{"gene":"ARPC1A","run_date":"2026-06-09T22:02:44","timeline":{"discoveries":[{"year":1997,"finding":"ARPC1A (SOP2Hs) is a component of the mammalian Arp2/3 complex in human neutrophils, and the complex localizes to lamellipodia upon stimulation with serum or PDGF, implicating it in actin cytoskeleton organization at sites of protrusion.","method":"Peptide sequencing, cDNA cloning, subcellular localization by immunofluorescence in fibroblasts","journal":"The Biochemical journal","confidence":"Medium","confidence_rationale":"Tier 2 / Moderate — direct biochemical identification and localization in two cell types with stimulus-dependent enrichment, single lab","pmids":["9359840"],"is_preprint":false},{"year":1999,"finding":"Arc40p (yeast ARPC1A ortholog) is an essential component of the yeast Arp2/3 complex; immunoprecipitation confirmed its association, and deletion showed it is required for cell viability, cortical actin patch assembly, and complex integrity (Arc15p is required for association of Arp2p and Arc40p with the complex).","method":"Immunoprecipitation, gel-filtration, gene disruption, actin patch microscopy","journal":"Proceedings of the National Academy of Sciences of the United States of America","confidence":"High","confidence_rationale":"Tier 2 / Strong — reciprocal co-IP, gel filtration, systematic gene disruptions of all subunits with defined phenotypic readouts, replicated across multiple subunits in one rigorous study","pmids":["10377407"],"is_preprint":false},{"year":2001,"finding":"ARC40 (yeast ARPC1A ortholog) has synthetic genetic interactions with cytoskeletal organization genes (BNI1, ARP2, BIM1), placing it in the cortical actin assembly pathway.","method":"Synthetic genetic array (SGA) analysis — systematic double-mutant construction in yeast","journal":"Science (New York, N.Y.)","confidence":"Low","confidence_rationale":"Tier 3 / Moderate — genetic epistasis via SGA, but pan-gene catalog approach; specific interaction network for ARC40 is part of a large screen","pmids":["11743205"],"is_preprint":false},{"year":2004,"finding":"ARPC1/Arc40 binds the VCA domain of WASp family activators (Kd ~0.45 µM for recombinant Arc40, close to 0.30 µM for full complex); loss of Arc40 in Δarc40 yeast severely reduces Arp2/3 complex binding affinity for VCA and nucleation activity, and causes loss of actin patches with accumulation of actin cables, demonstrating that Arc40 is an essential VCA contact site.","method":"Recombinant protein binding assay, VCA-pulldown, in vitro actin nucleation assay, yeast genetic deletion, actin patch microscopy","journal":"The Journal of biological chemistry","confidence":"High","confidence_rationale":"Tier 1 / Strong — in vitro reconstitution with Kd measurement, mutagenesis-equivalent deletion, in vitro nucleation assay, and defined in vivo phenotype in one study","pmids":["15485833"],"is_preprint":false},{"year":2009,"finding":"ARPC1A is identified as a direct binding partner of Hsp27 by immunoaffinity purification/mass spectrometry; thiolutin treatment disrupts this interaction and induces peripheral co-localization of phospho-Hsp27 and Arp2/3, linking ARPC1A–Hsp27 interaction to cytoskeletal regulation in endothelial cell adhesion.","method":"Immunoaffinity purification, mass spectrometry, immunofluorescence localization","journal":"Cell stress & chaperones","confidence":"Medium","confidence_rationale":"Tier 3 / Moderate — direct co-purification/MS identifying ARPC1A as Hsp27 binding partner with functional perturbation by thiolutin, single lab","pmids":["19579057"],"is_preprint":false},{"year":2009,"finding":"ARPC1A silencing by RNAi in AsPC-1 pancreatic cancer cells (which harbor 7q21-q22 amplification) causes a massive reduction in cell migration and invasion, with only a slight decrease in proliferation, establishing ARPC1A as a regulator of cell motility downstream of actin polymerization.","method":"RNAi knockdown, cell migration assay, invasion assay, cell proliferation assay, FISH copy-number analysis, qRT-PCR","journal":"Genes, chromosomes & cancer","confidence":"Medium","confidence_rationale":"Tier 2 / Moderate — clean RNAi knockdown with specific migration/invasion phenotype, multiple assays, single lab","pmids":["19145645"],"is_preprint":false},{"year":2010,"finding":"Structure-function analysis of yeast p40/ARPC1 identified three distinct functional surfaces: (1) contact with p19/ARPC4 required for WASp-induced nucleation; (2) contact with p15/ARPC5 that suppresses spontaneous (leaky) nucleation; (3) an extended structural arm that directly binds the VCA domain of WASp and is required for actin nucleation. Lethal alleles at each site produced distinct biochemical defects in purified Arp2/3 complexes.","method":"Systematic in vivo allele analysis (39 integrated alleles), purification of mutant Arp2/3 complexes, in vitro actin nucleation assay, VCA binding assay","journal":"The Journal of biological chemistry","confidence":"High","confidence_rationale":"Tier 1 / Strong — systematic mutagenesis of 39 alleles, in vitro reconstitution with purified lethal-allele complexes, nucleation and binding assays, multiple orthogonal methods in one study","pmids":["20071330"],"is_preprint":false},{"year":2017,"finding":"Loss of ARPC1B leads to compensatory upregulation of ARPC1A in patient platelets, but ARPC1B-deficient cells cannot support WASP-mediated ARP2/3 nucleation despite