STRUCTURE OF THE CA2-RETICULUM - A VIEW ALONG THE LIPID BILAYER AT 9-ANGSTROM RESOLUTION( PUMP OF SARCOPLASMIC)

We have used multilamellar crystals of the ATP-driven calcium pump fro m sarcoplasmic reticulum to address the structural effects of calcium binding to the enzyme. They are stacks of disk-shaped two-dimensional crystals. A density map projected along the lipid bilayer was obtained at 9-Angstrom resolution by frozen-hydrated electron microscopy. Alth ough only in projection, much more details of the structure were revea led than previously available, especially in the transmembrane region. Quantitative comparison was made with the model obtained from the tub ular crystals of this enzyme formed in the absence of calcium. Unexpec tedly large differences in conformation were found, particularly in th e cytoplasmic domain.