Modulation of the calpain autoproteolysis by calpastatin and phospholipids.

The Ca-induced autoproteolysis calpain proceeds through the sequential formation of two forms of active enzyme with molecular masses of 78 kD and 75 kD, respectively. The autolysed calpains are produced by the cleavage of the peptide bond between Ser15-Ala16 and then between Gly27-Leu28. Calpastatin reduces with high efficiency the transition from 78 kD to 75 kD calpain forms. At higher concentration also the first autolytic event is blocked. The data are consistent with the presence of two calpain forms with different susceptibility to calpastatin. Furthermore, calpain, once bound to phospholipid vesicles, undergoes autoproteolysis which preferentially accumulates the 78 kD species. These data provide new information on the activation process of calpain, indicating that a Ca-induced conformational change is the triggering event, followed by the appearance of the active 78 kD calpain which can be considered the preferential form of calpain at the membrane level.