Structural organization of (Na+ + K+)-ATPase in purified membranes.

Abstract

The structural organization of crystalline, membrane-bound (Na+ + K+)-ATPase [from canine kidney microsomes] was studied by negative staining and thin sectioning. The enzyme molecules were induced to form crystalline arrays within fragments of membrane by incubation in defined ionic conditions. The enzyme remained fully active after crystallization. Negative staining and computer processing of images of the crystalline specimens identified 2 discrete crystalline arrays. The dimensions of the unit cell of one of the arrays were large enough to accomodate an .alpha..beta. protomer; those of the other array, an (.alpha..beta.)2 diprotomer. Thin sections of the crystalline fraction contained a unique membrane complex that was formed from 2 apposed plasma membranes. The paired membranes in this complex were separated by a center-to-center space of 15 nm containing evenly spaced septa that connected the membrane surfaces; the overall thickness of the entire structure was 22-55 nm. The agglutinin from Ricinus communis, a lectin that binds to the carbohydrate moiety of the .beta.-subunit of (Na+ + K+)-ATPase, decorated the free surfaces of the complex. Therefore, this complex of paired membranes is the result of interactions between the cytoplasmic domains of the enzyme. From measurements of the dimensions of these structures, the overall length of the enzyme is estimated to .apprx. 11.5 nm along the axis perpendicular to the plane of the membrane, and the molecular protrudes more (.apprx. 5 nm) on the cytoplasmic surface than on the extracytoplasmic surface (.apprx. 2 nm).