Arabidopsis HMA2, a Divalent Heavy Metal-Transporting PIB-Type ATPase, Is Involved in Cytoplasmic Zn^2+ Homeostasis

PIB-type ATPases transport heavy metal ions (Cu+, Cu^2+, Zn^2+, Cd^2+, Co^2+, etc.) across biological membranes. Several members of this subfamily are present in plants. Higher plants are the only eukaryotes where putative Zn^2+-ATPases have been identified. We have cloned HMA2, a PIB-ATPase present in Arabidopsis (Arabidopsis thaliana), and functionally characterized this enzyme after heterologous expression in yeast (Saccharomyces cerevisiae). HMA2 is a Zn^2+-dependent ATPase that is also activated by Cd^2+ and, to a lesser extent, by other divalent heavy metals (Pb^2+, Ni^2+, Cu^2+, and Co^2+). The enzyme forms an acid-stable phosphorylated intermediate and is inhibited by vanadate. HMA2 interacts with Zn^2+ and Cd^2+ with high affinity (Zn^2+ K1/2 = 0.11 (plus-minus) 0.03 (mu)M and Cd2+ K1/2 = 0.031 (plus-minus) 0.007 (mu)M). However, its activity is dependent on millimolar concentrations of Cys in the assay media. Zn^2+ transport determinations indicate that the enzyme drives the outward transport of metals from the cell cytoplasm. Analysis of HMA2 mRNA suggests that the enzyme is present in all plant organs and transcript levels do not change in plants exposed to various metals. Removal of HMA2 full-length transcript results in Zn^2+ accumulation in plant tissues. hma2 mutant plants also accumulate Cd^2+ when exposed to this metal. These results suggest that HMA2 is responsible for Zn^2+ efflux from the cells and therefore is required for maintaining low cytoplasmic Zn^2+ levels and normal Zn^2+ homeostasis.