Translocation of C. elegans CED-4 to Nuclear Membranes During Programmed Cell Death

Abstract

The Caenorhabditis elegans Bcl-2-like protein CED-9 prevents programmed cell death by antagonizing the Apaf-1-like cell-death activator CED-4. Endogenous CED-9 and CED-4 proteins localized to mitochondria in wild-type embryos, in which most cells survive. By contrast, in embryos in which cells had been induced to die, CED-4 assumed a perinuclear localization. CED-4 translocation induced by the cell-death activator EGL-1 was blocked by a gain-of-function mutation in ced-9 but was not dependent on ced-3 function, suggesting that CED-4 translocation precedes caspase activation and the execution phase of programmed cell death. Thus, a change in the subcellular localization of CED-4 may drive programmed cell death.

CED-4 forms a 2 : 2 heterotetrameric complex with CED-9 until specifically displaced by EGL-1 or CED-13

“…4 In healthy cells, the adapter CED-4 associates with CED-9, the worm B-cell lymphoma-2 (Bcl-2) orthologue, on mitochondrial membranes. 5 When the cell receives a death signal, transcriptionally upregulated Bcl-2 homology domain 3 (BH3)-only protein EGL-1 binds CED-9. This, in turn, releases CED-4 allowing its oligomerisation and leads to the activation of the caspase CED-3 which is synthesised as an inactive zymogen.…”

“…However, it is not known whether the CED-4/CED-9 complex at the mitochondrial membrane contains CED-3 constitutively, although during cell death, CED-4 dissociates from CED-9 and translocates from the mitochondrial to the nuclear membrane. 5 This pathway of protein interactions has been extensively studied using a myriad of techniques. For example, interaction of CED-4 with CED-9 (and simultaneously with CED-3) has been demonstrated by co-immunoprecipitation of proteins expressed in various cell types, 6-8 the yeast two-hybrid system, [9][10][11] or by colocalization studies.…”

egl-1: a key activator of apoptotic cell death in C. elegans

“…This observation suggests that many cells in the developing embryo have the potential to undergo apoptosis during the embryonic wave of cell death. Similar to CED-4, and most probably to proCED-3, the anti-apoptotic CED-9 protein is present in many cells at this stage (Chen et al, 2000). CED-9 localizes to the outer mitochondrial membrane, and it is through the physical interaction between CED-9 and an asymmetric dimer of CED-4 that the ability of CED-4 to form an active apoptosome and to mediate proCED-3 activation is blocked (Spector et al, 1997;Chinnaiyan et al, 1997b;Wu et al, 1997b;Chen et al, 2000;Yan et al, 2005).…”

“…(The remaining 18 cell deaths take place during post-embryonic development (Sulston and Horvitz, 1977).) At least during these B400 min, the presence of the CED-4 protein and the expression of the ced-3 gene do not seem to be restricted to the 113 cells that are programed to die (Chen et al, 2000;Maurer et al, 2007). (Antibodies that detect endogenous proCED-3 or CED-3 protein in C. elegans have so far not been described.)…”

“…Specifically, EGL-1 binds to a surface area of the CED-9 protein ('EGL-1-binding site') which is distinct from the surface area of CED-9 required for its ability to interact with CED-4 dimers ('CED-4-binding site'). The interaction of EGL-1 with CED-9/ CED-4 complexes induces a dramatic structural change in CED-9, which alters its CED-4-binding site and results in the release of the CED-4 dimer (Conradt and Horvitz, 1998;del Peso et al, 1998del Peso et al, , 2000Chen et al, 2000;Yan et al, 2004). CED-4 dimers released from the mitochondria-localized EGL-1/CED-9 complex relocalize to perinuclear membranes and assemble into active apoptosomes (which most probably are homotetramers), capable of mediating proCED-3 activation (Irmler et al, 1997;Seshagiri and Miller, 1997 Yan et al, 2005).…”