The human anti-apoptotic proteins cIAP1 and cIAP2 bind but do not inhibit caspases

cIAPs (cellular inhibitor of apoptosis proteins) 1 and 2 are able to regulate apoptosis when ectopically expressed in recipient cells and probably also in vivo. Previous work suggested that this is at least partially due to direct caspase inhibition, mediated by two of the three baculovirus IAP repeat (BIR) domains that are contained in these proteins. In support of this we show that the BIR domains 2 and 3 of the two cIAPs are able to bind caspases-7 and -9. However, we demonstrate that neither of these BIR domains is able to inhibit caspases because of critical substitutions in the regions that target caspase inhibition in the X-linked IAP, a tight binding caspase inhibitor. The cIAP BIR domains can be converted to tight binding caspase inhibitors by substituting these critical residues with XIAP residues. Thus, cIAPs maintain protein scaffolds suitable for direct caspase inhibition but have lost or never acquired specific caspase inhibitory interaction sites. Consequently, although the binding function of the cIAP BIRs may be important for their physiologic function, caspase inhibition is not.