Affinage

AP1S1

AP-1 complex subunit sigma-1A · UniProt P61966

Round 2 corrected
Length
158 aa
Mass
18.7 kDa
Annotated
2026-04-28
52 papers in source corpus 11 papers cited in narrative 11 extracted findings

Mechanistic narrative

Synthesis pass · prose summary of the discoveries below

AP1S1 encodes σ1A, the small subunit of the heterotetrameric AP-1 clathrin adaptor complex, and is essential for cargo sorting at the trans-Golgi network and endosomal compartments. The σ1A subunit assembles with β1, γ, and μ1 into a core whose crystal structure reveals an inactive conformation analogous to AP-2; σ1A recognizes [DE]XXXL[LI] dileucine sorting motifs on cargo proteins including the copper ATPases ATP7A/ATP7B and EGFR, directing their intracellular trafficking (PMID:15377783, PMID:39269494, PMID:37659097). σ1A further nucleates a complex with ArfGAP1 and Rabex-5 that promotes Rab5/Vps34 PI3-kinase activity required for multivesicular body formation and early-to-late endosome maturation, and it maintains epithelial tight-junction integrity by controlling the localization of ZO-1 and claudin-3 (PMID:27411398, PMID:32306098). Loss-of-function mutations in AP1S1—whether truncating or missense alleles that prevent complex assembly—cause MEDNIK syndrome, a multisystem disorder with features of both copper deficiency and copper overload (PMID:19057675, PMID:39269494).

Mechanistic history

Synthesis pass · year-by-year structured walk · 9 steps
  1. 1991 Medium

    Identification of AP19 (σ1A) as the smallest subunit of the Golgi-localized AP-1 complex established that clathrin adaptor complexes at distinct membranes share conserved small-chain architecture.

    Evidence cDNA cloning from rodent brain with sequence homology analysis revealing conservation with AP-2 small chain and yeast ortholog

    PMID:2040623

    Open questions at the time
    • No functional data beyond sequence identity
    • Role within the AP-1 complex undefined
  2. 2004 High

    The 4-Å crystal structure of the AP-1 core revealed how σ1A integrates with β1, γ, and μ1 subunits, demonstrating an inactive conformation that must undergo rearrangement to engage cargo sorting signals.

    Evidence X-ray crystallography of intact AP-1 core with mutagenesis-validated TGN localization and liposome binding

    PMID:15377783

    Open questions at the time
    • Active conformation with bound cargo motif not resolved
    • Mechanism of activation on membranes unknown
  3. 2008 High

    Genetic evidence linked AP1S1 loss-of-function to MEDNIK syndrome and demonstrated that σ1A is required for skin and neural development, answering whether this adaptor subunit has non-redundant developmental roles.

    Evidence Human patient genetics (splice mutation in four families), zebrafish morpholino knockdown recapitulating phenotype, rescued by wild-type but not truncated human AP1S1 mRNA

    PMID:19057675

    Open questions at the time
    • Specific cargo(s) mislocalized in disease not identified
    • Whether paralog σ1B can partially compensate in specific tissues unknown
  4. 2014 Medium

    Connecting MEDNIK syndrome phenotypes to mislocalization of copper ATPases ATP7A and ATP7B explained how a single AP-1 sorting defect can produce concurrent copper deficiency and copper overload.

    Evidence Clinical and biochemical analysis of MEDNIK patients integrated with known AP-1 cargo-sorting function

    PMID:24754424

    Open questions at the time
    • Direct cell-biological demonstration of ATP7A/ATP7B mistrafficking in AP1S1-null cells not provided in this study
    • Relative contributions to Menkes- versus Wilson-like features unresolved
  5. 2016 High

    Discovery that σ1A nucleates a ternary complex with ArfGAP1 and Rabex-5 to drive Rab5/Vps34-dependent endosome maturation revealed a function for AP1S1 beyond cargo sorting—regulating multivesicular body biogenesis.

