Mutations at position I131 alter KirBac1.1 SF dynamics at low [Na]. (A) Representative smFRET trajectories in different [ion] for KirBac1.1 WT and I131M mutant. Fluorescence intensities of the donor Alexa Fluor 555 and acceptor Alexa Fluor 647 (AF555/647) are colored cyan and red, respectively; FRET is colored blue. FU, fluorescence uptake. (B) FRET histograms for AF555/647 fluorophores labeled at diagonal T120C sites in the SF loop in the WT and I131M protein, at increasing [Na⁺] from 0 to 5 mM on a background of 150 mM K⁺, as indicated (n indicates number of traces in each case); fits as in Fig. 6.