{"gene":"SFI1","run_date":"2026-04-28T20:42:07","timeline":{"discoveries":[{"year":2003,"finding":"Yeast Sfi1p localizes to the half-bridge of the spindle pole body (SPB) and contains multiple internal repeats that each bind one molecule of the centrin Cdc31p in a 1:1 ratio, such that a single Sfi1p molecule binds multiple centrin molecules. Temperature-sensitive sfi1 mutants show a defect in SPB duplication and genetic interactions with cdc31-1, establishing Sfi1p as an essential component required for SPB duplication. Human proteins containing Sfi1 repeats also bind centrin, demonstrating conservation of this centrin-binding motif.","method":"Centrin pull-down experiments, temperature-sensitive mutant analysis, immunofluorescence localization, genetic interaction with cdc31-1","journal":"The Journal of cell biology","confidence":"High","confidence_rationale":"Tier 1-2 — direct biochemical pull-down, mutagenesis, and genetic epistasis; foundational paper with 168 citations","pmids":["14504268"],"is_preprint":false},{"year":1999,"finding":"SFI1 is an essential yeast gene required for cell cycle progression through G2-M transition. Conditional sfi1 mutants arrest as doublets of undivided mother-daughter cells with a single nucleus and no mitotic spindles, indicating a role in mitotic spindle assembly. SFI1 was originally identified as a suppressor of cAMP pathway deficiency (fil1 mutant) but acts independently of the cAMP pathway.","method":"Conditional expression (galactose-inducible promoter), cell cycle arrest phenotype analysis, genetic epistasis with cAMP pathway mutants","journal":"Yeast (Chichester, England)","confidence":"Medium","confidence_rationale":"Tier 2 — clean conditional KO with defined cellular phenotype; single lab","pmids":["10455233"],"is_preprint":false},{"year":2007,"finding":"Novel sfi1 alleles in the C-terminal domain of Sfi1p (identified via mad1 synthetic lethal screen) cause duplicated SPBs that fail to separate (SPBs <0.3 µm apart), suggesting a role for the Sfi1p C-terminal domain in SPB splitting/bridge dissolution distinct from the earlier duplication step.","method":"Genetic screen (mad1 synthetic lethal), light and electron microscopy of SPB morphology","journal":"Molecular biology of the cell","confidence":"Medium","confidence_rationale":"Tier 2 — genetic screen plus EM structural analysis; single lab","pmids":["17392514"],"is_preprint":false},{"year":2014,"finding":"Cdk1 phosphorylation of Sfi1 licenses yeast centrosome (SPB) duplication to occur only once per cell cycle. Reducing Cdk1 phosphorylation by changing Sfi1 phosphorylation sites to non-phosphorylatable residues causes defects in SPB separation and inappropriate SPB reduplication during mitosis, with bipolar spindle assembly and chromosome segregation defects. The phosphatase Cdc14 opposes Cdk1 by dephosphorylating Sfi1 to activate duplication licensing.","method":"Phosphomimetic/phospho-null mutagenesis of Sfi1 phosphorylation sites, live-cell imaging, quantitative mass spectrometry of phosphorylation sites","journal":"PLoS genetics","confidence":"High","confidence_rationale":"Tier 1-2 — mutagenesis combined with functional readouts and MS identification of phosphosites; multiple orthogonal methods","pmids":["25340401"],"is_preprint":false},{"year":2014,"finding":"In fission yeast, Sfi1 is gradually recruited to SPBs throughout the cell cycle (not abruptly at duplication onset). Conserved tryptophan residues in the internal repeats are functionally essential: the Trp-to-Arg mutant (sfi1-M46) forms monopolar spindles, causes mitosis and cytokinesis defects, and associates preferentially with one daughter SPB during mitosis, resulting in failure of new SPB assembly in the SPB receiving insufficient Sfi1. These tryptophans are required for proper Sfi1 partitioning between daughter SPBs.","method":"Live-cell fluorescence microscopy (Sfi1-GFP), tryptophan-to-arginine mutagenesis, spindle assembly assays, electron microscopy","journal":"Molecular biology of the cell","confidence":"High","confidence_rationale":"Tier 1-2 — mutagenesis combined with