title: cIAP1 cooperatively inhibits procaspase-3 activation by the caspase-9 apoptosome.
abstract: Although early studies of inhibitor of apoptosis proteins (IAPs) suggested that cIAP1 directly binds and inhibits caspases similarly to X-linked IAP (XIAP), a recent one found that micromolar concentrations of cIAP1 only weakly inhibit caspase-3, -7, or -9. Here, we show that cIAP1 specifically and cooperatively blocks the cytochrome c-dependent apoptosome in vitro. Hence, cIAP1 prevented the activation of procaspase-3 but had no effect on the processing of procaspase-9 or the activity of prior activated caspase-3. Like cIAP1, XIAP had no effect on procaspase-9 processing and was a more potent inhibitor of procaspase-3 activation than of already activated caspase-3 activity. Inhibition of procaspase-3 activation depended on BIR2 and BIR3 of cIAP1 and was independent of BIR1, RING, CARD, and UBA domains. Smac prevented cIAP1 from inhibiting procaspase-3 activation and reversed the inhibition by prior addition of cIAP1. A procaspase-9 mutant (D315A) that cannot produce the p12 subunit was resistant to inhibition by cIAP1. Therefore, the N-terminal Ala-Thr-Pro-Phe motif of the p12 subunit of the caspase-9 apoptosome facilitates apoptosome blockade. Consequently, cIAP1 cooperatively interacts with oligomerized processed caspase-9 in the apoptosome and blocks procaspase-3 activation.
authors.affiliation: Department of Pharmacology and Toxicology, Schools of Medicine and Dentistry, University of Alabama at Birmingham, Alabama 35294-0019, USA.
authors.forename: Stephen P
authors.initials: SP
authors.lastname: Burke
authors.forename: Lucinda
authors.initials: L
authors.lastname: Smith
authors.forename: Jeffrey B
authors.initials: JB
authors.lastname: Smith
mesh_terms: D053478:Apoptosomes / Q000235:genetics / Q000378:metabolism*; D053148:Caspase 3 / Q000235:genetics / Q000378:metabolism*; D053453:Caspase 9 / Q000235:genetics / Q000378:metabolism*; D004789:Enzyme Activation / Q000502:physiology; D006801:Humans; D051034:Inhibitor of Apoptosis Proteins / Q000235:genetics / Q000378:metabolism*; D020125:Mutation, Missense; D055503:Protein Multimerization / Q000502:physiology*; D017434:Protein Structure, Tertiary; D051636:X-Linked Inhibitor of Apoptosis Protein / Q000235:genetics / Q000378:metabolism
chemical_list: D053478:Apoptosomes; D051034:Inhibitor of Apoptosis Proteins; D051636:X-Linked Inhibitor of Apoptosis Protein; C496866:XIAP protein, human; D053148:Caspase 3; D053453:Caspase 9