A Ca2+ Switch Aligns the Active Site of Calpain

Ca2+ signaling by calpains leads to controlled proteolysis during processes ranging from cytoskeleton remodeling in mammals to sex determination in nematodes. Deregulated Ca2+ levels result in aberrant proteolysis by calpains, which contributes to tissue damage in heart and brain ischemias as well as neurodegeneration in Alzheimerʹs disease. Here we show that activation of the protease core of μ calpain requires cooperative binding of two Ca2+ atoms at two non-EF-hand sites revealed in the 2.1 Å crystal structure. Conservation of the Ca2+ binding residues defines an ancestral general mechanism of activation for most calpain isoforms, including some that lack EF-hand domains. The protease region is not affected by the endogenous inhibitor, calpastatin, and may contribute to calpain-mediated pathologies when the core is released by autoproteolysis.