Calpains (intracellular calcium-activated cysteine proteinases): structure-activity relationships and involvement in normal and abnormal cellular metabolism.

1. Calpains (calcium-activated cysteine proteinases) have evolved by gene fusion events involving calmodulin-like genes, cysteine proteinase genes and other sequences of unknown origin. 2. The enzymes are composed of two non-identical subunits, each of which contains functional calcium-binding sequences. 3. Calpains are inhibited by the endogenous protein inhibitor, calpastatin and some calmodulin antagonists are also inhibitors of calpain. A number of synthetic proteinase inhibitors also inhibit calpains. 4. Calpains can be activated by phospholipids, an endogenous protein activator and some amino acid derivatives. 5. Various protein substrates for calpains have been recognized in vitro, but the identity of in situ substrates remains unclear. 6. Proposals have been made for calpain function, including involvement in signal transduction, platelet activation, cell fusion, mitosis and cytoskeleton and contractile protein turnover. 7. Calpain and calpastatin expression is altered in a number of abnormal states including muscular dystrophy, muscle denervation and tenotomy, hypertension and platelet abnormalities.