SWP7 is a secreted, non-catalytic spore wall protein of Nosema bombycis, the microsporidian agent of pebrine disease in the silkworm. It is a 287-residue precursor with an N-terminal signal peptide and is the founding member of its own protein family (Pfam PF29448 / InterPro IPR062164), with no catalytic domain or recognizable enzymatic signature. It was first recovered from the spore wall proteome as HSWP7 and later characterized directly; during sporogony it is found in the cytoplasm of sporonts and sporoblasts, and in mature spores it is deposited in the two-layered spore wall, where it is reported in both the proteinaceous exospore and the chitin-rich endospore, and additionally in the polar tube, the extrusion organelle through which the parasite injects its sporoplasm into a host cell. SWP7 forms a complex with the larger spore wall protein SWP9, demonstrated by immunoprecipitation, far-Western blotting, unreduced SDS-PAGE and yeast two-hybrid assays. The partnership is asymmetric. SWP9 acts as the scaffold, binding the deproteinated chitin spore coat and the polar tube proteins PTP1 and PTP2 directly, whereas recombinant SWP7 reaches the chitin coat only when SWP9 is already bound, does not bind PTP1 or PTP2, and is thought to be carried to the polar tube by SWP9. Functionally, masking SWP7 on the spore surface with anti-SWP7 antibody reduces spore adhesion to, and infection of, cultured host cells, placing SWP7 on the adhesive outer surface of the spore and implicating it in attachment of the spore to the host cell. Because microsporidia lack transfection and gene-knockout systems, antibody and recombinant-protein competition are the only functional assays applied to this protein, and no receptor or host-cell ligand has been identified.
Definition: The outer layer of the microsporidian spore wall, an electron-dense, mainly proteinaceous and glycoprotein-rich layer lying external to the endospore and to the plasma membrane.
Justification: SWP7 and other N. bombycis spore wall proteins are localized by immunoelectron microscopy to a specific layer of the spore wall, but GO:0031160 spore wall has no microsporidian children. Its existing children are GO:0005619 ascospore wall, GO:0043591 endospore external encapsulating structure (the bacterial endospore coat, not the microsporidian endospore), GO:0097514 sexual spore wall and GO:0097515 asexual spore wall, none of which fits. Layer-resolved localizations for this gene family therefore cannot be recorded at the granularity at which they are measured.
Parent term: spore wall
Supporting Evidence:
Definition: The inner layer of the microsporidian spore wall, an electron-lucent, chitin- and protein-rich layer lying between the exospore and the plasma membrane.
Justification: Needed for the same reason as microsporidian exospore, and distinct from GO:0043591 endospore external encapsulating structure, which describes the peptidoglycan and proteinaceous coat of a bacterial endospore and is not homologous to the chitinous microsporidian endospore. The deproteinated chitin spore coat used in the N. bombycis binding assays is this layer.
Parent term: spore wall
Supporting Evidence:
| GO Term | Evidence | Action | Reason |
|---|---|---|---|
| GO:0005515 protein binding | IPI PMID:25605761 Interaction and assembly of two novel proteins in the spore ... | REMOVE | Summary: SWP7 interacts with the spore wall protein SWP9 (R0MLT0), shown by four orthogonal assays. The interaction is real and important, but bare protein binding conveys no molecular function. Reason: The SWP7-SWP9 interaction is among the best-supported facts about this protein, demonstrated by immunoprecipitation, far-Western blotting, unreduced SDS-PAGE and yeast two-hybrid (PMID:25605761), and curated by UniProt as a SUBUNIT statement. Removal follows the repository's protein-binding policy and does not dispute the interaction: GO:0005515 is uninformative and slated for do-not-annotate. It is not replaced by a more specific molecular function because the evidence rules out the obvious candidates for SWP7 itself. Chitin binding belongs to SWP9, not to SWP7, because recombinant rSWP7-His reaches the deproteinated chitin spore coats only after rSWP9-GST is bound (PMID:25605761), so SWP7's association with the endospore chitin is piggybacked. Polar tube protein binding likewise belongs to SWP9, since coimmunoprecipitation, LC-MS/MS and yeast two-hybrid show that SWP9, but not SWP7, interacts