ABSTRACT
ZIP transporters (ZRT, IRT-like proteins) are involved in the transport of iron (Fe), zinc (Zn) and other divalent metal cations. The expression of IRT3, a ZIP transporter, is higher in the Zn/cadmium (Cd) hyperaccumulator Arabidopsis halleri than is that of its ortholog in Arabidopsis thaliana, which implies a positive association of its expression with Zn accumulation in A. halleri. IRT3 genes from both A. halleri and A. thaliana functionally complemented the Zn uptake mutant Spzrt1 in Schizosaccharomyces pombe; and Zn uptake double mutant zrt1zrt2, Fe-uptake mutant fet3fet4 and conferred Zn and Fe uptake activity in Saccharomyces cerevisiae. By contrast, the manganese (Mn) uptake mutant smf1 phenotypes were not rescued. Insufficient Cd uptake for toxicity was found. Expression of IRT3-green fluorescent protein (GFP) fusion proteins in Arabidopsis root protoplasts indicated localization of both IRT3 proteins in the plasma membrane. Overexpressing AtIRT3 in A. thaliana led to increased accumulation of Zn in the shoot and Fe in the root of transgenic lines. Therefore, IRT3 functions as a Zn and Fe-uptake transporter in Arabidopsis.
Subject(s)
Arabidopsis Proteins/genetics , Arabidopsis/genetics , Cation Transport Proteins/genetics , Gene Expression , Genes, Plant , Iron/metabolism , Membrane Transport Proteins/genetics , Zinc/metabolism , Arabidopsis/metabolism , Arabidopsis Proteins/metabolism , Biological Transport/genetics , Cadmium/metabolism , Cation Transport Proteins/metabolism , Cell Membrane/genetics , Manganese/metabolismABSTRACT
Abstract Zinc is an essential micronutrient, and yet it can be toxic when present in excess. Zinc acquisition and distribution are dependent on tightly controlled transport of Zn(2+) ions. Schizosaccharomyces pombe represents a second eukaryotic model to study cellular metal homeostasis. In several ways its micronutrient metabolism is fundamentally different from Saccharomyces cerevisiae. We identified the first Zn(2+)-uptake system in S. pombe and named it SpZrt1. Knock-out strains for all three ZIP (Zrt, Irt-like protein) transporters in fission yeast were constructed. Only zrt1Delta cells were unable to grow at low Zn(2+) and showed reduced (65)Zn(2+) uptake. Elemental profiles revealed a strong decrease in zinc accumulation. Cd(2+) ions inhibited uptake but Fe(2+) or Mn(2+) did not. Both mRNA abundance and protein amount are tightly regulated. Zrt1 activity is rapidly shut down upon transfer of zinc-deficient cells to zinc-replete conditions. In cells lacking Zhf, a transporter mediating endoplasmic reticulum storage of zinc, this response is about 100-fold more sensitive. Thus, removal of excess of zinc from the cytosol is largely Zhf dependent. Moreover, cells deficient for both transporters are no longer able to adjust to changing external Zn(2+) concentrations. Optimal growth is restricted to a narrow range of Zn(2+) concentrations, illustrating the fine balance between micronutrient deficiency and toxicity.
