Shift in conformational equilibrium underlies the oscillatory phosphoryl transfer reaction in the circadian clock

Document Type

Article

Publication Date

10-1-2021

Abstract

Oscillatory phosphorylation/dephosphorylation can be commonly found in a biological system as a means of signal transduction though its pivotal presence in the workings of circadian clocks has drawn significant interest: for example in a significant portion of the physiology of Syn-echococcus elongatus PCC 7942. The biological oscillatory reaction in the cyanobacterial circadian clock can be visualized through its reconstitution in a test tube by mixing three proteins—KaiA, KaiB and KaiC—with adenosine triphosphate and magnesium ions. Surprisingly, the oscillatory phosphorylation/dephosphorylation of the hexameric KaiC takes place spontaneously and almost indefinitely in a test tube as long as ATP is present. This autonomous post‐translational modification is tightly regulated by the conformational change of the C‐terminal peptide of KaiC called the “A-loop” between the exposed and the buried states, a process induced by the time‐course binding events of KaiA and KaiB to KaiC. There are three putative hydrogen‐bond forming residues of the A‐loop that are important for stabilizing its buried conformation. Substituting the residues with alanine enabled us to observe KaiB’s role in dephosphorylating hyperphosphorylated KaiC, independent of KaiA’s effect. We found a novel role of KaiB that its binding to KaiC induces the A‐loop toward its buried conformation, which in turn activates the autodephosphorylation of KaiC. In addition to its traditional role of sequestering KaiA, KaiB’s binding contributes to the robustness of cyclic KaiC phosphorylation by inhibiting it during the dephosphorylation phase, effectively shift-ing the equilibrium toward the correct phase of the clock.

Identifier

85117271354 (Scopus)

Publication Title

Life

External Full Text Location

https://doi.org/10.3390/life11101058

e-ISSN

20751729

Issue

10

Volume

11

Grant

1R15 GM137311‐01

Fund Ref

National Institutes of Health

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