The calcium pump from sarcoplasmic reticulum (Ca2+-ATPase) is typical of the large family of P-type cation pumps. These couple ATP hydrolysis with cation transport, generating cation gradients across membranes. Ca2+- ATPase specifically maintains the low cytoplasmic calcium concentration of resting muscle by pumping calcium into the sarcoplasmic reticulum; subsequent release is used to initiate contraction. No high-resolution structure of a P- type pump has yet been determined, although a 14-Å structure of Ca2+- ATPase, obtained by electron microscopy of frozen-hydrated, tubular crystals, showed a large cytoplasmic head connected to the transmembrane domain by a narrow stalk. We have now improved the resolution to 8 Å and can discern ten transmembrane α-helices, four of which continue into the stalk. On the basis of constraints from transmembrane topology, site-directed mutagenesis and disulphide crosslinking, we have made tentative assignments for these α- helices within the amino-acid sequence. A distinct cavity leads to the putative calcium-binding site, providing a plausible path for calcium release to the lumen of the sarcoplasmic reticulum.
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