Serine protease that shows proteolytic activity against a non-specific substrate beta-casein. Promotes or induces cell death either by direct binding to and inhibition of BIRC proteins (also called inhibitor of apoptosis proteins, IAPs), leading to an increase in caspase activity, or by a BIRC inhibition-independent, caspase-independent and serine protease activity-dependent mechanism. Can antagonize antiapoptotic activity of th by directly inducing the degradation of th (By similarity). Cleavage of non-polar aliphatic amino-acids at the P1 position, with a preference for Val, Ile and Met. At the P2 and P3 positions, Arg is selected most strongly with a secondary preference for other hydrophilic residues. Interacts with th/DIAP1 (via BIR 2 domain). Predominantly present in the intermembrane space. Released into the cytosol following apoptotic stimuli, such as UV treatment. The extramitochondrial protein does not diffuse throughout the cytosol but stays near the mitochondria. Belongs to the peptidase S1C family.