Calsequestrin is a high-capacity, moderate affinity, calcium-binding protein and thus acts as an internal calcium store in muscle (PubMed:3427023). Calcium ions are bound by clusters of acidic residues at the protein surface, especially at the interface between subunits. Can bind around 60 Ca(2+) ions. Regulates the release of lumenal Ca(2+) via the calcium release channel RYR2; this plays an important role in triggering muscle contraction. Plays a role in excitation-contraction coupling in the heart and in regulating the rate of heart beats. Interacts with ASPH (By similarity). Monomer, homodimer and homooligomer. Mostly monomeric in the absence of calcium. Forms higher oligomers in a calcium-dependent manner. Dimers associate to form tetramers, that then form linear homomer chains. Interacts with TRDN. This isoform of calsequestrin occurs in the sarcoplasmic reticulum's terminal cisternae luminal spaces of cardiac and slow skeletal muscle cells. Detected in heart muscle (at protein level). Phosphorylation in the C-terminus, probably by CK2, moderately increases calcium buffering capacity. N-glycosylated. Belongs to the calsequestrin family.