Calpain-2 (EC3.4.22.53,calcium-activated neutral protease II,m-calpain,milli-calpain) is an intracellular heterodimeric calcium-activated cysteine protease.[1][2] This enzymecatalyses the followingchemical reaction
Calpain-2 is a heterodimer of a catalytic subunit encoded by CAPN2 gene and a regulatory subunit CAPNS1.[1][4][5] The catalytic subunit consists of four domains: protease core 1 domain (PC1), protease core 2 domain (PC2), calpain-type beta-sandwich-like domain (CBSW), and penta EF-hand domain (PEF(L)).[3] The catalytic cleft is formed by PC1 and PC2 upon calcium binding.[6] The catalytic triad consists of residues C105, H262, and N286. Noteworthy, CAPN2 also contains an N-terminal anchor helix, which however is cleaved off upon protease activation.[7] It is believed to play a role in a regulation of catalytic activity.
The regulatory subunit consists of two domains: a glycine-rich domain (GR), and penta EF-hand domain (PEF(S)).[3] The interaction of PEF(S) and PEF(L) through an unpaired EF-hand motif causes dimerization of the two subunits. Calpain-2 heterodimer is highly homologous to calpain-1, which is formed by a catalytic CAPN1 and a regulatory CAPNS1 subunits.[3]
There is no known consensus sequence for calpain-2 proteolysis, but there is evidence for over 130 potential substrates.[8] Proteolytic cleavage by calpain-2 is regulated by presence of Ca2+ ions. It requires supraphysiological (low millimolar) concentration of Ca2+ for activation.[6] Intracellular concentration of Ca2+ (approx. 100 nM)[9] is insufficient for activating calpain-2, so activation occurs upon influx of ions from extracellular space or from endoplasmic reticulum. In addition, calpain-1/2 can be inhibited by calpastatin (encoded by the CAST gene) which binds to the PEF domains of the catalytic and regulatory subunits of calpains-1/2. It prohibits substrate binding to the active site through steric hindrance.[10]
Upregulation of calpain-2 is linked to increased aggressiveness of cancer.[11][12] There is evidence suggesting that the mechanism of action is through cleavage of substrates involved in cell migration, invasion, and sensitivity to chemotherapeutic agents.[13][14][15]
Previously used nomenclature used Roman numerals to denote calpain-2 domains starting from the N-terminus of CAPN2 and ending at C-terminus of CAPNS1. For example, PEF(L) and PEF(S) were referred to as Domain IV and Domain VI, respectively.[16]
^Dutt P, Arthur JS, Croall DE, Elce JS (October 1998). "m-Calpain subunits remain associated in the presence of calcium".FEBS Letters.436 (3):367–71.doi:10.1016/s0014-5793(98)01167-3.PMID9801150.
^Chou JS, Impens F, Gevaert K, Davies PL (July 2011). "m-Calpain activation in vitro does not require autolysis or subunit dissociation".Biochimica et Biophysica Acta (BBA) - Proteins and Proteomics.1814 (7):864–72.doi:10.1016/j.bbapap.2011.04.007.PMID21549862.
^Storr SJ, Carragher NO, Frame MC, Parr T, Martin SG (May 2011). "The calpain system and cancer".Nature Reviews. Cancer.11 (5):364–74.doi:10.1038/nrc3050.PMID21508973.S2CID23555255.
