title: Differential sodium and potassium transport selectivities of the rice OsHKT2;1 and OsHKT2;2 transporters in plant cells.
abstract: Na(+) and K(+) homeostasis are crucial for plant growth and development. Two HKT transporter/channel classes have been characterized that mediate either Na(+) transport or Na(+) and K(+) transport when expressed in Xenopus laevis oocytes and yeast. However, the Na(+)/K(+) selectivities of the K(+)-permeable HKT transporters have not yet been studied in plant cells. One study expressing 5' untranslated region-modified HKT constructs in yeast has questioned the relevance of cation selectivities found in heterologous systems for selectivity predictions in plant cells. Therefore, here we analyze two highly homologous rice (Oryza sativa) HKT transporters in plant cells, OsHKT2;1 and OsHKT2;2, that show differential K(+) permeabilities in heterologous systems. Upon stable expression in cultured tobacco (Nicotiana tabacum) Bright-Yellow 2 cells, OsHKT2;1 mediated Na(+) uptake, but little Rb(+) uptake, consistent with earlier studies and new findings presented here in oocytes. In contrast, OsHKT2;2 mediated Na(+)-K(+) cotransport in plant cells such that extracellular K(+) stimulated OsHKT2;2-mediated Na(+) influx and vice versa. Furthermore, at millimolar Na(+) concentrations, OsHKT2;2 mediated Na(+) influx into plant cells without adding extracellular K(+). This study shows that the Na(+)/K(+) selectivities of these HKT transporters in plant cells coincide closely with the selectivities in oocytes and yeast. In addition, the presence of external K(+) and Ca(2+) down-regulated OsHKT2;1-mediated Na(+) influx in two plant systems, Bright-Yellow 2 cells and intact rice roots, and also in Xenopus oocytes. Moreover, OsHKT transporter selectivities in plant cells are shown to depend on the imposed cationic conditions, supporting the model that HKT transporters are multi-ion pores.
authors.affiliation: Division of Biological Sciences, Cell and Developmental Biology Section, and Center for Molecular Genetics, University of California, San Diego, La Jolla, California 92093-0116, USA.
authors.forename: Xuan
authors.initials: X
authors.lastname: Yao
authors.forename: Tomoaki
authors.initials: T
authors.lastname: Horie
authors.forename: Shaowu
authors.initials: S
authors.lastname: Xue
authors.forename: Ho-Yin
authors.initials: HY
authors.lastname: Leung
authors.forename: Maki
authors.initials: M
authors.lastname: Katsuhara
authors.forename: Dennis E
authors.initials: DE
authors.lastname: Brodsky
authors.forename: Yan
authors.initials: Y
authors.lastname: Wu
authors.forename: Julian I
authors.initials: JI
authors.lastname: Schroeder
mesh_terms: D000818:Animals; D001692:Biological Transport; D027682:Cation Transport Proteins / Q000145:classification / Q000235:genetics / Q000378:metabolism*; D002460:Cell Line; D018506:Gene Expression Regulation, Plant / Q000502:physiology; D009865:Oocytes; D012275:Oryza / Q000235:genetics / Q000378:metabolism*; D010940:Plant Proteins / Q000145:classification / Q000235:genetics / Q000378:metabolism*; D011188:Potassium / Q000378:metabolism*; D012964:Sodium / Q000378:metabolism*; D014026:Nicotiana / Q000166:cytology / Q000235:genetics / Q000378:metabolism; D014981:Xenopus
chemical_list: D027682:Cation Transport Proteins; C434985:HKT2 protein, Oryza sativa; D010940:Plant Proteins; D012964:Sodium; D011188:Potassium