Fig. 2.

Dissecting the molecular basis of Ca²⁺ ultrasensitivity. (A) Lack of significant cooperativity in CaM activation of CaMKII. Plotted is the Ca²⁺-dependent CaMKII activity versus [CaM] (±SD, n = 2). (B) Modest Ca²⁺ ultrasensitivity (n_H ≈ 3) caused by cooperative binding of Ca²⁺ to CaM. Plotted is the CaM-dependent CaMKII activity versus [Ca²⁺] (±SD, n = 2). (C) Greater Ca²⁺ ultrasensitivity (n_H ≈ 5) induced by the dual requirement of CaM for autophosphorylation. Plotted is the maximum autonomy versus [Ca²⁺](±SD, n = 2). Experiments were performed at 0.2 μM (triangle, solid black line), 1.0μM (diamond, dotted line), and 5.0 μM (square, solid gray line) CaMKII.