SIMILARITY OF 3-DIMENSIONAL MICROCRYSTALS OF DETERGENT-SOLUBILIZED (NA+,K+)-ATPASE FROM PIG-KIDNEY AND CA2+-ATPASE FROM SKELETAL-MUSCLE SARCOPLASMIC-RETICULUM

Image analysis has been used to compare the projection structure of three-dimensional crystals of (Na+,K)-ATPase from pig kidney and the Ca2+-ATPase from rabbit muscle sarcoplasmic reticulum. These crystals, formed from detergent-solubilized protein in the presence of 20% glycerol and at a low detergent to protein ratio, crystallize in nearly identical unit cells in the space group C2. Average cell dimensions for the Ca2+-ATPase were a = 166.8 +/- 4.5 angstrom, b = 57.7 +/- 4.4 angstrom gamma 90-degrees while those for the (Na+,K+)-ATPase were a = 166.2 +/- 3.8angstrom, b = 54.25 +/- 3.5angstrom, gamma = 90-degrees. Their projected structures at the resolution of 25-angstrom resolution are indistinguishable. Thus, the (Na+,K+)-ATPase crystals appear to contain only the alpha chain of the alphabeta heterodimer found in native membranes. We conclude from this that the three-dimensional structure of the a chain of the (Na+,K+)-ATPase is very similar to that of the Ca2+-ATPase despite their relatively weak overall sequence homology.