{"gene":"NAA80","run_date":"2026-06-10T05:19:52","timeline":{"discoveries":[{"year":2018,"finding":"NAA80 is the N-terminal acetyltransferase (NAT) responsible for Nt-acetylating actin. NAA80-knockout cells display increased F/G-actin ratio, increased filopodia and lamellipodia formation, and accelerated cell motility. In vitro, loss of Nt-acetylation alters rates of actin filament depolymerization and elongation (including formin-driven elongation), while Arp2/3-mediated nucleation is mostly unaffected.","method":"In vitro acetyltransferase assays, NAA80 knockout cell lines, actin polymerization/depolymerization assays, formin elongation assays, Arp2/3 nucleation assays, cell motility assays, fluorescence microscopy","journal":"Proceedings of the National Academy of Sciences of the United States of America","confidence":"High","confidence_rationale":"Tier 1–2 / Strong — multiple orthogonal methods including in vitro enzymatic assay, KO cell lines with defined phenotypes, and reconstituted biochemical assays; replicated across two simultaneous independent PNAS papers","pmids":["29581253"],"is_preprint":false},{"year":2018,"finding":"NAA80 substrate specificity is primarily determined by interactions with acidic amino acids at positions 2 and 3 of the actin substrate (not positions 1 and 2 as in most NATs). The crystal structure of NAA80 in complex with a bisubstrate inhibitor reveals a fold similar to other NAT enzymes but with a more open substrate-binding region. In a yeast model lacking NatB, ectopic NAA80 expression partially restored Nt-acetylation of NatB substrates, demonstrating intrinsic posttranslational Nt-acetylation capacity.","method":"Crystal structure determination of NAA80–bisubstrate inhibitor complex, bisubstrate inhibitor development, yeast complementation (NatB-deficient strain), in vitro acetyltransferase assays, active-site analysis","journal":"Proceedings of the National Academy of Sciences of the United States of America","confidence":"High","confidence_rationale":"Tier 1 / Strong — crystal structure with functional validation by mutagenesis-equivalent inhibitor binding, replicated biochemical assays, yeast genetic model; independent simultaneous study corroborating actin as sole substrate","pmids":["29581307"],"is_preprint":false},{"year":2018,"finding":"NAT6 (NAA80/FUS2) specifically acetylates the N-terminal acidic residue of different mammalian actin isoforms (β-actin Asp2, γ-actin-1 Glu2, α-actin-1). Knockout of NAT6 in two human cell lines abolished N-terminal acetylation of mature β- and γ-actin, and complete acetylation was restored by re-expression of NAT6 or addition of recombinant NAT6 to cell extracts. NAA10 showed much less or no activity on these substrates in equivalent assays.","method":"NAT6 knockout in two human cell lines, recombinant protein activity assays on purified proteins and actin N-terminal peptides, mass spectrometry for acetylation state, cell extract complementation assays","journal":"The FEBS journal","confidence":"High","confidence_rationale":"Tier 1–2 / Strong — two independent cell line KOs, recombinant enzyme assays, rescue experiments, MS validation; independent replication of PNAS findings","pmids":["30028079"],"is_preprint":false},{"year":2020,"finding":"PFN2 (profilin 2) is a stable interaction partner of NAA80 identified by interaction proteomics and confirmed by analytical ultracentrifugation. PFN2 binding specifically increases the intrinsic catalytic activity of NAA80. NAA80 binds PFN2 through a proline-rich loop; deletion of this loop abrogates PFN2 binding. Small-angle X-ray scattering shows NAA80, actin, and PFN2 form a ternary complex. PFN2 binding promotes interaction between the globular domains of actin and NAA80, facilitating actin acetylation. The majority of cellular NAA80 is stably bound to PFN2, not actin, and this complex acetylates G-actin before incorporation into filaments.","method":"Interaction proteomics, analytical ultracentrifugation, enzyme activity assays, deletion mutagenesis, small-angle X-ray scattering (SAXS)","journal":"The Journal of biological chemistry","confidence":"High","confidence_rationale":"Tier 1–2 / Moderate — multiple orthogonal methods (interaction proteomics, AUC, SAXS, enzyme assays, mutagenesis) in a single study demonstrating mechanism of PFN2-NAA80 cooperation","pmids":["32978259"],"is_preprint":false},{"year":2020,"finding":"NAA80 knockout cells display fragmentation of the Golgi apparatus. Re-expression of wild-type NAA80 rescues Golgi fragmentation, but a catalytically dead NAA80 mutant neither restores actin Nt-acetylation nor Golgi structure. NAA80 KO cells also show dramatically increased F-actin levels, suggesting a causal link between actin modification state and Golgi organization.","method":"NAA80 knockout cell lines, rescue experiments with wild-type and catalytic dead NAA80 mutant, immunofluorescence microscopy of Golgi structure, live-cell imaging, F-actin quantification","journal":"Experimental cell research","confidence":"High","confidence_rationale":"Tier 2 / Moderate — KO plus catalytic dead mutant rescue experiment directly links enzymatic activity to Golgi phenotype with multiple orthogonal