bacterial-type flagellum filament

GO:0009420 · Appendage · Filament · Proposed

The long, thin external propeller of the bacterial-type flagellum, formed by polymerized flagellin. [DOI:10.1038/nrmicro1887]

Synonyms: flagellar filament; flagellin-based flagellum filament

Part of: bacterial-type flagellum (GO:0009288)

UniProt location names: UniProt Consortium, CC BY 4.0. Reference label and route sources.

Components

Component	Type	Part of / acts on	Grounding	Genes	Stoichiometry	Essential	Role
flagellin (FliC / FljB)	Protein	part of	`InterPro:IPR001492`	fliC, fljB	~20,000 subunits per filament; usually 11 protofilaments	ESSENTIAL	Polymerizes into the tubular, helical propeller filament of the bacterial flagellum. `DOI:10.1146/annurev.micro.57.030502.090832` Macnab 2003 reviews flagellin export through the central channel and polymerization under the FliD cap. `DOI:10.3390/biom11020186` Halte and Erhardt 2021 review the approximate number of flagellin subunits in a long bacterial flagellar filament. `DOI:10.1038/s41598-019-51440-1` Blum et al. 2019 resolved an in vivo flagellar filament with 11 protofilaments in Kurthia. `DOI:10.1038/s41467-026-71203-7` Qin et al. 2026 resolved Vibrio alginolyticus flagellin packing by cryo-EM as independent structural evidence for flagellin as the bacterial filament constituent.

Organism-specific protein examples

Named proteins in which each component's role was established, with the organism the evidence comes from. The component above is the taxon-agnostic family; these are the specific entries behind it.

flagellin (FliC / FljB)

Accession	Protein	Gene	Organism	Entry
`UniProtKB:P06179`	Flagellin	fliC	Salmonella enterica subsp. enterica serovar Typhimurium str. LT2 (`NCBITaxon:99287`)	REVIEWED
Role and evidence (1 reference) The single reviewed UniProtKB entry for gene fliC in Salmonella enterica subsp. enterica serovar Typhimurium str. LT2, a taxon this record names. Identifier for the component's declared gene symbol; it is not itself evidence that the protein is part of this structure. `https://www.uniprot.org/uniprotkb/P06179` UniProtKB reviewed (Swiss-Prot) entry P06179, the single reviewed entry for gene fliC in Salmonella enterica subsp. enterica serovar Typhimurium str. LT2. Establishes the accession and gene identity only; the component's membership in this structure rests on the record's own sources.
`UniProtKB:P52616`	Phase 2 flagellin	fljB	Salmonella enterica subsp. enterica serovar Typhimurium str. LT2 (`NCBITaxon:99287`)	REVIEWED
Role and evidence (1 reference) The single reviewed UniProtKB entry for gene fljB in Salmonella enterica subsp. enterica serovar Typhimurium str. LT2, a taxon this record names. Identifier for the component's declared gene symbol; it is not itself evidence that the protein is part of this structure. `https://www.uniprot.org/uniprotkb/P52616` UniProtKB reviewed (Swiss-Prot) entry P52616, the single reviewed entry for gene fljB in Salmonella enterica subsp. enterica serovar Typhimurium str. LT2. Establishes the accession and gene identity only; the component's membership in this structure rests on the record's own sources.

Taxonomic distribution

NCBITaxon:2 Bacteria — VARIABLE: Bacterial flagellar systems have a patchy distribution: a survey of 30,238 reference genomes from 131 phyla found FliPQR export markers in about 40%. This is genomic marker evidence, not direct observation of assembled external filaments. Marker presence does not establish this particular structure in every genome. This record retains the external-filament scope of GO:0009420; periplasmic spirochete filaments are curated separately. [DOI:10.1073/pnas.2512476122]
NCBITaxon:2157 Archaea — ABSENT: Archaea swim with archaella, which are type-IV-pilus-related appendages rather than homologs of bacterial flagellin filaments. [DOI:10.3389/fmicb.2015.00023]

Canonical examples

NCBITaxon:90371 Salmonella enterica subsp. enterica serovar Typhimurium — Classical genetic model for bacterial flagellum assembly, including filament growth through the central channel. [DOI:10.1146/annurev.micro.57.030502.090832]
NCBITaxon:83334 Escherichia coli O157:H7 — Source serotype for purified H7 flagellar filaments characterized by microscopy and immunoassay. [DOI:10.1128/jcm.26.7.1367-1372.1988]

Functions

helical propeller thrust — Motor-driven rotation of the helical filament generates thrust for swimming motility.
- DOI:10.1146/annurev.biochem.72.121801.161737 Berg 2003 reviews how rotation of a helical bacterial flagellar filament propels swimming cells.

