The roles of carbohydrate chains of the β-subunit on the functional expression of gastric H+,K+-ATPase

Gastric H+,K+-ATPase consists of alpha and beta-subunits. The alpha-subunit is the catalytic subunit, and the beta-subunit is a glycoprotein stabilizing the alpha/beta complex in the membrane as a functional enzyme. There are seven putative N-glycosylation sites on the beta-subunit. In this study, we examined the roles of the carbohydrate chains of the beta-subunit by expressing the alpha-subunit together with the beta-subunit in which one, several, or all of the asparagine residues in the N-glycosylation sites were replaced by glutamine. Removing any one of seven carbohydrate chains from the beta-subunit retained the H+,K+-ATPase activity. The effects of a series of progressive removals of carbohydrate chains on the H+,K+-ATPase activity were cumulative, and removal of all carbohydrate chains resulted in the complete loss of H+,K+-ATPase activity. Removal of any single carbohydrate chain did not affect the alpha/beta assembly; however, Little alpha/beta assembly was observed after removal of all the carbohydrate chains from the beta-subunit. In contrast, removal of three carbohydrate chains inhibited the surface delivery of the beta-subunit and the alpha-subunit assembled with the alpha-subunit, indicating that the surface delivery mechanism is more dependent on the carbohydrate chains than the expression of the H+,K+-ATPase activity and alpha/beta assembly.