FliK is the flagellar hook-length control protein that acts as a molecular ruler and terminator to control the length of the flagellar hook during assembly. FliK is intermittently exported through the flagellar type III secretion system (fT3SS) during hook assembly, and its C-terminal domain contacts the cytoplasmic domain of FlhB (FlhBC) to trigger the substrate-specificity switch from rod/hook-type substrates (e.g., FlgE) to filament-type substrates (e.g., FliC, FlgK, FlgL, FlgM). Loss of FliK causes a polyhook phenotype where hooks overgrow because the switch fails to occur. In B. subtilis, both FliK and the hook are approximately 25% longer than their Salmonella counterparts, indicating species-specific calibration of the length-control mechanism. FliK belongs to the FliK family characterized by an N-terminal export/assembly-interacting region, a flexible linker, and a C-terminal FlhB-binding domain.
| GO Term | Evidence | Action | Reason |
|---|---|---|---|
|
GO:0009424
bacterial-type flagellum hook
|
IEA
GO_REF:0000002 |
MODIFY |
Summary: This cellular component annotation indicates FliK is located in the flagellar hook. FliK is an export substrate of the flagellar type III secretion system (fT3SS) and is intermittently secreted through the basal body during hook assembly. However, FliK is not a structural component of the hook itself - the hook is composed of FlgE subunits. FliK transiently passes through the hook structure during secretion to sense hook length, but its primary localization is cytoplasmic until it is exported. The annotation is misleading as it suggests FliK is a stable component of the hook structure.
Reason: FliK is not a structural component of the flagellar hook - the hook is built from FlgE protein. FliK is a cytoplasmic protein that is intermittently exported through the fT3SS during hook assembly to control hook length. While FliK transiently passes through the hook region during secretion, it does not permanently reside there. A more accurate CC annotation would be cytoplasm or the flagellar secretion apparatus. The InterPro domain annotation (IPR001635) correctly identifies FliK as a hook-length control protein, but the mapping to the hook CC term is inappropriate.
Proposed replacements:
cytoplasm
bacterial-type flagellum secretion apparatus
Supporting Evidence:
PMID:22730131
We used the fluorescently labeled hook to demonstrate that FlgE is the hook structural protein and that FliK regulated hook length.
file:BACSU/fliK/fliK-deep-research-falcon.md
FliK is a cytosolic protein that is exported through the basal-body fT3SS during hook assembly; its export is reduced once hooks complete, consistent with a role as a periodic sensor/signal.
|
|
GO:0044780
bacterial-type flagellum assembly
|
IEA
GO_REF:0000002 |
ACCEPT |
Summary: This biological process annotation indicates FliK is involved in bacterial flagellum assembly. This is accurate - FliK is essential for proper flagellum assembly as it controls the length of the hook and triggers the substrate-specificity switch that enables filament assembly to proceed. Without FliK, hooks overgrow (polyhook phenotype) and the switch to filament-type export fails, preventing completion of functional flagella.
Reason: FliK is a core regulator of flagellum assembly. It controls hook length and triggers the critical switch from rod/hook substrate export to filament substrate export. Loss of FliK results in polyhook formation and failure to assemble functional flagella. This annotation accurately captures FliK's essential role in the flagellar assembly process.
Supporting Evidence:
PMID:22730131
We used the fluorescently labeled hook to demonstrate that FlgE is the hook structural protein and that FliK regulated hook length.
file:BACSU/fliK/fliK-deep-research-falcon.md
FliK is the dedicated flagellar hook-length control protein that times the transition from rod/hook-type export to filament-type export by the flagellar type III secretion system (fT3SS). Loss of FliK causes a polyhook phenotype because the export apparatus fails to switch substrates.
|
|
GO:0044781
bacterial-type flagellum organization
|
IEA
GO_REF:0000043 |
KEEP AS NON CORE |
Summary: This biological process annotation indicates FliK is involved in bacterial flagellum organization, which encompasses assembly, arrangement, and disassembly. This is a parent term of GO:0044780 (bacterial-type flagellum assembly) and is therefore broader and less informative than the more specific assembly term that is also annotated to FliK. The annotation is not incorrect but redundant given the more specific annotation.
Reason: While technically correct (FliK does participate in flagellum organization via its role in assembly), this term is broader and less informative than the more specific GO:0044780 (bacterial-type flagellum assembly) which is also annotated to FliK. The annotation is redundant but not incorrect. Keeping as non-core since the more specific assembly term better captures FliK's function.
Supporting Evidence:
PMID:22730131
We used the fluorescently labeled hook to demonstrate that FlgE is the hook structural protein and that FliK regulated hook length.
|
|
GO:0044780
bacterial-type flagellum assembly
|
IMP
PMID:25313396 FlgM is secreted by the flagellar export apparatus in Bacill... |
ACCEPT |
Summary: This IMP (Inferred from Mutant Phenotype) annotation indicates FliK is involved in bacterial flagellum assembly based on evidence from PMID:25313396 (Calvo & Kearns 2015). This paper demonstrates that FliK is part of the minimal subset of flagellar proteins required for FlgM secretion, placing FliK functionally in the export apparatus pathway. The paper explicitly describes FliK as "the substrate specificity switch regulator," confirming its role in the assembly process through triggering the substrate switch.
Reason: The IMP annotation is well-supported by the cited reference. PMID:25313396 demonstrates that FliK is required for FlgM secretion and explicitly names it as "the substrate specificity switch regulator FliK." This directly supports FliK's role in flagellum assembly - without FliK, the switch from hook to filament substrate export fails, preventing proper flagellum assembly.
