An unconventional IAP-binding motif revealed by target-assisted iterative screening (TAIS) of the BIR3-cIAP1 domain

Target-assisted iterative screening (TAIS) has been applied to a random phage-displayed peptide library in a search for novel ligands of the third baculovirus IAP ('inhibitors of apoptosis') repeat (BIR) domain of cIAP1. The peptides selected in the screen fall into two distinct specificity groups, one that conforms to a known IAP-binding motif (IBM) and another one that reveals a novel BIR domain interacting motif, NH(2)-SR(V/P)W. The biochemical profiling of selected sequences with synthetic peptides, which included alanine scanning and N- and C-terminal truncations as well as competition with the Smac peptide, suggests a major energetic contribution of tryptophan at the +4 position of peptide ligands to binding and identifies the latter together with the respective pocket on the BIR domain surface as a 'hot spot' of the interaction. A peptide featuring the novel motif selectively binds the full-length cIAP1 protein in cell lysates. A 'two-pocket' model of BIR domain recognition mechanism is proposed as the basis of differential BIR domain interactions with different IBMs.     

Structure and function analysis of peptide antagonists of melanoma inhibitor of apoptosis (ML-IAP)

Melanoma inhibitor of apoptosis (ML-IAP) is a potent anti-apoptotic protein that is upregulated in a number of melanoma cell lines but not expressed in most normal adult tissues. Overexpression of IAP proteins, such as ML-IAP or the ubiquitously expressed X-chromosome-linked IAP (XIAP), in human cancers has been shown to suppress apoptosis induced by a variety of stimuli. Peptides based on the processed N-terminus of Smac/DIABLO can negate the ability of overexpressed ML-IAP or XIAP to suppress drug-induced apoptosis. Such peptides have been demonstrated to bind to the single baculovirus IAP repeat (BIR) of ML-IAP and the third BIR of XIAP with similar high affinities (approximately 0.5 microM). Herein, we use phage-display of na?ve peptide libraries and synthetic peptides to investigate the peptide-binding properties of ML-IAP-BIR and XIAP-BIR3. X-ray crystal structures of ML-IAP-BIR in complex with Smac- and phage-derived peptides, together with peptide structure-activity-relationship data, indicate that the peptides can be modified to provide increased binding affinity and selectivity for ML-IAP-BIR relative to XIAP-BIR3. For instance, substitution of Pro3' in the Smac-based peptide (AVPIAQKSE) with (2S,3S)-3-methylpyrrolidine-2-carboxylic acid [(3S)-methyl-proline] results in a peptide with 7-fold greater affinity for ML-IAP-BIR and about 100-fold specificity for ML-IAP-BIR relative to XIAP-BIR3.     

Solution structure of a baculoviral inhibitor of apoptosis (IAP) repeat.

Members of the inhibitor of apoptosis (IAP) family of proteins are able to inhibit cell death following viral infection, during development or in cell lines in vitro. All IAP proteins bear one or more baculoviral IAP repeats (BIRs). Here we describe the solution structure of the third BIR domain from the mammalian IAP homolog B (MIHB/c-IAP-1). The BIR domain has a novel fold that is stabilized by zinc tetrahedrally coordinated by one histidine and three cysteine residues. The structure consists of a series of short alpha-helices and turns with the zinc packed in an unusually hydrophobic environment created by residues that are highly conserved among all BIRs.     

A highly conserved arginine is critical for the functional folding of inhibitor of apoptosis (IAP) BIR domains

The inhibitor of apoptosis (IAP) proteins are found in all animals and regulate apoptosis (programmed cell death) by binding and inhibiting caspase proteases. This inhibition is overcome by several apoptosis stimulators, including Drosophila Hid and mammalian Smac/DIABLO, which bind to 65-residue baculovirus IAP repeat (BIR) domains found in one to three copies in all IAPs. Virtually all BIRs contain three Cys and a His that bind zinc, a Gly in a tight turn, and an Arg. The functional and structural role of the Arg was investigated in isolated BIR domains from the baculovirus Orgyia pseudotsugata Op-IAP and the Drosophila DIAP1 proteins. Mutation of the Arg to either Ala or Lys abolished Hid and Smac binding to BIRs, despite the Hid/Smac binding site being located on the opposite side of the BIR domain from the Arg. The mutant BIR domains also exhibited weakened zinc binding, increased sensitivity to limited proteolysis, and altered circular dichroism spectra indicative of perturbed domain folding. Examination of known BIR structures indicates that the Arg side chain makes simultaneous bridging hydrogen bonds and a cation-pi interaction for which the Arg guanidino group is uniquely well suited. These interactions are likely critical for stabilizing the tertiary fold of BIR domains in all IAPs, explaining the conservation of this residue.     

