Ammonium ion transport by the AMT/Rh homolog TaAMT1;1 is stimulated by acidic pH

It is unclear how ammonia is transported by proteins from the Amt/Mep/Rh superfamily. We investigated this for the ammonium transporter TaAMT1;1 from wheat expressed in Xenopus oocytes by two-electrode voltage clamp and radio-labeled uptakes. Inward currents were activated by NH 4 + or methylammonium ions (MeA + ). Importantly, currents increased fivefold when the external pH was decreased from 7.4 to 5.5; this type of pH dependence is unique and is a strong indication of NH 4 + or MeA + transport. This was confirmed by the close correlation between the uptake of radio-labeled MeA + and MeA + -induced currents. Homology models of members of the Amt/Mep/Rh superfamily exhibited major divergences in their cytoplasmic regions. A point mutation in this region of TaAMT1;1 abolished the pH sensitivity and decreased the apparent affinities for NH 4 + and MeA + . We suggest a model where NH 4 + is transported as NH 3 and H + via separate pathways but the latter two recombine before leaving the protein.