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Journal of Biochemistry Advance Access originally published online on July 23, 2007
Journal of Biochemistry 2007 142(3):365-376; doi:10.1093/jb/mvm143
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© 2007 The Japanese Biochemical Society.

Characteristics of Mitochondrial Calpains

Taku Ozaki1, Hiroshi Tomita2, Makoto Tamai2 and Sei-ichi Ishiguro1,*

1Department of Biochemistry and Biotechnology, Division of Cell Technology, Faculty of Agriculture and Life Science, Hirosaki University, Hirosaki 036-8561; and 2Biomedical Engineering Research Organization, Tohoku University, Sendai 980-3780, Japan

*To whom correspondence should be addressed. Tel: +81-172-39-3780, Fax: +81-172-39-3780, E-mail: is1019{at}cc.hirosaki-u.ac.jp

Received May 24, 2007; Accepted June 12, 2007


  Abstract

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.

Key Words: apoptosis-inducing factor (AIF), calpastatin, mitochondrial calpain, mitochondrial intermembrane space, µ-calpain-like protease


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