Crystallization and X-ray crystallographic analysis of m-calpain, a Ca²⁺-dependent protease

The absolute requirement of Ca²⁺ for proteolytic activity is a feature unique to the calpains, a family of heterodimeric cysteine proteases. Conditions are described which give rise to diffraction-quality crystals of m-calpain in two crystal forms, P1 and P2₁. Data have been collected from native crystals of m-calpain in both P1 and P2₁ forms, to 2.6 and 2.15 Å, respectively. Selenomethionine-containing crystals have been grown in both forms, and anomalous data from the P2₁ selenomethionine enzyme provided the location of 17 of the 19 Se atoms in the protein.