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.