Low Na⁺–dependent kinetics of SF in KirBac1.1 WT and I131M. Concatenated smFRET trajectories of WT (gray) and I131M (green) in 150 mM KCl with zero Na⁺, 0.05 mM Na⁺, 0.5 mM Na⁺, or 5 mM Na⁺, as indicated, together with three state idealizations (red) corresponding to low, middle, and high FRET states.