Structural Basis of Ion Pumping by Ca²⁺-ATPase of the Sarcoplasmic Reticulum

The structures of the Ca²⁺-ATPase (SERCA1a) have been determined for five different states by X-ray crystallography. Detailed comparison of the structures in the Ca²⁺ bound form and unbound (but thapsigargin bound) form reveals that very large rearrangements of the transmembrane helices take place accompanying Ca²⁺ dissociation and binding and that they are mechanically linked with equally large movements of the cytoplasmic domains. The meanings of the rearrangements of the transmembrane helices and those of the cytoplasmic domains as well as the mechanistic roles of phosphorylation are now becoming clear. Furthermore, the roles of critical amino acid residues identified by extensive mutagenesis studies are becoming evident in terms of atomic structure.