AC   MF_01835;
DC   Protein; auto
TR   HAMAP; MF_01835; -; 1; level=0
XX
Names: KaiB
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ID   KAIB
DE   RecName: Full=Circadian clock oscillator protein KaiB;
GN   Name=kaiB;
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case not <OC:Prochlorococcus>
CC   -!- FUNCTION: Key component of the KaiABC oscillator complex, which
CC       constitutes the main circadian regulator in cyanobacteria. Complex
CC       composition changes during the circadian cycle to control KaiC
CC       phosphorylation. KaiA stimulates KaiC autophosphorylation, while KaiB
CC       sequesters KaiA, leading to KaiC autodephosphorylation. Phospho-Ser-431
CC       KaiC accumulation triggers binding of KaiB to form the KaiB(6):KaiC(6)
CC       complex, leading to changes in output regulators CikA and SasA. KaiB
CC       switches to a thioredoxin-like fold (KaiB(fs)) when bound to KaiC.
CC       KaiB(6):KaiC(6) formation exposes a site for KaiA binding that
CC       sequesters KaiA from KaiC, making the KaiC(6):KaiB(6):KaiA(12) complex
CC       that results in KaiC autodephosphorylation.
CC   -!- FUNCTION: A metamorphic protein which reversibly switches between an
CC       inactive tetrameric fold and a rare, thioredoxin-like monomeric fold
CC       (KaiB(fs)). KaiB(fs) binds phospho-KaiC, KaiA and CikA. KaiA and CikA
CC       compete for binding to KaiB(fs), and KaiB(fs) and SasA compete for
CC       binding to KaiC, thus the clock oscillator and output signal pathway
CC       are tightly coupled.
CC   -!- SUBUNIT: The KaiABC complex composition changes during the circadian
CC       cycle to control KaiC phosphorylation. Complexes KaiC(6), KaiA(2-
CC       4):KaiC(6), KaiB(6):KaiC(6) and KaiC(6):KaiB(6):KaiA(12) are among the
CC       most important forms, many form cooperatively. Undergoes a major
CC       conformational rearrangment; in the free state forms homotetramers as a
CC       dimer of dimers. When bound to the CI domain of KaiC switches to a
CC       monomeric thioredoxin-fold (KaiB(fs)). KaiB(fs) binds CikA, leading it
CC       to dephosphorylate phospho-RpaA.
CC   -!- DOMAIN: Has 2 forms, fold switches to a thioredoxin-like fold
CC       (KaiB(fs)) when bound to KaiC.
end case
case <OC:Prochlorococcus>
CC   -!- FUNCTION: Component of the KaiBC clock protein complex, which
CC       constitutes the main circadian regulator in cyanobacteria; it may
CC       modify the ATPase activity of KaiC.
CC   -!- FUNCTION: May be a metamorphic protein which natively switches between
CC       an inactive tetrameric fold and a rare, thioredoxin-like monomeric fold
CC       (KaiB(fs)). KaiB(fs) binds phospho-KaiC, and perhaps clock output
CC       effectors.
CC   -!- SUBUNIT: May undergo a major conformational rearrangment; in the free
CC       state forms homooligomers. When bound to KaiC switches to a monomeric
CC       thioredoxin-fold (KaiB(fs)). The active oscillator complex is probably
CC       KaiC(6):KaiB(6).
CC   -!- DOMAIN: Has 2 forms, fold switches to a thioredoxin-like fold
CC       (KaiB(fs)) when bound to KaiC.
CC   -!- MISCELLANEOUS: The kiaA gene has been eliminated from Prochlorococcus
CC       during genome streamlining. It has been suggested that the central
CC       oscillator in Prochlorococcus does not have to be as robust as in other
CC       cyanobacteria because the former live in specific niches of the Earth's
CC       oceans; they divide exactly once a day and at the same time. Thus gene
CC       loss and changes in kaiB function compared to other cyanobacteria, can
CC       occur.
end case
CC   -!- SIMILARITY: Belongs to the KaiB family.
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DR   Pfam; PF07689; KaiB; 1; trigger=no
DR   NCBIfam; TIGR02654; circ_KaiB; 1; trigger=no
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KW   Biological rhythms
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GO   GO:0007623; P:circadian rhythm
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FT   None
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Size: 102-119;
Related: None;
Template: Q79PF5; Q8YT41; P74645;
Scope:
 Bacteria; Cyanobacteriota
Fusion:
 Nter: None
 Cter: None
Duplicate: None
Plasmid: None
Comments: Prochlorococcus do not contain KaiA (which stimulates KaiC
 autophosphorylation activity), suggesting that regulation of circadian cycle is
 different in these bacteria.
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# Revision 1.15 2023/06/01
//