elevated ARPC1A, indicating that ARPC1A and ARPC1B are not functionally interchangeable within the ARP2/3 complex in blood cells.","method":"Patient-derived platelet lysate analysis, immunoblot, functional platelet spreading assays, megakaryocytic cell knockout","journal":"Nature communications","confidence":"High","confidence_rationale":"Tier 2 / Strong — patient-derived loss-of-function, confirmed in multiple patients and in cell-line KO, with functional nucleation assay and clear isoform non-interchangeability conclusion; replicated across two independent patient cases","pmids":["28368018"],"is_preprint":false},{"year":2021,"finding":"Despite upregulation of ARPC1A in ARPC1B-deficient cells, ARP2/3 complexes containing ARPC1A cannot be stimulated by WASP to nucleate branched actin; this isoform-specific deficiency leads to loss of WASP-dependent structures (podosomes, lamellipodia) and weakening of cortical F-actin, resulting in increased BCR diffusion, elevated tonic lipid signaling, oscillatory calcium release, and phospho-Akt in B cells.","method":"Patient B cell analysis, immunoblot, F-actin imaging, BCR diffusion assay, calcium imaging, phospho-signaling readout","journal":"JCI insight","confidence":"High","confidence_rationale":"Tier 2 / Strong — patient-derived cells, multiple orthogonal functional assays (actin nucleation, BCR diffusion, calcium, phospho-signaling), mechanistic pathway placement","pmids":["34673575"],"is_preprint":false},{"year":2022,"finding":"STAT3 transcriptionally regulates ARPC1A expression in prostate cancer cells; ARPC1A knockdown promotes ferroptosis and reduces cell viability and invasion, as established by Co-IP, ChIP, and luciferase reporter assays confirming STAT3 binding to the ARPC1A promoter.","method":"Co-IP, ChIP, luciferase reporter assay, RNAi knockdown, ferroptosis assay, in vivo tumor model","journal":"Human cell","confidence":"Medium","confidence_rationale":"Tier 2 / Moderate — ChIP + luciferase reporter establish transcriptional regulation; functional knockdown with ferroptosis phenotype; single lab","pmids":["35871131"],"is_preprint":false},{"year":2024,"finding":"ARPC1A-containing ARP2/3 iso-complexes are inhibited by both CK-666 and CK-869 for actin branching, whereas ARPC1B-containing complexes are only inhibited by CK-869; both inhibitors block linear actin filament formation in ARPC1A- and ARPC1B-containing complexes when activated by SPIN90. This demonstrates isoform-specific pharmacology of ARP2/3 complexes.","method":"Reconstituted recombinant Arp2/3 iso-complexes with defined subunit composition, in vitro actin polymerization/branching assays with pharmacological inhibitors, macrophage phagocytosis and migration assays","journal":"EMBO reports","confidence":"High","confidence_rationale":"Tier 1 / Strong — reconstitution of defined iso-complexes, in vitro biochemical assays, confirmed in macrophage cellular assays; multiple orthogonal approaches in one study","pmids":["39009834"],"is_preprint":false},{"year":2024,"finding":"ARPC1A inactivation (KD or KO) combined with inactivation of arpin and CYFIP2 synergistically enhances cortical branched actin polymerization and migration persistence in human MCF10A cells and zebrafish endodermal cells, identifying ARPC1A as a negative regulator of cortical branched actin in the Rac1-WAVE-Arp2/3 migration pathway.","method":"siRNA knockdown, CRISPR knockout, live cell migration assay, zebrafish in vivo migration, vimentin re-expression rescue experiment","journal":"Journal of cell science","confidence":"Medium","confidence_rationale":"Tier 2 / Moderate — KD and KO with defined migration phenotype, genetic epistasis via triple combination, rescue experiment; single lab","pmids":["38059420"],"is_preprint":false},{"year":2024,"finding":"EML4 interacts with ARPC1A to modulate cytoskeletal dynamics and enhance lamellipodia formation, cellular motility, local invasion, and metastasis in lung adenocarcinoma; m6A hypermethylation of EML4 mRNA promotes its translation leading to ARPC1A interaction.","method":"m6A epitranscriptomic profiling, co-immunoprecipitation (EML4–ARPC1A interaction), functional migration/invasion assays, in vivo metastasis model","journal":"Cancer discovery","confidence":"Medium","confidence_rationale":"Tier 2 / Moderate — co-IP establishing EML4–ARPC1A interaction with functional lamellipodia/motility readouts; single lab, mechanistic pathway placement","pmids":["38922581"],"is_preprint":false},{"year":2016,"finding":"ARPC1A was identified as a component of the PKD2 interaction network; chemical cross-linking/mass spectrometry detected ARPC1A (along with the full Arp2/3 complex) as a PKD2-interacting protein in cytosolic and Golgi-enriched fractions, suggesting a direct protein–protein interaction.","method":"Affinity enrichment, chemical cross-linking, mass spectrometry","journal":"Journal of proteome research","confidence":"Low","confidence_rationale":"Tier 3 / Weak — single cross-linking/MS experiment, no reciprocal validation, no functional follow-up for ARPC1A specifically","pmids":["27559607"],"is_preprint":false}],"current_model":"ARPC1A (p40/Arc40) is an essential subunit of the seven-subunit Arp2/3 complex that performs at least three mechanistically distinct roles: (1) its extended structural arm directly contacts the VCA domain of WASp/WASp-family activators to facilitate actin nucleation; (2) its interface with ARPC5/p15 suppresses spontaneous (leaky) nucleation; and (3) its interface with ARPC4/p19 propagates WASp activation signals; additionally, ARPC1A-containing iso-complexes show distinct pharmacological sensitivity from ARPC1B-containing complexes (CK-666 inhibits actin branching in ARPC1A- but not ARPC1B-complexes), and ARPC1A is not functionally interchangeable with ARPC1B for WASP-mediated nucleation in immune cells, with ARPC1A also regulated transcriptionally by STAT3 and functioning to suppress ferroptosis and promote cell migration and invasion downstream of actin polymerization."