FliW is a flagellar assembly factor that functions as a partner-switching protein regulating flagellin (Hag) homeostasis. Its primary molecular function is as an anti-CsrA protein - it binds CsrA allosterically and prevents CsrA from repressing translation of hag mRNA. FliW also binds directly to flagellin (Hag) in a mutually exclusive manner with CsrA. This establishes a partner-switching checkpoint mechanism: when cytoplasmic flagellin accumulates (before hook completion or after filament assembly is complete), FliW binds Hag and releases CsrA to repress hag translation. When flagellin is secreted through the type III secretion apparatus during filament assembly, FliW is freed to bind CsrA and relieve translational repression, allowing flagellin synthesis. This homeostatic autoregulation ensures flagellin is produced only when the hook-basal body is complete and the secretion apparatus is ready. FliW adopts a minimal beta-barrel-like fold with a highly negatively charged surface that provides electrostatic repulsion against RNA binding when bound to CsrA.
| GO Term | Evidence | Action | Reason |
|---|---|---|---|
| GO:0071978 bacterial-type flagellum-dependent swarming motility | IBA GO_REF:0000033 | ACCEPT | Summary: FliW is required for swarming motility. Mutants of fliW (yviF) in B. subtilis show strongly reduced motility (PMID:16936039). PMID:21895793 confirmed reduced swarming motility in fliW mutants. The IBA annotation based on phylogenetic inference is well-supported by experimental evidence in B. subtilis itself and is consistent with the conserved role of FliW in flagellar assembly across bacteria. Reason: This annotation is well-supported by direct experimental evidence. FliW mutants show strongly reduced swarming motility, which is a downstream phenotypic consequence of its role in flagellin homeostasis. While not the primary molecular function, it accurately describes a biological process FliW is involved in. Supporting Evidence: PMID:16936039 Mutants of orthologs in Bacillus subtilis and Campylobacter jejuni (yviF, CJ1075) showed strongly reduced motility. UniProtKB:P96503 Greatly reduced swarming motility, less flagellin file:BACSU/fliW/fliW-deep-research-falcon.md See deep research file for comprehensive analysis |
| GO:0005737 cytoplasm | IEA GO_REF:0000120 | ACCEPT | Summary: FliW functions as a cytoplasmic protein that interacts with cytoplasmic flagellin monomers and CsrA. UniProt indicates cytoplasmic localization based on HAMAP-Rule:MF_01185. The partner-switching mechanism requires FliW to be in the cytoplasm where it can bind to newly synthesized flagellin and regulate CsrA activity on hag mRNA. Reason: This localization is consistent with FliW's known molecular function. The protein must be cytoplasmic to perform its partner-switching role between flagellin and CsrA. The IEA annotation from UniProtKB-SubCell is accurate and well-supported by functional evidence. Supporting Evidence: PMID:21895793 secretion of flagellin (Hag) releases FliW protein from a FliW-Hag complex |
| GO:0006417 regulation of translation | IEA GO_REF:0000043 | ACCEPT | Summary: FliW regulates translation by antagonizing CsrA, the translational repressor of hag mRNA. This IEA annotation from UniProtKB keyword mapping is accurate but less specific than the IMP annotation for GO:0045727 (positive regulation of translation). FliW specifically acts as a positive regulator of hag translation by inhibiting CsrA's repressor activity. Reason: The annotation is technically correct - FliW is involved in regulation of translation. However, the more specific annotation GO:0045727 (positive regulation of translation) with IMP evidence is also present and captures the directionality of the effect more precisely. Both annotations are acceptable as they are at different levels of specificity. Supporting Evidence: PMID:21895793 FliW then binds to CsrA and relieves CsrA-mediated translational repression of hag for flagellin synthesis concurrent with filament assembly |
