Cleavage of human inhibitor of apoptosis protein XIAP results in fragmentswith distinct specificities for caspases

Several human inhibitor of apoptosis (IAP) family proteins function by dire ctly inhibiting specific caspases in a mechanism that does not require IAP cleavage. In this study, however, we demonstrate that endogenous XIAP is cl eaved into two fragments during apoptosis induced by the tumor necrosis fac tor family member Fas (CD95), The two fragments produced comprise the bacul oviral inhibitory repeat (BIR) 1 and 2 domains (BIR1-2) and the BIR3 and RI NG (BIR3-Ring) domains of XIAP, Overexpression of the BIR1-2 fragment inhib its Fas-induced apoptosis, albeit at significantly reduced efficiency compa red with full-length XIAP, In contrast, overexpression of the BIR3-Ring fra gment results in a slight enhancement of Fas-directed apoptosis, Thus, clea vage of XIAP may be one mechanism by which cell death programs circumvent t he anti-apoptotic barrier posed by XIAP, Interestingly, ectopic expression of the BIR3-Ring fragment resulted in nearly complete protection from Bar-i nduced apoptosis, Use of purified recombinant proteins revealed that BIR3-R ing is a specific inhibitor of caspase-9 whereas BIR1-2 is specific for cas pases 3 and 7, Therefore XIAP possesses two different caspase inhibitory ac tivities which can be attributed to distinct domains within XIAP, These dat a may provide an explanation for why IAPs have evolved with multiple BIR do mains.