title: A novel vanadium transporter of the Nramp family expressed at the vacuole of vanadium-accumulating cells of the ascidian Ascidia sydneiensis samea.
abstract: BACKGROUND
  Vanadium is an essential transition metal in biological systems. Several key proteins related to vanadium accumulation and its physiological function have been isolated, but no vanadium ion transporter has yet been identified.
  
  
  METHODS
  We identified and cloned a member of the Nramp/DCT family of membrane metal transporters (AsNramp) from the ascidian Ascidia sydneiensis samea, which can accumulate extremely high levels of vanadium in the vacuoles of a type of blood cell called signet ring cells (also called vanadocytes). We performed immunological and biochemical experiments to examine its expression and transport function.
  
  
  RESULTS
  Western blotting analysis showed that AsNramp was localized at the vacuolar membrane of vanadocytes. Using the Xenopus oocyte expression system, we showed that AsNramp transported VO(2+) into the oocyte as pH-dependent manner above pH 6, while no significant activity was observed below pH 6. Kinetic parameters (K(m) and V(max)) of AsNramp-mediated VO(2+) transport at pH 8.5 were 90nM and 9.1pmol/oocyte/h, respectively. A rat homolog, DCT1, did not transport VO(2+) under the same conditions. Excess Fe(2+), Cu(2+), Mn(2+), or Zn(2+) inhibited the transport of VO(2+). AsNramp was revealed to be a novel VO(2+)/H(+) antiporter, and we propose that AsNramp mediates vanadium accumulation coupled with the electrochemical gradient generated by vacuolar H(+)-ATPase in vanadocytes.
  
  
  CONCLUSIONS
  This is the first report of identification and functional analysis on a membrane transporter for vanadium ions.
authors.affiliation: Molecular Physiology Laboratory, Department of Biological Science, Graduate School of Science, Hiroshima University, 1-3-1 Kagamiyama, Higashi-Hiroshima 739-8526, Japan. ueki@hiroshima-u.ac.jp
authors.forename: Tatsuya
authors.initials: T
authors.lastname: Ueki
authors.forename: Nobuaki
authors.initials: N
authors.lastname: Furuno
authors.forename: Hitoshi
authors.initials: H
authors.lastname: Michibata
mesh_terms: D000595:Amino Acid Sequence; D000818:Animals; D001692:Biological Transport; D027682:Cation Transport Proteins / Q000032:analysis / Q000235:genetics / Q000378:metabolism*; D003001:Cloning, Molecular; D015870:Gene Expression; D026901:Membrane Transport Proteins / Q000032:analysis / Q000235:genetics / Q000378:metabolism*; D008969:Molecular Sequence Data; D051381:Rats; D016415:Sequence Alignment; D014561:Urochordata / Q000235:genetics / Q000378:metabolism*; D014617:Vacuoles / Q000378:metabolism*; D014639:Vanadium / Q000378:metabolism*; D014982:Xenopus laevis; D000099173:Natural Resistance-associated Macrophage Protein 1; D000099236:Solute Carrier Family 11, Member 2
chemical_list: D027682:Cation Transport Proteins; D026901:Membrane Transport Proteins; D014639:Vanadium; D000099173:Natural Resistance-associated Macrophage Protein 1; D000099236:Solute Carrier Family 11, Member 2