Induces caspases and apoptosis. Counters the protective effect of Bcl-2 (By similarity). Forms heterodimers either with the pro-apoptotic protein BAX or the anti-apoptotic protein Bcl-2. Intact BH3 motif is required by BIK, BID, BAK, BAD and BAX for their pro-apoptotic activity and for their interaction with anti-apoptotic members of the Bcl-2 family. Apoptotic members of the Bcl-2 family.