Penta-EF hand, calcium binding motifs, found in m-type calpain (CAPN2)
CAPN2, also termed millimolar-calpain (m-calpain), or calpain-2 catalytic subunit, or calcium-activated neutral proteinase 2 (CANP 2), or calpain large polypeptide L2, or calpain-2 large subunit, is a ubiquitously expressed 80-kDa Ca2+-dependent intracellular cysteine protease that contains a short N-terminal anchor helix, followed by a calpain cysteine protease (CysPc) domain, a C2-domain-like (C2L) domain, and a C-terminal Ca2+-binding penta-EF-hand (PEF) domain. The catalytic subunit CAPN2 in complex with a regulatory subunit encoded by CAPNS1 forms an m-calpain heterodimer. CAPN2 acts as the key protease responsible for N-methyl-d-aspartic acid (NMDA)-induced cytoplasmic polyadenylation element-binding protein 3 (CPEB3) degradation in neurons. It cleaves several components of the focal adhesion complex, such as FAK and talin, triggering disassembly of the complex at the rear of the cell. The stimulation of CAPN2 activity is required for Golgi antiapoptotic proteins (GAAPs) to promote cleavage of FA kinase (FAK), cell spreading, and enhanced migration. calpain 2 is also involved in the onset of glial differentiation. It regulates proliferation, survival, migration, and tumorigenesis of breast cancer cells through a PP2A-Akt-FoxO-p27(Kip1) signaling cascade. Its expression is associated with response to platinum based chemotherapy, progression-free and overall survival in ovarian cancer. Moreover, CAPN2 may play a role in fundamental mitotic functions, such as the maintenance of sister chromatid cohesion. The activation of CAPN2 plays an essential role in hippocampal synaptic plasticity and in learning and memory. In the eye, CAPN2, together with a lens-specific variant of CAPN3, is responsible for proteolytic cleavages of alpha and beta-crystallin. Overactivated alpha and beta-crystallin can lead to cataract formation. Sometimes, CAPN2 compensates for loss of CAPN1, and both calpain isoforms are involved in AngII-induced aortic aneurysm formation. The main phosphorylation sites in m-calpain are Ser50 and Ser369/Thr370.