Mechanism graphs

Flagellin protofilaments form a rotary propeller (FUNCTION)

Flagellin subunits pack into protofilaments that form a long tubular helix. When the basal body transmits torque through the hook, rotation of that helical filament generates thrust for swimming motility.

Subject	Predicate	Object	Evidence
flagellin	polymerizes into	flagellin protofilament	`DOI:10.1038/s41598-019-51440-1` Blum et al. 2019 resolved repeating flagellin subunits in an 11-protofilament Kurthia filament. `DOI:10.1038/s41467-026-71203-7` Qin et al. 2026 resolved Vibrio alginolyticus flagellin packing by cryo-EM.
flagellin protofilament	packs into	bacterial-type flagellum filament	`DOI:10.1038/s41598-019-51440-1` Blum et al. 2019 describe the Kurthia flagellar filament as a tube built from 11 protofilaments.
flagellin	extends	bacterial-type flagellum filament	`DOI:10.1146/annurev.micro.57.030502.090832` Macnab 2003 reviews filament growth by flagellin export through the flagellar channel and assembly beneath FliD.
flagellar rotation	turns	bacterial-type flagellum filament	`DOI:10.1146/annurev.biochem.72.121801.161737` Berg 2003 reviews the bacterial flagellar motor and helical filament as the rotary swimming organelle.
bacterial-type flagellum filament	generates	propulsive thrust	`DOI:10.1146/annurev.biochem.72.121801.161737` Berg 2003 reviews the hydrodynamic role of helical flagellar filaments in swimming.
propulsive thrust	enables	bacterial-type flagellum-dependent cell motility	`DOI:10.1146/annurev.biochem.72.121801.161737` Berg 2003 reviews swimming by rotating flagellar filaments.

Discussions

CURATION_TODO (RESOLVED): Should the hook, hook-filament junction, or FliD cap be curated as bacterial-type flagellum filament components? The filament record is scoped to the flagellin polymer. FlgE, FlgK/FlgL, and FliD build adjoining or terminal axial substructures: the hook and hook-filament junction are separate flagellar substructures, and FliD forms the GO:0009421 filament-cap substructure of the enclosing bacterial-type flagellum rather than a flagellin-filament component.

Evidence

DOI:10.1146/annurev.micro.57.030502.090832 Macnab RM 2003, How bacteria assemble flagella.
DOI:10.1146/annurev.biochem.72.121801.161737 Berg HC 2003, The rotary motor of bacterial flagella.
DOI:10.1038/s41598-019-51440-1 Blum et al. 2019, The wild-type flagellar filament of the Firmicute Kurthia at 2.8 A resolution in vivo.
DOI:10.1038/s41467-026-71203-7 Qin et al. 2026, The structure of the Vibrio alginolyticus flagellar filament suggests molecular mechanism for the rotation of sheathed flagella.

Curation history

2026-09-07T03:11:54Z claude · CREATED_RECORD — Scaffolded by scripts/new_record.py
2026-09-07T03:12:34Z claude · EXPANDED_RECORD — Added GO filament synonymy, bacterial-flagellum parthood, the flagellin polymer constituent with InterPro grounding, bacterial and archaeal taxonomic scope, Salmonella and Escherichia examples, helical propeller function, structural and assembly evidence, a flagellin-to-propeller causal graph, and a boundary discussion excluding the hook and cap.
2026-09-07T03:22:35Z claude · FIX_REVIEW_ISSUES — Resolved #230 by restoring the exact GO:0009420 definition text; resolved #231 by adding Zhu et al. 2026 Vibrio filament cryo-EM evidence to the flagellin component.
2026-09-08T07:38:47Z uniprot_genes · SEED_PROTEIN_EXAMPLES — #183: added 2 protein example(s) by resolving each of the component's declared gene symbols to the single reviewed UniProtKB entry for that symbol in a taxon this record names, deduplicated by accession. Symbols resolving to more than one reviewed entry were skipped rather than chosen between.
2026-09-09T02:54:06Z claude · ADD_SUBSTRUCTURE_LINK — Added the GO:0009421 bacterial-type flagellum filament cap record to the bacterial-type flagellum filament has_part list.
2026-09-09T04:27:53Z claude · ADDRESS_REVIEW — Resolved #289 by removing GO:0009421 from the GO:0009420 filament has_part list and clarifying that the FliD cap is a separate top-level flagellar substructure rather than a flagellin-filament component.
2026-10-10T22:10:21Z claude · ADDRESS_SAVED_REVIEW — Addressed #2147, #2148, #2149: Three filament sources have incorrect author attribution; Inherited definition places a nanometer dimension next to length; Bacteria-wide COMMON is not supported by a flagellated-lineage statement. Preserved historical events and PROPOSED status.

Source: data/structures/appendage/bacterial_type_flagellum_filament.yaml