Accelerates programmed cell death by binding to, and antagonizing the apoptosis repressor BCL2 or its adenovirus homolog E1B 19k protein. Under stress conditions, undergoes a conformation change that causes translocation to the mitochondrion membrane, leading to the release of cytochrome c that then triggers apoptosis. Promotes activation of CASP3, and thereby apoptosis.Oltvai Z.N., Cell 74:609-619(1993).Cuddeback S.M., J. Biol. Chem. 276:20559-20565(2001).Guo B., Nature 423:456-461(2003)