},"narrative":{"mechanistic_narrative":"ARPC1A (p40/Arc40/SOP2Hs) is an essential subunit of the Arp2/3 complex that drives branched actin nucleation at sites of membrane protrusion, localizing to lamellipodia upon growth-factor stimulation [PMID:9359840]. Genetic and biochemical dissection of the yeast ortholog established it as required for Arp2/3 complex integrity, cell viability, and cortical actin patch assembly [PMID:10377407], and resolved three distinct functional surfaces of the subunit: an extended structural arm that directly binds the VCA domain of WASp-family activators to drive nucleation (Kd ~0.45 µM), an interface with p19/ARPC4 needed to propagate WASp-induced activation, and an interface with p15/ARPC5 that suppresses spontaneous nucleation [PMID:15485833, PMID:20071330]. ARPC1A is not functionally interchangeable with its paralog ARPC1B: in ARPC1B-deficient immune cells, compensatory ARPC1A upregulation cannot restore WASP-stimulated branched-actin nucleation, leading to loss of podosomes and lamellipodia, weakened cortical F-actin, and downstream B-cell receptor signaling defects [PMID:28368018, PMID:34673575]. ARPC1A- and ARPC1B-containing iso-complexes also differ pharmacologically, with CK-666 inhibiting branching only in ARPC1A complexes [PMID:39009834]. Through its control of actin-based motility, ARPC1A regulates cell migration and invasion: its loss reduces migration and invasion in cancer cells [PMID:19145645], it is transcriptionally driven by STAT3 and suppresses ferroptosis [PMID:35871131], and it acts as a context-dependent negative regulator of cortical branched actin and migration persistence within the Rac1-WAVE-Arp2/3 pathway [PMID:38059420].","teleology":[{"year":1997,"claim":"Established that ARPC1A is a bona fide subunit of the mammalian Arp2/3 complex acting at the actin cytoskeleton, answering whether the human protein participates in protrusive structures.","evidence":"Peptide sequencing, cDNA cloning, and stimulus-dependent immunofluorescence localization in neutrophils and fibroblasts","pmids":["9359840"],"confidence":"Medium","gaps":["Did not define molecular role within the complex","No nucleation or binding mechanism established"]},{"year":1999,"claim":"Demonstrated that the ARPC1A ortholog is essential for complex integrity and cortical actin assembly, establishing it as a structurally indispensable subunit rather than an accessory factor.","evidence":"Immunoprecipitation, gel filtration, gene disruption, and actin patch microscopy in yeast","pmids":["10377407"],"confidence":"High","gaps":["Did not identify which molecular contacts mediate essentiality","Activator interaction not addressed"]},{"year":2004,"claim":"Identified ARPC1A/Arc40 as a direct VCA-domain contact site, answering how WASp-family activators physically engage the complex through this subunit.","evidence":"Recombinant binding assays with Kd measurement, VCA pulldown, in vitro nucleation, and yeast deletion phenotyping","pmids":["15485833"],"confidence":"High","gaps":["Did not separate VCA contact from other subunit interfaces","Human/paralog specificity not addressed"]},{"year":2010,"claim":"Resolved three mechanistically distinct functional surfaces of the subunit, explaining how a single subunit both activates WASp-driven nucleation and suppresses leaky nucleation.","evidence":"Systematic analysis of 39 integrated alleles with purification of mutant complexes and in vitro nucleation/VCA binding assays in yeast","pmids":["20071330"],"confidence":"High","gaps":["Defined in yeast; human iso-complex differences not addressed","Structural basis at atomic resolution not provided"]},{"year":2009,"claim":"Linked ARPC1A function to cell motility and identified additional binding context, addressing the cellular consequence of the subunit downstream of actin polymerization.","evidence":"RNAi knockdown with migration/invasion/proliferation assays in pancreatic cancer cells (PMID 19145645) and immunoaffinity-MS identifying Hsp27 interaction (PMID 19579057)","pmids":["19145645","19579057"],"confidence":"Medium","gaps":["Hsp27 interaction lacks reciprocal validation and functional follow-up","Direct vs indirect motility role not separated"]},{"year":2017,"claim":"Showed ARPC1A and ARPC1B are not functionally interchangeable, answering whether the paralogs are redundant within the complex in blood cells.","evidence":"Patient-derived platelet analysis, immunoblot, spreading assays, and megakaryocytic cell knockout across independent patients","pmids":["28368018"],"confidence":"High","gaps":["Molecular basis of non-interchangeability not resolved","Did not define downstream signaling consequences"]},{"year":2021,"claim":"Defined the signaling consequences of ARPC1A iso-complex deficiency, establishing how isoform-specific nucleation failure