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. Cleaves THAP5 and promotes its degradation during apoptosis. Isoform 2 seems to be proteolytically inactive. 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. Homotrimer. Interacts with MXI2. Interacts with THAP5 under apoptotic conditions. The mature protein, but not the precursor, binds to BIRC2/c-IAP1, BIRC3/c-IAP2 and XIAP/BIRC4. Interacts with BIRC6/bruce. Interacts with AREL1 (via HECT domain); in the cytoplasm following induction of apoptosis (PubMed:23479728). Predominantly present in the intermembrane space. Released into the cytosol following apoptotic stimuli, such as UV treatment, and stimulation of mitochondria with caspase-8 truncated BID/tBID. Ubiquitously expressed. The mature N-terminus is involved in the interaction with XIAP. The PDZ domain mediates interaction with MXI2. Autoproteolytically activated. The disease is caused by variants affecting the gene represented in this entry. Disease susceptibility is associated with variants affecting the gene represented in this entry. Belongs to the peptidase S1C family.