Distinct molecular bases for pH sensitivity of the guard cell K+ channels KST1 and KAT1

Acid-induced potassium uptake through K+ channels is a prerequisite for stomatal opening. Our previous studies identified a pore histidine as a major component of the acid activation mechanism of the potato guard cell K+ channel KST1 (1). Although this histidine is highly conserved among all plant K+ uptake channels cloned so far, the pH-dependent gating of the Arabidopsis thaliana guard cell K+ channel KAT1 was not affected by mutations of this histidine. In both channels, KST1 and KAT1, aspartate mutants in the K+ channel consensus sequence GYGD adjacent to the histidine (KST1-D269N and KAT1-D265N) were inhibited by a rise in the extracellular proton concentration. pH changes affected the half-maximal activation voltage V(1)/(2) of the KST1 mutant, whereas in the mutant channel KAT1-D265N an acid-induced decrease in the maximum conductance gmax indicated the presence of a proton block. In contrast to the wild type KST1, the S4-mutant channel KST1-R181Q exhibited an activation upon alcalization of the extracellular solution. From our electrophysiological studies on channel mutants with respect to the pore histidine as well as the aspartate, we conclude that the common proton-supported shift in the voltage dependence of KST1 and KAT1 is based on distinct molecular elements.