A Single-Pore Residue Renders the Arabidopsis Root Anion
Channel SLAH2 Highly Nitrate Selective |
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Authors: | Tobias Maierhofer Christof Lind Stefanie Hüttl S?nke Scherzer Melanie Papenfu? Judy Simon Khaled AS Al-Rasheid Peter Ache Heinz Rennenberg Rainer Hedrich Thomas D Müller Dietmar Geiger |
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Institution: | aUniversity of Würzburg, Institute for Molecular Plant Physiology and Biophysics, D-97082 Würzburg, Germany;bInstitute of Forest Sciences, Chair of Tree Physiology, University of Freiburg, 79110 Freiburg, Germany;cZoology Department, College of Science, King Saud University, Riyadh 11451, Saudi Arabia |
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Abstract: | In contrast to animal cells, plants use nitrate as a major source of nitrogen.
Following the uptake of nitrate, this major macronutrient is fed into the vasculature
for long-distance transport. The Arabidopsis thaliana shoot
expresses the anion channel SLOW ANION CHANNEL1 (SLAC1) and its homolog SLAC1
HOMOLOGOUS3 (SLAH3), which prefer nitrate as substrate but cannot exclude chloride
ions. By contrast, we identified SLAH2 as a nitrate-specific channel that is
impermeable for chloride. To understand the molecular basis for nitrate selection in
the SLAH2 channel, SLAC1 and SLAH2 were modeled to the structure of HiTehA, a
distantly related bacterial member. Structure-guided site-directed mutations
converted SLAC1 into a SLAH2-like nitrate-specific anion channel and vice versa. Our
findings indicate that two pore-occluding phenylalanines constrict the pore. The
selectivity filter of SLAC/SLAH anion channels is determined by the polarity of
pore-lining residues located on alpha helix 3. Changing the polar character of a
single amino acid side chain (Ser-228) to a nonpolar residue turned the
nitrate-selective SLAH2 into a chloride/nitrate-permeable anion channel. Thus, the
molecular basis of the anion specificity of SLAC/SLAH anion channels seems to be
determined by the presence and constellation of polar side chains that act in concert
with the two pore-occluding phenylalanines. |
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