The physiological effects of caesium (Cs) on living cells are poorly understood. Here, we examined the physiological role of Cs+ on the activity of the potassium transporters in E. coli. In the absence of potassium (K+), Kup-mediated Cs+ uptake partially supported cell growth, however, at a much lower rate than with sufficient K+. In K+-limited medium (0.1 mM), the presence of Cs+ (up to 25 mM) in the medium enhanced growth as much as control medium containing 1 mM K+. This effect depended on the maintenance of basal levels of intracellular K+ by other K+ uptake transporters. Higher amounts of K+ (1 mM) in the medium eliminated the positive effect of Cs+ on growth, and revealed the inhibitory effect of high Cs+ on the growth of wild-type E. coli. Cells lacking Kdp, TrkG and TrkH but expressing Kup grew less well when Cs+ was increased in the medium. A kdp mutant contained an increased ratio of Cs+/K+ in the presence of high Cs+ in the medium and consequently was strongly inhibited in growth. Taken together, under excess Cs+ conditions Kup-mediated Cs+ influx sustains cell growth, which is supported by intracellular K+ supplied by Kdp.
Membrane intrinsic transport systems play an important role in maintaining ion and pH homeostasis and forming the proton motive force in the cytoplasm and cell organelles. In most organisms, cation/proton antiporters (CPAs) mediate the exchange of K+, Na+ and Ca2+ for H+ across the membrane in response to a variety of environmental stimuli. The tertiary structure of the ion selective filter and the regulatory domains of Escherichia coli CPAs have been determined and a molecular mechanism of cation exchange has been proposed. Due to symbiogenesis, CPAs localized in mitochondria and chloroplasts of eukaryotic cells resemble prokaryotic CPAs. CPAs primarily contribute to keeping cytoplasmic Na+ concentrations low and controlling pH, which promotes the detoxification of electrophiles and formation of proton motive force across the membrane. CPAs in cyanobacteria and chloroplasts are regulators of photosynthesis and are essential for adaptation to high light or osmotic stress. CPAs in organellar membranes and in the plasma membrane also participate in various intracellular signal transduction pathways. This review discusses recent advances in our understanding of the role of CPAs in cyanobacteria and plant cells.
Arabidopsis thaliana contains five tandem-pore domain potassium channels, TPK1-TPK5 and the related one-pore domain potassium channel, KCO3. Although KCO3 is unlikely to be an active channel, it still has a physiological role in plant cells. TPK2 is most similar to KCO3 and both are localized to the tonoplast. However, their function remains poorly understood. Here, taking advantage of the similarities between TPK2 and KCO3, we evaluated Ca 2+ binding to the EF hands in TPK2, and the elements of KCO3 required for K + channel activity. Presence of both EFhand motifs in TPK2 resulted in Ca 2+ binding, but EF1 or EF2 alone failed to interact with Ca 2+ . The EF hands were not required for K + transport activity. EF1 contains two cysteines separated by two amino acids. Replacement of both cysteines with serines in TPK2 increased Ca 2+ binding. We generated a two-pore domain chimeric K + channel by replacing the missing pore region in KCO3 with a pore domain of TPK2. Alternatively, we generated two versions of simple one-pore domain K + channels by removal of an extra region from KCO3. The chimera and one of the simple onepore variants were functional channels. This strongly suggests that KCO3 is not a pseudogene and KCO3 retains components required for the formation of a functional K + channel and oligomerization. Our results contribute to our understanding of the structural properties required for K + channel activity.
Phototrophic bacteria face diurnal variations of environmental conditions such as light and osmolarity that affect their carbon metabolism and ability to generate organic compounds. The model cyanobacterium, Synechocystis sp. PCC 6803 forms a biofilm when it encounters extreme conditions like high salt stress, but the molecular mechanisms involved in perception of environmental changes that lead to biofilm formation are unknown. Here, we studied two two-component regulatory systems (TCSs) that contain diguanylate cyclases (DGCs), which produce the second messenger c-di-GMP, as potential components of the biofilm-inducing signaling pathway in Synechocystis.Analysis of single mutants provided evidence for involvement of the response regulators, Rre2 and Rre8 in biofilm formation. A bacterial two-hybrid assay showed that Rre2 and Rre8 each formed a TCS with a specific histidine kinase, Hik12 and Hik14, respectively. The in vitro assay showed that Rre2 had DGC activity regardless of its de/phosphorylation status, whereas Rre8 required phosphorylation for DGC activity. Hik14-Rre8 likely functioned as an inducible sensing system in response to environmental change. Biofilm assays with Synechocystis mutants suggested that pairs of hik12-rre2 and hik14-rre8 responded to high salinity-induced biofilm formation.Inactivation of hik12-rre2 and hik14-rre8 did not affect the performance of the light reactions of photosynthesis. These data suggest that Hik12-Rre2 and Hik14-Rre8 participate in biofilm formation in Synechocystis by regulating c-di-GMP production via the DGC activity of Rre2 and Rre8.
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