Identification of a Crucial Histidine Involved in Metal Transport Activity in the Arabidopsis Cation/H+ Exchanger CAX1

Toshiro Shigaki,Bronwyn J Barkla,Maria Cristina Miranda-Vergara,Jian Zhao,Omar Pantoja,Kendal D Hirschi

doi:10.1074/jbc.m503610200

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.

Highlights

In plants, yeast, and bacteria, cation/H؉ exchangers (CAXs) have been shown to translocate Ca2؉ and other metal ions utilizing the H؉ gradient
The aim of this study was to characterize the role of histidine residues in the translocation of metal ions in the Arabidopsis CAX1 transporter
Like other CAXs from a variety of different organisms, analysis of the 463-amino acid sequence suggests that the protein has 11 TMs (Fig. 1A)

Summary

Introduction

Yeast, and bacteria, cation/H؉ exchangers (CAXs) have been shown to translocate Ca2؉ and other metal ions utilizing the H؉ gradient. 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. The term CAX was first used to describe the cation exchangers CAX1 and CAX2 in Arabidopsis thaliana [5] Histidyl residues in many transporters have been shown to play critical roles in transporter function [11] This is the case for membrane proteins involved in Hϩ translocation or those that exhibit Hϩ sensing. All the characterized CAX transporters are about 400 amino acids long and are predicted to have 11 membrane-spanning domains with several conserved histidine residues.. All the characterized CAX transporters are about 400 amino acids long and are predicted to have 11 membrane-spanning domains with several conserved histidine residues. no methodical analysis has been undertaken on the functional role of these amino acids in CAX transporters

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Conclusion