Three-dimensional reconstruction of negatively stained crystals of the Ca2+-ATPase from muscle sarcoplasmic reticulum.

The structure of the Ca2+ transport ATPase from rabbit skeletal muscle sarcoplasmic reticulum has been determined to 25 A resolution by three-dimensional image reconstruction of crystalline membrane tubules induced through exposure to Na3VO4 and preserved for electron microscopy in negative stain. The crystalline arrays have projection symmetry p2 and consist of chains of Ca2+-ATPase dimers arranged in a right-handed helix. The density map shows protein features that project from the membrane surface into the cytoplasm. The luminal side of the membrane tubules is featureless, presumably because very little of the Ca2+-ATPase molecule projects into the luminal space. The cytoplasmic region of the Ca2+-ATPase molecule is pear-shaped, with a lobe oriented nearly parallel to the axis of the dimer ribbons, about 16 A above the surface of the membrane bilayer. The structure seen in the maps has a volume of 71,000 A3, corresponding to a molecular weight of 57,000. The two Ca2+-ATPase profiles that constitute a dimer are connected by a stain-excluding bridge that is oriented parallel with the axis of the tubule at a height of about 42 A above the surface of the bilayer.