Structural basis of IAP recognition by Smac/DIABLO

Genomic organization of the X-linked inhibitor of apoptosis and identification of a novel testis-specific transcript

Here we report the genomic organization and mapping of the X-linked inhibitor of apoptosis gene (BIRC4, also known as XIAP and hILP) and the identification of a closely related transcript. BIRC4 is located on Xq25 and is composed of seven exons. The intron/exon structure is highly conserved between the mouse homologue and its human counterpart. Four bands cross-react with a BIRC4 coding region probe on a genomic Southern blot. One of these cross-reactive bands encodes an intronless gene that expresses a 2.2-kb transcript solely in the testis. This testis-specific transcript contains a putative open reading frame (ORF) that is homologous to the carboxy-terminal end of BIRC4; overexpression of this ORF shows protective effects against BAX-induced apoptosis.

Signaling pathways in apoptosis as potential targets for cancer therapy

Genetic instability contributes to the origin of cancer as well as to the ability of cancer cells to become resistant to various therapies. Because of this, cytotoxic rather than cytostatic therapies might be most effective against this disease. Many oncogenes and tumor suppressors mediate their effects by interfering with or inducing apoptotic signaling. Thus, apoptotic pathways might be significantly altered in cancer cells relative to untransformed cells, and these differences might present a therapeutic window that can be exploited for development of cancer drugs.

Structural analysis of a functional DIAP1 fragment bound to grim and hid peptides

The inhibitor of apoptosis protein DIAP1 suppresses apoptosis in Drosophila, with the second BIR domain (BIR2) playing an important role. Three proteins, Hid, Grim, and Reaper, promote apoptosis, in part by binding to DIAP1 through their conserved N-terminal sequences. The crystal structures of DIAP1-BIR2 by itself and in complex with the N-terminal peptides from Hid and Grim reveal that these peptides bind a surface groove on DIAP1, with the first four amino acids mimicking the binding of the Smac tetrapeptide to XIAP. The next 3 residues also contribute to binding through hydrophobic interactions. Interestingly, peptide binding induces the formation of an additional alpha helix in DIAP1. Our study reveals the structural conservation and diversity necessary for the binding of IAPs by the Drosophila Hid/Grim/Reaper and the mammalian Smac proteins.

XIAP, the guardian angel

Controlling the activity of caspases is essential for the appropriate execution of cell death and the regulation of cell survival. One cellular inhibitor of apoptosis, XIAP, has emerged as a crucial regulator of caspases, and is itself subject to complex negative regulation.

Apoptotic death sensor: an organelle's alter ego?

Caspases are intracellular cysteine proteases that are primarily responsible for the stereotypic morphological and biochemical changes that are associated with apoptosis. Caspases are often activated by the apoptotic protease-activating factor 1 (APAF-1) apoptosome, a complex that is formed following mitochondrial release of cytochrome c in response to many death-inducing stimuli. Both pro- and anti-apoptotic BCL-2 family members regulate apoptosis, primarily by their effects on mitochondria, whereas many inhibitor of apoptosis proteins (IAPs) regulate apoptosis by directly inhibiting distinct caspases. Exposure of cells to chemicals and radiation, as well as loss of trophic stimuli, perturb cellular homeostasis and, depending on the type of cellular stress, particular or multiple organelles appear to 'sense' the damage and signal the cell to undergo apoptosis by stimulating the formation of unique and/or common caspase-activating complexes.

The mitochondrial apoptosome: a killer unleashed by the cytochrome seas

The caspase family of cysteine proteases have emerged as central regulators of apoptosis. Diverse cellular stresses trigger caspase activation by promoting release of mitochondrial components, including cytochrome c, into the cytoplasm. In turn, cytochrome c promotes the assembly of a caspase-activating complex termed the apoptosome. In this article, the apoptosome and its role in life and death decisions of cells are discussed.

