TLR4-MD-2 lipopolysaccharide sensing module

Recognition of bacterial lipopolysaccharide (LPS) by the TLR4-MD-2 receptor and its two adaptor branches. LBP and CD14 deliver monomeric LPS; MD-2 binds lipid A in its hydrophobic pocket and TLR4 transmits the signal; the receptor first signals through TIRAP-MYD88 at the plasma membrane and, after endocytosis, through TRAM-TRIF from endosomes. This is the module where GO:0001875 lipopolysaccharide immune receptor activity belongs.

MODULE:tlr4_md2_lps_signalingDRAFTABSTRACTSignaling Pathwaymodules/tlr4_md2_lps_signaling.yaml

toll-like receptor 4 signaling pathwayGO:0034142

PMID:19252480

TLR4 and MD-2 form the LPS-recognising heterodimer.

Toll-like receptor (TLR) 4 and myeloid differentiation factor 2 (MD-2) form a heterodimer that recognizes a common 'pattern' in structurally diverse LPS molecules

Reference representation for TLR4 LPS sensing. Unlike GO-CAMs 5f46c3b700001031 and 6413ac9800000654, it includes MD-2 and puts GO:0001875 on the TLR4-MD-2 complex; unlike 5fb9cc0600000727 and 5fce9b7300000030, it does not use GO:0001875 for TLR2 heterodimers (see modules/tlr2_heterodimer_lipopeptide_signaling.yaml).

6Nodes

3Parts

1Variant Sets

2Variants

9Annotons

2Connections

Derived QC

Recommended-field compliance

100.0% recommended fields populated

All recommended fields populated.

Module deep research

✗ none found

No MODULE:tlr4_md2_lps_signaling deep-research report alongside the module YAML.

Leaf nodes lacking representative members

✓ representative grounding skipped for abstract module.

Template conformance

✓ every declared conforms_to bundle matches its template motif.

Function consistency and core coverage

7 gene-function check(s) reflected in core functions · 0 conflict(s) · 3 evidence or coverage gap(s) (11 comparison(s))

Retained noncore functions are reported separately; they do not require promotion to core functions. Support from representative genes does not establish conservation across the whole family. Missing reviews or identifiers leave a claim unverified.

Annoton	Participant	Function	Assessment
lbp_carrier	LBP UniProtKB:P18428 Gene	molecular carrier activity GO:0140104	Review Missing No ai-review exists for this protein; its function and core-function coverage cannot yet be checked.
cd14_carrier	CD14 UniProtKB:P08571 Gene	molecular carrier activity GO:0140104	Core Supported The GO molecular function is reflected in core_functions (exact term or a reviewed subclass).
tlr4_md2_lps_receptor	PROTEIN_COMPLEX Gene	lipopolysaccharide immune receptor activity GO:0001875	Participant Unchecked No direct UniProt participant to compare. Abstract selectors and complex-wide activities cannot be inferred from donor or subunit gene reviews.
tlr4_md2_lps_receptor/tlr4_unit	TLR4 UniProtKB:O00206 Complex Unit	transmembrane signaling receptor activity GO:0004888	Annotation Supported A retained annotation or curated replacement supports this function, but core_functions does not yet reflect it.
tlr4_md2_lps_receptor/md2_unit	LY96 UniProtKB:Q9Y6Y9 Complex Unit	coreceptor activity GO:0015026	Core Supported The GO molecular function is reflected in core_functions (exact term or a reviewed subclass).
tlr4_tirap	TIRAP UniProtKB:P58753 Gene	signaling adaptor activity GO:0035591	Core Supported The GO molecular function is reflected in core_functions (exact term or a reviewed subclass).
tlr4_myd88	MYD88 UniProtKB:Q99836 Gene	signaling adaptor activity GO:0035591	Core Supported The GO molecular function is reflected in core_functions (exact term or a reviewed subclass).
tlr4_irak4	IRAK4 UniProtKB:Q9NWZ3 Gene	protein serine/threonine kinase activity GO:0004674	Core Supported The GO molecular function is reflected in core_functions (exact term or a reviewed subclass).
tlr4_tram	TICAM2 UniProtKB:Q86XR7 Gene	signaling adaptor activity GO:0035591	Review Missing No ai-review exists for this protein; its function and core-function coverage cannot yet be checked.
tlr4_trif	TICAM1 UniProtKB:Q8IUC6 Gene	signaling adaptor activity GO:0035591	Core Supported The GO molecular function is reflected in core_functions (exact term or a reviewed subclass).
tlr4_tbk1	TBK1 UniProtKB:Q9UHD2 Gene	protein serine/threonine kinase activity GO:0004674	Core Supported The GO molecular function is reflected in core_functions (exact term or a reviewed subclass).

