Plant Cell Advance Online Publication
Published on July 18, 2002; 10.1105/tpc.003244
Received March 20, 2002
Accepted May 6, 2002
Outer Pore Residues Control the H+ and K+ Sensitivity of the
Arabidopsis Potassium Channel AKT3
Dietmar Geiger 1, Dirk Becker 1, Benoit Lacombe 1, and Rainer Hedrich 1*
1
Julius-von-Sachs-Institute, Molecular Plant Physiology and Biophysics, Julius-von-Sachs-Platz
2, D-97082 Würzburg, Germany
* To whom correspondence should be addressed. E-mail: hedrich{at}botanik.uni-wuerzburg.de.
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.
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