live imaging and EM; multiple orthogonal methods","pmids":["25031431"],"is_preprint":false},{"year":2015,"finding":"In fission yeast, Sfi1 and Cdc31 form the SPB half-bridge that duplicates to promote new SPB biogenesis. Sfi1 accumulates in two distinct cell-cycle phases. Cdc31 phosphorylation on serine 15 (a Cdk1/Cdc2 consensus site) is required for dissociation of a significant pool of Sfi1 from the bridge and for timely SPB segregation at mitotic onset, indicating that Cdc31 N-terminus modulates the stability of Sfi1-Cdc31 arrays and timing of spindle bipolarity.","method":"Phosphomimetic/phospho-null mutagenesis of Cdc31 S15, quantitative live-cell fluorescence microscopy, mass spectrometry (phosphoproteomics)","journal":"Journal of cell science","confidence":"High","confidence_rationale":"Tier 1-2 — mutagenesis of phosphosite with clear functional phenotype and MS validation; multiple methods","pmids":["25736294"],"is_preprint":false},{"year":2015,"finding":"Kar1 (membrane-anchored SPB component) directly binds the C-terminal region of Sfi1 and localizes to the bridge center. Kar1 tethers the Sfi1-containing bridge to the nuclear envelope. In kar1Δ cells the bridge adopts an arched conformation. Sfi1 C-terminal dimerization in G1 is the licensing step for SPB duplication (bridge formation), and Cdc31 and Kar1 provide cross-links that stabilize the bridge and ensure timely SPB separation.","method":"Photo-activated localization microscopy (PALM), direct binding assays between Kar1 and Sfi1 C-terminal fragments, binding free energy calculations","journal":"The Journal of cell biology","confidence":"High","confidence_rationale":"Tier 1-2 — direct binding assays plus super-resolution microscopy and computational modeling; multiple orthogonal approaches","pmids":["26076691"],"is_preprint":false},{"year":2014,"finding":"CK2 phosphorylates human centrin 1 at T138 and human centrin 2 at T138 and S158. These phosphorylation events reduce or abolish centrin binding to Sfi1 (as well as to XPC and transducin β), as demonstrated by phosphomimetic centrin 2 (T138D-S158D) which completely abrogates Sfi1 binding. This places CK2 as a writer that negatively regulates centrin-Sfi1 interaction.","method":"In vitro CK2 phosphorylation assay, isothermal titration calorimetry (ITC) binding measurements, phosphomimetic mutagenesis","journal":"FEBS open bio","confidence":"High","confidence_rationale":"Tier 1 — in vitro kinase assay plus ITC binding quantification and phosphomimetic mutagenesis","pmids":["24918055"],"is_preprint":false},{"year":2019,"finding":"Mammalian SFI1 localizes to the centrosome during S phase and interacts with the deubiquitylase USP9X. SFI1 recruits USP9X to the centrosome, where USP9X deubiquitylates STIL (a critical regulator of centriole duplication), protecting it from proteasomal degradation. Loss of USP9X (from patient mutations) reduces STIL levels. This SFI1→USP9X→STIL axis promotes centriole duplication and underlies roles of both SFI1 and USP9X in human neurodevelopment.","method":"Co-immunoprecipitation, centrosome localization experiments, ubiquitylation/deubiquitylation assays, patient cell analysis, RNAi knockdown with phenotypic readout","journal":"The Journal of cell biology","confidence":"High","confidence_rationale":"Tier 2 — reciprocal co-IP, functional deubiquitylation assay, patient cell validation; multiple orthogonal methods","pmids":["31197030"],"is_preprint":false},{"year":2021,"finding":"The N-terminus of Sfi1 (N-Sfi1), including the first three Cdc31-binding sites, directly interacts with SPB components Spc29 and Spc42 to trigger daughter SPB (dSPB) assembly. Cdc31 binding to N-Sfi1 promotes Spc29 recruitment and is essential for satellite formation (earliest step of dSPB biogenesis). Phosphorylation of N-Sfi1 has an inhibitory effect, delaying dSPB biogenesis until G1.","method":"Yeast two-hybrid, co-immunoprecipitation, in vivo localization, phosphomutant analysis","journal":"The Journal of cell biology","confidence":"High","confidence_rationale":"Tier 2 — multiple binding assays (Y2H and co-IP) with