Virtuelle (Nano) Welten PDB Molecule of the Month 97: Circadian Clock Proteins

Exploring the Structure

The KaiC clock protein (PDB entry 2gbl) is composed of six identical subunits that form a barrel-shaped structure. The protein is shown from side here, with two subunits removed to show the tunnel that runs through the middle. The phosphates that tick off the hours of each day are added to a serine and a threonine on each subunit. They are buried deep inside the tunnel, near the binding site for ATP, shown here in green. KaiA stimulates KaiC to add these phosphate groups to itself, and KaiB blocks the action of KaiA, allowing KaiC to remove these phosphates from itself. The speed of these reactions are all calibrated so that the whole process takes 24 hours to complete.

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