(Na+,K+)-ATPase was studied by electron microscopy and image processing of negatively stained and freeze-dried and shadowed crystalline sheets induced by a number of inorganic salts. Extensive experiments have identified new conditions for optimum crystal formation. Two crystal forms have been observed, one with a monomer and the other with a dimer, in the unit cell. Both show the same structure for the enzyme monomer. The enzyme can also be crystallized after partial proteolysis of its alpha subunit by trypsin. The proteolysed enzyme crystallizes under the same conditions as the whole enzyme. Comparison of the mass distributions in the images of the intact and proteolysed enzyme has allowed the tentative identification of the location of the alpha subunit within the monomer. The relationship between the structure of the crystallized enzyme and that of the enzyme in its native form is discussed, as is its apparent close structural relationship to the calcium-ATPase.

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