SUMMARY: The maintenance of appropriate intracellular concentrations of alkali metal cations, principally K ⁺ and Na ⁺ , is of utmost importance for living cells, since they determine cell volume, intracellular pH, and potential across the plasma membrane, among other important cellular parameters. Yeasts have developed a number of strategies to adapt to large variations in the concentrations of these cations in the environment, basically by controlling transport processes. Plasma membrane high-affinity K ⁺ transporters allow intracellular accumulation of this cation even when it is scarce in the environment. Exposure to high concentrations of Na ⁺ can be tolerated due to the existence of an Na ⁺ , K ⁺ -ATPase and an Na ⁺ , K ⁺ /H ⁺ -antiporter, which contribute to the potassium balance as well. Cations can also be sequestered through various antiporters into intracellular organelles, such as the vacuole. Although some uncertainties still persist, the nature of the major structural components responsible for alkali metal cation fluxes across yeast membranes has been defined within the last 20 years. In contrast, the regulatory components and their interactions are, in many cases, still unclear. Conserved signaling pathways (e.g., calcineurin and HOG) are known to participate in the regulation of influx and efflux processes at the plasma membrane level, even though the molecular details are obscure. Similarly, very little is known about the regulation of organellar transport and homeostasis of alkali metal cations. The aim of this review is to provide a comprehensive and up-to-date vision of the mechanisms responsible for alkali metal cation transport and their regulation in the model yeast Saccharomyces cerevisiae and to establish, when possible, comparisons with other yeasts and higher plants.

We investigated the impact of the deletions of genes from the final steps in the biosynthesis of ergosterol (ERG6, ERG2, ERG3, ERG5, ERG4) on the physiological function of the Saccharomyces cerevisiae plasma membrane by a combination of biological tests and the diS-C₃(3) fluorescence assay. Most of the erg mutants were more sensitive than the wild type to salt stress or cationic drugs, their susceptibilities were proportional to the hyperpolarization of their plasma membranes. The different sterol composition of the plasma membrane played an important role in the short-term and long-term processes that accompanied the exposure of erg strains to a hyperosmotic stress (effect on cell size, pH homeostasis and survival of yeasts), as well as in the resistance of cells to antifungal drugs. The pleiotropic drug-sensitive phenotypes of erg strains were, to a large extent, a result of the reduced efficiency of the Pdr5 efflux pump, which was shown to be more sensitive to the sterol content of the plasma membrane than Snq2p. In summary, the erg4Δ and erg6Δ mutants exhibited the most compromised phenotypes. As Erg6p is not involved in the cholesterol biosynthetic pathway, it may become a target for a new generation of antifungal drugs.

Summary: Maintenance of intracellular K⁺ homeostasis is one of the crucial requisites for the survival of yeast cells. In Saccharomyces cerevisiae, the high K⁺ content corresponds to a steady state between simultaneous influx and efflux across the plasma membrane. One of the transporters formerly believed to extrude K⁺ from the yeast cells (besides Ena1‐4p and Nha1p) was named Kha1p and presumed as a putative plasma membrane K⁺/H⁺ antiporter. We prepared kha1 and tok1‐kha1 deletion strains in the B31 and MAB 2d background. Both the strains contain the ena1‐4 and nha1 deletions; that means they lack the main active sodium and potassium efflux systems. MAB 2d has additional trk1 and trk2 deletions, i.e. is impaired in active K⁺ uptake as well. We performed a large physiological study with these strains to specify the phenotype of kha1 deletion. In our experiments, no difference in K⁺ content or efflux was observed in strains lacking the KHA1 gene compared with control strains. Two main phenotype manifestations of the kha1 deletion were growth defect on high external pH and hygromycin sensitivity. The correlation between these phenotypes and the kha1 deletion was confirmed by plasmid complementation. Fluorescence microscopy of green fluorescent protein (GFP)‐tagged Kha1p showed that this antiporter is localized preferentially intracellularly (in contrast to the plasma membrane Na⁺/H⁺ antiporter Nha1p). Based on these findings, Kha1p is probably not localized in plasma membrane and does not mediate efflux of alkali metal cations from cells, but is important for the regulation of intracelular cation homeostasis and optimal pH control, similarly as the Nhx1p.

To maintain optimal intracellular concentrations of alkali–metal–cations, yeast cells use a series of influx and efflux systems. Nonconventional yeas