This group of cysteine peptidases belong to the MEROPS peptidase family C2 (calpain family, clan CA). A type example is calpain, which is an intracellular protease involved in many important cellular functions that are regulated by calcium . The protein is a complex of 2
polypeptide chains (light and heavy), with three known forms in mammals
[7845226, 2555341]: a highly calcium-sensitive (i.e., micro-molar range) form known as mu-calpain, mu-CANP or calpain I; a form sensitive to calcium in the milli-molar range, known as m-calpain, m-CANP or calpain II; and a third form, known as p94, which is found in skeletal muscle only .
All forms have identical light but different heavy chains. Both mu- and m-calpain are heterodimers containing an identical 28-kDa subunit and an 80-kDa subunit that shares 55-65% sequence homology between the two proteases [7845226, 2539381]. The crystallographic structure of m-calpain reveals six "domains" in the 80-kDa subunit:
- A 19-amino acid NH2-terminal sequence;
- Active site domain IIa;
- Active site domain IIb.
Domain 2 shows
low levels of sequence similarity to papain; although the catalytic His has
not been located by biochemical means, it is likely that calpain and papain
are related .
- Domain III;
- An 18-amino acid extended sequence linking domain III to domain IV;
- Domain IV, which resembles the penta EF-hand family of polypeptides, binds calcium and regulates activity . />]. Ca2+-binding causes a rearrangement of the protein backbone, the net effect of which is that a Trp side chain, which acts as a wedge between catalytic domains IIa and IIb in the apo state, moves away from the active site cleft allowing for the proper formation of the catalytic triad .
Calpain-like mRNAs have been identified in other organisms including bacteria, but the molecules encoded by these mRNAs have not been isolated, so little is known
about their properties. How calpain activity is regulated in these organisms cells is still unclear In metazoans, the activity of calpain is controlled by a single proteinase inhibitor, calpastatin . The calpastatin gene can produce eight or more calpastatin polypeptides ranging from 17 to 85 kDa by use of different promoters and alternative splicing events. The physiological significance of these different calpastatins is unclear, although all bind to three different places on the calpain molecule; binding to at least two of the sites is Ca2+ dependent. The calpains ostensibly participate in a variety of cellular processes including remodelling of cytoskeletal/membrane attachments, different signal transduction pathways, and apoptosis. Deregulated calpain activity following loss of Ca2+ homeostasis results in tissue damage in response to events such as myocardial infarcts, stroke, and brain trauma .