosome 1 of Synechococcus elongatus PCC 7942. (AUG-)
• Iwasaki H. et al. Physical interactions among circadian clock proteins KaiA, KaiB and KaiC in cyanobacteria. — EMBO J. (1999)
• Xu Y. et al. Cyanobacterial circadian clockwork: roles of KaiA, KaiB and the kaiBC promoter in regulating KaiC. — EMBO J. (2003)
• Kitayama Y. et al. KaiB functions as an attenuator of KaiC phosphorylation in the cyanobacterial circadian clock system. — EMBO J. (2003)
• Nakahira Y. et al. Global gene repression by KaiC as a master process of prokaryotic circadian system. — Proc. Natl. Acad. Sci. U.S.A. (2004)
• Iwasaki H. et al. A kaiC-interacting sensory histidine kinase, SasA, necessary to sustain robust circadian oscillation in cyanobacteria. — Cell (2000)