Fig. 1.

Gating charge displacement for neutralization mutants in the VSD of the human BK channel. (A) Primary amino acid sequence of the VSD (S1 to S4). Positively charged residues are colored blue, and negatively charged residues are colored red. Residues that were neutralized are highlighted with their number in the primary sequence. Letters represent symbols for amino acids. (B) Representative records of gating current of BK wt and neutralization mutants R207Q, R210Q, and R213Q. (C) Gating charge versus voltage ($Q(V)$) curves at zero internal calcium. Except for R210Q and R207Q, $Q(V)$ curves were constructed from the integration of the ON gating currents recorded after a depolarization from −90 mV to a voltage ranging between 300 mV and 400 mV depending on the mutant tested, in 10 mV steps. For R210Q and R207Q, the holding potential was +90 mV, and gating currents were recorded during the OFF current in the voltage range of 90 to −400 (R210Q) or −350 mV (R207Q) in 10 mV steps. (D) Half-activation voltage ($V_0$) and the apparent number of gating charges ($z_Q$) for the BK wt and for mutants bearing neutralizing substitutions were obtained from the fitting of the $Q(V)$ data to Eq. 8 in SI Appendix, Materials and Methods. (SI Appendix, Table 1 and Fig. S1 provides examples of raw gating currents). *P < 0.01. Error bars in C and D = SD.