Fig. 4.

Relative movement of the 667 sites is Ca²⁺ and voltage dependent. G–V curves (Upper) and the corresponding E–V curves (Lower) were determined synchronously at various free Ca²⁺ concentrations, for (A) BK-667CY heterotetramers, (B) mutants with reduced Ca²⁺ sensitivity at the Ca²⁺ bowl (BK-667/5D5A/CY), or (C) mutations at both the Ca²⁺ bowl and the RCK1 Ca²⁺ binding site (BK-667/5D5A/M513I/CY). Data are color-coded in all graphs according to free Ca²⁺ concentrations. The solid lines in A–C (Upper) represent fits to the Sweet and Cox (16) model with parameters shown in Table S1. Data points and error bars represent average ± SEM (n = 3–13).