Calsequestrin: a well-known but curious protein in skeletal muscle.

Calsequestrin (CASQ) was discovered in rabbit skeletal muscle tissues in 1971 and has been considered simply a passive Ca-buffering protein in the sarcoplasmic reticulum (SR) that provides Ca ions for various Ca signals. For the past three decades, physiologists, biochemists, and structural biologists have examined the roles of the skeletal muscle type of CASQ (CASQ1) in skeletal muscle and revealed that CASQ1 has various important functions as (1) a major Ca-buffering protein to maintain the SR with a suitable amount of Ca at each moment, (2) a dynamic Ca sensor in the SR that regulates Ca release from the SR to the cytosol, (3) a structural regulator for the proper formation of terminal cisternae, (4) a reverse-directional regulator of extracellular Ca entries, and (5) a cause of human skeletal muscle diseases. This review is focused on understanding these functions of CASQ1 in the physiological or pathophysiological status of skeletal muscle.