A axonemal microtubule

GO:0097649 · Cytoskeleton · Filament · Proposed

The complete 13-protofilament A tubule of the axonemal doublet microtubule. [GO:0097649]

Synonyms: A tubule

Part of: axonemal doublet microtubule (GO:0097545)

Has part: axonemal A tubule inner sheath (GO:0160111)

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

Taxonomic distribution

NCBITaxon:3055 Chlamydomonas reinhardtii — COMMON: Chlamydomonas flagellar doublets contain a complete 13-protofilament A-tubule that adjoins the incomplete B-tubule and carries doublet-associated axonemal complexes. [DOI:10.1016/j.cell.2019.09.030]

Canonical examples

NCBITaxon:3055 Chlamydomonas reinhardtii — Green-algal model whose A-tubule protofilament lattice and associated axonemal doublet proteins were resolved in native cilia. [DOI:10.1016/j.cell.2019.09.030]

Functions

A-tubule doublet scaffold — The complete A axonemal microtubule forms the 13-protofilament microtubule wall that supports B-tubule attachment, inner- and outer-junction contact, radial-spoke anchoring, and dynein-arm attachment.
- GO:0097649 GO defines the A axonemal microtubule as the complete 13-protofilament tubule of the axonemal outer doublet.
- DOI:10.1016/j.cell.2019.09.030 Ma et al. 2019 resolved the native Chlamydomonas doublet microtubule lattice with A-tubule-associated inner-sheath, inner-junction, outer-junction, ribbon and external densities.

Mechanism graphs

A axonemal microtubule supports the outer doublet (FUNCTION)

In Chlamydomonas, the complete A tubule is the 13-protofilament microtubule of the axonemal outer doublet, connects to the incomplete B tubule through inner and outer junction structures, and carries radial-spoke and dynein-arm attachments.

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	fuses with	A axonemal microtubule	`GO:0097649` GO defines the A tubule as a complete microtubule that fuses with the incomplete B tubule.
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 outer junction	contacts	A axonemal microtubule	`GO:0160114` GO defines the outer junction as joining B1 to A10 and A11 within an axonemal doublet.
radial spoke	attaches to	A axonemal microtubule	`GO:0097649` GO defines the A axonemal microtubule as the doublet microtubule that carries attached radial spokes. `DOI:10.1126/science.1128618` Nicastro et al. 2006 identified radial spokes on Chlamydomonas outer doublets.
inner dynein arm	attaches to	A axonemal microtubule	`GO:0097649` GO defines the A axonemal microtubule as the doublet microtubule that carries attached inner dynein arms. `GO:0036156` GO identifies the inner dynein arm as an axonemal structure on ciliary and flagellar outer doublets. `DOI:10.1126/science.1128618` Nicastro et al. 2006 resolved inner dynein arms on Chlamydomonas outer doublets.
outer dynein arm	attaches to	A axonemal microtubule	`GO:0097649` GO defines the A axonemal microtubule as the doublet microtubule that carries attached outer dynein arms. `GO:0036157` GO identifies the outer dynein arm as an axonemal structure on ciliary and flagellar outer doublets. `DOI:10.1126/science.1128618` Nicastro et al. 2006 resolved outer dynein arms on Chlamydomonas outer doublets.

Discussions

CURATION_TODO (OPEN): Resolve exact A1-A13 protofilament boundaries, non-inner-sheath A-tubule MIP positions, radial-spoke and dynein-arm attachment geometry, source-neutral tubulin-family groundings and copy numbers for the A axonemal microtubule. GO exactly identifies the A axonemal microtubule, but this pass curates only its coarse role in the outer doublet and leaves exact protofilament, MIP, radial-spoke and dynein-arm attachment placement unresolved. Reassessment on 2026-10-11: GO exactly identifies the A axonemal microtubule and GO:0160111 now captures the A-tubule inner sheath, but this pass curates only the A tubule's coarse role in the outer doublet and leaves exact protofilament, non-inner-sheath MIP, radial-spoke and dynein-arm attachment placement unresolved.

Evidence

GO:0097649 Gene Ontology cellular-component term for the A axonemal microtubule.
GO:0005879 Gene Ontology places the A axonemal microtubule under the axonemal microtubule class.
GO:0097545 Gene Ontology places the A axonemal microtubule within the axonemal doublet microtubule.
DOI:10.1016/j.cell.2019.09.030 Ma et al. 2019, 'Structure of the Decorated Ciliary Doublet Microtubule', Cell.
DOI:10.1126/science.1128618 Nicastro et al. 2006 resolved Chlamydomonas radial spokes and dynein arms on the axonemal outer doublets.

Curation history

2026-09-23T17:12:02Z codex · CREATED_RECORD — Scaffolded by scripts/new_record.py
2026-09-23T17:12:39Z codex · EXPANDED_RECORD — Added the A tubule synonym, axonemal-microtubule parentage, doublet parthood, Chlamydomonas scope, GO and DOI evidence, a nonmechanistic A-tubule graph, and an A-tubule protofilament/MIP TODO.
2026-09-23T17:23:44Z codex · ADDRESSED_REVIEW — Resolved PR #1411 issue #1412 by adding radial-spoke, inner-dynein-arm and outer-dynein-arm A-tubule attachment nodes and edges.
2026-09-23T18:12:28Z codex · ADD_A_TUBULE_INNER_SHEATH_CHILD — Resolved PR #1414 issue #1415 by linking GO:0160111 from the A-tubule parent and narrowing the remaining A-tubule MIP TODO.
2026-10-11T07:14:15Z codex · ADDRESS_SAVED_REVIEW — Addressed #2187: Restored the exact original discussion rationale and retained the complete inner-sheath reassessment, narrowed prompt and original posed date.

Source: data/structures/cytoskeleton/a_axonemal_microtubule.yaml