Abstract
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 Ca 2+ (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 ΔpH 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.
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