title: The Arabidopsis vacuolar malate channel is a member of the ALMT family.
abstract: In plants, malate is a central metabolite and fulfills a large number of functions. Vacuolar malate may reach very high concentrations and fluctuate rapidly, whereas cytosolic malate is kept at a constant level allowing optimal metabolism. Recently, a vacuolar malate transporter (Arabidopsis thaliana tonoplast dicarboxylate transporter, AttDT) was identified that did not correspond to the well-characterized vacuolar malate channel. We therefore hypothesized that a member of the aluminum-activated malate transporter (ALMT) gene family could code for a vacuolar malate channel. Using GFP fusion constructs, we could show that AtALMT9 (A. thaliana ALMT9) is targeted to the vacuole. Promoter-GUS fusion constructs demonstrated that this gene is expressed in all organs, but is cell-type specific as GUS activity in leaves was detected nearly exclusively in mesophyll cells. Patch-clamp analysis of an Atalmt9 T-DNA insertion mutant exhibited strongly reduced vacuolar malate channel activity. In order to functionally characterize AtALMT9 as a malate channel, we heterologously expressed this gene in tobacco and in oocytes. Overexpression of AtALMT9-GFP in Nicotiana benthamiana leaves strongly enhanced the malate current densities across the mesophyll tonoplasts. Functional expression of AtALMT9 in Xenopus oocytes induced anion currents, which were clearly distinguishable from endogenous oocyte currents. Our results demonstrate that AtALMT9 is a vacuolar malate channel. Deletion mutants for AtALMT9 exhibit only slightly reduced malate content in mesophyll protoplasts and no visible phenotype, indicating that AttDT and the residual malate channel activity are sufficient to sustain the transport activity necessary to regulate the cytosolic malate homeostasis.
authors.affiliation: Institute for Plant Biology, University of Zürich, CH-8008 Zürich, Switzerland.
authors.forename: Peter
authors.initials: P
authors.lastname: Kovermann
authors.forename: Stefan
authors.initials: S
authors.lastname: Meyer
authors.forename: Stefan
authors.initials: S
authors.lastname: Hörtensteiner
authors.forename: Cristiana
authors.initials: C
authors.lastname: Picco
authors.forename: Joachim
authors.initials: J
authors.lastname: Scholz-Starke
authors.forename: Silvia
authors.initials: S
authors.lastname: Ravera
authors.forename: Youngsook
authors.initials: Y
authors.lastname: Lee
authors.forename: Enrico
authors.initials: E
authors.lastname: Martinoia
mesh_terms: D000535:Aluminum / Q000378:metabolism / Q000494:pharmacology; D000818:Animals; D017360:Arabidopsis / Q000235:genetics / Q000378:metabolism*; D029681:Arabidopsis Proteins / Q000235:genetics / Q000378:metabolism* / Q000502:physiology; D001692:Biological Transport / Q000187:drug effects; D029382:Dicarboxylic Acid Transporters / Q000235:genetics / Q000378:metabolism / Q000502:physiology; D018506:Gene Expression Regulation, Plant; D005966:Glucuronidase / Q000235:genetics / Q000378:metabolism; D049452:Green Fluorescent Proteins / Q000235:genetics / Q000378:metabolism; D008293:Malates / Q000378:metabolism*; D008564:Membrane Potentials / Q000502:physiology; D009154:Mutation; D009865:Oocytes / Q000378:metabolism / Q000502:physiology; D027361:Organic Anion Transporters / Q000235:genetics / Q000378:metabolism* / Q000502:physiology; D018408:Patch-Clamp Techniques; D030821:Plants, Genetically Modified; D011993:Recombinant Fusion Proteins / Q000235:genetics / Q000378:metabolism; D014026:Nicotiana / Q000235:genetics / Q000378:metabolism; D014617:Vacuoles / Q000378:metabolism*; D014981:Xenopus
chemical_list: D029681:Arabidopsis Proteins; D029382:Dicarboxylic Acid Transporters; D008293:Malates; D027361:Organic Anion Transporters; D011993:Recombinant Fusion Proteins; C501257:malate transporter protein, Arabidopsis; D049452:Green Fluorescent Proteins; C030298:malic acid; D000535:Aluminum; D005966:Glucuronidase