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The Reactome event describes the apoptosome assembly with the stoichiometry of 4 procaspase-9 zymogens per 7 APAF1 molecules. The formation of 1:1 and other combinations of APAF1:CASP9(1-416) complexes is not shown.
This Reactome event describes the GSDME (1‑275) binding to mitochondrial cardiolipin leading to CYCS release from the mitochondria.
Different models have been proposed to explain CASP9 activation: the “proximity‑driven dimerization model” and the “induced conformation model”. The first models states that upon binding to heptameric APAF1, monomers of procaspase‑9 are brought into close proximity at a high concentration (Acehan et al. 2002; Renatus et al. 2001). This induces dimerization which is sufficient for CASP9 activation whereas autoprocessing within the apoptosome complex merely stabilizes CASP9 dimer (Boatright KM et al. 2003; Pop C et al. 2006). The “induced conformation model” is based on the observation that CASP9 has a much higher level of catalytic activity when it's bound to the apoptosome. The model suggests that a conformational change occurs at the active site of CASP9 upon binding to APAF1 thus inducing CASP9 homodimerization and stabilizing it in the catalytically active conformation (Shiozaki EN et al. 2002). CASP9 activation may also involve formation of a multimeric CARD:CARD assembly between APAF1 and procaspase‑9 (Hu Q et al. 2014).