Structural Mechanisms of Calpain
(in)Activation
Calpains are unique because they combine papain-like cysteine protease activity with a dependence on Ca2+ binding. The regulatory mechanisms governing calpain activity are complex and clearly different from conventional cysteine proteases. The crystal structure of m-calpain in the Ca2+-free form has revealed the structural basis for the inactivity of calpain in the absence of Ca2+. Without Ca2+, the active site of calpain is not assembled, indicating that Ca2+-binding would induce conformational changes that re-orient the protease domains to form a functional enzyme. The first structure of calpain has revealed an unusual thiol protease fold, which is associated with Ca2+-binding EF-hand domains through a C2-like b-sandwich domain and hetero-dimerization with the regulatory subunit. In addition, we have also solved the structure of the core catalytic domains in the presence of Ca2+, confirming that conformational change is indeed required to assemble an active calpain. The Ca2+-structure further has revealed that Ca2+ ions bind in the catalytic domains and play a vital role in the activation. These structural studies have provided many insights into the (in)activation of calpain.