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. Abbreviations: AMT, ammonium transporter; AmtB, ammonium transporter B; AQP1, aquaporin 1; BCECF, 2′,7′-bis-(2-carboxyethyl)-5(6)-carboxyfluorescein; CLC, chloride channel; GFP, green fluorescent protein; MeA, methylammonium; pHi, intracellular pH; pHo, extracellular pH; Rh, Rhesus; RhCG, Rh C glycoprotein; rmsd, root mean square deviation