Sodium and sulfate ion transport in yeast vacuoles

The intra-luminal acidic pH of endomembrane organelles is established by a proton pump, vacuolar H+-ATPase (V-ATPase), in combination with other ion transporter(s). The proton gradient (DeltapH) established in yeast vacuolar vesicles decreased and reached the lower value after the addition of alkaline cations including Na+. As expected, the uptake of Na-22(+) was coupled with DeltapH generated by V-ATPase. Disruption of NHX1 or NHA1, encoding known Na+/H+ antiporters, did not result in the loss of Na-22(+) uptake or the alkaline cation-dependent DeltapH decrease. Upon the addition of sulfate ions, the V-ATPase-dependent DeltapH in the vacuolar vesicles increased, but the membrane potential (DeltaPsi) decreased. Consistent with this observation, radioactive sulfate was transported into the vesicles with a K-m value of 0.07 mM. The transport activity was unaffected upon disruption of the putative genes coding for homologues of plasma membrane sulfate transporters. These results indicate that the vacuoles exhibit unique Na+/H+ antiport and sulfate transport, which regulate the luminal pH and ion homeostasis in yeast.