Effect of ethidium bromide and DEAE-dextran on divalent cation accumulation in yeast. Evidence for an ion-selective extrusion pump for divalent cations.

The larger accumulation of Mn2+ than of Sr2+ in Saccharomyces cerevisiae is ascribed to the operation of a specific extrusion pump, presumably a Ca2+ pump, which has a higher affinity for Sr2+ than for Mn2+. The differences in accumulation levels of Mn2+ and Sr2+ attained after prolonged incubation are completely abolished in cells of which the plasmamembrane has been permeabilized with the polybase DEAE-dextran under isotonic conditions. In the permeabilized cells Sr2+ and Mn2+ accumulation levels are attained as for Mn2+ in intact cells. It is suggested that the accumulation of divalent cations into the permeabilized cells mainly represents their accumulation into the vacuoles. Also the cationic dye ethidium abolishes the differences in Mn2+ and Sr2+ accumulation. The dye increases the accumulation of Sr2+ but decreases that of Mn2+ somewhat. It cannot be distinguished yet whether its action is due to an impairment of the efflux pump or to an increase in the permeability of the plasmamembrane facilitating the divalent cations to be accumulated into the vacuoles. Ethidium does not affect the initial rates of divalent cation uptake into the vacuoles, but it effectively reduces the ultimate accumulation of the divalent cations in the DEAE-dextran permeabilized cells, possibly by competing with the divalent cations for intravacuolar binding sites. Similar results are obtained for the accumulation of Ca2+. It is concluded that the efflux pump enables the yeast cell to regulate accumulation levels of the various divalent cations to different extents.