AP-3 adaptor complex

GO:0030123 · Other · Multiprotein complex · Proposed

A heterotetrameric AP-type membrane coat adaptor complex that consists of beta3, delta, mu3 and sigma3 subunits and is found associated with endosomal membranes. AP-3 does not appear to associate with clathrin in all organisms. In at least humans, the AP-3 complex can be heterogeneric due to the existence of multiple subunit isoforms encoded by different genes (beta3A and beta3B, mu3A and mu3B, and sigma3A and sigma3B). [GO:0030123]

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

Components

Component	Type	Part of / acts on	Grounding	Essential	Role
AP-3 beta3 subunit	Protein	part of	Reviewed label only (reviewed name; no accession asserted)	UNKNOWN	Large beta3 subunit of the heterotetrameric AP-3 adaptor complex. `GO:0030123` GO:0030123 defines the AP-3 adaptor complex as consisting of beta3, delta, mu3 and sigma3 subunits.
AP-3 delta subunit	Protein	part of	Reviewed label only (reviewed name; no accession asserted)	UNKNOWN	Large delta subunit of the heterotetrameric AP-3 adaptor complex. `GO:0030123` GO:0030123 defines the AP-3 adaptor complex as consisting of beta3, delta, mu3 and sigma3 subunits.
AP-3 mu3 subunit	Protein	part of	Reviewed label only (reviewed name; no accession asserted)	UNKNOWN	Medium mu3 subunit of the heterotetrameric AP-3 adaptor complex. `GO:0030123` GO:0030123 defines the AP-3 adaptor complex as consisting of beta3, delta, mu3 and sigma3 subunits.
AP-3 sigma3 subunit	Protein	part of	Reviewed label only (reviewed name; no accession asserted)	UNKNOWN	Small sigma3 subunit of the heterotetrameric AP-3 adaptor complex. `GO:0030123` GO:0030123 defines the AP-3 adaptor complex as consisting of beta3, delta, mu3 and sigma3 subunits.

Taxonomic distribution

NCBITaxon:5911 Tetrahymena thermophila — COMMON: Tetrahymena thermophila has an AP-3 adaptor complex required for mucocyst formation and maturation. [DOI:10.1091/mbc.e17-01-0018]

Canonical examples

NCBITaxon:5911 Tetrahymena thermophila — Ciliate model in which the AP-3 complex was genetically connected to formation and maturation of mucocysts. [DOI:10.1091/mbc.e17-01-0018]

Functions

AP-3-dependent mucocyst formation and maturation — Tetrahymena AP-3 is required for formation and maturation of mucocysts, candidate lysosome-related secretory organelles.
- DOI:10.1091/mbc.e17-01-0018 Kaur et al. 2017 identify the AP-3 complex as required for Tetrahymena mucocyst formation and maturation.

Mechanism graphs

Beta3, delta, mu3 and sigma3 subunits form AP-3 (FUNCTION)

The AP-3 adaptor complex is a heterotetramer associated with endosomal membranes and required for Tetrahymena mucocyst formation and maturation.

Subject	Predicate	Object	Evidence
AP-3 beta3 subunit	is part of	AP-3 adaptor complex	`GO:0030123` GO:0030123 lists beta3 as an AP-3 constituent.
AP-3 delta subunit	is part of	AP-3 adaptor complex	`GO:0030123` GO:0030123 lists delta as an AP-3 constituent.
AP-3 mu3 subunit	is part of	AP-3 adaptor complex	`GO:0030123` GO:0030123 lists mu3 as an AP-3 constituent.
AP-3 sigma3 subunit	is part of	AP-3 adaptor complex	`GO:0030123` GO:0030123 lists sigma3 as an AP-3 constituent.
AP-3 adaptor complex	associates with	endosomal membrane	`GO:0030123` GO:0030123 describes AP-3 as found associated with endosomal membranes.
AP-3 adaptor complex	is required for	mucocyst formation and maturation	`DOI:10.1091/mbc.e17-01-0018` Kaur et al. 2017 connect the AP-3 complex to Tetrahymena mucocyst formation and maturation.

Discussions

INTERPRETATION (RESOLVED): Use Tetrahymena AP-3 as AP-3 evidence without curating it as a mature mucocyst component. Kaur et al. 2017 support a required upstream role for AP-3 in Tetrahymena mucocyst formation and maturation. This anchors the AP-3 adaptor-complex record, while AP-3 remains outside the mucocyst component list because the cited role is biogenesis rather than stable mature-organelle membership.
INTERPRETATION (RESOLVED): Avoid modeling every AP-3 adaptor complex as a clathrin-coat constituent. The GO:0030123 definition explicitly notes that AP-3 does not appear to associate with clathrin in all organisms. This record therefore keeps the parent-level membrane-coat adaptor identity through GO:0030119 but avoids a universal AP-3 to clathrin edge.
CURATION_TODO (OPEN): Resolve exact source-neutral groundings for the AP-3 beta3, delta, mu3 and sigma3 subunit classes. GO gives the exact AP-3 complex identity and names its subunit classes, but exact source-neutral InterPro, Pfam or NCBIfam CURIEs for those AP-3-specific classes were not curated in this pass.

Evidence

GO:0030123 Gene Ontology cellular-component term for the AP-3 adaptor complex.
GO:0030119 Gene Ontology cellular-component parent term for AP-type membrane coat adaptor complexes.
DOI:10.1091/mbc.e17-01-0018 Kaur et al. 2017, An endosomal syntaxin and the AP-3 complex are required for formation and maturation of candidate lysosome-related secretory organelles (mucocysts) in Tetrahymena thermophila, Molecular Biology of the Cell.

Curation history

2026-09-16T12:22:59Z codex · CREATED_RECORD — Scaffolded by scripts/new_record.py
2026-09-16T12:24:18Z codex · EXPANDED_RECORD — Added AP-type parentage, reviewed-label-only beta3/delta/mu3/sigma3 subunits, Tetrahymena AP-3 distribution and function evidence, GO/DOI evidence, a nonmechanistic AP-3 topology graph, and mucocyst-boundary, clathrin-scope and subunit-grounding discussions.
2026-09-16T12:36:39Z codex · ADDRESSED_PR_REVIEW — Retargeted the AP-3 requirement graph edge from the mature mucocyst organelle to a mucocyst formation and maturation BIOLOGICAL_PROCESS node; addresses PR #1094 review issue #1095.

Source: data/structures/other/ap_3_adaptor_complex.yaml