Sodium or potassium efflux ATPase A fungal, bryophyte, and protozoal ATPase

The K+ and Na+ concentrations in living cells are strictly regulated at almost constant concentrations, high for K+ and low for Na+. Because these concentrations correspond to influx-efflux steady states, K+ and Na+ effluxes and the transporters involved play a central role in the physiology of cells, especially in environments with high Na+ concentrations where a high Na+ influx may be the rule. In eukaryotic cells two P-type ATPases are crucial in these homeostatic processes, the Na,K-ATPase of animal cells and the H+-ATPase of fungi and plants. In fungi, a third P-type ATPase, the ENA ATPase, was discovered nineteen years ago. Although for many years it was considered to be exclusively a fungal enzyme, it is now known to be present in bryophytes and protozoa. Structurally, the ENA (from exitus natru: exit of sodium) ATPase is very similar to the sarco/endoplasmic reticulum Ca2+ (SERCA) ATPase, and it probably exchanges Na+ (or K+) for H+. The same exchange is mediated by Na+ (or K+)/H+ antiporters. However, in eukaryotic cells these antiporters are electroneutral and their function depends on a Delta pH across the plasma membrane. Therefore, the current notion is that the ENA ATPase is necessary at high external pH values, where the antiporters cannot mediate uphill Na+ efflux. This occurs in some fungal environments and at some points of protozoa parasitic cycles, which makes the ENA ATPase a possible target for controlling fungal and protozoan parasites. Another technological application of the ENA ATPase is the improvement of salt tolerance in flowering plants. (C) 2010 Elsevier B.V. All rights reserved.