AAGAB (p34, alpha- and gamma-adaptin-binding protein) is a cytosolic assembly chaperone for the heterotetrameric AP-type membrane coat adaptor complexes. It is not a folding chaperone: its clients are already-folded adaptor subunits, and AAGAB works by enforcing an ordered assembly pathway and by protecting partially assembled intermediates from the degradation that otherwise destroys unassembled subunits. AAGAB has two functional modules - an N-terminal type I pseudoGTPase domain, catalytically inactive, that engages the small sigma subunits, and a C-terminal dimerisation domain that recognises the gamma subunit of AP-1 and the alpha subunit of AP-2 through a shared surface. It exists as a homodimer that converts to a monomer on binding adaptor subunits. AAGAB is required for assembly of AP-1 (trans-Golgi network to endosome transport), AP-2 (clathrin-mediated endocytosis) and AP-4 (ATG9A export from the TGN), but specifically not AP-3. For AP-2 it initiates assembly by stabilising the alpha and sigma2 subunits and then hands the intermediate off to CCDC32, so AAGAB is released and does not form part of the finished complex. Loss of AAGAB collapses adaptor assembly and broadly remodels surface protein homeostasis; truncating mutations that destabilise the C-terminal domain and abolish chaperone function cause autosomal dominant punctate palmoplantar keratoderma type 1A.
Definition: Binding to a free, unassembled subunit of a heterotetrameric AP-type membrane coat adaptor complex (GO:0030119) - for example the alpha or sigma2 subunit of AP-2, the gamma or sigma1/sigma3 subunit of AP-1, or the epsilon or sigma4 subunit of AP-4 - as distinct from binding the assembled adaptor complex.
Justification: GO's existing adaptor-binding terms are all complex-level: AP-1 (GO:0035650), AP-2 (GO:0035612) and AP-3 (GO:0035651) adaptor complex binding all exist, with no AP-4 equivalent, and all of them denote binding to the assembled heterotetramer. That is precisely what assembly chaperones do NOT do. AAGAB binds free, unassembled subunits and is displaced before the tetramer exists - the AAGAB:alpha:sigma2 intermediate "cannot recruit additional AP2 subunits" and is handed to CCDC32 (PMID:39145939) - so annotating it with a complex-binding term asserts the one interaction the published mechanism rules out. A single subunit-level term covers AAGAB's whole client set (AP-1 gamma/sigma, AP-2 alpha/sigma2, AP-4 epsilon/sigma4) at the grain the protein is actually selected for, and would serve other assembly chaperones such as CCDC32 equally. The name is scoped to AP-type membrane coat adaptor complex (GO:0030119) rather than clathrin adaptor complex (GO:0030131) because AP-4 is not a clathrin adaptor: GO:0030124 AP-4 adaptor complex is_a GO:0030119, and the only is_a descendants of GO:0030131 are AP-1 (GO:0030121) and AP-2 (GO:0030122). A clathrin-scoped name would exclude one of AAGAB's three clients. It replaces the earlier draft of this entry, which proposed an "AP-1 adaptor complex binding" term to fill a supposed AP-1 gap. That draft was wrong twice over: GO:0035650 already exists, so there was no AP-1 gap, and filling it would in any case have been the wrong altitude. Note the irony that GO has an AP-3 term, AP-3 being the one adaptor AAGAB pointedly does not act on, while AP-4, which AAGAB does chaperone, has no term at all.
Parent term: protein binding
Supporting Evidence:
Definition: Binding to one or more folded subunits of a protein complex in order to stabilise them and promote their ordered assembly into that complex, where the binding protein does not form part of the finished complex.
Justification: GO's chaperone molecular-function terms concern folding - GO:0044183 protein folding chaperone is explicitly annotated "a protein folding chaperone binds an unfolded protein to fold it". Assembly chaperones such as AAGAB act on subunits that are already folded, and their function is to impose an assembly order and prevent premature degradation of intermediates. The biological-process side is served by GO:0051131 chaperone-mediated protein complex assembly, but there is no corresponding molecular function, so proteins like AAGAB can only be annotated with binding terms that do not convey the chaperone activity. Notably, an automated pipeline grounding AAGAB's activity selected GO:0044183 for exactly this reason, which is incorrect.
