SF-loop conformation dependence on ion occupancies is shifted in KirBac1.1 I131M. (A and B) Fractional amplitudes from three-state fits to FRET histograms in Figs. 6 B and 7 B, with sigmoidal equation fits of [ion] dependence of fractional occupancy of high (F0.8) and low (F0.2) states for KirBac1.1 WT (gray) and KirBac1.1 I131M (green), for increasing [K⁺] concentrations on a high [Na⁺] background (A; F0.8: K_1/2 WT, 0.81 mM; K_1/2 I131M, 113.2 mM; F0.45 and F0.2: K_1/2 WT, 0.8 mM; K_1/2 I131M, 81.5 mM), and increasing [Na⁺] concentrations on a high [K⁺] background (B; F0.8: K_1/2 WT, 12.96 mM; K_1/2 I131M, 7.29 mM), medium (F0.45), and low (F0.2: K_1/2 WT, 15.35 mM; K_1/2 I131M, 4.76 mM). For this figure and Fig. 10, all datasets were split into three equal-sized groups and analyzed separately, to assess variability in the data, which are presented as mean ± SEM of the three data groups. *, P < 0.05 versus WT. All data are represented as mean ± SEM.