A SEQUENTIAL 2-STEP MECHANISM FOR THE PRODUCTION OF THE MATURE P17-P12 FORM OF CASPASE-3 IN-VITRO

The apoptotic cysteine protease, caspase-3, is expressed in cells as a n inactive 32 kDa precursor from which 17 kDa (p17) and 12 kDa (p12) s ubunits of the mature caspase-3 are proteolytically generated during a poptosis, Two amino acid sequences, ESMD down arrow S (amino acids 25- 29) and IETD down arrow S (amino acids 172-176), in the precursor have been defined as the cleavage sites for the production of the p17 and p12 subunits, Using a cell-free assay system, we demonstrate that the caspase-3 precursor appears to be cleaved first at the IETD down arrow S site, producing the p12 subunit and a 20-kDa (p20) peptide, Subsequ ently, the p20 is cleaved at the ESMD down arrow S site, generating th e mature p17 subunit, The cleavage at the IETD down arrow S site requi red a protease activity that was selectively inhibited by the peptide, Ac-IETD-CHO (acetyl-IETD-aldehyde), and other protease inhibitors, su ch as the cowpox viral serine protease inhibitor, CrmA, and N-alpha-to syl-L-phenylalanine chloromethyl ketone. The protease that catalyzed t he cleavage at the ESMD/S site was selectively inhibited by another pe ptide, Ac-ESMD-CHO (acetyl-ESMD-aldehyde). More interestingly, the cas pase-3 inhibitor, Ac-DEVD-CHO, but not the caspase-1 inhibitor, Ac-YVA D-CHO, also selectively inhibited the protease activity that cleaves a t the ESMD down arrow S site. This indicated that the cleavage at the ESMD down arrow site was either autocatalytic or that it required a ca spase-3-like activity. In summary, we demonstrate that production of t he p17:p12 form of caspase-3 is a sequential two-step process and appe ars to require two distinct enzymatic activities.