Determination of the sequence specificity of XIAP BIR domains by screening a combinatorial peptide library

Inhibitor of apoptosis (IAP) proteins regulate programmed cell death by inhibiting members of the caspase family of proteases. The X-chromosome-linked IAP (XIAP) contains three baculovirus IAP repeat (BIR) domains, which bind directly to the N-termini of target proteins including those of caspases-3, -7, and -9. In the present study, we defined the consensus sequences of the motifs that interact with the three BIR domains in an unbiased manner. A combinatorial peptide library containing four random residues at the N-terminus was constructed and screened using BIR domains as probes. We found that the BIR3 domain binds a highly specific motif containing an alanine or valine at the N-terminus (P1 position), an arginine or proline at the P3 position, and a hydrophobic residue (Phe, Ile, and Tyr) at the P4 position. The BIR2-binding motif is less stringent. Although it still requires an N-terminal alanine, it tolerates a wide variety of amino acids at P2-P4 positions. The BIR1 failed to bind to any peptides in the library. SPR analysis of individually synthesized peptides confirmed the library screening results. Database searches with the BIR2- and BIR3-binding consensus sequences revealed a large number of potential target proteins. The combinatorial library method should be readily applicable to other BIR domains or other types of protein modular domains.     

Functional analysis of the inhibitor of apoptosis genes in <Emphasis Type="Italic">Antheraea pernyi</Emphasis> nucleopolyhedrovirus

The inhibitor of apoptosis proteins (IAP) plays an important role in cell apoptosis. We cloned two novel IAP family members, Ap-iap1 and Ap-iap2, from Antheraea pernyi nucleopolyhedrovirus (ApNPV) genome. Ap-IAP1 contains two baculoviral IAP repeat (BIR) domains followed by a RING domain, but Ap-IAP2 has only one BIR domain and RING. The result of transient expression in Spodoptera frugiperda (Sf21) showed that Ap-iap1 blocked cell apoptosis induced by actinomycin D treatment and also rescued the p35 deficient Autographa californica nucleopolyhedrovirus (AcNPV) to replicate in Sf9 cells, while Ap-iap2 does not have this function. Several Ap-IAP1 truncations were constructed to test the activity of BIRs or RING motif to inhibit cell apoptosis. The results indicated that BIRs or RING of Ap-IAP1 had equally function to inhibit cell apoptosis. Therefore deletion of above both of the above domains could not block apoptosis induced by actinomycin D or rescue the replication of AcMNPVΔp35. We also screened two phage-display peptides that might interact with Ap-IAP1.     

The BIR motifs mediate dominant interference and oligomerization of inhibitor of apoptosis Op-IAP

The defining structural motif of the inhibitor of apoptosis (iap) protein family is the BIR (baculovirus iap repeat), a highly conserved zinc coordination domain of approximately 70 residues. Although the BIR is required for inhibitor-of-apoptosis (IAP) function, including caspase inhibition, its molecular role in antiapoptotic activity in vivo is unknown. To define the function of the BIRs, we investigated the activity of these structural motifs within Op-IAP, an efficient, virus-derived IAP. We report here that Op-IAP(1-216), a loss-of-function truncation which contains two BIRs but lacks the C-terminal RING motif, potently interfered with Op-IAP's capacity to block apoptosis induced by diverse stimuli. In contrast, Op-IAP(1-216) had no effect on apoptotic suppression by caspase inhibitor P35. Consistent with a mechanism of dominant inhibition that involves direct interaction between Op-IAP(1-216) and full-length Op-IAP, both proteins formed an immunoprecipitable complex in vivo. Op-IAP also self-associated. In contrast, the RING motif-containing truncation Op-IAP(183-268) failed to interact with or interfere with Op-IAP function. Substitution of conserved residues within BIR 2 caused loss of dominant inhibition by Op-IAP(1-216) and coincided with loss of interaction with Op-IAP. Thus, residues encompassing the BIRs mediate dominant inhibition and oligomerization of Op-IAP. Consistent with dominant interference by interaction with an endogenous cellular IAP, Op-IAP(1-216) also lowered the survival threshold of cultured insect cells. Taken together, these data suggest a new model wherein the antiapoptotic function of IAP requires homo-oligomerization, which in turn mediates specific interactions with cellular apoptotic effectors.     

