periplasmic flagellum

GO:0055040 · Appendage · Appendage · Proposed

A bacterial-type flagellar appendage of spirochetes whose filaments reside in the periplasmic space between the cytoplasmic and outer membranes, attach near the cell poles, and rotate to deform the cell cylinder for motility. [DOI:10.3390/biom10040550]

Synonyms: endoflagellum; periplasmic flagellar filament

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

Components

Component	Type	Part of / acts on	Grounding	Genes	Essential	Role
FlaB core flagellins	Protein	part of	Reviewed label only (reviewed name; no accession asserted)	flaB	ESSENTIAL	Major flagellin class that polymerizes into the core of the periplasmic flagellar filament. `DOI:10.3390/biom10040550` Nakamura 2020 reviews FlaB proteins as the conserved spirochete periplasmic flagellar core proteins. `DOI:10.1073/pnas.200221797` Motaleb et al. 2000 disrupted Borrelia burgdorferi flaB and showed that periplasmic flagella are required for both motility and the normal flat-wave cell shape.
FlaA flagellar proteins	Protein	part of	Reviewed label only (reviewed name; no accession asserted)	flaA	UNKNOWN	Flagellar constituents whose architecture varies by lineage. FlaA forms a sheath around the FlaB core in Brachyspira hyodysenteriae; Borrelia burgdorferi FlaA is a minor filament-associated protein with unresolved precise localization, and Leptospira sheath formation does not require FlaA in the cited deletion studies. `DOI:10.3390/biom10040550` Nakamura 2020 distinguishes Brachyspira FlaA sheath architecture from tentative basal FlaA localization in Borrelia and a Leptospira sheath that persists after flaA deletion, with FcpA a major sheath constituent. `DOI:10.1128/JB.180.9.2418-2425.1998` Ge et al. 1998 copurified FlaA with Borrelia burgdorferi periplasmic flagella but did not establish a sheath location. The proposed localization near the hook was explicitly tentative.
spirochete flagellar basal body motor	Protein complex	part of	Reviewed label only (reviewed name; no accession asserted)		ESSENTIAL	Polar rotary basal body that anchors the periplasmic filament and transmits torque to it. `DOI:10.1371/journal.pbio.3000050` Qin et al. 2018 used in situ cryoET of Borrelia burgdorferi to visualize the periplasmic flagellar basal body, collar, and distinct cytoplasmic ATPase complex.

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.

FlaA flagellar proteins

Accession	Protein	Gene	Organism	Entry
`UniProtKB:P70856`	Flagellar filament outer layer protein	flaA	Borreliella burgdorferi B31 (`NCBITaxon:224326`)	REVIEWED
Role and evidence (1 reference) The single reviewed UniProtKB entry for gene flaA in Borreliella burgdorferi B31, 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/P70856` UniProtKB reviewed (Swiss-Prot) entry P70856, the single reviewed entry for gene flaA in Borreliella burgdorferi B31. Establishes the accession and gene identity only; the component's membership in this structure rests on the record's own sources.

Taxonomic distribution

NCBITaxon:203691 Spirochaetota — COMMON: Periplasmic flagella are characteristic endoflagella of motile spirochetes and reside between the inner and outer membranes. [DOI:10.3390/biom10040550]
NCBITaxon:2 Bacteria — RARE: The organelle is a spirochete-specific form of the bacterial flagellum rather than a general bacterial appendage. [DOI:10.3390/biom10040550]

Canonical examples

NCBITaxon:139 Borreliella burgdorferi — Qin et al. imaged intact periplasmic flagella and basal-body motors in Borrelia burgdorferi cells by cryo-electron tomography. [DOI:10.1371/journal.pbio.3000050]

Functions

bacterial-type flagellum-dependent cell motility GO:0071973 — Internal flagellar rotation within the periplasm deforms the spirochete cell cylinder and produces swimming motility.
- DOI:10.3390/biom10040550 Nakamura 2020 reviews propulsion by rotation of periplasmic flagella inside the narrow spirochete periplasmic space.
- DOI:10.1073/pnas.200221797 Motaleb et al. 2000 found flaB mutants nonmotile and rod-shaped, but CCCP-immobilized wild-type cells retained flat-wave shape. The skeletal shape contribution therefore does not require rotation and is distinct from dynamic deformation during swimming.