Parent term: protein binding
Supporting Evidence:
| GO Term | Evidence | Action | Reason |
|---|---|---|---|
| GO:0005829 cytosol | IEA GO_REF:0000044 | ACCEPT | Summary: Electronic mapping from the UniProt "Cytoplasm, cytosol" subcellular location, which is itself experimentally supported (ECO:0000269|PubMed:23000146, ECO:0000269|PubMed:23064416). This is the correct core compartment: AP-2 assembly by AAGAB takes place in the cytosol, before the assembled adaptor is trafficked to the cell surface. Supporting Evidence: file:human/AAGAB/AAGAB-uniprot.txt SUBCELLULAR LOCATION: Cytoplasm, cytosol {ECO:0000269|PubMed:23000146, PMID:31353312 assembly of the AP2 adaptor complex in the cytosol, prior to the trafficking of the adaptor complex to the cell surface. |
| GO:0005515 protein binding | IPI PMID:25416956 A proteome-scale map of the human interactome network. | MODIFY | Summary: Interaction with AP2S1 (UniProtKB:P53680), the sigma2 subunit of the AP-2 adaptor complex, from a proteome-scale Y2H interactome map. Bare "protein binding" is uninformative, but this is not a spurious hit: AP2S1 is recovered as an AAGAB partner by seven independent large-scale screens using orthogonal methods, and the focused mechanistic literature shows the AAGAB N-terminal pseudoGTPase domain binds and stabilises exactly this sigma subunit during AP-2 assembly. Replace with the specific AP-2 adaptor complex binding term. Reason: Bare protein binding conveys nothing; the partner is an AP-2 subunit and a specific term (GO:0035612) exists. Interim stand-in only: AAGAB binds the free subunit and is displaced before the tetramer exists (PMID:39145939), so it does not bind an assembled AP-2 complex; the accurate term is the subunit-binding term filed under proposed_new_terms. Proposed replacements: AP-2 adaptor complex binding Supporting Evidence: PMID:31353312 AAGAB acts as a molecular chaperone to stabilize the Ξ± subunit monomer. PMID:34494650 AAGAB is an assembly chaperone regulating AP1 and AP2 clathrin adaptors. |
| GO:0005515 protein binding | IPI PMID:25416956 A proteome-scale map of the human interactome network. | MARK AS OVER ANNOTATED | Summary: Interaction with HEATR1 (UniProtKB:Q96ES5) from the same proteome-scale Y2H screen. Unlike the adaptor-subunit partners, this was recovered in only one study (AAGAB-uniprot.txt records NbExp=3 IntAct experiments, but all from that one report), unreplicated by any other screen, and with no mechanistic rationale: HEATR1 is a nucleolar ribosome-biogenesis factor with no described connection to clathrin adaptor assembly, and it does not appear in any focused AAGAB study. Treated as screen noise rather than a functional interaction. Reason: Isolated large-scale two-hybrid hit, unreplicated, biologically unconnected to AAGAB's established chaperone role. Deliberately given a different action from the other protein-binding entries on this gene, because those are reproducible adaptor-subunit interactions and this one is not. No supported_by is given: the argument is the absence of corroborating evidence, and no publication speaks to the HEATR1 hit specifically. |
| GO:0005515 protein binding | IPI PMID:25416956 A proteome-scale map of the human interactome network. | MODIFY | Summary: Interaction with AP1S3 (UniProtKB:Q96PC3), a sigma subunit of the AP-1 adaptor complex. Mechanistically expected: AAGAB promotes AP-1 assembly by binding and stabilising the gamma and sigma subunits of AP1. Modified to GO:0035650 AP-1 adaptor complex binding, the AP-1 counterpart of the term used for the AP-2 partners. Reason: Bare protein binding is uninformative; the partner is an AP-1 subunit, so the specific GO:0035650 AP-1 adaptor complex binding term applies. Interim stand-in only: AAGAB binds the free subunit and is displaced before the tetramer exists (PMID:39145939), so it does not bind an assembled AP-1 complex; the accurate term is the subunit-binding term filed under proposed_new_terms. Proposed replacements: AP-1 adaptor complex binding Supporting Evidence: PMID:34494650 AAGAB promotes AP1 assembly by binding and stabilizing the Ξ³ and Ο subunits of AP1 |