    Evidence Reciprocal co-immunoprecipitation, PI3-kinase activity assays, electron microscopy of endosomes, and genetic knockout models

    PMID:27411398

    Open questions at the time
    • Structural basis of the σ1A–ArfGAP1 interaction not determined
    • Whether this complex operates at all cell types or is neuron-specific unclear
  6. 2020 High

    CRISPR knockout in intestinal epithelial cells showed that AP1S1 is required for tight-junction protein localization and barrier integrity, explaining the congenital diarrhea in MEDNIK patients.

    Evidence AP1S1 KO in CaCo2 cells with TEER, permeability, 3D lumen formation assays; wild-type rescue versus non-rescue by L90P and E116K mutants

    PMID:32306098

    Open questions at the time
    • Identity of the direct AP-1 cargo responsible for tight-junction assembly unknown
    • In vivo intestinal validation lacking
  7. 2023 Medium

    Demonstration that AP1S1 loss routes EGFR to lysosomal degradation rather than recycling expanded its cargo repertoire and linked AP1S1 to receptor tyrosine kinase signaling and drug sensitivity in cancer cells.

    Evidence AP1S1 KO in lung cancer cell lines with EGFR degradation assays, phosphorylation readouts, and erlotinib sensitivity under variable matrix stiffness

    PMID:37659097

    Open questions at the time
    • Whether AP1S1 directly recognizes a sorting motif on EGFR or acts indirectly not determined
    • Single cancer cell line context
  8. 2024 High

    Functional analysis of the L90P missense variant showed it fails to assemble into the AP-1 complex and cannot bind dileucine motifs, unifying truncating and missense alleles under a common loss-of-function mechanism for MEDNIK syndrome.

    Evidence Complex assembly assays, dileucine sorting motif binding assays, and clinical phenotype correlation

    PMID:39269494

    Open questions at the time
    • Structural mechanism by which L90P disrupts assembly not resolved at atomic level
    • Genotype–phenotype correlation across full allelic spectrum incomplete
  9. 2025 Medium

    Neuronal knockdown of Ap1s1 revealed a role in protecting against cellular senescence and oxidative/Aβ-mediated stress, linking Golgi dispersion from AP-1 loss to neurodegeneration-relevant vulnerability.

    Evidence shRNA knockdown in Neuro-2a cells with senescence assays, H₂O₂ and Aβ toxicity, proteomic profiling, Golgi morphology imaging

    PMID:40954504

    Open questions at the time
    • Causal relationship between Golgi dispersion and senescence not established
    • In vivo neuronal phenotype not examined
    • Proteomic changes are correlative

Open questions

Synthesis pass · forward-looking unresolved questions
  • Key unresolved questions include the structural basis of σ1A cargo-motif recognition in the active conformation, the full inventory of AP-1/σ1A cargoes responsible for each MEDNIK tissue phenotype, and whether σ1A and σ1B have tissue-specific non-redundant functions beyond the brain.
  • Active-state structure of AP-1 with bound dileucine motif not available
  • Direct identification of mislocalized cargoes in patient-derived cells incomplete
  • Tissue-specific redundancy with σ1B not systematically tested

Mechanism profile

Synthesis pass · controlled-vocabulary classification · explore literature graph →
Molecular activity
GO:0060090 molecular adaptor activity 3 GO:0038024 cargo receptor activity 2
Localization
GO:0005794 Golgi apparatus 3 GO:0031410 cytoplasmic vesicle 2 GO:0005768 endosome 1
Pathway
R-HSA-5653656 Vesicle-mediated transport 5 R-HSA-9609507 Protein localization 3 R-HSA-1643685 Disease 2 R-HSA-382551 Transport of small molecules 1
Partners
AP1B1AP1G1AP1M1ARFGAP1ATP7AATP7BEGFRRABGEF1
Complex memberships
AP-1 adaptor complexAP-1/σ1A–ArfGAP1–Rabex-5 complex

