axonemal doublet microtubule

GO:0097545 · Cytoskeleton · Filament · Proposed

A cellular anatomical entity that is part of an axoneme consisting of a doublet microtubule. [GO:0097545]

Synonyms: axonemal outer doublet; axoneme outer doublet; DMT; outer-doublet microtubules; outer doublet

Part of: axoneme (GO:0005930)

Has part: A axonemal microtubule (GO:0097649); B axonemal microtubule (GO:0097650); axonemal microtubule doublet inner sheath (GO:0160110); axonemal microtubule doublet inner junction (GO:0160113); axonemal microtubule doublet outer junction (GO:0160114); axonemal microtubule doublet ribbon (GO:0160115)

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
axonemal alpha- and beta-tubulins	Protein	part of	Reviewed label only (reviewed name; no accession asserted)			ESSENTIAL	Tubulin subunits that form the A and B microtubule protofilament walls of the axonemal doublet. `GO:0097649` GO defines the A axonemal microtubule as a complete 13-protofilament microtubule in the axonemal outer doublet. `GO:0097650` GO defines the B axonemal microtubule as the incomplete 10-protofilament microtubule that fuses with the A tubule. `DOI:10.1016/j.cell.2019.09.030` Ma et al. 2019 resolved alpha/beta-tubulin protofilaments and associated proteins in native Chlamydomonas axonemal doublet microtubules.

Taxonomic distribution

NCBITaxon:3055 Chlamydomonas reinhardtii — COMMON: Chlamydomonas flagellar axonemes contain nine doublet microtubules whose tubulin lattice and luminal MIP architecture have been resolved by cryo-ET and cryo-EM. [DOI:10.1016/j.cell.2019.09.030]

Canonical examples

NCBITaxon:3055 Chlamydomonas reinhardtii — Green-algal model whose native axonemal doublet microtubules have been structurally resolved with associated MIPs and axonemal complexes. [DOI:10.1016/j.cell.2019.09.030]

Functions

axonemal doublet scaffold — The nine Chlamydomonas axonemal doublet microtubules provide the circumferential scaffold for axonemal dyneins, radial spokes, nexin links and flagellar bending.
- DOI:10.1126/science.1128618 Nicastro et al. 2006 resolved the 9+2 Chlamydomonas axoneme and its doublet-associated dynein, radial-spoke and linker densities by cryoelectron tomography.
- DOI:10.1016/j.cell.2019.09.030 Ma et al. 2019 resolved the native Chlamydomonas axonemal doublet with associated radial-spoke and nexin-dynein regulatory-complex densities.

Mechanism graphs

A- and B-tubules form axonemal doublets (FUNCTION)

In Chlamydomonas flagella, A- and B-tubules form axonemal doublet microtubules whose tubulin walls and inner sheath stabilize the 9+2 axonemal scaffold.

Subject	Predicate	Object	Evidence
A axonemal microtubule	is part of	axonemal doublet microtubule	`GO:0097649` GO places the A axonemal microtubule within the axonemal doublet microtubule.
B axonemal microtubule	is part of	axonemal doublet microtubule	`GO:0097650` GO places the B axonemal microtubule within the axonemal doublet microtubule.
axonemal alpha- and beta-tubulins	polymerize into	axonemal doublet microtubule	`GO:0097649` GO identifies the A tubule as a complete 13-protofilament microtubule. `GO:0097650` GO identifies the B tubule as an incomplete 10-protofilament microtubule.
axonemal microtubule doublet inner sheath	stabilizes	axonemal doublet microtubule	`GO:0160110` GO defines the axonemal microtubule doublet inner sheath as a MIP network that helps stabilize the doublet microtubule. `DOI:10.1038/s41467-019-09051-x` Owa et al. 2019 found that Chlamydomonas FAP45 and FAP52 bind the inside of B tubules and stabilize ciliary axonemes.
axonemal doublet microtubule	is part of	axoneme	`GO:0097545` GO defines the axonemal doublet microtubule as part of an axoneme. `DOI:10.1126/science.1128618` Nicastro et al. 2006 visualized the nine outer doublet microtubules of the Chlamydomonas 9+2 axoneme by cryoelectron tomography.

Discussions

CURATION_TODO (OPEN): Resolve exact source-neutral groundings for remaining doublet-specific accessory structures, and decide which ones need child records. GO now identifies multiple named doublet substructures, and cryo-EM has mapped many Chlamydomonas MIPs, but this first GO:0097545 pass curates only the coarse A/B doublet scaffold. Historical reassessment at git 1a6f714581ad: GO still identifies named inner-junction, outer-junction and ribbon substructures, but this GO:0097545 pass leaves those finer child records for later curation. Historical reassessment at git 5b2752fe91a2: GO still identifies named outer-junction and ribbon substructures, but this GO:0097545 pass leaves those finer child records and other doublet-specific accessory structures for later curation. Historical reassessment at git 8885f3fcea2c: GO still identifies the named outer-junction substructure, but this GO:0097545 pass leaves that finer child record and other doublet-specific accessory structures for later curation. Historical reassessment at git 16734f1265fe: GO:0160110 inner sheath, GO:0160113 inner junction, GO:0160114 outer junction and GO:0160115 ribbon now have child records, but this GO:0097545 pass leaves any other doublet-specific accessory structures for later curation.

