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AP2S1

AP-2 complex subunit sigma · UniProt P53680

Length
142 aa
Mass
17.0 kDa
Annotated
2026-06-09
20 papers in source corpus 7 papers cited in narrative 7 extracted findings
Cross-family judge vs UniProt: Affinage preferred faithfulness: 5/5 claims corpus-supported (100%)

Mechanistic narrative

Synthesis pass · prose summary of the discoveries below

AP2S1 encodes the σ2 (AP17) subunit of the heterotetrameric AP2 clathrin adaptor complex, which links clathrin to the plasma membrane and recognizes tyrosine- and dileucine-based sorting motifs of cargo proteins during clathrin-mediated endocytosis (PMID:23222959, PMID:9040778). Within the assembled complex, Arg15 of AP2σ2 contacts the dileucine motif of cargo, and missense mutations at this residue impair both AP2σ2 association with the other AP2 subunits (AP2α, AP2β2, AP2μ2) and its interaction with the calcium-sensing receptor (CaSR), reducing CaSR endocytosis and CaSR-mediated calcium signaling (PMID:23222959, PMID:33729479). These loss-of-function mutations cause familial hypocalciuric hypercalcemia type 3, recapitulated in Arg15Leu knock-in mice that display hypercalcaemia, hypermagnesaemia, and hypophosphataemia (PMID:33729479). The cargo-adaptor role extends beyond Arg15: variants at Arg10, Lys18, and Arg61 likewise disrupt AP2 complex formation and reduce general clathrin-mediated endocytosis [PMID:bio_10.1101_2024.07.22.24310683]. Independent of its endocytic function, AP2S1 also restrains amyloid precursor protein (APP) degradation by limiting RAB9-positive late endosome-to-LAMP1-positive lysosome fusion via VPS41, such that its depletion lowers APP and Aβ levels (PMID:36412210). AP2σ2 is essential for development, as homozygous Ap2s1 loss is embryonic lethal while heterozygotes are haplosufficient (PMID:29479578).

Mechanistic history

Synthesis pass · year-by-year structured walk · 7 steps
  1. 1996 Medium

    Established the molecular identity of AP2S1 as the small σ chain of the clathrin-associated AP-2 complex, placing it within the endocytic machinery.

    Evidence cDNA cloning and FISH chromosomal mapping to 19q13.2→q13.3

    PMID:9040778

    Open questions at the time
    • No functional assay of the encoded protein
    • Cargo specificity and complex assembly not addressed
  2. 1998 Medium

    Showed AP2S1 produces an alternatively spliced shorter isoform (AP17Δ) expressed alongside the canonical transcript, raising the possibility of isoform-specific function.

    Evidence cDNA library screening and variant-specific RT-PCR in leukocytes and leukemia cells

    PMID:9767099

    Open questions at the time
    • Functional consequence of the 38-aa deletion unknown
    • No evidence the isoform incorporates into AP2 complexes
  3. 2012 High

    Linked AP2σ2 cargo-recognition function to a human disease by showing Arg15 mutations impair CaSR endocytosis and calcium signaling, defining the cargo-motif contact mechanism.

    Evidence Cellular endocytosis and calcium-signaling assays in CaSR-expressing cells with CaSR dileucine-motif mutational analysis

    PMID:23222959

    Open questions at the time
    • Direct biochemical demonstration of AP2σ2–CaSR motif binding not yet shown
    • Effect on other AP2 subunit interactions not tested
  4. 2017 High

    Demonstrated AP2σ2 is essential for embryonic development and that a single functional allele suffices for normal calcium homeostasis, distinguishing dosage requirements from FHH3 dominant-negative effects.

    Evidence ENU mutagenesis, structural modeling, and in vivo mouse phenotyping of knockout and heterozygous animals

    PMID:29479578

    Open questions at the time
    • Tissue-specific developmental roles not resolved
    • Mechanism of embryonic lethality not defined
  5. 2021 High

    Provided reciprocal biochemical proof that the Arg15Leu mutation weakens both AP2 intra-complex assembly and AP2σ2–CaSR binding, and modeled FHH3 in vivo with pharmacological intervention.

    Evidence CRISPR/Cas9 knock-in mice, co-immunoprecipitation of AP2 subunits and CaSR, plasma biochemistry, and cinacalcet treatment

    PMID:33729479

    Open questions at the time
    • Cinacalcet did not restore the AP2σ2–CaSR interaction, leaving the molecular defect uncorrected
    • Structural basis of disrupted assembly not solved
  6. 2022 High

    Uncovered an endocytosis-independent role in which AP2S1 limits APP degradation by restraining late endosome-to-lysosome fusion, implicating it in amyloid handling.

    Evidence siRNA knockdown/overexpression with RAB9/LAMP1 colocalization, VPS41 epistasis, and AAV-shRNA delivery in APP/PS1 mice with cognitive assays

    PMID:36412210

    Open questions at the time
    • How AP2S1 mechanistically controls LE-lysosome fusion is undefined
    • Relationship between this role and canonical AP2 function unclear
  7. 2024 Medium

    Extended the cargo-adaptor mechanism beyond Arg15 by showing additional surface residues are required for AP2 complex assembly and general clathrin-mediated endocytosis.

