BIR2 domain of XIAP plays a marginal role in inhibition of executioner caspases

The ability of the irreversibly denatured XIAP-BIR2 domain versus the native protein in inhibition of executioner caspase-3 and -7 was investigated. The denatured protein that lacked the physical characteristics of the native protein inhibited caspase-7, while failing to inhibit caspase-3. Furthermore, the kinetics of association of the denatured protein with caspase-7 decreased substantially suggesting that the exposure of the linker is reduced. This was further confirmed by the decreased level of proteolysis at the linker by trypsin for the denatured protein. These results suggest that the essential moiety of the XIAP involved in inhibition is the linker joining BIR1 to BIR2 and that the BIR2 plays a marginal role in inhibition.