Identification of a crucial histidine involved in metal transport activity in the Arabidopsis cation/H+ exchanger CAX1.
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.
Plant Physiology Laboratories, United State Department of Agriculture/Agricultural Research Service Children's Nutrition Research Center, Department of Pediatrics, Baylor College of Medicine, Houston, Texas 77030, USA.
Toshiro
T
Shigaki
Bronwyn J
BJ
Barkla
Maria Cristina
MC
Miranda-Vergara
Jian
J
Zhao
Omar
O
Pantoja
Kendal D
KD
Hirschi
D000595:Amino Acid Sequence; D017920:Antiporters / Q000235:genetics* / Q000502:physiology*; D017360:Arabidopsis / Q000378:metabolism*; D001692:Biological Transport; D002104:Cadmium / Q000737:chemistry; D002118:Calcium / Q000378:metabolism; D002135:Calcium-Binding Proteins / Q000235:genetics* / Q000502:physiology*; D027682:Cation Transport Proteins / Q000737:chemistry / Q000235:genetics* / Q000502:physiology*; D002412:Cations; D002462:Cell Membrane / Q000378:metabolism; D017931:DNA Primers / Q000737:chemistry; D004305:Dose-Response Relationship, Drug; D005822:Genetic Vectors; D006639:Histidine / Q000737:chemistry*; D006863:Hydrogen-Ion Concentration; D007477:Ions; D007700:Kinetics; D008670:Metals / Q000737:chemistry; D008969:Molecular Sequence Data; D016297:Mutagenesis, Site-Directed; D009154:Mutation; D020836:Protein Structure, Quaternary; D011522:Protons; D012441:Saccharomyces cerevisiae / Q000378:metabolism; D017386:Sequence Homology, Amino Acid; D013997:Time Factors; D015032:Zinc / Q000737:chemistry
D017920:Antiporters; D002135:Calcium-Binding Proteins; D027682:Cation Transport Proteins; D002412:Cations; D017931:DNA Primers; D007477:Ions; D008670:Metals; D011522:Protons; C027614:calcium-hydrogen antiporters; D002104:Cadmium; D006639:Histidine; D015032:Zinc; D002118:Calcium