Dissociation and Aggregation of Calpain in the Presence of Calcium

Calpain is a heterodimeric Ca2+-dependent cysteine protease consisting of a large (80 kDa) catalytic subunit and a small (28 kDa) regulatory subunit. The effects of Ca2+ on the enzyme include activation, aggregation, and autolysis. They may also include subunit dissociation, which has been the subject of some debate. Using the inactive C105S-80k/21k form of calpain to eliminate autolysis, we have studied its disassociation and aggregation in the presence of Ca2+ and the inhibition of its aggregation by means of crystallization, light scattering, and sedimentation. Aggregation, as assessed by light scattering, depended on the ionic strength and pH of the buffer, on the Ca2+ concentration, and on the presence or absence of calpastatin. At low ionic strength, calpain aggregated rapidly in the presence of Ca2+, but this was fully reversible by EDTA. With Ca2+ in 0.2 m NaCl, no aggregation was visible but ultracentrifugation showed that a mixture of soluble high molecular weight complexes was present. Calpastatin prevented aggregation, leading instead to the formation of a calpastatin-calpain complex. Crystallization in the presence of Ca2+ gave rise to crystals mixed with an amorphous precipitate. The crystals contained only the small subunit, thereby demonstrating subunit dissociation, and the precipitate was highly enriched in the large subunit. Reversible dissociation in the presence of Ca2+ was also unequivocally demonstrated by the exchange of slightly different small subunits between μ-calpain and m-calpain. We conclude that subunit dissociation is a dynamic process and is not complete in most buffer conditions unless driven by factors such as crystal formation or autolysis of active enzymes. Exposure of the hydrophobic dimerization surface following subunit dissociation may be the main factor responsible for Ca2+-induced aggregation of calpain. It is likely that dissociation serves as an early step in calpain activation by releasing the constraints upon protease domain I.