Uniport of NH4+by the Root Hair Plasma Membrane Ammonium Transporter LeAMT1;1

• Published in: The Journal of biological chemistry Vol. 277; no. 16; pp. 13548 - 13555
• Main Authors: Ludewig, Uwe, von Wirén, Nico, Frommer, Wolf B.
• Format: Journal Article
• Language: English
• Published: United States Elsevier Inc 19.04.2002
• Subjects: active transport; Ammonia; Ammonia - metabolism; ammonium compounds; Animals; Biological Transport; Carrier Proteins; Carrier Proteins - chemistry; Carrier Proteins - physiology; Cation Transport Proteins; Cell Membrane; Cell Membrane - metabolism; chemistry; Dose-Response Relationship, Drug; electric current; Electrophysiology; Female; gene expression; gene transfer; Hydrogen-Ion Concentration; Kinetics; Lycopersicon esculentum - metabolism; metabolism; nutrient transport; oocytes; Oocytes - metabolism; physiology; Plant Proteins; Plant Roots; Plant Roots - metabolism; plasma membrane; Plasmids; Plasmids - metabolism; Potassium; Potassium - metabolism; Protons; Quaternary Ammonium Compounds; Quaternary Ammonium Compounds - metabolism; root hairs; Sodium; Sodium - metabolism; Solanum lycopersicum; Solanum lycopersicum var. lycopersicum; Xenopus; Xenopus laevis
• ISSN: 0021-9258; 1083-351X
• DOI: 10.1074/jbc.M200739200

The transport of ammonium/ammonia is a key process for the acquisition and metabolism of nitrogen. Ammonium transport is mediated by the AMT/MEP/Rh family of membrane proteins which are found in microorganisms, plants, and animals, including the Rhesus blood group antigens in humans. Although ammonium transporters from all kingdoms have been functionally expressed and partially characterized, the transport mechanism, as well as the identity of the true substrate (NH4+or NH3) remains unclear. Here we describe the functional expression and characterization of LeAMT1;1, a root hairammonium transporter from tomato (Lycopersicon esculentum) in Xenopus oocytes. Micromolar concentrations of external ammonium were found to induce concentration- and voltage-dependent inward currents in oocytes injected with LeAMT1;1 cRNA, but not in water-injected control oocytes. The NH4+-induced currents were more than 3-fold larger than methylammonium currents and were not subject to inhibition by Na+ or K+. The voltage dependence of the affinity of LeAMT1;1 toward its substrate strongly suggests that charged NH4+, rather than NH3, is the true transport substrate. Furthermore, ammonium transport was independent of the external proton concentration between pH 5.5 and pH 8.5. LeAMT1;1 is concluded to mediate potential-driven NH4+uptake and retrieval depending on root membrane potential and NH4+concentration gradient.