TLN is the talin of the homoscleromorph sponge Oscarella pearsei, a 2531-residue cytoskeletal adaptor with the domain organization of animal talins: an N-terminal FERM head, followed by a talin rod made of helical bundles that carry vinculin-binding sites, and a C-terminal I/LWEQ actin-binding domain. It also has a predicted phosphotyrosine-binding domain just after the FERM domain that is absent from mouse talin. A peptide corresponding to its principal vinculin-binding site (residues 598-621) binds the D1 domain of sponge vinculin (VIN1) with nanomolar affinity and activates F-actin binding by VIN1, reproducing the talin-vinculin activation mechanism of vertebrate focal adhesions. By homology with talins of other animals, TLN is expected to bind integrin beta-subunit cytoplasmic tails and membrane lipids through its head and actin filaments through its rod and C-terminus, linking integrin-based cell-matrix adhesions to the actin cytoskeleton; these activities have not been tested for the sponge protein.
| GO Term | Evidence | Action | Reason |
|---|---|---|---|
| GO:0001726 ruffle | IEA GO_REF:0000002 | MARK AS OVER ANNOTATED | Summary: InterPro2GO mapping from the talin central domain. No localization data exist for sponge talin. Reason: Ruffle is a peripheral, context-dependent location of talin in cultured vertebrate cells, and a domain signature is a poor predictor of it. The characteristic talin location (focal adhesion / anchoring junction) is already annotated, and no ruffle-like structure has been described for O. pearsei talin. |
| GO:0003779 actin binding | IEA GO_REF:0000002 | ACCEPT | Summary: InterPro2GO mapping from the I/LWEQ actin-binding domain, which TLN has at its C-terminus. Reason: The I/LWEQ domain is the conserved talin F-actin-binding module and is present in TLN; untested in sponge but well-founded by homology. Supporting Evidence: PMID:29880641 As in their mammalian counterparts, Op talin has an N-terminal FERM domain followed by a talin middle domain, vinculin-binding sites, and an I/WLEQ domain. |
| GO:0005178 integrin binding | IEA GO_REF:0000118 | ACCEPT | Summary: TreeGrafter (PANTHER) propagation from the talin family node. Integrin beta tail binding by the talin FERM head is the defining talin activity. Reason: TLN retains the FERM head and the talin domain architecture, and sponges are inside the animal clade in which talin-integrin linkage is conserved. No target-specific evidence of loss. Not experimentally tested in sponge. Supporting Evidence: PMID:29880641 integrins anchor to the actin cytoskeleton through interactions with a large number of cytoplasmic proteins including talin, vinculin, and paxillin |
| GO:0005200 structural constituent of cytoskeleton | IEA GO_REF:0000002 | MARK AS OVER ANNOTATED | Summary: InterPro2GO mapping from the talin central domain. Reason: Talin is an adaptor that links integrins and membrane to actin filaments and recruits vinculin; it is not a polymer-forming or lattice structural constituent of the cytoskeleton. Its activity is better described by integrin binding, actin filament binding and vinculin binding, all of which are annotated. |
| GO:0005515 protein binding | IPI PMID:29880641 Analysis of a vinculin homolog in a sponge (phylum Porifera)... | MODIFY | Summary: ITC with a synthetic peptide of TLN residues 598-621 (the homolog of the mouse talin vinculin-binding site) and recombinant VIN1 or its D1 domain shows direct binding; the peptide also activates VIN1 F-actin binding. Reason: Generic protein binding is uninformative. The partner is vinculin and the tested segment is a vinculin-binding site, so vinculin binding (GO:0017166) describes the function. Caveat: evidence is from a peptide, not full-length TLN, whose vinculin-binding sites in other talins are cryptic until the rod is unfolded by force. Proposed replacements: vinculin binding Supporting Evidence: PMID:29880641 Full-length Op vinculin and Op vinculin D1 bound Op talin peptide PMID:29880641 In a MUSCLE alignment Mm talin 605–628 correspond to residues 598–621 in Op talin |
| GO:0005737 cytoplasm | IEA GO_REF:0000118 | ACCEPT | Summary: TreeGrafter propagation; talin is a cytoplasmic protein. Reason: Correct, if general, for a soluble cytoskeletal adaptor. |
| GO:0005856 cytoskeleton | IEA GO_REF:0000044 | ACCEPT | Summary: UniProt subcellular location by similarity (UniProtKB:P0CE95); no sponge data. Reason: Talins associate with the actin cytoskeleton throughout eukaryotes; general and consistent with the conserved actin-binding domains. |