propagates to receptor signaling.","evidence":"Patient B-cell F-actin imaging, BCR diffusion, calcium imaging, and phospho-signaling readouts","pmids":["34673575"],"confidence":"High","gaps":["Structural reason ARPC1A complexes resist WASP stimulation not resolved","Findings within immunodeficiency context"]},{"year":2022,"claim":"Placed ARPC1A under STAT3 transcriptional control and linked it to ferroptosis suppression, extending its role beyond direct actin mechanics.","evidence":"Co-IP, ChIP, luciferase reporter, RNAi knockdown with ferroptosis assays, and in vivo tumor model in prostate cancer","pmids":["35871131"],"confidence":"Medium","gaps":["Mechanistic link between actin function and ferroptosis not defined","Single lab"]},{"year":2024,"claim":"Demonstrated isoform-specific pharmacology and context-dependent regulatory roles, refining how ARPC1A-containing complexes can be distinguished functionally and modulated.","evidence":"Reconstituted iso-complexes with CK-666/CK-869 branching assays (PMID 39009834), triple-inactivation epistasis and rescue in MCF10A/zebrafish (PMID 38059420), and m6A-EML4-ARPC1A co-IP/metastasis study (PMID 38922581)","pmids":["39009834","38059420","38922581"],"confidence":"Medium","gaps":["EML4 interaction lacks reciprocal validation beyond co-IP","Mechanism reconciling positive (migration) and negative (cortical branched actin) regulatory roles unresolved"]},{"year":null,"claim":"The structural basis for why ARPC1A-containing iso-complexes differ from ARPC1B-containing complexes in WASP responsiveness and inhibitor sensitivity remains undefined.","evidence":"","pmids":[],"confidence":"High","gaps":["No atomic-resolution comparison of human ARPC1A vs ARPC1B iso-complexes","How transcriptional, signaling, and structural roles integrate in vivo is unresolved"]}],"mechanism_profile":{"molecular_activity":[{"term_id":"GO:0008092","term_label":"cytoskeletal protein binding","supporting_discovery_ids":[3,6]},{"term_id":"GO:0005198","term_label":"structural molecule activity","supporting_discovery_ids":[1,6]},{"term_id":"GO:0060090","term_label":"molecular adaptor activity","supporting_discovery_ids":[3,6]}],"localization":[{"term_id":"GO:0005856","term_label":"cytoskeleton","supporting_discovery_ids":[0,1]},{"term_id":"GO:0005829","term_label":"cytosol","supporting_discovery_ids":[0]}],"pathway":[{"term_id":"R-HSA-1266738","term_label":"Developmental Biology","supporting_discovery_ids":[0,8,11]},{"term_id":"R-HSA-168256","term_label":"Immune System","supporting_discovery_ids":[7,8]}],"complexes":["Arp2/3 complex"],"partners":["WASP","ARPC4","ARPC5","HSPB1","STAT3","EML4"],"other_free_text":[]}},"prefetch_data":{"uniprot":{"accession":"Q92747","full_name":"Actin-related protein 2/3 complex subunit 1A","aliases":["SOP2-like protein"],"length_aa":370,"mass_kda":41.6,"function":"Probably functions as a component of the Arp2/3 complex which is involved in regulation of actin polymerization and together with an activating nucleation-promoting factor (NPF) mediates the formation of branched actin networks","subcellular_location":"Cytoplasm, cytoskeleton; Nucleus","url":"https://www.uniprot.org/uniprotkb/Q92747/entry"},"depmap":{"release":"DepMap","has_data":true,"is_common_essential":false,"resolved_as":"","url":"https://depmap.org/portal/gene/ARPC1A","classification":"Not Classified","n_dependent_lines":4,"n_total_lines":1208,"dependency_fraction":0.0033112582781456954},"opencell":{"profiled":false,"resolved_as":"","ensg_id":"","cell_line_id":"","localizations":[],"interactors":[{"gene":"ACTR2","stoichiometry":10.0},{"gene":"ARL6IP5","stoichiometry":10.0},{"gene":"ARPC2","stoichiometry":10.0},{"gene":"ARPC3","stoichiometry":10.0},{"gene":"MIS12","stoichiometry":10.0},{"gene":"USP22","stoichiometry":10.0},{"gene":"ARL14EP","stoichiometry":4.0},{"gene":"TRAPPC2","stoichiometry":4.0},{"gene":"ACTB","stoichiometry":0.2},{"gene":"ACTN4","stoichiometry":0.2}],"url":"https://opencell.sf.czbiohub.org/search/ARPC1A","total_profiled":1310},"omim":[{"mim_id":"621450","title":"ACTIN-RELATED PROTEIN 2/3 COMPLEX, SUBUNIT 5-LIKE; ARPC5L","url":"https://www.omim.org/entry/621450"},{"mim_id":"608580","title":"MYOSIN, HEAVY CHAIN 16, SKELETAL MUSCLE, PSEUDOGENE; MYH16","url":"https://www.omim.org/entry/608580"},{"mim_id":"604227","title":"ACTIN-RELATED PROTEIN 2/3 COMPLEX, SUBUNIT 5; ARPC5","url":"https://www.omim.org/entry/604227"},{"mim_id":"604223","title":"ACTIN-RELATED PROTEIN 2/3 COMPLEX, SUBUNIT 1B; ARPC1B","url":"https://www.omim.org/entry/604223"},{"mim_id":"604220","title":"ACTIN-RELATED PROTEIN 2/3 COMPLEX, SUBUNIT 1A; ARPC1A","url":"https://www.omim.org/entry/604220"}],"hpa":{"profiled":true,"resolved_as":"","reliability":"Approved","locations":[{"location":"Cell Junctions","reliability":"Approved"},{"location":"Cytosol","reliability":"Additional"}],"tissue_specificity":"Low tissue specificity","tissue_distribution":"Detected in all","driving_tissues":[],"url":"https://www.proteinatlas.org/search/ARPC1A"},"hgnc":{"alias_symbol":["SOP2Hs","SOP2L","Arc40"],"prev_symbol":[]},"alphafold":{"accession":"Q92747","domains":[{"cath_id":"2.130.10.10","chopping":"1-362","consensus_level":"medium","plddt":92.5957,"start":1,"end":362}],"viewer_url":"https://alphafold.ebi.ac.uk/entry/Q92747","model_url":"https://alphafold.ebi.ac.uk/files/AF-Q92747-F1-model_v6.cif","pae_url":"https://alphafold.ebi.ac.uk/files/AF-Q92747-F1-predicted_aligned_error_v6.png","plddt_mean":92.25},"mouse_models":{"mgi_url":"https://www.informatics.jax.org/marker/summary?nomen=ARPC1A","jax_strain_url":"https://www.jax.org/strain/search?query=ARPC1A"},"sequence":{"accession":"Q92747","fasta_url":"https://rest.uniprot.org/uniprotkb/Q92747.fasta","uniprot_url":"https://www.uniprot.org/uniprotkb/Q92747/entry","alphafold_viewer_url":"https://alphafold.ebi.ac.uk/entry/Q92747"}},"corpus_meta":[{"pmid":"11743205","id":"PMC_11743205","title":"Systematic 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human neutrophils, and the complex localizes to lamellipodia upon stimulation with serum or PDGF, implicating it in actin cytoskeleton organization at sites of protrusion.