Gene-review completeness (9/12 grounded genes reviewed)

5 complete review(s) · 8 with deep research · 3 missing review · 1 reviewed but lacking deep research

Gene	Review	Complete	Deep research
CD14 P08571	✓	✓	✓
IRAK4 Q9NWZ3	✓	✓	✓
IRF3 Q14653	✓	189/190	✗
LY96 Q9Y6Y9	✓	✓	✓
MYD88 Q99836	✓	228/229	✓
LBP P18428	✗	—	—
IRAK1 P51617	✗	—	—
TICAM2 Q86XR7	✗	—	—
TBK1 Q9UHD2	✓	✓	✓
TICAM1 Q8IUC6	✓	✓	✓
TIRAP P58753	✓	108/109	✓
TLR4 O00206	✓	274/275	✓

Tree

TLR4-MD-2 LPS sensingSignaling Pathwaytlr4_md2_lps_signaling

Parts
1. 1. ligand delivery optional
2. Lipid-ligand delivery by LBP and CD14Transport Steplipid_ligand_delivery
  
  Annotons
  - LBP lipid carrier Gene Product: LBP
  - CD14 lipid carrier Gene Product: CD14
3. 2. receptor recognition
4. TLR4-MD-2 receptorProtein Complextlr4_md2_receptor
  
  Annotons
  - TLR4-MD-2 LPS receptor Protein Complex: lipopolysaccharide receptor complex
5. 3. adaptor branches
6. TLR4 adaptor branchesRegulatory Steptlr4_adaptor_branches
  
  Variants
  1. Adaptor pair and compartment by sorting adaptor pair and signaling compartment (One Or More)
    
    TIRAP-MYD88 branch (plasma membrane)Regulatory Steptlr4_myd88_branch
    
    Annotons
    
    TIRAP sorting adaptor Gene Product: TIRAP
    
    MYD88 adaptor Gene Product: MYD88
    
    IRAK4 kinase Gene Product: IRAK4
    
    TRAM-TRIF branch (endosome)Regulatory Steptlr4_trif_branch
    
    Annotons
    
    TRAM sorting adaptor Gene Product: TICAM2
    
    TRIF adaptor Gene Product: TICAM1
    
    TBK1 kinase Gene Product: TBK1

Details

Context

mammalsNCBITaxon:40674

TLR4-MD-2 LPS sensingSignaling Pathwaytlr4_md2_lps_signaling

toll-like receptor 4 signaling pathwayGO:0034142

Context

mammalsNCBITaxon:40674

Connections

lipid_ligand_delivery -> tlr4_md2_receptor Provides Input For

Carriers hand monomeric LPS to MD-2.

tlr4_md2_receptor -> tlr4_adaptor_branches Causes

The LPS-induced receptor dimer recruits the sorting adaptors.

Part 1: ligand delivery (optional)

Lipid-ligand delivery by LBP and CD14Transport Steplipid_ligand_delivery

Soluble LBP and CD14 bind the acylated ligand and hand it to the receptor; neither stays in the signaling complex. Delivery enhances sensitivity but is not strictly required, so the step is optional.

Annotons

LBP lipid carrier

lbp_carrier

Participant: Gene Product: LBP

Gene Product:

LBPUniProtKB:P18428

Function

molecular carrier activityGO:0140104

Cargo: lipopolysaccharideCHEBI:16412

Locations

extracellular regionGO:0005576

Extracellular acute-phase carrier that extracts the acylated ligand and transfers it onward.

PMID:27986454

LBP and CD14 both catalyse LPS transfer to TLR4-MD-2.

LPS transfer to TLR4-MD2 is catalyzed by both LPS binding protein (LBP) and CD14

PMID:27986454

CD14 hands a single LPS molecule to TLR4-MD-2.

