Figure 3. Activation of CaM.

a) The plot shows the probability for a CaM molecule with a fully occupied C-lobe at t=0 to have the C-lobe fully occupied by Ca²⁺ as a function of time in the presence of 100 nM [Ca²⁺] (solid line). It is clear that the C-lobe remains activated with high probability for tens of ms after it binds the second Ca²⁺. In contrast, if the k_off of the C-lobe would not have decreased upon binding the 2^nd Ca²⁺ (dashed line), the lobe would remain activated for less than 1 ms. This indicates that the switch in k_off of the C-lobe is essential for a sustained lifetime of a primed C-lobe. b) The plot shows the probability for a CaM molecule with no Ca²⁺ bound to the N-lobe at t=0 to have the N-lobe fully occupied with Ca²⁺ as it dwells in a nanodomain with 100 μM [Ca²⁺] (D_CaM =50 nm²μs⁻¹, closed line). It is quite likely that the N-lobe will be activated within the size of a nanodomain. However, if the N-lobe would not have increased k_on after binding the 1^st Ca²⁺ (dashed line), the domain size needed for a fully occupied N-lobe would be substantially larger. c) Plot showing the probability for a CaM molecule to become fully occupied by Ca²⁺ as it dwells in a domain with 100 μM [Ca²⁺]. If the molecule has no Ca²⁺ bound at t=0, full activation is unlikely (black line), whereas if the C-lobe is primed (has both binding sites occupied) at t=0, activation within the nanodomain is likely (green line). Yellow areas give approximate size for a nanodomain.