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CELL BIOLOGY AND SIGNAL TRANSDUCTION

Regulation of NH₄⁺ Transport by Essential Cross Talk between AMT Monomers through the Carboxyl Tails^{1,[C],[W],[OA]}

Zentrum für Molekularbiologie der Pflanzen, Pflanzenphysiologie, Universität Tübingen, D–72076 Tuebingen, Germany

Ammonium transport across plant plasma membranes is facilitated by AMT/Rh-type ammonium transporters (AMTs), which also have homologs in most organisms. In the roots of the plant Arabidopsis (Arabidopsis thaliana), AMTs have been identified that function directly in the high-affinity NH₄⁺ acquisition from soil. Here, we show that AtAMT1;2 has a distinct role, as it is located in the plasma membrane of the root endodermis. AtAMT1;2 functions as a comparatively low-affinity NH₄⁺ transporter. Mutations at the highly conserved carboxyl terminus (C terminus) of AMTs, including one that mimics phosphorylation at a putative phosphorylation site, impair NH₄⁺ transport activity. Coexpressing these mutants along with wild-type AtAMT1;2 substantially reduced the activity of the wild-type transporter. A molecular model of AtAMT1;2 provides a plausible explanation for the dominant inhibition, as the C terminus of one monomer directly contacts the neighboring subunit. It is suggested that part of the cytoplasmic C terminus of a single monomer can gate the AMT trimer. This regulatory mechanism for rapid and efficient inactivation of NH₄⁺ transporters may apply to several AMT members to prevent excess influx of cytotoxic ammonium.

The author responsible for distribution of materials integral to the findings presented in this article in accordance with the policy described in the Instructions for Authors (www.plantphysiol.org) is: Uwe Ludewig (uwe.ludewig{at}zmbp.uni-tuebingen.de).

^[C] Some figures in this article are displayed in color online but in black and white in the print edition.

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www.plantphysiol.org/cgi/doi/10.1104/pp.106.094243

^* Corresponding author; e-mail uwe.ludewig{at}zmbp.uni-tuebingen.de; fax 49–7071–29–3287.

Received December 5, 2006; accepted February 22, 2007; published March 2, 2007.

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