A7RHZ4 is the single MyD88 (myeloid differentiation primary response protein 88) homologue of the starlet sea anemone Nematostella vectensis, encoded by gene model NEMVEDRAFT_v1g82163. The UniProt sequence is an N-terminally incomplete predicted fragment of 245 residues comprising a death domain followed by a C-terminal TIR (Toll/interleukin-1 receptor) domain, the defining domain architecture of MyD88-family adaptors, and it lacks a signal peptide or transmembrane segment, consistent with a cytoplasmic protein. By homology with vertebrate and insect MyD88, it is expected to act as the cytosolic adaptor that binds, through TIR-TIR contacts, the cytoplasmic TIR domain of the single Nematostella Toll-like receptor (Nv-TLR) and possibly of cnidarian IL-1 receptor-like proteins, and to recruit downstream death-domain kinases, leading to activation of the Nematostella NF-kappaB transcription factor. The Nematostella TLR TIR domain binds human MYD88 in vitro and Nv-TLR activates NEMO-dependent NF-kappaB in human cells in response to bacterial products, and MyD88 knockdown in the cnidarian Hydra impairs responses to bacteria. Transcripts of MyD88 and the other predicted TLR-to-NF-kappaB pathway components are present in nematosomes, motile multicellular structures that can engulf Vibrio coralliilyticus, but the Nematostella MyD88 protein itself has not been studied experimentally.
| GO Term | Evidence | Action | Reason |
|---|---|---|---|
| GO:0002755 MyD88-dependent toll-like receptor signaling pathway | IEA GO_REF:0000002 | ACCEPT | Summary: InterPro2GO mapping from the MyD88 family signature (IPR017281). A7RHZ4 has the death domain plus TIR architecture of MyD88. Nematostella encodes a single TLR (Nv-TLR), whose TIR domain binds human MYD88 and which activates NF-kappaB in human cells in response to heat-killed Vibrio coralliilyticus and flagellin. In Hydra, MyD88 knockdown impairs bacterial sensing and defence. Reason: This is the defining process of MyD88 and the conditions for transfer are met: the organism has a Toll-like receptor with a TIR domain that engages MyD88, and a cnidarian MyD88 loss-of-function study supports a role in TLR-dependent responses to bacteria. The GO definition states that toll-like receptors bind microbial pattern motifs directly; for Nv-TLR this is supported only by responses to flagellin and heat-killed bacteria in a reconstituted human-cell system, not by a binding assay, so the annotation is an inference, appropriate at IEA. The generic MyD88-dependent term is the right level. Numbered, ligand-specific TLR pathway terms (for example GO:0034142 TLR4 signalling) should not be transferred: the similarity of the Nv-TLR TIR to TLR4 says nothing about LPS recognition. Supporting Evidence: PMID:29109290 we show that the intracellular Toll/IL-1 receptor (TIR) domain of Nv-TLR can interact with the human TLR adapter proteins MAL and MYD88 PMID:29109290 heat-inactivated V. coralliilyticus and bacterial flagellin can activate a reconstituted Nv-TLR-to-NF-ΞΊB pathway in human cells PMID:17437634 These include a single MyD88 homolog (NvMyD88) and a protein (NvTLR-1) clearly related to members of the Toll/TLR family (Figure 2). PMID:23112184 we use a MyD88 loss-of-function approach in Hydra to demonstrate that recognition of bacteria is an ancestral function of TLR signaling |
| GO:0005737 cytoplasm | IEA GO_REF:0000044 | ACCEPT | Summary: Subcellular-location mapping from an ARBA rule. The protein has no signal peptide or transmembrane segment; MyD88-family adaptors are cytoplasmic and are recruited to the cytoplasmic face of receptor TIR domains. Reason: Consistent with the domain architecture and with the conserved function of MyD88 as a cytosolic adaptor. In a survey of actiniarian transcriptomes, none of the MyD88 predicted proteins had a transmembrane domain. Supporting Evidence: PMID:27806695 All TLR and IL-1R-like predicted peptides had transmembrane domains (TMDs) and all MyD88 and NF-ΞΊB predicted peptides did not. |
