The roles of the Na,K-ATPase beta 1 subunit in pump sorting and epithelial integrity

In epithelial MDCK cells, the Na,K-ATPase is co-localized with adherens junctions in all stages of monolayer formation starting from initiation of cell–cell contact. The Na,K-ATPase and adherens junction proteins stay partially co-localized even after internalization due to disruption of intercellular contacts by Ca2+ deprivation. Similar to adherens junction proteins, the Na,K-ATPase is resistant to extraction with non-ionic detergent, suggesting pump association with the cytoskeleton. In contrast, the heterodimer formed by expressed unglycosylated Na,K-ATPase β1 subunit and the endogenous α1 subunit is easily dissociated from the adherens junctions and cytoskeleton by detergent extraction. The MDCK cells in which half of the endogenous β1 subunits in the lateral membrane are substituted by unglycosylated β1 subunits display a slower rate of cell-to-cell contact formation and decreased ability to both spread over the surface and migrate. The lack of N-glycans in the Na,K-ATPase β1 subunit results in an impairment of mature cell–cell junctions as detected by an increase in the paracellular permeability of the MDCK cell monolayers and by a decrease in resistance of adherens junction proteins to extraction by a non-ionic detergent. Therefore the N-glycans of the Na,K-ATPase β1 subunit are important for retention of the pump at the sites of cell–cell contact. Moreover, they are important for the integrity and stability of cell–cell junctions in mature epithelia. In addition, N-glycans contribute to the formation of cell–cell contacts between surface-attached dispersed cells by mediating lamellipodia formation and stabilizing the newly formed adherens junctions.