HCO₃/⁺-dependent conformational change in gastric parietal cell AE2, a glycoprotein naturally lacking sialic acid

Although the AE1 chloride/bicarbonate exchanger of the red blood cell is among the most thoroughly investigated of membrane transport proteins, less is known about the related AE2 polypeptide of parietal cells. We have studied enzymatic deglycosylation of native AE2 polypeptide in gastric mucosal membranes from pig and rabbit. Deglycosylation of AE2 was maximal at low ionic strength. Deglycosylation of AE2 in membranes was preferentially inhibited by bicarbonate compared with other anions. This inhibition was maximal at alkaline pH and was not evident after detergent solubilization of AE2. Deglycosylation of AE2 increased its susceptibility to proteolytic degradation, but the presence of bicarbonate protected against this degradation. Bicarbonate failed to inhibit deglycosylation of the membrane glycoproteins AE1 and gastric H⁺-K⁺-adenosinetriphosphatase β-subunit or deglycosylation of the soluble glycoproteins fetuin and ribonuclease B. These data suggest that bicarbonate induces a conformational change in AE2 that can protect the polypeptide from deglycosylation and proteolysis. Pig AE2 was purified in sodium dodecyl sulfate, and its monosaccharide composition was determined after blotting onto polyvinylidene fluoride membrane. AE2 was found to be devoid of sialic acid, with a composition suggestive of the presence of lactosamine-type chains.