CIRCADIAN RHYTHMS - Cyanobacteria - page 310

Figure 1: Three-dimensional structures of the cyanobacterial clock proteins KaiA, KaiB, and KaiC. (A) Crystal structure of
the domain-swapped KaiA dimer from S. elongatus , PDB-ID 1R8J, http://www.rcsb.org (Ye et al ., 2004). (B) Crystal structure
of the KaiB tetramer from Synechocystis , PDB-ID 1WWJ (Hitomi et al . 2005). (C) Crystal structure of the KaiC hexamer from
S. elongatus , PDB-ID 2GBL (Pattanayek et al ., 2006). Subunits are colored differently and N- and C-terminal ends are labelled.
ATP molecules bound between subunits in the N-terminal CI and C-terminal CII halves of KaiC were omitted. The images
were produced with the UCSF Chimera package (Pettersen et al ., 2004). Illustration courtesy Martin Egli, Department of
Biological Sciences, Vanderbilt University, Nashville, Tennessee, USA.
Color image of this figure appears in the color plate section at the end of the topic.
KaiB consists of 108 amino acid residues and has a molecular weight of 12 kDa (Iwase et al .,
2004). NMR structure analysis of KaiB from Synechocystis sp. strain PCC 6803 revealed it to be a
tetramer (Fig. 1B). This homotetrameric structure of KaiB forms an open square that is suitable for
protein-protein interaction with positively charged residues being located around the perimeter.
KaiB is localized on the phospholipid-rich membrane and gets translocated into the cytosol to
interact with other Kai components, KaiA and KaiC. Further, the sensory domain of KaiB resembles
thioredoxin family of proteins with β-α-β topology similar to the sensory domain of SasA (Hitomi
et al ., 2005). However, studies on X-ray crystal structure (at a resolution of 2.6 A°) of KaiB from
Thermosynechococcus elongatus BP-1 revealed an overall shape of the tetramer to be an elongated
hexagonal plate with a single positively charged cleft fl anked by two negatively charged ridges. The
biological function of KaiB thus seems to be dependent on the positively charged cleft fl anked by
negatively charged ridges. This has been further confi rmed by the site-directed mutagenesis of kaiB
where substitution of residues Lys-11 or Lys-13 by Ala in the cleft region or deletion of C-terminal
residues from 95-108 forming the ridge, dampens the in vivo circadian rhythms (Iwase et al ., 2005).
KaiC belongs to the family of DNA-binding ATPase superfamily of proteins, ATPase (DnaB) and
a DNA recombinase (RecA; Leipe et al ., 2000). It forms a hexamer in vitro (Mori et al ., 2002; Hayashi
et al ., 2003). Mori et al . (2002) reported a weak DNA-binding activity of KaiC protein. KaiC consists
of 519 amino acid residues and has a duplicated tandem structure, i.e. the fi rst and second halves of
the KaiC amino acid sequences are similar. These two regions are designated as CI and CII domains
that possess Walker's motifs (also called as P-loops). Two KaiA-binding sites are present, one in the
middle (at approximately 240 to 260 aa residues) and the second at 500 to 519 aa residues. Three
dimensional structure of KaiC hexamers has been studied by electron cryomicroscopy. Each KaiC
monomer is a dumb-bell-shaped structure composed of two domains divided by relatively narrow