with PTP1 and PTP2 (PMID:28031263). The one structural statement about SWP7 is hedged by the authors themselves ("may have important structural capacities"), which is not a basis for asserting a structural molecule activity here. Supporting Evidence: PMID:25605761 immunoprecipitation, far-Western blotting, unreduced SDS-PAGE, and yeast two-hybrid data demonstrated that SWP7 interacted with SWP9 PMID:25605761 binding of the recombinant protein rSWP7-His to the DCSCs is dependent on the combination of rSWP9-glutathione S-transferase (GST) with the DCSCs PMID:28031263 demonstrated that NbSWP9, but not NbSWP7, interacts with NbPTP1 and NbPTP2 file:NOSB1/SWP7/SWP7-uniprot.txt SUBUNIT: Interacts with SWP9. {ECO:0000269|PubMed:25605761}. |
| GO:0005737 cytoplasm | IDA PMID:25605761 Interaction and assembly of two novel proteins in the spore ... | KEEP AS NON CORE | Summary: SWP7 is present in the cytoplasm of sporonts and sporoblasts before it is deposited in the wall; a staging location on the route to the mature spore wall, not the site of its function. Reason: The UniProt curator, who read the full text, records Cytoplasm for SWP7 with ECO:0000269|PubMed:25605761 and the developmental note that it is localized to the cytoplasm of sporonts and sporoblasts before appearing in the wall and polar tube of mature spores. The abstract of PMID:25605761 is ambiguous on this point, since its "respectively" could be read as assigning sporont cytoplasm to SWP9 alone; the full text is not accessible (the cached record is abstract-only and PMC4363453 is not open access), so the curator is not second-guessed. The reading is independently confirmed by immunoelectron microscopy in the follow-up study, where NbSWP7 was found randomly in the cytoplasm of sporoblasts (PMID:28031263). Kept as non-core because cytoplasmic residence is a biosynthetic intermediate stage of a protein whose function is executed in the spore wall, and because GO offers no more specific term for this pre-sporal compartment. Supporting Evidence: PMID:28031263 NbSWP7 was found randomly in the cytoplasm file:NOSB1/SWP7/SWP7-uniprot.txt Note=Localized to the cytoplasm of sporonts and sporoblasts and to the spore wall and polar tube of mature spores. |
| GO:0005737 cytoplasm | IEA GO_REF:0000044 | KEEP AS NON CORE | Summary: Electronic transfer of the UniProt subcellular location keyword SL-0086 (Cytoplasm), which itself rests on the curated ECO:0000269 statement from PMID:25605761. Reason: This row is a vocabulary mapping, not an independent assay, so it inherits the standing of the curated Cytoplasm statement it transfers. That statement is experimentally grounded (ECO:0000269|PubMed:25605761) and corroborated by immunoelectron microscopy in PMID:28031263, so the mapping is sound at the granularity it offers. Kept as non-core for the same reason as the IDA row it mirrors, namely that the cytoplasm is where SWP7 transits rather than where it acts. Supporting Evidence: file:NOSB1/SWP7/SWP7-uniprot.txt SUBCELLULAR LOCATION: Cytoplasm {ECO:0000269|PubMed:25605761}. PMID:28031263 NbSWP7 was found randomly in the cytoplasm |
| GO:0031160 spore wall | IDA PMID:25605761 Interaction and assembly of two novel proteins in the spore ... | ACCEPT | Summary: SWP7 is a constituent of the mature spore wall, colocalizing there with SWP9; this is the protein's defining location and GO:0031160 is the deepest term GO offers for it. Reason: Spore wall residence is the central, repeatedly demonstrated localization for SWP7. It was first recovered in the N. bombycis spore wall proteome as one of fourteen hypothetical spore wall proteins (PMID:18563739, the reference UniProt cites for identification by mass spectrometry and subcellular location), then localized directly by immunofluorescence and immunoelectron microscopy to the exospore and endospore of mature spores and shown to colocalize with SWP9 in the wall (PMID:25605761), and confirmed again by immunoelectron microscopy in PMID:28031263. The papers resolve the location to individual wall layers, but GO cannot express that, because the only children of GO:0031160 are GO:0005619 ascospore wall, GO:0043591 endospore external encapsulating structure (a bacterial structure, not the microsporidian endospore), GO:0097514 sexual spore wall and GO:0097515 asexual spore wall (checked via OLS, 2026-10-29). GO:0031160 is therefore as precise as the ontology allows, and the missing exospore and endospore terms are filed under proposed_new_terms rather than forced onto an ill-fitting child. Supporting Evidence: PMID:25605761 Both SWP7 and SWP9 are specifically colocalized to the spore wall in mature spores. PMID:25605761 SWP7 and SWP9 are mainly localized to the exospore and endospore of mature spores and the cytoplasm of sporonts, respectively. PMID:28031263 both NbSWP9 and NbSWP7 were located in the polar tube and spore wall of N. bombycis mature spores |