Evidence

Reading pass · 11 per-paper findings extracted from the source corpus
Year Finding Method Journal Conf PMIDs
1991 AP19 (the protein product of AP1S1) was identified as the smallest polypeptide chain component of the AP-1 clathrin-associated protein complex located at the Golgi apparatus of mammalian cells. cDNA cloning from mouse brain predicted a protein of 158 amino acids (Mr 18,733). Sequence comparison revealed that AP19 is highly related to AP17 (the small chain of AP-2 at the plasma membrane), and a yeast homolog (Yap17p) was identified, establishing the evolutionary conservation of this subunit. cDNA cloning from rat/mouse brain libraries, nucleotide sequencing, sequence homology analysis The Journal of biological chemistry Medium 2040623
2004 The crystal structure of the AP-1 complex core was solved at 4-Å resolution, revealing that the intact sigma1 (σ1A, encoded by AP1S1) small chain together with the medium chain μ1 and N-terminal fragments of the large chains β1 and γ constitute the core. The molecular architecture closely resembles that of AP-2, and the structure represents an 'inactive' conformation with respect to tyrosine-based cargo sorting signal binding, establishing the structural basis for AP-1 complex assembly. X-ray crystallography at 4-Å resolution with directed mutagenesis validation of TGN localization Proceedings of the National Academy of Sciences of the United States of America High 15377783
2008 Loss-of-function mutation in AP1S1 (a splice mutation causing a premature stop codon in the σ1A subunit of AP-1) was identified as causing MEDNIK syndrome in humans. Knockdown of Ap1s1 in zebrafish via antisense morpholino oligonucleotides recapitulated the disease phenotype (impaired skin formation, reduced pigmentation, severe motility deficits due to impaired neural network development). Rescue with wild-type human AP1S1 mRNA but not the truncated mutant form confirmed loss-of-function, establishing a critical role for AP1S1 in development of skin and spinal cord through its function in AP-1-mediated vesicular trafficking. Antisense morpholino knockdown in zebrafish, mRNA rescue experiments, genetic analysis in human patients PLoS genetics High 19057675
2014 AP1S1 (σ1A subunit of AP-1) was shown to direct intracellular trafficking of the copper-transporting ATPases ATP7A and ATP7B between the trans-Golgi network and other organelles. Loss of AP1S1 function in MEDNIK syndrome disrupts this trafficking, producing combined clinical and biochemical signs of both Menkes disease (copper deficiency) and Wilson's disease (copper overload in liver), establishing AP1S1 as a regulator of copper homeostasis through its cargo-sorting role for ATP7A and ATP7B. Clinical and biochemical analysis of MEDNIK patients combined with mechanistic interpretation of AP-1 function in copper pump trafficking Annals of the New York Academy of Sciences Medium 24754424
2016 AP-1/σ1A (the AP-1 complex containing σ1A encoded by AP1S1) forms a complex with ArfGAP1 and Rabex-5. σ1A binds ArfGAP1 (with higher affinity for brain-specific ArfGAP1), and this AP-1/σ1A–ArfGAP1–Rabex-5 complex promotes increased endosomal Rabex-5 and enhanced Rab5(GTP)-stimulated Vps34 PI3-kinase activity, which is essential for multivesicular body (MVB) endosome formation. In contrast, σ1B (the paralog) binds Rabex-5 directly, preventing AP-1/σ1A complex formation with Rabex-5 and reducing endosomal Rabex-5. This differential regulation by σ1A versus σ1B controls early endosome maturation into MVB late endosomes, coordinating synaptic vesicle protein recycling and degradation. Co-immunoprecipitation, binding affinity studies, PI3-kinase activity assays, electron microscopy of endosomes, genetic knockout models Scientific reports High 27411398
