Abstract.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 Zn2+ 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 Zn2+-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 Zn2+, the channel mutant KST1-H160A in the S3-S4 linker lost most of its Zn2+ sensitivity. Since both histidines alter the susceptibility of KST1 to Zn2+, 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.