Gastric H+, K+ -ATPase comprised of alpha- and beta-subunits was functionally expressed in an animal cell-line. When glutamic acid (345) of the alpha-subunit was mutated to glutamine, the affinity of K+ decreased 10-fold, indicating that this residue in the 4th transmembrane domain engages in the determination of the K+ affinity. The roles of other residues are also discussed. The number of the binding site of proton pump inhibitors such as omeprazole and E3810 (rabeprazole) is 1, which is contrary to the proposal of 2 or 3 by other researchers. The reason of this discrepancy is explained. The interaction between 2 or 4 alpha-subunits was shown to be necessary for the function of this pump. Finally, recent topics about H+ -ATPase are discussed.