    Evidence Transferrin uptake CME assay, Co-IP of AP2 subunits, and quantitative interaction proteomics (preprint)

    PMID:bio_10.1101_2024.07.22.24310683

    Open questions at the time
    • Preprint not yet peer-reviewed
    • Functional role of the intersectin-1 interaction not validated
    • Clinical correlation of these variants not established

Open questions

Synthesis pass · forward-looking unresolved questions
  • How AP2S1's canonical endocytic adaptor function and its endosome-lysosome fusion role are mechanistically coordinated, and the structural basis of cargo-motif recognition, remain open.
  • No high-resolution structure of AP2σ2 bound to a cargo dileucine motif in the timeline
  • Whether the APP/fusion role requires the assembled AP2 complex is unknown

Mechanism profile

Synthesis pass · controlled-vocabulary classification · explore literature graph →
Molecular activity
GO:0060090 molecular adaptor activity 3 GO:0038024 cargo receptor activity 1
Localization
GO:0005764 lysosome 1 GO:0005768 endosome 1 GO:0005886 plasma membrane 1
Pathway
R-HSA-1643685 Disease 2 R-HSA-5653656 Vesicle-mediated transport 2
Partners
AP2A1AP2B1AP2M1CASRITSN1VPS41
Complex memberships
AP2 adaptor complex

Evidence

Reading pass · 7 per-paper findings extracted from the source corpus
Year Finding Method Journal Conf PMIDs
2012 AP2S1 encodes the σ2 subunit of the AP2 heterotetramer (α, β, μ, σ), which links clathrin to vesicle membranes and binds tyrosine- and dileucine-based motifs of membrane-associated cargo proteins during clathrin-mediated endocytosis. Missense mutations at Arg15 of AP2σ2, which forms key contacts with dileucine-based motifs of CCV cargo proteins, impair CaSR endocytosis, reduce CaSR-mediated intracellular signaling, and decrease sensitivity of CaSR-expressing cells to extracellular calcium, likely through loss of interaction with a C-terminal CaSR dileucine-based motif. Cellular expression assays, CaSR endocytosis assays, calcium signaling assays in CaSR-expressing cells, mutational analysis of the CaSR dileucine motif Nature genetics High 23222959
1996 The human AP2S1 gene (symbol CLAPS2) encodes AP17, the small (sigma) chain of the clathrin-associated AP-2 complex, and maps to chromosome 19q13.2→q13.3. cDNA cloning, chromosomal assignment by fluorescence in situ hybridization Cytogenetics and cell genetics Medium 9040778
1998 AP2S1 (CLAPS2) undergoes alternative splicing to produce a variant transcript encoding AP17Δ, a 142 aa protein lacking 38 aa of the canonical AP17; both transcripts are expressed in leukocytes and leukemia cells. cDNA library screening, complete coding sequence determination, RT-PCR with variant-specific primers Gene Medium 9767099
2017 ENU-induced deletion of 17 evolutionarily conserved amino acids forming part of the AP2σ α1-helix, α1-β3 loop, and β3 strand (del17 splice-site variant) results in a non-functional AP2σ. Homozygous Ap2s1 knockout mice are non-viable and die between embryonic days 3.5 and 9.5, demonstrating that AP2σ is essential for embryonic patterning and organogenesis. Heterozygous mice are haplosufficient with normal calcium homeostasis. ENU mutagenesis screen, 3D structural modeling, cellular expression of missense variants, CaSR-mediated signaling assays, in vivo mouse phenotyping (plasma biochemistry, urinary excretion, hormone measurements) JBMR plus High 29479578
2021 The AP2S1 p.Arg15Leu mutation impairs protein-protein interactions between AP2σ2 and the other AP2 complex subunits (AP2α, AP2β2, AP2μ2), and also reduces the AP2σ2–CaSR interaction, as demonstrated by co-immunoprecipitation. CRISPR/Cas9 knock-in mice harboring p.Arg15Leu recapitulate FHH3 with hypercalcaemia, hypermagnesaemia, and hypophosphataemia; cinacalcet reduced plasma calcium and PTH in these mice but did not restore the diminished AP2σ2–CaSR interaction in vitro. CRISPR/Cas9 knock-in mouse generation, co-immunoprecipitation (AP2σ2 with AP2 subunits and CaSR), in vivo plasma biochemistry, cinacalcet treatment of mice, in vitro signaling assay Human molecular genetics High 33729479
2022 AP2S1 regulates the degradation of amyloid precursor protein (APP) through a mechanism involving late endosome (LE)-to-lysosome fusion rather than endocytosis per se. Knockdown of AP2S1 promoted translocation of APP from RAB9-positive late endosomes to LAMP1-positive lysosomes and enhanced LE-lysosome fusion; this was prevented by silencing VPS41, a component required for LE-lysosome fusion. AAV-mediated AP2S1 knockdown in hippocampus of APP/PS1 mice reduced APP and Aβ levels and improved cognitive function. siRNA knockdown and overexpression in APP695-expressing cells, confocal morphological colocalization (RAB9, LAMP1 markers), VPS41 co-knockdown epistasis, AAV-mediated shRNA delivery in APP/PS1 mice, Western blotting, cognitive behavioral assays Traffic (Copenhagen, Denmark) High 36412210
2024 Five AP2S1 variants at residues other than Arg15 (p.Arg10Trp, p.Arg10Gln, p.Lys18Glu, p.Lys18Asn, p.Arg61His) decrease cell viability, reduce clathrin-mediated endocytosis (transferrin uptake assay), and disrupt interactions between AP2σ2 and other AP2 complex subunits, thereby impairing AP2 complex formation. The p.Arg10Trp variant additionally shows reduced interactions with 44 human proteins including intersectin-1, a component required for clathrin-coated pit formation and synaptic vesicle dynamics. Cell viability assays, transferrin uptake CME assay, co-immunoprecipitation of AP2 subunits, quantitative proteomics/interaction profiling (mass spectrometry with intersectin-1) bioRxivpreprint Medium bio_10.1101_2024.07.22.24310683

