ciliary pocket collar

GO:1990900 · Cytoskeleton · Multiprotein complex · Proposed

A constriction site at the junction of the plasma, flagellar and flagellar pocket membranes where the flagellum emerges from the cell body. Observed in some unicellular eukaryotic species such as Chlamydomonas, Giardia and Trypanosoma. [GO:1990900]

Synonyms: cilium pocket collar; flagellar pocket collar; flagellum pocket collar; FPC

Part of: flagellar pocket (GO:0020016)

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
BILBO1 flagellar pocket collar scaffold	Protein	part of	Reviewed label only (reviewed name; no accession asserted)	BILBO1	ESSENTIAL	A calcium-binding, polymer-forming scaffold protein that forms the first characterized framework for the trypanosome FPC. `DOI:10.1371/journal.pbio.0060105` Bonhivers et al. 2008 identified BILBO1 as a Trypanosoma brucei flagellar pocket cytoskeletal protein and showed that BILBO1 RNAi blocks new FPC and flagellar-pocket biogenesis. `DOI:10.1371/journal.ppat.1004654` Florimond et al. 2015 showed that the BILBO1 coiled-coil domain can target BILBO1 to the FPC and is sufficient to drive BILBO1 polymer formation.
FPC4 microtubule-binding FPC protein	Protein	part of	Reviewed label only (reviewed name; no accession asserted)	FPC4		A BILBO1-interacting protein that localizes to the FPC and hook complex and is proposed to link the collar to microtubule-associated cytoskeletal elements. `DOI:10.1371/journal.ppat.1006710` Albisetti et al. 2017 identified FPC4 as a BILBO1 partner whose C-terminal domain interacts with the BILBO1 N-terminal domain and whose N-terminal domain binds microtubules. `DOI:10.1074/jbc.RA119.010768` Vidilaseris et al. 2020 mapped the BILBO1 N-terminal-domain surface used by the FPC4 BILBO1-binding domain.
BILBO2 flagellar pocket collar partner	Protein	part of	Reviewed label only (reviewed name; no accession asserted)	BILBO2, Tb927.6.3240		A second BILBO-family FPC component that partners with BILBO1 and FPC4 in a tripartite FPC protein complex. `DOI:10.1371/journal.ppat.1009329` Isch et al. 2021 characterized BILBO2/Tb927.6.3240 as a BILBO1 partner and FPC component whose N-terminal domain also binds the FPC4 BILBO1-binding domain.

Taxonomic distribution

NCBITaxon:5691 Trypanosoma brucei — COMMON: BILBO1, FPC4 and BILBO2 have been characterized as proteins associated with the flagellar pocket collar in Trypanosoma brucei. [DOI:10.1371/journal.ppat.1009329]

Canonical examples

NCBITaxon:5691 Trypanosoma brucei — Trypanosome model in which BILBO1 RNAi established that FPC assembly is required for flagellar-pocket biogenesis. [DOI:10.1371/journal.pbio.0060105]

Functions

flagellar pocket neck scaffolding — The FPC forms a cytoskeletal collar at the flagellar pocket neck where the flagellum exits the cell body.
- DOI:10.3390/cells5010009 Perdomo et al. 2016 reviewed the T. brucei FPC as an essential cytoskeletal component at the neck of the flagellar pocket.
flagellar pocket biogenesis — BILBO1-dependent FPC assembly supplies a cytoskeletal framework required to build the trypanosome flagellar pocket.
- DOI:10.1371/journal.pbio.0060105 Bonhivers et al. 2008 showed that BILBO1 RNAi prevents formation of new FPC and FP structures in procyclic T. brucei and is lethal in procyclic and bloodstream forms.
pocket-hook microtubule association — FPC4 binds microtubules in vitro and localizes at the interface of the FPC, hook complex and proximal microtubule quartet.
- DOI:10.1371/journal.ppat.1006710 Albisetti et al. 2017 localized FPC4 to the FPC, hook complex and possible proximal microtubule quartet and separately found that its N-terminal domain binds microtubules.

Mechanism graphs

BILBO1 polymers recruit partner proteins at the FPC (ASSEMBLY)

BILBO1 dimerizes and polymerizes into a scaffold at the flagellar pocket collar. FPC4 binds the BILBO1 N-terminal domain and microtubules, while BILBO2 adds a second BILBO-family FPC component that shares the FPC4 binding interface.

