Purity >90% by SDS-PAGE. Calsequestrin is the major calcium storage protein of the sarcoplasmic reticulum. Intraluminar Ca2+ binds to calsequestrin during diastole to prevent Ca2+ precipitation and to lower its free ionic concentration to facilitate efficient storage. During systole, Calsequestrin coordinately releases ~40--50 Ca2+ ions per molecule for each contraction-relaxation cycle by an uncertain mechanism. Calsequestrin has been shown to be of major importance in the regulation of cardiac excitation-contraction coupling.