Structure of a calpain Ca²⁺-binding domain reveals a novel EF-hand and Ca²⁺-induced conformational changes

The crystal structure of a Ca²⁺-binding domain (dVI) of rat m-calpain has been determined at 2.3 Å resolution, both with and without bound Ca²⁺. The structures reveal a unique fold incorporating five EF-hand motifs per monomer, three of which bind calcium at physiological calcium concentrations, with one showing a novel EF-hand coordination pattern. This investigation gives us a first view of the calcium-induced conformational changes, and consequently an insight into the mechanism of calcium induced activation in calpain. The crystal structures reveal a dVI homodimer which provides a preliminary model for the subunit dimerization in calpain.