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

The absolute requirement of Ca2+ for proteolytic activity is a feature uniq ue to the calpains, a family of heterodimeric cysteine proteases. Condition s are described which give rise to diffraction-quality crystals of m-calpai n in two crystal forms, P1 and P2(1). Data have been collected from native crystals of m-calpain in both P1 and P2(1) forms, to 2.6 and 2.15 Angstrom, respectively. Selenomethionine-containing crystals have been grown in both forms, and anomalous data from the P2(1) selenomethionine enzyme provided the location of 17 of the 19 Se atoms in the protein.