A novel vanadium transporter of the Nramp family expressed at the vacuole of vanadium-accumulating cells of the ascidian Ascidia sydneiensis samea

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 VO²⁺ 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 VO²⁺ transport at pH 8.5 were 90 nM and 9.1 pmol/oocyte/h, respectively. A rat homolog, DCT1, did not transport VO²⁺ under the same conditions. Excess Fe²⁺, Cu²⁺, Mn²⁺, or Zn²⁺ inhibited the transport of VO²⁺. AsNramp was revealed to be a novel VO²⁺/H⁺ antiporter, and we propose that AsNramp mediates vanadium accumulation coupled with the electrochemical gradient generated by vacuolar H⁺-ATPase in vanadocytes.

General Significance

This is the first report of identification and functional analysis on a membrane transporter for vanadium ions.