ALTERED NA-CEREVISIAE CELLS EXPRESSING THE BACTERIAL CATION ANTIPORTER NHAA( AND LI+ HOMEOSTASIS IN SACCHAROMYCES)

The bacterial Na+(Li+)/H+ antiporter NhaA has been expressed in the ye ast Saccharomyces cerevisiae. NhaA was present in both the plasma memb rane and internal membranes, and it conferred lithium but not sodium t olerance. In cells containing the yeast Ena1-4 (Na+, Li+) extrusion AT Pase, the extra lithium tolerance conferred by NhaA was dependent on a functional vacuolar H+ ATPase and correlated with an increase of lith ium in an intracellular pool which exhibited slow efflux of cations. I n yeast mutants without (Na+, Li+) ATPase, lithium tolerance conferred by NhaA was not dependent on a functional vacuolar H+ ATPase and corr elated with a decrease of intracellular lithium. NhaA was able to conf er sodium tolerance and to decrease intracellular sodium accumulation in a double mutant devoid of both plasma membrane (Na+, Li+) ATPase an d vacuolar H+ ATPase. These results indicate that the bacterial antipo rter NhaA expressed in yeast is functional at both the plasma membrane acid the vacuolar membrane. The phenotypes conferred by its expressio n depend on the functionality of plasma membrane (Na+, Li+) ATPase and vacuolar H+ ATPase.