functional rescue and phosphomutant analysis; multiple orthogonal methods","pmids":["33523111"],"is_preprint":false},{"year":2022,"finding":"Human SFI1 is a centriolar protein that localizes to the distal end of the centriole, where it associates with a pool of Centrin. Both SFI1 and Centrin are recruited early during procentriole assembly. Depletion of SFI1 causes loss of the distal Centrin pool without altering centriole duplication per se; instead, the SFI1/Centrin complex is essential for centriolar architecture, CEP164 distribution, and CP110 removal during ciliogenesis.","method":"Expansion microscopy (ExM), siRNA depletion, fluorescence localization, ciliogenesis assays","journal":"The EMBO journal","confidence":"High","confidence_rationale":"Tier 2 — super-resolution expansion microscopy with functional depletion phenotypes; multiple orthogonal approaches in human cells","pmids":["36125182"],"is_preprint":false},{"year":2025,"finding":"C2CD3 depletion in human centrioles destabilizes a luminal ring network composed of C2CD3/SFI1/centrin-2/CEP135/NA14, revealing that SFI1 is a structural component of the luminal ring at the distal centriole. This network connects the centriolar lumen, distal microtubule cap, and appendages in an 'in-to-out' molecular hub.","method":"Ultrastructure Expansion Microscopy (U-ExM), iterative U-ExM, in situ cryo-electron tomography, siRNA depletion","journal":"bioRxiv","confidence":"Medium","confidence_rationale":"Tier 1 — cryo-ET and U-ExM structural data; preprint, not yet peer-reviewed","pmids":["bio_10.1101_2025.06.17.660204"],"is_preprint":true}],"current_model":"SFI1 is a conserved centrin-binding scaffold protein that, through its multiple internal repeat motifs, recruits centrin (Cdc31/CETN) in a 1:1 stoichiometry per repeat to form the SPB half-bridge/bridge in yeast and the distal centriolar ring structure in humans; in yeast, SPB duplication is licensed by Cdk1 phosphorylation of Sfi1 (promoting bridge dimerization and SPB separation) and counteracted by Cdc14 dephosphorylation, while CK2 phosphorylation of centrin negatively regulates centrin–Sfi1 binding; in mammalian cells, SFI1 localizes USP9X to the centrosome during S phase to deubiquitylate and stabilize STIL for centriole duplication, and the SFI1/Centrin complex at the centriole distal end is dispensable for duplication but essential for centriolar architecture, CEP164 positioning, and CP110 removal during ciliogenesis."},"narrative":{"teleology":[{"year":1999,"claim":"Establishing that SFI1 is an essential gene required for progression through mitosis resolved its role beyond its original identification as a suppressor of cAMP pathway deficiency, placing it in the cell division machinery.","evidence":"Conditional sfi1 mutants in budding yeast arrest with undivided nuclei and no mitotic spindles","pmids":["10455233"],"confidence":"Medium","gaps":["Molecular target of Sfi1 unknown at this stage","No mechanism for spindle assembly failure identified"]},{"year":2003,"claim":"Identification of Sfi1p as a multi-centrin scaffold at the SPB half-bridge established the molecular basis by which centrin is organized into functional arrays and explained why Sfi1 is required for SPB duplication.","evidence":"Centrin pull-down, localization to the SPB half-bridge, temperature-sensitive mutant phenotypes, and genetic interaction with cdc31-1 in budding yeast; conservation confirmed with human Sfi1 repeat–centrin binding","pmids":["14504268"],"confidence":"High","gaps":["No structural information on the repeat–centrin interface","Mechanism of SPB duplication initiation not resolved"]},{"year":2007,"claim":"Demonstrating that the C-terminal domain of Sfi1 controls SPB separation (bridge splitting) rather than initial duplication revealed functionally distinct roles for different Sfi1 domains in the centrosome cycle.","evidence":"C-terminal sfi1 alleles from a mad1 synthetic-lethal screen yield duplicated but unseparated SPBs by light and electron microscopy","pmids":["17392514"],"confidence":"Medium","gaps":["Identity of the C-terminal binding partner