Extended Data Fig. 5. Divalent cation dependence of Lettuce fluorescence.

a, Fluorescence of Lettuce-DFHBI-1T as a function of Mg²⁺ concentration (in the presence of 150 mM K⁺). The half-maximal activity (K_1/2) is at 1.4 mM Mg²⁺ (mean ± s.d.; n = 3). b, Fluorescence activation of Lettuce–DFHBI-1T in 150 mM K⁺ alone, or supplemented with 10 mM Mg²⁺, Ca²⁺, or Mn²⁺ (mean ± s.d., n = 3). * denotes P= 0.0002 (two-sided t-test). No significancy (P= 0.251) between K⁺ + Mg²⁺ and K⁺ + Mn²⁺ (two-sided t-test). c, Electrostatic potential of Lettuce mapped on its molecular surface. Color ramp from 0 to +20 k_BT (white to red). d, Green sphere shows Mg²⁺ ion bound in the P1.1 loop. e, Molecular surface of Lettuce (same color scheme as c) showing the P1.2 loop at its center. f, Green sphere shows Mg²⁺ ion bound in the P1.2 loop.