Apoptosome

The apoptosome is a large quaternary protein structure formed in the process of apoptosis. Its formation is triggered by the release of from the in response to an internal (intrinsic) or external (extrinsic) cell death stimulus. Stimuli can vary from DNA damage and viral infection to developmental cues such as those leading to the degradation of a tadpole's tail.

In mammalian cells, once is released, it binds to the cytosolic protein Apaf-1 to facilitate the formation of apoptosome. An early biochemical study suggests a two-to-one ratio of cytochrome c to apaf-1 for apoptosome formation. However, recent structural studies suggest the cytochrome c to apaf-1 ratio is one-to-one. It has also been shown that the nucleotide dATP as third component binds to apaf-1, however its exact role is still debated. The mammalian apoptosome had never been crystallized, but a human APAF-1/cytochrome-c apoptosome has been imaged at lower (2 nm) resolution by cryogenic transmission electron microscopy 10 years ago, revealing a wheel-like particle with 7-fold symmetry. Recently, a medium resolution (9.5 Ångström) structure of human apoptosome was also solved by cryo-electron microscopy, which allows unambiguous inference for positions of all the APAF-1 domains (CARD, NBARC and WD40) and cytochrome c. There is also now a crystal structure of the monomeric, inactive Apaf-1 subunit (PDB 3SFZ).

Once formed, the apoptosome can then recruit and activate the inactive pro-caspase-9. Once activated, this initiator caspase can then activate effector caspases and trigger a cascade of events leading to apoptosis.

The term Apoptosome was first introduced in Yoshihide Tsujimoto's 1998 paper "Role of Bcl-2 family proteins in apoptosis: apoptosomes or ?". However, the Apoptosome was known before this time as a ternary complex. This complex involved caspase-9 and Bcl-XL which each bound a specific Apaf-1 domain. The formation of this complex was then believed to play a regulatory role in mammalian cell death. In December of the same year, a further article was released in The Journal of Biological Chemistry stating that Apaf-1 is the regulator of apoptosis, through activation of procaspase-9.

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