Molecular requirements for the cell-surface expression of multisubunit ion-transporting ATPases. Identification of protein domains that participate in Na,K-ATPase and H,K-ATPase subunit assembly

The ion-transporting H,K-ATPase and Na,K-ATPase enzymes are each composed of an α and a β subunit. It is known that assembly of the α and β subunits of the Na,K-ATPase is necessary for the cell-surface delivery of the active enzyme. We have examined the molecular domains involved in the assembly of the H,K-ATPase and Na,K-ATPase α and β subunits by expressing individual subunits and subunit chimeras in transiently transfected COS-1 cells. Our results demonstrate that the H,K-ATPase α subunit requires its β subunit for efficient cell-surface expression, as determined by indirect immunofluorescence. The H,K-ATPase β protein appears to be able to get to the cell surface unaccompanied by any α subunit and appears to localize as well to a population of intracellular vesicles. We find that a transfected chimera encoding the NH₂-terminal half of the H,K-ATPase α subunit and the COOH-terminal half of the Na,K-ATPase α subunit appears to assemble with the endogenous Na,K-ATPase β subunit and to reach the plasmalemma. Transfection of the complementary α chimera requires coexpression with the H,K-ATPase β subunit in order to attain surface delivery. Thus, it is the COOH-terminal half of the α subunit that specifies assembly with a particular β subunit.