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). {ECO:0000250|UniProtKB:Q99ML1, ECO:0000269|PubMed:11463391, ECO:0000269|PubMed:23340338}. [Isoform 3]: Does not affect cell growth. . This is the function of Ensembl gene identifier ENSG00000105327 (BBC3, BCL2 binding component 3).