axonemal microtubule doublet inner junction

GO:0160113 · Cytoskeleton · Multiprotein complex · Proposed

A structure that joins the B10 protofilament of the B tubule to the A1 protofilament of the A tubule within an axonemal microtubule doublet. [GO:0160113]

Part of: axonemal doublet microtubule (GO:0097545)

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
inner-junction proteins	Protein	part of	Reviewed label only (reviewed name; no accession asserted)			UNKNOWN	Protein families that build the inner-junction bridge between the A- and B-tubules of an axonemal doublet. `GO:0160113` GO defines the axonemal microtubule doublet inner junction by its position between the B-tubule B10 protofilament and the A-tubule A1 protofilament. `DOI:10.1091/mbc.E13-08-0464` Yanagisawa et al. 2014 identified Chlamydomonas FAP20 as an inner-junction component by mutant electron microscopy and cryo-ET localization. `DOI:10.1091/mbc.E19-01-0063` Dymek et al. 2019 used Chlamydomonas pacrg and fap20 mutants to show that PACRG and FAP20 form the inner-junction bridge along all nine ciliary doublets. `DOI:10.7554/eLife.52760` Khalifa et al. 2020 combined Chlamydomonas and Tetrahymena structural and proteomic evidence to assign FAP276, FAP106 and FAP126 to inner-junction or inner-junction-associated positions.

Taxonomic distribution

NCBITaxon:3055 Chlamydomonas reinhardtii — COMMON: Chlamydomonas flagellar doublets contain an inner-junction bridge whose FAP20 and PACRG constituents were localized by mutant and cryo-electron tomography studies. [DOI:10.1091/mbc.E19-01-0063]

Canonical examples

NCBITaxon:3055 Chlamydomonas reinhardtii — Green-algal model in which FAP20, PACRG and additional inner-junction-associated proteins have been localized within axonemal doublet microtubules. [DOI:10.7554/eLife.52760]

Functions

A/B-tubule junction stability — The inner-junction protein bridge connects the B-tubule B10 protofilament to the A-tubule A1 protofilament and helps stabilize the axonemal doublet for ciliary and flagellar motility.
- GO:0160113 GO defines the inner junction as the structure that joins the B10 protofilament to A1 within an axonemal doublet.
- DOI:10.1091/mbc.E13-08-0464 Yanagisawa et al. 2014 reported that Chlamydomonas fap20 axonemes lack the inner junction and are liable to disintegrate during beating.
- DOI:10.1091/mbc.E19-01-0063 Dymek et al. 2019 found that loss of PACRG and FAP20 removes the inner-junction bridge and reduces dynein-driven microtubule sliding.

Mechanism graphs

Inner-junction proteins bridge A- and B-tubules (FUNCTION)

In Chlamydomonas flagella, inner-junction proteins form the bridge that links B10 to A1 within each axonemal doublet and helps stabilize the doublet during motility.

Subject	Predicate	Object	Evidence
inner-junction proteins	form	axonemal microtubule doublet inner junction	`DOI:10.1091/mbc.E19-01-0063` Dymek et al. 2019 localized PACRG and FAP20 to the inner-junction bridge of Chlamydomonas axonemal doublets. `DOI:10.7554/eLife.52760` Khalifa et al. 2020 extended the inner-junction map with FAP276, FAP106 and FAP126 assignments.
axonemal microtubule doublet inner junction	contacts	A axonemal microtubule	`GO:0160113` GO defines the inner junction as joining the B10 protofilament to the A1 protofilament.
axonemal microtubule doublet inner junction	contacts	B axonemal microtubule	`GO:0160113` GO defines the inner junction as joining the B10 protofilament to the A1 protofilament.
axonemal microtubule doublet inner junction	stabilizes	axonemal doublet microtubule	`DOI:10.1091/mbc.E13-08-0464` Yanagisawa et al. 2014 found that fap20 mutant axonemes lack the inner junction and are structurally unstable during beating. `DOI:10.1091/mbc.E19-01-0063` Dymek et al. 2019 showed that Chlamydomonas pacrg/fap20 defects remove the inner-junction bridge and perturb ciliary motility.

Discussions

CURATION_TODO (OPEN): Resolve exact FAP20, PACRG, tektin and other inner-junction protein-family groundings, memberships and copy numbers. Primary Chlamydomonas and Tetrahymena studies identify specific inner-junction and inner-junction-associated proteins, but this pass did not verify source-neutral family CURIEs for those constituents.

Evidence

GO:0160113 Gene Ontology cellular-component term for the axonemal microtubule doublet inner junction.
GO:0097545 GO places the axonemal microtubule doublet inner junction within the axonemal doublet microtubule.
DOI:10.1091/mbc.E13-08-0464 Yanagisawa et al. 2014, 'FAP20 is an inner junction protein of doublet microtubules essential for both the planar asymmetrical waveform and stability of flagella in Chlamydomonas', Molecular Biology of the Cell.
DOI:10.1091/mbc.E19-01-0063 Dymek et al. 2019, 'PACRG and FAP20 form the inner junction of axonemal doublet microtubules and regulate ciliary motility', Molecular Biology of the Cell.
DOI:10.7554/eLife.52760 Khalifa et al. 2020, 'The inner junction complex of the cilia is an interaction hub that involves tubulin post-translational modifications', eLife.

Curation history

2026-09-23T12:48:43Z codex · CREATED_RECORD — Scaffolded by scripts/new_record.py
2026-09-23T12:51:03Z codex · EXPANDED_RECORD — Added doublet parthood, a reviewed-label-only inner-junction protein component, Chlamydomonas scope, A/B-junction stability function, GO and DOI evidence, a nonmechanistic inner-junction bridge graph, and an inner-junction protein-grounding TODO.

Source: data/structures/cytoskeleton/axonemal_microtubule_doublet_inner_junction.yaml