Apoptosoma

Apoptosoma is not a widely used term in current biology literature. The proteolytic activation complex that initiates caspase-9 activation in the intrinsic pathway of apoptosis is normally referred to as the apoptosome. Occasional appearances of the form "apoptosoma" are generally regarded as nonspecific or historical variants rather than standard nomenclature.

Composition and structure: The apoptosome is a large, wheel-like, heptameric complex formed in the cytosol after mitochondrial outer membrane permeabilization. It comprises cytochrome c, the cytosolic adaptor APAF1 (APAF1 in humans), and several procaspase-9 molecules. Binding of cytochrome c to APAF1 in the presence of dATP or ATP induces APAF1 oligomerization into the apoptosome, which creates recruitment sites for procaspase-9 via CARD-CARD interactions.

Mechanism of activation: Within the apoptosome, procaspase-9 molecules are activated and then cleave downstream executioner caspases such as caspase-3 and caspase-7, establishing the proteolytic cascade that leads to apoptosis. The complex thus couples mitochondrial signals of stress to cellular demolition.

Regulation and context: Apoptosome activity is modulated by inhibitors of apoptosis (IAPs); mitochondrial proteins such as SMAC/DIABLO antagonize IAPs to promote caspase activation. The Bcl-2 family and p53 pathways influence upstream events like mitochondrial permeabilization, thereby controlling apoptosome formation.

Evolution and distribution: The apoptosome has been most extensively studied in mammals; some organisms lack a direct APAF1 homolog or use functionally analogous factors, reflecting diversity in apoptotic regulation across eukaryotes. In summary, apoptosome is the standard term for the complex, while apoptosoma is not the accepted nomenclature.