mediating splitting unknown","Regulation of the splitting step not addressed"]},{"year":2014,"claim":"Showing that Cdk1 phosphorylation of Sfi1 licenses once-per-cycle SPB duplication, counteracted by Cdc14 dephosphorylation, established a phospho-regulatory switch ensuring centrosome copy-number control.","evidence":"Phosphomimetic and phospho-null Sfi1 mutants in budding yeast combined with mass spectrometry and live-cell imaging","pmids":["25340401"],"confidence":"High","gaps":["How phosphorylation physically blocks or permits C-terminal dimerization not structurally resolved","Contribution of additional kinases not excluded"]},{"year":2014,"claim":"Identification that conserved tryptophan residues in Sfi1 repeats are essential for proper Sfi1 partitioning between daughter SPBs linked Sfi1 stoichiometry to spindle bipolarity in fission yeast.","evidence":"Trp-to-Arg mutagenesis with live-cell imaging and EM in S. pombe","pmids":["25031431"],"confidence":"High","gaps":["Whether the tryptophans mediate centrin binding or intra-Sfi1 contacts not distinguished","Mechanism of asymmetric partitioning unclear"]},{"year":2014,"claim":"CK2 phosphorylation of centrin at T138/S158 was shown to abolish centrin–Sfi1 binding, revealing an additional kinase-mediated layer of regulation of the Sfi1–centrin array independent of Cdk1.","evidence":"In vitro CK2 kinase assay and isothermal titration calorimetry with phosphomimetic human centrin 2","pmids":["24918055"],"confidence":"High","gaps":["In vivo relevance of CK2-mediated centrin phosphorylation for centrosome biology not demonstrated","Whether CK2 regulation is cell-cycle dependent is unknown"]},{"year":2015,"claim":"Characterization of Kar1 as a direct binding partner of the Sfi1 C-terminus that tethers the bridge to the nuclear envelope, combined with evidence that C-terminal Sfi1 dimerization in G1 is the licensing step for SPB duplication, provided a structural model of bridge formation.","evidence":"Super-resolution PALM imaging, direct binding assays between Kar1 and Sfi1 C-terminal fragments, and bridge morphology in kar1Δ cells in budding yeast","pmids":["26076691"],"confidence":"High","gaps":["Atomic-resolution structure of the Sfi1 C-terminal dimer interface lacking","How Cdk1 phosphorylation physically disrupts the dimer not determined"]},{"year":2015,"claim":"Demonstrating that Cdc31 phosphorylation on S15 promotes Sfi1 dissociation from the bridge in fission yeast established that centrin phosphorylation actively regulates bridge dynamics and timing of SPB segregation.","evidence":"Phosphomimetic/phospho-null Cdc31 S15 mutagenesis with quantitative live-cell fluorescence and mass spectrometry in S. pombe","pmids":["25736294"],"confidence":"High","gaps":["Whether mammalian centrin phosphorylation plays an equivalent role at the centriole unknown","Relationship between CK2-mediated (T138) and Cdk-mediated (S15) phosphorylation not integrated"]},{"year":2019,"claim":"Discovery that mammalian SFI1 recruits the deubiquitylase USP9X to the centrosome to stabilize STIL extended Sfi1's function beyond structural scaffolding to active regulation of centriole duplication via ubiquitin signaling.","evidence":"Reciprocal co-immunoprecipitation, deubiquitylation assays, patient cell analysis, and RNAi knockdown in human cells","pmids":["31197030"],"confidence":"High","gaps":["Domain of SFI1 that binds USP9X not mapped","Whether the SFI1–USP9X axis operates in non-neural tissues not tested"]},{"year":2021,"claim":"Identification of the Sfi1 N-terminus as the direct binding partner of SPB core components Spc29 and Spc42 resolved how Sfi1–Cdc31 arrays nucleate the earliest step of daughter SPB biogenesis (satellite formation), with N-terminal phosphorylation acting as an inhibitory timer.","evidence":"Yeast two-hybrid, co-immunoprecipitation, in vivo localization, and phosphomutant analysis in budding yeast","pmids":["33523111"],"confidence":"High","gaps":["Identity of the kinase responsible for inhibitory N-terminal phosphorylation not fully resolved","Stoichiometry of the N-Sfi1/Spc29/Spc42 assembly