HCO3--DEPENDENT CONFORMATIONAL CHANGE IN GASTRIC PARIETAL-CELL AE2, AGLYCOPROTEIN NATURALLY LACKING SIALIC-ACID

Although the AE1 chloride/bicarbonate exchanger of the red blood cell is among the most thoroughly investigated of membrane transport protei ns, less is known about the related AE2 polypeptide of parietal cells. We have studied enzymatic deglycosylation of native AE2 polypeptide i n gastric mucosal membranes from pig and rabbit. Deglycosylation of AE 2 was maximal at low ionic strength. Deglycosylation of AE2 in membran es was preferentially inhibited by bicarbonate compared with other ani ons. This inhibition was maximal at alkaline pH and was not evident af ter detergent solubilization of AE2. Deglycosylation of AE2 increased its susceptibility to proteolytic degradation, but the presence of bic arbonate protected against this degradation. Bicarbonate failed to inh ibit deglycosylation of the membrane glycoproteins AE1 and gastric H+- K+-adenosinetriphosphatase beta-subunit or deglycosylation of the solu ble glycoproteins fetuin and ribonuclease B. These data suggest that b icarbonate induces a conformational change in AE2 that can protect the polypeptide from deglycosylation and proteolysis. Pig AE2 was purifie d in sodium dodecyl sulfate, and its monosaccharide composition was de termined after blotting onto polyvinylidene fluoride membrane. AE2 was found to be devoid of sialic acid, with a composition suggestive of t he presence of lactosamine-type chains.