Susceptibility of the guard-cell K(+)-uptake channel KST1 to Zn(2+) requires histidine residues in the S3-S4 linker and in the channel pore

Potassium channels are inhibited by several mono- and divalent cations. To identify sites involved in the interaction between K(+) channels and cationic effectors, we expressed the potato (Solanum tuberosum L.) guard-cell K(+)-uptake channel KST1 in Xenopus oocytes. This channel was reversibly blocked by extracellular Zn(2+) in the micromolar range. In the presence of this heavy metal, steady-state currents were reduced in a pH-dependent but voltage-independent manner. Since Zn(2+)-inhibition was less effective at elevated external proton concentrations, we generated alanine mutants with respect to both extracellular histidines in KST1. Whereas substitution of the pore histidine H271 resulted in a reduced blockade by Zn(2+), the channel mutant KST1-H160A in the S3-S4 linker lost most of its Zn(2+ )sensitivity. Since both histidines alter the susceptibility of KST1 to Zn(2+), the block may predominantly result from these two sites. We thus conclude that the S3-S4 linker is involved in the formation of the outer pore.