Specific inhibition of calpain (calcium-dependent cysteine protease). Plays a key role in postmortem tenderization of meat and have been proposed to be involved in muscle protein degradation in living tissue (By similarity). Each of the four flexible inhibitory domains can inhibit one calcium-bound calpain molecule by occupying both sides of the active site. Belongs to the protease inhibitor I27 (calpastatin) family.