A low-resolution three-dimensional model of membrane-bound H,K-ATPase from pig gastric mucosa has been reconstructed by electron microscopy and image processing of two-dimensional crystals in negative stain. The crystal formation is induced by magnesium and vanadate, which stabilize the E-2 conformation of the enzyme. The unit cell, with a size of a = b = 123 Angstrom, gamma = 90 degrees, has tetragonal p4 symmetry. There are four separate alpha beta protomers within each unit cell, The high-contrast region is limited to the cytoplasmic part of the protein, The total volume of the observed asymmetric protein domain corresponds to a molecular mass of 80-90 kDa. It consists mainly of a large pear-shaped domain measuring 60 x 45 Angstrom(2), With a height of 50 A as measured perpendicular to the membrane plane. A small stalk segment, 20 Angstrom in length, forms a connection to the transmembrane region. (C) 1997 Academic Press.