Biochemical basis of a 24-hour clock: Circadian clocks keep organisms in synch with such daily cycles as illumination, activity, and food availability. The circadian clock in cyanobacteria has the necessary 24-hour period despite its three component proteins having biochemical activities that occur on a much faster time scale. Abe et al. focused on the cyanobacterial clock component KaiC, an adenosine triphosphatase (ATPase) that can autophosphorylate and autodephosphorylate. The slow ATPase activity of KaiC, which is linked to a peptide isomerisation, provided the slow kinetics that set the speed of the 24-hour clock. Chang et al. found that another clock component, KaiB, also has slow changes in its protein conformation that help to set the oscillation period of the clock and its signaling output. Science , this issue pp. 312 and 324

The circadian clock in cyanobacteria persists even without the transcription/translation feedbacks proposed for eukaryotic systems. The period of the cyanobacterial clock is tuned to the circadian range by the ATPase activity of a clock protein known as KaiC. Here, we provide structural evidence on how KaiC ticks away 24 h while coupling the ATPase activity in its N‐terminal ring to the phosphorylation state in its C‐terminal ring. During the phosphorylation cycle, the C‐terminal domains of KaiC are repositioned in a stepwise manner to affect global expansion and contraction motions of the C‐terminal ring. Arg393 of KaiC has a critical function in expanding the C‐terminal ring and its replacement with Cys affects the temperature compensation of the period—a fundamental property of circadian clocks. The conformational ticking of KaiC observed here in solution serves as a timing cue for assembly/disassembly of other clock proteins (KaiA and KaiB), and is interlocked with its auto‐inhibitory ATPase underlying circadian periodicity of cyanobacteria.

Conformational studies on amyloid β peptide (Aβ) in aqueous solution are complicated by its tendency to aggregate. In this study, we determined the atomic‐level structure of Aβ₂₈₋₄₂ in an aqueous environment. We fused fragments of Aβ, residues 10–24 (Aβ₁₀₋₂₄) or 28–42 (Aβ₂₈₋₄₂), to three positions in the C‐terminal region of ribonuclease HII from a hyperthermophile, Thermococcus kodakaraensis (Tk‐RNase HII). We then examined the structural properties in an aqueous environment. The host protein, Tk‐RNase HII, is highly stable and the C‐terminal region has relatively little interaction with other parts. CD spectroscopy and thermal denaturation experiments demonstrated that the guest amyloidogenic sequences did not affect the overall structure of the Tk‐RNase HII. Crystal structure analysis of Tk‐RNase HII₁₋₁₉₇–Aβ₂₈₋₄₂ revealed that Aβ₂₈₋₄₂ forms a β conformation, whereas the original structure in Tk‐RNase HII₁₋₂₁₃ was α helix, suggesting β‐structure formation of Aβ₂₈₋₄₂ within full‐length Aβ in aqueous solution. Aβ₂₈₋₄₂ enhanced aggregation of the host protein more strongly than Aβ₁₀₋₂₄. These results and other reports suggest that after proteolytic cleavage, the C‐terminal region of Aβ adopts a β conformation in an aqueous environment and induces aggregation, and that the central region of Aβ plays a critical role in fibril formation. This study also indicates that this fusion technique is useful for obtaining structural information with atomic resolution for amyloidogenic peptides in aqueous environments.

Misfolding of mutant Cu/Zn‐superoxide dismutase (SOD1) is a pathological hallmark in a familial form of amyotrophic lateral sclerosis. Pathogenic mutations have been proposed to monomerize SOD1 normally adopting a homodimeric configuration and then trigger abnormal oligomerization of SOD1 proteins. Despite this, a misfolded conformation of SOD1 leading to the oligomerization at physiological conditions still remains ambiguous. Here, we show that, around the body temperature (∼37°C), mutant SOD1 maintains a dimeric configuration but lacks most of its secondary structures. Also, such an abnormal SOD1 dimer with significant structural disorder was prone to irreversibly forming the oligomers crosslinked via disulfide bonds. The disulfide‐crosslinked oligomers of SOD1 were detected in the spinal cords of the diseased mice expressing mutant SOD1. We hence propose an alternative pathway of mutant SOD1 misfolding that is responsible for oligomerization in the pathologies of the disease.