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

Proc Natl Acad Sci U S A. 2014 Jan 28;111(4):1379-84. doi: 10.1073/pnas.1314326111. Epub 2014 Jan 13.


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.

Keywords: ion mobility spectrometry; native MS; protein–protein docking.

Publication types

  • Research Support, Non-U.S. Gov't

MeSH terms

  • Bacterial Proteins / metabolism*
  • Circadian Rhythm Signaling Peptides and Proteins / metabolism*
  • Circadian Rhythm*
  • Cyanobacteria / physiology*
  • Mass Spectrometry
  • Phosphorylation
  • Protein Binding
  • Protein Conformation


  • Bacterial Proteins
  • Circadian Rhythm Signaling Peptides and Proteins
  • KaiB protein, cyanobacteria
  • KaiC protein, cyanobacteria