PLANT-SPECIFIC K+ TRANSPORTERS WITH DISTINCT PROPERTIES AND THEIR EMERGING ROLES IN ENDOMEMBRANE TRAFFICKING
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Plant growth, development and survival are dependent on the dynamic endomembrane system that modulates the delivery of various cargos to their destination or removal of components for degradation in time and in space. Yet the molecular determinants that regulate this dynamic and intricate machinery in plants are poorly understood. The multiplicity of <underline>C</underline>ation/<underline>H</underline><super>+</super> e<underline>X</underline>changers (CHX) genes in higher plants, but not in metazoans, raises questions about their transport activity and suggests roles characteristic to plant life. Five CHX proteins from <italic>Arabidopsis thaliana</italic> were implicated in pH homeostasis as each rescued growth of alkaline-sensitive yeast strains, though with distinct properties. Each CHX improved growth of K<super>+</super>-uptake deficient strain at different pHs. Moreover, CHX17, 18 and 19 conferred hygromycin B resistance when CHX20 could not. Although both CHX17 and CHX20 mediated K<super>+</super> uptake when expressed in <italic>Escherichia coli</italic>; their properties suggested differential modes of transport. CHX20, but not CHX17, acidified cytoplasmic pH and alkalinized vacuolar pH in yeast. CHX17 was more effective than CHX20 in reducing secretion of a vacuolar lumen protein in yeast. As these CHXs were localized to yeast endomembranes, results suggested that CHXs differentially modulate pH and K<super>+</super> homeostasis of intracellular compartments which affect protein sorting. CHX20-tagged with fluorescent protein was localized to ER, whereas CHX16, 17, 18 and 19 localized to prevacuolar compartments (PVC) and to plasma membrane (PM) <italic>in planta</italic>. Brefeldin A diminished PM-associated CHX17, whereas wortmannin caused formation of ring-like structures of CHX17-bound compartments. When full-length CHX17 (820 residues) was truncated, CHX17(1-472) partially restored yeast growth on alkaline medium. However, the truncated protein was localized to Golgi, suggesting a role of the C tail in sorting CHX17 from Golgi to PVC and PM. These results suggest that CHX17 is associated with a subset of endosomes that traffick among PVC, vacuole and PM in cells of whole plants. Together, the results support a model where CHX17 and its homologs modulate localized pH and K<super>+</super> environment of distinct endomembrane compartments and so affect membrane trafficking and cargo sorting through the endocytic and/or secretory pathways. CHXs are proposed to facilitate cell wall modifications as plants adapted to dry land.