Calpain activation by cooperative Ca2+ binding at two non-EF-hand sites

The active site residues in calpain are mis-aligned in the apo, Ca2+-free form. Alignment for catalysis requires binding of Ca2+ to two non-EF-hand sites, one in each of the core domains I and II. Using domain swap constructs between the protease cores of the mu and m isoforms (which have different Ca2+ requirements) and structural and biochemical characterization of site-directed mutants, we have deduced the order of Ca2+ binding and the basis of the cooperativity between the two sites. Ca2+ binds first to the partially preformed site in domain I. Knockout of this site through D106A substitution eliminates binding to this domain as shown by the crystal structure of D106A muI-II. However, at elevated Ca2+ concentrations this mutant still forms the double salt bridge that links the two Ca2+ sites and becomes nearly as active as muI-II. Elimination of the bridge in E333A muI-II has a more drastic effect on enzyme action, especially at low Ca2+ concentrations. Domain II Ca2+ binding appears essential, because Ca2+-coordinating side-chain mutants E302R and D333A have severely impaired muI-II activation and activity. The introduction of mutations into the whole heterodimeric enzyme that eliminate the salt bridge or Ca2+ binding to domain II produce similar phenotypes, suggesting that the protease core Ca2+ switch is crucial and cannot be overridden by Ca2+ binding to other domains.