The β-subunit of the rabbit H,K-ATPase: A glycoprotein with all terminal lactosamine units capped with α-linked galactose residues

The β-subunit of the gastric H,K-ATPase is the most abundant glycoprotein in the tubulovesicular compartment of the acid-secreting parietal cells. The oligosaccharides of the β-subunit have been shown to contain fucose, N- acetylglucosamine, mannose, galactose, and N-acetylgalactosamine. Previous studies have shown that the rabbit β-subunit is devoid of N-acetylneuraminic acid. Here we report the structural features of the N-linked oligosaccharides of the β-subunit from rabbit H,K-ATPase. We used glycosidase digestions and analysis by high-pH anion-exchange chromatography with pulsed amperometric detection and matrix-assisted laser desorption/ionization mass spectrometry to analyze the peptide-N⁴-(N-acetyl-β-D-glucosaminyl)asparagine amidase (PNGase F)- and endo-β-N-acetylglucosaminidase H (Endo H)-released oligosaccharides. The studies showed that the oligosaccharides of the β- subunit are a mixture of both oligomannosidic and lactosamine-type structures. The high-mannose structures were identified as Man₅MansGlcNAc₂ species. A striking finding was that all the branches of the lactosamine- type structures were terminated with Galα→Galβ→GlcNAc extensions. All of the lactosamine-type structures were found to be core fucosylated and some of them contained one to three lactosamine repeats. We propose that a part of the adaptation of the gastric β-subunit to the acidic environment of the stomach is through providing acid-stable terminal residues on the oligosaccharides.