| GO:0031160 spore wall | IDA PMID:25605761 Interaction and assembly of two novel proteins in the spore ... | ACCEPT | Summary: CACAO row for the same claim as the UniProt IDA row. The cited paper localizes SWP7 to the mature spore wall by microscopy and colocalization with SWP9; the term matches the assay exactly. Reason: CACAO row (GOA ASSIGNED BY = CACAO, 2015-02-10). What the paper shows for this gene: immunofluorescence and immunoelectron localization of SWP7 to the exospore and endospore of mature spores, and specific colocalization with SWP9 in the wall of mature spores (PMID:25605761 abstract; the full text, which carries the figures, is not accessible here because PMC4363453 is not open access, and the GONUTS wiki page for this PMID is offline). The term matches that result: it is neither downstream of the assay nor a phenotype read as a location, and it is at the right granularity because GO has no microsporidian exospore or endospore term under GO:0031160. The claim is independently reconfirmed by immunoelectron microscopy in PMID:28031263, so this is not a single-assay localization. It duplicates the UniProt IDA row for the same term, which reinforces rather than weakens the claim and is never grounds for removal. Supporting Evidence: PMID:25605761 Both SWP7 and SWP9 are specifically colocalized to the spore wall in mature spores. PMID:28031263 both NbSWP9 and NbSWP7 were located in the polar tube and spore wall of N. bombycis mature spores |
| GO:0031160 spore wall | IEA GO_REF:0000044 | ACCEPT | Summary: Electronic transfer of UniProt subcellular location SL-0254 (Spore wall), which rests on curated ECO:0000269 statements from two independent papers. Reason: The mapped UniProt statement cites both PMID:18563739 (spore wall proteome plus immunolocalization) and PMID:25605761 (direct characterization), so the transferred location is doubly grounded experimentally. The mapping adds no granularity beyond the root spore wall term, but none is available in GO, so the row is accurate as written and belongs with the core location. Supporting Evidence: file:NOSB1/SWP7/SWP7-uniprot.txt Spore wall {ECO:0000269|PubMed:18563739, ECO:0000269|PubMed:25605761}. PMID:18563739 14 hypothetical spore wall proteins (HSWPs) or peptides were obtained in total |
| GO:0044099 polar tube | IDA PMID:25605761 Interaction and assembly of two novel proteins in the spore ... | KEEP AS NON CORE | Summary: SWP7 is detected in the polar tube of mature spores, but it gets there by riding on SWP9 rather than through any demonstrated polar-tube activity of its own. Reason: The location is sound. UniProt curates Spore polar tube for SWP7 with ECO:0000269|PubMed:25605761, and immunoelectron microscopy in the follow-up paper independently places NbSWP7 in the polar tube of mature spores (PMID:28031263). It is kept as non-core rather than accepted as a core location because the same follow-up paper shows SWP7 plays no part in polar tube assembly or function. SWP9, but not SWP7, binds PTP1 and PTP2; the authors attribute SWP7's polar tube residence to its interaction with SWP9; and on germination it is SWP9, not SWP7, that accumulates at the anchoring disk, with only anti-SWP9 reducing germination. The polar tube is therefore a secondary, SWP9-dependent destination beside the spore wall, which is where the protein's demonstrated role lies. Supporting Evidence: PMID:28031263 both NbSWP9 and NbSWP7 were located in the polar tube and spore wall of N. bombycis mature spores PMID:28031263 The reason why NbSWP7 was localized to the polar tube may be due to the interaction between NbSWP9 and NbSWP7 file:NOSB1/SWP7/SWP7-uniprot.txt Spore polar tube {ECO:0000269|PubMed:25605761}. |
