Oligomeric Apaf-1 mediates the cytochrome c-dependent autocatalytic activation of pro-caspase-9 (Apaf-3), leading to the activation of caspase-3 and apoptosis. This activation requires ATP (By similarity). Monomer. Oligomerizes to a heptameric ring, known as the apoptosome, upon binding of cytochrome c and dATP. Oligomeric Apaf-1 and pro-caspase-9 bind to each other via their respective NH2-terminal CARD domains and consecutively mature caspase-9 is released from the complex (By similarity). Interacts with UACA. It may also interact with Bcl-XL. Interacts with APIP. Interacts (via CARD and NACHT domains) with NAIP/BIRC1 (via NACHT domain) (By similarity). Interacts with CIAO2A (By similarity). Highly expressed in lung and spleen, weakly in brain and kidney and not detectable in liver. High levels in embryonic brain and liver from 11.5 dpc to 17.5 dpc. The CARD domain mediates interaction with APIP. The monomeric form is autoinhibited in a closed conformation through a bound ADP at the nucleotide binding site. Exchange of ADP for ATP and binding of cytochrome c trigger a large conformational change where the first WD repeat region swings out, allowing the NB-ARC domain to rotate and expose the contact areas for oligomerization. Physiological concentrations of calcium ions negatively affect the assembly of apoptosome by inhibiting nucleotide exchange in the monomeric form. Major isoform.