Regulation of effector caspases by inhibitor of apoptosis (IAP) proteins
Apoptosis is a biologically essential phenomenon executed in large part by caspases. Members of the caspase family are activated at different points during apoptosis to proteolyze specific substrates. Given that both excessive and insufficient apoptosis is related to the pathogenesis of various diseases, proper regulation of caspases and apoptosis is necessary for the health of living organisms. Inhibitor of apoptosis (IAP) proteins are endogenous inhibitors of caspases, and since XIAP, the prototypical IAP, binds to and inhibits caspases, all IAPs have been speculated to engage in similar inhibition mechanisms. However, in this dissertation, I demonstrate that cIAP1 binds to the effector caspases-3 and -7, through distinct mechanisms. cIAP1 readily binds to and ubiquitinates, but dos not directly inhibit the activity of fully mature caspase-7. By contrast, cIAP1 does not bind to caspase-3. cIAP1 binding to caspase-7 is mediated primarily by the N-terminus of the large subunit of caspase-7. An AKPD motif located on the N-terminus of caspase-7 is involved in the proteasome-mediated degradation of caspase-7 in cells, thereby decreasing the sensitivity of these cells to apoptosis. Thus, I demonstrate for the first time that cIAP1 is capable of inhibiting caspase-dependent apoptosis through indirect regulation of caspase activity.