X-linked Inhibitor of Apoptosis Protein (XIAP)

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X-linked Inhibitor of Apoptosis Protein (XIAP)

<table class="infobox infobox-protein">
<tr>
<th class="infobox-header" colspan="2">X-linked Inhibitor of Apoptosis Protein (XIAP)</th>
</tr>
<tr>
<td class="label">Interactor</td>
<td>Function</td>
</tr>
<tr>
<td class="label">[Caspase-3](/proteins/casp3-protein)</td>
<td>Inhibition</td>
</tr>
<tr>
<td class="label">[Caspase-7](/proteins/casp7-protein)</td>
<td>Inhibition</td>
</tr>
<tr>
<td class="label">[Caspase-9](/proteins/casp9-protein)</td>
<td>Inhibition</td>
</tr>
<tr>
<td class="label">SMAC/DIABLO</td>
<td>Antagonist</td>
</tr>
<tr>
<td class="label">TAB1</td>
<td>NF-κB activation</td>
</tr>
<tr>
<td class="label">TAK1</td>
<td>Kinase signaling</td>
</tr>
<tr>
<td class="label">NOD2</td>
<td>Bacterial sensing</td>
</tr>
<tr>
<td class="label">cIAP1/cIAP2</td>
<td>Heterodimerization</td>
</tr>
<tr>
<td class="label">Associated Diseases</td>
<td><a href="/wiki/als" style="color:#ef9a9a">Als</a>, <a href="/wiki/alzheimer" style="color:#ef9a9a">Alzheimer</a>, <a href="/wiki/amyotrophic-lateral-sclerosis" style="color:#ef9a9a">Amyotrophic Lateral Sclerosis</a>, <a href="/wiki/autoimmune" style="color:#ef9a9a">Autoimmune</a>, <a href="/wiki/breast-cancer" style="color:#ef9a9a">Breast Cancer</a></td>
</tr>
<tr>
<td class="label">KG Connections</td>
<td><a href="/atlas" style="color:#4fc3f7">131 edges</a></td>
</tr>
</table>

...

X-linked Inhibitor of Apoptosis Protein (XIAP)

<div style="float: right; margin: 0 0 1em 1em; padding: 1em; background: #f8f9fa; border: 1px solid #ddd; border-radius: 8px; font-size: 0.9em; max-width: 300px;">
<h3 style="margin-top: 0; border-bottom: 1px solid #ccc;">XIAP Protein</h3>
<table style="width: 100%; border-collapse: collapse;">
<tr><td style="padding: 4px 8px;"><strong>Gene</strong></td><td>[XIAP](/genes/xiap)</td></tr>
<tr><td style="padding: 4px 8px;"><strong>UniProt ID</strong></td><td>[P98170](https://www.uniprot.org/uniprot/P98170)</td></tr>
<tr><td style="padding: 4px 8px;"><strong>PDB Structures</strong></td><td>[1G3F](https://www.rcsb.org/structure/1G3F), [1I4O](https://www.rcsb.org/structure/1I4O)</td></tr>
<tr><td style="padding: 4px 8px;"><strong>Molecular Weight</strong></td><td>56.6 kDa</td></tr>
<tr><td style="padding: 4px 8px;"><strong>Amino Acids</strong></td><td>497</td></tr>
<tr><td style="padding: 4px 8px;"><strong>Subcellular Location</strong></td><td>Cytosol</td></tr>
<tr><td style="padding: 4px 8px;"><strong>Protein Family</strong></td><td>IAP family (BIR domain)</td></tr>
</table>
</div>

Overview

X-linked Inhibitor of [Apoptosis](/entities/apoptosis) Protein (XIAP), encoded by the [XIAP](/genes/xiap) gene on chromosome Xq25, is the most potent endogenous inhibitor of caspases and a key regulator of programmed cell death[@deveraux1999]. As a member of the inhibitor of apoptosis (IAP) protein family, XIAP contains baculoviral IAP repeat (BIR) domains that directly bind and inhibit caspases-3, -7, and -9, making it a critical determinant of cell survival under stress conditions[@salvesen2002].

In the nervous system, XIAP expression protects [neurons](/entities/neurons) from various apoptotic stimuli including oxidative stress, excitotoxicity, and protein aggregation[@liston2003]. Dysregulation of XIAP has been implicated in neurodegenerative diseases, where insufficient XIAP activity contributes to neuronal loss, while excessive XIAP may impair clearance of damaged cells[@galban2010].

