The S1-S2 linker determines the distinct pH sensitivity between ZmK2.1 and KAT1

Efficient stomatal opening requires activation of KAT-type K(+) channels, which mediate K(+) influx into guard cells. Most KAT-type channels are functionally facilitated by extracellular acidification. However, despite sequence and structural homologies, the maize counterpart of Arabidopsis KAT1 (ZmK2.1) is resistant to pH activation. To understand the structural determinant that results in the differential pH activation of these counterparts, we analysed chimeric channels and channels with point mutations for ZmK2.1 and its closest Arabidopsis homologue KAT1. Exchange of the S1-S2 linkers altered the pH sensitivity between the two channels, suggesting that the S1-S2 linker is essentially involved in the pH sensitivity. The effects of D92 mutation within the linker motif together with substitution of the first half of the linker largely resemble the effects of substitution of the complete linker. Topological modelling predicts that one of the two cysteines located on the outer face section of the S5 domain may serve as a potential titratable group that interacts with the S1-S2 linker. The difference between ZmK2.1 and KAT1 is predicted to be the result of the distance of the stabilized linkers from the titratable group. In KAT1, residue K85 within the linker forms a hydrogen bond with C211 that enables the pH activation; conversely, the linker of ZmK2.1 is distantly located and thus does not interact with the equivalent titration group (C208). Thus, in addition to the known structural contributors to the proton activation of KAT channels, we have uncovered a previously unidentified component that is strongly involved in this complex proton activation network.