The roles of carbohydrate chains of the beta-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 stabilizi ng 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, w e examined the roles of the carbohydrate chains of the beta-subunit by expr essing the alpha-subunit together with the beta-subunit in which one, sever al, or all of the asparagine residues in the N-glycosylation sites were rep laced 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 w ere cumulative, and removal of all carbohydrate chains resulted in the comp lete loss of H+,K+-ATPase activity. Removal of any single carbohydrate chai n did not affect the alpha/beta assembly; however, Little alpha/beta assemb ly was observed after removal of all the carbohydrate chains from the beta- subunit. In contrast, removal of three carbohydrate chains inhibited the su rface delivery of the beta-subunit and the alpha-subunit assembled with the alpha-subunit, indicating that the surface delivery mechanism is more depe ndent on the carbohydrate chains than the expression of the H+,K+-ATPase ac tivity and alpha/beta assembly.