Neuronal apoptosis-inhibitory protein does not interact with Smac and requires ATP to bind caspase-9

The neuronal apoptosis-inhibitory protein (NAIP) is the founding member of the mammalian family of inhibitor of apoptosis (IAP) proteins (also known as BIRC proteins) and has been shown to be antiapoptotic both in vivo and in vitro. The 160-kDa NAIP contains three distinct regions: an amino-terminal cluster of three baculoviral inhibitory repeat (BIR) domains, a central nucleotide binding oligomerization domain (NOD), and a carboxyl-terminal leucine-rich repeat (LRR) domain. The presence of the NOD and LRR domains renders NAIP unique among the IAPs and suggests that NAIP activity is regulated in a manner distinct from that of other members of the family. In this report, we examined the interaction of various regions of NAIP with caspase-9 and Smac. Recombinant NAIPs with truncations of the carboxyl-terminal LRR or NOD-LRR regions bound to caspase-9. In contrast, the full-length protein did not, suggesting some form of structural autoregulation. However, the association of the wild type full-length protein with caspase-9 was observed when interaction analysis was performed in the presence of ATP. Furthermore, mutation of the NAIP ATP binding pocket allowed full-length protein to interact with caspase-9. Thus, we conclude that NAIP binds to caspase-9 with a structural requirement for ATP and that in the absence of ATP the LRR domain negatively regulates the caspase-9-inhibiting activity of the BIR domains. Interestingly, and in contrast to the X-chromosome-linked inhibitor of apoptosis protein (XIAP), NAIP-mediated inhibition of caspase-9 was not countered by a peptide containing an amino-terminal IAP binding motif (IBM). Consistent with this observation was the failure of Smac protein to interact with the NAIP BIR domains. These results demonstrate that NAIP is distinct from the other IAPs, both in demonstrating a ligand-dependent caspase-9 interaction and in demonstrating a distinct mechanism of inhibition.     

Structural basis of caspase inhibition by XIAP: differential roles of the linker versus the BIR domain

The inhibitor of apoptosis proteins (IAPs) represent the only endogenous caspase inhibitors and are characterized by the presence of baculoviral IAP repeats (BIRs). Here, we report the crystal structure of the complex between human caspase-7 and XIAP (BIR2 and the proceeding linker). The structure surprisingly reveals that the linker is the only contacting element for the caspase, while the BIR2 domain is invisible in the crystal. The linker interacts with and blocks the substrate groove of the caspase in a backward fashion, distinct from substrate recognition. Structural analyses suggest that the linker is the energetic and specificity determinant of the interaction. Further biochemical characterizations clearly establish that the linker harbors the major energetic determinant, while the BIR2 domain serves as a regulatory element for caspase binding and Smac neutralization.     

Crystal structure of the BIR1 domain of XIAP in two crystal forms

X-linked inhibitor of apoptosis (XIAP) is a potent negative regulator of apoptosis. It also plays a role in BMP signaling, TGF-beta signaling, and copper homeostasis. Previous structural studies have shown that the baculoviral IAP repeat (BIR2 and BIR3) domains of XIAP interact with the IAP-binding-motifs (IBM) in several apoptosis proteins such as Smac and caspase-9 via the conserved IBM-binding groove. Here, we report the crystal structure in two crystal forms of the BIR1 domain of XIAP, which does not possess this IBM-binding groove and cannot interact with Smac or caspase-9. Instead, the BIR1 domain forms a conserved dimer through the region corresponding to the IBM-binding groove. Structural and sequence analyses suggest that this dimerization of BIR1 in XIAP may be conserved in other IAP family members such as cIAP1 and cIAP2 and may be important for the action of XIAP in TGF-beta and BMP signaling and the action of cIAP1 and cIAP2 in TNF receptor signaling.     

