Apoptotic protease-activating factor 1 (Apaf-1) is a key player in the mitochondrial pathway of apoptosis and composed of three main domains: the N-terminal caspase recruitment domain (CARD), the nucleotide-binding and oligomerization domain (NOD), and the C-terminal WD40-repeat domain (two b-propellers, WD1 and WD2). The NOD is subdivided into the NB-ARC domain and the helical domain. Apaf-1 forms fully inhibited conformation in the absence of cytochrome c with ADP bound in the nucleotide binding site. The NB-ARC domain is held in place through interactions with the b-propeller WD1 and with the helical domain of the NOD, respectively. The presence of a gamma-phosphate in the nucleotide binding site leads to the formation of a salt bridge between R265 and ATP, which is a prerequisite for subsequent rotation of the NB-ARC domain. Binding of cytochrome c between the b-propellers causes propeller WD1 to swing out of its resting position, thereby releasing the attachment of the NB-ARC domain to the regulatory domain. Rotation of the NB-ARC domain exposes the contact areas for oligomerization and relocates the CARD. Apoptotic oligomerization of open Apaf-1 monomers arranges the flexibly tethered CARDs in the central hub of the apoptosomal wheel, where they bind procaspase-9 molecules for activation of the death cascade.