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. 2020 Jun 4;16(6):e9355. doi: 10.15252/msb.20199355

Figure 3. Substrate competition explains KaiC phosphorylation ultrasensitivity.

Figure 3

  1. The predicted stimulus–response relation of the total steady‐state KaiC phosphorylation level as a function of %ATP and [KaiA].
  2. Experimentally determined stimulus–response function of KaiC at three %ATP conditions; the curves are based on refitting the best fit of the full model to the steady‐state measurements.
  3. Similar to B, but for KaiC S431A, which has only one phosphorylation site; the curves are based on independent fits to a simple phenomenological substrate competition model.
  4. Cross sections of the stimulus–response relation at three %ATP, computed using the full model (left) and model –n (right).
  5. Posterior distributions for the shape measures of the stimulus–response functions at 25% ATP predicted by the full model and model –n. The contours represent the 68% and 95% HDRs, and the gray stars represent the model best fits. The shapes of the stimulus–response functions are quantified using two metrics: EC10, which quantifies threshold‐like behavior, and EC90–EC10, which quantifies switch‐like behavior. The shape measures of the experimentally determined stimulus–response function at 25% ATP is shown as the yellow star. The dashed line represents (EC10, EC90–EC10) = (K/9, 80K/9), which characterizes the shape of a hyperbolic stimulus–response function [A]/(K + [A]) that has no switching or thresholding.
  6. The stimulus–response functions of KaiC S431A at 100% ATP in the presence of KaiC S431A/T432A (AA; left) and S431E/T432E (EE; right) phosphomimetic mutants to probe the effect of kinetic competition on KaiC phosphorylation.
  7. The relations between EC50 (the midpoint of a stimulus–response function) and KaiC AA/EE concentrations quantified using the curves shown in (F).

Source data are available online for this figure.