Promotes apoptosis by activating caspases in the cytochrome c/Apaf-1/caspase-9 pathway. Acts by opposing the inhibitory activity of inhibitor of apoptosis proteins (IAP). Inhibits the activity of BIRC6/bruce by inhibiting its binding to caspases. ; [Isoform 3]: Attenuates the stability and apoptosis-inhibiting activity of XIAP/BIRC4 by promoting XIAP/BIRC4 ubiquitination and degradation through the ubiquitin-proteasome pathway. Also disrupts XIAP/BIRC4 interacting with processed caspase-9 and promotes caspase-3 activation. ; [Isoform 1]: Defective in the capacity to down-regulate the XIAP/BIRC4 abundance.
