Calsequestrin is a high-capacity, moderate affinity, calcium-binding protein and thus acts as an internal calcium store in muscle . Calcium ions are bound by clusters of acidic residues at the protein surface, often at the interface between subunits. Can bind around 80 Ca(2+) ions . Regulates the release of lumenal Ca(2+) via the calcium release channel RYR1; this plays an important role in triggering muscle contraction. Negatively regulates store-operated Ca(2+) entry (SOCE) activity .
Subcellular locations: Endoplasmic reticulum, Sarcoplasmic reticulum, Sarcoplasmic reticulum lumen, Sarcoplasmic reticulum membrane, Mitochondrion matrix
This isoform of calsequestrin occurs in the sarcoplasmic reticulum's terminal cisternae luminal spaces of fast skeletal muscle cells. Preferentially forms linear and round aggregates in the endoplasmic reticulum (ER) of resting cells . In a minority of cells, homogeneously detected in the ER lumen . Colocalizes with STIM1 at endoplasmic reticulum in response to a depletion of intracellular calcium .
Expressed in myoblasts (at protein level).