Supporting Evidence:
PMID:25313396
FlgM secretion is strongly enhanced by, but does not strictly require, hook-basal body completion and instead demands a minimal subset of flagellar proteins that includes the FliF/FliG basal body proteins, the flagellar type III export apparatus components FliO, FliP, FliQ, FliR, FlhA, and FlhB, and the substrate specificity switch regulator FliK.
|
|
GO:0071978
bacterial-type flagellum-dependent swarming motility
|
IMP
PMID:25313396 FlgM is secreted by the flagellar export apparatus in Bacill... |
KEEP AS NON CORE |
Summary: This IMP annotation indicates FliK is involved in bacterial swarming motility based on PMID:25313396. While FliK is essential for functional flagella (through its role in hook length control and substrate switching), and functional flagella are required for swarming motility, this annotation may represent an indirect/downstream effect rather than a direct role in swarming. FliK's primary function is in flagellum assembly (hook length control), not in the motility process itself. The paper's focus is on FlgM secretion and flagellar assembly, not specifically on swarming motility assays.
Reason: FliK is indirectly required for swarming motility because it is essential for proper flagellum assembly. Without FliK, hooks overgrow and flagella are non-functional, which would impair swarming. However, FliK does not directly participate in the motility mechanism itself - its core function is hook length control and triggering the substrate-specificity switch. This annotation captures a downstream phenotypic effect rather than FliK's primary molecular function. The assembly annotation (GO:0044780) more directly reflects FliK's biological role.
Supporting Evidence:
PMID:25313396
FlgM secretion is strongly enhanced by, but does not strictly require, hook-basal body completion and instead demands a minimal subset of flagellar proteins that includes the FliF/FliG basal body proteins, the flagellar type III export apparatus components FliO, FliP, FliQ, FliR, FlhA, and FlhB, and the substrate specificity switch regulator FliK.
file:BACSU/fliK/fliK-deep-research-falcon.md
FliK is the dedicated flagellar hook-length control protein that times the transition from rod/hook-type export to filament-type export by the flagellar type III secretion system (fT3SS). Loss of FliK causes a polyhook phenotype because the export apparatus fails to switch substrates.
|
|
GO:0071978
bacterial-type flagellum-dependent swarming motility
|
IMP
PMID:22730131 Molecular characterization of the flagellar hook in Bacillus... |
KEEP AS NON CORE |
Summary: This IMP annotation indicates FliK is involved in bacterial swarming motility based on PMID:22730131 (Courtney et al. 2012). This paper directly demonstrates in B. subtilis that "FliK regulated hook length" and shows that mutants affecting hook assembly result in reduced motility. However, like the other swarming annotation, this represents an indirect effect - FliK's primary function is hook length control, and the motility defect is a downstream consequence of faulty flagellum assembly.
Reason: While PMID:22730131 directly demonstrates FliK's role in B. subtilis flagellum biology, its core finding is that "FliK regulated hook length." The swarming motility defect in fliK mutants is an indirect consequence of improper hook assembly rather than a direct role in the swarming process. The annotation is not incorrect but reflects a downstream phenotype rather than FliK's primary molecular function. Retaining as non-core because the experimental evidence does support the connection, even if indirect.
Supporting Evidence:
PMID:22730131
We used the fluorescently labeled hook to demonstrate that FlgE is the hook structural protein and that FliK regulated hook length.
PMID:22730131
All mutants defective in hook completion resulted in homogeneously reduced sigma(D)-dependent gene expression due to the action of the anti-sigma factor FlgM.
|
|
GO:0005515
protein binding
|
NAS | NEW |
Summary: Added to align core_functions with existing annotations.
Reason: Core function term not present in existing_annotations.
|
Q: What is the precise mechanism by which FliK senses hook length in B. subtilis? Is it through physical spanning of the export channel (molecular ruler model) or through export frequency and stochastic interactions (molecular clock model)? Understanding this in B. subtilis, where hook and FliK are approximately 25% longer than Salmonella, could illuminate species-specific calibration of the length control system.
Q: Does FliK interact with FlgD (hook cap) in B. subtilis as it does in Salmonella? What is the role of FlhO and FlhP (B. subtilis-specific proteins) in the FliK-mediated switching mechanism? B. subtilis has unique hook assembly factors not found in Gram-negative bacteria, and understanding how these interact with the conserved FliK-FlhB switching mechanism could reveal Gram-positive-specific adaptations.
Experiment: Measure hook length distributions in B. subtilis fliK deletion, overexpression, and truncation mutants using cryo-EM or fluorescent labeling to quantify the length control function. Expected outcomes: fliK deletion should produce polyhooks (elongated hooks), overexpression should produce shorter hooks, and C-terminal truncations should disrupt switching without affecting secretion.
Experiment: Co-immunoprecipitation or bacterial two-hybrid to test FliK-FlhB interaction in B. subtilis, and test whether FlhO/FlhP affect this interaction. Expected outcome: Should detect FliK-FlhB interaction dependent on the FliK C-terminal domain, and may reveal whether FlhO/FlhP modulate the switch timing.
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template_file: templates/gene_research_go_focused.md
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organism: BACSU
gene_id: fliK
gene_symbol: fliK
uniprot_accession: P23451
protein_description: 'RecName: Full=Probable flagellar hook-length control protein;'
gene_info: Name=fliK; OrderedLocusNames=BSU16270;
organism_full: Bacillus subtilis (strain 168).
protein_family: Belongs to the FliK family. .
protein_domains: Flag_hook_Flik. (IPR001635); Flagellar_hook_control-like_C. (IPR021136);
FliK-like_C_sf. (IPR038610); Flg_hook (PF02120)
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BEFORE YOU BEGIN RESEARCH: You MUST verify you are researching the CORRECT gene/protein. Gene symbols can be ambiguous, especially for less well-characterized genes from non-model organisms.
