Low K⁺–dependent kinetics of SF in KirBac1.1 WT and I131M. Concatenated smFRET trajectories of WT (gray) and I131M (green) in 150 mM NaCl with zero K⁺, 0.5 mM K⁺, 5 mM K⁺, and 100 mM Na⁺ + 50 mM K⁺, as indicated, together with three state idealizations (red) corresponding to low, middle, and high FRET states.