Subsequently, the single LPS molecule bound to CD14 was transferred to TLR4-MD2 in a TLR4-dependent manner

CD14 lipid carrier

cd14_carrier

Participant: Gene Product: CD14

Gene Product:

CD14UniProtKB:P08571

Function

molecular carrier activityGO:0140104

Cargo: lipopolysaccharideCHEBI:16412

Locations

plasma membraneGO:0005886 extracellular regionGO:0005576

GPI-anchored and soluble carrier; it cannot signal across the membrane, so it is a carrier, not a receptor.

PMID:27986454

LBP and CD14 both catalyse LPS transfer to TLR4-MD-2.

LPS transfer to TLR4-MD2 is catalyzed by both LPS binding protein (LBP) and CD14

PMID:27986454

CD14 hands a single LPS molecule to TLR4-MD-2.

Subsequently, the single LPS molecule bound to CD14 was transferred to TLR4-MD2 in a TLR4-dependent manner

Part 2: receptor recognition

TLR4-MD-2 receptorProtein Complextlr4_md2_receptor

MD-2 binds lipid A; LPS then bridges two TLR4-MD-2 units into an m-shaped dimer that brings the TLR4 TIR domains together.

Annotons

TLR4-MD-2 LPS receptor

tlr4_md2_lps_receptor

Participant: Protein Complex: lipopolysaccharide receptor complex

Protein Complex:

lipopolysaccharide receptor complexGO:0046696

Active units:

TLR4

Participant: Gene Product: TLR4

Gene Product:

TLR4UniProtKB:O00206

Role: Transmembrane signaling chain; does not bind LPS on its own but transmits the signal through its TIR domain.

Function:

transmembrane signaling receptor activityGO:0004888 TLR4 alone transmits the signal across the membrane; LPS recognition needs MD-2, so the LPS receptor term belongs to the complex.

MD-2 (LY96)

Participant: Gene Product: LY96

Gene Product:

LY96UniProtKB:Q9Y6Y9

Role: Extracellular co-receptor whose hydrophobic pocket binds lipid A.

Function:

coreceptor activityGO:0015026 Binds lipid A in its hydrophobic pocket and confers LPS responsiveness on TLR4; matches the LY96 gene review (coreceptor activity, contributing to GO:0001875).

Function

lipopolysaccharide immune receptor activityGO:0001875

Processes

toll-like receptor 4 signaling pathwayGO:0034142

Locations

plasma membraneGO:0005886

The LPS receptor: binding by MD-2, signal transmission by TLR4. CD14 and LBP deliver ligand but are not part of it.

PMID:22532668

MD-2 binds lipid A directly.

MD-2 has a unique hydrophobic cavity that directly binds to lipid A, the active center of LPS.

PMID:22532668

TLR4 needs MD-2 to bind LPS.

TLR4 alone does not directly bind LPS and requires the coreceptor MD-2

PMID:19252480

LPS induces an m-shaped dimer of TLR4-MD-2-LPS.

LPS binding induced the formation of an m-shaped receptor multimer composed of two copies of the TLR4-MD-2-LPS complex arranged symmetrically

Part 3: adaptor branches

TLR4 adaptor branchesRegulatory Steptlr4_adaptor_branches

TLR4 is the only TLR that uses both adaptor pairs, from two compartments in sequence.

Variant set: Adaptor pair and compartment by sorting adaptor pair and signaling compartment (One Or More)

PMID:18222170

TLR4 switches to TRIF signaling after relocating to endosomes.

These results suggest that TLR4 activates TRIF-signaling in endosome/lysosome after relocation from the cell surface.

The MyD88 branch signals from the plasma membrane first; after endocytosis the TRIF branch signals from endosomes. Both normally fire in macrophages.

TIRAP-MYD88 branch (plasma membrane)Regulatory Steptlr4_myd88_branch

TIRAP recruits MYD88 to TLR4 at the plasma membrane; MYD88 recruits IRAK4, leading to NF-kappaB activation.

Annotons

TIRAP sorting adaptor

tlr4_tirap

Participant: Gene Product: TIRAP

Gene Product:

TIRAPUniProtKB:P58753

Function

signaling adaptor activityGO:0035591

Processes

MyD88-dependent toll-like receptor signaling pathwayGO:0002755

Locations

plasma membraneGO:0005886

PIP2-anchored sorting adaptor that brings MYD88 to TLR4.

