Fig 1.
(A) Structure of CaM (PDB 1CLL), shown in blue, with two Ca2+ ions (gold) at each terminus. (B) Structure of Ca2+/CaM (PDB 2JZI) bound to a calcineurin (CaN) peptide (red). (C) Schematic of CaM interactions with downstream binding partners. CaM may bind Ng in the absence of Ca2+. In the presence of Ca2+, CaM binds to CaN, CaMKII, NOS, MLCK, and AC1 and AC8.
Fig 2.
Model of Ca2+/CaM interactions.
(A) Reversible binding of Ca2+ binding to CaM (blue). (B) Reversible binding of Ca2+ to CaM bound to a given binding partner, denoted with ‘B’ (green). (C) Reversible binding of a given binding partner to any state of Ca2+/CaM (orange).
Table 1.
Significant PRCCs for initial protein concentration parameters.
Table 2.
Significant PRCCs for rate parameters.
Fig 3.
Competition for CaM alters binding dynamics.
Time-course of CaM binding partners bound to various states of CaM for 1 second of 10 Hz Ca2+ flux: CaM0 (blue), CaM2N (red), CaM2C (green), CaM4 (purple), and CaMtot (orange). CaMtot is the sum of all CaM-bound states for a given protein. The concentration of each species is normalized against its maximum value of CaMtot. Solid lines denote the isolated model. Dotted lines denote the competitive model. The differences between isolated and competitive behavior are more significant for some CaM binding partners than others.
Fig 4.
Competition tunes activation frequencies.
(A) and (B) show normalized activation of CaM as a function of frequency for the isolated and competitive models, respectively. Red denotes peak activation; blue denotes minimal activation. Frequency windows of peak activation tend to narrow and shift for many of the binding partners in the competitive case. Indeed, (C) indicates a sharpening of activation frequency windows as an increase in specificity in the competitive model, at least for most proteins. Specificity is Sb multiplied by 100 percent.
Fig 5.
Competitive tuning explains intermolecular crosstalk.
(A) Simulations of CaMKII phosphorylation in our isolated model with and without inclusion of Ng. (B) Simulations of CaMKII phosphorylation in our competitive model with and without Ng. (C) CaMKII activity in WT and Ng-/- knockout mice from Krucker et al. Simulations were performed to replicate the experimental method of Krucker et al. as closely as possible. (D) The average bound concentration (Cb) of each CaM binding protein in semi-isolated models as a function of Ng concentration. AC8-Ct and AC8-Nt exhibit the greatest relative change in CaM-binding (Cb, Eq 1) as Ng concentration decreases. (E) The average bound concentration (Cb) of each CaM binding protein in the competitive model as a function of Ng concentration. For a decreasing Ng concentration, AC8-Ct and AC8-Nt again exhibit the greatest relative change in CaM-binding. (F) Comparing the semi-isolated (dotted traces) to the competitive (solid traces) model shows that only in the competitive model does summed AC8 (AC8-Nt + AC8-Ct, dark red) mirror the loss in CaM-CaMKII binding as Ng concentration decreases.