Anabaena circadian clock proteins KaiA and KaiB reveal a potential common binding site to their partner KaiC

103

142

Circadian clocks are widespread endogenous mechanisms that control the temporal pattern of diverse biological processes, including gene transcription. KaiA is the positive element of the cyanobacterial clock because KaiA overexpression elevates transcription levels of clock components. Recently, we showed that the structure of KaiA is that of a domain-swapped homodimer. The N-terminal domain is a pseudo-receiver; thus, it is likely to be involved in signal transduction in the clock-resetting pathway. The C-terminal domain of KaiA is structurally novel and enhances the KaiC autokinase activity directly. Here, we report the NMR structure of the C-terminal domain of KaiA (ThKaiA180C) in complex with a KaiC-derived peptide from the cyanobacterium Thermosynechococcus elongatus BP-1. The protein-peptide interface is revealed to be different from a model that was proposed earlier, is stabilized by a combination of hydrophobic and electrostatic interactions, and includes many residues known to produce a circadian-period phenotype upon substitution. Although the structure of the monomeric subunit of ThKaiA180C is largely unchanged upon peptide binding, the intersubunit dimerization angle changes. It is proposed that modulation of the C-terminal KaiA domain dimerization angle regulates KaiA-KaiC interactions.

Section: Discussioncontrasting

confidence: 52%

“…In addition, the x-ray structure of KaiA from Anabaena sp. PCC 7120 (essentially an independent C-terminal KaiA domain) was recently solved (21). Individually, the x-ray structures of the monomeric subunits of KaiA are virtually identical to the NMR structures, although the dimerization angle of the C-terminal domain differs by Ϸ20° (20).…”

mentioning

confidence: 99%

Structure of the C-terminal domain of the clock protein KaiA in complex with a KaiC-derived peptide: Implications for KaiC regulation

Vakonakis

LiWang

2004

103

142

“…KaiC forms a hexamer with a double ring structure, the CI and CII domain each forming one ring, stacked together (4,25,26). Free KaiB has been crystallized as a tetramer in four studies and has once been described as a dimer (27)(28)(29)(30)(31)(32). It was proposed to bind to KaiC as a tetramer (33), dimer (27), or, more recently, as a monomer (32).…”

mentioning

confidence: 99%

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

Snijder

Burnley

Wiegard

et al. 2014

Significance The Kai system is a widely studied model in theoretical biology and systems biology. It is to date the only known circadian clock that can be reconstituted in vitro. Essential to the rhythmicity of the system is the formation of the KaiC–KaiB complex. Many aspects of this interaction, such as the mode of binding of KaiB, the stoichiometry of the interaction, and the exact binding interfaces have long remained ambiguous. We present a mass spectrometry-based structural model of the KaiC–KaiB interaction that answers many of these outstanding questions on the basis of direct experimental evidence. This structural model sheds light on the intricate workings of the in vitro oscillator.

“…The three-dimensional structures of the proteins encoded by the kai genes have been determined (3,(5)(6)(7)(8)(9)(10). The Kai proteins interact with each other (11,12) to form large complexes in vivo in which KaiC is the core (13).…”

mentioning

confidence: 99%

Identification of key phosphorylation sites in the circadian clock protein KaiC by crystallographic and mutagenetic analyses

Mori

Pattanayek

et al. 2004

134

192

In cyanobacteria, KaiC is an essential hexameric clock protein that forms the core of a circadian protein complex. KaiC can be phosphorylated, and the ratio of phospho-KaiC to non-phospho-KaiC is correlated with circadian period. Structural analyses of KaiC crystals identify three potential phosphorylation sites within a 10-Å radius of the ATP binding regions that are at the T432, S431, and T426 residues in the KaiCII domains. When these residues are mutated by alanine substitution singly or in combination, KaiC phosphorylation is altered, and circadian rhythmicity is abolished. These alanine substitutions do not prevent KaiC from hexamerizing. Intriguingly, the ability of KaiC overexpression to repress its own promoter is also not prevented by alanine substitutions at these sites, implying that the capability of KaiC to repress its promoter is not sufficient to allow the clockwork to oscillate. The KaiC structure and the mutational analysis suggest that S431 and T426 may share a phosphate that can shuttle between these two residues. Because the phosphorylation status of KaiC oscillates over the daily cycle, and KaiC phosphorylation is essential for clock function as shown here, daily modulations of KaiC activity by phosphorylation at T432 and S431͞T426 seem to be key components of the circadian clockwork in cyanobacteria.