Fig. 1. Schematic representation of the domain architecture of the classical calpains. The 80 kDa large subunits can be divided into four domains, plus a short linker that might be important for transducing conformational changes throughout the molecule upon calcium binding (T). The N-terminal -helix makes up domain I, which interacts with the small subunits before undergoing intermolecular autolysis on activation. Protease activity is contained within domain II, which is further divided into subdomains (IIa and IIb) that make up the two halves of the active site. Domain III comprises a C2-like domain that harbors sites for phosphorylation and phospholipid binding. Five consecutive EF-hand motifs make up domain IV and contribute to the calcium binding of the large subunits and to dimerization with the small subunits. Domain VI of the small subunits has a similar arrangement; the first four EF hands participate in calcium binding and the last motif interacts with the large subunit. The small subunits also contain a highly flexible, glycine-rich region called domain V. Calpain 1 and calpain 2 large subunits are phosphorylated at several sites in domains I-III; some of these residues are conserved and some are isoform specific.