PMID:16751103

TIRAP delivers MyD88 to activated TLR4.

TIRAP then functions to facilitate MyD88 delivery to activated TLR4 to initiate signal transduction.

MYD88 adaptor

tlr4_myd88

Participant: Gene Product: MYD88

Gene Product:

MYD88UniProtKB:Q99836

Function

signaling adaptor activityGO:0035591

Processes

MyD88-dependent toll-like receptor signaling pathwayGO:0002755

Couples the receptor to IRAK kinases.

PMID:9430229

MyD88 recruits IRAK to the receptor complex.

MyD88 binds to both IRAK (IL-1 receptor-associated kinase) and the heterocomplex (the signaling complex) of the two receptor chains and thereby mediates the association of IRAK with the receptor.

IRAK4 kinase

tlr4_irak4

Participant: Gene Product: IRAK4

Gene Product:

IRAK4UniProtKB:Q9NWZ3

Function

protein serine/threonine kinase activityGO:0004674

Targets: IRAK1UniProtKB:P51617

Processes

MyD88-dependent toll-like receptor signaling pathwayGO:0002755

Receptor-proximal kinase of the myddosome.

PMID:11960013

IRAK4 phosphorylates IRAK1.

Here we show that recombinant IRAK-1 is a substrate for IRAK-4, whereas IRAK-1 is not able to phosphorylate IRAK-4 in vitro

TRAM-TRIF branch (endosome)Regulatory Steptlr4_trif_branch

After LPS-induced endocytosis, TRAM bridges TLR4 to TRIF on endosomes; TRIF recruits TBK1, which activates IRF3 and type I interferon production.

Annotons

TRAM sorting adaptor

tlr4_tram

Participant: Gene Product: TICAM2

Gene Product:

TICAM2UniProtKB:Q86XR7

Function

signaling adaptor activityGO:0035591

Processes

TRIF-dependent toll-like receptor signaling pathwayGO:0035666

Locations

plasma membraneGO:0005886 endosome membraneGO:0010008

Bridges TLR4 to TRIF and moves with TLR4 into endosomes.

PMID:18222170

TRAM recruits TRIF.

Here, we show that TRAM recruits TRIF to the plasma membrane.

PMID:18222170

LPS drives TLR4-TRAM into endosomes.

LPS induces upregulation of TLR4-association with TRAM and their subsequent translocation into endosome/lysosome

TRIF adaptor

tlr4_trif

Participant: Gene Product: TICAM1

Gene Product:

TICAM1UniProtKB:Q8IUC6

Function

signaling adaptor activityGO:0035591

Processes

TRIF-dependent toll-like receptor signaling pathwayGO:0035666

Locations

endosome membraneGO:0010008

Adaptor that recruits TBK1, TRAF6 and RIPK1 and presents IRF3 for phosphorylation.

PMID:25736436

TRIF binds RIP1, TRAF6 and TBK1.

TRIF contains multiple conserved domains that are responsible for further association with downstream molecules receptor-interacting protein 1 (RIP1), TRAF6, and TANK-binding kinase 1 (TBK1).

PMID:25636800

TRIF activates IRF3.

We further show that TRIF, an adaptor protein in Toll-like receptor signaling, activates IRF3 through a similar phosphorylation-dependent mechanism.

TBK1 kinase

tlr4_tbk1

Participant: Gene Product: TBK1

Gene Product:

TBK1UniProtKB:Q9UHD2

Function

protein serine/threonine kinase activityGO:0004674

Targets: IRF3UniProtKB:Q14653

Processes

TRIF-dependent toll-like receptor signaling pathwayGO:0035666

Kinase recruited by TRIF that phosphorylates IRF3.

file:human/TBK1/TBK1-uniprot.txt

TBK1 is a serine/threonine kinase in innate immune signaling.

Serine/threonine kinase that plays an essential role in regulating inflammatory responses to foreign agents

PMID:25736436

TRIF recruits TBK1.

TRIF contains multiple conserved domains that are responsible for further association with downstream molecules receptor-interacting protein 1 (RIP1), TRAF6, and TANK-binding kinase 1 (TBK1).