Ammonium ion transport by the AMT/Rh homologue LeAMT1;1.
AMT (ammonium transporter)/Rh (Rhesus) ammonium transporters/channels are identified in all domains of life and fulfil contrasting functions related either to ammonium acquisition or excretion. Based on functional and crystallographic high-resolution structural data, it was recently proposed that the bacterial AmtB (ammonium transporter B) is a gas channel for NH3 [Khademi, O'Connell, III, Remis, Robles-Colmenares, Miercke and Stroud (2004) Science 305, 1587-1594; Zheng, Kostrewa, Berneche, Winkler and Li (2004) Proc. Natl. Acad. Sci. U.S.A. 101, 17090-17095]. Key residues, proposed to be crucial for NH3 conduction, and the hydrophobic, but obstructed, pore were conserved in a homology model of LeAMT1;1 from tomato. Transport by LeAMT1;1 was affected by mutations of residues that were predicted to constitute the aromatic recruitment site for NH4+ at the external pore entrance. Despite the structural similarities, LeAMT1;1 was shown to transport only the ion; each transported 14C-methylammonium molecule carried a single positive elementary charge. Similarly, NH4+ (or H+/NH3) was transported, but NH3 conduction was excluded. It is concluded that related proteins and a similar molecular architecture can apparently support contrasting transport mechanisms.
Mayer
Maria
M
Zentrum für Molekularbiologie der Pflanzen (ZMBP), Pflanzenphysiologie, Universität Tübingen, Auf der Morgenstelle 1, 72076 Tübingen, Germany.
Dynowski
Marek
M
Ludewig
Uwe
U
D000818:Animals; D027682:Cation Transport Proteins / Q000235:genetics / Q000378:metabolism*; D005260:Female; D008744:Methylamines / Q000378:metabolism; D009154:Mutation; D009865:Oocytes / Q000378:metabolism; D010940:Plant Proteins / Q000235:genetics / Q000378:metabolism*; D000644:Quaternary Ammonium Compounds / Q000378:metabolism*; D014981:Xenopus
D027682:Cation Transport Proteins; D008744:Methylamines; D010940:Plant Proteins; D000644:Quaternary Ammonium Compounds; C088952:ammonium transporters, plant; C027451:methylamine