The human anti-apoptotic proteins cIAP1 and cIAP2 bind but do not inhibit caspases

cIAPs (cellular inhibitor of apoptosis proteins) 1 and 2 are able to regulate apoptosis when ectopically expressed in recipient cells and probably also in vivo. Previous work suggested that this is at least partially due to direct caspase inhibition, mediated by two of the three baculovirus IAP repeat (BIR) domains that are contained in these proteins. In support of this we show that the BIR domains 2 and 3 of the two cIAPs are able to bind caspases-7 and -9. However, we demonstrate that neither of these BIR domains is able to inhibit caspases because of critical substitutions in the regions that target caspase inhibition in the X-linked IAP, a tight binding caspase inhibitor. The cIAP BIR domains can be converted to tight binding caspase inhibitors by substituting these critical residues with XIAP residues. Thus, cIAPs maintain protein scaffolds suitable for direct caspase inhibition but have lost or never acquired specific caspase inhibitory interaction sites. Consequently, although the binding function of the cIAP BIRs may be important for their physiologic function, caspase inhibition is not.     

Recognition specificity of individual EH domains of mammals and yeast.

The Eps homology (EH) domain is a recently described protein binding module that is found, in multiple or single copies, in several proteins in species as diverse as human and yeast. In this work, we have investigated the molecular details of recognition specificity mediated by this domain family by characterizing the peptide-binding preference of 11 different EH domains from mammal and yeast proteins. Ten of the eleven EH domains could bind at least some peptides containing an Asn-Pro-Phe (NPF) motif. By contrast, the first EH domain of End3p preferentially binds peptides containing an His-Thr/Ser-Phe (HT/SF) motif. Domains that have a low affinity for the majority of NPF peptides reveal some affinity for a third class of peptides that contains two consecutive amino acids with aromatic side chains (FW or WW). This is the case for the third EH domain of Eps15 and for the two N-terminal domains of YBL47c. The consensus sequences derived from the peptides selected from phage-displayed peptide libraries allows for grouping of EH domains into families that are characterized by different NPF-context preference. Finally, comparison of the primary sequence of EH domains with similar or divergent specificity identifies a residue at position +3 following a conserved tryptophan, whose chemical characteristics modulate binding preference.     

An exegesis of IAPs: salvation and surprises from BIR motifs.

The BIR (baculovirus IAP repeat) motif is a conserved sequence of approximately 70 amino acids that was identified originally in the 'inhibitor of apoptosis' (IAP) family of proteins. BIR-containing proteins (BIRPs) are found in viruses, yeast and metazoans. Recent genetic analysis of a nematode BIRP demonstrated an essential role in cytokinesis instead of apoptosis. It is likely that BIRs originated in eukaryotes to serve a role in cytokinesis and/or mitotic spindle function during cell division and that, with gene duplication, the more recent adaptation of some BIRPs to the regulation of apoptosis was possible. IAPs interact with a variety of proteins, including members of the caspase protease family. This article discusses current research on the structure and function of the BIR motifs and how it could provide insight into the function of BIRPs in cell division.     

ML-IAP, a novel inhibitor of apoptosis that is preferentially expressed in human melanomas

BACKGROUND: Inhibitors of apoptosis (IAPs) are a family of cell death inhibitors found in viruses and metazoans. All IAPs have at least one baculovirus IAP repeat (BIR) motif that is essential for their anti-apoptotic activity. IAPs physically interact with a variety of pro-apoptotic proteins and inhibit apoptosis induced by diverse stimuli. This allows them to function as sensors and inhibitors of death signals that emanate from a variety of pathways. RESULTS: Here we report the characterization of ML-IAP, a novel human IAP that contains a single BIR and RING finger motif. ML-IAP is a powerful inhibitor of apoptosis induced by death receptors and chemotherapeutic agents, probably functioning as a direct inhibitor of downstream effector caspases. Modeling studies of the structure of the BIR domain revealed it to closely resemble the fold determined for the BIR2 domain of X-IAP. Deletion and mutational analysis demonstrated that integrity of the BIR domain was required for anti-apoptotic function. Tissue survey analysis showed expression in a number of embryonic tissues and tumor cell lines. In particular, the majority of melanoma cell lines expressed high levels of ML-IAP in contrast to primary melanocytes, which expressed undetectable levels. These melanoma cells were significantly more resistant to drug-induced apoptosis. CONCLUSIONS: ML-IAP, a novel human IAP, inhibits apoptosis induced by death receptors and chemotherapeutic agents. The BIR of ML-IAP possesses an evolutionarily conserved fold that is necessary for anti-apoptotic activity. Elevated expression of ML-IAP renders melanoma cells resistant to apoptotic stimuli and thereby potentially contributes to the pathogenesis of this malignancy.     

