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 (By similarity). 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 (By similarity). Monomer; increases in response to a depletion of intracellular calcium. Homodimer. Homotetramer and homopolymer. Can form linear homooligomers. Ca(2+) ions promote oligomerization. Interacts (via C-terminal end and preferentially with the monomeric form) with STIM1; this interaction increases in response to a depletion of intracellular calcium, decreases both STIM1 aggregation and clustering, interaction of STIM1 with ORAI1 and store-operated Ca(2+) entry (SOCE) activity. Interacts with ASPH and TRDN. This isoform of calsequestrin occurs in the sarcoplasmic reticulum's terminal cisternae luminal spaces of fast skeletal muscle cells (PubMed:22049211). 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 (By similarity). Detected in skeletal muscle (at protein level). Detected in skeletal muscle. N-glycosylated. Mice are viable and fertile, but have a lower body weight than wild-type, due to a reduction in fast-twitch muscle mass. Fast-twitch muscle from mutant mice exhibits slower contraction kinetics and requires more time to achieve peak tension and to achieve half-relaxation after a contraction. Fast-twitch muscle fibers from mutant mice show a strikingly altered structure of the calcium release units in the sarcoplasmic reticulum with a strongly increased number of ryanodine receptors, plus narrower sarcoplasmic reticulum cisternae. In addition, the number of mitochondria is increased in mutant muscle. Mutant muscle fibers show smaller calcium transients upon electrical stimulation and release less Ca(2+) in response to caffeine. Belongs to the calsequestrin family. Truncated N-terminus.