Essential mediator of p53/TP53-dependent and p53/TP53-independent apoptosis. Promotes partial unfolding of BCL2L1 and dissociation of BCL2L1 from p53/TP53, releasing the bound p53/TP53 to induce apoptosis. Regulates ER stress-induced neuronal apoptosis. Interacts with MCL1 and BCL2A1 (By similarity). Interacts (via BH3 domain) with BCL2 and BCL2L1/BCL-XL (By similarity). Interacts (via BH3 domain) with NOL3/ARC (via CARD domain); this interaction prevents BBC3 association with BCL2 and results in CASP8 activation (PubMed:17998337). Localized to the mitochondria in order to induce cytochrome c release. By DNA damage, glucocorticoid treatment, growth factor deprivation and p53. The BH3 motif is intrinsically disordered in the absence of a binding partner but folds upon binding (By similarity). Folds when bound to BCL2L1 (By similarity). Also folds when bound to MCL1 (By similarity). Belongs to the Bcl-2 family.