Caspase cleaved BID targets mitochondria and is required for cytochrome c release, while BCL-X-L prevents this release but not tumor necrosis factor-R1/Fas death

"BH3 domain only" members of the BCL-2 family including the pro-apoptotic m olecule BID represent candidates to connect with proximal signal transducti on. Tumor necrosis factor alpha (TNF alpha) treatment induced a caspase-med iated cleavage of cytosolic, inactive p22 BID at internal Asp sites to yiel d a major p15 and minor p13 and p11 fragments. p15 BID translocates to mito chondria as an integral membrane protein. p15 BID within cytosol targeted n ormal mitochondria and released cytochrome c. Immunodepletion of p15 BID pr events cytochrome c release, in vivo, anti-Fas Ab results in the appearance of p15 BID in the cytosol of hepatocytes which translocates to mitochondri a where it releases cytochrome c. Addition of activated caspase-8 to normal cytosol generates p15 BID which is also required in this system for releas e of cytochrome c. In the presence of BCL-X-L/BCL-2, TNF alpha still induce d BID cleavage and p15 BID became an integral mitochondrial membrane protei n. However, BCC X-L/BCL-2 prevented the release of cytochrome c, yet other aspects of mitochondrial dysfunction still transpired and cells died noneth eless. Thus, while BID appears to be required for the release of cytochrome c in the TNF death pathway, the release of cytochrome c may not be require d for cell death.