| GO:0005515 protein binding | IPI PMID:28514442 Architecture of the human interactome defines protein commun... | MODIFY | Summary: Interaction with AP1G1 isoform 2 (UniProtKB:O43747-2), the gamma1 subunit of AP-1, from a large-scale interactome study. This is the interaction AAGAB is named for - "alpha- and gamma-adaptin-binding protein" - and structural work shows the AAGAB C-terminal domain recognises AP1-gamma and AP2-alpha through a shared surface. Modified to GO:0035650 AP-1 adaptor complex binding. Reason: Bare protein binding; partner is the AP-1 gamma subunit, the eponymous AAGAB client. Interim stand-in only: AAGAB binds the free subunit and is displaced before the tetramer exists (PMID:39145939), so it does not bind an assembled AP-1 complex; the accurate term is the subunit-binding term filed under proposed_new_terms. Proposed replacements: AP-1 adaptor complex binding Supporting Evidence: PMID:34494650 AAGAB promotes AP1 assembly by binding and stabilizing the Ξ³ and Ο subunits of AP1 file:human/AAGAB/AAGAB-uniprot.txt SUBUNIT: Associated with AP-1 and AP-2 complexes. |
| GO:0005515 protein binding | IPI PMID:28514442 Architecture of the human interactome defines protein commun... | MODIFY | Summary: Second independent large-scale recovery of the AAGAB-AP2S1 (UniProtKB:P53680) interaction. Same assessment as the other AP-2 sigma2 entries: a reproducible, mechanistically central interaction recorded under an uninformative term. Reason: Bare protein binding; partner is the AP-2 sigma2 subunit. Interim stand-in only: AAGAB binds the free subunit and is displaced before the tetramer exists (PMID:39145939), so it does not bind an assembled AP-2 complex; the accurate term is the subunit-binding term filed under proposed_new_terms. Proposed replacements: AP-2 adaptor complex binding Supporting Evidence: PMID:31353312 assembly of the AP2 adaptor complex begins with the AAGAB-Ξ±-subunit dimer. |
| GO:0005515 protein binding | IPI PMID:29892012 An interactome perturbation framework prioritizes damaging m... | MODIFY | Summary: AAGAB-AP2S1 (UniProtKB:P53680) interaction recovered again in an interactome perturbation study of damaging missense mutations. The reproducibility of this particular pair across methodologically distinct screens is itself informative and matches the structurally defined pseudoGTPase-sigma interface. Reason: Bare protein binding; partner is the AP-2 sigma2 subunit. Interim stand-in only: AAGAB binds the free subunit and is displaced before the tetramer exists (PMID:39145939), so it does not bind an assembled AP-2 complex; the accurate term is the subunit-binding term filed under proposed_new_terms. Proposed replacements: AP-2 adaptor complex binding Supporting Evidence: PMID:31353312 AAGAB acts as a specific chaperone for the Ξ± subunit monomer. |
| GO:0005515 protein binding | IPI PMID:32296183 A reference map of the human binary protein interactome. | MODIFY | Summary: Interaction with AP1G1 isoform 2 (UniProtKB:O43747-2) in the human binary interactome reference map. Same assessment as the other AP-1 gamma entry. Reason: Bare protein binding; partner is the AP-1 gamma1 subunit. Interim stand-in only: AAGAB binds the free subunit and is displaced before the tetramer exists (PMID:39145939), so it does not bind an assembled AP-1 complex; the accurate term is the subunit-binding term filed under proposed_new_terms. Proposed replacements: AP-1 adaptor complex binding Supporting Evidence: PMID:34494650 AAGAB promotes AP1 assembly by binding and stabilizing the Ξ³ and Ο subunits of AP1 |
