Uptake and accumulation of Mn2+ and Sr2+ in Saccharomyces cerevisiae

Initial uptake of Mn2+ and Sr2+ in the yeast Saccharomyces cerevisiae was studied in order to investigate the selectivity of the divalent cation uptake system and the possible involvement of the plasma-membrane ATPase in this uptake. The initial uptake rates of the two ions were not significantly different. This ruled out a direct role of the plasma-membrane ATPase, since this ATPase is specific for Mn2+ compared to Sr2+. After 1 h uptake, Mn2+ had accumulated 10-times more than Sr2+. Influx of Mn2+ and Sr2+ remained unchanged during that time, however. The differences in accumulation level found for Mn2+ and Sr2+ could be ascribed to a greater efflux of Sr2+ as compared with Mn2+. Probably this greater efflux of Sr2+ was only apparent, since differential extraction of the yeast cells revealed that Mn2+ is more compartmentalised than Sr2+, giving rise to a lower relative cytoplasmic Mn2+ concentration.