2020 Loss of AP1S1 function causes an intestinal epithelial barrier defect. In CaCo2 intestinal cell AP1S1 knockout lines, tight-junction proteins ZO-1 and claudin-3 showed altered localization, transepithelial electrical resistance was decreased, dextran permeability was increased, and lumen formation in 3D cultures was abnormal. Re-expression of wild-type AP1S1 reverted these abnormalities, while expression of AP1S1 carrying missense mutations (p.Leu90Pro or p.Glu116Lys) did not rescue, indicating these missense variants are loss-of-function alleles that specifically impair epithelial barrier integrity. CRISPR/Cas9 knockout of AP1S1 in CaCo2 cells, stable rescue expression, immunofluorescence localization of tight-junction proteins, transepithelial electrical resistance measurement, dextran permeability assay, 3D culture lumen formation Human genetics High 32306098
2023 AP1S1 regulates EGFR intracellular trafficking under stiff matrix conditions. Knockout of AP1S1 in non-small cell lung cancer cells caused lysosomal degradation of EGFR (rather than recycling), leading to suppressed EGF-induced ALK phosphorylation. AP1S1 expression was upregulated under stiff matrix conditions, and its loss increased sensitivity of TKI-resistant H1975 cells to erlotinib, establishing AP1S1 as a component of the EGFR recycling pathway. AP1S1 knockout in lung cancer cell lines, lysosomal degradation assays, phosphorylation assays, drug sensitivity assays (erlotinib), matrix stiffness manipulation Journal of cellular physiology Medium 37659097
2024 The AP1S1 missense variant c.269T>C (σ1A L90P) is largely unable to assemble into the AP-1 complex and fails to bind [DE]XXXL[LI] dileucine-type sorting motifs, resulting in loss-of-function. Functional analyses demonstrated that this variant, previously thought to cause only non-syndromic congenital diarrhea, actually causes full MEDNIK syndrome, establishing that both truncating and missense AP1S1 variants produce AP-1 dysfunction by impairing σ1A assembly into the complex and cargo-motif recognition. Functional assembly assays, dileucine sorting motif binding assays, patient clinical correlation, protein structural analysis Journal of molecular medicine (Berlin, Germany) High 39269494
2025 In vitro mRNA splicing experiments confirmed that the AP1S1 splice-site variant c.430-1G>A causes a single base-pair deletion in exon 5 of the mRNA, resulting in a frameshift (p.Glu144ArgfsTer83) that alters the protein structure and disrupts AP1S1 function. Three-dimensional structural reconstruction of the mutant protein predicted conformational changes consistent with loss of function. In vitro mRNA splicing experiments with mutant plasmid, gene sequencing, 3D structural reconstruction International journal of genomics Medium 40901618
2025 Knockdown of Ap1s1 in neuronal cells (N2a) induced cellular senescence without directly impairing viability, but exacerbated neuronal vulnerability to oxidative stress (H₂O₂) and Aβ toxicity, manifesting as Golgi dispersion and reduced survival. Proteomic profiling following Ap1s1 depletion implicated dysregulation of rRNA modifications and Golgi-associated vesicle biogenesis, placing Ap1s1 at the nexus of Golgi function and neuronal stress responses. shRNA knockdown in Neuro-2a cells, senescence assays, H₂O₂ and Aβ toxicity assays, proteomic profiling, Golgi morphology imaging Alzheimer's research & therapy Medium 40954504
2026 NSCLC-derived extracellular vesicle miRNA hsa-let-7b-5p suppresses M2 macrophage polarization by targeting and inhibiting AP1S1 expression. In vitro experiments showed that the hsa-let-7b-5p mimic suppressed M2 polarization of TAMs via the AP1S1/p53 signaling axis, attenuating proliferation, migration, and invasion of NSCLC cells, establishing AP1S1 as a positive regulator of M2 macrophage polarization acting through p53 signaling. miRNA mimic transfection, flow cytometry for macrophage polarization, RT-qPCR, Western blot, Transwell invasion/migration assays, Ki-67 staining, bioinformatics International journal of genomics Low 41551936

