Essential mediator of p53/TP53-dependent and p53/TP53-independent apoptosis (PubMed:11463391, PubMed:23340338). Promotes partial unfolding of BCL2L1 and dissociation of BCL2L1 from p53/TP53, releasing the bound p53/TP53 to induce apoptosis (PubMed:23340338). Regulates ER stress-induced neuronal apoptosis (By similarity). Interacts with MCL1 and BCL2A1 (By similarity). Interacts (via BH3 domain) with BCL2 (PubMed:11463391). Interacts with BCL2L1/BCL-XL (PubMed:23340338). Interacts (via BH3 domain) with NOL3/ARC (via CARD domain); this interaction prevents BBC3 association with BCL2 and results in CASP8 activation (By similarity). Localized to the mitochondria in order to induce cytochrome c release. Ubiquitously expressed. Up-regulated by TP53 (PubMed:11463392, PubMed:11572983). Up-regulated by DNA damage, glucocorticoid treatment and growth factor deprivation (PubMed:11572983). Up-regulated by ER stress in a DDIT3/CHOP-dependent manner (PubMed:22761832). The BH3 motif is intrinsically disordered in the absence of a binding partner but folds upon binding (PubMed:23340338). Folds when bound to BCL2L1 (PubMed:23340338). Also folds when bound to MCL1 (By similarity). Belongs to the Bcl-2 family.