Requirement of both the second and third BIR domains for the relief of X-linked inhibitor of apoptosis protein (XIAP)-mediated caspase inhibition by Smac

The inhibitor of apoptosis proteins (IAP) are endogenous caspase inhibitors in the metazoan and characterized by the presence of baculoviral IAP repeats (BIR). X-linked IAP (XIAP) contains three BIR domains and directly inhibits effector caspases such as caspase-7 via a linker_BIR2 fragment and initiator caspases such as caspase-9 via the BIR3 domain. A mitochondrial protein Smac/DIABLO, which is released during apoptosis, antagonizes XIAP-mediated caspase inhibition by interacting directly with XIAP. Here, using glutathione S-transferase pulldown and caspase activity assay, we show that Smac is ineffective in relieving either caspase-7 or caspase-9 inhibition by XIAP domain fragments. In addition, Smac forms a ternary complex with caspase-7 and linker_BIR2, suggesting that Smac/linker_BIR2 interaction does not sterically exclude linker_BIR2/caspase-7 interaction. However, Smac is effective in removing caspase-7 and caspase-9 inhibition by XIAP fragments containing both the BIR2 and BIR3 domains. Surface plasmon resonance measurements show that Smac interacts with the BIR2 or BIR3 domain in micromolar dissociation constants. On the other hand, Smac interacts with an XIAP construct containing both BIR2 and BIR3 domains in a subnanomolar dissociation constant by the simultaneous interaction of the Smac dimer with the BIR2 and BIR3 domains of a single XIAP molecule. This 2:1 Smac/XIAP interaction not only possesses enhanced affinity but also sterically excludes XIAP/caspase-7 interaction, demonstrating the requirement of both BIR2 and BIR3 domains for Smac to relieve XIAP-mediated caspase inhibition.     

Targeting the X-linked inhibitor of apoptosis protein through 4-substituted azabicyclo[5.3.0]alkane smac mimetics. Structure, activity, and recognition principles

The X-linked inhibitor of apoptosis protein (XIAP) is overexpressed in several malignant cells where it prevents apoptosis by binding to, and blocking, the activation of caspase-3, -7, and -9. Human XIAP (479 residues) is composed of three tandem-repeated baculoviral IAP repeat (BIR) domains (BIR1-3), and by a C-terminal RING domain. Smac-DIABLO [second mitochondria-derived activator of caspases (Smac)-direct IAP binding protein with low pI (DIABLO)], the natural antagonist of XIAP, binds through its N-terminal sequence AVPI to the same surface groove, in the BIR domains, that binds caspases. Synthetic compounds mimicking such tetrapeptide motif effectively block the interaction between IAP and active caspases, thus triggering apoptosis. Peptidomimetics based on an azabicyclo[x.y.0]alkane scaffolds, have been shown to bind the BIR3 domain of XIAP with micromolar to nanomolar affinities, thus presenting attractive features for drug lead optimization. Here we report a study on three newly synthesized Smac mimetics, which have been characterized in their complexes with XIAP BIR3 domain through X-ray crystallography and molecular modelling/docking simulations. Based on analysis of the crystal structures, we show that specific substitutions at the 4-position of the azabicyclo[5.3.0]alkane scaffold results in sizeable effects on the peptidomimetic-BIR3 domain affinity. By means of functional, biophysical and simulative approaches we also propose that the same Smac mimetics can bind XIAP BIR2 domain at a location structurally related to the BIR3 domain AVPI binding groove. Details of the XIAP-Smac mimetic recognition principles highlighted by this study are discussed in light of the drug-like profile of the three (potentially proapoptotic) compounds developed that show improved performance in ADMET (adsorption, distribution, metabolism, excretion and toxicity) tests.     

Amsacta moorei Entomopoxvirus inhibitor of apoptosis suppresses cell death by binding Grim and Hid

Inhibitor of apoptosis (iap) genes have been identified in the genomes of two independent families of insect viruses, the Baculoviridae and the Entomopoxvirinae. In this report, we examined the functional attributes of the Amsacta moorei entomopoxvirus-encoded IAP protein (AMV-IAP). The binding specificity of the individual baculoviral IAP repeat (BIR) domains of AMV-IAP was investigated by using a random-peptide, phage display library, and sequences similar to the amino termini of proapoptotic Drosophila proteins in the Reaper/Hid/Grim family were identified. Furthermore, the BIR domains of AMV-IAP protein were demonstrated to bind the mammalian IAP inhibitor Smac through the AVPI tetrapeptide sequence, suggesting that the peptide binding pocket and groove found in the insect and mammalian IAPs is conserved in this viral protein. Interaction analysis implicated BIR1 as the high-affinity site for Grim, while BIR2 interacted more strongly with Hid. Both Grim and Hid were demonstrated to interact with AMV-IAP in vivo, and Grim- or Hid-induced cell death was suppressed when AMV-IAP was coexpressed.     