Source papers

Stage 0 corpus · 52 papers · ranked by NIH iCite citations
Year Title Journal Citations PMID
2012 Insights into RNA biology from an atlas of mammalian mRNA-binding proteins. Cell 1718 22658674
2002 Generation and initial analysis of more than 15,000 full-length human and mouse cDNA sequences. Proceedings of the National Academy of Sciences of the United States of America 1479 12477932
2015 The BioPlex Network: A Systematic Exploration of the Human Interactome. Cell 1118 26186194
2017 Architecture of the human interactome defines protein communities and disease networks. Nature 1085 28514442
2015 A human interactome in three quantitative dimensions organized by stoichiometries and abundances. Cell 1015 26496610
2020 A reference map of the human binary protein interactome. Nature 849 32296183
2007 Large-scale mapping of human protein-protein interactions by mass spectrometry. Molecular systems biology 733 17353931
2021 Dual proteome-scale networks reveal cell-specific remodeling of the human interactome. Cell 705 33961781
2012 A census of human soluble protein complexes. Cell 689 22939629
2011 Phylogenetic-based propagation of functional annotations within the Gene Ontology consortium. Briefings in bioinformatics 656 21873635
2000 Clathrin. Annual review of biochemistry 471 10966473
2004 The status, quality, and expansion of the NIH full-length cDNA project: the Mammalian Gene Collection (MGC). Genome research 438 15489334
2022 OpenCell: Endogenous tagging for the cartography of human cellular organization. Science (New York, N.Y.) 432 35271311
1999 A novel clathrin adaptor complex mediates basolateral targeting in polarized epithelial cells. Cell 432 10535737
2000 Three ways to make a vesicle. Nature reviews. Molecular cell biology 410 11252894
2015 Panorama of ancient metazoan macromolecular complexes. Nature 407 26344197
2000 Secretory protein trafficking and organelle dynamics in living cells. Annual review of cell and developmental biology 380 11031247
2011 HIV-1 envelope glycoprotein biosynthesis, trafficking, and incorporation. Journal of molecular biology 362 21762802
2001 Structure--function relationships in HIV-1 Nef. EMBO reports 317 11463741
2017 Genome-wide CRISPR screen identifies HNRNPL as a prostate cancer dependency regulating RNA splicing. Proceedings of the National Academy of Sciences of the United States of America 282 28611215
2000 A family of proteins with gamma-adaptin and VHS domains that facilitate trafficking between the trans-Golgi network and the vacuole/lysosome. The Journal of cell biology 275 10747088
1998 Interaction of HIV-1 Nef with the cellular dileucine-based sorting pathway is required for CD4 down-regulation and optimal viral infectivity. Proceedings of the National Academy of Sciences of the United States of America 246 9736718
2002 Cooperation of GGAs and AP-1 in packaging MPRs at the trans-Golgi network. Science (New York, N.Y.) 217 12215646
1998 A dileucine motif in HIV-1 Nef is essential for sorting into clathrin-coated pits and for downregulation of CD4. Current biology : CB 211 9811611
1998 A dileucine motif in HIV-1 Nef acts as an internalization signal for CD4 downregulation and binds the AP-1 clathrin adaptor. Current biology : CB 205 9811606
2015 A deep proteomics perspective on CRM1-mediated nuclear export and nucleocytoplasmic partitioning. eLife 198 26673895
2004 HIV-1 Nef disrupts MHC-I trafficking by recruiting AP-1 to the MHC-I cytoplasmic tail. The Journal of cell biology 189 15569716
2003 The DNA sequence of human chromosome 7. Nature 188 12853948
1999 Interactions of the cytoplasmic domains of human and simian retroviral transmembrane proteins with components of the clathrin adaptor complexes modulate intracellular and cell surface expression of envelope glycoproteins. Journal of virology 166 9882340
