The inhibitor of apoptosis protein-binding domain of Smac is not essentialfor its proapoptotic activity

Smac/DIABLO, a recently identified inhibitor of apoptosis protein (IAP)-bin ding protein, is released from the mitochondria during apoptosis and report edly potentiates apoptosis by relieving the inhibition of IAPs on caspases. We now describe the molecular characterization of Smac beta, an alternativ ely spliced form of Smac, which lacks the mitochondrial-targeting sequence found in Smac and has a cortical distribution in both human embryonic kidne y 293 and breast epithelial tumor MCF-7 cells. Smac beta, which binds IAPs in vitro, does not bind IAPs in intact cells due to cellular processing and removal of its NH2-terminal IAP-binding domain. Despite its inability to i nteract with IAPs in cells, processed Smac beta is proapoptotic, as demonst rated by its ability to potentiate apoptosis induced by both death receptor and chemical stimuli. Furthermore, expression of a NH2-terminally truncate d Smac mutant (Delta 75), which lacks the entire TAP-interacting domain, po tentiates apoptosis to the same extent as Smac and Smac beta. Our data supp ort the hypothesis that the main proapoptotic function of Smac and Smac bet a is due to a mechanism other than IAP binding.