Peptides targeting caspase inhibitors

Here we report on the identification of peptides targeting the X-inhibitor of apoptosis protein (XIAP). XIAP functions as a caspase inhibitor and is a member of the inhibitors of apoptosis (IAP) family of proteins. IAPs are often overexpressed in cancers and leukemias and are associated with an unfavorable clinical prognosis. We have selected peptides from a phage library by using recombinant full-length human XIAP or a fragment containing only the baculovirus IAP repeat 2 (BIR2) domain. A consensus motif, C(D/E/P)(W/F/Y)-acid/basic-XC, was recovered from two independent screenings by using different libraries. Phage-displaying variations of the consensus sequence bound specifically to the BIR2 domain of XIAP but not to other IAPs. The interaction was specific as it could be blocked by the cognate synthetic peptides in a dose-dependent manner. Phage displaying the XIAP-binding motif CEFESC bound to the BIR2 domain of XIAP with an estimated dissociation constant of 1.8 nm as determined by surface plasmon resonance. Protein-protein interaction assays revealed that caspase-3 and caspase-7 (but not caspase-8) blocked the binding of the CEFESC phage to XIAP, indicating that this peptide targets a domain within XIAP that is related to the caspase-binding site. In fact, the sequence EFES is homologous to a loop unique to the executioner caspase-3 and caspase-7 that are targeted by XIAP. Finally, we demonstrated that an internalizing version of the XIAP-binding peptide identified in our screenings (PFKQ) can induce programmed cell death in leukemia cells. Peptides interacting with XIAP could serve as prototypes for the design of low molecular weight modulators of apoptosis.     

Characterization of a heterodimeric Smac-based peptide that features sequences specific to both the BIR2 and BIR3 domains of the X-linked inhibitor of apoptosis protein

XIAP, an important regulator of apoptosis, has emerged as a target for the development of cancer therapeutics. The homodimeric Smac protein simultaneously binds to both the BIR2 and BIR3 domains of XIAP. Peptide-based dimeric compounds that mimic the binding mode of Smac show promise as XIAP antagonists. Herein we characterize the first example of a Smac mimetic that incorporates a peptide sequence specific for BIR2. We show that the tetrapeptide motif Ala-Glu-Ala-Val has a higher affinity for BIR2 than the BIR3-specific sequence Ala-Val-Pro-Phe, and we compare the binding characteristics of a heterodimeric peptide containing both tetrapeptide motifs to those of a homodimeric peptide featuring only AVPF. Despite the enhanced affinity of AEAV (relative to AVPF) for BIR2, the heterodimeric peptide displays only a slightly higher affinity for XIAP relative to its homodimeric counterpart. Enhanced affinity of both dimers relative to the tetrapeptide AVPF is largely maintained even when the BIR2 binding groove is modified, implying that hydrophobic contacts afforded by the second peptide motif need not necessarily be made at the BIR2 binding groove to contribute substantial binding energy. Finally, we use mutagenesis to show that the difference in sequence specificity observed between the two domains is primarily owing to steric bulk introduced at the BIR2 site by lysine 206. Replacement of K206 at BIR2 with glycine, the corresponding residue in BIR3, restores the majority of the affinity for the AVPF motif exhibited by BIR3. The implications of these finding in the development of XIAP antagonists are discussed. ? 2011 Wiley Periodicals, Inc. Biopolymers (Pept Sci) 98: 122-130, 2012.     

The evolution of BIR domain and its containing proteins

BIR domain and its containing proteins play critical roles in cell apoptosis and cell division. Here several lines of novelty were revealed based on a comprehensive evolutionary analysis of BIR domains in 11 representative organisms. First, the type II BIR domains in Survivin and Bruce showed more conservation compared with the type I BIR domains in the inhibitors of apoptosis proteins (IAPs). Second, cIAP was derived from a XIAP duplicate and emerged just after the divergence of invertebrates and vertebrates. Third, the three BIR domains of NAIP displayed significantly elevated evolutionary rates compared with the BIR domains in other IAPs.