Direct inhibition of caspase 3 is dispensable for the anti-apoptotic activity of XIAP

XIAP is a mammalian inhibitor of apoptosis protein (IAP), To determine resi dues within the second baculoviral IAP repeat (BIR2) required for inhibitio n of caspase 3, we screened a library of BIR2 mutants for loss of the abili ty to inhibit caspase 3 toxicity in the yeast Schizosaccharomyces pombe. Fo ur of the mutations, not predicted to affect the structure of the BIR fold, clustered together on the N-terminal region that flanks BIR2, suggesting t hat this is a site of interaction with caspase 3. Introduction of these mut ations into full-length XIAP reduced caspase 3 inhibitory activity up to 50 0-fold, but did not affect its ability to inhibit caspase 9 or interact wit h the IAP antagonist DIABLO. Furthermore, these mutants retained full abili ty to inhibit apoptosis in transfected cells, demonstrating that although X IAP is able to inhibit caspase 3, this activity is dispensable for inhibiti on of apoptosis by XIAP in vivo.