DO NOT PROCEED WITH RESEARCH ON A DIFFERENT GENE. Instead:
- State clearly: "The gene symbol 'fliK' is ambiguous or literature is limited for this specific protein"
- Explain what you found (e.g., "Found extensive literature on a different gene with the same symbol in a different organism")
- Describe the protein based ONLY on the UniProt information provided above
- Suggest that the protein function can be inferred from domain/family information
Please provide a comprehensive research report on the gene fliK (gene ID: fliK, UniProt: P23451) in BACSU.
The research report should be a detailed narrative explaining the function, biological processes, and localization of the gene product. Citations should be given for all claims.
You should prioritize authoritative reviews and primary scientific literature when conducting research. You can supplement
this with annotations you find in gene/protein databases, but these can be outdated or inaccurate.
We are specifically interested in the primary function of the gene - for enzymes, what reaction is catalyzed, and what is the substrate specificity? For transporters, what is the substrate? For structural proteins or adapters, what is the broader structural role? For signaling molecules, what is the role in the pathway.
We are interested in where in or outside the cell the gene product carries out its function.
We are also interested in the signaling or biochemical pathways in which the gene functions. We are less interested in broad pleiotropic effects, except where these elucidate the precise role.
Include evidence where possible. We are interested in both experimental evidence as well as inference from structure, evolution, or bioinformatic analysis. Precise studies should be prioritized over high-throughput, where available.
Question: You are an expert researcher providing comprehensive, well-cited information.
Provide detailed information focusing on:
1. Key concepts and definitions with current understanding
2. Recent developments and latest research (prioritize 2023-2024 sources)
3. Current applications and real-world implementations
4. Expert opinions and analysis from authoritative sources
5. Relevant statistics and data from recent studies
Format as a comprehensive research report with proper citations. Include URLs and publication dates where available.
Always prioritize recent, authoritative sources and provide specific citations for all major claims.
BEFORE YOU BEGIN RESEARCH: You MUST verify you are researching the CORRECT gene/protein. Gene symbols can be ambiguous, especially for less well-characterized genes from non-model organisms.
DO NOT PROCEED WITH RESEARCH ON A DIFFERENT GENE. Instead:
- State clearly: "The gene symbol 'fliK' is ambiguous or literature is limited for this specific protein"
- Explain what you found (e.g., "Found extensive literature on a different gene with the same symbol in a different organism")
- Describe the protein based ONLY on the UniProt information provided above
- Suggest that the protein function can be inferred from domain/family information
Please provide a comprehensive research report on the gene fliK (gene ID: fliK, UniProt: P23451) in BACSU.
The research report should be a detailed narrative explaining the function, biological processes, and localization of the gene product. Citations should be given for all claims.
You should prioritize authoritative reviews and primary scientific literature when conducting research. You can supplement
this with annotations you find in gene/protein databases, but these can be outdated or inaccurate.
We are specifically interested in the primary function of the gene - for enzymes, what reaction is catalyzed, and what is the substrate specificity? For transporters, what is the substrate? For structural proteins or adapters, what is the broader structural role? For signaling molecules, what is the role in the pathway.
We are interested in where in or outside the cell the gene product carries out its function.
We are also interested in the signaling or biochemical pathways in which the gene functions. We are less interested in broad pleiotropic effects, except where these elucidate the precise role.
Include evidence where possible. We are interested in both experimental evidence as well as inference from structure, evolution, or bioinformatic analysis. Precise studies should be prioritized over high-throughput, where available.
Plan (abbreviated)
- Verified identity: fliK (UniProt P23451) in Bacillus subtilis 168 encodes a FliK-family flagellar hook-length control protein with Flag_hook_Flik/FliK-like C-terminal domains. No conflicting gene symbol usage in B. subtilis was found (see sources below). (mukherjee2014thestructureand pages 3-4, waters2007theflikprotein pages 6-7)
- Collected recent and foundational literature; extracted function, mechanism, localization, partners; compiled 2023 updates; gathered quantitative data.
Research report: Bacillus subtilis fliK (UniProt P23451)
1) Key concepts and definitions
- Core role. FliK is the dedicated flagellar hook-length control protein that times the transition from rod/hook-type export to filament-type export by the flagellar type III secretion system (fT3SS). Loss of FliK causes a polyhook phenotype because the export apparatus fails to switch substrates. In the conserved model, FliK is intermittently secreted during hook growth and signals completion by engaging FlhB to trigger substrate-specificity switching. URL (review): https://doi.org/10.1110/ps.072785407 (May 2007). URL (B. subtilis-focused review): https://doi.org/10.1146/annurev-genet-120213-092406 (Nov 2014). URL (recent synthesis): https://doi.org/10.1128/ecosalplus.esp-0011-2023 (Dec 2023). (waters2007theflikprotein pages 1-2, mukherjee2014thestructureand pages 3-4, minamino2023structureassemblyand pages 1-3)
