title: Ammonium ion transport by the AMT/Rh homolog TaAMT1;1 is stimulated by acidic pH.
abstract: 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.
authors.affiliation: Nordic Centre for Water Imbalance Related Disorders, Department of Cellular and Molecular Medicine, Blegdamsvej 3C, University of Copenhagen, Copenhagen, 2200 N, Denmark.
authors.forename: Rikke
authors.initials: R
authors.lastname: Søgaard
authors.forename: Magnus
authors.initials: M
authors.lastname: Alsterfjord
authors.forename: Nanna
authors.initials: N
authors.lastname: Macaulay
authors.forename: Thomas
authors.initials: T
authors.lastname: Zeuthen