Cation selectivity evaluation in NavAb N49K/T206A, S178T ^NK/TA , S178A ^NK/TA , and S178G ^NK/TA mutants. (A–D) Current–voltage relationship plots (left) and representative currents (right) used to evaluate the permeability of K⁺, Cs⁺, and Ca²⁺ relative to that of Na⁺. Background (BG) indicates NavAb N49K/T206A mutant without the Ser178 mutation. Currents were measured at 150 mM [K⁺]_out (green), 150 mM [Cs⁺]_out (sky blue), and 100 mM [Ca²⁺]_out (orange), respectively, and 150 mM [Na⁺]_in. Currents were generated by a step pulse from a −140 mV holding potential to −20 mV, followed by ramp pulses to different voltage values. The time course of the change of membrane potentials is shown at the bottom of the respective current traces.