Key Points
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Proteolytic pathways that execute apoptosis are initiated by the death-inducing signalling complex (DISC) or the apoptosome. The DISC is essentially a classic ligand-dependent transmembrane receptor complex that integrates extracellular death signals. By contrast, the apoptosome represents a soluble mechanism-based receptor platform that integrates intracellular death signals.
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The central building block of the apoptosome is the multidomain protein Apaf-1, which consists of three fundamental units: a sensor for an apoptotic stimulus (WD40 repeats), an oligomerization unit (NB-ARC) and a domain (caspase-recruitment domain (CARD)) that recruits its target, the apical caspase-9. The NB-ARC region is homologous to the AAA+ type of ATPases, but unlike many members of this family Apaf-1 uses its ATPase function as a regulatory tool.
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The formation of the apoptosome is dependent on cytochrome c and the nucleotide triphosphates ATP or 2′-deoxy ATP (dATP), the latter being more effective and most likely the natural agent.
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Apaf-1 exists as a compact nucleotide-triphosphate-bound monomer, autolocked by its WD40 region. Findings from recent structural and biochemical studies can be put together to reveal the following sequence of events in apoptosome formation. First, upon the egress of the apoptotic signalling protein cytochrome c from mitochondria, it binds to the WD40 region of Apaf-1, causing hydrolysis of nucleotide triphosphate and release of the lock. Second, Apaf-1 adopts a semi-open configuration, but its nucleotide-binding and oligomerization domain (NOD; also referred to as NB-ARC) remains in an autoinhibited conformation. Third, the final opening of Apaf-1 and its subsequent oligomerization is dependent on the exchange of nucleotide diphosphate for triphosphate in the NB-ARC region of Apaf-1.
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The functional oligomerized apoptosome is a wheel-like structure that consists of a heptameric arrangement of Apaf-1 molecules. The NB-ARC region provides the scaffold of the oligomer, whereas the CARD domains are arranged in a ring-like manner near the centre of the assembly, generating a platform competent to activate capase-9.
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The sensor–oligomerizer–effector structure of the soluble Apaf-1 receptor is conserved in a related family of proteins, the NOD-like receptors (NLRs), which possess a domain that is closely related to the NB-ARC region of Apaf-1. These proteins, implicated in the regulation of innate immunity, can form similar soluble receptor platforms to activate their respective caspase or kinase targets.
Abstract
Recent work on the initial switches that trigger cell death has revealed surprising inventions of nature that ensure the ordered suicide of a cell that has been selected for demise. Particularly intriguing is how a signal — the release of cytochrome c from the mitochondria — is translated into the activation of the death cascade, which leads to a point of no return. Now there is new understanding of how this crucial process is delicately handled by a cytosolic signalling platform known as the apoptosome. The formation of the apoptosome and the activation of its effector, caspase-9, reveals a sophisticated mechanism that might be more common than was initially thought.
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Acknowledgements
We thank C. Akey for kindly providing images of the apoptosome as well as the model of the apoptosome, and we thank J. Heuser for the electron-microscopy image of the locked apoptosome. We also thank A. Lupas and A. Diemand for providing the coordinates for the AAA+-like model of the apoptosome. Further thanks go to current or former members of our laboratories and colleagues, for their support and advice. We apologize to our colleagues whose important contributions were inadvertently overlooked or cited only indirectly due to space limitations. S.J.R. is a fellow of the Leukemia and Lymphoma Society and the V Foundation.
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DATABASES
Protein Data Bank
FURTHER INFORMATION
Glossary
- Zymogen
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Pro-form or precursor of an enzyme. In the context of this review, refers to the latent form of a caspase before its activation.
- Extrinsic pathway
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Apoptotic pathway that initiates cell death following an extracellular signal. Binding of a death ligand to a death receptor triggers the formation of the death-inducing signalling complex (DISC). This leads to caspase-8 (or caspase-10) activation and subsequent caspase-3 activation and cell death.
- Death-inducing signalling complex
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A group of cellular factors that are recruited to the intracellular domain of the cell-surface receptor CD95/Fas/Apo-1 after ligand binding.
- Intrinsic pathway
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Apoptotic pathway that initiates cell death following an intracellular signal. Stress signals from within the cell lead to the release of mitochondrial factors, among them cytochrome c. This triggers apoptosome formation, leading to caspase-9 activation and subsequent caspase-3 activation and cell death.
- AAA+
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Extended superfamily of ATPases associated with a variety of cellular activities. Characterized by their extended P-loop ATPase domain that is capable of forming ring-like oligomers.
- Caspase-recruitment domain
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(CARD). Similar to the pyrin domain, a small helical death domain that is involved in protein–protein interactions. The CARD is vital for the interaction of Apaf-1 with caspase-9; it is also found in several nucleotide-binding and oligomerization domain (NOD)-like receptors and other caspases.
- WD40 repeat
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A repeat sequence of ∼40 amino acids, which often terminates in Trp–Asp. WD40 repeats form circular β-propeller structures that are implicated in various functions by building the scaffold for protein–protein interactions.
- Winged-helix domain
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A motif usually found in DNA-binding proteins. In Apaf-1, the winged-helix domain directly interacts with the bound ADP of the autoinhibited state. Here, the winged-helix domain might have a role in sensing the bound nucleotide. It is a key structural element for the transformation of the autoinhibited state of Apaf-1 into the apoptosome.
- Superhelical domain
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In Apaf-1, several helices form a higher-order structure, which is referred to as a superhelical domain, similar to those found in Armadillo-repeat proteins.
- Toll-like receptor family
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Family of transmembrane receptors with a central role in the innate immune system. They are important sensors of pathogen-associated molecular patterns (PAMPs) and trigger the induction of inflammatory responses.
- Leu-rich repeats
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Consist of several repeats of a 20–30 amino-acid motif, which show conserved patterns containing Leu residues. These repeats form horseshoe-like structures that serve various functions including protein–protein interaction and pathogen recognition.
- Pyrin domain
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Globular α-helical domain that belongs to the 'DEATH fold'. A protein–protein interaction domain that is found in several nucleotide-binding and oligomerization domain (NOD)-like receptors. The DEATH fold refers to a structural motif present in several proteins that regulate apoptosis or inflammation. Examples include the death domain, death effector domain, caspase-recruitment domain and pyrin domain, each of which acts as a recruitment module for the formation of higher-order complexes.
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Riedl, S., Salvesen, G. The apoptosome: signalling platform of cell death. Nat Rev Mol Cell Biol 8, 405–413 (2007). https://doi.org/10.1038/nrm2153
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DOI: https://doi.org/10.1038/nrm2153
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