MOLECULAR-BASIS OF PLANT-SPECIFIC ACID ACTIVATION OF K+ UPTAKE CHANNELS

During stomatal opening potassium uptake into guard cells and K+ chann el activation is tightly coupled to proton extrusion. The pH sensor of the K+ uptake channel in these motor cells has, however, not yet been identified, Electrophysiological investigations on the voltage-gated, inward rectifying K+ channel in guard cell protoplasts from Solanum t uberosum (KST1), and the kst1 gene product expressed in Xenopus oocyte s revealed that pH dependence is an intrinsic property of the channel protein, Whereas extracellular acidification resulted in a shift of th e voltage-dependence toward less negative voltages, the single-channel conductance was pH-insensitive. Mutational analysis allowed us to rel ate this acid activation to both extracellular histidines in KST1. One histidine is located within the linker between the transmembrane heli ces S3 and S4 (H160), and the other within the putative pore-forming r egion P between S5 and S6 (H271). When both histidines were substitute d by alanines the double mutant completely lost its pH sensitivity. Am ong the single mutants, replacement of the pore histidine, which is hi ghly conserved in plant K+ channels, increased or even inverted the pH sensitivity of KST1. From our molecular and biophysical analyses we c onclude that both extracellular sites are part of the pH sensor in pla nt K+ uptake channels.