Subject	Predicate	Object	Evidence
BILBO1	dimerizes into	BILBO1 antiparallel dimer	`DOI:10.1074/jbc.M114.554659` Vidilaseris et al. 2014 dissected BILBO1 domains and linked the coiled coil and leucine zipper to dimerization and filament formation.
BILBO1 antiparallel dimer	polymerizes into	BILBO1 polymer	`DOI:10.1074/jbc.M114.554659` Vidilaseris et al. 2014 connected BILBO1 leucine-zipper interdimer interactions with extended in-vitro filament assembly.
BILBO1 polymer	scaffolds	ciliary pocket collar	`DOI:10.1371/journal.ppat.1004654` Florimond et al. 2015 showed that BILBO1 has intrinsic polymer-forming properties and that its coiled-coil domain targets BILBO1 to the FPC.
FPC4	binds	BILBO1	`DOI:10.1371/journal.ppat.1006710` Albisetti et al. 2017 identified FPC4 as a BILBO1 partner and mapped the interaction to the FPC4 C-terminal domain and BILBO1 N-terminal domain.
FPC4	binds	microtubules	`DOI:10.1371/journal.ppat.1006710` Albisetti et al. 2017 localized FPC4 to the FPC and hook complex, noted possible proximal microtubule-quartet localization, and found that the FPC4 N-terminal domain binds microtubules.
BILBO2	binds	FPC4	`DOI:10.1371/journal.ppat.1009329` Isch et al. 2021 demonstrated BILBO1, BILBO2 and FPC4 interactions and solved the BILBO2 NTD-FPC4 complex.
ciliary pocket collar	is required for	flagellar pocket biogenesis	`DOI:10.1371/journal.pbio.0060105` Bonhivers et al. 2008 showed that BILBO1 RNAi prevents new FPC and flagellar pocket biogenesis in procyclic trypanosomes.

Discussions

CURATION_TODO (OPEN): Can BILBO1, BILBO2 or FPC4 be grounded to exact whole-protein families? InterPro currently resolves only the BILBO N-terminal domain term InterPro:IPR040747 / Pfam:PF18281 from the BILBO-family search used for this record. The domain is shared by BILBO1 and BILBO2 and should not be asserted as a whole-protein family for either component; no exact FPC4 family accession was curated.

Evidence

DOI:10.1371/journal.pbio.0060105 Bonhivers et al. 2008, Biogenesis of the trypanosome endo-exocytotic organelle is cytoskeleton mediated, PLoS Biology.
DOI:10.1371/journal.ppat.1004654 Florimond et al. 2015, BILBO1 is a scaffold protein of the flagellar pocket collar in the pathogen Trypanosoma brucei, PLOS Pathogens.
DOI:10.3390/cells5010009 Perdomo, Bonhivers and Robinson 2016, The Trypanosome Flagellar Pocket Collar and Its Ring Forming Protein-TbBILBO1, Cells.
DOI:10.1371/journal.ppat.1006710 Albisetti et al. 2017, Interaction between the flagellar pocket collar and the hook complex via a novel microtubule-binding protein in Trypanosoma brucei, PLOS Pathogens.
DOI:10.1371/journal.ppat.1009329 Isch et al. 2021, Structural and functional studies of the first tripartite protein complex at the Trypanosoma brucei flagellar pocket collar, PLOS Pathogens.
DOI:10.1074/jbc.M114.554659 Vidilaseris et al. 2014, Assembly mechanism of Trypanosoma brucei BILBO1, a multidomain cytoskeletal protein, Journal of Biological Chemistry.
DOI:10.1074/jbc.RA119.010768 Vidilaseris et al. 2020, Crystal structure of the N-terminal domain of the trypanosome flagellar protein BILBO1 reveals a ubiquitin fold with a long structured loop for protein binding, Journal of Biological Chemistry.

Curation history

2026-09-09T19:34:41Z claude · CREATED_RECORD — Scaffolded by scripts/new_record.py
2026-09-09T19:35:57Z claude · EXPANDED_RECORD — Added GO synonymy and ciliary-pocket parthood; curated BILBO1, FPC4 and BILBO2 as flagellar pocket collar constituents; added eukaryotic and Trypanosoma taxonomic scope, a T. brucei canonical example, FPC scaffold and biogenesis functions, core literature evidence, a BILBO1/FPC4/BILBO2 assembly graph, and a grounding discussion for the unresolved FPC protein families.
2026-09-10T01:38:03Z claude · ADDRESS_QC — Split the BILBO1 assembly graph into BILBO1 dimerization and BILBO1-dimer polymerization edges so the graph no longer points a dimerization predicate directly at the final polymer node.
2026-09-10T01:42:07Z claude · ADDRESS_REVIEW — Resolved #295 by narrowing the FPC4 graph edge to microtubule binding plus FPC/hook/proximal-MtQ localization; resolved #296 by removing the broad Eukaryota taxonomic-distribution row that was backed only by GO term scope.
2026-10-11T23:33:47Z codex · ADDRESS_SAVED_REVIEW — Addressed #2267, #2268: corrected the three-protein occurrence citation and qualified the proposed FPC4 native linking role.

Source: data/structures/cytoskeleton/ciliary_pocket_collar.yaml