Structure and Domain Architecture

XIAP is composed of three N-terminal BIR domains and a C-terminal RING finger domain[@chai2001]:

BIR Domains

BIR1 Domain (Residues 21-97)

Structural scaffold: Does not directly inhibit caspases
TAB1 binding: Mediates interaction with TAK1-binding protein 1
[NF-κB](/entities/nf-kb) activation: Participates in inflammatory signaling

BIR2 Domain (Residues 153-209)

Caspase-3 and -7 inhibition: The primary anti-caspase domain
Linker region: Contains the critical Ile-Thr-Glu-Gln (ITEQ) motif that blocks caspase active sites
Surface groove: Binds caspase dimerization interface

BIR3 Domain (Residues 258-310)

Caspase-9 inhibition: Selectively binds caspase-9 monomers
SMAC/DIABLO binding: Contains IAP-binding motif (IBM) groove
Allosteric regulation: Disrupts caspase-9 homodimerization

RING Finger Domain (Residues 422-490)

E3 ubiquitin ligase activity: Catalyzes auto-ubiquitination and substrate ubiquitination
Protein degradation: Targets caspases and itself for proteasomal degradation
Zinc coordination: C3HC4 motif coordinates two zinc ions

UBA Domain (Residues 362-399)

Ubiquitin-associated domain: Binds ubiquitin chains
Regulatory function: May modulate E3 ligase activity

Normal Function in the Nervous System

Apoptosis Inhibition

XIAP blocks the intrinsic apoptotic pathway through direct caspase inhibition[@riedl2001]:

Caspase-9 inhibition: BIR3 domain prevents apoptosome-mediated caspase-9 activation

Caspase-3/7 inhibition: BIR2 domain blocks executioner caspase activity

Substrate protection: Prevents cleavage of PARP, lamin, and other apoptotic substrates

Neuroprotection

In neurons, XIAP provides survival advantages[@perrelet2004]:

Stress resistance: Protects against oxidative damage and ER stress
Excitotoxicity protection: Limits [NMDA receptor](/entities/nmda-receptor)-mediated death pathways
Developmental refinement: Regulates neuronal numbers during development

Inflammatory Regulation

XIAP modulates neuroinflammation via[@krieg2009]:

NF-κB pathway: Activates inflammatory gene expression through TAB1-TAK1 complex
NOD2 signaling: Participates in bacterial sensing and response
Cytokine production: Influences IL-1β and TNF-α levels

Role in Neurodegeneration

Alzheimer's Disease

XIAP has complex roles in AD[@schubert2012]:

Neuroprotective Functions

Inhibits [Aβ](/proteins/amyloid-beta)-induced apoptosis: Blocks caspase activation by Aβ oligomers
Protects against [tau](/proteins/tau) toxicity: Reduces caspase-mediated tau cleavage
Preserves mitochondrial integrity: Prevents cytochrome c release

Pathogenic Considerations

Impaired clearance: May prevent death of dysfunctional neurons
Inflammatory activation: XIAP-mediated NF-κB may exacerbate neuroinflammation
Caspase-6 paradox: Does not efficiently inhibit caspase-6, which cleaves tau

Parkinson's Disease

XIAP protects dopaminergic neurons[@eberhardt2000]:

MPTP protection: XIAP overexpression reduces nigrostriatal degeneration
[α-synuclein](/proteins/alpha-synuclein) toxicity: Limits caspase activation in α-synuclein models
Mitochondrial protection: Preserves PINK1/parkin pathway function

Huntington's Disease

In HD models[@hermel2004]:

Caspase-3 inhibition: Reduces [huntingtin](/proteins/huntingtin) cleavage and toxicity
Striatal protection: Preserves medium spiny neurons
Enhanced survival: Gene therapy with XIAP shows benefit in models

Amyotrophic Lateral Sclerosis

XIAP affects motor neuron survival[@inoue2003]:

SOD1 models: Variable effects depending on disease stage
[TDP-43](/mechanisms/tdp-43-proteinopathy) pathology: May limit caspase activation
SMAC mimetics: Sensitize motor neurons to apoptosis

Spinal Muscular Atrophy

XIAP upregulation is protective in SMA[@azzouz2004]:

Motor neuron survival: Reduces apoptosis in SMA models
Combined therapy: Works synergistically with SMN-enhancing treatments

Therapeutic Targeting

SMAC/DIABLO Mimetics

SMAC mimetics displace caspases from XIAP to promote apoptosis[@fulda2012]:

IAP antagonists: Bind BIR2/BIR3 IBM grooves
Clinical applications: Cancer therapy (promote tumor cell death)
Neurological concern: May increase neuronal vulnerability

XIAP Enhancers

Approaches to boost XIAP neuroprotection[@siegel2006]:

Gene therapy: AAV-XIAP delivery to vulnerable neurons
Small molecule stabilizers: Compounds that enhance XIAP-caspase binding
Transcriptional activation: Upregulate endogenous XIAP expression

XIAP Modulators in Clinical Trials

LCL161: Oral SMAC mimetic (cancer trials)
Birinapant: Bivalent SMAC mimetic (cancer trials)
GDC-0152: IAP antagonist (cancer trials)

Targeting Challenges

Key considerations for XIAP modulation[@gyrdhansen2010]:

Cell type specificity: Neurons vs. glia vs. cancer cells
Temporal window: Timing of intervention critical
Compensation: Other IAPs may compensate for XIAP loss
Inflammatory effects: NF-κB activation may be detrimental

Protein-Protein Interactions

Summary

XIAP is the most potent endogenous caspase inhibitor, playing a crucial role in regulating neuronal survival and death. Its ability to directly inhibit caspases-3, -7, and -9 makes it a key determinant of cellular fate under stress conditions. While XIAP overexpression can protect neurons in neurodegenerative disease models, the therapeutic window is complex due to its inflammatory functions and the need to balance neuroprotection with appropriate removal of damaged cells.

