A calpain can be described as a protein which belongs to a family of fully calcium-dependent proteolytic enzymes that can be expressed around the entire body in a range of different mammals and other organisms. Calpain molecules facilitate the formation of PMP through a process of degradation of structural proteins (the actin-binding protein and the chain of myosin).
Recent research and investigation into sequence data definitively proves that calpain has in fact been present throughout years upon years of evolution, being identified and located in nearly all eukaryotes and some bacteria. It’s said that 15 genes inside the human genome actually encode a calpain-like protease domain, and it’s interesting to note that some human calpains (especially those with non-classical domain structures) are eerily similar to calpain homologs that have previously been seen in evolutionarily distant and dissimilar organisms. 3D structural analyses have enabled scientists to observe and assess calpain's unique mechanism of activation - the calpain protease molecule comprises of two central domains which fuse together to form a functional protease, but this process can only happen when it’s bound to a Ca2+ via complete amino acids.
Calpain homologs have been identified in nearly all forms of eukaryotes, and even inside certain strains of bacteria, too. The influence of calpain activity on pathophysiological phenomena has been reported in the past, but often as nothing more than an aggravating factor. As a result, although it has been accepted that regulation of calpain is essential for normal cell function, questions still remain surrounding when and how CAPN is actually activated.