Evidence

GO:0097545 Gene Ontology cellular-component term for the axonemal doublet microtubule.
GO:0097649 Gene Ontology cellular-component term for the A axonemal microtubule part.
GO:0097650 Gene Ontology cellular-component term for the B axonemal microtubule part.
GO:0160110 Gene Ontology cellular-component term for the MIP network of the axonemal microtubule doublet inner sheath.
GO:0160113 Gene Ontology cellular-component term for the inner junction that links the B10 and A1 protofilaments of an axonemal doublet.
GO:0160114 Gene Ontology cellular-component term for the outer junction that links the B1 protofilament to A10/A11 in an axonemal doublet.
GO:0160115 Gene Ontology cellular-component term for the axonemal doublet ribbon made from protofilaments A11, A12 and A13 plus associated MIPs.
GO:0005930 Gene Ontology places the axonemal doublet microtubule within the axoneme.
DOI:10.1126/science.1128618 Nicastro et al. 2006, 'The molecular architecture of axonemes revealed by cryoelectron tomography', Science.
DOI:10.1038/s41467-019-09051-x Owa et al. 2019, 'Inner lumen proteins stabilize doublet microtubules in cilia and flagella', Nature Communications.
DOI:10.1016/j.cell.2019.09.030 Ma et al. 2019, 'Structure of the Decorated Ciliary Doublet Microtubule', Cell.

Curation history

2026-09-23T11:45:10Z codex · CREATED_RECORD — Scaffolded by scripts/new_record.py
2026-09-23T11:46:26Z codex · EXPANDED_RECORD — Added GO synonyms, axoneme parthood, A- and B-tubule part links, label-only tubulin and microtubule-inner-protein components, Chlamydomonas scope, GO and DOI evidence, a nonmechanistic doublet scaffold graph, and a MIP/substructure grounding TODO.
2026-09-23T11:56:15Z codex · ADDRESSED_REVIEW — Resolved PR #1390 review issue #1391 by preserving exact GO inner-sheath, inner-junction, outer-junction and ribbon identifiers in the unresolved doublet-substructure TODO.
2026-09-23T11:57:07Z codex · FIX_REVIEW_YAML_ALIASES — Removed YAML aliases from duplicated doublet-substructure TODO evidence.
2026-09-23T12:17:31Z codex · ADD_AXONEMAL_INNER_SHEATH_PART — Linked GO:0160110 as an axonemal doublet substructure, replaced the label-only microtubule-inner-protein graph node with the GO-grounded inner sheath, and narrowed the doublet-substructure TODO.
2026-09-23T12:17:57Z codex · FIX_INNER_SHEATH_TODO_ATTACHMENT — Removed the stale microtubule-inner-protein component attachment from the parent doublet-substructure TODO after moving MIP curation into the GO:0160110 child record.
2026-09-23T12:29:50Z codex · ADDRESSED_REVIEW — Resolved PR #1393 review issue #1394 by removing broad inner-sheath MIP DOI evidence from the remaining parent doublet-substructure TODO.
2026-09-23T12:51:03Z codex · ADD_AXONEMAL_INNER_JUNCTION_PART — Linked GO:0160113 as an axonemal doublet substructure and narrowed the remaining doublet-substructure TODO to the outer junction, ribbon and other doublet-specific accessories.
2026-09-23T13:01:51Z codex · ADDRESSED_REVIEW — Resolved PR #1396 review issue #1397 by narrowing the parent doublet graph scope note after GO:0160113 inner-junction curation.
2026-09-23T13:22:42Z codex · ADD_AXONEMAL_RIBBON_PART — Linked GO:0160115 as an axonemal doublet substructure and narrowed the remaining doublet-substructure TODO to the outer junction and other doublet-specific accessories.
2026-09-23T13:56:28Z codex · ADD_AXONEMAL_OUTER_JUNCTION_PART — Linked GO:0160114 as an axonemal doublet substructure and narrowed the remaining doublet-substructure TODO to other doublet-specific accessories.
2026-10-11T08:56:24Z codex · ADDRESS_SAVED_REVIEW — Addressed #2198: restored original and intermediate discussion rationales with revision context; preserved the current prompt, task lifecycle, original dates, scientific claims and child-record boundaries.

Source: data/structures/cytoskeleton/axonemal_doublet_microtubule.yaml