Characterization of Na,K-ATPase and H,K-ATPase enzymes with glycosylation-deficient β-subunit variants by voltage-clamp fluorometry in Xenopus oocytes

The role of N-linked glycosylation of beta-subunits in the functional properties of the oligomeric beta-type ATPases Na,K- and H,K-ATPase has been examined by expressing glycosylation-deficient Asn-to-Gln beta-variants in Xenopus oocytes. For both ATPases, the absence of the huge N-linked oligosaccharide moiety on the beta-subunit does not affect alpha/beta coassembly, plasma membrane delivery or functional activity of the holoenzyme. Whereas this is in line with several previous glycosylation studies on Na,K-ATPase, this is the first report showing that the cell surface delivery and enzymatic activity of the gastric H,K-ATPase is unaffected by the lack of N-linked glycosylation. Sulfhydryl-specific labeling of introduced cysteine reporter sites with the environmentally sensitive fluorophore tetramethylrhodamine-6-maleimide (TMRM) upon expression in Xenopus oocytes enabled us to further investigate potential effects of the N-glycans on more subtle enzymatic properties, like the distribution between E1P/E2P states of the catalytic cycle and the kinetics of the E1P/E2P conformational transition under presteady state conditions. For both Na,K-ATPase and H,K-ATPase, we observed differences in neither the voltage-dependent E1P/E2P ratio nor the kinetics of the E1P/E2P transition between holoenzymes comprising glycosylated and glycosylation-deficient beta-subunits. We conclude that the N-linked glycans on these essential accessory subunits of oligomeric beta-type ATPases are dispensable for proper folding, membrane stabilization of the alpha-subunit and transport function itself. Glycosylation is rather important for other cellular functions not relevant in the oocyte expression system, such as intercellular interactions or basolateral versus apical targeting in polarized cells, as demonstrated in other expression systems.