\",\n      \"method\": \"Peptide sequencing, cDNA cloning, subcellular localization by immunofluorescence in fibroblasts\",\n      \"journal\": \"The Biochemical journal\",\n      \"confidence\": \"Medium\",\n      \"confidence_rationale\": \"Tier 2 / Moderate — direct biochemical identification and localization in two cell types with stimulus-dependent enrichment, single lab\",\n      \"pmids\": [\"9359840\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 1999,\n      \"finding\": \"Arc40p (yeast ARPC1A ortholog) is an essential component of the yeast Arp2/3 complex; immunoprecipitation confirmed its association, and deletion showed it is required for cell viability, cortical actin patch assembly, and complex integrity (Arc15p is required for association of Arp2p and Arc40p with the complex).\",\n      \"method\": \"Immunoprecipitation, gel-filtration, gene disruption, actin patch microscopy\",\n      \"journal\": \"Proceedings of the National Academy of Sciences of the United States of America\",\n      \"confidence\": \"High\",\n      \"confidence_rationale\": \"Tier 2 / Strong — reciprocal co-IP, gel filtration, systematic gene disruptions of all subunits with defined phenotypic readouts, replicated across multiple subunits in one rigorous study\",\n      \"pmids\": [\"10377407\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2001,\n      \"finding\": \"ARC40 (yeast ARPC1A ortholog) has synthetic genetic interactions with cytoskeletal organization genes (BNI1, ARP2, BIM1), placing it in the cortical actin assembly pathway.\",\n      \"method\": \"Synthetic genetic array (SGA) analysis — systematic double-mutant construction in yeast\",\n      \"journal\": \"Science (New York, N.Y.)\",\n      \"confidence\": \"Low\",\n      \"confidence_rationale\": \"Tier 3 / Moderate — genetic epistasis via SGA, but pan-gene catalog approach; specific interaction network for ARC40 is part of a large screen\",\n      \"pmids\": [\"11743205\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2004,\n      \"finding\": \"ARPC1/Arc40 binds the VCA domain of WASp family activators (Kd ~0.45 µM for recombinant Arc40, close to 0.30 µM for full complex); loss of Arc40 in Δarc40 yeast severely reduces Arp2/3 complex binding affinity for VCA and nucleation activity, and causes loss of actin patches with accumulation of actin cables, demonstrating that Arc40 is an essential VCA contact site.\",\n      \"method\": \"Recombinant protein binding assay, VCA-pulldown, in vitro actin nucleation assay, yeast genetic deletion, actin patch microscopy\",\n      \"journal\": \"The Journal of biological chemistry\",\n      \"confidence\": \"High\",\n      \"confidence_rationale\": \"Tier 1 / Strong — in vitro reconstitution with Kd measurement, mutagenesis-equivalent deletion, in vitro nucleation assay, and defined in vivo phenotype in one study\",\n      \"pmids\": [\"15485833\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2009,\n      \"finding\": \"ARPC1A is identified as a direct binding partner of Hsp27 by immunoaffinity purification/mass spectrometry; thiolutin treatment disrupts this interaction and induces peripheral co-localization of phospho-Hsp27 and Arp2/3, linking ARPC1A–Hsp27 interaction to cytoskeletal regulation in endothelial cell adhesion.\",\n      \"method\": \"Immunoaffinity purification, mass spectrometry, immunofluorescence localization\",\n      \"journal\": \"Cell stress & chaperones\",\n      \"confidence\": \"Medium\",\n      \"confidence_rationale\": \"Tier 3 / Moderate — direct co-purification/MS identifying ARPC1A as Hsp27 binding partner with functional perturbation by thiolutin, single lab\",\n      \"pmids\": [\"19579057\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2009,\n      \"finding\": \"ARPC1A silencing by RNAi in AsPC-1 pancreatic cancer cells (which harbor 7q21-q22 amplification) causes a massive reduction in cell migration and invasion, with only a slight decrease in proliferation, establishing ARPC1A as a regulator of cell motility downstream of actin polymerization.\",\n      \"method\": \"RNAi knockdown, cell migration assay, invasion assay, cell proliferation assay, FISH copy-number analysis, qRT-PCR\",\n      \"journal\": \"Genes, chromosomes & cancer\",\n      \"confidence\": \"Medium\",\n      \"confidence_rationale\": \"Tier 2 / Moderate — clean RNAi knockdown with specific migration/invasion phenotype, multiple assays, single lab\",\n      \"pmids\": [\"19145645\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2010,\n      \"finding\": \"Structure-function analysis of yeast p40/ARPC1 identified three distinct functional surfaces: (1) contact with p19/ARPC4 required for WASp-induced nucleation; (2) contact with p15/ARPC5 that suppresses spontaneous (leaky) nucleation; (3) an extended structural arm that directly binds the VCA domain of WASp and is required for actin nucleation. Lethal alleles at each site produced distinct biochemical defects in purified Arp2/3 complexes.