As mentioned earlier, in some cells the extrinsic apoptotic pathway recruits the intrinsic pathway to amplify the caspase cascade to kill the cell. The BH3-only protein Bid is the link between the two pathways. Bid is normally inactive. However, when death receptors activate the extrinsic pathway in some cells, the initiator caspase, caspase-8, cleaves Bid, producing an active form of Bid that trans-locates to the outer mitochondrial membrane and inhibits anti-apoptotic Bcl2 family proteins, thereby amplifying the death signal.
Figure 18–10 How pro-apoptotic BH3only and anti-apoptotic Bcl2 family proteins regulate the intrinsic pathway of apoptosis. (A) In the absence of an apoptotic stimulus, anti-apoptotic Bcl2 family proteins bind to and inhibit the effector Bcl2 family proteins on the mitochondrial outer membrane (and in the cytosol—not shown). (B) In the presence of an apoptotic stimulus, Bh3-only proteins are activated and bind to the anti-apoptotic Bcl2 family proteins so that they can no longer inhibit the effector Bcl2 family proteins; the latter then become activated, aggregate in the outer mitochondrial membrane, and promote the release of intermembrane mitochondrial proteins into the cytosol. Some activated Bh3only proteins may stimulate mitochondrial protein release more directly by binding to and activating the effector Bcl2 family proteins. Although not shown, the antiapoptotic Bcl2 family proteins are bound to the mitochondrial surface.
Because activation of a caspase cascade leads to certain death, the cell employs multiple robust mechanisms to ensure that these proteases are activated only when appropriate. One line of defense is provided by a family of proteins called inhibitors of apoptosis (IAPs). These proteins were first identified in certain insect viruses (baculoviruses), which encode IAP proteins to prevent a host cell that is infected by the virus from killing itself by apoptosis. It is now known that most animal cells also make IAP proteins.
All IAPs have one or more BIR (baculovirus IAP repeat) domains, which enable them to bind to and inhibit activated caspases. Some IAPs also polyubiquitylate caspases, marking the caspases for destruction by proteasomes. In this way, the IAPs set an inhibitory threshold that caspases must overcome to trigger apoptosis.
In Drosophila at least, the inhibitory barrier provided by IAPs can be neutralized by anti-IAP proteins, which are produced in response to various apoptotic stimuli. There are numerous anti-IAPs in flies, including Reaper, Grim, and Hid, and their only structural similarity is their short, N-terminal, IAP-binding motif, which binds to the BIR domain of IAPs, preventing the domain from binding to a caspase. Deletion of the three genes encoding Reaper, Grim, and Hid blocks apoptosis in flies. Conversely, inactivation of one of the two genes that encode IAPs in Drosophila causes all of the cells in the developing fly embryo to undergo apoptosis. Clearly, the balance between IAPs and anti-IAPs is tightly regulated and is crucial for controlling apoptosis in the fly.
The role of mammalian IAP and anti-IAP proteins in apoptosis is less clear. Anti-IAPs are released from the mitochondrial intermembrane space when the intrinsic pathway of apoptosis is activated, blocking IAPs in the cytosol and thereby promoting apoptosis. However, mice appear to develop normally if they are missing either the major mammalian IAP (called XIAP) or the two known mammalian anti-IAPs (called Smac/Diablo and Omi). Worms do not even contain a caspaseinhibiting IAP protein. Apparently, the tight control of caspase activity is achieved by different mechanisms in different animals.