ATP-induced hexameric ring structure of the cyanobacterial circadian clock protein KaiC.
BACKGROUND
KaiA, KaiB and KaiC are cyanobacterial circadian clock proteins. KaiC contains two ATP/GTP-binding Walker's motif As, and mutations in these regions affect the clock oscillations.


RESULTS
ATP induced the hexamerization of KaiC. The Km value for the ATP for the hexamerization was 1.9 micro m. Triphosphate nucleotides bound to the two Walker's motif As, and their binding functioned cooperatively for the hexamerization. An unhydrolysable substrate, 5'-adenylylimidodiphosphate (AMPPNP), also induced the hexamerization, indicating that nucleotide binding, but not its hydrolysis, is essential for the hexamerization. Mutations in each of the two Walker's motif As that affect the clock phenotype increased the Km value for ATP and inhibited the hexamerization. Thus, the KaiC hexamerization seems to be necessary for its clock function. The KaiC hexamer has the shape of a hexagonal pot with a diameter and height of approximately 100 A and with a relatively large cavity (73 A deep and 18-34 A wide) inside. This pot-shaped structure suggests that KaiC functions in a similar manner to F1-ATPase, helicase or ATP-dependent protease/chaperon, all of which have dynamic activities inside the central cavity of their hexameric rings.


CONCLUSIONS
ATP-induced KaiC hexamerization is necessary for the clock function of KaiC.
Hayashi
Fumio
F
Center for Gene Research, Nagoya University, Furo, Chikusa, Nagoya, 464-8602, Japan.
Suzuki
Hirofumi
H
Iwase
Ryo
R
Uzumaki
Tatsuya
T
Miyake
Asako
A
Shen
Jian-Ren
JR
Imada
Katsumi
K
Furukawa
Yukio
Y
Yonekura
Koji
K
Namba
Keiichi
K
Ishiura
Masahiro
M
D000255:Adenosine Triphosphate / Q000378:metabolism*; D000595:Amino Acid Sequence; D001426:Bacterial Proteins / Q000737:chemistry* / Q000378:metabolism / Q000648:ultrastructure; D002940:Circadian Rhythm / Q000235:genetics*; D056925:Circadian Rhythm Signaling Peptides and Proteins; D008854:Microscopy, Electron; D008969:Molecular Sequence Data; D011108:Polymers / Q000737:chemistry; D016415:Sequence Alignment
D001426:Bacterial Proteins; D056925:Circadian Rhythm Signaling Peptides and Proteins; C114271:KaiC protein, cyanobacteria; D011108:Polymers; D000255:Adenosine Triphosphate