The cyanobacterial clock proteins KaiA, KaiB and SasA are proposed as regulators of the circadian rhythm in cyanobacteria [ PUBMED:12441347 PUBMED:12727878 ]. Mutations in both proteins have been reported to alter or abolish circadian rhythmicity. … expand

The cyanobacterial clock proteins KaiA, KaiB and SasA are proposed as regulators of the circadian rhythm in cyanobacteria [ PUBMED:12441347 PUBMED:12727878 ]. Mutations in both proteins have been reported to alter or abolish circadian rhythmicity. KaiB adopts an α-β meander motif and is found to be a dimer [ PUBMED:15313603 ]. KaiB was originally discovered from the cyanobacterium Synechococcus as part of the circadian clock gene cluster, kaiABC. KaiB attenuates KaiA-enhanced KaiC autokinase activity by interacting with KaiA-KaiC complexes in a circadian fashion [ PUBMED:15071498 PUBMED:12727879 ]. KaiB is membrane-associated as well as cytosolic. The amount of membrane-associated protein peaks in the evening (at circadian time (CT) 12-16) while the cytosolic form peaks later (at CT 20). The rhythmic localisation of KaiB may function in regulating the formation of Kai complexes. SasA is a sensory histidine kinase which associates with KaiC [ PUBMED:10786837 ]. Although it is not an essential oscillator component, it is important in enhancing kaiABC expression and is important in metabolic growth control under day/night cycle conditions. SasA contains an N-terminal sensory domain with a TRX fold which is involved in the SasA-KaiC interaction [ PUBMED:12449424 ]. This domain shows high sequence similarity with KaiB [ PUBMED:15014139 ]. However, the KaiB structure does not show a classical TRX fold. The N-terminal half of KaiB shares the same β-α-β topology as TRX, but the topology of its C-terminal half diverges.