Alanine Scanning Mutagenesis of Oxygen-Containing Amino Acids in the Transmembrane Region of the Na,K-ATPase

Oxygen-containing amino acids in the transmembrane region of the Na,K-ATPase α subunit were studied to identify residues involved in Na+ and/or K+ coordination by the enzyme. Conserved residues located in the polar face of transmembrane helices were selected using helical wheel and topological models of the enzyme. Alanine substitution of these residues were introduced into an ouabain-resistant sheep α1 isoform and expressed in HeLa cells. The capacity to generate essential Na+ and K+ gradients and thus support cell growth was used as an initial indication of the functionality of heterologous enzymes. Enzymes carrying alanine substitution of Ser94, Thr136, Ser140, Gln143, Glu144, Glu282, Thr334, Thr338, Thr340, Ser814, Tyr817, Glu818, Glu821, Ser822, Gln854, and Tyr994 supported cell growth, while those carrying substitutions Gln923Ala, Thr955Ala, and Asp995Ala did not. To study the effects of these latter replacements on cation binding, they were introduced into the wild-type α1 sheep isoform and expressed in mouse NIH3T3 cells where [3H]ouabain binding was utilized to probe the heterologous proteins. These substitutions did not affect ouabain, K+, or Na+ binding. Expression levels of these enzymes were similar to that of control. However, the level of Gln923Ala-, Thr955Ala-, or Asp995Ala-substituted enzyme at the plasma membrane was significantly lower than that of the wild-type isoform. Thus, these substitutions appear to impair the maturation process or targeting of the enzyme to the plasma membrane, but not cation–enzyme interactions. These results complete previous studies which have identified Ser755, Asp804, and Asp808 as absolutely essential for Na+ and K+ transport by the enzyme. Thus, it is significant that most transmembrane conserved-oxygen-containing residues in the Na,K-ATPase can be replaced without substantially affecting cation–enzyme interactions to the extent of preventing enzyme function. Consequently, other chemical groups, aromatic rings or backbone carbonyls, should be considered in models of cation-binding sites.