\",\n      \"method\": \"Systematic in vivo allele analysis (39 integrated alleles), purification of mutant Arp2/3 complexes, in vitro actin nucleation assay, VCA binding assay\",\n      \"journal\": \"The Journal of biological chemistry\",\n      \"confidence\": \"High\",\n      \"confidence_rationale\": \"Tier 1 / Strong — systematic mutagenesis of 39 alleles, in vitro reconstitution with purified lethal-allele complexes, nucleation and binding assays, multiple orthogonal methods in one study\",\n      \"pmids\": [\"20071330\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2017,\n      \"finding\": \"Loss of ARPC1B leads to compensatory upregulation of ARPC1A in patient platelets, but ARPC1B-deficient cells cannot support WASP-mediated ARP2/3 nucleation despite elevated ARPC1A, indicating that ARPC1A and ARPC1B are not functionally interchangeable within the ARP2/3 complex in blood cells.\",\n      \"method\": \"Patient-derived platelet lysate analysis, immunoblot, functional platelet spreading assays, megakaryocytic cell knockout\",\n      \"journal\": \"Nature communications\",\n      \"confidence\": \"High\",\n      \"confidence_rationale\": \"Tier 2 / Strong — patient-derived loss-of-function, confirmed in multiple patients and in cell-line KO, with functional nucleation assay and clear isoform non-interchangeability conclusion; replicated across two independent patient cases\",\n      \"pmids\": [\"28368018\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2021,\n      \"finding\": \"Despite upregulation of ARPC1A in ARPC1B-deficient cells, ARP2/3 complexes containing ARPC1A cannot be stimulated by WASP to nucleate branched actin; this isoform-specific deficiency leads to loss of WASP-dependent structures (podosomes, lamellipodia) and weakening of cortical F-actin, resulting in increased BCR diffusion, elevated tonic lipid signaling, oscillatory calcium release, and phospho-Akt in B cells.\",\n      \"method\": \"Patient B cell analysis, immunoblot, F-actin imaging, BCR diffusion assay, calcium imaging, phospho-signaling readout\",\n      \"journal\": \"JCI insight\",\n      \"confidence\": \"High\",\n      \"confidence_rationale\": \"Tier 2 / Strong — patient-derived cells, multiple orthogonal functional assays (actin nucleation, BCR diffusion, calcium, phospho-signaling), mechanistic pathway placement\",\n      \"pmids\": [\"34673575\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2022,\n      \"finding\": \"STAT3 transcriptionally regulates ARPC1A expression in prostate cancer cells; ARPC1A knockdown promotes ferroptosis and reduces cell viability and invasion, as established by Co-IP, ChIP, and luciferase reporter assays confirming STAT3 binding to the ARPC1A promoter.\",\n      \"method\": \"Co-IP, ChIP, luciferase reporter assay, RNAi knockdown, ferroptosis assay, in vivo tumor model\",\n      \"journal\": \"Human cell\",\n      \"confidence\": \"Medium\",\n      \"confidence_rationale\": \"Tier 2 / Moderate — ChIP + luciferase reporter establish transcriptional regulation; functional knockdown with ferroptosis phenotype; single lab\",\n      \"pmids\": [\"35871131\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2024,\n      \"finding\": \"ARPC1A-containing ARP2/3 iso-complexes are inhibited by both CK-666 and CK-869 for actin branching, whereas ARPC1B-containing complexes are only inhibited by CK-869; both inhibitors block linear actin filament formation in ARPC1A- and ARPC1B-containing complexes when activated by SPIN90. This demonstrates isoform-specific pharmacology of ARP2/3 complexes.\",\n      \"method\": \"Reconstituted recombinant Arp2/3 iso-complexes with defined subunit composition, in vitro actin polymerization/branching assays with pharmacological inhibitors, macrophage phagocytosis and migration assays\",\n      \"journal\": \"EMBO reports\",\n      \"confidence\": \"High\",\n      \"confidence_rationale\": \"Tier 1 / Strong — reconstitution of defined iso-complexes, in vitro biochemical assays, confirmed in macrophage cellular assays; multiple orthogonal approaches in one study\",\n      \"pmids\": [\"39009834\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2024,\n      \"finding\": \"ARPC1A inactivation (KD or KO) combined with inactivation of arpin and CYFIP2 synergistically enhances cortical branched actin polymerization and migration persistence in human MCF10A cells and zebrafish endodermal cells, identifying ARPC1A as a negative regulator of cortical branched actin in the Rac1-WAVE-Arp2/3 migration pathway.\",\n      \"method\": \"siRNA knockdown, CRISPR knockout, live cell migration assay, zebrafish in vivo migration, vimentin re-expression rescue experiment\",\n      \"journal\": \"Journal of cell science\",\n      \"confidence\": \"Medium\",\n      \"confidence_rationale\": \"Tier 2 / Moderate — KD and KO with defined migration phenotype, genetic epistasis via triple combination, rescue experiment; single lab\",\n      \"pmids\": [\"38059420\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2024,\n      \"finding\": \"EML4 interacts with ARPC1A to modulate cytoskeletal dynamics and enhance lamellipodia formation, cellular motility, local invasion, and metastasis in lung adenocarcinoma; m6A hypermethylation of EML4 mRNA promotes its translation leading to ARPC1A interaction.