# Apoptosome
The apoptosome is a multimeric protein complex that forms in the cytosol of a cell undergoing intrinsic apoptosis and activates initiator caspases, the proteases that execute programmed cell death. In mammals it is a heptameric platform built from seven copies of apoptotic protease-activating factor 1 (Apaf-1) bound to cytochrome c, and it recruits and activates caspase-9.[1](https://pmc.ncbi.nlm.nih.gov/articles/PMC4691890/) • [2](https://www.ncbi.nlm.nih.gov/mesh/68053478) The name was introduced for the tripartite complex of Apaf-1, caspase-9, and cytochrome c assembled in the presence of dATP.[3](https://era.ed.ac.uk/server/api/core/bitstreams/da75009e-215f-4757-a8b7-0eb4753a81c2/content)
| Key fact | Detail |
|---|---|
| Composition (human) | Seven Apaf-1 molecules, seven bound cytochrome c molecules, and dATP; the assembled complex exceeds 1.0 MDa[1](https://pmc.ncbi.nlm.nih.gov/articles/PMC4691890/) • [4](https://comptes-rendus.academie-sciences.fr/biologies/item/10.1016/j.crvi.2004.08.006.pdf) |
| Trigger | Release of cytochrome c from mitochondria during intrinsic apoptosis, followed by binding to Apaf-1 and nucleotide exchange[5](https://pmc.ncbi.nlm.nih.gov/articles/PMC3644875/) |
| Function | Binds procaspase-9 via CARD-CARD interactions and activates it, forming a caspase-9 holoenzyme that cleaves effector caspases such as caspase-3[6](https://pmc.ncbi.nlm.nih.gov/articles/PMC5320974/) |
| Structure | Intact mammalian apoptosome solved at 3.8 Å resolution by single-particle cryo-EM; central hub formed by nucleotide-binding oligomerization domains, spokes by WD40 repeats[1](https://pmc.ncbi.nlm.nih.gov/articles/PMC4691890/) • [6](https://pmc.ncbi.nlm.nih.gov/articles/PMC5320974/) |
| Orthologs | CED-4 in *C. elegans* (quasi 8-fold symmetry), Dark in *Drosophila* (single rings with eight CARDs), Apaf-1 in humans[5](https://pmc.ncbi.nlm.nih.gov/articles/PMC3644875/) |
| Disease link | Mechanisms that inactivate the apoptosome have been reported in different cancer types, contributing to apoptosis evasion and chemoresistance[7](https://www.annualreviews.org/docserver/fulltext/pathmechdis/20/1/annurev-pathmechdis-051222-115023.pdf?accname=guest&checksum=04E4D8D0323325E58743DDC4AEDD005A&expires=1778216910&id=id) • [8](https://www.nature.com/articles/4402265) |
## Position in the intrinsic apoptosis pathway
Apoptosis is executed by caspases, cysteine-dependent aspartate-specific proteases synthesized as inactive zymogens. [Initiator caspases](https://www.edgepedia.org/initiator-caspases) such as caspase-9 are monomeric and require an activation platform, while effector caspases such as caspase-3 are activated by proteolytic cleavage carried out by an upstream initiator.[6](https://pmc.ncbi.nlm.nih.gov/articles/PMC5320974/) • [9](https://pmc.ncbi.nlm.nih.gov/articles/PMC3962096/)
In the intrinsic pathway, pro-apoptotic [Bcl-2 family](https://www.edgepedia.org/bcl-2-family) members are thought to form pores in the outer mitochondrial membrane through which cytochrome c and other effectors pass into the cytoplasm. Cytosolic cytochrome c then binds to Apaf-1, triggering nucleotide exchange and stepwise assembly of the heptameric apoptosome.[5](https://pmc.ncbi.nlm.nih.gov/articles/PMC3644875/) Many proapoptotic stimuli act at this point, promoting cytochrome c release and apoptosome formation as a central locus of apoptotic control.[10](https://www.sciencedirect.com/science/article/pii/S1534580706001699)
The apoptosome is one of several adaptor platforms that activate initiator caspases in mammalian cells: caspase-9 relies on the Apaf-1 apoptosome, caspase-8 on the death-inducing signaling complex (DISC), and caspase-2 on the PIDDosome.[11](https://www.nature.com/articles/4402028)
