Neutralization of Smac/Diablo by inhibitors of apoptosis (IAPs). A caspase-independent mechanism for apoptotic inhibition

J Biol Chem. 2004 Dec 3;279(49):51082-90. doi: 10.1074/jbc.M408655200. Epub 2004 Sep 15.


Numerous members of the IAP family can suppress apoptotic cell death in physiological settings. Whereas certain IAPs directly inhibit caspases, the chief proteolytic effectors of apoptosis, the protective effects of other IAPs do not correlate well with their caspase inhibitory activities, suggesting the involvement of alternative cytoprotective abilities. To examine this issue, we have characterized the protective effects of an ancestral, baculoviral IAP (Op-IAP) in mammalian cells. We show that although Op-IAP potently inhibited Bax-mediated apoptosis in human cells, Op-IAP failed to directly inhibit mammalian caspases. However, Op-IAP efficiently bound the IAP antagonist Smac/Diablo, thereby preventing Smac/Diablo-mediated inhibition of cellular IAPs. Whereas reduction of Smac/Diablo protein levels in the absence of Op-IAP prevented Bax-mediated apoptosis, overexpression of Smac/Diablo neutralized Op-IAP-mediated protection, and an Op-IAP variant unable to bind Smac/Diablo failed to prevent apoptosis. Finally, Op-IAP catalyzed the ubiquitination of Smac/Diablo, an activity that contributed to Op-IAP-mediated inhibition of apoptosis. These data show that cytoprotective IAPs can inhibit apoptosis through the neutralization of IAP antagonists, rather than by directly inhibiting caspases.

Publication types

  • Research Support, Non-U.S. Gov't
  • Research Support, U.S. Gov't, P.H.S.

MeSH terms

  • Amino Acid Sequence
  • Apoptosis Regulatory Proteins
  • Apoptosis*
  • Carrier Proteins / chemistry
  • Carrier Proteins / physiology*
  • Caspase 3
  • Caspases / metabolism*
  • Cell Line
  • Cell Survival
  • Dose-Response Relationship, Drug
  • Glutathione Transferase / metabolism
  • Green Fluorescent Proteins / metabolism
  • Humans
  • Inhibitor of Apoptosis Proteins
  • Intracellular Signaling Peptides and Proteins
  • Mitochondrial Proteins / chemistry
  • Mitochondrial Proteins / physiology*
  • Molecular Sequence Data
  • Plasmids / metabolism
  • Protein Binding
  • Protein Structure, Tertiary
  • Proteins / metabolism
  • RNA Interference
  • RNA, Small Interfering / metabolism
  • Sequence Homology, Amino Acid
  • Time Factors
  • Transfection
  • Ubiquitin / metabolism
  • Viral Proteins / metabolism
  • Viral Proteins / physiology*
  • X-Linked Inhibitor of Apoptosis Protein


  • Apoptosis Regulatory Proteins
  • Carrier Proteins
  • DIABLO protein, human
  • Inhibitor of Apoptosis Proteins
  • Intracellular Signaling Peptides and Proteins
  • Mitochondrial Proteins
  • Proteins
  • RNA, Small Interfering
  • Ubiquitin
  • Viral Proteins
  • X-Linked Inhibitor of Apoptosis Protein
  • XIAP protein, human
  • inhibitor of apoptosis, Nucleopolyhedrovirus
  • Green Fluorescent Proteins
  • Glutathione Transferase
  • CASP3 protein, human
  • Caspase 3
  • Caspases