BH3-dependent and independent activation of BAX and BAK in mitochondrial apoptosis

•BH3s initiate apoptosis by activating BAX/BAK or inactivating anti-death BCL-2s.•BID, BIM, PUMA, and NOXA directly induce stepwise bimodal activation of BAX and BAK.•BH3-independent autoactivation of BAX/BAK occurs when anti-death BCL-2s are decreased.•Anti-death BCL-2 members sequester BH3s or BH3-exposed BAX/BAK to inhibit apoptosis.•BCL-2 family interactions dictate the efficacy of BH3 mimetics in cancer therapy. Mitochondria play key roles in mammalian apoptosis, a highly regulated genetic program of cell suicide. Multiple apoptotic signals culminate in mitochondrial outer membrane permeabilization (MOMP), which not only couples the mitochondria to the activation of caspases but also initiates caspase-independent mitochondrial dysfunction. The BCL-2 family proteins are central regulators of MOMP. Multidomain pro-apoptotic BAX and BAK are essential effectors responsible for MOMP, whereas anti-apoptotic BCL-2, BCL-XL, and MCL-1 preserve mitochondrial integrity. The third BCL-2 subfamily of proteins, BH3-only molecules, promotes apoptosis by either activating BAX and BAK or inactivating BCL-2, BCL-XL, and MCL-1. Through an interconnected hierarchical network of interactions, the BCL-2 family proteins integrate developmental and environmental cues to dictate the survival versus death decision of cells by regulating the integrity of the mitochondrial outer membrane. Over the past 30 years, research on the BCL-2-regulated apoptotic pathway has not only revealed its importance in both normal physiological and disease processes, but has also resulted in the first anti-cancer drug targeting protein–protein interactions.