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Outer Pore Residues Control the H⁺ and K⁺ Sensitivity of the Arabidopsis Potassium Channel AKT3

Dietmar Geiger, Dirk Becker, Benoit Lacombe and Rainer Hedrich
The Plant Cell
Vol. 14, No. 8 (Aug., 2002), pp. 1859-1868
Stable URL: http://www.jstor.org/stable/3871680
Page Count: 10
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Since scans are not currently available to screen readers, please contact JSTOR User Support for access. We'll provide a PDF copy for your screen reader.
Outer Pore Residues Control the H⁺ and K⁺ Sensitivity of the Arabidopsis Potassium Channel AKT3
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Abstract

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 ${\rm K}^{+}\text{-conducting}$ 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.

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