| GO:0007165 signal transduction | IEA GO_REF:0000002 | ACCEPT | Summary: InterPro2GO mapping from the TIR (IPR000157) and death (IPR000488) domains, both signalling-interaction modules. Reason: Broad but correct for a death-domain/TIR adaptor. The more specific GO:0002755 row carries the informative process. Supporting Evidence: PMID:27806695 The TLR and IL-1R-like pathways both interact with MyD88 via their TIR domain during signal transduction |
| GO:0043123 positive regulation of canonical NF-kappaB signal transduction | IEA GO_REF:0000002 | ACCEPT | Summary: InterPro2GO mapping from the MyD88 family signature. Nematostella has an NF-kappaB protein that, unlike the p100/p105 precursors, lacks C-terminal IkappaB-like repeats, and separately encoded IkappaB proteins, i.e. the substrate architecture of canonical IkappaB-regulated signalling. Nv-TLR activates NF-kappaB in human cells through the NEMO-dependent canonical route. Reason: MyD88 promotes NF-kappaB activation throughout the animals in which it has been studied, and the components needed for a canonical, IkappaB-regulated route exist in Nematostella. The transfer is reasonable at IEA. Caveats, recorded rather than acted on: no experiment has tested NvMyD88 itself, and the NEMO dependence shown for Nv-TLR was measured with human downstream components. Supporting Evidence: PMID:17120026 Nv-NF-kappaB lacks the C-terminal IkappaB-like sequences present in all other NF-kappaB proteins. There are, however, two IkappaB-like genes in Nematostella encoded by loci distinct from Nv-nfkb. PMID:29109290 Neither Nv-TLR nor Hu-TLR4 activated the NF-ΞΊBβsite luciferase reporter in 293 cells in which NEMO expression was genetically ablated |
| GO:0070976 TIR domain binding | IEA GO_REF:0000002 | ACCEPT | Summary: InterPro2GO mapping from the MyD88 family signature. MyD88 binds receptor TIR domains through its own TIR domain; A7RHZ4 retains an intact TIR domain (residues 104-236). Cnidarian TLR TIR domains (Nematostella, the coral Orbicella) bind MyD88 in vitro. Reason: This is the core molecular function of a MyD88 adaptor and the most informative MF available for it. Direct binding by the Nematostella protein has not been tested, but cnidarian TLR TIR domains engage MyD88, and the domain is conserved. Supporting Evidence: PMID:29109290 we show that the intracellular Toll/IL-1 receptor (TIR) domain of Nv-TLR can interact with the human TLR adapter proteins MAL and MYD88 PMID:29080785 it can interact in vitro with the human TLR4 adapter MYD88 |
Loading supporting contentβ¦
Download this section (compressed HTML)Q: Does NvMyD88 also act downstream of the Nematostella IL-1R-like TIR receptors? If so, which ligands activate them? Vertebrate interleukin-1-mediated signalling terms should not be transferred unless an IL-1-type ligand-receptor system is shown to exist in cnidarians.
Q: Is the full-length NvMyD88 gene model longer at the N-terminus than the UniProt fragment, and is the death domain complete?
Q: Does Nv-TLR bind flagellin or other microbial molecules directly, as the GO definition of toll-like receptor signalling requires, or is it activated indirectly?
Q: Morpholino knockdown shows that Nv-TLR is needed for early development, a role the authors propose is NF-kappaB-independent (PMID:29109290). Does NvMyD88 mediate this developmental function, or does Nv-TLR signal through another adaptor during development?
Experiment: Co-immunoprecipitation or GST pulldown of full-length NvMyD88 with the Nv-TLR TIR domain, and co-expression of NvMyD88 with Nv-TLR in MYD88-deficient human cells followed by an NF-kappaB reporter assay.
Hypothesis: NvMyD88 binds the Nv-TLR TIR domain and can replace human MYD88 in coupling Nv-TLR to NF-kappaB.
Type: protein interaction and reconstitution assay
Experiment: Morpholino or CRISPR knockdown of NvMyD88 in Nematostella juveniles followed by Vibrio coralliilyticus or Pseudomonas aeruginosa infection, with measurement of survival and Nv-NF-kappaB nuclear localisation.
Hypothesis: NvMyD88 is required for bacteria-induced NF-kappaB activation and defence in vivo, as MyD88 is in Hydra.
Type: in vivo loss-of-function
Loading supporting contentβ¦
Download this section (compressed HTML)Loading supporting contentβ¦
Download this section (compressed HTML)Loading supporting contentβ¦
Download this section (compressed HTML)