The positive charge at position 538 is more important than that at 537 in controlling channel activation. (A–F) Typical current recordings from oocytes expressing various R537 and K538 single and double mutations. Arrows represent the zero current level. (A–C) Oocytes were held at −80 mV and subjected to either 2-s (A) or 10-s (B and C) pulses to +60 mV in 10-mV increments; tail currents were recorded at −60 mV. (D–F) Oocytes were held at −130 mV and subjected to 2-s pulses to 0 or 20 mV in 10-mV increments; tail currents were recorded at −130 mV. (G) Comparison of the effects of R537 mutations (R/K/Q) on the G-V relationship when a Lys residue is present at position 538. (H) Comparison of the effects of R537 (R/K/Q) mutations when the residue at position 538 is an Arg. (I) Comparison of the effects of R537 mutations (R/K/Q) on the G-V relationship when a Q residue is present at position 538. Lines in G–I represent fits of the data to a Boltzmann function. n values and Boltzmann parameters are summarized in Table 1. (J) Plot of the τ_act values against the electrochemical potential for activation for the R537 and K538 double mutants. Data points in G–J represent mean ± SEM (error bars).