Alkali Metal Cation Transport and Homeostasis in Yeasts

Abstract

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.

Caesium accumulation in yeast and plants is selectively repressed by loss of the SNARE Sec22p/SEC22

“…The corresponding cytosolic K þ concentration is B63 mM. Furthermore, the specificity of Cs þ sequestration to the vacuole is supported by the consideration that even minor changes in the vacuolar K þ concentration of sec22D would substantially decrease the total K þ level (which has not been observed) provided that the cytosolic K þ homeostasis must be preserved to retain a wild-type-like growth and viability in the sec22D mutant 26 .…”

Osmoadaptation Strategy of the Most Halophilic Fungus, Wallemia ichthyophaga, Growing Optimally at Salinities above 15% NaCl

“…This powers all the secondarily active symporters and antiporters necessary for the maintenance of homeostasis of intracellular K ϩ and Na ϩ . Hence, the lower pH at higher salinity might occur due to higher demands for the proton motive force (37,38).…”

Wide-Ranging Effects of the Yeast Ptc1 Protein Phosphatase Acting Through the MAPK Kinase Mkk1

“…However, deletion of MKK2 had no effect. Because toxic cation tolerance of yeast cells strongly correlates with changes in the expression of ENA1 (Arino et al 2010), it is conceivable that partial restoration of normal ENA1 function could explain the improvement in Li + tolerance observed in the double mkk1 ptc1 mutant (Figure 2). In agreement with this notion, deletion of MKK2 neither increases the activity of the ENA1 promoter in ptc1 cells nor improves tolerance to LiCl.…”