The Crystallization of the Ca2+-ATPase of Sarcoplasmic Reticulum

The concentration of Ca²⁺-ATPase in sarcoplasmic reticulum of fast-twitch skeletal muscle is very high and there are good indications that in the native membrane a significant portion of the ATPase molecules are present in oligomeric form. The conditions required for the crystallization of the Ca²⁺-ATPase provided good stabilization of the solubilized enzyme with retention of ATPase activity over several months. Of particular importance are the concentration of Ca²⁺, the pH, and the presence of glycerol. Vesicles with extensive crystalline arrays usually acquire an elongated tubular shape. The vanadate-induced crystals consist of chains of Ca²⁺-ATPase dimers wound in a right-handed helix around the tubules. The formation of the vanadate-induced crystals is promoted by inside-positive membrane potential, suggesting an influence of transmembrane potential on the conformation of the Ca²⁺-ATPase. The crystals are disrupted by Ca²⁺ at a concentration sufficient to saturate the high affinity Ca²⁺ binding sites of the Ca²⁺-ATPase.