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.