ENFORCED DIMERIZATION OF BAX RESULTS IN ITS TRANSLOCATION, MITOCHONDRIAL DYSFUNCTION AND APOPTOSIS

Expression of the pro-apoptotic molecule BAX has been shown to induce cell death, While BAX forms both homo- and heterodimers, questions rem ain concerning its native conformation in vivo and which moiety is fun ctionally active. Here we demonstrate that a physiologic death stimulu s, the withdrawal of interleukin-3 (IL-3), resulted in the translocati on of monomeric BAX from the cytosol to the mitochondria where it coul d be cross-linked as a BAX homodimer, In contrast, cells protected by BCL-2 demonstrated a block in this process in that BAX did not redistr ibute or homodimerize in response to a death signal. To test the funct ional consequence of BAX dimerization, we expressed a chimeric FKBP-BA X molecule. Enforced dimerization of FKBP-BAX by the bivalent ligand F K1012 resulted in its translocation to mitochondria and induced apopto sis, Caspases were activated yet caspase inhibitors did not block deat h; cytochrome c was not released detectably despite the induction of m itochondrial dysfunction, Moreover, enforced dimerization of BAX overr ode the protection by BCL-X-L and IL-3 to kill cells. These data suppo rt a model in which a death signal results in the activation of BAX. T his conformational change in BAX manifests in its translocation, mitoc hondrial membrane insertion and homodimerization, and a program of mit ochondrial dysfunction that results in cell death.