Despite the fact that mucus and bicarbonate are important macroscopic components of the gastric mucosal barrier, severe acidic and peptic conditions surely exist at the apical membrane of gastric glandular cells, and these membranes must have highly specialized adaptations to oppose external insults. Parietal cells abundantly express the heterodimeric, acid-pumping H,K-ATPase in their apical membranes. Its ß-subunit (HKß), a glycoprotein with >70% of its mass and all its oligosaccharides on the extracellular side, may play a protective role. Here we show that the extracellular domain of HKß is highly resistant to trypsin in the native state (much more than that of the structurally related Na,K-ATPase ß-subunit) and requires denaturation to expose tryptic sites. Native HKß also resists other proteases, such as chymotrypsin and V8 protease, which hydrolyze at hydrophobic and anionic amino acids, respectively. Removal of terminal -anomeric-linked galactose does not appreciably alter tryptic sensitivity of HKß. However, full deglycosylation makes HKß much more susceptible to all proteases tested, including pepsin at pH < 2.0. We propose that (i) intrinsic folding of HKß, (ii) bonding forces between subunits, and (iii) oligosaccharides on HKß provide a luminal protein domain that resists gastric lytic conditions. Protein folding that protects susceptible charged amino acids and is maintained by disulfide bonding and hydrophilic oligosaccharides, would provide a stable structure in the face of large pH changes. The H,K-ATPase is an obvious model, but other gastric luminally exposed proteins are likely to possess analogous protective specializations.