STRUCTURE OF THE CALCIUM-PUMP FROM SARCOPLASMIC-RETICULUM AT 8-ANGSTROM RESOLUTION

The calcium pump from sarcoplasmic reticulum (Ca2+-ATPase) is typical of the large family of P-type cation pumps. These couple ATP hydrolysi s with cation transport, generating cation gradients across membranes. Ca2+-ATPase specifically maintains the low cytoplasmic calcium concen tration of resting muscle by pumping calcium into the sarcoplasmic ret iculum; subsequent release is used to initiate contraction. No high-re solution structure of a P-type pump has yet been determined, although a 14-Angstrom structure of Ca2+-ATPase, obtained by electron microscop y of frozen-hydrated, tubular crystals(1), showed a large cytoplasmic head connected to the transmembrane domain by a narrow stalk. We have now improved the resolution to 8 Angstrom and can discern ten transmem brane ol-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 th ese alpha-helices within the amino-acid sequence. A distinct cavity le ads to the putative calcium-binding site, providing a plausible path f or calcium release to the lumen of the sarcoplasmic reticulum.