Molecular characterization of mitochondrial apoptosis-inducing factor

Mitochondria play a key part in the regulation of apoptosis (cell death)(1, 2). Their intermembrane space contains several proteins that are liberated through the outer membrane in order to participate in the degradation phase of apoptosis(3-9). Here we report the identification and cloning of an apo ptosis-inducing factor, AIF(5), which is sufficient to induce apoptosis of isolated nuclei. AIF is a flavoprotein of relative molecular mass 57,000 wh ich shares homology with the bacterial oxidoreductases; it is normally conf ined to mitochondria but translocates to the nucleus when apoptosis is indu ced. Recombinant AIF causes chromatin condensation in isolated nuclei and l arge-scale fragmentation of DNA. It induces purified mitochondria to releas e the apoptogenic proteins cytochrome c and caspase-9. Microinjection of AI F into the cytoplasm of intact cells induces condensation of chromatin, dis sipation of the mitochondrial transmembrane potential, and exposure of phos phatidylserine in the plasma membrane. None of these effects is prevented b y the wide-ranging caspase inhibitor known as Z-VAD.fmk. Overexpression of Bcl-2, which controls the opening of mitochondrial permeability transition pores, prevents the release of AIF from the mitochondrion but does not affe ct its apoptogenic activity. These results indicate that AIF is a mitochond rial effector of apoptotic cell death.