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Description

"Calpains I & II are intracellular cysteine proteinases distributed in various tissues and cells. Calpain I & II are also known as mu- and m-calpains, respectively, based on their calcium requirement, with calpain I being active at micromolar Ca+2 concentrations and calpain II at millimolar Ca+2 concentrations. Both calpains are composed of a large catalytic (80 kDa) and a small regulatory (30 kDa) subunit, and each subunit contains a calmodulinlike domain at the C-terminus. Upon Ca2+ binding to the calmodulin-like domains, calpains become activated for catalyzing both substrate proteolytic degradation as well as autolysis. Fourteen calpain large subunits have been identified in human (calpain 1-3 & 5-15 encoded by CAPN1-3 & 5-15 gene) and two small subunits encoded by the CAPNS1 & CAPSN2 genes. The calpain small subunit 1 encoded by CAPNS1 is also known as calpain 4. To date, calpain I & II remain the best characterized members of the calpain family. Calpain activity was initially characterized as ""calcium-activated neutral protease"" (CANP) detected in brain, lens of the eye, and other tissues. The name ""calpain"" was later adopted as a hybrid name from two well-known proteins at the time, calmodulin and papain."