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. Purity >90% as determined by SDS-PAGE.