Plays a role in the mitochondrial apoptotic process (PubMed:10772918, PubMed:16113678, PubMed:18948948, PubMed:21199865, PubMed:21458670, PubMed:25609812, PubMed:8358790, PubMed:8521816, PubMed:11060313, PubMed:16199525). Under normal conditions, BAX is largely cytosolic via constant retrotranslocation from mitochondria to the cytosol mediated by BCL2L1/Bcl-xL, which avoids accumulation of toxic BAX levels at the mitochondrial outer membrane (MOM) (PubMed:21458670). 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 (PubMed:11060313, PubMed:16199525, PubMed:10772918, PubMed:16113678, PubMed:18948948, PubMed:21199865, PubMed:21458670, PubMed:25609812, PubMed:8358790, PubMed:8521816). Promotes activation of CASP3, and thereby apoptosis (PubMed:11060313, PubMed:16199525, PubMed:10772918, PubMed:16113678, PubMed:18948948, PubMed:21199865, PubMed:21458670, PubMed:25609812, PubMed:8358790, PubMed:8521816). Homodimer. Forms higher oligomers under stress conditions. Forms heterooligomers with BAK (PubMed:29531808). Interacts with BCL2L11. Interaction with BCL2L11 promotes BAX oligomerization and association with mitochondrial membranes, with subsequent release of cytochrome c. Forms heterodimers with BCL2, BCL2L1 isoform Bcl-X(L), BCL2L2, MCL1 and A1 (PubMed:25609812). Interacts with SH3GLB1. Interacts with humanin; forms fibers with humanin which results in BAX conformational changes and sequestering of BAX into the fibers, preventing BAX activation (PubMed:12732850, PubMed:31690630). Interacts with SFN and YWHAZ; the interaction occurs in the cytoplasm. Under stress conditions, JNK-mediated phosphorylation of SFN and YWHAZ, releases BAX to mitochondria. Interacts with RNF144B, which regulates the ubiquitin-dependent stability of BAX. Interacts with CLU under stress conditions that cause a conformation change leading to BAX oligomerization and association with mitochondria. Does not interact with CLU in unstressed cells. Interacts with FAIM2/LFG2. Interacts with RTL10/BOP. Interacts (via a C-terminal 33 residues) with NOL3 (via CARD domain); inhibits BAX activation and translocation and consequently cytochrome c release from mitochondria. Interacts with GIMAP3/IAN4 and GIMAP5/IAN5; this interaction is increased, when cells initiate apoptosis upon IL2 withdrawal (PubMed:16509771). Interacts with IRF3; the interaction is direct, increases upon Sendai virus infection and mediates the formation of the apoptosis complex TOMM70:HSP90AA1:IRF3:BAX (PubMed:25609812). Interacts with MOAP1, facilitating BAX-dependent mitochondrial outer membrane permeabilization and apoptosis (PubMed:11060313, PubMed:16199525). Interacts with BCL2L10/BCL-B (PubMed:23235460). Interacts with BCL2A1 and BCL2L1 isoform Bcl-X(L). (Microbial infection) Interacts with adenovirus E1B 19K protein; this interaction blocks BAX oligomerization (PubMed:11462023). (Microbial infection) Interacts with human cytomegalovirus/HHV-5 protein vMIA/UL37. (Microbial infection) Interacts with enterovirus protein 2B; this interaction activates BAX-induced apoptosis. Colocalizes with 14-3-3 proteins in the cytoplasm. Under stress conditions, undergoes a conformation change that causes release from JNK-phosphorylated 14-3-3 proteins and translocation to the mitochondrion membrane. Upon Sendai virus infection, recruited to the mitochondrion through interaction with IRF3 (PubMed:25609812). Expressed in a wide variety of tissues. Isoform Psi is found in glial tumors. Isoform Alpha is expressed in spleen, breast, ovary, testis, colon and brain, and at low levels in skin and lung. Isoform Sigma is expressed in spleen, breast, ovary, testis, lung, colon, brain and at low levels in skin. Isoform Alpha and isoform Sigma are expressed in pro-myelocytic leukemia, histiocytic lymphoma, Burkitt's lymphoma, T-cell lymphoma, lymphoblastic leukemia, breast adenocarcinoma, ovary adenocarcinoma, prostate carcinoma, prostate adenocarcinoma, lung carcinoma, epidermoid carcinoma, small cell lung carcinoma and colon adenocarcinoma cell lines. 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.