axonemal microtubule doublet outer junction

GO:0160114 · Cytoskeleton · Multiprotein complex · Proposed

A junctional axonemal doublet substructure that links the B-tubule B1 protofilament to A-tubule protofilaments A10 and A11. [GO:0160114]

Part of: axonemal doublet microtubule (GO:0097545)

Components

Component	Type	Part of / acts on	Grounding	Genes	Stoichiometry	Essential	Role
outer-junction proteins	Protein	part of	Reviewed label only (reviewed name; no accession asserted)			UNKNOWN	Microtubule inner proteins and wedge-associated proteins that span the B1-A10/A11 outer junction and reinforce B-tubule attachment to the A-tubule. `DOI:10.1016/j.cell.2019.09.030` Ma et al. 2019 localized Chlamydomonas FAP127, FAP53 and FAP141 helices across the B1-to-A10/A11 outer-junction cleft. `DOI:10.1038/s41467-023-37868-0` Kubo et al. 2023 modeled Tetrahymena CFAP77, OJ2 and OJ3 proteins at the axonemal doublet outer junction.

NCBITaxon:5911 Tetrahymena thermophila — COMMON: Tetrahymena ciliary axonemal doublets contain CFAP77, OJ2 and OJ3 at the outer junction in native doublet cryo-EM structures. [DOI:10.1038/s41467-023-37868-0]

NCBITaxon:5911 Tetrahymena thermophila — Ciliate model used to identify CFAP77, OJ2 and OJ3 outer-junction proteins and test CFAP77 loss in ciliary motility and doublet-integrity assays. [DOI:10.1038/s41467-023-37868-0]

outer-junction B-tubule stabilization — Outer-junction proteins span the A10, A11, B1 and B2 protofilaments of the axonemal doublet and help keep the incomplete B-tubule stably attached to the A-tubule.
- GO:0160114 GO defines the outer junction as the structure joining B1 to A10 and A11 in the axonemal doublet.
- DOI:10.1038/s41467-023-37868-0 Kubo et al. 2023 modeled TtCFAP77 and OJ2 across A10-A12/B1-B2 protofilaments and found B1-B2 dissociation without both proteins in molecular dynamics simulations.

Outer-junction proteins occupy the B1-A10/A11 boundary of the axonemal doublet and help stabilize the outer side of the B-tubule attachment site.

Subject	Predicate	Object	Evidence
outer-junction proteins	form	axonemal microtubule doublet outer junction	`DOI:10.1016/j.cell.2019.09.030` Ma et al. 2019 mapped FAP127, FAP53 and FAP141 across the Chlamydomonas outer-junction cleft. `DOI:10.1038/s41467-023-37868-0` Kubo et al. 2023 identified CFAP77, OJ2 and OJ3 at the Tetrahymena outer junction.
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.
axonemal microtubule doublet outer junction	contacts	B axonemal microtubule	`GO:0160114` GO defines the outer junction as the site where B1 joins the A-tubule protofilaments A10 and A11.
axonemal microtubule doublet outer junction	stabilizes	axonemal doublet microtubule	`DOI:10.1038/s41467-023-37868-0` Kubo et al. 2023 found B1-B2 protofilaments dissociated from A10-A12 when TtCFAP77 and OJ2 were omitted from molecular dynamics simulations; CFAP77A/B knockout mildly reduced motility with occasional doublet damage.

CURATION_TODO (OPEN): Resolve exact CFAP77, OJ2, OJ3, FAP127, FAP53, FAP141 and other axonemal doublet outer-junction protein-family groundings, memberships and copy numbers. Primary Chlamydomonas and Tetrahymena doublet structures identify outer-junction protein classes, but this pass did not verify source-neutral family CURIEs for the mixed conserved and species-specific constituents.

GO:0160114 Gene Ontology cellular-component term for the axonemal microtubule doublet outer junction.
GO:0097545 GO places the axonemal microtubule doublet outer junction 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:10.1038/s41467-023-37868-0 Kubo et al. 2023, 'Native doublet microtubules from Tetrahymena thermophila reveal the importance of outer junction proteins', Nature Communications.

2026-09-23T13:55:40Z codex · CREATED_RECORD — Scaffolded by scripts/new_record.py
2026-09-23T13:56:28Z codex · EXPANDED_RECORD — Added doublet parthood, a reviewed-label-only outer-junction protein component, Tetrahymena scope, B-tubule stabilization function, GO and DOI evidence, a nonmechanistic outer-junction graph, and an outer-junction protein-grounding TODO.
2026-09-23T14:08:32Z codex · ADDRESS_CFAP77_REVIEW — #1401: narrowed TtCFAP77/OJ2 simulation and CFAP77 knockout wording to avoid treating CFAP77 loss alone as a direct full-outer-junction destabilization assay.