Mitochondria--the suicide organelles.

One of the near-to-invariant hallmarks of early apoptosis (programmed cell death) is mitochondrial membrane permeabilization (MMP). It appears that mitochondria fulfill a dual role during the apoptotic process. On the one hand, they integrate multiple different pro-apoptotic signal transducing cascades into a common pathway initiated by MMP. On the other hand, they coordinate the catabolic reactions accompanying late apoptosis by releasing soluble proteins that are normally sequestered within the intermembrane space. In a recent study, Li et al. described a nuclear transcription factor (Nur77/TR1/NGFI-B) that can translocate to mitochondrial membranes to induce MMP. Moreover, two groups identified a novel intermembrane protein (Smac/DIABLO) that specifically neutralizes the inhibitor of apoptosis (IAP) proteins, thereby facilitating the activation of caspases, a class of proteases activated during apoptosis. These findings refine our knowledge how MMP connects to the cellular suicide machinery.