Fig. 7. Model of the life cycle of the Na⁺/K⁺-ATPase subunits in HeLa cells. The results obtained in this study, combined with those of other previous studies, are summarized. The - and β-subunits first co-translationally assemble in the ER. The assembled enzymes leave the ER for the Golgi network, where they are subjected to sugar modifications for maturation. Once they reach the plasma membrane, the subunits dissociate and the unassembled β-subunit undergoes ubiquitylation, endocytosis and fast degradation, whereas the -subunit remains in the plasma membrane and is subjected to slow degradation. As a result, the plasma membrane contains more -subunit than β-subunit. Reassembly of the subunits may occur in the plasma membrane, although it is not clear how this assembly-disassembly is regulated.