title: Characterization of three novel members of the Arabidopsis thaliana equilibrative nucleoside transporter (ENT) family.
abstract: Research on metabolism of nucleotides and their derivatives has gained increasing interest in the recent past. This includes de novo synthesis, analysis of salvage pathways, breakdown and transport of nucleotides, nucleosides and nucleobases. To perform a further step towards the analysis of nucleoside transport in Arabidopsis, we incubated leaf discs with various radioactively labelled nucleosides. Leaf cells imported labelled nucleosides and incorporated these compounds into RNA, but not into DNA. Furthermore, we report on the biochemical properties of three so far uncharacterized members of the Arabidopsis ENT (equilibrative nucleoside transporter) family (AtENT4, AtENT6 and AtENT7). After heterologous expression in yeast, all three proteins exhibited broad substrate specificity and transported the purine nucleosides adenosine and guanosine, as well as the pyrimidine nucleosides cytidine and uridine. The apparent K(m) values were in the range 3-94 microM, and transport was inhibited most strongly by deoxynucleosides, and to a smaller extent by nucleobases. Typical inhibitors of mammalian ENT proteins, such as dilazep and NBMPR (nitrobenzylmercaptopurine ribonucleoside, also known as nitrobenzylthioinosine) surprisingly exerted almost no effect on Arabidopsis ENT proteins. Transport mediated by the AtENT isoforms differed in pH-dependency, e.g. AtENT7 was not affected by changes in pH, AtENT3, 4 and 6 exhibited a less pronounced pH-dependency, and AtENT1 activity was clearly pH-dependent. Using a GFP (green fluorescent protein)-fusion protein transiently expressed in tobacco leaf protoplasts, a localization of AtENT6 in the plant plasma membrane has been revealed.
authors.affiliation: Universität Kaiserslautern, Pflanzenphysiologie, Postfach 3049, D-67653 Kaiserslautern, Germany.
authors.forename: Alexandra
authors.initials: A
authors.lastname: Wormit
authors.forename: Michaela
authors.initials: M
authors.lastname: Traub
authors.forename: Martin
authors.initials: M
authors.lastname: Flörchinger
authors.forename: H Ekkehard
authors.initials: HE
authors.lastname: Neuhaus
authors.forename: Torsten
authors.initials: T
authors.lastname: Möhlmann
mesh_terms: D017360:Arabidopsis / Q000378:metabolism*; D029681:Arabidopsis Proteins / Q000502:physiology*; D001692:Biological Transport / Q000187:drug effects / Q000502:physiology; D002258:Carbonyl Cyanide m-Chlorophenyl Hydrazone / Q000494:pharmacology; D033705:Equilibrative Nucleoside Transport Proteins / Q000037:antagonists & inhibitors / Q000502:physiology*; D007476:Ionophores / Q000494:pharmacology; D007700:Kinetics; D018515:Plant Leaves / Q000378:metabolism; D011684:Purine Nucleosides / Q000378:metabolism; D011741:Pyrimidine Nucleosides / Q000378:metabolism; D011993:Recombinant Fusion Proteins / Q000502:physiology; D013347:Subcellular Fractions / Q000378:metabolism; D013379:Substrate Specificity; D014665:Vasodilator Agents / Q000494:pharmacology
chemical_list: D029681:Arabidopsis Proteins; D033705:Equilibrative Nucleoside Transport Proteins; D007476:Ionophores; D011684:Purine Nucleosides; D011741:Pyrimidine Nucleosides; D011993:Recombinant Fusion Proteins; D014665:Vasodilator Agents; D002258:Carbonyl Cyanide m-Chlorophenyl Hydrazone