## Structure and assembly
**Apaf-1 domain architecture.** Apaf-1 contains, in sequence, an N-terminal caspase recruitment domain (CARD), a nucleotide-binding oligomerization domain comprising a nucleotide-binding domain, a small helical domain, a winged-helix domain, and a large helical domain, and at the C terminus two β-propellers of seven and eight WD40 repeats.[6](https://pmc.ncbi.nlm.nih.gov/articles/PMC5320974/) In normal cells Apaf-1 exists as an autoinhibited monomer.[1](https://pmc.ncbi.nlm.nih.gov/articles/PMC4691890/)
**Activation by cytochrome c and dATP.** [Cytochrome c](https://www.edgepedia.org/cytochrome-c) binds to the β-propellers of Apaf-1, releasing autoinhibition through specific interactions with the WD40 repeats. Binding of cytochrome c also stimulates exchange of a bound nucleoside diphosphate (ADP or dADP) for a nucleoside triphosphate (ATP or dATP), and Apaf-1 then oligomerizes into the heptameric apoptosome.[1](https://pmc.ncbi.nlm.nih.gov/articles/PMC4691890/) • [5](https://pmc.ncbi.nlm.nih.gov/articles/PMC3644875/) [Nucleotide](https://www.edgepedia.org/nucleotide) hydrolysis is not required for caspase activation, because dATP, not dADP, is associated with the mature apoptosome.[12](https://mcb.berkeley.edu/courses/mcb230/jk_pdfs/Jiang%26Wang.pdf)
**Cryo-EM structures.** An intact mammalian apoptosome was determined at 3.8 Å resolution by single-particle cryo-electron microscopy, showing how monomeric Apaf-1 assembles into the functional heptamer. The central hub of the platform is formed by the nucleotide-binding oligomerization domains and the spokes by the WD40 repeats.[1](https://pmc.ncbi.nlm.nih.gov/articles/PMC4691890/) • [6](https://pmc.ncbi.nlm.nih.gov/articles/PMC5320974/) A model of the active heptameric platform built from these data contains 49 domains, seven cytochrome c molecules, and three to four well-ordered procaspase-9 CARDs.[13](https://www.bumc.bu.edu/ppb/apoptosome/)
## Activation of caspase-9
Caspase-9 exists as a monomeric, inactive zymogen, and intrachain cleavage alone is insufficient for its activation; the cleaved enzyme is catalytically active only when bound to the apoptosome, which is why the apoptosome-caspase-9 assembly is described as a holoenzyme.[6](https://pmc.ncbi.nlm.nih.gov/articles/PMC5320974/) Procaspase-9 binds the platform through homotypic CARD-CARD interactions.[5](https://pmc.ncbi.nlm.nih.gov/articles/PMC3644875/)
A cryo-EM structure of the holoenzyme showed that the CARDs of Apaf-1 and caspase-9 assemble in two ways: a 4:4 complex of four Apaf-1 CARDs and four caspase-9 CARDs docks onto the central hub, and a 2:1 heterotrimeric complex binds the periphery of the hub in about 15% of particles. The interface between the CARD complex and the central hub is required for caspase-9 activation, and mutations weakening that interface reduced caspase-9 activity without preventing apoptosome assembly. Unexpectedly, the CARD of free caspase-9 strongly inhibits its own proteolytic activity, so the apoptosome activates caspase-9 at least in part by sequestering this inhibitory [CARD domain](https://www.edgepedia.org/card-domain) while also stimulating the catalytic activity of the protease domain.[6](https://pmc.ncbi.nlm.nih.gov/articles/PMC5320974/)
Once activated, caspase-9 cleaves and activates the effector caspases, chiefly caspase-3 and caspase-7, which dismantle the cell.[6](https://pmc.ncbi.nlm.nih.gov/articles/PMC5320974/)
## Apoptosomes across species
Apoptosome assemblies occur across metazoans but differ in architecture. In *Caenorhabditis elegans*, the CED-4 apoptosome assembles with quasi 8-fold symmetry from lateral CED-4 dimers after they are released from an inhibited CED-9 complex by EGL-1; because ATP is already bound to each CED-4 molecule, nucleotide exchange is not needed for assembly. In *Drosophila*, the Apaf-1 homolog Dark binds dATP and assembles into single rings carrying eight CARDs, though inactive double rings have also been observed at high concentration.[5](https://pmc.ncbi.nlm.nih.gov/articles/PMC3644875/) • [14](https://spj.science.org/doi/10.5936/csbj.201204007) The Dark apoptosome activates the caspase-9 homolog Dronc and is essential for stress-induced apoptosis in the fly.[5](https://pmc.ncbi.nlm.nih.gov/articles/PMC3644875/) • [15](https://academic.oup.com/evlett/article/8/2/267/7424387)
A notable difference is the role of cytochrome c. In mammals it is required to activate Apaf-1, but comparative studies argue against a direct interaction of cytochrome c with the *Drosophila* apoptosome, and in the developing fly retina cytochrome c is not essential for apoptosis execution, though it helps apoptosis proceed on time.[16](https://www.imrpress.com/journal/FBL/14/11/10.2741/3517/pdf) These differences reveal evolutionary divergence in apoptosome formation and procaspase activation mechanisms among animals.[15](https://academic.oup.com/evlett/article/8/2/267/7424387)
