B axonemal microtubule

GO:0097650 · Cytoskeleton · Filament · Proposed

The incomplete 10-protofilament B tubule of the axonemal doublet microtubule. [GO:0097650]

Synonyms: B tubule

Part of: axonemal doublet microtubule (GO:0097545)

Has part: axonemal B tubule inner sheath (GO:0160112)

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 an incomplete 10-protofilament B-tubule that fuses with the complete A-tubule through inner and outer junctions. [DOI:10.1016/j.cell.2019.09.030]

Canonical examples

NCBITaxon:3055 Chlamydomonas reinhardtii — Green-algal model whose native B-tubule protofilament lattice and junctional contacts were resolved in axonemal doublets. [DOI:10.1016/j.cell.2019.09.030]

Functions

B-tubule doublet scaffold — The incomplete B axonemal microtubule contributes ten protofilaments to the axonemal outer-doublet wall and fuses with the A tubule at the inner and outer junctions.
- GO:0097650 GO defines the B axonemal microtubule as the incomplete 10-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, including B-tubule protofilaments and the inner- and outer-junction regions.

Mechanism graphs

B axonemal microtubule completes the outer doublet (FUNCTION)

In Chlamydomonas, the incomplete B tubule fuses with the complete A tubule and contacts inner- and outer-junction structures at the B10 and B1 protofilaments.

Subject	Predicate	Object	Evidence
B axonemal microtubule	is part of	axonemal doublet microtubule	`GO:0097650` GO places the B axonemal microtubule within the axonemal doublet microtubule.
B axonemal microtubule	fuses with	A axonemal microtubule	`GO:0097650` GO defines the B tubule as an incomplete microtubule that fuses with the complete A tubule.
axonemal microtubule doublet inner junction	contacts	B axonemal microtubule	`GO:0160113` GO defines the inner junction as joining B10 to A1 within an axonemal doublet.
axonemal microtubule doublet outer junction	contacts	B axonemal microtubule	`GO:0160114` GO defines the outer junction as the site where B1 joins A10 and A11.

Discussions

CURATION_TODO (OPEN): Resolve exact B1-B10 protofilament boundaries, non-inner-sheath B-tubule MIP positions, source-neutral tubulin-family groundings and copy numbers for the B axonemal microtubule. GO exactly identifies the B axonemal microtubule, but this pass curates only its coarse role in the outer doublet and leaves exact protofilament and MIP placement unresolved. Historical reassessment at git 871db9b2b9d6: GO exactly identifies the B axonemal microtubule and GO:0160112 now captures the B-tubule inner sheath, but this pass curates only the B tubule's coarse role in the outer doublet and leaves exact protofilament and non-inner-sheath MIP placement unresolved.

Evidence

GO:0097650 Gene Ontology cellular-component term for the B axonemal microtubule.
GO:0005879 Gene Ontology places the B axonemal microtubule under the axonemal microtubule class.
GO:0097545 Gene Ontology places the B 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.

Curation history

2026-09-23T17:42:23Z codex · CREATED_RECORD — Scaffolded by scripts/new_record.py
2026-09-23T17:43:10Z codex · EXPANDED_RECORD — Added the B tubule synonym, axonemal-microtubule parentage, doublet parthood, Chlamydomonas scope, GO and DOI evidence, a nonmechanistic B-tubule graph, and a B-tubule protofilament/MIP TODO.
2026-09-23T18:30:46Z codex · ADD_B_TUBULE_INNER_SHEATH_CHILD — Linked GO:0160112 from the B-tubule parent and narrowed the remaining B-tubule MIP TODO.
2026-10-11T09:48:28Z codex · ADDRESS_SAVED_REVIEW — Addressed #2202: Restored the exact original B-tubule rationale and retained the later inner-sheath reassessment with revision context, preserving the current prompt and scientific scope.

Source: data/structures/cytoskeleton/b_axonemal_microtubule.yaml