Ion transporters in the root nodule of Medicago truncatula: potassium transporters

The transporter proteins were mistargeted and partly depleted from plasma membrane of mature infected cells, this phenomenon may contribute to the potassium loss by symbiosomes during their development and senescence.

Comparative Functional Analysis of Class II Potassium Transporters, SvHKT2;1, SvHKT2;2, and HvHKT2;1, on Ionic Transport and Salt Tolerance in Transgenic Arabidopsis

Class II high-affinity potassium transporters (HKT2s) mediate Na+–K+ cotransport and Na+/K+ homeostasis under K+-starved or saline conditions. Their functions have been studied in yeast and X. laevis oocytes; however, little is known about their respective properties in plant cells. In this study, we characterized the Na+ and K+ transport properties of SvHKT2;1, SvHKT2;2 and HvHKT2;1 in Arabidopsis under different ionic conditions. The differences were detected in shoot K+ accumulation and root K+ uptake under salt stress conditions, K+ accumulation in roots and phloem sap under K+-starved conditions, and shoot and root Na+ accumulation under K+-starved conditions among the HKT2s transgenic lines and WT plants. These results indicate the diverse ionic transport properties of these HKT2s in plant cells, which could not be detected using yeast or X. laevis oocytes. Furthermore, Arabidopsis expressing HKT2s showed reduced salt tolerance, while over-expression of HvHKT2;1 in barley, which has the ability to sequestrate Na+, showed enhanced salt tolerance by accumulating Na+ in the shoots. These results suggest that the coordinated enhancement of Na+ accumulation and sequestration mechanisms in shoots could be a promising strategy to confer salt tolerance to glycophytes.

Small Molecule Channels Harness Membrane Potential to Concentrate Potassium in trk1Δtrk2Δ Yeast

ABSTRACTMany protein ion channels harness membrane potential to move ions in opposition to their chemical gradient. Deficiencies of such proteins cause several human diseases, including cystic fibrosis, Bartter Syndrome Type II, and proximal renal tubular acidosis. Using yeast as a readily manipulated eukaryotic model system, we asked whether, in the context of a deficiency of such protein ion channel function in vivo, small molecule channels could similarly harness membrane potential to concentrate ions. In yeast, Trk potassium transporters use membrane potential to move potassium ions from a compartment of relatively low concentration outside cells (∼15mM) to one of >10 times higher concentration inside (150-500mM). trk1Δtrk2Δ yeast are missing these potassium transporters and thus cannot concentrate potassium or grow in standard media. Here we show that potassium permeable, but not potassium selective, small molecule ion channels formed by the natural product amphotericin B can harness membrane potential to concentrate potassium in trk1Δtrk2Δ cells and thereby restore growth. This finding expands the list of potential human channelopathies that might be addressed by a molecular prosthetics approach.

The KupA and KupB Proteins of Lactococcus lactis IL1403 Are Novel c-di-AMP Receptor Proteins Responsible for Potassium Uptake

ABSTRACT Cyclic di-AMP (c-di-AMP) is a second messenger involved in diverse metabolic processes, including osmolyte uptake, cell wall homeostasis, and antibiotic and heat resistance. In Lactococcus lactis, a lactic acid bacterium which is used in the dairy industry and as a cell factory in biotechnological processes, the only reported interaction partners of c-di-AMP are the pyruvate carboxylase and BusR, the transcription regulator of the busAB operon for glycine betaine uptake. However, recent studies uncovered a major role of c-di-AMP in the control of potassium homeostasis, and potassium is the signal that triggers c-di-AMP synthesis. In this study, we have identified KupA and KupB, which belong to the Kup/HAK/KT family, as novel c-di-AMP binding proteins. Both proteins are high-affinity potassium transporters, and their transport activities are inhibited by binding of c-di-AMP. Thus, in addition to the well-studied Ktr/Trk potassium channels, KupA and KupB represent a second class of potassium transporters that are subject to inhibition by c-di-AMP. IMPORTANCE Potassium is an essential ion in every living cell. Even though potassium is the most abundant cation in cells, its accumulation can be toxic. Therefore, the level of potassium has to be tightly controlled. In many Gram-positive bacteria, the second messenger cyclic di-AMP plays a key role in the control of potassium homeostasis by binding to potassium transporters and regulatory proteins and RNA molecules. In the lactic acid bacterium Lactococcus lactis, none of these conserved c-di-AMP-responsive molecules are present. In this study, we demonstrate that the KupA and KupB proteins of L. lactis IL1403 are high-affinity potassium transporters and that their transport activity is inhibited by the second messenger c-di-AMP.