Insight into cyanobacterial circadian timing from structural details of the KaiB–KaiC interaction

• Published in: Proceedings of the National Academy of Sciences - PNAS Vol. 111; no. 4; pp. 1379 - 1384
• Main Authors: Snijder, Joost, Burnley, Rebecca J., Wiegard, Anika, Melquiond, Adrien S. J., Bonvin, Alexandre M. J. J., Axmann, Ilka M., Heck, Albert J. R.
• Format: Journal Article
• Language: English
• Published: United States National Academy of Sciences 28-01-2014; National Acad Sciences
• Subjects: Adenosine triphosphatases; Bacterial Proteins - metabolism; Biological Sciences; CII region; Circadian Rhythm; Circadian Rhythm Signaling Peptides and Proteins - metabolism; Crystal oscillators; Cyanobacteria; Cyanobacteria - physiology; Deuterium; Dimers; Mass Spectrometry; Mass spectroscopy; Monomers; Phosphorylation; Protein Binding; Protein Conformation; Proteins; Stoichiometry; ion mobility spectrometry; native MS; protein–protein docking
• ISSN: 0027-8424; 1091-6490
• DOI: 10.1073/pnas.1314326111

Circadian timing in cyanobacteria is determined by the Kai system consisting of KaiA, KaiB, and KaiC. Interactions between Kai proteins change the phosphorylation status of KaiC, defining the phase of circadian timing. The KaiC–KaiB interaction is crucial for the circadian rhythm to enter the dephosphorylation phase but it is not well understood. Using mass spectrometry to characterize Kai complexes, we found that KaiB forms monomers, dimers, and tetramers. The monomer is the unit that interacts with KaiC, with six KaiB monomers binding to one KaiC hexamer. Hydrogen–deuterium exchange MS reveals structural changes in KaiC upon binding of KaiB in both the CI and CII domains, showing allosteric coupling upon KaiB binding. Based on this information we propose a model of the KaiB–KaiC complex and hypothesize that the allosteric changes observed upon complex formation relate to coupling KaiC ATPase activity with KaiB binding and to sequestration of KaiA dimers into KaiCBA complexes.