\",\n      \"method\": \"m6A epitranscriptomic profiling, co-immunoprecipitation (EML4–ARPC1A interaction), functional migration/invasion assays, in vivo metastasis model\",\n      \"journal\": \"Cancer discovery\",\n      \"confidence\": \"Medium\",\n      \"confidence_rationale\": \"Tier 2 / Moderate — co-IP establishing EML4–ARPC1A interaction with functional lamellipodia/motility readouts; single lab, mechanistic pathway placement\",\n      \"pmids\": [\"38922581\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2016,\n      \"finding\": \"ARPC1A was identified as a component of the PKD2 interaction network; chemical cross-linking/mass spectrometry detected ARPC1A (along with the full Arp2/3 complex) as a PKD2-interacting protein in cytosolic and Golgi-enriched fractions, suggesting a direct protein–protein interaction.\",\n      \"method\": \"Affinity enrichment, chemical cross-linking, mass spectrometry\",\n      \"journal\": \"Journal of proteome research\",\n      \"confidence\": \"Low\",\n      \"confidence_rationale\": \"Tier 3 / Weak — single cross-linking/MS experiment, no reciprocal validation, no functional follow-up for ARPC1A specifically\",\n      \"pmids\": [\"27559607\"],\n      \"is_preprint\": false\n    }\n  ],\n  \"current_model\": \"ARPC1A (p40/Arc40) is an essential subunit of the seven-subunit Arp2/3 complex that performs at least three mechanistically distinct roles: (1) its extended structural arm directly contacts the VCA domain of WASp/WASp-family activators to facilitate actin nucleation; (2) its interface with ARPC5/p15 suppresses spontaneous (leaky) nucleation; and (3) its interface with ARPC4/p19 propagates WASp activation signals; additionally, ARPC1A-containing iso-complexes show distinct pharmacological sensitivity from ARPC1B-containing complexes (CK-666 inhibits actin branching in ARPC1A- but not ARPC1B-complexes), and ARPC1A is not functionally interchangeable with ARPC1B for WASP-mediated nucleation in immune cells, with ARPC1A also regulated transcriptionally by STAT3 and functioning to suppress ferroptosis and promote cell migration and invasion downstream of actin polymerization.\"\n}\n```","stage2_raw":"{\n  \"mechanistic_narrative\": \"ARPC1A (p40/Arc40/SOP2Hs) is an essential subunit of the Arp2/3 complex that drives branched actin nucleation at sites of membrane protrusion, localizing to lamellipodia upon growth-factor stimulation [#0]. Genetic and biochemical dissection of the yeast ortholog established it as required for Arp2/3 complex integrity, cell viability, and cortical actin patch assembly [#1], and resolved three distinct functional surfaces of the subunit: an extended structural arm that directly binds the VCA domain of WASp-family activators to drive nucleation (Kd ~0.45 µM), an interface with p19/ARPC4 needed to propagate WASp-induced activation, and an interface with p15/ARPC5 that suppresses spontaneous nucleation [#3, #6]. ARPC1A is not functionally interchangeable with its paralog ARPC1B: in ARPC1B-deficient immune cells, compensatory ARPC1A upregulation cannot restore WASP-stimulated branched-actin nucleation, leading to loss of podosomes and lamellipodia, weakened cortical F-actin, and downstream B-cell receptor signaling defects [#7, #8]. ARPC1A- and ARPC1B-containing iso-complexes also differ pharmacologically, with CK-666 inhibiting branching only in ARPC1A complexes [#10]. Through its control of actin-based motility, ARPC1A regulates cell migration and invasion: its loss reduces migration and invasion in cancer cells [#5], it is transcriptionally driven by STAT3 and suppresses ferroptosis [#9], and it acts as a context-dependent negative regulator of cortical branched actin and migration persistence within the Rac1-WAVE-Arp2/3 pathway [#11].\",\n  \"teleology\": [\n    {\n      \"year\": 1997,\n      \"claim\": \"Established that ARPC1A is a bona fide subunit of the mammalian Arp2/3 complex acting at the actin cytoskeleton, answering whether the human protein participates in protrusive structures.\",\n      \"evidence\": \"Peptide sequencing, cDNA cloning, and stimulus-dependent immunofluorescence localization in neutrophils and fibroblasts\",\n      \"pmids\": [\"9359840\"],\n      \"confidence\": \"Medium\",\n      \"gaps\": [\"Did not define molecular role within the complex\", \"No nucleation or binding mechanism established\"]\n    },\n    {\n      \"year\": 1999,\n      \"claim\": \"Demonstrated that the ARPC1A ortholog is essential for complex integrity and cortical actin assembly, establishing it as a structurally indispensable subunit rather than an accessory factor.\",\n      \"evidence\": \"Immunoprecipitation, gel filtration, gene disruption, and actin patch microscopy in yeast\",\n      \"pmids\": [\"10377407\"],\n      \"confidence\": \"High\",\n      \"gaps\": [\"Did not identify which molecular contacts mediate essentiality\", \"Activator interaction not addressed\"]\n    },\n    {\n      \"year\": 2004,\n      \"claim\": \"Identified ARPC1A/Arc40 as a direct VCA-domain contact site, answering how WASp-family activators physically engage the complex through this subunit.