Injected cytochrome c induces apoptosis

Mitochondria, the cell's energy-producing organelles, are thought to play a central role in mediating apoptosis, or programmed cell death. Mitochondrial morphology remains intact during the process, the apoptosis-blocking protein Bcl-2 is localized in the outer mitochondrial membrane, several critical steps in the process require ATP, and mitochondrial ‘megachannels’ or ‘permeability transitions’ open during apoptosis. It has been suggested that mitochondrial constituents leak out through these permeability transition channels, and activate the apoptotic machinery in the cytosol. Here we show that the injection into cells of a key mitochondrial protein, cytochrome c, activates apoptosis.