Cloning and characterization of a novel Mg²⁺/H⁺ exchanger

Cellular functions require adequate homeostasis of several divalent metal cations, including Mg²⁺ and Zn²⁺. Mg²⁺ the most abundant free divalent cytoplasmic cation, is essential for many enzymatic reactions, while Zn²⁺ is a structural constituent of various enzymes. Multicellular organisms have to balance not only the intake of Mg²⁺ and Zn²⁺, but also the distribution of these ions to various organs. To date, genes encoding Mg²⁺ transport proteins have not been cloned from any multicellular organism. We report here the cloning and characterization of an Arabidopsis thaliana transporter, designated AtMHX, which is localized in the vacuolar membrane and functions as an electrogenic exchanger of protons with Mg²⁺ and Zn²⁺ ions. Functional homologs of AtMHX have not been cloned from any organism. Ectopic overexpression of AtMHX in transgenic tobacco plants render them sensitive to growth on media containing elevated levels of Mg²⁺ or Zn²⁺, but does not affect the total amounts of these minerals in shoots of the transgenic plants. AtMHX mRNA is mainly found at the vascular cylinder, and a large proportion of the mRNA is localized in close association with the xylem tracheary elements. This localization suggests that AtMHX may control the partitioning of Mg²⁺ and Zn²⁺ between the various plant organs.