Three-dimensional structure of (Na+ + K+)-ATPase revealed by electron microscopy of two-dimensional crystals

Prolonged incubation of membrane fragments containing homogeneous (Na+ + K+)-ATPase with Mg2+, K+ and VO-3 at 4 degrees C resulted in formation of two-dimensional crystals of this enzyme with unit cell parameters: a = 66 A, b = 118 A, gamma = 108 degrees. The crystals correspond to the two-sided plane group p21. By combining tilted electron microscopic views of the crystals, a three-dimensional structure of (Na+ + K+)- ATPase was calculated at approximately 20 A resolution. The unit cell is formed by two (alpha beta)-promoters which are in contact in their central parts. The structure was compared with chemical modification and immunochemical data; the arrangement of intra- and extramembrane domains was proposed.