Binding of calcium ions to the calpain dimer composed of the calpain catalytic subunit of 80 kDa and a calpain regulatory subunit of 30 kDa enables conformation change that results in formation of a functional catalytic center and also promotes relocalization of the calpain complex to the plasma membrane (Lin et al. 1997, Strobl et al. 2000, Schad et al. 2002). Calpain complexes involving the neuronally expressed mu-calpain (CAPN1) and m-calpain (CAPN2) catalytic subunits (Lee et al. 2000) are shown in this activation reaction.