Plant Physiology Preview
Published on October 8, 2004; 10.1104/pp.104.046292
Received May 28, 2004
Returned for revision July 19, 2004
Accepted July 23, 2004
Arabidopsis HMA2, a Divalent Heavy Metal-Transporting PIB-Type ATPase, Is Involved in Cytoplasmic Zn2+ Homeostasis
Elif Eren and José M. Argüello *
Department of Chemistry and Biochemistry, Worcester Polytechnic Institute, Worcester, Massachusetts 01609
* Corresponding author; email: arguello{at}wpi.edu.
PIB-type ATPases transport heavy metal ions (Cu+, Cu2+, Zn2+, Cd2+, Co2+, etc.) across biological membranes. Several members of this subfamily are present in plants. Higher plants are the only eukaryotes where putative Zn2+-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 Zn2+-dependent ATPase that is also activated by Cd2+ and, to a lesser extent, by other divalent heavy metals (Pb2+, Ni2+, Cu2+, and Co2+). The enzyme forms an acid-stable phosphorylated intermediate and is inhibited by vanadate. HMA2 interacts with Zn2+ and Cd2+ with high affinity (Zn2+ K1/2 = 0.11 ± 0.03 µM and Cd2+ K1/2 = 0.031 ± 0.007 µM). However, its activity is dependent on millimolar concentrations of Cys in the assay media. Zn2+ 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 Zn2+ accumulation in plant tissues. hma2 mutant plants also accumulate Cd2+ when exposed to this metal. These results suggest that HMA2 is responsible for Zn2+ efflux from the cells and therefore is required for maintaining low cytoplasmic Zn2+ levels and normal Zn2+ homeostasis.
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