Mechanism graphs

Periplasmic flagellar rotation drives dynamic cell-body deformation during swimming (FUNCTION)

FlaB builds the periplasmic filament; FlaA association and sheath architecture differ across spirochete lineages. The polar basal body drives filament rotation and dynamic cell-body deformation during swimming. This motility pathway is distinct from the rotation-independent skeletal contribution to resting Borrelia shape.

Subject	Predicate	Object	Evidence
FlaB core flagellins	polymerizes into	periplasmic flagellar filament	`DOI:10.3390/biom10040550` Nakamura 2020 reviews the conserved FlaB core of spirochete periplasmic flagellar filaments.
FlaA flagellar proteins	associates with	periplasmic flagellar filament	`DOI:10.1128/JB.180.9.2418-2425.1998` Ge et al. copurified Borrelia FlaA with flagella but left its precise localization unresolved. `DOI:10.3390/biom10040550` Nakamura distinguishes Brachyspira FlaA sheath architecture, tentative basal localization in Borrelia and Leptospira sheath retention after FlaA deletion.
periplasmic flagellar filament	is part of	periplasmic flagellum	`DOI:10.1371/journal.pbio.3000050` Qin et al. 2018 resolved filaments connected to polar periplasmic flagellar motors in Borrelia burgdorferi.
spirochete flagellar basal body motor	drives	flagellar rotation	`DOI:10.1371/journal.pbio.3000050` Qin et al. 2018 imaged Borrelia burgdorferi periplasmic flagella attached to basal bodies with the spirochete collar and cytoplasmic ATPase complex.
flagellar rotation	turns	periplasmic flagellar filament	`DOI:10.3390/biom10040550` Nakamura 2020 reviews rotation of endoflagella inside the spirochete periplasmic space.
flagellar rotation	produces	dynamic spirochete cell-body deformation during swimming	`DOI:10.3390/biom10040550` Section 4.1 reviews wave propagation driven by periplasmic flagellar rotation in Borrelia and Brachyspira. This swimming mechanism is distinct from maintenance of resting flat-wave shape. `DOI:10.1073/pnas.200221797` Boundary evidence, not proof of this active edge: Motaleb et al. found that CCCP-paralyzed Borrelia cells retain flat-wave shape. The resting skeletal contribution therefore does not require rotation.
dynamic spirochete cell-body deformation during swimming	drives	bacterial-type flagellum-dependent cell motility	`DOI:10.3390/biom10040550` Nakamura 2020 reviews fluid and surface propulsion driven by periplasmic flagellar rotation inside spirochetes.

Evidence

DOI:10.3390/biom10040550 Nakamura 2020, Spirochete Flagella and Motility, Biomolecules.
DOI:10.1371/journal.pbio.3000050 Qin et al. 2018, Cryo-electron tomography of periplasmic flagella in Borrelia burgdorferi reveals a distinct cytoplasmic ATPase complex, PLOS Biology.
DOI:10.1073/pnas.200221797 Motaleb et al. 2000, Borrelia burgdorferi periplasmic flagella have both skeletal and motility functions, Proceedings of the National Academy of Sciences.
DOI:10.1016/j.tim.2022.09.010 San Martin et al. 2023, Diving into the complexity of the spirochetal endoflagellum, Trends in Microbiology.

Curation history

2026-09-06T11:38:55Z claude · CREATED_RECORD — Scaffolded by scripts/new_record.py
2026-09-06T11:40:14Z claude · EXPANDED_RECORD — Added periplasmic flagellum synonyms, bacterial flagellum parentage, FlaB, FlaA, and basal-body motor components, spirochete taxonomy, a Borrelia B31 example, a motility function, record evidence, and a cell-deformation graph.
2026-09-06T11:41:34Z claude · TIGHTENED_SCOPE — Moved the Borrelia canary to species-level NCBITaxon:139 and marked FlaA sheath essentiality unknown to avoid a broad spirochete assertion.
2026-09-06T12:37:44Z claude · FIXED_CAUSAL_GRAPH — Replaced the direct filament-to-deformation edge with flagellar rotation edges so the periplasmic graph places cell-body deformation downstream of an active rotation process.
2026-09-08T07:38:54Z 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.
2026-10-11T04:58:45Z codex · ADDRESS_SAVED_REVIEW — Addressed #2173 and #2174: qualified lineage-dependent FlaA architecture, removed generic sheathed-filament assertions, and separated static skeletal shape evidence from rotation-driven swimming deformation while retaining the active pathway introduced for #203.

Source: data/structures/appendage/periplasmic_flagellum.yaml