A conserved XIAP-interaction motif in caspase-9 and Smac/DIABLO regulates caspase activity and apoptosis

Abstract

X-linked inhibitor-of-apoptosis protein (XIAP) interacts with caspase-9 and inhibits its activity, whereas Smac (also known as DIABLO) relieves this inhibition through interaction with XIAP. Here we show that XIAP associates with the active caspase-9-Apaf-1 holoenzyme complex through binding to the amino terminus of the linker peptide on the small subunit of caspase-9, which becomes exposed after proteolytic processing of procaspase-9 at Asp315. Supporting this observation, point mutations that abrogate the proteolytic processing but not the catalytic activity of caspase-9, or deletion of the linker peptide, prevented caspase-9 association with XIAP and its concomitant inhibition. We note that the N-terminal four residues of caspase-9 linker peptide share significant homology with the N-terminal tetra-peptide in mature Smac and in the Drosophila proteins Hid/Grim/Reaper, defining a conserved class of IAP-binding motifs. Consistent with this finding, binding of the caspase-9 linker peptide and Smac to the BIR3 domain of XIAP is mutually exclusive, suggesting that Smac potentiates caspase-9 activity by disrupting the interaction of the linker peptide of caspase-9 with BIR3. Our studies reveal a mechanism in which binding to the BIR3 domain by two conserved peptides, one from Smac and the other one from caspase-9, has opposing effects on caspase activity and apoptosis.

Specific downregulation of bcl-2 and xIAP by RNAi enhances the effects of chemotherapeutic agents in MCF-7 human breast cancer cells

“…The current model for the antiapoptotic function of IAP relies on their capacity to block final activation of caspases and/or enzymatic activity of fully activated caspases. 31,32 Therefore, in the absence of an apoptotic stimulus leading to caspase activation, the IAP function would not occur. Consequently, a role of XIAP in the maintenance of cell viability suggests that low levels of caspase activation may occur in homeostatic conditions.…”

MAPK signaling in neurodegeneration: influences of flavonoids and of nitric oxide

“…Secondly, at the level of maintaining the calcium homeostasis shown to be implicated in MAPK activation [60,87,234] and NOS activity. Thirdly, flavonoids might modulate the mPT believed to be important in apoptotic cell death by either opening a gateway for cytochrome c release from the mitochondria [74,202] or by activating other mitochondrial-related proapoptotic factors such as DIABLO/smac [72,195]. Inhibitors of the mPTP opening like cyclosporin A bind to cyclophilin D which is associated with the adenine nucleotide transporter (ANT), part of the multi protein complex of the mPTP [200], and modulate mitochondrial depolarization [229], cytochrome c translocation [230], and cell death [74,230].…”

Genetic characterization of two gain-of-function alleles of the effector caspase DrICE in Drosophila

“…There are several possibilities. For example, because IAPs require effector caspases for inhibitory activity, 55,56 it may be possible that a wt/class C heterotetramer does not produce as much active DIAP1 compared with a wild-type tetramer. Specifically, the N terminus of DIAP1 is an intramolecular inhibitor of DIAP1, keeping DIAP1 in an auto-inhibited conformation, unable to bind and inhibit DrICE.…”