The H,K-ATPase responsible for gastric acidification is a heterodimeric (α and β subunit) P-type ATPase, an integral protein of parietal cell apical membranes, which promotes the electroneutral exchange of K+ for protons, is stimulated by K+ and is inhibited by 2-methyl-8-(phenylmethoxy)imidazo[1,2-α]pyridine-3-acetonitrile (SCH 28080). Hydropathy analysis of the catalytic α subunit has been interpreted in terms of four N-terminal transmembrane domains, a cytoplasmically oriented segment containing ATP binding and phosphorylation sites, and a C-terminal region with four or six putative transmembrane domains. Several lines of evidence implicate the C-terminal region of P-type ATPases in cation-binding and occlusion, conformational changes, and interactions with the β subunit (HKβ), making the definition of topology a prerequisite for understanding the structural basis of these functions. Influenza haemagglutinin epitopes (YPYDVPDYA; flu tag) were inserted in predicted hydrophilic segments of the α subunit (HKα) to establish the membrane orientation of two amino acids with different predicted topologies in the C-terminal four- and six-transmembrane models. Wild-type and mutated HKα and HKβ cDNA species were expressed in insect cells (Sf9) via recombinant baculovirus infection, and expression of H,K-ATPase was verified by immunoblotting with HKα- and HKβ-specific and flu-tag-specific antibodies. Functional assays showed K+-stimulated, SCH 28080-sensitive ATPase activity, confirming neo-native topology in H,K-ATPase heterodimers expressed in Sf9 cells. The topology of flu tags was determined by microsomal protease protection assays in Sf9 cells and immunolabelling of HKα and HKβ in intact and permeabilized Sf9 cells. In addition, MS of native H,K-ATPase tryptic peptides identified cytoplasmically oriented HKα residues. The results indicated cytoplasmic exposure of Leu844 and Phe996, and luminal exposure of Pro898, leading to a revised secondary structure model of the C-terminal third of HKα.

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