- Mechanistic definition. FliK comprises an N-terminal export/assembly-interacting region and a C-terminal signaling region that engages the cytoplasmic domain of FlhB (FlhBC). FliK export events within a window of hook lengths permit FliK C-terminus to bind FlhBC and initiate the switch. URL: https://doi.org/10.1110/ps.072785407 (May 2007). (waters2007theflikprotein pages 6-7, waters2007theflikprotein pages 8-9)
- Substrate-specificity switching. FlhB contains a conserved NPTH autocleavage site; interaction with mature FliK and the state of the hook facilitate cleavage-dependent switching from exporting rod/hook substrates (e.g., FlgE) to filament/late substrates (e.g., FlgK/FlgL/FliC and anti-sigma factor FlgM secretion). URL: https://doi.org/10.1128/ecosalplus.esp-0011-2023 (Dec 2023). (minamino2023structureassemblyand pages 20-22)
2) Recent developments and latest research (priority 2023–2024)
- 2023 review consolidating structural and mechanistic updates. Minamino & Kinoshita (EcoSal Plus, Dec 2023) summarize high-resolution cryo-EM advances in flagellar assemblies and detail FliK domain boundaries (FliKN 1–207; flexible linker 208–267; FliKC 268–405) and their roles in FlhB engagement and switching. They also review FlhB NPTH autocleavage, FlhA ring dynamics, and chaperone docking for filament-class export. URL: https://doi.org/10.1128/ecosalplus.esp-0011-2023 (Dec 2023). (minamino2023structureassemblyand pages 20-22, minamino2023structureassemblyand pages 1-3)
- Model refinement away from a strict “physical ruler.” Deletion and overexpression analyses (mainly Salmonella) indicate FliK acts as a terminator that limits maximal hook length and triggers switching, with export frequency and interactions with hook/cap influencing effective hook lengths, rather than a strict linear ruler in all species. Preprint synthesis: https://doi.org/10.1101/2020.03.25.007062 (Mar 2020). Contemporary reviews still note conserved FliK–FlhB switching principles applicable across bacteria, including Gram-positives like B. subtilis. (minamino2023structureassemblyand pages 29-30, minamino2023structureassemblyand pages 20-22)
3) Current applications and real-world implementations
- Tuning hook length and export switching is a practical handle for engineering motility and secretion behaviors in bacteria. Studies demonstrate that altering FliK levels/length shortens or lengthens hooks via changing the probability of successful switching events, a principle used to modulate motility phenotypes in model systems. URL: https://doi.org/10.1110/ps.072785407 (May 2007). (waters2007theflikprotein pages 8-9)
- While B. subtilis applications are less documented than Gram-negative models, B. subtilis retains the conserved fT3SS core (FlhA/FlhB/FliPQR) and uses similar assembly rules; thus, synthetic biology strategies targeting FliK–FlhB interactions and timing are expected to translate with genus-specific calibration. URL: https://doi.org/10.1146/annurev-genet-120213-092406 (Nov 2014); https://doi.org/10.1128/ecosalplus.esp-0011-2023 (Dec 2023). (mukherjee2014thestructureand pages 3-4, minamino2023structureassemblyand pages 1-3)
4) Expert opinions and authoritative analysis
- B. subtilis overview (Mukherjee & Kearns, 2014) describes FliK secretion during hook assembly with a model in which the N-terminus is exported and interacts with the nascent hook, a central region spans the conduit, and the C-terminus remains transiently cytoplasmic to probe and signal via FlhB; they note longer FliK and hook in B. subtilis relative to Salmonella. URL: https://doi.org/10.1146/annurev-genet-120213-092406 (Nov 2014). (mukherjee2014thestructureand pages 3-4)
- Comprehensive cross-species review (Waters et al., 2007) weighs multiple mechanistic models and synthesizes binding and export requirements for signaling; it underscores that export of FliK is essential for signaling and that C-terminal regions mediate FlhB engagement. URL: https://doi.org/10.1110/ps.072785407 (May 2007). (waters2007theflikprotein pages 6-7, waters2007theflikprotein pages 1-2)
- Recent synthesis (Minamino & Kinoshita, 2023) integrates cryo-EM and biochemical insights into a unified view of FliK–FlhB switching and downstream export-chaperone interactions. URL: https://doi.org/10.1128/ecosalplus.esp-0011-2023 (Dec 2023). (minamino2023structureassemblyand pages 20-22)
5) Relevant statistics and data from recent and foundational studies
- Hook length benchmarks. In Salmonella, wild-type hook length is ~55 nm (often cited as 55 ± 6 nm), with FliK overexpression shortening hooks and low FliK causing polyhooks due to failed switching; these data define the window in which FliK export events can trigger the switch. URL: https://doi.org/10.1110/ps.072785407 (May 2007). (waters2007theflikprotein pages 8-9)
- FliK secretion/export counts. In Salmonella, roughly 30–100 FliK molecules are exported per cell during assembly, consistent with intermittent sampling of hook length/state. URL: https://doi.org/10.1046/j.1365-2958.1999.01597.x (Oct 1999). (minamino1999fliktheprotein pages 4-6)
- B. subtilis genus-specific differences. B. subtilis has a Gram-positive envelope and uses related but distinct stator systems and hook architecture; reviews note its hook and FliK are each approximately 25% longer than those of Salmonella, implying species-calibrated length targets and timing without altering the core FliK–FlhB switching principle. URL: https://doi.org/10.1146/annurev-genet-120213-092406 (Nov 2014). (mukherjee2014thestructureand pages 3-4)
Functional annotation for B. subtilis FliK (UniProt P23451)
- Primary function. A non-enzymatic assembly factor that controls hook length and triggers the fT3SS substrate-specificity switch from rod/hook to filament-class substrates. This prevents overgrowth of the hook and enables filament assembly and late gene expression. URL: https://doi.org/10.1128/ecosalplus.esp-0011-2023 (Dec 2023); https://doi.org/10.1110/ps.072785407 (May 2007). (minamino2023structureassemblyand pages 20-22, waters2007theflikprotein pages 1-2)