2004 Crystal structure of the clathrin adaptor protein 1 core. Proceedings of the National Academy of Sciences of the United States of America 164 15377783
2008 Disruption of AP1S1, causing a novel neurocutaneous syndrome, perturbs development of the skin and spinal cord. PLoS genetics 132 19057675
2014 AP1S1 defect causing MEDNIK syndrome: a new adaptinopathy associated with defective copper metabolism. Annals of the New York Academy of Sciences 50 24754424
1991 AP17 and AP19, the mammalian small chains of the clathrin-associated protein complexes show homology to Yap17p, their putative homolog in yeast. The Journal of biological chemistry 50 2040623
2020 AP1S1 missense mutations cause a congenital enteropathy via an epithelial barrier defect. Human genetics 31 32306098
1993 Cloning of the YAP19 gene encoding a putative yeast homolog of AP19, the mammalian small chain of the clathrin-assembly proteins. Biochimica et biophysica acta 21 8373805
2018 MEDNIK syndrome with a frame shift causing mutation in AP1S1 gene and literature review of the clinical features. Metabolic brain disease 18 30244301
2016 AP-1/σ1A and AP-1/σ1B adaptor-proteins differentially regulate neuronal early endosome maturation via the Rab5/Vps34-pathway. Scientific reports 17 27411398
2023 Downregulation of AP1S1 causes the lysosomal degradation of EGFR in non-small cell lung cancer. Journal of cellular physiology 13 37659097
2007 Isolation and identification of newly isolated antagonistic Streptomyces sp. strain AP19-2 producing chromomycins. Journal of microbiology (Seoul, Korea) 13 18176531
2023 Isolation, identification, degradation mechanism and exploration of active enzymes in the ochratoxin A degrading strain Acinetobacter pittii AP19. Journal of hazardous materials 11 38150759
1996 Cloning and expression of a plant homologue of the small subunit of the Golgi-associated clathrin assembly protein AP19 from Camptotheca acuminata. Plant molecular biology 11 9002613
2022 The Overexpression and Clinical Significance of AP1S1 in Breast Cancer. Cancer management and research 10 35463798
2023 Clinicopathologic Features of IDEDNIK (MEDNIK) Syndrome in a Term Infant: Histopathologic Features of the Gastrointestinal Tract and Report of a Novel AP1S1 Variant. Pediatric and developmental pathology : the official journal of the Society for Pediatric Pathology and the Paediatric Pathology Society 5 37278357
2022 The Single Nucleotide Polymorphisms of AP1S1 are Associated with Risk of Esophageal Squamous Cell Carcinoma in Chinese Population. Pharmacogenomics and personalized medicine 5 35321090
2004 Existence of a tungsten-binding protein in Acidithiobacillus ferrooxidans AP19-3. Journal of bioscience and bioengineering 5 16233646
1997 Molecular characterization of the AP19 gene family in Arabidopsis thaliana: components of the Golgi AP-1 clathrin assembly protein complex. Plant molecular biology 5 9426606
2024 Revising pathogenesis of AP1S1-related MEDNIK syndrome: a missense variant in the AP1S1 gene as a causal genetic lesion. Journal of molecular medicine (Berlin, Germany) 4 39269494
2025 Clinical and Genetic Functional Validation of a Novel AP1S1 Mutation Causing MEDNIK Syndrome. International journal of genomics 2 40901618
2026 Mechanism of Non-Small Cell Lung Cancer-Derived Extracellular Vesicle miRNA hsa-let-7b-5p Targeting AP1S1 to Regulate M2 Macrophage Polarization. International journal of genomics 0 41551936
2025 Ap1s1 reduction in the aging brain heightens neuronal vulnerability to amyloid-β and oxidative stress in Alzheimer's pathogenesis. Alzheimer's research & therapy 0 40954504
2025 IDEDNIK syndrome: a newly recognized rare genetic disorder caused by AP1S1 and AP1B1 mutations. Frontiers in neurology 0 41404470
2025 Feeding-Triggered Seizures in a Newborn with AP1S1-Related MEDNIK Syndrome: Expanding the Phenotype of a Hyper-Rare Disease. Journal of clinical medicine 0 41517358