Calpain large subunit, domain III superfamily (IPR036213)
Short name: Calpain_III_sf
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 [PMID: 2539381,PMID: 11517928]. The protein is a complex of 2 polypeptide chains (light and heavy), with eleven known active peptidases in humans and two non-peptidase homologues known as calpamodulin and androglobin [PMID: 21864727]. These include 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 [PMID: 2555341].
All forms have identical light but different heavy chains. Both mu- and m-calpain are heterodimers containing an identical 28kDa subunit and an 80kDa subunit that shares 55-65% sequence homology between the two proteases [PMID: 7845226, PMID: 2539381]. The crystallographic structure of m-calpain reveals six "domains" in the 80kDa subunit [PMID: 9396712,PMID: 11328585]:
- 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 [PMID: 7845226].
- 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 [PMID: 7845226]. 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 [PMID: 11914728].
This superfamily describes domain III. Calpains are activated via rearrangement of the catalytic domain II induced by cooperative binding of Ca2+ to several sites of the molecule. A cluster of acidic residues in domain III, the acidic loop, has been proposed to function as part of an electrostatic switch in the activation process [PMID: 15180595].
- SSF49758 (SSF49758)