Ca2+ dependency of calpain 3 (p94) activation

Calpain 3, commonly called p94 in the literature, is the abundant skeletal muscle-specific calpain that is genetically linked to limb girdle muscular dystrophy type 2A. Recently, we showed that p94's insertion sequence 1 (IS1) is a propeptide that must be autoproteolytically cleaved to provide access of substrates and inhibitors to the enzyme's active site. Removal of IS1 from the core of p94 by recombinant methods produced a fully active enzyme. Here we have resolved the discrepancies in the literature about the Ca2+ requirement of p94 using the protease core. Even at substoichiometric levels of Ca2+, and in competition with EDTA, autoproteolyzed enzyme slowly accumulated. Because the initial autoproteolytic cleavage is an intramolecular reaction, transient binding of two Ca2+ ions to the core would be sufficient to promote the reaction that is facilitated by having the scissile peptide lying close to the active site cysteine. The second autolytic cleavage was much slower and required higher Ca2+ levels, consistent with it being an intermolecular reaction. Other metal ions such as Na+, K+, and Mg2+ cannot substitute for Ca2+ in catalyzing the intramolecular autoproteolysis of the p94 core or in the subsequent hydrolysis of exogenous substrates. These metal ions increase moderately the activity of this enzyme but only at very high concentrations. Thus, the proteolytic activity of the core of p94 and its deletion mutant lacking NS and IS1 was shown to be strictly Ca2+-dependent. We propose a two-stage model of activation of the proteolytic core of p94.