To maintain optimal intracellular concentrations of alkali-metal-cations, yeast cells use a series of influx and efflux systems. Nonconventional yeast species have at least three different types of efficient transporters that ensure potassium uptake and accumulation in cells. Most of them have Trk uniporters and Hak K(+)-H(+) symporters and a few yeast species also have the rare K(+) (Na(+))-uptake ATPase Acu. To eliminate surplus potassium or toxic sodium cations, various yeast species use highly conserved Nha Na(+) (K(+))/H(+) antiporters and Na(+) (K(+))-efflux Ena ATPases. The potassium-specific yeast Tok1 channel is also highly conserved among various yeast species and its activity is important for the regulation of plasma membrane potential.
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