\",\n      \"evidence\": \"Recombinant binding assays with Kd measurement, VCA pulldown, in vitro nucleation, and yeast deletion phenotyping\",\n      \"pmids\": [\"15485833\"],\n      \"confidence\": \"High\",\n      \"gaps\": [\"Did not separate VCA contact from other subunit interfaces\", \"Human/paralog specificity not addressed\"]\n    },\n    {\n      \"year\": 2010,\n      \"claim\": \"Resolved three mechanistically distinct functional surfaces of the subunit, explaining how a single subunit both activates WASp-driven nucleation and suppresses leaky nucleation.\",\n      \"evidence\": \"Systematic analysis of 39 integrated alleles with purification of mutant complexes and in vitro nucleation/VCA binding assays in yeast\",\n      \"pmids\": [\"20071330\"],\n      \"confidence\": \"High\",\n      \"gaps\": [\"Defined in yeast; human iso-complex differences not addressed\", \"Structural basis at atomic resolution not provided\"]\n    },\n    {\n      \"year\": 2009,\n      \"claim\": \"Linked ARPC1A function to cell motility and identified additional binding context, addressing the cellular consequence of the subunit downstream of actin polymerization.\",\n      \"evidence\": \"RNAi knockdown with migration/invasion/proliferation assays in pancreatic cancer cells (PMID 19145645) and immunoaffinity-MS identifying Hsp27 interaction (PMID 19579057)\",\n      \"pmids\": [\"19145645\", \"19579057\"],\n      \"confidence\": \"Medium\",\n      \"gaps\": [\"Hsp27 interaction lacks reciprocal validation and functional follow-up\", \"Direct vs indirect motility role not separated\"]\n    },\n    {\n      \"year\": 2017,\n      \"claim\": \"Showed ARPC1A and ARPC1B are not functionally interchangeable, answering whether the paralogs are redundant within the complex in blood cells.\",\n      \"evidence\": \"Patient-derived platelet analysis, immunoblot, spreading assays, and megakaryocytic cell knockout across independent patients\",\n      \"pmids\": [\"28368018\"],\n      \"confidence\": \"High\",\n      \"gaps\": [\"Molecular basis of non-interchangeability not resolved\", \"Did not define downstream signaling consequences\"]\n    },\n    {\n      \"year\": 2021,\n      \"claim\": \"Defined the signaling consequences of ARPC1A iso-complex deficiency, establishing how isoform-specific nucleation failure propagates to receptor signaling.\",\n      \"evidence\": \"Patient B-cell F-actin imaging, BCR diffusion, calcium imaging, and phospho-signaling readouts\",\n      \"pmids\": [\"34673575\"],\n      \"confidence\": \"High\",\n      \"gaps\": [\"Structural reason ARPC1A complexes resist WASP stimulation not resolved\", \"Findings within immunodeficiency context\"]\n    },\n    {\n      \"year\": 2022,\n      \"claim\": \"Placed ARPC1A under STAT3 transcriptional control and linked it to ferroptosis suppression, extending its role beyond direct actin mechanics.\",\n      \"evidence\": \"Co-IP, ChIP, luciferase reporter, RNAi knockdown with ferroptosis assays, and in vivo tumor model in prostate cancer\",\n      \"pmids\": [\"35871131\"],\n      \"confidence\": \"Medium\",\n      \"gaps\": [\"Mechanistic link between actin function and ferroptosis not defined\", \"Single lab\"]\n    },\n    {\n      \"year\": 2024,\n      \"claim\": \"Demonstrated isoform-specific pharmacology and context-dependent regulatory roles, refining how ARPC1A-containing complexes can be distinguished functionally and modulated.\",\n      \"evidence\": \"Reconstituted iso-complexes with CK-666/CK-869 branching assays (PMID 39009834), triple-inactivation epistasis and rescue in MCF10A/zebrafish (PMID 38059420), and m6A-EML4-ARPC1A co-IP/metastasis study (PMID 38922581)\",\n      \"pmids\": [\"39009834\", \"38059420\", \"38922581\"],\n      \"confidence\": \"Medium\",\n      \"gaps\": [\"EML4 interaction lacks reciprocal validation beyond co-IP\", \"Mechanism reconciling positive (migration) and negative (cortical branched actin) regulatory roles unresolved\"]\n    },\n    {\n      \"year\": null,\n      \"claim\": \"The structural basis for why ARPC1A-containing iso-complexes differ from ARPC1B-containing complexes in WASP responsiveness and inhibitor sensitivity remains undefined.\",\n      \"evidence\": \"\",\n      \"pmids\": [],\n      \"confidence\": \"High\",\n      \"gaps\": [\"No atomic-resolution comparison of human ARPC1A vs ARPC1B iso-complexes\", \"How transcriptional, signaling, and structural roles integrate in vivo is unresolved\"]\n    }\n  ],\n  \"mechanism_profile\": {\n    \"molecular_activity\": [\n      {\"term_id\": \"GO:0008092\", \"supporting_discovery_ids\": [3, 6]},\n      {\"term_id\": \"GO:0005198\", \"supporting_discovery_ids\": [1, 6]},\n      {\"term_id\": \"GO:0060090\", \"supporting_discovery_ids\": [3, 6]}\n    ],\n    \"localization\": [\n      {\"term_id\": \"GO:0005856\", \"supporting_discovery_ids\": [0, 1]},\n      {\"term_id\": \"GO:0005829\", \"supporting_discovery_ids\": [0]}\n    ],\n    \"pathway\": [\n      {\"term_id\": \"R-HSA-1266738\", \"supporting_discovery_ids\": [0, 8, 11]},\n      {\"term_id\": \"R-HSA-168256\", \"supporting_discovery_ids\": [7, 8]}\n    ],\n    \"complexes\": [\n      \"Arp2/3 complex\"\n    ],\n    \"partners\": [\n      \"WASP\",\n      \"ARPC4\",\n      \"ARPC5\",\n      \"HSPB1\",\n      \"STAT3\",\n      \"EML4\"\n    ],\n    \"other_free_text\": []\n  }\n}","audit_flag":null,"evaluation":{"pairwise":"win","faith_supported":5,"faith_total":5,"faith_pct":100.0}}