Abstract. The structure of Na,K-ATPase has been studied by electron microscopy and image reconstruction. A three-dimensional structure of this enzyme has been obtained to an overall resolution of 2.5 nm using data from specimens of negatively stained dimer sheets tilted through a range of angles +60 °.The reconstruction shows a complex mass distribution consisting of ribbons of paired molecules extending ,',,6.0 nm from the cytoplasmic side of the membrane. The molecular envelope consists of a massive "body" with "lobe" and "arm" structures projecting from it. The body has a columnar shape and is tilted with respect to the plane of the membrane. The region of interaction responsible for dimer formation is located between two bodies and is clearly visible in the reconstruction. It has been identified as a segment in the amino-terminal portion of the ct subunit. The arms that interconnect the ribbons are located close to the membrane and are most probably formed by the 13 subunits.
NA,K-ATPaSe constitutes the ion pump that couples the hydrolysis of ATP to the transmembrane transport of sodium and potassium ions. It is primarily responsible for the establishment and maintenance of the gradients of Na + and K ÷ across the plasma membrane of eukaryotic cells. The transmembrane potential produced by this process is used by the cell to power other transport processes. The enzyme is built from two polypeptide chains (7,14,16,20), of which ¢t (Mr 110,000) is the catalytic subunit and 13 (mr 50,000-60,000) is a glycoprotein whose function has yet to be determined.The availability of purified specimens of Na,K-ATPase has allowed extensive studies of its various properties (for reviews see references 5, 10, 17, 32). The recent progress in structural studies of the enzyme has been particularly impressive. The amino acid sequence of ¢t (18, 34) and 13 (19,29,33) have been determined from various tissues. Furthermore, the discovery of crystallization schemes that produced regular arrays of the enzyme in membrane fragments (35) initiated structural studies by electron microscopy and image processing. Structural analyses of the enzyme in projection have been reported (11,23,42) as well as preliminary data on its three-dimensional structure (13,27,31). In this laboratory we have tentatively mapped the domains in the projection image of the enzyme that correspond to its two subunits (23, 25), and have found reliable crystallization methods to produce regular sheets of the enzyme in ap2 lattice (26).In this paper we present a three-dimensional structural study of Na,K-ATPase to an overall resolution of 2.5 nm. The reconstruction shows the structure of the enzyme on the cgtoplasmic side of the membrane, and reveals that it consists of tilted columns of mass interconnected by a complex set of bridges. These bridges can be identified with the bonds responsible for dimer formation and the subsequent ribbon-ribbon interaction. A segment of the amino-terminal of the ct subunit can be located within the structure and a tentative identific...
