Plays a role in the mitochondrial apoptosic process. Upon arrival of cell death signals, promotes mitochondrial outer membrane (MOM) permeabilization by oligomerizing to form pores within the MOM. This releases apoptogenic factors into the cytosol, including cytochrome c, promoting the activation of caspase 9 which in turn processes and activates the effector caspases. Homodimer. Formation of the homodimer is zinc-dependent (PubMed:17157251). Forms heterodimers with BCL2 and BCL2L1 isoform Bcl-X(L) (PubMed:9020082). Forms heterooligomers with BAX (PubMed:29531808). Interacts with BCL2A1 (By similarity). Interacts with RTL10/BOP (PubMed:23055042). Interacts with VDAC1 (PubMed:25296756). Interacts with GIMAP3/IAN4 and GIMAP5/IAN5 (PubMed:16509771). (Microbial infection) Interacts with vaccinia virus protein F1. (Microbial infection) Interacts with myxoma virus protein M11L. (Microbial infection) Interacts with Epstein-Barr virus protein BALF1. (Microbial infection) Interacts with adenovirus protein E1B 19K. Expressed in a wide variety of tissues, with highest levels in the heart and skeletal muscle. 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. Belongs to the Bcl-2 family.