Modulation of H(+),K(+)-ATPase activity by the molecular chaperone ERp57 highly expressed in gastric parietal cells

ERp57 is a ubiquitous ER chaperone that has disulfide isomerase activity. Here, we found that both ERp57 and gastric H(+),K(+)-ATPase are expressed in a sample derived from the apical canalicular membranes of parietal cells. Overexpression of ERp57 in HEK293 cells stably expressing H(+),K(+)-ATPase significantly increased the ATPase activity without changing the expression level of H(+),K(+)-ATPase. Interestingly, overexpression of a catalytically inactive mutant of ERp57 (C57S/C60S/C406S/C409S) in the cells also increased H(+),K(+)-ATPase activity. In contrast, knockdown of endogenous ERp57 in H(+),K(+)-ATPase-expressing cells significantly decreased ATPase activity without changing the expression level of H(+),K(+)-ATPase. Overexpression and knockdown of ERp57 had no significant effect on the expression and function of Na(+),K(+)-ATPase. These results suggest that ERp57 positively regulates H(+),K(+)-ATPase activity apart from its chaperoning function.