title: Plant-specific cation/H+ exchanger 17 and its homologs are endomembrane K+ transporters with roles in protein sorting.
abstract: The complexity of intracellular compartments in eukaryotic cells evolved to provide distinct environments to regulate processes necessary for cell proliferation and survival. A large family of predicted cation/proton exchangers (CHX), represented by 28 genes in Arabidopsis thaliana, are associated with diverse endomembrane compartments and tissues in plants, although their roles are poorly understood. We expressed a phylogenetically related cluster of CHX genes, encoded by CHX15-CHX20, in yeast and bacterial cells engineered to lack multiple cation-handling mechanisms. Of these, CHX16-CHX20 were implicated in pH homeostasis because their expression rescued the alkaline pH-sensitive growth phenotype of the host yeast strain. A smaller subset, CHX17-CHX19, also conferred tolerance to hygromycin B. Further differences were observed in K(+)- and low pH-dependent growth phenotypes. Although CHX17 did not alter cytoplasmic or vacuolar pH in yeast, CHX20 elicited acidification and alkalization of the cytosol and vacuole, respectively. Using heterologous expression in Escherichia coli strains lacking K(+) uptake systems, we provide evidence for K(+) ((86)Rb) transport mediated by CHX17 and CHX20. Finally, we show that CHX17 and CHX20 affected protein sorting as measured by carboxypeptidase Y secretion in yeast mutants grown at alkaline pH. In plant cells, CHX20-RFP co-localized with an endoplasmic reticulum marker, whereas RFP-tagged CHX17-CHX19 co-localized with prevacuolar compartment and endosome markers. Together, these results suggest that in response to environmental cues, multiple CHX transporters differentially modulate K(+) and pH homeostasis of distinct intracellular compartments, which alter membrane trafficking events likely to be critical for adaptation and survival.
authors.affiliation: Department of Cell Biology and Molecular Genetics and the Maryland Agricultural Experiment Station, University of Maryland, College Park, Maryland 20742, USA.
authors.forename: Salil
authors.initials: S
authors.lastname: Chanroj
authors.forename: Yongxian
authors.initials: Y
authors.lastname: Lu
authors.forename: Senthilkumar
authors.initials: S
authors.lastname: Padmanaban
authors.forename: Kei
authors.initials: K
authors.lastname: Nanatani
authors.forename: Nobuyuki
authors.initials: N
authors.lastname: Uozumi
authors.forename: Rajini
authors.initials: R
authors.lastname: Rao
authors.forename: Heven
authors.initials: H
authors.lastname: Sze
mesh_terms: D000220:Adaptation, Biological / Q000502:physiology; D000900:Anti-Bacterial Agents / Q000494:pharmacology; D017920:Antiporters / Q000235:genetics / Q000378:metabolism*; D017360:Arabidopsis / Q000235:genetics / Q000378:metabolism*; D029681:Arabidopsis Proteins / Q000235:genetics / Q000378:metabolism*; D004351:Drug Resistance / Q000502:physiology; D011992:Endosomes / Q000235:genetics / Q000378:metabolism*; D004926:Escherichia coli / Q000235:genetics / Q000378:metabolism; D006706:Homeostasis / Q000502:physiology; D006863:Hydrogen-Ion Concentration; D006921:Hygromycin B / Q000378:metabolism / Q000494:pharmacology; D011188:Potassium / Q000378:metabolism*; D011994:Recombinant Proteins / Q000235:genetics / Q000378:metabolism; D012441:Saccharomyces cerevisiae / Q000235:genetics / Q000378:metabolism; D017923:Sodium-Hydrogen Exchangers / Q000235:genetics / Q000378:metabolism*
chemical_list: D000900:Anti-Bacterial Agents; D017920:Antiporters; D029681:Arabidopsis Proteins; C493886:CHX17 protein, Arabidopsis; C521945:CHX20 protein, Arabidopsis; D011994:Recombinant Proteins; D017923:Sodium-Hydrogen Exchangers; D006921:Hygromycin B; D011188:Potassium