The Amino-terminal Transmembrane Region of the β Subunit is Important for Na⁺/K⁺-ATPase Complex Stability

Abstract

Heterologous expression of the α and β subunits of Na⁺/K⁺-ATPase allows for the expression of altered forms of the Na⁺/K⁺-pump which may be useful in determining the structural basis of pump function. Most systems for heterologous expression are limited by the low abundance of protein expression, the difficulty in producing assembled complexes with enzymatic activity, and a background of endogenous Na⁺/K⁺ ATPase activity. While the level of heterologous expression of Na⁺/K⁺-ATPase in yeast (Saccharomyces cerevisiae) is limited, a major advantage of this system is the lack of an endogenous Na⁺/K⁺-ATPase enzyme. Thus, activities which are unique to the Na⁺/K⁺-ATPase can be assayed with virtually no background. In particular, the ability of Na⁺/K⁺-ATPase to bind cardiac glycosides such as ouabain with high affinity has proven to be a reliable and sensitive marker for the assembly of functional enzyme complexes in yeast. Ouabain binding depends on phosphorylation of the enzyme and is sensitive to both Na⁺ and K⁺ which are thought to cause conformational shifts in Na⁺/K⁺-ATPase structure related to the transport of ions across the cell membrane. (1)