- Mechanism. FliK is intermittently secreted during hook assembly; successful export events permit the C-terminal domain to contact FlhB’s cytoplasmic domain (FlhBC), promoting FlhB NPTH autocleavage and reconfiguration of the export apparatus to accept filament-type substrates. Export frequency and interactions with hook-associated proteins modulate the effective “length-readout,” and the B. subtilis system is expected to be calibrated to its longer native hook. URL: https://doi.org/10.1128/ecosalplus.esp-0011-2023 (Dec 2023); https://doi.org/10.1110/ps.072785407 (May 2007); https://doi.org/10.1146/annurev-genet-120213-092406 (Nov 2014). (minamino2023structureassemblyand pages 20-22, waters2007theflikprotein pages 8-9, mukherjee2014thestructureand pages 3-4)
- Domain/family features. FliK-family proteins commonly have an N-terminal export/assembly-interacting region, a flexible linker, and a compact C-terminal domain that binds FlhBC; an intrinsically disordered C-tail modulates this interaction. B. subtilis FliK belongs to this family, consistent with UniProt domain annotations (Flag_hook_Flik/FliK-like C). URL: https://doi.org/10.1128/ecosalplus.esp-0011-2023 (Dec 2023); https://doi.org/10.1110/ps.072785407 (May 2007). (minamino2023structureassemblyand pages 20-22, waters2007theflikprotein pages 6-7)
- Cellular localization and secretion dynamics. FliK is a cytosolic protein that is exported through the basal-body fT3SS during hook assembly; its export is reduced once hooks complete, consistent with a role as a periodic sensor/signal. URL: https://doi.org/10.1046/j.1365-2958.1999.01597.x (Oct 1999); https://doi.org/10.1146/annurev-genet-120213-092406 (Nov 2014). (minamino1999fliktheprotein pages 4-6, mukherjee2014thestructureand pages 3-4)
- Interacting partners and pathway context. The critical receptor is FlhB (cytoplasmic domain, FlhBC). FliK interacts with hook-associated proteins (e.g., FlgD strongly, FlgE weakly in Salmonella) that help position the switching interaction, and switching engages FlhA and chaperone docking for filament-type export. The B. subtilis fT3SS core (FlhA/FlhB/FliPQR) is conserved. URLs: https://doi.org/10.1110/ps.072785407 (May 2007); https://doi.org/10.1128/ecosalplus.esp-0011-2023 (Dec 2023); https://doi.org/10.1146/annurev-genet-120213-092406 (Nov 2014). (waters2007theflikprotein pages 8-9, minamino2023structureassemblyand pages 20-22, mukherjee2014thestructureand pages 3-4)
Ambiguity check and organism verification
- The gene symbol “fliK” in Bacillus subtilis 168 refers to the hook-length control protein, consistent with UniProt P23451 and conserved literature usage. No alternative Bacillus gene with the same symbol was found in the surveyed sources; cross-species results (Salmonella/E. coli) were used only to inform conserved mechanisms. (mukherjee2014thestructureand pages 3-4, waters2007theflikprotein pages 6-7)
Key references with URLs and dates
- Mukherjee S, Kearns DB. The structure and regulation of flagella in Bacillus subtilis. Annual Review of Genetics. Nov 2014. URL: https://doi.org/10.1146/annurev-genet-120213-092406. (mukherjee2014thestructureand pages 3-4)
- Waters RC, O’Toole PW, Ryan KA. The FliK protein and flagellar hook-length control. Protein Science. May 2007. URL: https://doi.org/10.1110/ps.072785407. (waters2007theflikprotein pages 6-7, waters2007theflikprotein pages 1-2, waters2007theflikprotein pages 8-9)
- Minamino T, Kinoshita M. Structure, Assembly, and Function of Flagella Responsible for Bacterial Locomotion. EcoSal Plus. Dec 2023. URL: https://doi.org/10.1128/ecosalplus.esp-0011-2023. (minamino2023structureassemblyand pages 1-3, minamino2023structureassemblyand pages 20-22)
- Minamino T et al. FliK is exported during hook assembly. Molecular Microbiology. Oct 1999. URL: https://doi.org/10.1046/j.1365-2958.1999.01597.x. (minamino1999fliktheprotein pages 4-6)
- Guse A, Rohde M, Erhardt M. Controlling minimal and maximal hook-length of the bacterial flagellum. bioRxiv. Mar 2020. URL: https://doi.org/10.1101/2020.03.25.007062. (minamino2023structureassemblyand pages 29-30)
Limitations and outlook
- Direct, quantitative characterization of B. subtilis FliK–FlhB biochemistry (e.g., binding constants, in vivo B. subtilis hook-length distributions and switch timing) is less abundant than for Salmonella/E. coli; however, authoritative B. subtilis reviews and conserved-domain logic indicate that the same ruler-and-switch framework applies, calibrated to Bacillus-specific hook dimensions and export dynamics. Future work leveraging cryo-EM and in vivo single-molecule export measurements in B. subtilis is expected to clarify quantitative parameters while preserving the conserved mechanism summarized here. (mukherjee2014thestructureand pages 3-4, minamino2023structureassemblyand pages 20-22)
References
(mukherjee2014thestructureand pages 3-4): Sampriti Mukherjee and Daniel B. Kearns. The structure and regulation of flagella in bacillus subtilis. Annual review of genetics, 48:319-40, Nov 2014. URL: https://doi.org/10.1146/annurev-genet-120213-092406, doi:10.1146/annurev-genet-120213-092406. This article has 225 citations and is from a domain leading peer-reviewed journal.
(waters2007theflikprotein pages 6-7): Richard C. Waters, Paul W. O'Toole, and Kieran A. Ryan. The flik protein and flagellar hook‐length control. Protein Science, 16:769-780, May 2007. URL: https://doi.org/10.1110/ps.072785407, doi:10.1110/ps.072785407. This article has 76 citations and is from a peer-reviewed journal.
(waters2007theflikprotein pages 1-2): Richard C. Waters, Paul W. O'Toole, and Kieran A. Ryan. The flik protein and flagellar hook‐length control. Protein Science, 16:769-780, May 2007. URL: https://doi.org/10.1110/ps.072785407, doi:10.1110/ps.072785407. This article has 76 citations and is from a peer-reviewed journal.
(minamino2023structureassemblyand pages 1-3): Tohru Minamino and Miki Kinoshita. Structure, assembly, and function of flagella responsible for bacterial locomotion. EcoSal Plus, Dec 2023. URL: https://doi.org/10.1128/ecosalplus.esp-0011-2023, doi:10.1128/ecosalplus.esp-0011-2023. This article has 43 citations and is from a poor quality or predatory journal.
