Fig. 1

In vitro characterization of GEPIIs. a Schematic representation of the mechanism of K⁺ sensing by FRET-based GEPIIs. Predicted conformational rearrangement of GEPIIs in the absence and presence of K⁺. b Scheme (upper panel) and spectral properties (lower panel) of CFP and FRET fluorescence of purified GEPII 1.0 at increasing K⁺ concentrations. Purified GEPII 1.0 was excited at 413.4 ± 8 nm. c Scheme (upper panel) and emission spectra (lower panel) of the K⁺-insensitive lc-BON GEPII 1.0 in vitro. d Schematic overviews of differently mutated and engineered GEPIIs. e Normalized EC₅₀ curves for K⁺ of GEPIIs in vitro (n = 4 for each construct). f K⁺ selectivity of lc-LysM GEPII 1.0 (n = 3 for each ion). FRET ratio signals are plotted against increasing concentrations of different cations