Calpain is an intracellular Ca2+-dependent cysteine protease (EC 18.104.22.168; Clan CA, family C02). Calpain has limited proteolytic activity; it transforms or modulates the structure and/or activity of its substrates. It is, therefore, referred to as a "modulator protease". Calpains are found in almost all eukaryotes and a few eubacteria, but not in archaebacteria. In the human genome, 15 genes (CAPN1-3, 5-16) encode a calpain-like protease (CysPc) domain along with several different functional domains.
Calpain has long been an enigmatic enzyme, although it is involved in a variety of biological phenomena. Recent progress in calpain genetics combined with biochemistry and structural biology studies has highlighted numerous physiological contexts in which the functions of calpain are of great significance. Genetic studies have shown that calpain deficiencies can cause a variety of adverse consequences in many different organisms, including lethality, muscular dystrophies, and gastropathy. These findings provide the basis for ongoing and future studies on the physiological role of calpains.
Since the first calpain was discovered in 1964 by Gordon Guroff, many other researchers have contributed to their study, resulting in nomenclature that is sometimes inconsistent. This site summarizes the current knowledge about the structure and nomenclature of calpains, and attempts to find a consensus.