Failure of Bcl-2 family members to interact with Apaf-1 in normal and apoptotic cells

Abstract

CED-9 blocks programmed cell death (apoptosis) in the nematode C. elegans by binding to and neutralizing CED-4, an essential activator of the aspartate-directed cysteine protease (caspase) CED-3. In mammals, the CED-9 homologs Bcl-2 and Bcl-x L also block apoptosis by interfering with the activation of CED-3-like caspases. However, it is unknown whether this occurs by binding to the CED-4 homolog Apaf-1. Whilst two groups previously detected an interaction between Bcl-x L and Apaf-1 in immunoprecipitates, 1,2 another group found no interaction between Apaf-1 and any of ten individual members of the Bcl-2 family using the same experimental approach. 3 In this study, we aimed to resolve this discrepancy by monitoring the binding of Apaf-1 to three Bcl-2 family members within cells. Using immunofluorescence and Western blot analysis, we show that whilst Apaf-1 is a predominantly cytoplasmic protein, Bcl-2, Bcl-x L and Bax mostly reside on nuclear/ER and mitochondrial membranes. This pattern of localization is maintained when the proteins are co-expressed in both normal and apoptotic cells, suggesting that Bcl-2, Bcl-x L or Bax do not significantly sequester cytoplasmic Apaf-1 to intracellular membranes. In addition, we confirm that Apaf-1 does not interact with Bcl-2 and Bcl-x L in immunoprecipitates. Based on these data, we propose that Apaf-1 is not a direct, physiological target of Bcl-2, Bcl-x L or Bax. Cell Death and Differentiation (2000) 7, 947 ± 954.

BH3-only proteins: orchestrating cell death

“…Is the function of its mammalian Bcl-2-like counterparts primarily to buffer the action of BH3-only proteins, or like CED-9, do they act instead to control downstream mediators? Whereas mammalian prosurvival proteins do not appear to interact directly with the mammalian CED-4 homolog Apaf-1, 42,43 specific interactions with Bax 44 and Bak 45 are observed. In a manner akin to EGL-1, which indirectly triggers CED-4 activation by neutralizing CED-9, might mammalian BH3-only proteins also activate Bax/Bak indirectly by neutralizing the prosurvival Bcl-2 proteins, rather than directly as proposed ( Figure 1a)?…”

Living with death: the evolution of the mitochondrial pathway of apoptosis in animals

“…Furthermore, despite early reports to the contrary 20,21 it is now accepted that mammalian Bcl-2 homologs do not functionally interact with APAF-1. [22][23][24] Given these significant differences, it is intriguing that the antiapoptotic functions of both Bcl-2 and CED-9 involve the mitochondria. Indeed, the functions of the mammalian Bcl-2 family members and those of CED-9/EGL-1 overlap in a number of interesting ways: despite sharing only around 22% homology with CED-9, expression of Bcl-2 inhibits apoptosis in C.elegans, 25 and Bcl-2 expression was found to rescue the excess of apoptosis found in CED-9 loss-of-function worms.…”

The role of mitochondria in apoptosis induction in Caenorhabditis elegans: more than just innocent bystanders?

“…The idea that a direct pathway also functions in mammals fell out of favor mainly because of the lack of evidence for a physical interaction between antiapoptotic Bcl-2 proteins and Apaf-1. 25 However, the recent analysis of mice expressing a mutant cytochrome c protein unable to bind to Apaf-1 (and hence, most likely unable to activate the apoptosome), suggests that one or more cytochrome c-independent, Apaf-1-dependent pathways do exist for the activation of caspases in mammals. 26 Mitochondria have long been thought to not play a role in apoptosis induction in C. elegans; however, the recent studies on mitochondrial fragmentation have revealed that they are more than just innocent bystanders.…”