title: PvAMT1;1, a highly selective ammonium transporter that functions as H+/NH4(+) symporter.
abstract: One of the main forms of nitrogen assimilated by microorganisms and plants is ammonium, despite its toxicity at low millimolar concentrations. Ammonium absorption has been demonstrated to be carried out by highly selective plasma membrane-located transporters of the AMT/MEP/Rh family and characterized by the presence of a well conserved hydrophobic pore through which ammonia is proposed to move. However, uncertainties exist regarding the exact chemical species transported by these membrane proteins, which can be in the form of either hydrophobic ammonia or charged ammonium. Here, we present the characterization of PvAMT1;1 from the common bean and demonstrate that it mediates the high affinity (micromolar), rapidly saturating (1 mM) electrogenic transport of ammonium. Activity of the transporter is enhanced by low extracellular pH, and associated with this acidic pH stimulation are changes in the reversal potential and cytoplasm acidification, indicating that PvAMT1;1 functions as an H(+)/NH(4)(+) symporter. Mutation analysis of a unique histidine present in PvAMT1;1 (H125R) leads to the stimulation of ammonium transport by decreasing the K(m) value by half and by increasing the V(max) 3-fold, without affecting the pH dependence of the symporter. In contrast, mutation of the first conserved histidine within the channel modifies the properties of PvAMT1;1, increasing its K(m) and V(max) values and transforming it into a pH-independent mechanism.
authors.affiliation: Departamento de Biología Molecular de Plantas, Instituto de Biotecnología, Universidad Nacional Autónoma de México, A. P. 510-3, Colonia Miraval, Cuernavaca, Morelos 62250, México.
authors.forename: Carlos
authors.initials: C
authors.lastname: Ortiz-Ramirez
authors.forename: Silvia I
authors.initials: SI
authors.lastname: Mora
authors.forename: Jorge
authors.initials: J
authors.lastname: Trejo
authors.forename: Omar
authors.initials: O
authors.lastname: Pantoja
mesh_terms: D027682:Cation Transport Proteins / Q000378:metabolism*; D003593:Cytoplasm; D055724:Electrophysiological Phenomena; D006863:Hydrogen-Ion Concentration; D009154:Mutation; D027805:Phaseolus / Q000378:metabolism*; D010940:Plant Proteins / Q000378:metabolism*; D000644:Quaternary Ammonium Compounds / Q000494:pharmacology; D027981:Symporters / Q000378:metabolism*
chemical_list: D027682:Cation Transport Proteins; D010940:Plant Proteins; D000644:Quaternary Ammonium Compounds; D027981:Symporters; C088952:ammonium transporters, plant