Contributes to the regulation of lumenal Ca2+ release via the sarcoplasmic reticulum calcium release channels RYR1 and RYR2, a key step in triggering skeletal and heart muscle contraction. Required for normal organization of the triad junction, where T-tubules and the sarcoplasmic reticulum terminal cisternae are in close contact. Required for normal skeletal muscle strength. Plays a role in excitation-contraction coupling in the heart and in regulating the rate of heart beats. Homooligomer of variable subunit number; disulfide-linked. Interacts with CASQ1 and RYR1 in skeletal muscle (By similarity). Interacts with CASQ2. Detected in heart (at protein level). Skeletal and cardiac muscle. Phosphorylated by CaMK2. N-glycosylated.