| GO:0005515 protein binding | IPI PMID:32296183 A reference map of the human binary protein interactome. | MODIFY | Summary: Interaction with AP2A2 (UniProtKB:O94973), the alpha2 subunit of AP-2. This is the other half of the eponymous alpha/gamma-adaptin binding activity, and the subunit that AAGAB stabilises as a monomer to nucleate AP-2 assembly. Reason: Bare protein binding; partner is the AP-2 alpha subunit. Interim stand-in only: AAGAB binds the free subunit and is displaced before the tetramer exists (PMID:39145939), so it does not bind an assembled AP-2 complex; the accurate term is the subunit-binding term filed under proposed_new_terms. Proposed replacements: AP-2 adaptor complex binding Supporting Evidence: PMID:31353312 AAGAB acts as a molecular chaperone to stabilize the Ξ± subunit monomer. |
| GO:0005515 protein binding | IPI PMID:32296183 A reference map of the human binary protein interactome. | MODIFY | Summary: AAGAB-AP2S1 (UniProtKB:P53680) interaction in the human binary interactome reference map. Same assessment as the other AP-2 sigma2 entries. Reason: Bare protein binding; partner is the AP-2 sigma2 subunit. Interim stand-in only: AAGAB binds the free subunit and is displaced before the tetramer exists (PMID:39145939), so it does not bind an assembled AP-2 complex; the accurate term is the subunit-binding term filed under proposed_new_terms. Proposed replacements: AP-2 adaptor complex binding Supporting Evidence: PMID:31353312 AAGAB acts as a specific chaperone for the Ξ± subunit monomer. |
| GO:0005515 protein binding | IPI PMID:32296183 A reference map of the human binary protein interactome. | MODIFY | Summary: Interaction with AP1S3 isoform 2 (UniProtKB:Q96PC3-2), an AP-1 sigma subunit. Same assessment as the other AP-1 sigma entry; AAGAB's pseudoGTPase domain engages the small sigma subunits of both AP-1 and AP-2. Reason: Bare protein binding; partner is an AP-1 sigma subunit. Interim stand-in only: AAGAB binds the free subunit and is displaced before the tetramer exists (PMID:39145939), so it does not bind an assembled AP-1 complex; the accurate term is the subunit-binding term filed under proposed_new_terms. Proposed replacements: AP-1 adaptor complex binding Supporting Evidence: PMID:34494650 AAGAB promotes AP1 assembly by binding and stabilizing the Ξ³ and Ο subunits of AP1 |
| GO:0005515 protein binding | IPI PMID:33961781 Dual proteome-scale networks reveal cell-specific remodeling... | MODIFY | Summary: AAGAB-AP2A2 (UniProtKB:O94973) interaction from the BioPlex affinity-purification interactome. Independent, orthogonal (AP-MS rather than two-hybrid) recovery of the AP-2 alpha subunit interaction. Reason: Bare protein binding; partner is the AP-2 alpha subunit. Interim stand-in only: AAGAB binds the free subunit and is displaced before the tetramer exists (PMID:39145939), so it does not bind an assembled AP-2 complex; the accurate term is the subunit-binding term filed under proposed_new_terms. Proposed replacements: AP-2 adaptor complex binding Supporting Evidence: PMID:31353312 AAGAB acts as a molecular chaperone to stabilize the Ξ± subunit monomer. |
| GO:0005515 protein binding | IPI PMID:33961781 Dual proteome-scale networks reveal cell-specific remodeling... | MODIFY | Summary: AAGAB-AP2S1 (UniProtKB:P53680) interaction from the BioPlex AP-MS interactome. Same assessment as the other AP-2 sigma2 entries. Reason: Bare protein binding; partner is the AP-2 sigma2 subunit. Interim stand-in only: AAGAB binds the free subunit and is displaced before the tetramer exists (PMID:39145939), so it does not bind an assembled AP-2 complex; the accurate term is the subunit-binding term filed under proposed_new_terms. Proposed replacements: AP-2 adaptor complex binding Supporting Evidence: PMID:31353312 AAGAB acts as a specific chaperone for the Ξ± subunit monomer. |