## Regulation, disease, and current picture
Because the apoptosome sits downstream of mitochondrial outer-membrane permeabilization, it is a key regulatory point. Its activity can be influenced by proteins such as XIAP, Smac, and prothymosin-α, and targeting apoptosome-dependent, post-mitochondrial stages of apoptosis has been discussed as a rational approach to cancer treatment.[8](https://www.nature.com/articles/4402265) Mechanisms that inactivate the apoptosome have been reported in different cancer types, helping tumor cells evade apoptosis during tumorigenesis.[7](https://www.annualreviews.org/docserver/fulltext/pathmechdis/20/1/annurev-pathmechdis-051222-115023.pdf?accname=guest&checksum=04E4D8D0323325E58743DDC4AEDD005A&expires=1778216910&id=id)
Structural work has resolved how Apaf-1 is released from autoinhibition and how the platform engages caspase-9, but the full activation program of the holoenzyme remains an active subject of study.[13](https://www.bumc.bu.edu/ppb/apoptosome/) A 2025 study using in-cell imaging found that large, transient Apaf1 assemblies constitute the apoptosome in cells, suggesting that the dynamic spatiotemporal organization of apoptosome components regulates the progression of apoptosis.[17](https://pubmed.ncbi.nlm.nih.gov/41136486/)
## References
1. [Atomic structure of the apoptosome: mechanism of cytochrome c- and dATP-mediated activation of Apaf-1, PMC](https://pmc.ncbi.nlm.nih.gov/articles/PMC4691890/)
2. [Apoptosomes, MeSH, NCBI](https://www.ncbi.nlm.nih.gov/mesh/68053478)
3. [Clarke MCH (2002), University of Edinburgh thesis](https://era.ed.ac.uk/server/api/core/bitstreams/da75009e-215f-4757-a8b7-0eb4753a81c2/content)
4. [Apoptose et complexe apoptosomal, Comptes Rendus Biologies (2004)](https://comptes-rendus.academie-sciences.fr/biologies/item/10.1016/j.crvi.2004.08.006.pdf)
5. [Apoptosome structure, assembly and procaspase activation, PMC](https://pmc.ncbi.nlm.nih.gov/articles/PMC3644875/)
6. [Mechanistic insights into caspase-9 activation by the structure of the apoptosome holoenzyme, PNAS (2017), PMC](https://pmc.ncbi.nlm.nih.gov/articles/PMC5320974/)
7. [Annual Review of Pathology: Mechanisms of Disease (2025)](https://www.annualreviews.org/docserver/fulltext/pathmechdis/20/1/annurev-pathmechdis-051222-115023.pdf?accname=guest&checksum=04E4D8D0323325E58743DDC4AEDD005A&expires=1778216910&id=id)
8. [Big wheel keeps on turning: apoptosome regulation and its role in chemoresistance, Cell Death & Differentiation](https://www.nature.com/articles/4402265)
9. [Structural biology of the intrinsic cell death pathway, Computational and Structural Biotechnology Journal (2012), PMC](https://pmc.ncbi.nlm.nih.gov/articles/PMC3962096/)
10. [The Apoptosome: Physiological, Developmental, and Pathological Modes of Regulation, Developmental Cell (2006)](https://www.sciencedirect.com/science/article/pii/S1534580706001699)
11. [Apoptosome: a platform for the activation of initiator caspases, Cell Death & Differentiation](https://www.nature.com/articles/4402028)
12. [Jiang X, Wang X. Cytochrome C-Mediated Apoptosis, Annual Review of Biochemistry (2004), PDF](https://mcb.berkeley.edu/courses/mcb230/jk_pdfs/Jiang%26Wang.pdf)
13. [The apoptosome: a cell death signaling platform, Boston University Pharmacology, Physiology & Biophysics](https://www.bumc.bu.edu/ppb/apoptosome/)
14. [Structural biology of the intrinsic cell death pathway: what do we know and what is missing?, CSBJ](https://spj.science.org/doi/10.5936/csbj.201204007)
15. [Intrinsic apoptosis is evolutionarily divergent among metazoans, Evolution Letters](https://academic.oup.com/evlett/article/8/2/267/7424387)
16. [Evolution of the mitochondrial apoptotic pathway, Functional Biology](https://www.imrpress.com/journal/FBL/14/11/10.2741/3517/pdf)
17. [Large transient assemblies of Apaf1 constitute the apoptosome in cells, PubMed (2025)](https://pubmed.ncbi.nlm.nih.gov/41136486/)
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*Topic: Encyclopedia › Life and health › Biological foundations › Biochemistry and metabolism › Enzyme classes and activities › Proteolytic and peptidase enzymes › Proteases by catalytic mechanism › Cysteine proteases › Caspases and apoptotic proteolysis › Caspase activation pathways*
*Initially written Sep 17, 2026 · Reviewed: Oct 21, 2026 · Edited: Oct 15, 2026; Oct 21, 2026 · Last review: Oct 21, 2026*
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License: Edgepedia Community License 1.0, https://www.edgepedia.org/edgepedia/license