readouts","pmids":["32209306"],"is_preprint":false},{"year":2021,"finding":"The final maturation state of β-actin is Nt-acetylation by NAA80 (yielding Ac-DDDI-). Using NAA80-lacking cells and targeted proteomics/mass spectrometry, Nt-arginylation of β-actin (RDDI-) previously claimed as a competing modification could not be confirmed in wildtype cells. Only a very minor level of arginylation of cleaved β-actin was detectable in NAA80-lacking cells but not in wildtype, establishing NAA80 as the terminal modifier that prevents arginylation.","method":"NAA80 knockout cells, targeted proteomics, mass spectrometry-based Nt-modification profiling, comparison with commercially available antibody detection","journal":"Journal of molecular biology","confidence":"High","confidence_rationale":"Tier 1–2 / Moderate — state-of-the-art targeted proteomics with KO model and orthogonal antibody comparison; rigorous negative result on arginylation mechanistically establishes NAA80 as terminal modifier","pmids":["34896361"],"is_preprint":false},{"year":2000,"finding":"The human FUS2 (NAA80) protein has homology to the catalytic domain of acetyltransferases, can acetylate protein N-termini using a ping-pong mechanism, shows substrate specificity, and localizes to the cytoplasm as shown by GFP-tagging experiments.","method":"Sequence homology analysis, in vitro N-terminal acetyltransferase assay, ping-pong kinetic mechanism determination, GFP-fusion subcellular localization","journal":"Oncogene","confidence":"Medium","confidence_rationale":"Tier 2–3 / Weak — single lab, enzyme activity demonstrated but substrate not yet defined as actin; cytoplasmic localization confirmed by GFP tagging","pmids":["10644992"],"is_preprint":false},{"year":2021,"finding":"Individuals with homozygous NAA80 p.(Leu130Pro) variant show ~50% decrease in actin acetylation, confirming NAA80 is required for actin N-terminal acetylation in vivo. Patient-derived fibroblasts and PBMCs showed increased migration, increased filopodia counts, and increased polymerized actin, consistent with NAA80 KO cell phenotypes. The variant destabilizes the NAA80 protein, reducing protein availability.","method":"Patient fibroblasts and PBMCs from individuals with NAA80 variant, mass spectrometry for actin acetylation, cell migration assays, filopodia counting, F-actin quantification, molecular structure-based protein stability prediction confirmed biochemically","journal":"Brain communications","confidence":"Medium","confidence_rationale":"Tier 2 / Moderate — human patient cells with defined variant confirming in vivo function; multiple cellular phenotype readouts consistent with prior KO studies, but single family/variant","pmids":["34805998"],"is_preprint":false},{"year":2024,"finding":"Zebrafish Naa80 acetylates both muscle and non-muscle actins in vivo and in vitro with preference for actin N-termini. Naa80 knockout zebrafish exhibit abnormal inner ear development, small otoliths, and impaired response to sound, but show normal development, morphology, and muscle function otherwise, demonstrating that actin N-terminal acetylation is essential for normal hearing.","method":"Zebrafish naa80 knockout model, in vitro acetyltransferase assays with purified Naa80, mass spectrometry for acetylation state, auditory/inner ear phenotype assays, morphological analysis","journal":"Life science alliance","confidence":"Medium","confidence_rationale":"Tier 2 / Moderate — KO animal model with defined phenotype, confirmed by in vitro enzymatic assays; single lab study","pmids":["39384430"],"is_preprint":false}],"current_model":"NAA80 (also known as NAT6/FUS2) is the sole N-terminal acetyltransferase for animal actins, acting posttranslationally on processed actin N-termini through substrate recognition primarily mediated by acidic residues at positions 2 and 3; it forms a stable ternary complex with profilin 2 (PFN2) and G-actin, whereby PFN2 binding via NAA80's proline-rich loop enhances catalytic activity and promotes actin acetylation before filament incorporation, and this modification critically controls actin filament dynamics (depolymerization and formin-driven elongation), cytoskeletal organization, Golgi integrity, cell motility, and hearing in vivo."},"narrative":{"mechanistic_narrative":"NAA80 (NAT6/FUS2) is the dedicated N-terminal acetyltransferase for animal actins, posttranslationally acetylating the processed, acidic N-termini of muscle and non-muscle actin isoforms (β-actin Asp2, γ-actin Glu2, α-actin) and thereby controlling actin filament dynamics and cytoskeletal organization [PMID:29581253, PMID:30028079]. Unlike most NATs, its substrate specificity is dictated by acidic residues at positions 2 and 3 of actin rather than positions 1–2, and a crystal structure reveals a NAT-like fold with an unusually open substrate-binding region [PMID:29581307]. Acetylation is the terminal maturation event of β-actin (yielding Ac-DDDI-) and prevents N-terminal arginylation [PMID:34896361]. Catalysis is enhanced by a stable interaction with profilin 2 (PFN2): NAA80 binds PFN2 through a proline-rich loop, and NAA80, actin, and PFN2 assemble into a ternary complex in which PFN2 boosts intrinsic catalytic activity and promotes acetylation of G-actin before filament incorporation [PMID:32978259]. Functionally, loss of NAA80 raises the F/G-actin ratio, increases filopodia and lamellipodia, accelerates cell motility, and fragments the Golgi apparatus in a manner rescued only by catalytically active enzyme [PMID:29581253, PMID:32209306]. The enzyme is required in vivo: a homozygous human p.