To maintain an optimum cytoplasmic K+/Na+ ratio, cells employ three distinct strategies: 1) strict discrimination among alkali metal cations at the level of influx, 2) efficient efflux of toxic cations from cells, and 3) selective sequestration ofcations in organelles. Cation efflux and influx are mediated
in cells by systems with different substrate specificities and diverse mechanisms, e.g. ATPases, symporters, antiporters, and channels. Simple eukaryotic yeast Saccharomyces cerevisiae cells proved to be an excellent model for studying the transport properties and physiological function of alkali-metal-cation transporters, and the existence of mutant strains lacking their own transport systems provided an efficient tool for a molecular study of alkali-metal-cation tr
ansporters from higher eukaryotes upon their expression in yeast cells.