C2 axonemal microtubule

GO:1990720 · Cytoskeleton · Filament · Proposed

The asymmetric C2 singlet microtubule of the axonemal central pair. [GO:1990720]

Synonyms: C2 tubule

Part of: axonemal central pair (GO:0097540)

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

Taxonomic distribution

NCBITaxon:3055 Chlamydomonas reinhardtii — COMMON: Chlamydomonas flagellar central pair complexes contain an asymmetric C2 singlet microtubule with structurally distinct C2 projections. [DOI:10.1002/cm.21094]

Canonical examples

NCBITaxon:3055 Chlamydomonas reinhardtii — Green-algal model whose C2 microtubule and C2-associated central-pair projections were resolved by cryoelectron tomography and subtomogram averaging. [DOI:10.1002/cm.21094]

Functions

C2 projection scaffold — The C2 axonemal microtubule carries the structurally distinct C2a, C2b, C2c, C2d and C2e central-pair projections.
- GO:1990720 GO distinguishes the C2 axonemal microtubule from C1 by the differing protein components of their projections.
- DOI:10.1002/cm.21094 Carbajal-Gonzalez et al. 2013 resolved the Chlamydomonas and Strongylocentrotus C2 microtubule projections, including newly assigned 2d and 2e densities.

Mechanism graphs

C2 axonemal microtubule carries C2 projections (FUNCTION)

In Chlamydomonas, the C2 singlet microtubule is one member of the axonemal central pair and carries C2-specific projections while the central bridge links it to C1.

Subject	Predicate	Object	Evidence
C2 axonemal microtubule	is part of	axonemal central pair	`GO:1990720` GO places the C2 axonemal microtubule within the axonemal central pair.
C2 central-pair projections	project from	C2 axonemal microtubule	`DOI:10.1002/cm.21094` Carbajal-Gonzalez et al. 2013 resolved C2a, C2b, C2c, C2d and C2e projection densities on the C2 microtubule.
axonemal central bridge	connects to	C1 axonemal microtubule	`GO:1990717` GO defines the axonemal central bridge as connecting the central pair of single microtubules.
axonemal central bridge	connects to	C2 axonemal microtubule	`GO:1990717` GO defines the axonemal central bridge as two bridges linking the two central-pair microtubules.

Discussions

CURATION_TODO (OPEN): Resolve exact source-neutral boundaries, C2 microtubule-wall constituents, MIP-C2a/MIP-C2b identities, protein-family groundings and copy numbers for the C2 axonemal microtubule and C2a/C2b/C2c/C2d/C2e projection subcomplexes. Carbajal-Gonzalez et al. 2013 resolved five named C2 projection densities and two C2 microtubule inner proteins, but this pass curates only the broad GO:1990720 C2 axonemal microtubule scaffold and leaves exact C2 wall, inner protein and projection subcomplex boundaries unresolved.

Evidence

GO:1990720 Gene Ontology cellular-component term for the C2 axonemal microtubule.
GO:0005879 Gene Ontology places the C2 axonemal microtubule under the axonemal microtubule class.
GO:0097540 Gene Ontology places the C2 axonemal microtubule within the axonemal central pair.
DOI:10.1002/cm.21094 Carbajal-Gonzalez et al. 2013, 'Conserved structural motifs in the central pair complex of eukaryotic flagella', Cytoskeleton.

Curation history

2026-09-23T16:52:43Z codex · CREATED_RECORD — Scaffolded by scripts/new_record.py
2026-09-23T16:53:43Z codex · EXPANDED_RECORD — Added the C2 tubule synonym, axonemal-microtubule parentage, central-pair parthood, Chlamydomonas scope, GO and DOI evidence, a nonmechanistic C2 projection graph, and a C2 projection/MIP subcomplex TODO.

Source: data/structures/cytoskeleton/c2_axonemal_microtubule.yaml