(Leu130Pro) variant reduces actin acetylation and recapitulates the migration and actin phenotypes [PMID:34805998], and zebrafish naa80 knockouts show defective inner ear development and impaired hearing [PMID:39384430].","teleology":[{"year":2000,"claim":"Before any substrate was known, FUS2/NAA80 was established as a cytoplasmic acetyltransferase, framing it as an enzyme in search of a function.","evidence":"sequence homology analysis, in vitro NAT assay with ping-pong kinetics, and GFP-fusion localization","pmids":["10644992"],"confidence":"Medium","gaps":["Physiological substrate undefined","No structural or cellular role established"]},{"year":2018,"claim":"Identifying actin as the substrate answered what NAA80 does, defining it as the sole NAT for animal actins and linking acetylation to filament dynamics.","evidence":"in vitro acetyltransferase assays, NAA80-knockout human cell lines, reconstituted polymerization/depolymerization, formin elongation and Arp2/3 nucleation assays, and MS validation across independent studies","pmids":["29581253","30028079"],"confidence":"High","gaps":["Mechanism of cytoskeletal control downstream of acetylation not fully resolved","In vivo organismal consequences not yet tested"]},{"year":2018,"claim":"A crystal structure and yeast complementation explained how NAA80 recognizes its unusual substrate, showing specificity arises from acidic residues at positions 2 and 3 via an open binding cleft.","evidence":"crystal structure of NAA80–bisubstrate inhibitor complex, active-site analysis, and ectopic expression in a NatB-deficient yeast strain","pmids":["29581307"],"confidence":"High","gaps":["No structure of the productive NAA80–actin or ternary complex","Determinants of isoform preference not fully mapped"]},{"year":2020,"claim":"Discovery of PFN2 as a stable partner answered how NAA80 reaches and efficiently acetylates G-actin, defining a profilin-assisted ternary mechanism acting before filament incorporation.","evidence":"interaction proteomics, analytical ultracentrifugation, SAXS, enzyme activity assays, and proline-rich-loop deletion mutagenesis","pmids":["32978259"],"confidence":"High","gaps":["High-resolution structure of the ternary complex lacking","Role of other profilin isoforms not addressed"]},{"year":2020,"claim":"Catalytic-dead rescue established that NAA80 enzymatic activity, not mere presence, maintains Golgi integrity, connecting actin modification state to organelle organization.","evidence":"NAA80 KO cells with wild-type versus catalytic-dead rescue, Golgi immunofluorescence/live imaging, and F-actin quantification","pmids":["32209306"],"confidence":"High","gaps":["Molecular link between actin acetylation and Golgi structure unresolved","Effects on Golgi-associated trafficking not measured"]},{"year":2021,"claim":"Targeted proteomics resolved a competing-modification controversy, establishing NAA80 acetylation as the terminal β-actin maturation state that precludes N-terminal arginylation.","evidence":"NAA80 KO cells with targeted MS-based Nt-modification profiling and antibody comparison","pmids":["34896361"],"confidence":"High","gaps":["Functional consequence of the residual arginylation seen only in KO cells unclear"]},{"year":2021,"claim":"A human homozygous variant demonstrated NAA80's requirement for actin acetylation in vivo and tied loss of function to a human phenotype.","evidence":"patient fibroblasts/PBMCs with p.(Leu130Pro), MS for acetylation, migration and filopodia assays, F-actin quantification, and biochemical stability assessment","pmids":["34805998"],"confidence":"Medium","gaps":["Single family/variant","Genotype–clinical phenotype relationship not fully defined"]},{"year":2024,"claim":"A zebrafish knockout defined an organismal role, showing actin N-terminal acetylation is essential for inner ear development and hearing despite otherwise normal development and muscle.","evidence":"naa80 knockout zebrafish, in vitro acetyltransferase assays, MS, and auditory/morphological phenotyping","pmids":["39384430"],"confidence":"Medium","gaps":["Cellular mechanism linking acetylation to hair-cell/otolith function unknown","Single lab study"]},{"year":null,"claim":"How actin N-terminal acetylation is mechanistically translated into specific filament behaviors, Golgi maintenance, and tissue-specific phenotypes such as hearing remains unresolved.","evidence":"","pmids":[],"confidence":"Medium","gaps":["No structure of the productive NAA80–actin complex","Causal chain from acetylation to organelle and sensory phenotypes uncharacterized"]}],"mechanism_profile":{"molecular_activity":[{"term_id":"GO:0016740","term_label":"transferase activity","supporting_discovery_ids":[0,1,2,6]},{"term_id":"GO:0140096","term_label":"catalytic activity, acting on a protein","supporting_discovery_ids":[0,2]}],"localization":[{"term_id":"GO:0005829","term_label":"cytosol","supporting_discovery_ids":[6]},{"term_id":"GO:0005794","term_label":"Golgi