| GO:0044099 polar tube | IDA PMID:25605761 Interaction and assembly of two novel proteins in the spore ... | KEEP AS NON CORE | Summary: CACAO row for the same claim as the UniProt IDA row. The polar tube localization is in the cited paper's full text rather than its abstract, and is confirmed by a second paper, so the row stands, but as a secondary location. Reason: CACAO row (GOA ASSIGNED BY = CACAO, 2015-02-10). What the paper shows for this gene: the abstract of PMID:25605761 does not mention the polar tube at all, so this row must rest on a figure in the full text, which is not accessible here (PMC4363453 is not open access and the GONUTS page for this PMID returns only an offline notice). The claim is nonetheless verifiable from two independent sources. UniProt's curator records Spore polar tube for SWP7 with ECO:0000269|PubMed:25605761 after reading that full text, and immunoelectron microscopy in PMID:28031263 independently places NbSWP7 in the polar tube of mature spores. The term is the right one and the right granularity, GO:0044099 being a microsporidia-specific term with no children. It is kept as non-core rather than accepted because SWP7 is carried there by SWP9 and has no demonstrated polar-tube function, since SWP9 but not SWP7 binds PTP1 and PTP2 and only anti-SWP9 blocks germination. This is a case where an abstract-only cache would have made the row look unsupported; the curator-deference rule and the follow-up paper both say otherwise. Supporting Evidence: file:NOSB1/SWP7/SWP7-uniprot.txt Spore polar tube {ECO:0000269|PubMed:25605761}. PMID:28031263 both NbSWP9 and NbSWP7 were located in the polar tube and spore wall of N. bombycis mature spores |
| GO:0044099 polar tube | IEA GO_REF:0000044 | KEEP AS NON CORE | Summary: Electronic transfer of UniProt subcellular location SL-0436 (Spore polar tube), resting on the curated ECO:0000269 statement from PMID:25605761. Reason: A vocabulary mapping of a curated, experimentally grounded location, independently corroborated by immunoelectron microscopy in PMID:28031263. Kept at the same standing as the IDA rows it mirrors, the location being real but secondary to the spore wall and dependent on SWP9. Supporting Evidence: file:NOSB1/SWP7/SWP7-uniprot.txt Spore polar tube {ECO:0000269|PubMed:25605761}. PMID:28031263 both NbSWP9 and NbSWP7 were located in the polar tube and spore wall of N. bombycis mature spores |
| GO:0075004 adhesion of symbiont spore to host | IDA PMID:25605761 Interaction and assembly of two novel proteins in the spore ... | ACCEPT | Summary: Anti-SWP7 antibody reduces adhesion of N. bombycis spores to cultured host cells, placing SWP7 on the adhesive spore surface. This is the protein's core biological role and the term matches the readout exactly. Reason: CACAO row (GOA ASSIGNED BY = CACAO, 2015-02-10). What the paper shows for this gene: an antibody and recombinant-protein competition experiment in which rSWP9-GST, anti-SWP9 and anti-SWP7 antibodies each decreased spore adhesion to, and infection of, the host cell; the authors conclude the two proteins play significant roles in modulating host cell adherence and infection in vitro. UniProt curates the same result as "Involved in adherence of spores to the host cell surface and in infection efficiency" with ECO:0000269|PubMed:25605761, and the entry carries keyword Cell adhesion. The term matches the assay and is on the correct branch, OLS recording that GO:0075004 must only be used for symbiont gene products, which is what SWP7 is. Notably the student annotated the direct readout, adhesion, and did not also annotate the downstream infection phenotype, so this is not a phenotype read as a process. On the participation test, blocking a surface-exposed exospore protein with its own antibody and losing adhesion indicates that SWP7 forms part of the adhesive interface rather than being merely required upstream, which is what the involved_in claim asserts; the antibody-masking design cannot by itself exclude steric masking of a neighbouring adhesin, which is recorded as a knowledge gap. Microsporidia have no transfection or knockout system, so competition assays of this kind are the only functional handle available for the protein. Supporting Evidence: PMID:25605761 anti-SWP9, and anti-SWP7 antibodies decreased spore adhesion and infection of the host cell PMID:25605761 play significant roles in modulating host cell adherence and infection in vitro file:NOSB1/SWP7/SWP7-uniprot.txt FUNCTION: Involved in adherence of spores to the host cell surface and in infection efficiency. {ECO:0000269|PubMed:25605761}. |
