Calsequestrin is a high-capacity, moderate affinity, calcium-binding protein and thus acts as an internal calcium store in muscle (PubMed:28895244). Calcium ions are bound by clusters of acidic residues at the protein surface, often at the interface between subunits. Can bind around 80 Ca2+ ions (PubMed:28895244). Regulates the release of lumenal Ca2+ via the calcium release channel RYR1; this plays an important role in triggering muscle contraction. Negatively regulates store-operated Ca2+ entry (SOCE) activity (PubMed:27185316).