A mechanistic insight into caspase-7 inhibition by BIR1-2 domains of XIAP and cIAP1

Previous studies failed to demonstrate any role for the BIR1 domain of the inhibitor of apoptosis proteins (IAPs) in inhibition of executioner caspases. In this study, XIAP-BIR1-2 and c-IAP1-BIR1-2 domains have been used to investigate the role of BIR1 in the inhibition of caspase-7. Kinetic analysis confirmed that caspase-7 was inhibited in an uncompetitive manner at lower concentrations of XIAP-BIR1-2, whereas the inhibition was switched to the mixed type mode at higher concentrations of the inhibitor. In contrast, cIAP1-BIR1-2 inhibited caspase-7 in a mixed type mode at all examined concentrations. These data suggest that the presence of BIR1 is essential for inhibition of caspase-7 by cIAP1. Far-UV CD and fluorescence spectroscopy experiments showed that despite similar secondary structures, XIAP-BIR1-2 and cIAP1-BIR1-2 have different biophysical properties. BIR1-2 domain of XIAP was found to be more flexible than cIAP1, which may be the reason behind differences in their kinetic properties.