apparatus","supporting_discovery_ids":[4]}],"pathway":[{"term_id":"R-HSA-392499","term_label":"Metabolism of proteins","supporting_discovery_ids":[0,2]}],"complexes":["NAA80–PFN2–actin ternary complex"],"partners":["PFN2","ACTB","ACTG1"],"other_free_text":[]}},"prefetch_data":{"uniprot":{"accession":"Q93015","full_name":"N-alpha-acetyltransferase 80","aliases":["N-acetyltransferase 6","Protein fusion-2","Protein fus-2"],"length_aa":286,"mass_kda":31.4,"function":"N-alpha-acetyltransferase that specifically mediates the acetylation of the acidic amino terminus of processed forms of beta- and gamma-actin (ACTB and ACTG, respectively) (PubMed:29581253, PubMed:30028079). N-terminal acetylation of processed beta- and gamma-actin regulates actin filament depolymerization and elongation (PubMed:29581253). In vivo, preferentially displays N-terminal acetyltransferase activity towards acid N-terminal sequences starting with Asp-Asp-Asp and Glu-Glu-Glu (PubMed:29581253, PubMed:30028079). In vitro, shows high activity towards Met-Asp-Glu-Leu and Met-Asp-Asp-Asp (PubMed:10644992, PubMed:29581307). May act as a tumor suppressor (PubMed:10644992)","subcellular_location":"Cytoplasm, cytosol","url":"https://www.uniprot.org/uniprotkb/Q93015/entry"},"depmap":{"release":"DepMap","has_data":true,"is_common_essential":false,"resolved_as":"","url":"https://depmap.org/portal/gene/NAA80","classification":"Not Classified","n_dependent_lines":0,"n_total_lines":1208,"dependency_fraction":0.0},"opencell":{"profiled":false,"resolved_as":"","ensg_id":"","cell_line_id":"","localizations":[],"interactors":[],"url":"https://opencell.sf.czbiohub.org/search/NAA80","total_profiled":1310},"omim":[{"mim_id":"620830","title":"AURONEURODENTAL SYNDROME; ANDS","url":"https://www.omim.org/entry/620830"},{"mim_id":"620093","title":"ACTIN MATURATION PROTEASE; ACTMAP","url":"https://www.omim.org/entry/620093"},{"mim_id":"607073","title":"N-ALPHA-ACETYLTRANSFERASE 80, NatH CATALYTIC SUBUNIT; NAA80","url":"https://www.omim.org/entry/607073"}],"hpa":{"profiled":true,"resolved_as":"","reliability":"","locations":[],"tissue_specificity":"Tissue enhanced","tissue_distribution":"Detected in all","driving_tissues":[{"tissue":"testis","ntpm":59.4}],"url":"https://www.proteinatlas.org/search/NAA80"},"hgnc":{"alias_symbol":["FUS2"],"prev_symbol":["NAT6"]},"alphafold":{"accession":"Q93015","domains":[{"cath_id":"3.40.630.30","chopping":"62-200_276-286","consensus_level":"medium","plddt":94.3937,"start":62,"end":286}],"viewer_url":"https://alphafold.ebi.ac.uk/entry/Q93015","model_url":"https://alphafold.ebi.ac.uk/files/AF-Q93015-F1-model_v6.cif","pae_url":"https://alphafold.ebi.ac.uk/files/AF-Q93015-F1-predicted_aligned_error_v6.png","plddt_mean":74.75},"mouse_models":{"mgi_url":"https://www.informatics.jax.org/marker/summary?nomen=NAA80","jax_strain_url":"https://www.jax.org/strain/search?query=NAA80"},"sequence":{"accession":"Q93015","fasta_url":"https://rest.uniprot.org/uniprotkb/Q93015.fasta","uniprot_url":"https://www.uniprot.org/uniprotkb/Q93015/entry","alphafold_viewer_url":"https://alphafold.ebi.ac.uk/entry/Q93015"}},"corpus_meta":[{"pmid":"29581253","id":"PMC_29581253","title":"NAA80 is actin's N-terminal acetyltransferase and regulates cytoskeleton assembly and cell motility.","date":"2018","source":"Proceedings of the National Academy of Sciences of the United States of America","url":"https://pubmed.ncbi.nlm.nih.gov/29581253","citation_count":160,"is_preprint":false},{"pmid":"29581307","id":"PMC_29581307","title":"Structural determinants and cellular environment define processed actin as the sole substrate of the N-terminal acetyltransferase NAA80.","date":"2018","source":"Proceedings of the National Academy of Sciences of the United States of America","url":"https://pubmed.ncbi.nlm.nih.gov/29581307","citation_count":62,"is_preprint":false},{"pmid":"7559752","id":"PMC_7559752","title":"Fus2 localizes near the site of cell fusion and is required for both cell fusion and nuclear alignment during zygote formation.","date":"1995","source":"The Journal of cell biology","url":"https://pubmed.ncbi.nlm.nih.gov/7559752","citation_count":60,"is_preprint":false},{"pmid":"30028079","id":"PMC_30028079","title":"NAT6 acetylates the N-terminus of different forms of actin.","date":"2018","source":"The FEBS journal","url":"https://pubmed.ncbi.nlm.nih.gov/30028079","citation_count":39,"is_preprint":false},{"pmid":"11929860","id":"PMC_11929860","title":"Characterization of the murine hyaluronidase gene region reveals complex organization and cotranscription of Hyal1 with downstream genes, Fus2 and Hyal3.","date":"2002","source":"The Journal of biological chemistry","url":"https://pubmed.ncbi.nlm.nih.gov/11929860","citation_count":30,"is_preprint":false},{"pmid":"32978259","id":"PMC_32978259","title":"PFN2 and NAA80 cooperate to efficiently acetylate the N-terminus of actin.","date":"2020","source":"The Journal of biological chemistry","url":"https://pubmed.ncbi.nlm.nih.gov/32978259","citation_count":26,"is_preprint":false},{"pmid":"34805998","id":"PMC_34805998","title":"NAA80 bi-allelic missense variants result in high-frequency hearing loss, muscle weakness and