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Structural basis of IAP recognition by Smac/DIABLO

Abstract

Apoptosis is an essential process in the development and homeostasis of all metazoans1,2,3,4. The inhibitor-of-apoptosis (IAP) proteins suppress cell death by inhibiting the activity of caspases; this inhibition is performed by the zinc-binding BIR domains5,6 of the IAP proteins. The mitochondrial protein Smac/DIABLO promotes apoptosis by eliminating the inhibitory effect of IAPs through physical interactions7,8,9. Amino-terminal sequences in Smac/DIABLO are required for this function, as mutation of the very first amino acid leads to loss of interaction with IAPs and concomitant loss of Smac/DIABLO function9. Here we report the high-resolution crystal structure of Smac/DIABLO complexed with the third BIR domain (BIR3) of XIAP. Our results show that the N-terminal four residues (Ala-Val-Pro-Ile) in Smac/DIABLO recognize a surface groove on BIR3, with the first residue Ala binding a hydrophobic pocket and making five hydrogen bonds to neighbouring residues on BIR3. These observations provide a structural explanation for the roles of the Smac N terminus as well as the conserved N-terminal sequences in the Drosophila proteins Hid/Grim/Reaper. In conjunction with other observations, our results reveal how Smac may relieve IAP inhibition of caspase-9 activity. In addition to explaining a number of biological observations, our structural analysis identifies potential targets for drug screening.

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Figure 1: Schematic representation of the Smac(Phe33→Asp)/XIAP-BIR3 structures.
Figure 2: Binding interface between Smac and XIAP-BIR3.
Figure 3: Proposed mechanism for the relief of XIAP inhibition of caspase-9 by Smac.

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Acknowledgements

We thank C. Ogata for help at the NSLS-X4A beamline; F. Hughson for critically reading the manuscript; S. Kyin for peptide sequencing, DNA synthesis and mass spectroscopy; and N. Hunt for secretarial assistance. This research is supported by start-up funds from Princeton University (Y.S.) and the Howard Hughes Medical Institute (X.W.). Y.S. is a Searle Scholar and a Rita Allen Scholar.

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Correspondence to Yigong Shi.

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Wu, G., Chai, J., Suber, T. et al. Structural basis of IAP recognition by Smac/DIABLO. Nature 408, 1008–1012 (2000). https://doi.org/10.1038/35050012

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