Outer pore residues control the H+ and K+ sensitivity of the Arabidopsis potassium channel AKT3

The Arabidopsis phloem channel AKT3 is the founder of a subfamily of shaker-like plant potassium channels characterized by weak rectification, Ca2+ block, proton inhibition, and, as shown in this study, K+ sensitivity. In contrast to inward-rectifying, acid-activated K+ channels of the KAT1 family, extracellular acidification decreases AKT3 currents at the macroscopic and single-channel levels. Here, we show that two distinct sites within the outer mouth of the K+-conducting pore provide the molecular basis for the pH sensitivity of this phloem channel. After generation of mutant channels and functional expression in Xenopus oocytes, we identified the His residue His-228, which is proximal to the K+ selectivity filter (GYGD) and the distal Ser residue Ser-271, to be involved in proton susceptibility. Mutations of these sites, H228D and S271E, drastically reduced the H+ and K+ sensitivity of AKT3. Although in K+-free bath solutions outward K+ currents were abolished completely in wild-type AKT3, S271E as well as the AKT3-HDSE double mutant still mediated K+ efflux. We conclude that the pH- and K+-dependent properties of the AKT3 channel involve residues in the outer mouth of the pore. Both properties, H+ and K+ sensitivity, allow the fine-tuning of the phloem channel and thus seem to represent important elements in the control of membrane potential and sugar loading.