Apopain/CPP32 Cleaves Proteins That Are Essential for Cellular Repair: A Fundamental Principle of Apoptotic Death By Livia Casciola-Rosen,** DonaldW. Nicholson,II Tae Chongfl

Summary Proteolysis mediated by the interleukin 1(3-converting enzyme (ICE) homologues is an impor- tant mechanism of the apoptotic process. The ICE homologue apopain/CPP-32/Yama (subse- quently referred to as apopain) cleaves poly(ADP-ribose)polymerase (PAR.P) early during apop- tosis. Additional apoptosis-specific protein cleavages have been observed in which the direct involvement of ICE-like proteases has been postulated. These substrates include the 70-kD pro- tein component of the Ul-ribonucleoprotein (U1-70kD), and the catalytic subunit of the DNA- dependent protein kinase (DNA-PKcs). The present studies demonstrate that U1-70kD and DNA-PK~ are excellent substrates for apopain, with cleavage occurring at sites that are highly similar to the cleavage site within PAR.P. The fragments generated from isolated protein sub- strates by apopain are identical to those observed in intact apoptotic cells, in apoptotic cell ex- tracts, and in normal cell extracts to which apopain has been added. Like PARP, cleavage of these substrates in apoptotic cell extracts is abolished by nanomolar concentrations of Ac- DEVD-CHO and micromolar amounts of Ac-YVAD-CHO, confirming the involvement of apopain or an apopain-like activity. We propose that a central function of apopain or similar homologues in apoptosis is the cleavage of nuclear repair proteins, thereby abolishing their crit- ical homeostatic functions.