| GO:0005515 protein binding | IPI PMID:33961781 Dual proteome-scale networks reveal cell-specific remodeling... | MODIFY | Summary: AAGAB-AP1S3 (UniProtKB:Q96PC3) interaction from the BioPlex AP-MS interactome. Same assessment as the other AP-1 sigma entries. Reason: Bare protein binding; partner is an AP-1 sigma subunit. Interim stand-in only: AAGAB binds the free subunit and is displaced before the tetramer exists (PMID:39145939), so it does not bind an assembled AP-1 complex; the accurate term is the subunit-binding term filed under proposed_new_terms. Proposed replacements: AP-1 adaptor complex binding Supporting Evidence: PMID:34494650 AAGAB promotes AP1 assembly by binding and stabilizing the Ξ³ and Ο subunits of AP1 |
| GO:0005515 protein binding | IPI PMID:35271311 OpenCell: Endogenous tagging for the cartography of human ce... | MODIFY | Summary: AAGAB-AP2S1 (UniProtKB:P53680) interaction recovered by OpenCell, using endogenous split-mNeonGreen tagging and immunoprecipitation - that is, at native expression levels rather than by overexpression. This is the methodologically strongest of the high-throughput recoveries of this pair. Reason: Bare protein binding; partner is the AP-2 sigma2 subunit, here detected at endogenous expression levels. Interim stand-in only: AAGAB binds the free subunit and is displaced before the tetramer exists (PMID:39145939), so it does not bind an assembled AP-2 complex; the accurate term is the subunit-binding term filed under proposed_new_terms. Proposed replacements: AP-2 adaptor complex binding Supporting Evidence: PMID:31353312 AAGAB acts as a specific chaperone for the Ξ± subunit monomer. |
| GO:0005515 protein binding | IPI PMID:40205054 Multimodal cell maps as a foundation for structural and func... | MODIFY | Summary: AAGAB-AP2S1 (UniProtKB:P53680) interaction from a multimodal cell-map study - the seventh independent large-scale recovery of this pair. Same assessment as the other AP-2 sigma2 entries. Reason: Bare protein binding; partner is the AP-2 sigma2 subunit. Interim stand-in only: AAGAB binds the free subunit and is displaced before the tetramer exists (PMID:39145939), so it does not bind an assembled AP-2 complex; the accurate term is the subunit-binding term filed under proposed_new_terms. Proposed replacements: AP-2 adaptor complex binding Supporting Evidence: PMID:31353312 AAGAB acts as a specific chaperone for the Ξ± subunit monomer. |
| GO:0005829 cytosol | IDA GO_REF:0000052 | ACCEPT | Summary: Direct immunofluorescence localisation (Human Protein Atlas) to the cytosol. Agrees with the UniProt subcellular location and with the mechanistic requirement that AP-2 assembly happens in the cytosol before delivery of the assembled adaptor to the cell surface. Core localisation. Supporting Evidence: PMID:31353312 assembly of the AP2 adaptor complex in the cytosol, prior to the trafficking of the adaptor complex to the cell surface. |
| GO:0005737 cytoplasm | IDA GO_REF:0000054 | KEEP AS NON CORE | Summary: Correct but redundant: "cytoplasm" is a parent of the "cytosol" term already annotated twice with better evidence. Kept as non-core rather than modified, since it is not wrong, merely less informative. Supporting Evidence: file:human/AAGAB/AAGAB-uniprot.txt SUBCELLULAR LOCATION: Cytoplasm, cytosol {ECO:0000269|PubMed:23000146, |
| GO:0051131 chaperone-mediated protein complex assembly | IMP PMID:34494650 AAGAB is an assembly chaperone regulating AP1 and AP2 clathr... | NEW | Summary: Proposed new annotation, and the single most important gap in this gene's record: GOA currently gives AAGAB no biological process at all. AAGAB is an assembly chaperone that governs the ordered assembly of the heterotetrameric AP-1, AP-2 and AP-4 clathrin adaptor complexes, binding and stabilising the small sigma subunits through its N-terminal pseudoGTPase domain and the gamma/alpha subunits through its C-terminal dimerisation domain. Loss of AAGAB causes the unassembled subunits to be degraded and abolishes complex formation. Crucially for term choice, AAGAB is released before the tetramer is complete - for AP-2 it hands its alpha/sigma2 intermediate to CCDC32 - so it satisfies the defining clause of GO:0051131, "mediated by chaperone molecules that do not form part of the