From primordial clocks to circadian oscillators

By W. Pitsawong, R.A.P. Padua, Tim Grant, Marc Hoemberger, Renee Otten, N. Bradshaw, N. Grigorieff, Dorothee Kern. 2023

30 citations recorded by the source corpus.

Circadian rhythms play an essential part in many biological processes, and only three prokaryotic proteins are required to constitute a true post-translational circadian oscillator^ 1 . The evolutionary history of the three Kai proteins indicates that KaiC is the oldest member and a central component of the clock^ 2 . Subsequent additions of KaiB and KaiA regulate the phosphorylation state of KaiC for time synchronization. The canonical KaiABC system in cyanobacteria is well understood^ 3 – 6 , but little is known about more ancient systems that only possess KaiBC. However, there are reports that they might exhibit a basic, hourglass-like timekeeping mechanism^ 7 – 9 . Here we investigate the primordial circadian clock in Rhodobacter sphaeroides , which contains only KaiBC, to elucidate its inner workings despite missing KaiA. Using a combination of X-ray crystallography and cryogenic electron microscopy, we find a new dodecameric fold for KaiC, in which two hexamers are held together by a coiled-coil bundle of 12 helices. This interaction is formed by the carboxy-terminal extension of KaiC and serves as an ancient regulatory moiety that is later superseded by KaiA. A coiled-coil register shift between daytime and night-time conformations is connected to phosphorylation sites through a long-range allosteric network that spans over 140 Å. Our kinetic data identify the difference in the ATP-to-ADP ratio between day and night as the environmental cue that drives the clock. They also unravel mechanistic details that shed light on the evolution of self-sustained oscillators. X-ray, cryo-EM and kinetic studies of the circadian oscillator KaiBC from the photosynthetic bacterium Rhodobacter sphaeroides shed light on the evolution of self-regulating oscillators.

Subfields: Cellular and Molecular Neuroscience, Molecular Biology, Plant Science

Citations in the corpus

• Predicting multiple conformations via sequence clustering and AlphaFold2 (2023, neutral): “KaiB is a circadian-rhythm protein found in cyanobacteria 5,31 and proteo-bacteria 32 that adopts two conformations with distinct secondary structures as part of its function: during the day, it primarily adopts the ground-state conformation, which has a secondary structure of βαββααβ that is not…”