Genetics of Apoptosis

biochemical data suggest that RPR, HID, and GRIM promote cell death by
disrupting DIAP1-caspase interactions and that DIAP1 is required to block apoptosis-
inducing caspase activity (Wang, S.L. et al., 1999; Goyal et al., 2000). RPR, HID,
and GRIM contain a short region of homology in their N-termini called the RHG
motif, which allows binding to DIAP1 and DIAP2. Loss of DIAP1 function results
in global early embryonic cell death and a large increase in DIAP1-inhibitable caspase
activity, and DIAP1 is still required for cell survival when expression of rpr, hid, and
grim is eliminated (Wang, S.L. et al., 1999). DIAP1 and DIAP2 interact with a
number of long and short prodomain Drosophila caspases, including DRONC,
DRICE, DCP-1, and STRICA, but not DAMM (Fraser et al., 1997; Hawkins et al.,
2000; Meier et al., 2000; Quinn et al., 2000; Doumanis et al., 2001; Harvey et al.,
2001). In mammals, the proteins SMAC/DIABLO and HtrA2 are released during
apoptosis from the mitochondria into the cytoplasm, where they bind to IAPs and
prevent their ability to inhibit caspase activity (Verhagen et al., 2000, 2002; Du et
al., 2000; Suzuki et al., 2001; Hedge et al., 2002; Martin et al., 2002; van Loo et al.,
2002). SMAC and HtrA2 seem to function similarly to RPR, HID, and GRIM, and
contain the characteristic tetrapeptide RHG motifs in the mature protein that binds
to the BIR domain of IAPs. Structural data show that the binding of SMAC to the
BIR2 and BIR3 domains of XIAP disrupts its ability to inhibit the caspases by steric
hindrance (Chai et al., 2001b).

6.3

Regulation by Bcl-2 family

In C. elegans, the CED-4 adapter is required for the activation of CED-3, whereas
CED-9 negatively regulates this process by directly interacting with CED-4, and
perhaps sequestering it (see Chapter 10). However, EGL-1 binds CED-9, thereby
acting as a trigger for CED-4-dependent CED-3 activation (Conradt and Horvitz,
1998; del Peso et al., 1998). Both EGL-1 and CED-9 homologs, BH3-only proteins
and Bcl-2-like proteins, respectively, also play pivotal roles in the regulation of
caspase-9 activation in mammals (see Chapter 4 for details of the Bcl-2 gene family).
However, unlike the C.elegans CED-9, mammalian Bcl-2 family members do not
interact with Apaf-1; rather, they seem to regulate the release of cytochrome c. In
addition to the EGL-1 and CED-9 homologs, mammals also have proapoptotic Bcl-2-
like proteins (Bax, Bak, and Bok) that contain three BH (Bcl-2 homology) domains
(Chapter 4). The antiapoptotic members of this family, such as Bcl-2 and Bcl-xL,
prevent cytochrome c release, while the proapoptotic members such, as Bax, and BH3-
only proteins (e.g., Bid, Bim, Bmf, Bad, and others) promote it (Green and Reed,
1998; Adams and Cory, 2001). Following apoptotic signaling, many of the Bcl-2
family proteins undergo conformational changes and translocate to the outer
membrane of the mitochondria. The exact mechanisms by which Bcl-2 family
proteins regulate cytochrome c release are still a matter of considerable debate; these
mechanisms are discussed in detail in Chapter 4.

42 GENETICS OF APOPTOSIS