What the structure of a calcium pump tells us about its mechanism

The report of the crystal structure of the Ca2+-ATPase of skeletal muscle sarcoplasmic reticulum in its Ca2+-bound form [Toyoshima, Nakasako and Ogawa (2000) Nature (London) 405, 647–655] provides an opportunity to interpret much kinetic and mutagenic data on the ATPase in structural terms. There are no large channels leading from the cytoplasmic surface to the pair of high-affinity Ca2+ binding sites within the transmembrane region. One possible access pathway involves the charged residues in transmembrane a-helix M1, with a Ca2+ ion passing through the first site to reach the second site. The Ca2+-ATPase also contains a pair of binding sites for Ca2+ that are exposed to the lumen. In the four-site model for transport, phosphorylation of the ATPase leads to transfer of the two bound Ca2+ ions from the cytoplasmic to the lumenal pair of sites. In the alternating four-site model for transport, phosphorylation leads to release of the bound Ca2+ ions directly from the cytoplasmic pair of sites, linked to closure of the pair of lumenal binding sites. The lumenal pair of sites could involve a cluster of conserved acidic residues in the loop between M1 and M2. Since there is no obvious pathway from the high-affinity sites to the lumenal surface of the membrane, transport of Ca2+ ions must involve a significant change in the packing of the transmembrane a-helices. The link between the phosphorylation domain and the pair of high-affinity Ca2+ binding sites is probably provided by two small helices, P1 and P2, in the phosphorylation domain, which contact the loop between transmembrane a-helices M6 and M7.