• Na+/K+-exchanging ATPase;
  • cardiac glycoside;
  • ouabain;
  • random mutagenesis;
  • drug interaction

Random mutagenesis with ouabain selection has been used to comprehensively scan the extracellular and transmembrane domains of the α1 subunit of the sheep Na+/K+-ATPase for amino acid residues that alter ouabain sensitivity. The four random mutant libraries used in this study include all of the transmembrane and extracellular regions of the molecule as well as 75% of the cytoplasmic domains. Through an extensive number of HeLa cell transfections of these libraries and subsequent ouabain selection, 24 ouabain-resistant clones have been identified. All previously described amino acids that confer ouabain resistance were identified, confirming the completeness of this random mutagenesis screen. The amino acid substitutions that confer the greatest ouabain resistance, such as Gln111[RIGHTWARDS ARROW]Arg, Asp121[RIGHTWARDS ARROW]Gly, Asp121[RIGHTWARDS ARROW]Glu, Asn122[RIGHTWARDS ARROW]Asp, and Thr797[RIGHTWARDS ARROW]a were identified more than once in this study. This extensive survey of the extracellular and transmembrane regions of the Na+/K+-ATPase molecule has identified two new regions of the molecule that affect ouabain sensitivity: the H4 and the H10 transmembrane regions. The new substitutions identified in this study are Leu330[RIGHTWARDS ARROW]Gln, Ala331[RIGHTWARDS ARROW]Gly, Thr338[RIGHTWARDS ARROW]a, and Thr338[RIGHTWARDS ARROW]Asn in the H4 transmembrane domain and Phe982[RIGHTWARDS ARROW]Ser in the H10 transmembrane domain. These substitutions confer modest increases in the concentration of cardiac glycoside needed to produce 50% inhibition of activity (IC50 values), 3.1–-7.9-fold difference. The results of this extensive screening of the Na+/K+-ATPase α1 subunit to identify amino acids residues that are important in ouabain sensitivity further supports our hypothesis that the H1-H2 and H4-H8 regions represent the major binding sites for the cardiac glycoside class of drugs.