ciliary transition fiber

GO:0097539 · Appendage · Multiprotein complex · Proposed

A nine-bladed, propeller-like protein complex that links the distal end of the basal body and the cilium to the plasma membrane. Functions in protein sorting and gating (i.e. active and passive transport of proteins in and out of the cilium). [GO:0097539]

Image (not hosted — OTHER): hal-03817184:figure3

Figure 3: TFK1 immunogold labeling near transition-fiber tips in procyclic trypanosomes. A-B are longitudinal and C-D transverse views. Main scale bars are 200 nm; inset bars are 100 nm. TEM · Trypanosoma brucei (NCBITaxon:5691) · Miharisoa Rijatiana Ramanantsalama, Nicolas Landrein, Elina Casas, Benedicte Salin, Corinne Blancard, Melanie Bonhivers, Derrick R. Robinson and Denis Dacheux; Journal of Cell Science, 2022; accepted manuscript deposited as HAL hal-03817184. · OTHER · source: OTHER hal-03817184:figure3 · DOI:10.1242/jcs.259893

Synonyms: centriolar distal appendage; cilial transition fiber; cilial transition fibre; ciliary transition fibre; cilium transition fiber; cilium transition fibre; distal appendage of basal body; distal appendage of centriole; distal appendage of mother centriole; transition fiber; transition fibre

Part of: ciliary basal body (GO:0036064)

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
transition fibre kinetoplastid-specific protein 1	Protein	part of	Reviewed label only (reviewed name; no accession asserted)	TFK1	UNKNOWN	Resident of the transition-fiber matrix region in Trypanosoma brucei. RNAi leaves gross fiber-region architecture intact; bloodstream-form lethality does not establish assembly essentiality. `DOI:10.1242/jcs.259893` Figures 2-3 localize TFK1 between CEP164C puncta and distal to RP2. Figure 5 tests its CC1 targeting region and self-interaction; RNAi is not a null allele.
CEP164 family protein	Protein	part of	Reviewed label only (reviewed name; no accession asserted)	CEP164A, CEP164B, CEP164C	UNKNOWN	Spatially and temporally differentiated transition-fiber proteins. The trypanosome paralogues are not interchangeable: CEP164C is associated with older flagella, whereas A and B also label the new flagellum base. `DOI:10.1242/jcs.259893` Figure 2 resolves a punctate CEP164C pattern distinct from the TFK1 matrix pattern. `DOI:10.1242/jcs.261692` Spatial mapping and cell-cycle analysis distinguish CEP164A, B and C; localization does not prove that every paralogue is required for fiber assembly.
Protein XRP2	Protein	part of	`InterPro:IPR039093`	RP2, XRP2	UNKNOWN	A proximal, approximately ring-like resident of the trypanosome transition-fiber region, spatially distinct from TFK1. `DOI:10.1242/jcs.259893` Figure 2 uses endogenously TY1-tagged RP2 to distinguish its distribution from TFK1. `InterPro:IPR039093` Authority-verified family type and canonical label Protein XRP2.
Fas-binding factor 1	Protein	part of	`InterPro:IPR033561`	FBF1	UNKNOWN	Transition-fiber-associated protein with an approximately ninefold arrangement around the trypanosome basal body. It supports normal new-flagellum assembly, but its exact placement in blade versus matrix and fiber-assembly requirement remain unresolved here. `DOI:10.1242/jcs.261692` Figure 1H expansion microscopy resolves the FBF1 pattern; Figure 5D shows strongly shortened new flagella after depletion. Neither establishes a specific molecular gating interaction in trypanosomes. `InterPro:IPR033561` Authority-verified FBF1 family identity; its mammalian and nematode mechanistic annotations are not transferred to the trypanosome.

