Polarized membrane distribution of potassium-dependent ion pumps in epithelial cells: Different roles of the N-glycans of their β subunits

The Na,K-ATPases and the H,K-ATPases are two potassium-dependent homologous heterodimeric P2-type pumps that catalyze active transport of Na+ in exchange for K+ (Na,K-ATPase) or H+ in exchange for K+ (H,K-ATPase). The ubiquitous Na,K-ATPase maintains intracellular ion balance and membrane potential. The gastric H,K-ATPase is responsible for acid secretion by the parietal cell of the stomach. Both pumps consist of a catalytic α-subunit and a glycosylated β-subunit that is obligatory for normal pump maturation and trafficking. Individual N-glycans linked to the β-subunits of the Na,K-ATPase and H,K-ATPase are important for stable membrane integration of their respective α subunits, folding, stability, subunit assembly, and enzymatic activity of the pumps. They are also essential for the quality control of unassembled β-subunits that results in either the exit of the subunits from the ER or their ER retention and subsequent degradation. Overall, the importance of N-glycans for the␣maturation and quality control of the H,K-ATPase is greater than that of the Na,K-ATPase. The roles of individual N-glycans of the β-subunits in the post-ER trafficking, membrane targeting and plasma membrane retention of the Na,K-ATPase and H,K-ATPase are different. The Na,K-ATPase β1-subunit is the major β-subunit isoform in cells with lateral location of the pump. All three N-glycans of the Na,K-ATPase β1-subunit are important for the lateral membrane retention of the pump due to glycan-mediated interaction between the β1-subunits of the two neighboring cells in the cell monolayer and cytosolic linkage of the α-subunit to the cytoskeleton. This intercellular β1–β1 interaction is also important for formation of cell–cell contacts. In contrast, the N-glycans unique to the Na,K-ATPase β2-subunit,which has up to eight N-glycosylation sites, contain apical sorting information. This is consistent with the apical location of the Na,K-ATPase in normal and malignant epithelial cells with high abundance of the β2-subunit. Similarly, all seven N-glycans of the gastric H,K-ATPase β-subunit determine apical sorting of this subunit.