developmental delay.","date":"2021","source":"Brain communications","url":"https://pubmed.ncbi.nlm.nih.gov/34805998","citation_count":24,"is_preprint":false},{"pmid":"30534344","id":"PMC_30534344","title":"Actin polymerization and cell motility are affected by NAA80-mediated posttranslational N-terminal acetylation of actin.","date":"2018","source":"Communicative & integrative biology","url":"https://pubmed.ncbi.nlm.nih.gov/30534344","citation_count":18,"is_preprint":false},{"pmid":"34896361","id":"PMC_34896361","title":"The Final Maturation State of β-actin Involves N-terminal Acetylation by NAA80, not N-terminal Arginylation by ATE1.","date":"2021","source":"Journal of molecular biology","url":"https://pubmed.ncbi.nlm.nih.gov/34896361","citation_count":16,"is_preprint":false},{"pmid":"32209306","id":"PMC_32209306","title":"N-terminal acetylation of actin by NAA80 is essential for structural integrity of the Golgi apparatus.","date":"2020","source":"Experimental cell research","url":"https://pubmed.ncbi.nlm.nih.gov/32209306","citation_count":12,"is_preprint":false},{"pmid":"10644992","id":"PMC_10644992","title":"The putative tumour suppressor Fus-2 is an N-acetyltransferase.","date":"2000","source":"Oncogene","url":"https://pubmed.ncbi.nlm.nih.gov/10644992","citation_count":11,"is_preprint":false},{"pmid":"15036368","id":"PMC_15036368","title":"Characterization of a new SNP c767A/T (Arg222Trp) in the candidate TSG FUS2 on human chromosome 3p21.3: prevalence in Asian populations and analysis of association with nasopharyngeal cancer.","date":"2004","source":"Molecular and cellular probes","url":"https://pubmed.ncbi.nlm.nih.gov/15036368","citation_count":3,"is_preprint":false},{"pmid":"15276206","id":"PMC_15276206","title":"Structural and expression analysis of the porcine FUS2 gene.","date":"2004","source":"Gene","url":"https://pubmed.ncbi.nlm.nih.gov/15276206","citation_count":2,"is_preprint":false},{"pmid":"39384430","id":"PMC_39384430","title":"Naa80 is required for actin N-terminal acetylation and normal hearing in zebrafish.","date":"2024","source":"Life science alliance","url":"https://pubmed.ncbi.nlm.nih.gov/39384430","citation_count":2,"is_preprint":false},{"pmid":"37408560","id":"PMC_37408560","title":"Optimized bisubstrate inhibitors for the actin N-terminal acetyltransferase NAA80.","date":"2023","source":"Frontiers in chemistry","url":"https://pubmed.ncbi.nlm.nih.gov/37408560","citation_count":2,"is_preprint":false},{"pmid":"21176460","id":"PMC_21176460","title":"[SNP767A/T of FUS2 gene and lung cancer risk in Chinese population].","date":"2006","source":"Zhongguo fei ai za zhi = Chinese journal of lung cancer","url":"https://pubmed.ncbi.nlm.nih.gov/21176460","citation_count":0,"is_preprint":false}],"cost":{"stage1":{"model":"claude-sonnet-4-6","input_tokens":9593,"output_tokens":2849,"usd":0.035757,"stage1_stop_reason":"end_turn"},"stage2":{"model":"claude-opus-4-8","input_tokens":10148,"output_tokens":2758,"usd":0.059845,"stage2_stop_reason":"end_turn"},"total_usd":0.095602,"stage1_batch_id":"msgbatch_013a8chtv3AWyTqv4z4sEF4C","stage2_batch_id":"msgbatch_01AHsuW63vinsvuTgzmQ3n62","note":"batch pricing = 50% of standard"},"stage1_raw":"```json\n{\n  \"discoveries\": [\n    {\n      \"year\": 2018,\n      \"finding\": \"NAA80 is the N-terminal acetyltransferase (NAT) responsible for Nt-acetylating actin. NAA80-knockout cells display increased F/G-actin ratio, increased filopodia and lamellipodia formation, and accelerated cell motility. In vitro, loss of Nt-acetylation alters rates of actin filament depolymerization and elongation (including formin-driven elongation), while Arp2/3-mediated nucleation is mostly unaffected.\",\n      \"method\": \"In vitro acetyltransferase assays, NAA80 knockout cell lines, actin polymerization/depolymerization assays, formin elongation assays, Arp2/3 nucleation assays, cell motility assays, fluorescence microscopy\",\n      \"journal\": \"Proceedings of the National Academy of Sciences of the United States of America\",\n      \"confidence\": \"High\",\n      \"confidence_rationale\": \"Tier 1–2 / Strong — multiple orthogonal methods including in vitro enzymatic assay, KO cell lines with defined phenotypes, and reconstituted biochemical assays; replicated across two simultaneous independent PNAS papers\",\n      \"pmids\": [\"29581253\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2018,\n      \"finding\": \"NAA80 substrate specificity is primarily determined by interactions with acidic amino acids at positions 2 and 3 of the actin substrate (not positions 1 and 2 as in most NATs). The crystal structure of NAA80 in complex with a bisubstrate inhibitor reveals a fold similar to other NAT enzymes but with a more open substrate-binding region. In a yeast model lacking NatB, ectopic NAA80 expression partially restored Nt-acetylation of NatB substrates, demonstrating intrinsic posttranslational Nt-acetylation capacity.