(waters2007theflikprotein pages 8-9): Richard C. Waters, Paul W. O'Toole, and Kieran A. Ryan. The flik protein and flagellar hook‐length control. Protein Science, 16:769-780, May 2007. URL: https://doi.org/10.1110/ps.072785407, doi:10.1110/ps.072785407. This article has 76 citations and is from a peer-reviewed journal.
(minamino2023structureassemblyand pages 20-22): Tohru Minamino and Miki Kinoshita. Structure, assembly, and function of flagella responsible for bacterial locomotion. EcoSal Plus, Dec 2023. URL: https://doi.org/10.1128/ecosalplus.esp-0011-2023, doi:10.1128/ecosalplus.esp-0011-2023. This article has 43 citations and is from a poor quality or predatory journal.
(minamino2023structureassemblyand pages 29-30): Tohru Minamino and Miki Kinoshita. Structure, assembly, and function of flagella responsible for bacterial locomotion. EcoSal Plus, Dec 2023. URL: https://doi.org/10.1128/ecosalplus.esp-0011-2023, doi:10.1128/ecosalplus.esp-0011-2023. This article has 43 citations and is from a poor quality or predatory journal.
(minamino1999fliktheprotein pages 4-6): Tohru Minamino, Bertha González‐Pedrajo, Kenta Yamaguchi, Shin‐Ichi Aizawa, and Robert M. Macnab. Flik, the protein responsible for flagellar hook length control in salmonella, is exported during hook assembly. Molecular Microbiology, 34:295-304, Oct 1999. URL: https://doi.org/10.1046/j.1365-2958.1999.01597.x, doi:10.1046/j.1365-2958.1999.01597.x. This article has 181 citations and is from a domain leading peer-reviewed journal.
id: P23451
gene_symbol: fliK
product_type: PROTEIN
status: COMPLETE
taxon:
id: NCBITaxon:224308
label: Bacillus subtilis (strain 168)
description: >-
FliK is the flagellar hook-length control protein that acts as a molecular ruler
and terminator
to control the length of the flagellar hook during assembly. FliK is intermittently
exported
through the flagellar type III secretion system (fT3SS) during hook assembly, and
its C-terminal
domain contacts the cytoplasmic domain of FlhB (FlhBC) to trigger the substrate-specificity
switch from rod/hook-type substrates (e.g., FlgE) to filament-type substrates (e.g.,
FliC, FlgK,
FlgL, FlgM). Loss of FliK causes a polyhook phenotype where hooks overgrow because
the switch
fails to occur. In B. subtilis, both FliK and the hook are approximately 25% longer
than their
Salmonella counterparts, indicating species-specific calibration of the length-control
mechanism.
FliK belongs to the FliK family characterized by an N-terminal export/assembly-interacting
region,
a flexible linker, and a C-terminal FlhB-binding domain.
existing_annotations:
- term:
id: GO:0009424
label: bacterial-type flagellum hook
evidence_type: IEA
original_reference_id: GO_REF:0000002
review:
summary: >-
This cellular component annotation indicates FliK is located in the flagellar
hook.
FliK is an export substrate of the flagellar type III secretion system (fT3SS)
and
is intermittently secreted through the basal body during hook assembly. However,
FliK
is not a structural component of the hook itself - the hook is composed of
FlgE subunits.
FliK transiently passes through the hook structure during secretion to sense
hook length,
but its primary localization is cytoplasmic until it is exported. The annotation
is
misleading as it suggests FliK is a stable component of the hook structure.
action: MODIFY
reason: >-
FliK is not a structural component of the flagellar hook - the hook is built
from FlgE
protein. FliK is a cytoplasmic protein that is intermittently exported through
the fT3SS
during hook assembly to control hook length. While FliK transiently passes
through the
hook region during secretion, it does not permanently reside there. A more
accurate CC
annotation would be cytoplasm or the flagellar secretion apparatus. The InterPro
domain
annotation (IPR001635) correctly identifies FliK as a hook-length control
protein, but
the mapping to the hook CC term is inappropriate.
proposed_replacement_terms:
- id: GO:0005737
label: cytoplasm
- id: GO:0120102
label: bacterial-type flagellum secretion apparatus
additional_reference_ids:
- PMID:22730131
supported_by:
- reference_id: PMID:22730131
supporting_text: "We used the fluorescently labeled hook to demonstrate
that FlgE is the hook structural protein and that FliK regulated hook
length."
- reference_id: file:BACSU/fliK/fliK-deep-research-falcon.md
supporting_text: "FliK is a cytosolic protein that is exported through the
basal-body fT3SS during hook assembly; its export is reduced once hooks
complete, consistent with a role as a periodic sensor/signal."
- term:
id: GO:0044780
label: bacterial-type flagellum assembly
evidence_type: IEA
original_reference_id: GO_REF:0000002
review:
summary: >-
This biological process annotation indicates FliK is involved in bacterial
flagellum
assembly. This is accurate - FliK is essential for proper flagellum assembly
as it
controls the length of the hook and triggers the substrate-specificity switch
that
enables filament assembly to proceed. Without FliK, hooks overgrow (polyhook
phenotype)
and the switch to filament-type export fails, preventing completion of functional
flagella.
action: ACCEPT
reason: >-
FliK is a core regulator of flagellum assembly. It controls hook length and
triggers
the critical switch from rod/hook substrate export to filament substrate export.
Loss of FliK results in polyhook formation and failure to assemble functional
flagella.
This annotation accurately captures FliK's essential role in the flagellar
assembly
process.
supported_by:
- reference_id: PMID:22730131
supporting_text: "We used the fluorescently labeled hook to demonstrate
that FlgE is the hook structural protein and that FliK regulated hook
length."
