title: Knockouts of Physcomitrella patens CHX1 and CHX2 transporters reveal high complexity of potassium homeostasis.
abstract: This study aims to increase our understanding of the functions of CHX transporters in plant cells using the model plant Physcomitrella patens, in which four CHX genes have been identified, PpCHX1-PpCHX4. Two of these genes, PpCHX1 and PpCHX2, are expressed at approximately the same level as the PpACT5 gene, but the other two genes show an extremely low expression. PpCHX1 and PpCHX2 restored growth of Escherichia coli mutants on low K(+)-containing media, suggesting that they mediated K+ uptake that may be energized by symport with H+. In contrast, these genes suppressed the defect associated with the kha1 mutation in Saccharomyces cerevisiae, which suggests that they might mediate K+/H+ antiport. PpCHX1-green fluorescent protein (GFP) fusion protein transiently expressed in P. patens protoplasts co-localized with a Golgi marker. In similar experiments, the PpCHX2-GFP protein appeared to localize to tonoplast and plasma membrane. We constructed the ΔPpchx1 and ΔPpchx2 single mutant lines, and the ΔPpchx2 ΔPphak1 double mutant. Single mutant plants grew normally under all the conditions tested and exhibited normal K+ and Rb+ influxes; the ΔPpchx2 mutation did not increase the defect of ΔPphak1 plants. In long-term experiments, ΔPpchx2 plants showed slightly higher Rb+ retention than wild-type plants, which suggests that PpCHX2 mediates the transfer of Rb+ either from the vacuole to the cytosol or from the cytosol to the external medium in parallel with other transporters. The distinction between these two possibilities is technically difficult. We suggest that K+ transporters of several families are involved in the pH homeostasis of organelles by mediating either K+/H+ antiport or K(+)-H(+) symport.
authors.affiliation: Centro de Biotecnología y Genómica de Plantas, Universidad Politécnica de Madrid, Campus de Montegancedo, Carretera M-40, km 38, 28223 Pozuelo de Alarcón, Madrid, Spain.
authors.forename: Shady A
authors.initials: SA
authors.lastname: Mottaleb
authors.forename: Alonso
authors.initials: A
authors.lastname: Rodríguez-Navarro
authors.forename: Rosario
authors.initials: R
authors.lastname: Haro
mesh_terms: D000595:Amino Acid Sequence; D019068:Bryopsida / Q000235:genetics / Q000378:metabolism*; D027682:Cation Transport Proteins / Q000145:classification / Q000235:genetics / Q000378:metabolism*; D002462:Cell Membrane / Q000378:metabolism; D018506:Gene Expression Regulation, Plant; D055786:Gene Knockout Techniques; D005816:Genetic Complementation Test; D006056:Golgi Apparatus / Q000378:metabolism; D049452:Green Fluorescent Proteins / Q000235:genetics / Q000378:metabolism; D006706:Homeostasis; D017136:Ion Transport / Q000235:genetics; D007700:Kinetics; D008969:Molecular Sequence Data; D009154:Mutation; D010802:Phylogeny; D010940:Plant Proteins / Q000235:genetics / Q000378:metabolism*; D011188:Potassium / Q000378:metabolism*; D027961:Potassium-Hydrogen Antiporters / Q000235:genetics / Q000378:metabolism; D011523:Protoplasts / Q000378:metabolism; D020133:Reverse Transcriptase Polymerase Chain Reaction; D012413:Rubidium / Q000378:metabolism; D012441:Saccharomyces cerevisiae / Q000235:genetics / Q000254:growth & development / Q000378:metabolism; D029701:Saccharomyces cerevisiae Proteins / Q000235:genetics / Q000378:metabolism; D017386:Sequence Homology, Amino Acid
chemical_list: D027682:Cation Transport Proteins; C421963:KHA1 protein, S cerevisiae; D010940:Plant Proteins; D027961:Potassium-Hydrogen Antiporters; D029701:Saccharomyces cerevisiae Proteins; D049452:Green Fluorescent Proteins; D012413:Rubidium; D011188:Potassium
keywords: CHX transporters; Endomembranes; Potassium fluxes