Figure 2. In vitro properties of purified NTnC protein.

(a) Absorbance, excitation and emission spectra of NTnC in Ca²⁺-free and Ca²⁺-bound states. (b) Intensity and dynamic range of NTnC as a function of pH. The dynamic range (fold) at each pH value was determined as the ratio of NTnC fluorescence intensity in the absence of Ca²⁺ to that in the presence of Ca²⁺. (c) Ca²⁺ titration curves for NTnC and GCaMP6s in the absence or presence of 1 mM MgCl₂. (d) Observed Ca²⁺ association rates at moderate Ca²⁺ concentrations (in the range of 0–350 nM) overlaid with the fitting curves (k_obs = k_on × [Ca²⁺]ⁿ + k_off, see Table 1 for the fitting parameters k_on and n). (e) Maturation curves for NTnC in the Ca²⁺-free (green line) and Ca²⁺-bound (grey line) states and for mEGFP (red line). (f) Photobleaching curves for NTnC and GCaMP6s in Ca²⁺-free and Ca²⁺-bound states and for mEGFP. Error represents the standard error of the estimate for the average of three records.