Characteristics of Mitochondrial Calpains

Calpains are considered to be cytoplasmic enzymes, although several studies have shown that calpain-like protease activities also exist in mitochondria. We partially purified mitochondrial calpain from swine liver mitochondria and characterized. Only one type of mitochondrial calpain was detected by the column chromatographies. The mitochondrial calpain was stained with anti-μ-calpain and calpain small subunit antibodies. The susceptibility of mitochondrial calpain to calpain inhibitors and the optimum pH differ from those of cytosolic μ- and m-calpains. The Ca2+-dependency of mitochondrial calpain was similar to that of cytosolic μ-calpain. Therefore, we named the protease mitochondrial μ-like calpain. In zymogram analysis, two types of caseinolytic enzymes existed in mitochondria and showed different mobilities from cytosolic μ- and m-calpains. The upper major band was stained with anti-μ-calpain and calpain small subunit antibodies (mitochondrial calpain I, mitochondrial μ-like calpain). The lower band was stained only with anti-calpain small subunit antibody (mitochondrial calpain II, unknown mitochondrial calpain). Calpastatin was not detected in mitochondrial compartments. The mitochondrial calpain processed apoptosis-inducing factor (AIF) to truncated AIF (tAIF), releasing tAIF into the intermembrane space. These results indicate that mitochondrial calpain, which differs from μ- and m-calpains, seems to be a ubiquitous calpain and may play a role in mitochondrial apoptotic signalling.