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

Acid-induced potassium uptake through Kt channels is a prerequisite for sto matal opening. Our previous studies identified a pore histidine as a major component of the acid activation mechanism of the potato guard cell K+ chan nel 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+ cha nnel 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 Vim of the KST1 mu tant, whereas in the mutant channel KAT1-D265N an acid-induced decrease in the maximum conductance g(max) indicated the presence of a proton block. In contrast to the wild type KST1, the S4-mutant chan nel KST1-R181Q exhibite d an activation upon alcalization of the extracellular solution. From our e lectrophysiological studies on channel mutants with respect to the pore his tidine as well as the aspartate, we conclude that the common proton-support ed shift in the voltage dependence of KST1 and KAT1 is based on distinct mo lecular elements.