Highly sensitive tryptophan fluorescence probe for detecting rhythmic conformational changes of KaiC in the cyanobacterial circadian clock system. Issue 14 (29th July 2022)

• Record Type: Journal Article
• Authors: Mukaiyama, Atsushi; Furuike, Yoshihiko; Yamashita, Eiki; Akiyama, Shuji
• Is Part Of: Biochemical journal. Volume 479:Issue 14(2022)
• Page Start: 1505
• Page End: 1515
• Publication Date: 2022-07-29
• Subjects: circadian clock -- cyanobacteria -- KaiC -- solution structural change -- Trp fluorescence; Biochemistry -- Periodicals; 572
• DOI: 10.1042/BCJ20210544
• Languages: English
• ISSNs: 0264-6021

Abstract

Abstract : KaiC, a core protein of the cyanobacterial circadian clock, consists of an N-terminal CI domain and a C-terminal CII domain, and assembles into a double-ring hexamer upon binding with ATP. KaiC rhythmically phosphorylates and dephosphorylates its own two adjacent residues Ser431 and Thr432 at the CII domain with a period of ∼24 h through assembly and disassembly with the other clock proteins, KaiA and/or KaiB. In this study, to understand how KaiC alters its conformation as the source of circadian rhythm, we investigated structural changes of an inner-radius side of the CII ring using time-resolved Trp fluorescence spectroscopy. A KaiC mutant harboring a Trp fluorescence probe at a position of 419 exhibited a robust circadian rhythm with little temperature sensitivity in the presence of KaiA and KaiB. Our fluorescence observations show a remarkable environmental change at the inner-radius side of the CII ring during circadian oscillation. Crystallographic analysis revealed that a side chain of Trp at the position of 419 was oriented toward a region undergoing a helix–coil transition, which is considered to be a key event to allosterically regulate the CI ring that plays a crucial role in determining the cycle period. The present study provides a dynamical insight into how KaiC generates circadian oscillation.