Does the Kai clock rotate?

suggests that an ancient circadian clock rotates much like a hand on an analogue clock. His analysis of cyanobacterial clock protein structures reveals a similarity with the F 1-ATPase rotary motor. The cyanobacterial clock is controlled by the KaiA, KaiB, and KaiC proteins, which form a complex at night that falls apart in the day. Mutations causing more stable complexes correspond to a longer periodicity. But little is known about how the timing of complex formation is controlled. Wang analyzed the recently solved structures of the Kai proteins to suggest a mechanism. When KaiC is ATP-bound, the Kai complex is stable. But when ATP is bumped off by autophosphorylation near the ATP-binding site, the complex falls apart. Autophosphorylation is stimulated in vitro by KaiA. Wang realized that the ATP-binding domains of KaiC hexamers resemble the F 1-ATPase ring. He proposes that KaiA fits inside the ring much like the ␥␦ subunits fit inside the F 1-ATPase ring. Based on previous structures, KaiA dimers are too large to fit inside the ring. But Wang proposes that KaiA dimers must first be activated by the extension of helical domains. This proposed extension makes KaiA dimers resemble ␥␦ subunits. The need for KaiA activation would also explain the two-hour delay between the rise in KaiA and KaiC levels at dusk and complex formation. The stimulus for such KaiA activation is unclear. In Wang's model, KaiA is expected to contact at most two KaiC monomers at a time and stimulate their autokinase J Stiffening under pressure etworks of biological filaments have just enough flexibility to stiffen as they are strained, according to findings from Cornelis Storm, Natural networks, such as collagen gels or cytoskeletal webs, have the ability to increase their stiffness with increasing strain. This unique feature is an advantage over most synthetic fibers. If blood vessels were made of rubber tubing, for example, the pressure from a heart beat would vastly increase vessel diameter. But collagen's nonlinear elasticity prevents such a drastic endothelial deformation. This ability is usually explained by the heterogeneous nature of biological gels—perhaps tauter filaments take over at increasing strains. But in vitro measurements by Storm et al. now show that uniform biopolymer gels also exhibit nonlinear elasticity. The authors then produced a mathematical model that explains this behavior based on the char-N activity. Autophosphorylation would both displace ATP and provide energy for the rotation of KaiA to new …