- reference_id: file:BACSU/fliK/fliK-deep-research-falcon.md
supporting_text: "FliK is the dedicated flagellar hook-length control protein
that times the transition from rod/hook-type export to filament-type export
by the flagellar type III secretion system (fT3SS). Loss of FliK causes
a polyhook phenotype because the export apparatus fails to switch substrates."
- term:
id: GO:0044781
label: bacterial-type flagellum organization
evidence_type: IEA
original_reference_id: GO_REF:0000043
review:
summary: >-
This biological process annotation indicates FliK is involved in bacterial
flagellum
organization, which encompasses assembly, arrangement, and disassembly. This
is a
parent term of GO:0044780 (bacterial-type flagellum assembly) and is therefore
broader and less informative than the more specific assembly term that is
also
annotated to FliK. The annotation is not incorrect but redundant given the
more
specific annotation.
action: KEEP_AS_NON_CORE
reason: >-
While technically correct (FliK does participate in flagellum organization
via its
role in assembly), this term is broader and less informative than the more
specific
GO:0044780 (bacterial-type flagellum assembly) which is also annotated to
FliK.
The annotation is redundant but not incorrect. Keeping as non-core since the
more
specific assembly term better captures FliK's function.
supported_by:
- reference_id: PMID:22730131
supporting_text: "We used the fluorescently labeled hook to demonstrate
that FlgE is the hook structural protein and that FliK regulated hook
length."
- term:
id: GO:0044780
label: bacterial-type flagellum assembly
evidence_type: IMP
original_reference_id: PMID:25313396
review:
summary: >-
This IMP (Inferred from Mutant Phenotype) annotation indicates FliK is involved
in
bacterial flagellum assembly based on evidence from PMID:25313396 (Calvo &
Kearns 2015).
This paper demonstrates that FliK is part of the minimal subset of flagellar
proteins
required for FlgM secretion, placing FliK functionally in the export apparatus
pathway.
The paper explicitly describes FliK as "the substrate specificity switch regulator,"
confirming its role in the assembly process through triggering the substrate
switch.
action: ACCEPT
reason: >-
The IMP annotation is well-supported by the cited reference. PMID:25313396
demonstrates
that FliK is required for FlgM secretion and explicitly names it as "the substrate
specificity switch regulator FliK." This directly supports FliK's role in
flagellum
assembly - without FliK, the switch from hook to filament substrate export
fails,
preventing proper flagellum assembly.
supported_by:
- reference_id: PMID:25313396
supporting_text: "FlgM secretion is strongly enhanced by, but does not strictly
require, hook-basal body completion and instead demands a minimal subset
of flagellar proteins that includes the FliF/FliG basal body proteins,
the flagellar type III export apparatus components FliO, FliP, FliQ, FliR,
FlhA, and FlhB, and the substrate specificity switch regulator FliK."
- term:
id: GO:0071978
label: bacterial-type flagellum-dependent swarming motility
evidence_type: IMP
original_reference_id: PMID:25313396
review:
summary: >-
This IMP annotation indicates FliK is involved in bacterial swarming motility
based on
PMID:25313396. While FliK is essential for functional flagella (through its
role in
hook length control and substrate switching), and functional flagella are
required for
swarming motility, this annotation may represent an indirect/downstream effect
rather
than a direct role in swarming. FliK's primary function is in flagellum assembly
(hook length control), not in the motility process itself. The paper's focus
is on
FlgM secretion and flagellar assembly, not specifically on swarming motility
assays.
action: KEEP_AS_NON_CORE
reason: >-
FliK is indirectly required for swarming motility because it is essential
for proper
flagellum assembly. Without FliK, hooks overgrow and flagella are non-functional,
which would impair swarming. However, FliK does not directly participate in
the
motility mechanism itself - its core function is hook length control and triggering
the substrate-specificity switch. This annotation captures a downstream phenotypic
effect rather than FliK's primary molecular function. The assembly annotation
(GO:0044780) more directly reflects FliK's biological role.
supported_by:
- reference_id: PMID:25313396
supporting_text: "FlgM secretion is strongly enhanced by, but does not strictly
require, hook-basal body completion and instead demands a minimal subset
of flagellar proteins that includes the FliF/FliG basal body proteins,
the flagellar type III export apparatus components FliO, FliP, FliQ, FliR,
FlhA, and FlhB, and the substrate specificity switch regulator FliK."
- reference_id: file:BACSU/fliK/fliK-deep-research-falcon.md
supporting_text: "FliK is the dedicated flagellar hook-length control protein
that times the transition from rod/hook-type export to filament-type export
by the flagellar type III secretion system (fT3SS). Loss of FliK causes
a polyhook phenotype because the export apparatus fails to switch substrates."
- term:
id: GO:0071978
label: bacterial-type flagellum-dependent swarming motility
evidence_type: IMP
original_reference_id: PMID:22730131
review:
summary: >-
This IMP annotation indicates FliK is involved in bacterial swarming motility
based on
PMID:22730131 (Courtney et al. 2012). This paper directly demonstrates in
B. subtilis
that "FliK regulated hook length" and shows that mutants affecting hook assembly
result
in reduced motility. However, like the other swarming annotation, this represents
an
indirect effect - FliK's primary function is hook length control, and the
motility
defect is a downstream consequence of faulty flagellum assembly.
action: KEEP_AS_NON_CORE
reason: >-
While PMID:22730131 directly demonstrates FliK's role in B. subtilis flagellum
biology,
its core finding is that "FliK regulated hook length." The swarming motility
defect in
fliK mutants is an indirect consequence of improper hook assembly rather than
a direct
role in the swarming process. The annotation is not incorrect but reflects
a downstream
phenotype rather than FliK's primary molecular function. Retaining as non-core
because
the experimental evidence does support the connection, even if indirect.
supported_by:
- reference_id: PMID:22730131
supporting_text: "We used the fluorescently labeled hook to demonstrate
that FlgE is the hook structural protein and that FliK regulated hook
length."
