Different Transport Mechanisms in Plant and Human AMT/Rh-type Ammonium Transporters

The conserved family of AMT/Rh proteins facilitates ammonium transport across animal, plant, and microbial membranes. A bacterial homologue, AmtB, forms a channel-like structure and appears to function as an NH(3) gas channel. To evaluate the function of eukaryotic homologues, the human RhCG glycoprotein and the tomato plant ammonium transporter LeAMT1;2 were expressed and compared in Xenopus oocytes and yeast. RhCG mediated the electroneutral transport of methylammonium (MeA), which saturated with K(m) = 3.8 mM at pH(o) 7.5. Uptake was strongly favored by increasing the pH(o) and was inhibited by ammonium. Ammonium induced rapid cytosolic alkalinization in RhCG-expressing oocytes. Additionally, RhCG expression was associated with an alkali-cation conductance, which was not significantly permeable to NH(4) (+) and was apparently uncoupled from the ammonium transport. In contrast, expression of the homologous LeAMT1;2 induced pH(o)-independent MeA(+) uptake and specific NH(4) (+) and MeA(+) currents that were distinct from endogenous currents. The different mechanisms of transport, including the RhCG-associated alkali-cation conductance, were verified by heterologous expression in appropriate yeast strains. Thus, homologous AMT/Rh-type proteins function in a distinct manner; while LeAMT1;2 carries specifically NH(4) (+), or cotransports NH(3)/H(+), RhCG mediates electroneutral NH(3) transport.