Two kinds of BIR-containing protein - inhibitors of apoptosis, or required for mitosis

The baculoviral IAP repeat (BIR) is a zinc-binding fold. Some BIR-containing proteins (BIRCs), including several from insect viruses, insects and vertebrates, are inhibitors of cell death and act by binding to active caspases. Their ability to do so can be antagonized by pro-apoptotic insect proteins such as Grim, HID and Reaper, or the mammalian protein Diablo/Smac. Members of one structurally distinct subgroup of BIR-containing proteins, which are present in yeasts and Caenorhabditis elegans as well as insects and vertebrates, do not act as caspase inhibitors; instead, they are required for chromosome segregation and cytokinesis, and act in concert with inner centromere protein (INCENP) homologues and aurora kinase homologues.

Viruses and apoptosis

Apoptosis, or programmed cell death, is essential in development and homeostasis in multi-cellular organisms. It is also an important component of the cellular response to injury. Many cells undergo apoptosis in response to viral infection, with a consequent reduction in the release of progeny virus. Viruses have therefore evolved multiple distinct mechanisms for modulating host cell apoptosis. Viruses may interfere with either the highly conserved 'effector' mechanisms of programmed cell death or regulatory mechanisms specific to mammalian cells. In addition to conferring a selective advantage to the virus, the capacity to prevent apoptosis has an essential role in the transformation of the host cell by oncogenic viruses. This article provides a focussed review of apoptosis and illustrates how the study of viruses has informed our understanding of this process. Selected mechanisms by which viral gene products interfere with cell death are discussed in detail and used to illustrate the general principles of the interactions between viruses and apoptosis.

Structural basis for the inhibition of caspase-3 by XIAP

The molecular mechanism(s) that regulate apoptosis by caspase inhibition remain poorly understood. The main endogenous inhibitors are members of the IAP family and are exemplified by XIAP, which regulates the initiator caspase-9, and the executioner caspases-3 and -7. We report the crystal structure of the second BIR domain of XIAP (BIR2) in complex with caspase-3, at a resolution of 2.7 A, revealing the structural basis for inhibition. The inhibitor makes limited contacts through its BIR domain to the surface of the enzyme, and most contacts to caspase-3 originate from the N-terminal extension. This lies across the substrate binding cleft, but in reverse orientation compared to substrate binding. The mechanism of inhibition is due to a steric blockade prohibitive of substrate binding, and is distinct from the mechanism utilized by synthetic substrate analog inhibitors.

Structural basis of caspase-7 inhibition by XIAP

The inhibitor of apoptosis (IAP) proteins suppress cell death by inhibiting the catalytic activity of caspases. Here we present the crystal structure of caspase-7 in complex with a potent inhibitory fragment from XIAP at 2.45 A resolution. An 18-residue XIAP peptide binds the catalytic groove of caspase-7, making extensive contacts to the residues that are essential for its catalytic activity. Strikingly, despite a reversal of relative orientation, a subset of interactions between caspase-7 and XIAP closely resemble those between caspase-7 and its tetrapeptide inhibitor DEVD-CHO. Our biochemical and structural analyses reveal that the BIR domains are dispensable for the inhibition of caspase-3 and -7. This study provides a structural basis for the design of the next-generation caspase inhibitors.

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

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.

Inhibitor of apoptosis proteins and their relatives: IAPs and other BIRPs

SUMMARY

Apoptosis is a physiological cell death process important for development, homeostasis and the immune defence of multicellular animals. The key effectors of apoptosis are caspases, cysteine proteases that cleave after aspartate residues. The inhibitor of apoptosis (IAP) family of proteins prevent cell death by binding to and inhibiting active caspases and are negatively regulated by IAP-binding proteins, such as the mammalian protein DIABLO/Smac. IAPs are characterized by the presence of one to three domains known as baculoviral IAP repeat (BIR) domains and many also have a RING-finger domain at their carboxyl terminus. More recently, a second group of BIR-domain-containing proteins (BIRPs) have been identified that includes the mammalian proteins Bruce and Survivin as well as BIR-containing proteins in yeasts and Caenorhabditis elegans. These Survivin-like BIRPs regulate cytokinesis and mitotic spindle formation. In this review, we describe the IAPs and other BIRPs, their evolutionary relationships and their subcellular and tissue localizations.