The location of the ATP-binding site of a P-type ion pump, Ca2+- ATPase from rabbit sarcoplasmic reticulum, was examined by cryoelectron microscopy. A nonhydrolyzable analog of ATP, β,γ-bidentate chromium (III) complex of ATP (CrATP), was used to stabilize the enzyme in the Ca2+- occluded state. Tubular crystals were then induced by vanadate in the presence of EGTA, keeping CrATP bound to the enzyme. The three-dimensional structures of the crystals were determined at 14 Å resolution by cryoelectron microscopy and helical image analysis. Statistical comparison of the structures with and without CrATP showed clear and significant differences at the groove proposed previously as the ATP-binding pocket.
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