Recent evidence suggests that CPP32 is an essential component of an aspartate-specific cysteine protease (ASCP) cascade responsible for apoptosis execution in mammalian cells. Activation of CPP32 could lead to activation of other downstream ASCPs, resulting in late morphological changes such as lamin cleavage and DNA fragmentation, observed in cells undergoing apoptosis. Here we describe the identification and cloning of a novel human ASCP named Mch6 from Jurkat T lymphocytes. We demonstrate that the pro-enzymes of Mch6 and the lamin-cleaving enzyme Mch2alpha are substrates for mature CPP32. Site-directed mutagenesis revealed that CPP32 processes pro-Mch6 preferentially at Asp330 to generate two subunits of molecular masses 37 kDa (p37) and 10 kDa (p10). However, CPP32 processes pro-Mch2alpha at three aspartate processing sites (Asp23, Asp179, and Asp193) to produce the large (p18) and small (p11) subunits of the mature Mch2alpha enzyme. The CPP32-processed Mch2alpha is capable of cleaving the VEIDN lamin cleavage site, indicating that CPP32 can, in fact, activate pro-Mch2alpha. Granzyme B at a concentration that allows processing and activation of CPP32 failed to process pro-Mch2alpha. However, incubation of pro-Mch2alpha with granzyme B in the presence of a cellular extract containing pro-CPP32 resulted in activation of pro-CPP32 and subsequent processing of pro-Mch2alpha. Interestingly, granzyme B can also process pro-Mch6 but at a site N-terminal to that cleaved by CPP32. These data suggest that Mch2alpha and Mch6 are downstream proteases activated in CPP32- and granzyme B-mediated apoptosis. This is the first demonstration of a protease cascade involving granzyme B, CPP32, Mch2alpha, and Mch6 and evidence that the lamin-cleaving enzyme Mch2 is a target of mature CPP32.
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