Ca2+‐Binding domain VI of rat calpain is a homodimer in solution: Hydrodynamic, crystallization and preliminary X‐ray diffraction studies

Abstract
The 21‐kDa calcium‐binding domain (VI) of the small subunit of rat calpain II has been expressed in Escherichia coli, purified, and crystallized. Two orthorhombic crystal forms have been obtained: space group P212121 with a = 50.3, b = 56.5, c = 141.3 Å; and space group C2221 with a = 69.4, b = 73.9, c = 157.4 Å. Diffraction data have been collected to 2.4 Å. Sedimentation equilibrium, dynamic light scattering, and gel‐permeation chromatography indicate that domain VI exists as a homodimer in solution. In accordance with the protein's behavior in solution, each crystal form contains two molecules per asymmetric unit. Screening for heavy‐atom derivatives is in progress. To decrease the sensitivity to mercurials and to aid in the search for useful derivatives, Cys‐to‐Ser mutants have been prepared, expressed, and crystallized.