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 s arcoplasmic 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 ar e no large channels leading from the cytoplasmic surface to the pair of hig h-affinity Ca2+ binding sites within the transmembrane region. One possible access pathway involves the charged residues in transmembrane x-helix M1, with a Ca2+ ion passing through the first site to reach the second site. Th e Ca2+-ATPase also contains a pair of binding sites for Ca2+ that are expos ed to the lumen. In the four-site model for transport, phosphorylation of t he 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 trans port, phosphorylation leads to release of the bound Ca2+ ions directly from the cytoplasmic pair of sites, linked to closure of the pair of lumenal bi nding 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 p athway 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 x-helices. The link between the phosphorylation domain a nd 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 x-helices M6 and M7.