Growth at High pH and Sodium and Potassium Tolerance in Media above the Cytoplasmic pH Depend on ENA ATPases in Ustilago maydis

Potassium and Na+ effluxes across the plasma membrane are crucial processes for the ionic homeostasis of cells. In fungal cells, these effluxes are mediated by cation/H+ antiporters and ENA ATPases. We have cloned and studied the functions of the two ENA ATPases of Ustilago maydis, U. maydis Ena1 (UmEna1) and UmEna2. UmEna1 is a typical K+ or Na+ efflux ATPase whose function is indispensable for growth at pH 9.0 and for even modest Na+ or K+ tolerances above pH 8.0. UmEna1 locates to the plasma membrane and has the characteristics of the low-Na+/K+-discrimination ENA ATPases. However, it still protects U. maydis cells in high-Na+ media because Na+ showed a low cytoplasmic toxicity. The UmEna2 ATPase is phylogenetically distant from UmEna1 and is located mainly at the endoplasmic reticulum. The function of UmEna2 is not clear, but we found that it shares several similarities with Neurospora crassa ENA2, which suggests that endomembrane ENA ATPases may exist in many fungi. The expression of ena1 and ena2 transcripts in U. maydis was enhanced at high pH and at high K+ and Na+ concentrations. We discuss that there are two modes of Na+ tolerance in fungi: the high-Na+-content mode, involving ENA ATPases with low Na+/K+ discrimination, as described here for U. maydis, and the low-Na+-content mode, involving Na+-specific ENA ATPases, as in Neurospora crassa.