finished complex". The specificity of the effect (AP-1, AP-2 and AP-4 but explicitly not AP-3) argues against this being a generic stability phenomenon. Reason: Fills a total biological-process vacuum with a term that exactly matches the published mechanism, including the "not part of the finished complex" requirement. Supporting Evidence: PMID:34494650 AAGAB is an assembly chaperone regulating AP1 and AP2 clathrin adaptors. PMID:35976721 we report that the alpha- and gamma-adaptin-binding protein (AAGAB, also known as p34) binds to and stabilizes the AP-4 Ξ΅ and Ο4 subunits, thus promoting complex assembly. PMID:36598941 Alpha and gamma adaptin-binding protein (AAGAB) is a chaperone governing the assembly of the heterotetrameric adaptor complexes 1 and 2 (AP1 and AP2) involved in clathrin-mediated membrane trafficking. file:human/AAGAB/AAGAB-deep-research-affinage.md AAGAB (p34) is an assembly chaperone for heterotetrameric clathrin adaptor protein complexes, nucleating the formation of AP1, AP2, and AP4 |
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Download this section (compressed HTML)Q: Why is AP-3 exempt? AAGAB is required for AP-1, AP-2 and AP-4 assembly but explicitly not for AP-3. Identifying what distinguishes the AP-3 subunits - a different chaperone, or an intrinsically spontaneous assembly - would sharpen the mechanism and indicate whether a parallel AP-3 assembly chaperone remains to be found.
Q: Is the reported AAGAB-NEDD4-1-PTEN/SHIP2 axis in hypoxic-ischaemic injury a genuinely separate function, or a downstream consequence of impaired adaptor assembly? It comes from a single group in rodent models, and NEDD4-1 substrate ubiquitination is plausibly affected indirectly by altered endocytic trafficking.
Q: Does AAGAB have clients beyond the AP complexes? Its architecture - a pseudoGTPase domain reading small subunits plus a dimerisation domain reading large subunits - is generic enough that other heteromeric complexes could use the same chaperone.
Q: Why is haploinsufficiency for a chaperone of three ubiquitous adaptor complexes tolerated everywhere except palms and soles? Understanding the tissue restriction of PPKP1A would reveal which adaptor-dependent cargo is rate-limiting in acral keratinocytes.
Experiment: Quantitative interaction proteomics on endogenously tagged AAGAB under conditions that trap transient chaperone-client complexes (crosslinking, or a substrate-trap AAGAB mutant), comparing wild type with the PPKP1A C-terminal truncations, to enumerate clients systematically rather than by candidate.
Hypothesis: AAGAB is a general assembly chaperone whose client range extends beyond AP-1/AP-2/AP-4.
Type: interaction proteomics
Experiment: Swap the sigma-subunit surfaces between AP-1/AP-2 and AP-3 and test whether AP-3 sigma chimeras become AAGAB-dependent for assembly, and whether AP-1/AP-2 sigma chimeras carrying AP-3 surfaces become AAGAB-independent.
Hypothesis: AP-3 assembly is AAGAB-independent because its sigma subunit lacks the pseudoGTPase-domain binding determinants.
Type: structure-function
Experiment: Compare surface-proteome remodelling on AAGAB depletion between palmoplantar and truncal primary keratinocytes, identifying cargo whose surface level is disproportionately AAGAB-sensitive in acral cells.
Hypothesis: The acral restriction of PPKP1A reflects a specific adaptor-dependent cargo in palmoplantar keratinocytes.
Type: cell biology
Experiment: Test in vitro whether purified AAGAB can refold chemically denatured AP-2 sigma2, versus whether it only binds and stabilises already-folded sigma2 and accelerates its incorporation into the tetramer. A negative refolding result with a positive assembly result would formally justify the proposed assembly-chaperone molecular function term.
Hypothesis: AAGAB's molecular function is assembly chaperoning and not folding chaperoning.
Type: in vitro reconstitution
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