| GO:0044780 bacterial-type flagellum assembly | IEA GO_REF:0000120 | ACCEPT | Summary: FliW is involved in bacterial-type flagellum assembly through its role in regulating flagellin homeostasis. PMID:16936039 characterized it as a conserved assembly factor of the bacterial flagellum. PMID:21895793 showed that fliW mutants have fewer, shorter flagella demonstrating a clear role in flagellar assembly. The IEA annotation from InterPro:IPR003775 (Flagellar_assembly_factor_FliW) is well-supported. Reason: This annotation accurately describes FliW's involvement in flagellum assembly. The protein is named Flagellar assembly factor FliW and experimental evidence confirms its role. While FliW's specific mechanism is regulatory (controlling flagellin synthesis), this contributes directly to assembly. Supporting Evidence: PMID:16936039 TP0658 is a conserved assembly factor for the bacterial flagellum UniProtKB:P96503 Fewer, shorter flagella assemble |
| GO:0044781 bacterial-type flagellum organization | IEA GO_REF:0000043 | ACCEPT | Summary: FliW contributes to bacterial-type flagellum organization through its role in coordinating flagellin production with assembly. This IEA annotation from UniProtKB keyword mapping (KW-1005, Bacterial flagellum biogenesis) is accurate. The term organization encompasses the broader cellular process including assembly and homeostatic control of flagellum components. Reason: This annotation is accurate and complementary to GO:0044780 (assembly). FliW's checkpoint function ensures proper temporal organization of flagellum assembly by coupling flagellin synthesis to the completion of the hook-basal body structure. Supporting Evidence: UniProtKB:P96503 Partner switching by flagellin between FliW and CsrA provides a flagellar assembly checkpoint to tightly control the timing of flagellin synthesis |
| GO:0045727 positive regulation of translation | IMP PMID:21895793 CsrA-FliW interaction governs flagellin homeostasis and a ch... | ACCEPT | Summary: FliW positively regulates translation of hag mRNA by antagonizing CsrA, the translational repressor. PMID:21895793 demonstrated that FliW binds to CsrA and relieves CsrA-mediated translational repression. PMID:27516547 further showed FliW binds CsrA at an allosteric site and non-competitively inhibits CsrA binding to hag RNA. The IMP evidence is well-supported by the mutant phenotype showing decreased flagellin levels in fliW mutants. Reason: This is a core function of FliW. By binding and inhibiting the translational repressor CsrA, FliW positively regulates translation of hag mRNA. This annotation accurately captures the regulatory outcome of FliW's molecular activity. The experimental evidence is strong. Supporting Evidence: PMID:21895793 FliW then binds to CsrA and relieves CsrA-mediated translational repression of hag for flagellin synthesis concurrent with filament assembly UniProtKB:P96503 An antagonist to translational regulator CsrA, it binds CsrA at an allosteric site and non-competitively inhibits CsrA binding to hag RNA |
| GO:0071978 bacterial-type flagellum-dependent swarming motility | IMP PMID:16936039 Novel conserved assembly factor of the bacterial flagellum. | ACCEPT | Summary: This IMP annotation is based on PMID:16936039, which reported that mutants of orthologs in Bacillus subtilis and Campylobacter jejuni showed strongly reduced motility. The fliW (yviF) mutant phenotype directly demonstrates involvement in swarming motility. This is a duplicate of the IBA annotation for the same term but with direct experimental evidence. Reason: Experimental evidence from mutant analysis directly supports FliW's involvement in swarming motility. While this is a downstream phenotypic effect rather than the primary molecular function, the annotation accurately describes a biological process affected by loss of FliW function. Supporting Evidence: PMID:16936039 Mutants of orthologs in Bacillus subtilis and Campylobacter jejuni (yviF, CJ1075) showed strongly reduced motility |
| GO:0071978 bacterial-type flagellum-dependent swarming motility | IMP PMID:21895793 CsrA-FliW interaction governs flagellin homeostasis and a ch... | ACCEPT | Summary: This IMP annotation is based on PMID:21895793, which provides evidence for motility defects in fliW mutants. This paper also showed that loss of motility and flagellar assembly are suppressed by deletion of csrA, demonstrating that the motility defect is due to CsrA-mediated repression of flagellin in the absence of FliW. This is consistent with the partner-switching model. Reason: This annotation provides additional experimental evidence supporting FliW's role in swarming motility. The rescue of the phenotype by csrA deletion confirms the mechanistic basis. While a duplicate term with the same evidence code, it represents independent experimental validation. Supporting Evidence: UniProtKB:P96503 Greatly reduced swarming motility, less flagellin |
