Granzyme B-mediated cytochrome c release is regulated by the Bcl-2 family members Bid and Bax

Cytotoxic T lymphocytcs (CTLs) destroy target cells through a mechanism inv olving the exocytosis of cytolytic granule components including granzyme B (grB) and perforin, which have been shown to induce apoptosis through caspa se activation. However, grB has also been linked with caspase-independent d isruption of mitochondrial function. Wt show here that cytochrome c release requires the direct proteolytic cleavage of Bid by grB to generate a 14-kD grB-truncated product (gtBid) that translocates to mitochondria. In rum, g tBid recruits Bar to mitochondria through a caspase-independent mechanism w here it becomes integrated into the membrane and induces cytochrome c relea se. Our results provide evidence for a new pathway by which CTLs inflict da mage and explain the caspase-independent mechanism of mitochondrial dysfunc tion.