Submitted to: Journal of Biological Chemistry
Publication Type: Peer reviewed journal
Publication Acceptance Date: 7/29/2005
Publication Date: 8/26/2005
Citation: Shigake, T., Barkla, B.J., Miranda-Vergara, M.C., Zhao, J., Pantoja, O., Hirschi, K. 2005. Identification of a crucial histidine involved in metal transport activity in the Arabidopsis cation/h+ exchanger cax1. Journal of Biological Chemistry. 280(34):30136-30142. Interpretive Summary: The ability to alter transporter function is an important goal. If we can manipulate the transporter we can bring healthy nutrients into the plant and exclude potentially toxic compounds. Here we have altered a transporter to be more exclusive in the compounds it can transport. These experiments were all performed in test tubes. The next step will be to see if this is true at the whole plant level.
Technical Abstract: In plants, yeast, and bacteria, cation/H+ exchangers (CAXs) have been shown to translocate Ca2+ and other metal ions utilizing the H+ gradient. The best characterized of these related transporters is the plant vacuolar localized CAX1. We have used site-directed mutagenesis to assess the impact of altering the seven histidine residues to alanine within Arabidopsis CAX1. The mutants were expressed in a Saccharomyces cerevisiae strain that is sensitive to Ca2+ and other metals. By utilizing a yeast growth assay, the H338A mutant was the only mutation that appeared to alter Ca2+ transport activity. The CAX1 His338 residue is conserved among various CAX transporters and may be located within a filter for cation selection. We proceeded to mutate His338 to every other amino acid residue and utilized yeast growth assays to estimate the transport properties of the 19 CAX mutants. Expression of 16 of these His338 mutants could not rescue any of the metal sensitivities. However, expression of H338N, H338Q, and H338K allowed for some growth on media containing Ca2+. Most interestingly, H338N exhibited increased tolerance to Cd2+ and Zn2+. Endomembrane fractions from yeast cells were used to measure directly the transport of H338N. Although the H338N mutant demonstrated 25% of the wild type Ca2+/H+ transport, it showed an increase in transport for both Cd2+ and Zn2+ reflected in a decrease in the Km for these substrates. This study provides insights into the CAX cation filter and novel mechanisms by which metals may be partitioned across membranes.