not quantified"]},{"year":2022,"claim":"Super-resolution mapping of human SFI1 to the centriole distal end, and demonstration that its depletion disrupts centriolar architecture, CEP164 positioning, and ciliogenesis without blocking duplication, defined a mammalian structural role for SFI1/Centrin distinct from its duplication-licensing function.","evidence":"Expansion microscopy, siRNA depletion, and ciliogenesis assays in human cells","pmids":["36125182"],"confidence":"High","gaps":["How SFI1/Centrin loss specifically impairs CP110 removal is mechanistically unclear","Whether SFI1's structural and USP9X-recruiting functions are separable not tested"]},{"year":null,"claim":"Key open questions include how SFI1's structural scaffold role at the centriole distal end relates to its S-phase role in USP9X recruitment, what the atomic structure of the SFI1–centrin array looks like in situ, and whether mammalian SFI1 phosphoregulation mirrors the yeast Cdk1/Cdc14 licensing switch.","evidence":"","pmids":[],"confidence":"High","gaps":["No in situ or high-resolution structure of the full-length SFI1–centrin array","Mammalian phosphoregulation of SFI1 during the centrosome cycle uncharacterized","Relationship between centriole distal-end architecture and ciliogenesis signaling not mechanistically resolved"]}],"mechanism_profile":{"molecular_activity":[{"term_id":"GO:0005198","term_label":"structural molecule activity","supporting_discovery_ids":[0,4,6,10]},{"term_id":"GO:0060090","term_label":"molecular adaptor activity","supporting_discovery_ids":[8,9]}],"localization":[{"term_id":"GO:0005815","term_label":"microtubule organizing center","supporting_discovery_ids":[0,2,4,5,6,8,9,10]}],"pathway":[{"term_id":"R-HSA-1640170","term_label":"Cell Cycle","supporting_discovery_ids":[1,3,4,5]},{"term_id":"R-HSA-1852241","term_label":"Organelle biogenesis and maintenance","supporting_discovery_ids":[0,6,9,10]}],"complexes":["Sfi1-centrin (Sfi1-Cdc31) half-bridge/bridge array"],"partners":["CETN2","CDC31","USP9X","KAR1","SPC29","SPC42","STIL","CEP164"],"other_free_text":[]},"mechanistic_narrative":"SFI1 is a conserved centrin-binding scaffold protein that organizes centrosome/spindle pole body (SPB) architecture and licenses centrosome duplication across eukaryotes. Its multiple internal repeats each bind one molecule of centrin (Cdc31/CETN2) in a 1:1 stoichiometry, forming elongated arrays that constitute the SPB half-bridge in yeast and the distal centriolar ring in human cells; Cdk1 phosphorylation of the Sfi1 C-terminus drives bridge dimerization and SPB separation, while the phosphatase Cdc14 opposes this to reset duplication licensing, and CK2 phosphorylation of centrin negatively regulates centrin–Sfi1 binding [PMID:14504268, PMID:25340401, PMID:24918055]. In budding yeast, the Sfi1 N-terminus recruits SPB components Spc29 and Spc42 to initiate daughter SPB assembly, with Cdc31 binding promoting Spc29 recruitment [PMID:33523111]. In mammalian cells, SFI1 localizes to the centriole distal end where it recruits USP9X to deubiquitylate and stabilize STIL for centriole duplication during S phase, and the SFI1/Centrin complex is dispensable for duplication per se but essential for centriolar architecture, CEP164 positioning, and CP110 removal during ciliogenesis [PMID:31197030, PMID:36125182]."},"prefetch_data":{"uniprot":{"accession":"A8K8P3","full_name":"Protein SFI1 homolog","aliases":[],"length_aa":1242,"mass_kda":147.7,"function":"Plays a role in the dynamic structure of centrosome-associated contractile fibers via its interaction with CETN2","subcellular_location":"Cytoplasm, cytoskeleton, microtubule organizing center, centrosome, centriole","url":"https://www.uniprot.org/uniprotkb/A8K8P3/entry"},"depmap":{"release":"DepMap","has_data":true,"is_common_essential":false,"resolved_as":"","url":"https://depmap.org/portal/gene/SFI1","classification":"Not Classified","n_dependent_lines":3,"n_total_lines":1208,"dependency_fraction":0.0024834437086092716},"opencell":{"profiled":false,"resolved_as":"","ensg_id":"","cell_line_id":"","localizations":[],"interactors":[{"gene":"CETN2","stoichiometry":0.2},{"gene":"CETN3","stoichiometry":0.2}],"url":"https://opencell.sf.czbiohub.org/search/SFI1","total_profiled":1310},"omim":[{"mim_id":"612765","title":"SFI1 