\",\n      \"method\": \"Crystal structure determination of NAA80–bisubstrate inhibitor complex, bisubstrate inhibitor development, yeast complementation (NatB-deficient strain), in vitro acetyltransferase assays, active-site analysis\",\n      \"journal\": \"Proceedings of the National Academy of Sciences of the United States of America\",\n      \"confidence\": \"High\",\n      \"confidence_rationale\": \"Tier 1 / Strong — crystal structure with functional validation by mutagenesis-equivalent inhibitor binding, replicated biochemical assays, yeast genetic model; independent simultaneous study corroborating actin as sole substrate\",\n      \"pmids\": [\"29581307\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2018,\n      \"finding\": \"NAT6 (NAA80/FUS2) specifically acetylates the N-terminal acidic residue of different mammalian actin isoforms (β-actin Asp2, γ-actin-1 Glu2, α-actin-1). Knockout of NAT6 in two human cell lines abolished N-terminal acetylation of mature β- and γ-actin, and complete acetylation was restored by re-expression of NAT6 or addition of recombinant NAT6 to cell extracts. NAA10 showed much less or no activity on these substrates in equivalent assays.\",\n      \"method\": \"NAT6 knockout in two human cell lines, recombinant protein activity assays on purified proteins and actin N-terminal peptides, mass spectrometry for acetylation state, cell extract complementation assays\",\n      \"journal\": \"The FEBS journal\",\n      \"confidence\": \"High\",\n      \"confidence_rationale\": \"Tier 1–2 / Strong — two independent cell line KOs, recombinant enzyme assays, rescue experiments, MS validation; independent replication of PNAS findings\",\n      \"pmids\": [\"30028079\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2020,\n      \"finding\": \"PFN2 (profilin 2) is a stable interaction partner of NAA80 identified by interaction proteomics and confirmed by analytical ultracentrifugation. PFN2 binding specifically increases the intrinsic catalytic activity of NAA80. NAA80 binds PFN2 through a proline-rich loop; deletion of this loop abrogates PFN2 binding. Small-angle X-ray scattering shows NAA80, actin, and PFN2 form a ternary complex. PFN2 binding promotes interaction between the globular domains of actin and NAA80, facilitating actin acetylation. The majority of cellular NAA80 is stably bound to PFN2, not actin, and this complex acetylates G-actin before incorporation into filaments.\",\n      \"method\": \"Interaction proteomics, analytical ultracentrifugation, enzyme activity assays, deletion mutagenesis, small-angle X-ray scattering (SAXS)\",\n      \"journal\": \"The Journal of biological chemistry\",\n      \"confidence\": \"High\",\n      \"confidence_rationale\": \"Tier 1–2 / Moderate — multiple orthogonal methods (interaction proteomics, AUC, SAXS, enzyme assays, mutagenesis) in a single study demonstrating mechanism of PFN2-NAA80 cooperation\",\n      \"pmids\": [\"32978259\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2020,\n      \"finding\": \"NAA80 knockout cells display fragmentation of the Golgi apparatus. Re-expression of wild-type NAA80 rescues Golgi fragmentation, but a catalytically dead NAA80 mutant neither restores actin Nt-acetylation nor Golgi structure. NAA80 KO cells also show dramatically increased F-actin levels, suggesting a causal link between actin modification state and Golgi organization.\",\n      \"method\": \"NAA80 knockout cell lines, rescue experiments with wild-type and catalytic dead NAA80 mutant, immunofluorescence microscopy of Golgi structure, live-cell imaging, F-actin quantification\",\n      \"journal\": \"Experimental cell research\",\n      \"confidence\": \"High\",\n      \"confidence_rationale\": \"Tier 2 / Moderate — KO plus catalytic dead mutant rescue experiment directly links enzymatic activity to Golgi phenotype with multiple orthogonal readouts\",\n      \"pmids\": [\"32209306\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2021,\n      \"finding\": \"The final maturation state of β-actin is Nt-acetylation by NAA80 (yielding Ac-DDDI-). Using NAA80-lacking cells and targeted proteomics/mass spectrometry, Nt-arginylation of β-actin (RDDI-) previously claimed as a competing modification could not be confirmed in wildtype cells. Only a very minor level of arginylation of cleaved β-actin was detectable in NAA80-lacking cells but not in wildtype, establishing NAA80 as the terminal modifier that prevents arginylation.\",\n      \"method\": \"NAA80 knockout cells, targeted proteomics, mass spectrometry-based Nt-modification profiling, comparison with commercially available antibody detection\",\n      \"journal\": \"Journal of molecular biology\",\n      \"confidence\": \"High\",\n      \"confidence_rationale\": \"Tier 1–2 / Moderate — state-of-the-art targeted proteomics with KO model and orthogonal antibody comparison; rigorous negative result on arginylation mechanistically establishes NAA80 as terminal modifier\",\n      \"pmids\": [\"34896361\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2000,\n      \"finding\": \"The human FUS2 (NAA80) protein has homology to the catalytic domain of acetyltransferases, can acetylate protein N-termini using a ping-pong mechanism, shows substrate specificity, and localizes to the cytoplasm as shown by GFP-tagging experiments.