The Na+/K+ ATPase pump is a tetramer consisting of:
Purified sodium potassium ATPase from kidney outer medulla analyzed by SDS PAGE reveals catalytic alpha subunit about 112 kilodaltons with ten membrane spans and beta subunit about 55 kilodaltons glycosylated single pass plus small FXYD regulatory subunit 6 to 10 kilodaltons modulating kinetics. Early radiation inactivation, freeze fracture and crosslinking experiments indicated functional unit larger than heterodimer. Kinetic studies combined with electron microscopy suggested association of two heterodimers. High resolution structures show extracellular domain of beta contacting alpha of same protomer and alpha-alpha contacts between neighboring protomers forming dimer of heterodimers. Each alpha-beta heterodimer already capable of ouabain sensitive ATP hydrolysis and ion exchange, but in native membrane they often arrange as tetramer comprising two alpha and two beta subunits (alpha-beta)2 via noncovalent interactions mediated by extracellular loops and transmembrane helices M7 M10. This tetramer may facilitate cooperative interactions and stable membrane insertion. Some tissues contain alpha-beta protomer alone active, yet biochemical preparations typically show tetrameric assembly as predominant oligomeric state consistent with (alpha2 beta2) stoichiometry.
Ref: Jorgensen et al., Biochim Biophys Acta, Na+/K+ ATPase Tetrameric Structure 2 Alpha 2 Beta Assembly.