| GO:0005886 plasma membrane | IEA GO_REF:0000118 | ACCEPT | Summary: TreeGrafter propagation; talin heads associate with the inner face of the plasma membrane at integrin adhesions. Reason: Consistent with the conserved FERM head (membrane- and integrin-binding) in TLN. Not tested in sponge. |
| GO:0005925 focal adhesion | IEA GO_REF:0000120 | ACCEPT | Summary: Combined InterPro/PANTHER automatic annotation. Focal adhesion is the principal location of talin in other animals. Reason: Talin is a core focal adhesion component across animals. Focal-adhesion-like structures in O. pearsei are not defined molecularly; VIN1, the talin partner, is found in filopodia of migratory cells that the authors interpret as cell-ECM contacts. Accepted as the family-level location. Supporting Evidence: PMID:29880641 The AJ (containing cadherins and catenins) and FAs (comprising integrins, talin, and paxillin) differ in protein composition |
| GO:0005938 cell cortex | IEA GO_REF:0000044 | ACCEPT | Summary: UniProt subcellular location by similarity (UniProtKB:P0CE95); no sponge data. Reason: Talins act at the membrane-cortex interface; general and plausible. |
| GO:0007155 cell adhesion | IEA GO_REF:0000002 | ACCEPT | Summary: InterPro2GO mapping from the alpha-catenin/vinculin-like superfamily (talin rod bundles). Reason: Talin does part of the work of integrin-mediated adhesion (it links and activates integrins and recruits vinculin and actin), so the general term is appropriate for a talin with conserved domain architecture. Supporting Evidence: PMID:29880641 integrins anchor to the actin cytoskeleton through interactions with a large number of cytoplasmic proteins including talin, vinculin, and paxillin |
| GO:0030036 actin cytoskeleton organization | IEA GO_REF:0000118 | KEEP AS NON CORE | Summary: TreeGrafter propagation from the talin family node. Reason: Talin anchors actin filaments at adhesions and so contributes to actin organization, but this is a consequence of its adaptor role rather than its core function; untested in sponge. |
| GO:0051015 actin filament binding | IEA GO_REF:0000002 | ACCEPT | Summary: InterPro2GO mapping. Talins bind F-actin through rod and I/LWEQ domains. Reason: TLN has the I/LWEQ domain and talin rod; conserved activity, not tested in sponge. |
| GO:0070161 anchoring junction | IEA GO_REF:0000117 | ACCEPT | Summary: ARBA machine-learning rule; general parent of focal adhesion. Reason: Correct at this general level and consistent with the focal adhesion annotation. |
| GO:0098609 cell-cell adhesion | IEA GO_REF:0000118 | MODIFY | Summary: TreeGrafter propagation from the talin family node. Reason: Talin's established role is in integrin-based attachment of cells to the extracellular matrix, whereas cell-cell contacts (adherens junctions) are built on cadherins and catenins. Nothing links O. pearsei talin to cell-cell contacts. The essence (adhesion) is right, but cell-matrix adhesion is the appropriate process. Propagation Review Root cause: TERM SCOPING PROBLEM Failure modes: GRANULARITY MISMATCH Sources checked: PANTHER:PTN001072690 · talin family node SUPPORTS SOURCE BUT NOT TARGET The family-node adhesion term is right in essence, but talin acts at integrin-matrix adhesions; cell-cell adhesion is the wrong sibling for this sponge protein. Proposed replacements: cell-matrix adhesion Supporting Evidence: PMID:29880641 The AJ (containing cadherins and catenins) and FAs (comprising integrins, talin, and paxillin) differ in protein composition |
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Download this section (compressed HTML)Q: Does full-length O. pearsei talin localize with VIN1 in the filopodia of migratory mesohyl cells and at basal contacts with the collagen IV-containing basal lamina, i.e. are these focal-adhesion-like contacts?
Q: What does the additional PTB domain after the FERM domain of sponge talin bind?
Experiment: Raise an antibody to O. pearsei talin and co-stain with VIN1, integrin beta and phalloidin in sponge tissue.
Type: Immunolocalization
Experiment: Test binding of the recombinant TLN FERM head to O. pearsei integrin beta cytoplasmic tails and to PIP2-containing membranes, and map vinculin binding across rod fragments with and without force (magnetic tweezers).
Type: Binding assay
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