| GO:0075004 adhesion of symbiont spore to host | IMP PMID:25605761 Interaction and assembly of two novel proteins in the spore ... | ACCEPT | Summary: The same antibody-blocking experiment as the IDA row, submitted a second time under IMP. The claim is correct and retained; the evidence code is not supportable, since no SWP7 mutant exists. Reason: CACAO row (GOA ASSIGNED BY = CACAO, 2015-03-03). What the paper shows for this gene is the same single functional experiment behind the companion IDA row, namely antibody and recombinant-protein competition reducing spore adhesion and infection. The term is right and the claim is the same claim, so this row reinforces rather than duplicates in any removable sense, and it is accepted. The defect is the evidence code. IMP requires a mutated, knocked-down or variant form of the gene product, and there is none; microsporidia are not genetically tractable, no SWP7 mutant or RNAi knockdown appears in PMID:25605761 or anywhere in the N. bombycis spore wall literature (PMID:28031263, PMID:29522765), and the only perturbation reported is extracellular antibody masking, which is what the IDA row already encodes. This is recorded as a source defect against the submitting group rather than as a REMOVE, because removing a row to deduplicate a true claim is not a sanctioned use of that action. Supporting Evidence: PMID:25605761 anti-SWP9, and anti-SWP7 antibodies decreased spore adhesion and infection of the host cell file:NOSB1/SWP7/SWP7-uniprot.txt FUNCTION: Involved in adherence of spores to the host cell surface and in infection efficiency. {ECO:0000269|PubMed:25605761}. |
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Download this section (compressed HTML)Q: Does purified SWP7 bind any host-cell surface component directly, or is the anti-SWP7 adhesion block a steric effect on a neighbouring adhesin in the exospore?
Suggested experts: Donglin Yang, Guoqing Pan, Zeyang Zhou
Q: Is SWP7 required for spore wall integrity, or is it purely a client deposited by the SWP9 scaffold?
Suggested experts: Donglin Yang, Guoqing Pan
Q: Is the SWP7 family (Pfam PF29448) conserved across microsporidia beyond Nosema, and do the orthologues occupy the same wall layers and partner with an SWP9 orthologue?
Suggested experts: Guoqing Pan, Zeyang Zhou
Q: Should GO acquire microsporidian exospore and endospore cellular-component terms under GO:0031160, given that the whole N. bombycis spore wall protein literature reports layer-resolved immunoelectron localizations?
Experiment: Compare adhesion of N. bombycis spores to cultured BmN or Sf9 cells in the presence of purified rSWP7 versus rSWP9 and versus an irrelevant recombinant protein of the same tag and size, and in parallel assay direct binding of immobilized rSWP7 to intact host cells and to host cell surface extracts (ELISA or pull-down followed by LC-MS/MS of the retained host proteins). A competition block by soluble rSWP7 plus identification of a retained host ligand would show direct adhesion; a block by anti-SWP7 alone, with no soluble-protein competition and no ligand, would indicate steric masking.
Hypothesis: SWP7 is itself an adhesin that contacts the host cell surface.
Type: adhesion competition and receptor identification
Experiment: Use a system in which SWP7 abundance can be lowered or its deposition blocked (RNAi or antibody pre-loading in an in vitro sporogony system, or heterologous expression of the SWP9-SWP7 pair on chitin coats), then measure wall integrity directly by transmission electron microscopy of exospore and endospore thickness and layering, resistance to hot alkaline and chitinase treatment, and germination rate. Reconstitute in vitro by layering purified rSWP9 and rSWP7 onto deproteinated chitin spore coats and testing whether SWP7 addition alters coat mechanical or chemical resistance.
Hypothesis: SWP7 contributes structural integrity to the spore wall rather than merely being deposited in it.
Type: wall integrity and in vitro reconstitution
Experiment: Perform quantitative immunoelectron microscopy on mature spores with anti-SWP7 and anti-SWP9 double labelling, counting gold particles per layer and measuring the SWP7 to SWP9 ratio in exospore versus endospore across sporogony stages. A constant ratio in both layers supports co-deposition as a complex; divergent ratios imply separate targeting routes and a distinct exospore function.
Hypothesis: SWP7 occupies both the exospore and the endospore as a single population carried by SWP9, rather than two independently targeted pools.
Type: quantitative immunoelectron microscopy
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