Significance Apoptosis is crucial for immune system function and limiting tumor development. Because BAK and BAX are essential effectors of apoptosis, understanding how they are activated to form the oligomeric mitochondrial pores that kill cells is a major goal of the field. We define a requirement for two sites on mitochondrial BAK for its interaction with, and activation by, BCL-2 homology 3 (BH3)-only proteins during apoptosis and determine that binding of BH3-only proteins at a distal site promotes exposure of a canonical site to allow terminal BAK activation and homooligomerization. Additionally, we provide insight into how BAK and BAX kill cells, identifying that the oligomeric pore is limited to interactions between the BH3 domain and canonical groove and does not involve additional protein interfaces.