Programmed Cell Death and Its Regulation in C. e/egans
3.5 Elucidating the Molecular Mechanism
of the Programmed Cell-Death Process
41
The genes ced-9, ced-4, and ced-3 form the central programmed celldeath pathway in C. elegans and so far, there is no evidence of additional
ced-9-, ced-4-, or ced-3-like genes with a role in programmed cell death
in this organism. This is in contrast to vertebrates in which a family of
ced-9-like genes exists (i.e., the family of bcl-2-like genes) and in which
over ten CED-3/ICE-like cell-death proteases have been identified. The
C. elegans cell-death pathway can therefore be considered the "basic
model" for the programmed cell-death process.
Using various biochemical analyses, the yeast two-hybrid system,
and the expression of the CED-9, CED-4, and CED-3 proteins in heterologous systems, such as mammalian or insect cells, data have been
gathered over the last few years which reveal some aspects of the
molecular mechanism of the cell-death process. While most of the
results still need to be confirmed in C. elegans, the following model is
emerging from these results (Fig. IB). The protease activity of the
CED-3 caspase is required for killing (Xue et al. 1996; Seshagiri and
Miller 1997; Chinnaiyan et al. 1997a; Wu et al. 1997b; Hisahara et al.
1998). Because CED-3, just like other caspases, is synthesized as an
inactive zymogen, proCED-3, the activation of CED-3 is an important
step in the cell-death process. It has been shown that caspase zymogens
can be activated through auto-catalysis (reviewed by Nicholson and
Thornberry 1997; Cryns and Yuan 1998). However, the cleavage of
proCED-3 to the mature, active CED-3 caspase is greatly enhanced by
the CED-4 protein (Seshagiri and Miller 1997; Wu et al. 1997b; Chinnaiyan et al. 1997b) which can interact with proCED-3 (Chinnaiyan et
al. 1997a; Wu et al. 1997b; Irmler et al. 1997). This interaction appears
to be mediated through two distinct domains of CED-4: an N-terminal
domain of CED-4 interacts with the catalytic domain of proCED-3 and
the P-Ioop region of CED-4, a nucleotide-binding domain, interacts with
the pro-domain of proCED-3 (Chaudhary et al. 1998). The enhancement
of proCED-3 cleavage and activation by the CED-4 protein has been
proposed to come about through the oligomerization of CED-4 which
would allow bound proCED-3 molecules to cleave and activate each
other (Yang et al. 1998). It has also been suggested that the interaction
between the P-Ioop region of CED-4 and the pro-domain of proCED-3
3.5 Elucidating the Molecular Mechanism
of the Programmed Cell-Death Process
41
The genes ced-9, ced-4, and ced-3 form the central programmed celldeath pathway in C. elegans and so far, there is no evidence of additional
ced-9-, ced-4-, or ced-3-like genes with a role in programmed cell death
in this organism. This is in contrast to vertebrates in which a family of
ced-9-like genes exists (i.e., the family of bcl-2-like genes) and in which
over ten CED-3/ICE-like cell-death proteases have been identified. The
C. elegans cell-death pathway can therefore be considered the "basic
model" for the programmed cell-death process.
Using various biochemical analyses, the yeast two-hybrid system,
and the expression of the CED-9, CED-4, and CED-3 proteins in heterologous systems, such as mammalian or insect cells, data have been
gathered over the last few years which reveal some aspects of the
molecular mechanism of the cell-death process. While most of the
results still need to be confirmed in C. elegans, the following model is
emerging from these results (Fig. IB). The protease activity of the
CED-3 caspase is required for killing (Xue et al. 1996; Seshagiri and
Miller 1997; Chinnaiyan et al. 1997a; Wu et al. 1997b; Hisahara et al.
1998). Because CED-3, just like other caspases, is synthesized as an
inactive zymogen, proCED-3, the activation of CED-3 is an important
step in the cell-death process. It has been shown that caspase zymogens
can be activated through auto-catalysis (reviewed by Nicholson and
Thornberry 1997; Cryns and Yuan 1998). However, the cleavage of
proCED-3 to the mature, active CED-3 caspase is greatly enhanced by
the CED-4 protein (Seshagiri and Miller 1997; Wu et al. 1997b; Chinnaiyan et al. 1997b) which can interact with proCED-3 (Chinnaiyan et
al. 1997a; Wu et al. 1997b; Irmler et al. 1997). This interaction appears
to be mediated through two distinct domains of CED-4: an N-terminal
domain of CED-4 interacts with the catalytic domain of proCED-3 and
the P-Ioop region of CED-4, a nucleotide-binding domain, interacts with
the pro-domain of proCED-3 (Chaudhary et al. 1998). The enhancement
of proCED-3 cleavage and activation by the CED-4 protein has been
proposed to come about through the oligomerization of CED-4 which
would allow bound proCED-3 molecules to cleave and activate each
other (Yang et al. 1998). It has also been suggested that the interaction
between the P-Ioop region of CED-4 and the pro-domain of proCED-3