| GO:1902021 regulation of bacterial-type flagellum-dependent cell motility | IMP PMID:16936039 Novel conserved assembly factor of the bacterial flagellum. | ACCEPT | Summary: FliW regulates bacterial-type flagellum-dependent cell motility through its control of flagellin homeostasis. PMID:16936039 showed that fliW mutants have strongly reduced motility. While this term is broader than swarming motility, it accurately captures FliW's regulatory role in motility. The term is appropriate because FliW does not directly participate in motility but regulates it through controlling flagellin availability. Reason: This regulatory term is appropriate because FliW's role is indeed regulatory - it controls the availability of flagellin for flagellum assembly rather than directly participating in motility mechanics. The term distinguishes the regulatory function from direct involvement. Supporting Evidence: PMID:16936039 Mutants of orthologs in Bacillus subtilis and Campylobacter jejuni (yviF, CJ1075) showed strongly reduced motility |
| GO:1902210 positive regulation of bacterial-type flagellum assembly | IMP PMID:21895793 CsrA-FliW interaction governs flagellin homeostasis and a ch... | ACCEPT | Summary: FliW positively regulates bacterial-type flagellum assembly by relieving CsrA-mediated translational repression of flagellin when the hook-basal body is complete. PMID:21895793 showed that fliW mutants have fewer, shorter flagella and this phenotype is suppressed by deletion of csrA. The positive regulation is mediated through antagonizing CsrA to allow flagellin synthesis. Reason: This annotation accurately describes FliW's function as a positive regulator of flagellum assembly. The mutant phenotype (fewer, shorter flagella) and its suppression by csrA deletion directly support this annotation. FliW promotes assembly by enabling flagellin production at the appropriate time. Supporting Evidence: UniProtKB:P96503 Fewer, shorter flagella assemble UniProtKB:P96503 Loss of motility and flagellar assembly are suppressed by deletion of csrA |
| GO:0140678 molecular function inhibitor activity | IDA PMID:27516547 FliW antagonizes CsrA RNA binding by a noncompetitive allost... | NEW | Summary: FliW functions as a molecular function inhibitor by allosterically binding to CsrA and inhibiting its RNA-binding activity. PMID:27516547 demonstrated that FliW antagonizes CsrA RNA binding by a noncompetitive allosteric mechanism - FliW binds to CsrA at a site distinct from the RNA-binding site and induces conformational changes that prevent RNA binding. This is a core molecular function of FliW. A more specific child term may be warranted if one exists for translation repressor inhibitor activity. Reason: The existing GO annotations for FliW lack any molecular function (MF) terms. The only existing annotations are for cellular component (cytoplasm) and biological process terms. FliW's primary molecular activity is inhibiting the molecular function of CsrA (translation repressor activity). GO:0140678 accurately describes this inhibitory activity. This is a significant gap in the current annotation set that should be addressed. Supporting Evidence: UniProtKB:P96503 An antagonist to translational regulator CsrA, it binds CsrA at an allosteric site and non-competitively inhibits CsrA binding to hag RNA PMID:21895793 Here we discover FliW, the first protein antagonist of CsrA activity that constitutes a partner switching mechanism to control flagellin synthesis |
| GO:0005515 protein binding | NAS | NEW | Summary: Added to align core_functions with existing annotations. Reason: Core function term not present in existing_annotations. |
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Download this section (compressed HTML)Q: Does FliW have additional targets beyond CsrA, given that CsrA is a pleiotropic regulator of multiple cellular processes beyond flagellin?
Q: What determines the relative affinities of FliW for CsrA vs flagellin, and how are these tuned for optimal homeostatic control?
Experiment: Structural characterization of the FliW-CsrA complex by X-ray crystallography or cryo-EM to visualize the allosteric mechanism by which FliW inhibits CsrA RNA binding.
Experiment: Quantitative proteomics or RNA-seq in fliW mutants vs wild-type to identify additional CsrA targets that may be affected by FliW-mediated regulation.
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