Canonical examples

NCBITaxon:5691 Trypanosoma brucei — The 2022 study examines procyclic and bloodstream SmOx 427 cells, with high-resolution localization and domain targeting experiments in procyclic cells. [DOI:10.1242/jcs.259893]

Functions

basal-body membrane linkage — The radial transition-fiber complex links the distal basal body and cilium to the plasma membrane.
- GO:0097539 The exact cellular-component definition supplies this anatomical linking function; no individual protein is assigned direct lipid binding.
support of new flagellum assembly — Transition-fiber-associated proteins contribute to normal new-flagellum assembly in procyclic trypanosomes.
- DOI:10.1242/jcs.261692 RNAi of FBF1 and five other mapped proteins impairs new-flagellum assembly. This does not isolate fiber docking, prove loss of every blade, or establish a specific IFT-entry reaction.

Mechanism graphs

TFK1 domain-dependent targeting to transition fibers (ASSEMBLY)

Experimentally delimited component recruitment and self-interaction in the trypanosome transition-fiber assembly context.

Subject	Predicate	Object	Evidence
transition fibre kinetoplastid-specific protein 1	supports	TFK1 self-interaction in yeast two-hybrid	`DOI:10.1242/jcs.259893` Figure 5C, full-length versus full-length, with positive and negative assay controls.
TFK1 CC1-containing T4 fragment	supports	TFK1 self-interaction in yeast two-hybrid	`DOI:10.1242/jcs.259893` Figure 5C compares the domain constructs; CC1-containing T2 and T4 give interaction signals.
TFK1 CC1-containing T4 fragment	is sufficient for targeting to	ciliary transition fiber	`DOI:10.1242/jcs.259893` Figure 5B and supplementary Figure S5 compare localized CC1-containing constructs with non-retained fragments. This is not autonomous de novo fiber assembly.

Discussions

CURATION_TODO (RESOLVED): Separate transition fibers from the surrounding basal body and distal transition-zone linkers. Adjacent records mention this structure but do not denote the discrete radial appendage complex.
CURATION_TODO (OPEN): Resolve exact CEP164 and TFK1 families, strain-paired protein examples, and the remaining mapped component classes. The four curated families are not a complete inventory. Thirty proteins were mapped in 2024, in addition to previously studied RP2 and TFK1; localization, constitutive membership and assembly essentiality require separate judgements.
CURATION_TODO (OPEN): Determine native binding partners, self-association requirements, blade assembly dependencies and molecular gating reactions. Domain targeting, protein-depletion phenotypes and whole-fiber construction are different claims.
CURATION_TODO (RESOLVED): Do not infer a tubulin-binding reaction from basal-body YL1/2 staining. YL1/2 recognition of RP2 confounds interpreting disappearance of the basal-body signal as loss of a tubulin pool.

Evidence

GO:0097539 Exact structure definition and synonym scopes; distinct from transition-zone Y-links.
DOI:10.1242/jcs.259893 Ramanantsalama et al. 2022. Accepted manuscript, Results, Discussion, relevant Methods and Figures 2, 3 and 5 inspected.
PMID:35588197 PubMed bibliographic identity of the TFK1 primary study.
DOI:10.1242/jcs.261692 Ahmed et al. 2024. Official PMC11190437.1 JATS inspected with its metadata checksum verified; Figure 1 also inspected visually.
PMID:38572631 PubMed bibliographic identity of the trypanosome transition-fiber mapping study.

Curation history

2026-10-04T06:53:41Z codex · CREATED_RECORD — Scaffolded by scripts/new_record.py
2026-10-04T06:57:12Z codex · EXPANDED_RECORD — Added exact GO synonymy and basal-body parthood, four bounded component families, a trypanosome example, functions, TFK1 targeting and self-interaction graph, link-only immunogold microscopy and explicit assembly, grounding and antibody limitations.
2026-10-04T07:16:16Z codex · CORRECTED_EVIDENCE_SCOPE — Addressed review issue #2018: distinguish demonstrated yeast two-hybrid CC1/full-length interaction from the proposed endogenous targeting route.

Source: data/structures/appendage/ciliary_transition_fiber.yaml