bacterial-type flagellum hook

GO:0009424 · Appendage · Filament · Proposed

The portion of the bacterial-type flagellum that connects the filament to the basal body. [GO:0009424]

Synonyms: flagellar hook; flagellin-based flagellum hook

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
hook protein FlgE	Protein	part of	Reviewed label only (reviewed name; no accession asserted)	flgE		ESSENTIAL	Polymerizes into the short, curved tubular hook that connects the basal body to the external filament. `DOI:10.1146/annurev.micro.57.030502.090832` Macnab 2003 reviews the hook as the FlgE-built flexible connector between the flagellar basal body and filament. `DOI:10.1038/ncomms13425` Matsunami et al. 2016 resolved the Campylobacter jejuni hook as a helical tube built from FlgE. `DOI:10.3390/biom9090462` Horvath et al. 2019 resolved the Salmonella flagellar hook and modeled FlgE intermolecular contacts that support universal-joint mechanics.

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.

hook protein FlgE

Accession	Protein	Gene	Organism	Entry
`UniProtKB:P0A1J1`	Flagellar hook protein FlgE	flgE	Salmonella enterica subsp. enterica serovar Typhimurium str. LT2 (`NCBITaxon:99287`)	REVIEWED
Role and evidence (1 reference) The single reviewed UniProtKB entry for gene flgE 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/P0A1J1` UniProtKB reviewed (Swiss-Prot) entry P0A1J1, the single reviewed entry for gene flgE 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 FlgE hooks. Marker presence does not establish this particular structure in every genome. [DOI:10.1073/pnas.2512476122]
NCBITaxon:2157 Archaea — ABSENT: Archaeal archaella are type-IV-pilus-related appendages and do not use the FlgE hook of bacterial flagella. [DOI:10.3389/fmicb.2015.00023]

Canonical examples

NCBITaxon:197 Campylobacter jejuni — Source species for the complete cryo-EM structure of a bacterial flagellar hook. [DOI:10.1038/ncomms13425]
NCBITaxon:90371 Salmonella enterica subsp. enterica serovar Typhimurium — Model species used to resolve hook intermolecular interactions and native hook supercoiling. [DOI:10.3390/biom9090462]

Functions

universal-joint torque transmission — The hook bends while remaining torsionally stiff enough to transmit motor rotation from the basal body to the helical filament.
- DOI:10.1038/s41467-019-13252-9 Kato et al. 2019 resolved the native supercoiled Salmonella hook and linked its FlgE lattice to universal-joint bending and torque transmission.
- DOI:10.3390/biom9090462 Horvath et al. 2019 analyzed Salmonella FlgE contacts that explain hook flexibility during universal-joint function.

Mechanism graphs

FlgE builds the torque-transmitting hook (FUNCTION)

FlgE subunits polymerize into a short curved tube. The resulting hook connects the basal body and filament, bends as the filament changes angle, and transmits motor torque into rotation of the filament propeller.

Subject	Predicate	Object	Evidence
hook protein FlgE	polymerizes into	bacterial-type flagellum hook	`DOI:10.1038/ncomms13425` Matsunami et al. 2016 resolved a complete Campylobacter jejuni hook built from polymerized FlgE.
hook protein FlgE	forms	FlgE intermolecular interfaces	`DOI:10.3390/biom9090462` Horvath et al. 2019 modeled Salmonella FlgE intermolecular domain interactions in the hook.
FlgE intermolecular interfaces	stabilize	bacterial-type flagellum hook	`DOI:10.1038/s41467-019-13252-9` Kato et al. 2019 linked the native supercoiled hook lattice to a flexible universal joint.
bacterial-type flagellum hook	connects	bacterial-type flagellum basal body	`DOI:10.1146/annurev.micro.57.030502.090832` Macnab 2003 reviews the hook as the axial substructure connecting the basal body to the external filament.
bacterial-type flagellum hook	connects	bacterial-type flagellum filament	`DOI:10.1146/annurev.micro.57.030502.090832` Macnab 2003 reviews assembly of the hook, hook-filament junction and filament.
bacterial-type flagellum hook	transmits torque to	bacterial-type flagellum filament	`DOI:10.1038/s41467-019-13252-9` Kato et al. 2019 resolved the native hook conformation used to explain universal-joint torque transmission.
bacterial-type flagellum hook	transmits torque into	bacterial flagellar rotation	`DOI:10.1038/s41467-019-13252-9` Kato et al. 2019 resolved the native hook conformation used to explain universal-joint torque transmission.
bacterial 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 flagellar rotation	drives	bacterial-type flagellum-dependent cell motility	`DOI:10.1146/annurev.biochem.72.121801.161737` Berg 2003 reviews swimming by rotating helical flagellar filaments.

Discussions

CURATION_TODO (RESOLVED): Should FlgD, FlgK, or FlgL be curated as bacterial-type flagellum hook components? The GO:0009424 hook record is scoped to the FlgE tube. FlgD forms the hook cap during assembly, and FlgK/FlgL form the hook-filament junction after hook completion; each is adjacent to, rather than part of, the mature hook.

Evidence

DOI:10.1146/annurev.micro.57.030502.090832 Macnab RM 2003, How bacteria assemble flagella.
DOI:10.1038/ncomms13425 Matsunami H et al. 2016, Complete structure of the bacterial flagellar hook reveals extensive set of stabilizing interactions.
DOI:10.3390/biom9090462 Horvath P, Kato T, Miyata T and Namba K 2019, Structure of Salmonella flagellar hook reveals intermolecular domain interactions for the universal joint function.
DOI:10.1038/s41467-019-13252-9 Kato T et al. 2019, Structure of the native supercoiled flagellar hook as a universal joint.

Curation history

2026-09-07T03:32:32Z claude · CREATED_RECORD — Scaffolded by scripts/new_record.py
2026-09-07T03:33:33Z claude · EXPANDED_RECORD — Added GO hook synonymy, bacterial-flagellum parthood, the FlgE hook protein component with the reviewed label-only grounding decision, bacterial and archaeal taxonomic scope, Campylobacter and Salmonella examples, universal-joint torque-transmission function, structural hook evidence, a FlgE-to-filament causal graph, and a boundary discussion excluding the hook cap and hook-filament junction.
2026-09-07T04:11:02Z claude · FIX_REVIEW_ISSUES — Resolved #233 by localizing the FlgE label-only grounding note to the standalone hook scope; resolved #234 by inserting an explicit flagellar-rotation process between hook torque transmission and motility.
2026-09-08T07:38:48Z uniprot_genes · SEED_PROTEIN_EXAMPLES — #183: added 1 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.
2026-10-10T22:10:27Z claude · ADDRESS_SAVED_REVIEW — Addressed #2151: Bacteria-wide COMMON is not supported by a flagellated-lineage statement. Preserved historical events and PROPOSED status.

Source: data/structures/appendage/bacterial_type_flagellum_hook.yaml