\",\n      \"method\": \"Sequence homology analysis, in vitro N-terminal acetyltransferase assay, ping-pong kinetic mechanism determination, GFP-fusion subcellular localization\",\n      \"journal\": \"Oncogene\",\n      \"confidence\": \"Medium\",\n      \"confidence_rationale\": \"Tier 2–3 / Weak — single lab, enzyme activity demonstrated but substrate not yet defined as actin; cytoplasmic localization confirmed by GFP tagging\",\n      \"pmids\": [\"10644992\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2021,\n      \"finding\": \"Individuals with homozygous NAA80 p.(Leu130Pro) variant show ~50% decrease in actin acetylation, confirming NAA80 is required for actin N-terminal acetylation in vivo. Patient-derived fibroblasts and PBMCs showed increased migration, increased filopodia counts, and increased polymerized actin, consistent with NAA80 KO cell phenotypes. The variant destabilizes the NAA80 protein, reducing protein availability.\",\n      \"method\": \"Patient fibroblasts and PBMCs from individuals with NAA80 variant, mass spectrometry for actin acetylation, cell migration assays, filopodia counting, F-actin quantification, molecular structure-based protein stability prediction confirmed biochemically\",\n      \"journal\": \"Brain communications\",\n      \"confidence\": \"Medium\",\n      \"confidence_rationale\": \"Tier 2 / Moderate — human patient cells with defined variant confirming in vivo function; multiple cellular phenotype readouts consistent with prior KO studies, but single family/variant\",\n      \"pmids\": [\"34805998\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2024,\n      \"finding\": \"Zebrafish Naa80 acetylates both muscle and non-muscle actins in vivo and in vitro with preference for actin N-termini. Naa80 knockout zebrafish exhibit abnormal inner ear development, small otoliths, and impaired response to sound, but show normal development, morphology, and muscle function otherwise, demonstrating that actin N-terminal acetylation is essential for normal hearing.\",\n      \"method\": \"Zebrafish naa80 knockout model, in vitro acetyltransferase assays with purified Naa80, mass spectrometry for acetylation state, auditory/inner ear phenotype assays, morphological analysis\",\n      \"journal\": \"Life science alliance\",\n      \"confidence\": \"Medium\",\n      \"confidence_rationale\": \"Tier 2 / Moderate — KO animal model with defined phenotype, confirmed by in vitro enzymatic assays; single lab study\",\n      \"pmids\": [\"39384430\"],\n      \"is_preprint\": false\n    }\n  ],\n  \"current_model\": \"NAA80 (also known as NAT6/FUS2) is the sole N-terminal acetyltransferase for animal actins, acting posttranslationally on processed actin N-termini through substrate recognition primarily mediated by acidic residues at positions 2 and 3; it forms a stable ternary complex with profilin 2 (PFN2) and G-actin, whereby PFN2 binding via NAA80's proline-rich loop enhances catalytic activity and promotes actin acetylation before filament incorporation, and this modification critically controls actin filament dynamics (depolymerization and formin-driven elongation), cytoskeletal organization, Golgi integrity, cell motility, and hearing in vivo.\"\n}\n```","stage2_raw":"{\n  \"mechanistic_narrative\": \"NAA80 (NAT6/FUS2) is the dedicated N-terminal acetyltransferase for animal actins, posttranslationally acetylating the processed, acidic N-termini of muscle and non-muscle actin isoforms (β-actin Asp2, γ-actin Glu2, α-actin) and thereby controlling actin filament dynamics and cytoskeletal organization [#0, #2]. Unlike most NATs, its substrate specificity is dictated by acidic residues at positions 2 and 3 of actin rather than positions 1–2, and a crystal structure reveals a NAT-like fold with an unusually open substrate-binding region [#1]. Acetylation is the terminal maturation event of β-actin (yielding Ac-DDDI-) and prevents N-terminal arginylation [#5]. Catalysis is enhanced by a stable interaction with profilin 2 (PFN2): NAA80 binds PFN2 through a proline-rich loop, and NAA80, actin, and PFN2 assemble into a ternary complex in which PFN2 boosts intrinsic catalytic activity and promotes acetylation of G-actin before filament incorporation [#3]. Functionally, loss of NAA80 raises the F/G-actin ratio, increases filopodia and lamellipodia, accelerates cell motility, and fragments the Golgi apparatus in a manner rescued only by catalytically active enzyme [#0, #4]. The enzyme is required in vivo: a homozygous human p.(Leu130Pro) variant reduces actin acetylation and recapitulates the migration and actin phenotypes [#7], and zebrafish naa80 knockouts show defective inner ear development and impaired hearing [#8].\",\n  \"teleology\": [\n    {\n      \"year\": 2000,\n      \"claim\": \"Before any substrate was known, FUS2/NAA80 was established as a cytoplasmic acetyltransferase, framing it as an enzyme in search of a function.\",\n      \"evidence\": \"sequence homology analysis, in vitro NAT assay with ping-pong kinetics, and GFP-fusion localization\",\n      \"pmids\": [\"10644992\"],\n      \"confidence\": \"Medium\",\n      \"gaps\": [\"Physiological substrate undefined\", \"No structural or cellular role established\"]\n    },\n    {\n      \"year\": 2018,\n      \"claim\": \"Identifying actin as the substrate answered what NAA80 does, defining it as the sole NAT for animal actins and linking acetylation to filament dynamics.