axonemal central pair

GO:0097540 · Cytoskeleton · Filament · Proposed

A pair of asymmetric C1 and C2 singlet microtubules at the center of the 9+2 axonemal central apparatus. [GO:0097540]

Synonyms: axonemal microtubule central pair; axoneme central pair; axoneme microtubule central pair; central pair; central-pair microtubules

Part of: axonemal central apparatus (GO:1990716)

Has part: C1 axonemal microtubule (GO:1990719); C2 axonemal microtubule (GO:1990720)

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

Taxonomic distribution

• NCBITaxon:3055 Chlamydomonas reinhardtii — COMMON: Chlamydomonas flagellar axonemes contain a central pair complex with C1 and C2 singlet microtubules, associated projections and a central bridge. [DOI:10.1002/cm.21094]

Canonical examples

• NCBITaxon:3055 Chlamydomonas reinhardtii — Green-algal model whose flagellar central pair complex has been resolved in situ by cryoelectron tomography and subtomogram averaging. [DOI:10.1002/cm.21094]

Functions

• C1/C2 central microtubule core — The axonemal central pair supplies the asymmetric C1/C2 singlet-microtubule core from which central-pair projections extend and around which the bridge network is organized.
  • GO:0097540 GO defines the axonemal central pair as the inner microtubule pair of the 9+2 axoneme.
  • DOI:10.1002/cm.21094 Carbajal-Gonzalez et al. 2013 resolved the Chlamydomonas central pair complex with asymmetric C1 and C2 microtubules, bridges and projections.

Mechanism graphs

C1 and C2 form the axonemal central pair (FUNCTION)

In Chlamydomonas, asymmetric C1 and C2 singlet microtubules form the axonemal central pair that carries projections and is bridged within the central apparatus.

Discussions

• CURATION_TODO (RESOLVED): Add exact C1 and C2 axonemal microtubule child records. GO exactly identifies the C1 and C2 axonemal microtubules, but this pass only curates their parent GO:0097540 central pair and leaves the individual asymmetric microtubules for later records. Historical narrowing at git 03c7f770a3a5: GO exactly identifies the C2 axonemal microtubule, but this pass leaves the second asymmetric microtubule of GO:0097540 for a later record.

Evidence

• GO:0097540 Gene Ontology cellular-component term for the axonemal central pair.
• GO:1990716 Gene Ontology places the axonemal central pair within the axonemal central apparatus.
• GO:1990719 Gene Ontology cellular-component term for the C1 axonemal microtubule.
• GO:1990720 Gene Ontology cellular-component term for the C2 axonemal microtubule.
• DOI:10.1002/cm.21094 Carbajal-Gonzalez et al. 2013, 'Conserved structural motifs in the central pair complex of eukaryotic flagella', Cytoskeleton.

Curation history

• 2026-09-23T15:49:11Z codex · CREATED_RECORD — Scaffolded by scripts/new_record.py
• 2026-09-23T15:50:17Z codex · EXPANDED_RECORD — Added GO synonyms, central-apparatus parthood, exact C1 and C2 has_part links, Chlamydomonas scope, GO and DOI evidence, a nonmechanistic central-pair architecture graph, and a C1/C2 child record TODO.
• 2026-09-23T16:00:55Z codex · ADDRESSED_REVIEW — Resolved PR #1406 review issue #1407 by keeping exact named projection subcomplex boundaries in the central-pair graph scope note.
• 2026-09-23T16:16:58Z codex · ADD_C1_AXONEMAL_MICROTUBULE_CHILD — Narrowed the axonemal central-pair TODO and graph scope note after splitting GO:1990719 into a C1 axonemal microtubule child record.
• 2026-09-23T16:53:43Z codex · ADD_C2_AXONEMAL_MICROTUBULE_CHILD — Removed the axonemal central-pair C2 child TODO and narrowed the parent graph scope note after splitting GO:1990720 into a C2 axonemal microtubule child record.
• 2026-10-11T08:09:53Z codex · ADDRESS_SAVED_REVIEW — Addressed #2196: Restored the deleted central-pair discussion with original identity, rationale and posed date, retained its intermediate narrowing, and resolved only child creation with explicit handoff of open component tasks.

Source: data/structures/cytoskeleton/axonemal_central_pair.yaml