The structural basis of calcium transport by the calcium pump

The sarcoplasmic reticulum Ca²⁺-ATPase, a P-type ATPase, has a critical role in muscle function and metabolism. Here we present functional studies and three new crystal structures of the rabbit skeletal muscle Ca²⁺-ATPase, representing the phosphoenzyme intermediates associated with Ca²⁺ binding, Ca²⁺ translocation and dephosphorylation, that are based on complexes with a functional ATP analogue, beryllium fluoride and aluminium fluoride, respectively. The structures complete the cycle of nucleotide binding and cation transport of Ca²⁺-ATPase. Phosphorylation of the enzyme triggers the onset of a conformational change that leads to the opening of a luminal exit pathway defined by the transmembrane segments M1 through M6, which represent the canonical membrane domain of P-type pumps. Ca²⁺ release is promoted by translocation of the M4 helix, exposing Glu309, Glu771 and Asn796 to the lumen. The mechanism explains how P-type ATPases are able to form the steep electrochemical gradients required for key functions in eukaryotic cells.