VSD-I is the main contributor to Ca_V1.1 pore opening. A Ca_V allosteric model (scheme in Fig. 6) was used to simultaneously fit steady-state and kinetics data in both WT and R174W channels. An error function was minimized using a Markov chain Monte Carlo sampling as in Savalli et al. (2016). Only parameters characterizing VSD-I activity could vary independently for WT and R174W channels. (A and B) Experimental G(V) and F(V) data points are shown superimposed to the model predictions (black lines) for WT (A) and R174W (B) channels. (C–F) Ionic currents (C and E) and fluorescence traces (D and F) from each VSD (normalized to the steady-state probability of activation) in both WT (C and D) and R174W (E and F) channels are shown superimposed with model predictions (black lines, best solutions out of 1.5 ∙ 10⁶ trials from three replicas). Shaded areas depict all solutions within the 95% CI. Solution uniqueness and CIs are described in Fig. 8 and Table S1.