title: The potassium transporters HAK2 and HAK3 localize to endomembranes in Physcomitrella patens. HAK2 is required in some stress conditions.
abstract: The function of HAK transporters in high-affinity K+ uptake in plants is well established; this study aims to demonstrate that some transporters of the same family play important roles in endomembranes. The PpHAK2-PpHAK4 genes of Physcomitrella patens encode three transporters of high sequence similarity. Quantitative PCR showed that PpHAK2 and PpHAK3 transcripts are expressed at approximately the same level as the PpACT5 gene, while the expression of PpHAK4 seems to be restricted to specific conditions that have not been determined. KHA1 is an endomembrane K+/H+ antiporter of Saccharomyces cerevisiae, and the expression of the PpHAK2 cDNA, but not that of PpHAK3, suppressed the defect of a kha1 mutant. Transient expression of the PpHAK2-green fluorescent protein (GFP) and PpHAK3-GFP fusion proteins in P. patens protoplasts localized to the endoplasmic reticulum and Golgi complex, respectively. To determine the function of PpHAK2 and PpHAK3 in planta, we constructed ΔPphak2 and ΔPphak2 ΔPphak3 plants. ΔPphak2 plants were normal under all of the conditions tested except under K+ starvation or at acidic pH in the presence of acetic acid, whereupon they die. The defect observed under K+ starvation was suppressed by the presence of Na+. We propose that PpHAK2 may encode either a K(+)-H(+) symporter or a K+/H+ antiporter that mediates the transfer of H+ from the endoplasmic reticulum lumen to the cytosol. PpHAK2 may be a model of the second function of HAK transporters in plant cells. The disruption of the PpHAK3 gene in ΔPphak2 plants showed no effect.
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. rosario.haro@upm.es
authors.forename: Rosario
authors.initials: R
authors.lastname: Haro
authors.forename: Ana
authors.initials: A
authors.lastname: Fraile-Escanciano
authors.forename: Pablo
authors.initials: P
authors.lastname: González-Melendi
authors.forename: Alonso
authors.initials: A
authors.lastname: Rodríguez-Navarro
mesh_terms: D000595:Amino Acid Sequence; D019068:Bryopsida / Q000235:genetics / Q000378:metabolism* / Q000648:ultrastructure; D027682:Cation Transport Proteins / Q000235:genetics / Q000378:metabolism*; D003600:Cytosol / Q000378:metabolism; D004721:Endoplasmic Reticulum / Q000378:metabolism; D018506:Gene Expression Regulation, Plant; D006056:Golgi Apparatus / Q000378:metabolism; D049452:Green Fluorescent Proteins / Q000235:genetics / Q000378:metabolism; D007425:Intracellular Membranes / Q000378:metabolism; D008854:Microscopy, Electron; D008969:Molecular Sequence Data; D009154:Mutation; 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; D017386:Sequence Homology, Amino Acid; D013312:Stress, Physiological / Q000235:genetics; D027981:Symporters / Q000235:genetics / Q000378:metabolism
chemical_list: D027682:Cation Transport Proteins; D010940:Plant Proteins; D027961:Potassium-Hydrogen Antiporters; D027981:Symporters; D049452:Green Fluorescent Proteins; D011188:Potassium
keywords: Endomembranes; HAK transporters; Potassium fluxes