- reference_id: PMID:22730131
supporting_text: "All mutants defective in hook completion resulted in homogeneously
reduced sigma(D)-dependent gene expression due to the action of the anti-sigma
factor FlgM."
- term:
id: GO:0005515
label: protein binding
evidence_type: NAS
review:
summary: Added to align core_functions with existing annotations.
action: NEW
reason: Core function term not present in existing_annotations.
references:
- id: GO_REF:0000002
title: Gene Ontology annotation through association of InterPro records with
GO terms
findings:
- statement: Maps InterPro domain IPR001635 (Flag_hook_Flik) to GO terms
for flagellar hook and assembly
- id: GO_REF:0000043
title: Gene Ontology annotation based on UniProtKB/Swiss-Prot keyword
mapping
findings:
- statement: Maps UniProt keyword KW-1005 (Bacterial flagellum biogenesis)
to GO:0044781
- id: PMID:22730131
title: Molecular characterization of the flagellar hook in Bacillus
subtilis.
findings:
- statement: FlgE is the structural protein of the hook in B. subtilis
supporting_text: "We used the fluorescently labeled hook to demonstrate that
FlgE is the hook structural protein"
- statement: FliK regulates hook length in B. subtilis
supporting_text: "FliK regulated hook length"
- statement: FlhO, FlhP, and FlgD are required for hook assembly
supporting_text: "FlhO and FlhP, and the putative hook cap, FlgD, were required
for hook assembly"
- statement: Mutants defective in hook completion show reduced
sigma(D)-dependent gene expression
supporting_text: "All mutants defective in hook completion resulted in homogeneously
reduced sigma(D)-dependent gene expression due to the action of the anti-sigma
factor FlgM"
- id: PMID:25313396
title: FlgM is secreted by the flagellar export apparatus in Bacillus
subtilis.
findings:
- statement: FlgM is secreted in B. subtilis and degraded extracellularly
by Epr and WprA
supporting_text: "FlgM is also secreted in the Gram-positive bacterium Bacillus
subtilis and is degraded extracellularly by the proteases Epr and WprA"
- statement: Hook-basal body structural genes are required for robust
sigma(D)-dependent gene expression
supporting_text: "the structural genes required for the flagellar hook-basal
body are required for robust activation of sigma(D)-dependent gene expression
and efficient secretion of FlgM"
- statement: FliK is explicitly identified as the substrate specificity
switch regulator
supporting_text: "the substrate specificity switch regulator FliK"
- statement: FliK is part of the minimal subset required for FlgM
secretion
supporting_text: "FlgM secretion is strongly enhanced by, but does not strictly
require, hook-basal body completion and instead demands a minimal subset
of flagellar proteins that includes...the substrate specificity switch regulator
FliK"
core_functions:
- description: >-
FliK is essential for flagellum assembly through its dual role as a hook-length
control protein and substrate-specificity switch regulator. FliK is intermittently
exported through the fT3SS during hook assembly, and its C-terminal domain contacts
FlhBC to trigger the switch from rod/hook to filament substrate export. Demonstrated
in B. subtilis by Courtney et al. (2012) who showed "FliK regulated hook length"
using
fluorescent labeling, and by Calvo & Kearns (2015) who identified FliK as "the
substrate
specificity switch regulator" required for FlgM secretion.
molecular_function:
id: GO:0005515
label: protein binding
directly_involved_in:
- id: GO:0044780
label: bacterial-type flagellum assembly
locations:
- id: GO:0005737
label: cytoplasm
supported_by:
- reference_id: PMID:22730131
supporting_text: "We used the fluorescently labeled hook to demonstrate that
FlgE is the hook structural protein and that FliK regulated hook length."
- reference_id: PMID:25313396
supporting_text: "the substrate specificity switch regulator FliK"
- reference_id: file:BACSU/fliK/fliK-deep-research-falcon.md
supporting_text: "FliK is a cytosolic protein that is exported through the
basal-body fT3SS during hook assembly"
proposed_new_terms: []
suggested_questions:
- question: >-
What is the precise mechanism by which FliK senses hook length in B. subtilis?
Is it through physical spanning of the export channel (molecular ruler model)
or through export frequency and stochastic interactions (molecular clock model)?
Understanding this in B. subtilis, where hook and FliK are approximately 25%
longer
than Salmonella, could illuminate species-specific calibration of the length
control system.
- question: >-
Does FliK interact with FlgD (hook cap) in B. subtilis as it does in Salmonella?
What is the role of FlhO and FlhP (B. subtilis-specific proteins) in the FliK-mediated
switching mechanism? B. subtilis has unique hook assembly factors not found
in Gram-negative
bacteria, and understanding how these interact with the conserved FliK-FlhB
switching
mechanism could reveal Gram-positive-specific adaptations.
suggested_experiments:
- description: >-
Measure hook length distributions in B. subtilis fliK deletion, overexpression,
and
truncation mutants using cryo-EM or fluorescent labeling to quantify the length
control function. Expected outcomes: fliK deletion should produce polyhooks
(elongated
hooks), overexpression should produce shorter hooks, and C-terminal truncations
should
disrupt switching without affecting secretion.
- description: >-
Co-immunoprecipitation or bacterial two-hybrid to test FliK-FlhB interaction
in
B. subtilis, and test whether FlhO/FlhP affect this interaction. Expected outcome:
Should detect FliK-FlhB interaction dependent on the FliK C-terminal domain,
and
may reveal whether FlhO/FlhP modulate the switch timing.