Thermodynamically Induced Conformational Changes of the Cyanobacterial Circadian Clock Protein KaiB

Site-directed spin labeling electron spin resonance (ESR) was applied to investigate the local environment of the cyanobacterial circadian clock protein KaiB. We prepared five cysteine residue-substituted mutants of KaiB labeled with maleimide spin label (MSL). By comparing the ESR spectra of KaiBs carrying MSL at different positions (Thr64, Lys67, Tyr94, Gly98, and Ala101), local conformational changes were identified. The ESR spectra of MSL-T64C and MSL-K67C showed the relatively slow motion of MSL characterized by tau = 79 and 59 ns at 4A degrees C, respectively. The spectra of MSL-Y94C, MSL-G98C and MSL-A101C showed relatively fast motion characterized by tau = 8.0, 4.1 and 3.1 ns at 4A degrees C, respectively. These differences were explained by the local environments of the position in KaiB. On incubation at 40A degrees C for 24 h, all ESR spectra of the labeled KaiBs changed, which can be explained by the structural relaxation of KaiB.