Calpain-2

Structure

Calpain-2 is a heterodimer of a catalytic subunit encoded by CAPN2 gene and a regulatory subunit CAPNS1. 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)). The catalytic cleft is formed by PC1 and PC2 upon calcium binding. 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. 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)). 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.

Properties

There is no known consensus sequence for calpain-2 proteolysis, but there is evidence for over 130 potential substrates. Proteolytic cleavage by calpain-2 is regulated by presence of Ca2+ ions. It requires supraphysiological (low millimolar) concentration of Ca2+ for activation. Intracellular concentration of Ca2+ (approx. 100 nM) 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.

Calpain-2 in Cancer

Upregulation of calpain-2 is linked to increased aggressiveness of cancer. There is evidence suggesting that the mechanism of action is through cleavage of substrates involved in cell migration, invasion, and sensitivity to chemotherapeutic agents.

Domain Nomenclature

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.

References

References

1. (January 2000). "The crystal structure of calcium-free human m-calpain suggests an electrostatic switch mechanism for activation by calcium". Proceedings of the National Academy of Sciences of the United States of America.
2. (October 2002). "Origins of the difference in Ca2+ requirement for activation of mu- and m-calpain". The Biochemical Journal.
3. (January 2012). "Calpains: an elaborate proteolytic system". Biochimica et Biophysica Acta (BBA) - Proteins and Proteomics.
4. (December 1999). "Crystal structure of calpain reveals the structural basis for Ca(2+)-dependent protease activity and a novel mode of enzyme activation". The EMBO Journal.
5. (October 1998). "m-Calpain subunits remain associated in the presence of calcium". FEBS Letters.
6. (March 2002). "A Ca(2+) switch aligns the active site of calpain". Cell.
7. (July 2011). "m-Calpain activation in vitro does not require autolysis or subunit dissociation". Biochimica et Biophysica Acta (BBA) - Proteins and Proteomics.
8. (April 2011). "GPS-CCD: a novel computational program for the prediction of calpain cleavage sites". PLOS ONE.
9. (2008). "Extracellular calcium as an integrator of tissue function". The International Journal of Biochemistry & Cell Biology.
10. (November 2008). "Calcium-bound structure of calpain and its mechanism of inhibition by calpastatin". Nature.
11. (May 2011). "The calpain system and cancer". Nature Reviews. Cancer.
12. (October 2012). "Calpain-2 expression is associated with response to platinum based chemotherapy, progression-free and overall survival in ovarian cancer". Journal of Cellular and Molecular Medicine.
13. (September 2005). "Regulating cell migration: calpains make the cut". Journal of Cell Science.
14. (August 2016). "Calpain Genetic Disruption and HSP90 Inhibition Combine To Attenuate Mammary Tumorigenesis". Molecular and Cellular Biology.
15. (September 2018). "Genetic disruption of calpain-1 and calpain-2 attenuates tumorigenesis in mouse models of HER2+ breast cancer and sensitizes cancer cells to doxorubicin and lapatinib". Oncotarget.
16. "Structure and nomenclature / Calpain Research Portal: Calpain Structure and Nomenclature".