External Links

[UniProt: P98170](https://www.uniprot.org/uniprot/P98170)
[NCBI Gene: 331](https://www.ncbi.nlm.nih.gov/gene/331)
[IAP Superfamily Database](https://www.uniprot.org/docs/iap.txt)

References

[Deveraux QL, Reed JC, IAP family proteins—suppressors of apoptosis (1999)](https://doi.org/10.1101/gad.13.3.239)

[Salvesen GS, Duckett CS, IAP proteins: blocking the road to death's door (2002)](https://doi.org/10.1038/nrm830)

[Liston P, Fong WG, Korneluk RG, The inhibitors of apoptosis: there is more to life than Bcl2 (2003)](https://doi.org/10.1038/sj.onc.1207101)

[Galban S, Duckett CS, XIAP as a ubiquitin ligase in cellular signaling (2010)](https://doi.org/10.1038/cdd.2009.81)

[Chai J, Shiozaki E, Srinivasula SM, et al, Structural basis of caspase-7 inhibition by XIAP (2001)](https://doi.org/10.1016/S0092-8674(01)

[Riedl SJ, Renatus M, Schwarzenbacher R, et al, Structural basis for the inhibition of caspase-3 by XIAP (2001)](https://doi.org/10.1016/S0092-8674(01)

[Perrelet D, Ferri A, MacKenzie AE, et al, XIAP overexpression in motor neurons in vivo is protective against excitotoxicity (2004)](https://doi.org/10.1016/j.nbd.2004.08.014)

[Krieg A, Correa RG, Garrison JB, et al, XIAP mediates NOD signaling via interaction with RIP2 (2009)](https://doi.org/10.1073/pnas.0907973106)

[Schubert D, The potentiation of proteotoxic stress as a novel therapeutic approach for treating Alzheimer's disease (2012)](https://doi.org/10.1016/B978-0-12-394816-8.00001-2)

[Eberhardt O, Coelln RV, Kugler S, et al, Protection by synergistic effects of adenovirus-mediated X-chromosome-linked inhibitor of apoptosis and glial cell line-derived neurotrophic factor gene transfer in the 1-methyl-4-phenyl-1,2,3,6-tetrahydropyridine model of Parkinson's disease (2000)](https://doi.org/10.1523/JNEUROSCI.20-24-09126.2000)

[Hermel E, Gafni J, Propp SS, et al, Specific caspase interactions and amplification are involved in selective neuronal vulnerability in Huntington's disease (2004)](https://doi.org/10.1038/sj.cdd.4401358)

[Inoue H, Tsukita K, Iwasato T, et al, The crucial role of caspase-9 in the disease progression of a transgenic ALS mouse model (2003)](https://doi.org/10.1093/emboj/cdg634)

[Azzouz M, Le T, Ralph GS, et al, Lentivector-mediated SMN replacement in a mouse model of spinal muscular atrophy (2004)](https://doi.org/10.1172/JCI22922)

[Fulda S, Vucic D, Targeting IAP proteins for therapeutic intervention in cancer (2012)](https://doi.org/10.1038/nrd3627)

[Siegel RM, Caspases at the crossroads of immune-cell life and death (2006)](https://doi.org/10.1038/nri1807)

[Gyrd-Hansen M, Meier P, IAPs: from caspase inhibitors to modulators of NF-κB, inflammation and cancer (2010)](https://doi.org/10.1038/nrc2889)

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X-linked Inhibitor of Apoptosis Protein (XIAP)

X-linked Inhibitor of Apoptosis Protein (XIAP)

X-linked Inhibitor of Apoptosis Protein (XIAP)

Overview

Structure and Domain Architecture

BIR Domains

BIR1 Domain (Residues 21-97)

BIR2 Domain (Residues 153-209)

BIR3 Domain (Residues 258-310)

RING Finger Domain (Residues 422-490)

UBA Domain (Residues 362-399)

Normal Function in the Nervous System

Apoptosis Inhibition

Neuroprotection

Inflammatory Regulation

Role in Neurodegeneration

Alzheimer's Disease

Neuroprotective Functions

Pathogenic Considerations

Parkinson's Disease

Huntington's Disease

Amyotrophic Lateral Sclerosis

Spinal Muscular Atrophy

Therapeutic Targeting

SMAC/DIABLO Mimetics

XIAP Enhancers

XIAP Modulators in Clinical Trials

Targeting Challenges

Protein-Protein Interactions

Summary

See Also

External Links

References