\",\n      \"evidence\": \"in vitro acetyltransferase assays, NAA80-knockout human cell lines, reconstituted polymerization/depolymerization, formin elongation and Arp2/3 nucleation assays, and MS validation across independent studies\",\n      \"pmids\": [\"29581253\", \"30028079\"],\n      \"confidence\": \"High\",\n      \"gaps\": [\"Mechanism of cytoskeletal control downstream of acetylation not fully resolved\", \"In vivo organismal consequences not yet tested\"]\n    },\n    {\n      \"year\": 2018,\n      \"claim\": \"A crystal structure and yeast complementation explained how NAA80 recognizes its unusual substrate, showing specificity arises from acidic residues at positions 2 and 3 via an open binding cleft.\",\n      \"evidence\": \"crystal structure of NAA80–bisubstrate inhibitor complex, active-site analysis, and ectopic expression in a NatB-deficient yeast strain\",\n      \"pmids\": [\"29581307\"],\n      \"confidence\": \"High\",\n      \"gaps\": [\"No structure of the productive NAA80–actin or ternary complex\", \"Determinants of isoform preference not fully mapped\"]\n    },\n    {\n      \"year\": 2020,\n      \"claim\": \"Discovery of PFN2 as a stable partner answered how NAA80 reaches and efficiently acetylates G-actin, defining a profilin-assisted ternary mechanism acting before filament incorporation.\",\n      \"evidence\": \"interaction proteomics, analytical ultracentrifugation, SAXS, enzyme activity assays, and proline-rich-loop deletion mutagenesis\",\n      \"pmids\": [\"32978259\"],\n      \"confidence\": \"High\",\n      \"gaps\": [\"High-resolution structure of the ternary complex lacking\", \"Role of other profilin isoforms not addressed\"]\n    },\n    {\n      \"year\": 2020,\n      \"claim\": \"Catalytic-dead rescue established that NAA80 enzymatic activity, not mere presence, maintains Golgi integrity, connecting actin modification state to organelle organization.\",\n      \"evidence\": \"NAA80 KO cells with wild-type versus catalytic-dead rescue, Golgi immunofluorescence/live imaging, and F-actin quantification\",\n      \"pmids\": [\"32209306\"],\n      \"confidence\": \"High\",\n      \"gaps\": [\"Molecular link between actin acetylation and Golgi structure unresolved\", \"Effects on Golgi-associated trafficking not measured\"]\n    },\n    {\n      \"year\": 2021,\n      \"claim\": \"Targeted proteomics resolved a competing-modification controversy, establishing NAA80 acetylation as the terminal β-actin maturation state that precludes N-terminal arginylation.\",\n      \"evidence\": \"NAA80 KO cells with targeted MS-based Nt-modification profiling and antibody comparison\",\n      \"pmids\": [\"34896361\"],\n      \"confidence\": \"High\",\n      \"gaps\": [\"Functional consequence of the residual arginylation seen only in KO cells unclear\"]\n    },\n    {\n      \"year\": 2021,\n      \"claim\": \"A human homozygous variant demonstrated NAA80's requirement for actin acetylation in vivo and tied loss of function to a human phenotype.\",\n      \"evidence\": \"patient fibroblasts/PBMCs with p.(Leu130Pro), MS for acetylation, migration and filopodia assays, F-actin quantification, and biochemical stability assessment\",\n      \"pmids\": [\"34805998\"],\n      \"confidence\": \"Medium\",\n      \"gaps\": [\"Single family/variant\", \"Genotype–clinical phenotype relationship not fully defined\"]\n    },\n    {\n      \"year\": 2024,\n      \"claim\": \"A zebrafish knockout defined an organismal role, showing actin N-terminal acetylation is essential for inner ear development and hearing despite otherwise normal development and muscle.\",\n      \"evidence\": \"naa80 knockout zebrafish, in vitro acetyltransferase assays, MS, and auditory/morphological phenotyping\",\n      \"pmids\": [\"39384430\"],\n      \"confidence\": \"Medium\",\n      \"gaps\": [\"Cellular mechanism linking acetylation to hair-cell/otolith function unknown\", \"Single lab study\"]\n    },\n    {\n      \"year\": null,\n      \"claim\": \"How actin N-terminal acetylation is mechanistically translated into specific filament behaviors, Golgi maintenance, and tissue-specific phenotypes such as hearing remains unresolved.\",\n      \"evidence\": \"\",\n      \"pmids\": [],\n      \"confidence\": \"Medium\",\n      \"gaps\": [\"No structure of the productive NAA80–actin complex\", \"Causal chain from acetylation to organelle and sensory phenotypes uncharacterized\"]\n    }\n  ],\n  \"mechanism_profile\": {\n    \"molecular_activity\": [\n      {\"term_id\": \"GO:0016740\", \"supporting_discovery_ids\": [0, 1, 2, 6]},\n      {\"term_id\": \"GO:0140096\", \"supporting_discovery_ids\": [0, 2]}\n    ],\n    \"localization\": [\n      {\"term_id\": \"GO:0005829\", \"supporting_discovery_ids\": [6]},\n      {\"term_id\": \"GO:0005794\", \"supporting_discovery_ids\": [4]}\n    ],\n    \"pathway\": [\n      {\"term_id\": \"R-HSA-392499\", \"supporting_discovery_ids\": [0, 2]}\n    ],\n    \"complexes\": [\"NAA80–PFN2–actin ternary complex\"],\n    \"partners\": [\"PFN2\", \"ACTB\", \"ACTG1\"],\n    \"other_free_text\": []\n  }\n}","audit_flag":null,"evaluation":{"pairwise":"win","faith_supported":6,"faith_total":6,"faith_pct":100.0}}