Calcium ATPase is a member of the P-type ATPases that transport ions across the membrane against a concentration gradient. Here we have solved the crystal structure of the calcium ATPase of skeletal muscle sarcoplasmic reticulum (SERCA1a) at 2.6 A resolution with two calcium ions bound in the transmembrane domain, which comprises ten alpha-helices. The two calcium ions are located side by side and are surrounded by four transmembrane helices, two of which are unwound for efficient coordination geometry. The cytoplasmic region consists of three well separated domains, with the phosphorylation site in the central catalytic domain and the adenosine-binding site on another domain. The phosphorylation domain has the same fold as haloacid dehalogenase. Comparison with a low-resolution electron density map of the enzyme in the absence of calcium and with biochemical data suggests that large domain movements take place during active transport.

In skeletal muscle, calcium ions are transported (pumped) against a concentration gradient from the cytoplasm into the sarcoplasmic reticulum, an intracellular organelle. This causes muscle cells to relax after cytosolic calcium increases during excitation. The Ca(2+) ATPase that carries out this pumping is a representative P-type ion-transporting ATPase. Here we describe the structure of this ion pump at 3.1 A resolution in a Ca(2+)-free (E2) state, and compare it with that determined previously for the Ca(2+)-bound (E1Ca(2+)) state. The structure of the enzyme stabilized by thapsigargin, a potent inhibitor, shows large conformation differences from that in E1Ca(2+). Three cytoplasmic domains gather to form a single headpiece, and six of the ten transmembrane helices exhibit large-scale rearrangements. These rearrangements ensure the release of calcium ions into the lumen of sarcoplasmic reticulum and, on the cytoplasmic side, create a pathway for entry of new calcium ions.

Sodium-potassium ATPase is an ATP-powered ion pump that establishes concentration gradients for Na(+) and K(+) ions across the plasma membrane in all animal cells by pumping Na(+) from the cytoplasm and K(+) from the extracellular medium. Such gradients are used in many essential processes, notably for generating action potentials. Na(+), K(+)-ATPase is a member of the P-type ATPases, which include sarcoplasmic reticulum Ca(2+)-ATPase and gastric H(+), K(+)-ATPase, among others, and is the target of cardiac glycosides. Here we describe a crystal structure of this important ion pump, from shark rectal glands, consisting of alpha- and beta-subunits and a regulatory FXYD protein, all of which are highly homologous to human ones. The ATPase was fixed in a state analogous to E2.2K(+).P(i), in which the ATPase has a high affinity for K(+) and still binds P(i), as in the first crystal structure of pig kidney enzyme at 3.5 A resolution. Clearly visualized now at 2.4 A resolution are coordination of K(+) and associated water molecules in the transmembrane binding sites and a phosphate analogue (MgF(4)(2-)) in the phosphorylation site. The crystal structure shows that the beta-subunit has a critical role in K(+) binding (although its involvement has previously been suggested) and explains, at least partially, why the homologous Ca(2+)-ATPase counter-transports H(+) rather than K(+), despite the coordinating residues being almost identical.

P-type ATPases are ATP-powered ion pumps that establish ion concentration gradients across cell and organelle membranes. Here, we describe the crystal structure of the Ca2+ pump of skeletal muscle sarcoplasmic reticulum, a representative member of the P-type ATPase superfamily, with an ATP analogue, a Mg2+ and two Ca2+ ions in the respective binding sites. In this state, the ATP analogue reorganizes the three cytoplasmic domains (A, N and P), which are widely separated without nucleotide, by directly bridging the N and P domains. The structure of the P-domain itself is altered by the binding of the ATP analogue and Mg2+. As a result, the A-domain is tilted so that one of the transmembrane helices moves to lock the cytoplasmic gate of the transmembrane Ca2+-binding sites. This appears to be the mechanism for occluding the bound Ca2+ ions, before releasing them into the lumen of the sarcoplasmic reticulum.