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.