Source papers

Stage 0 corpus · 20 papers · ranked by NIH iCite citations
Year Title Journal Citations PMID
2012 Mutations in AP2S1 cause familial hypocalciuric hypercalcemia type 3. Nature genetics 191 23222959
1999 Localization of familial benign hypercalcemia, Oklahoma variant (FBHOk), to chromosome 19q13. American journal of human genetics 85 9915958
2014 Codon Arg15 mutations of the AP2S1 gene: common occurrence in familial hypocalciuric hypercalcemia cases negative for calcium-sensing receptor (CASR) mutations. The Journal of clinical endocrinology and metabolism 33 24731014
2015 Cinacalcet Treatment in an Adolescent With Concurrent 22q11.2 Deletion Syndrome and Familial Hypocalciuric Hypercalcemia Type 3 Caused by AP2S1 Mutation. The Journal of clinical endocrinology and metabolism 20 25993639
2017 Stepwise CaSR, AP2S1, and GNA11 sequencing in patients with suspected familial hypocalciuric hypercalcemia. Endocrine 18 28176280
2017 N-ethyl-N-nitrosourea-Induced Adaptor Protein 2 Sigma Subunit 1 (Ap2s1) Mutations Establish Ap2s1 Loss-of-Function Mice. JBMR plus 16 29479578
2016 AP2S1 and GNA11 mutations - not a common cause of familial hypocalciuric hypercalcemia. European journal of endocrinology 16 27913609
2013 Identification of AP2S1 mutation and effects of low calcium formula in an infant with hypercalcemia and hypercalciuria. The Journal of clinical endocrinology and metabolism 16 24081735
2022 AP2S1 regulates APP degradation through late endosome-lysosome fusion in cells and APP/PS1 mice. Traffic (Copenhagen, Denmark) 15 36412210
2021 Ap2s1 mutation causes hypercalcaemia in mice and impairs interaction between calcium-sensing receptor and adaptor protein-2. Human molecular genetics 15 33729479
2014 Mutational analysis of the adaptor protein 2 sigma subunit (AP2S1) gene: search for autosomal dominant hypocalcemia type 3 (ADH3). The Journal of clinical endocrinology and metabolism 12 24708097
2014 Analysis of AP2S1, a calcium-sensing receptor regulator, in familial and sporadic isolated hypoparathyroidism. The Journal of clinical endocrinology and metabolism 7 24423332
2020 Clinical and Biochemical Features in a Case of Familial Hypocalciuric Hypercalcemia Type 3 with AP2S1 Gene Mutation in Codon Arg15His. Case reports in pediatrics 6 32047691
2019 Cinacalcet sustainedly prevents pancreatitis in a child with a compound heterozygous SPINK1/AP2S1 mutation. Pancreatology : official journal of the International Association of Pancreatology (IAP) ... [et al.] 6 31391146
2019 A Hong Kong Chinese kindred with familial hypocalciuric hypercalcaemia caused by AP2S1 mutation. F1000Research 4 31723423
1996 Human CLAPS2 encoding AP17, a small chain of the clathrin-associated protein complex: cDNA cloning and chromosomal assignment to 19q13.2-->q13.3. Cytogenetics and cell genetics 4 9040778
1998 A novel spliced transcript of human CLAPS2 encoding a protein alternative to clathrin adaptor protein AP17. Gene 2 9767099
2020 Familial hypocalciuric hypercalcaemia type 3: AP2S1 missense mutation. BMJ case reports 1 33168530
2026 Single-cell extracellular vesicle-program scoring maps immunometabolic rewiring and immune crosstalk of mesenchymal stromal cells in intervertebral disc degeneration, prioritizing AP2S1 and CSTB. Frontiers in immunology 0 42253998
2025 Successful Treatment With Evocalcet Against Familial Hypocalciuric Hypercalcemia Type 3 (FHH3) Identified by AP2S1 Gene Mutation (p.Arg15Leu). Case reports in endocrinology 0 39949382

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