Feedback Inhibition of Ammonium Uptake by a Phospho-Dependent Allosteric Mechanism in Arabidopsis

• Published in: The Plant cell Vol. 21; no. 11; pp. 3610 - 3622
• Main Authors: Lanquar, Viviane, Loqué, Dominique, Hörmann, Friederike, Yuan, Lixing, Bohner, Anne, Engelsberger, Wolfgang R, Lalonde, Sylvie, Schulze, Waltraud X, von Wirén, Nicolaus, Frommer, Wolf B
• Format: Journal Article
• Language: English
• Published: United States American Society of Plant Biologists 01.11.2009
• Subjects: Allosteric Regulation - physiology; Amino Acid Sequence - physiology; Ammonium; Antiserum; Arabidopsis - genetics; Arabidopsis - metabolism; Autoreceptors - metabolism; Cation Transport Proteins - chemistry; Cation Transport Proteins - metabolism; Cell membranes; Conserved Sequence - physiology; Feedback, Physiological - physiology; Homeostasis - physiology; Inactivation; Nitrogen; Phosphorus - metabolism; Phosphorylation; Phosphotransferases - metabolism; Plant cells; Plant Proteins - chemistry; Plant Proteins - metabolism; Plant roots; Plant Roots - metabolism; Plants; Protein Structure, Tertiary - physiology; Protein Subunits - metabolism; Proteins; Quaternary ammonium compounds; Quaternary Ammonium Compounds - metabolism; Rhizosphere; Roots; Seedlings; Threonine - metabolism; Time Factors; Yeasts
• ISSN: 1040-4651; 1532-298X
• DOI: 10.1105/tpc.109.068593

The acquisition of nutrients requires tight regulation to ensure optimal supply while preventing accumulation to toxic levels. Ammonium transporter/methylamine permease/rhesus (AMT/Mep/Rh) transporters are responsible for ammonium acquisition in bacteria, fungi, and plants. The ammonium transporter AMT1;1 from Arabidopsis thaliana uses a novel regulatory mechanism requiring the productive interaction between a trimer of subunits for function. Allosteric regulation is mediated by a cytosolic C-terminal trans-activation domain, which carries a conserved Thr (T460) in a critical position in the hinge region of the C terminus. When expressed in yeast, mutation of T460 leads to inactivation of the trimeric complex. This study shows that phosphorylation of T460 is triggered by ammonium in a time- and concentration-dependent manner. Neither Gln nor L-methionine sulfoximine-induced ammonium accumulation were effective in inducing phosphorylation, suggesting that roots use either the ammonium transporter itself or another extracellular sensor to measure ammonium concentrations in the rhizosphere. Phosphorylation of T460 in response to an increase in external ammonium correlates with inhibition of ammonium uptake into Arabidopsis roots. Thus, phosphorylation appears to function in a feedback loop restricting ammonium uptake. This novel autoregulatory mechanism is capable of tuning uptake capacity over a wide range of supply levels using an extracellular sensory system, potentially mediated by a transceptor (i.e., transporter and receptor).