The pH sensor of the plant K⁺-uptake channel KAT1 is built from a sensory cloud rather than from single key amino acids

The uptake of potassium ions (K⁺) accompanied by an acidification of the apoplasm is a prerequisite for stomatal opening. The acidification (approximately 2–2.5 pH units) is perceived by voltage-gated inward potassium channels (K_in) that then can open their pores with lower energy cost. The sensory units for extracellular pH in stomatal K_in channels are proposed to be histidines exposed to the apoplasm. However, in the Arabidopsis thaliana stomatal K_in channel KAT1, mutations in the unique histidine exposed to the solvent (His²⁶⁷) do not affect the pH dependency. We demonstrate in the present study that His²⁶⁷ of the KAT1 channel cannot sense pH changes since the neighbouring residue Phe²⁶⁶ shifts its pK_a to undetectable values through a cation–π interaction. Instead, we show that Glu²⁴⁰ placed in the extracellular loop between transmembrane segments S5 and S6 is involved in the extracellular acid activation mechanism. Based on structural models we propose that this region may serve as a molecular link between the pH- and the voltage-sensor. Like Glu²⁴⁰, several other titratable residues could contribute to the pH-sensor of KAT1, interact with each other and even connect such residues far away from the voltage-sensor with the gating machinery of the channel.