X-linked inhibitor of apoptosis protein (XIAP) inhibits caspase-3 and-7 indistinct modes

The inhibitor of apoptosis proteins (IAP) regulates cell death by inhibitin g caspases. The region of X-linked (X) IAP containing the second baculoviru s IAP repeat domain (BIR2) is sufficient for inhibiting caspase-3 and -7. I n this study,we found that the modes of inhibition of these two caspases we re different: caspase-3 is inhibited in a competitive manner whereas caspas e-7 inhibition occurs through a mixed competitive and noncompetitive mechan ism. Binding assays revealed that the inhibition of caspase-3 by XIAP was t otally dependent on the interaction between the active site of caspase-3 an d the linker region between the BIR1 and BIR2 domains of XIAP. In contrast, the active site and the NP2-terminal region of caspase-7 bound to the link er region and the BIR2, respectively. Moreover the BIR2 with a mutated link er region, which inhibited caspase-3 very weakly, still bound to and inhibi ted caspase-7. Furthermore, a chimeric caspase-7/3 comprising the NH2-termi nal portion of caspase-7 and COOH-terminal portion of caspase-3 was inhibit ed by XIAP by a mixed competitive and noncompetitive mechanism. Our results suggest that the Linker region between BIR1 and BIR2 domains is responsibl e for active site-directed, competitive inhibition of both caspase-3 and -7 , whereas the BIR2 itself is involved in noncompetitive inhibition of caspa se-7.