A membrane transport protein that exchanges sodium ions (Na⁺) for potassium ions (K⁺) across the plasma membrane of a eukaryotic cell. It exports three Na⁺ ions for every two K⁺ ions imported. This maintains the differential concentrations of each ion across the plasma membrane so that the cell normally has a low Na⁺ and a high K⁺ concentration relative to the extracellular fluid. This differential is vital to cellular function, e.g. in establishing the resting potential of a neuron. The process requires energy in the form of ATP, being a form of active transport.