CENTRIN-BINDING PROTEIN; SFI1","url":"https://www.omim.org/entry/612765"},{"mim_id":"300710","title":"ALOPECIA, ANDROGENETIC, 2; AGA2","url":"https://www.omim.org/entry/300710"},{"mim_id":"188035","title":"PRO-PLATELET BASIC PROTEIN-LIKE 1; PPBPL1","url":"https://www.omim.org/entry/188035"}],"hpa":{"profiled":true,"resolved_as":"","reliability":"","locations":[],"tissue_specificity":"Tissue enhanced","tissue_distribution":"Detected in many","driving_tissues":[{"tissue":"brain","ntpm":25.5}],"url":"https://www.proteinatlas.org/search/SFI1"},"hgnc":{"alias_symbol":["KIAA0542","PISD","PPP1R139"],"prev_symbol":[]},"alphafold":{"accession":"A8K8P3","domains":[],"viewer_url":"https://alphafold.ebi.ac.uk/entry/A8K8P3","model_url":"https://alphafold.ebi.ac.uk/files/AF-A8K8P3-F1-model_v6.cif","pae_url":"https://alphafold.ebi.ac.uk/files/AF-A8K8P3-F1-predicted_aligned_error_v6.png","plddt_mean":66.38},"mouse_models":{"mgi_url":"https://www.informatics.jax.org/marker/summary?nomen=SFI1","jax_strain_url":"https://www.jax.org/strain/search?query=SFI1"},"sequence":{"accession":"A8K8P3","fasta_url":"https://rest.uniprot.org/uniprotkb/A8K8P3.fasta","uniprot_url":"https://www.uniprot.org/uniprotkb/A8K8P3/entry","alphafold_viewer_url":"https://alphafold.ebi.ac.uk/entry/A8K8P3"}},"corpus_meta":[{"pmid":"30488656","id":"PMC_30488656","title":"The homozygous variant c.797G>A/p.(Cys266Tyr) in PISD is associated with a Spondyloepimetaphyseal dysplasia with large epiphyses and disturbed mitochondrial function.","date":"2018","source":"Human mutation","url":"https://pubmed.ncbi.nlm.nih.gov/30488656","citation_count":61,"is_preprint":false,"source_track":"pubmed_title"},{"pmid":"30858161","id":"PMC_30858161","title":"PISD is a mitochondrial disease gene causing skeletal dysplasia, cataracts, and white matter changes.","date":"2019","source":"Life science alliance","url":"https://pubmed.ncbi.nlm.nih.gov/30858161","citation_count":57,"is_preprint":false,"source_track":"pubmed_title"},{"pmid":"25031431","id":"PMC_25031431","title":"Regulation of spindle pole body assembly and cytokinesis by the centrin-binding protein Sfi1 in fission yeast.","date":"2014","source":"Molecular biology of the cell","url":"https://pubmed.ncbi.nlm.nih.gov/25031431","citation_count":33,"is_preprint":false,"source_track":"pubmed_title"},{"pmid":"25340401","id":"PMC_25340401","title":"Licensing of yeast centrosome duplication 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conditional depletion causes arrest as doublets with a single nucleus and no mitotic spindle, indicating a role in mitotic spindle assembly.\",\n      \"method\": \"Conditional depletion (galactose-inducible promoter), light microscopy, cell cycle analysis\",\n      \"journal\": \"Yeast\",\n      \"confidence\": \"Medium\",\n      \"confidence_rationale\": \"Tier 2 — KO with defined cellular phenotype (G2/M arrest, monopolar spindle), single lab\",\n      \"pmids\": [\"10455233\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2007,\n      \"finding\": \"The C-terminal domain of Sfi1p (budding yeast) is required for spindle pole body (SPB) splitting after duplication is complete; novel C-terminal sfi1 alleles produce duplicated but unseparated SPBs (<0.3 µm apart), suggesting a role in bridge splitting.\",\n      \"method\": \"Genetic screen (mad1 synthetic lethal), light and electron microscopy of sfi1 mutants\",\n      \"journal\": \"Molecular biology of the cell\",\n      \"confidence\": \"Medium\",\n      \"confidence_rationale\": \"Tier 2 — multiple orthogonal microscopy methods, defined structural phenotype, single lab\",\n      \"pmids\": [\"17392514\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2014,\n      \"finding\": \"Sfi1 (budding yeast) is a substrate of Cdk1; Cdk1 phosphorylation of Sfi1 promotes SPB separation and prevents premature SPB reduplication, while the phosphatase Cdc14 dephosphorylates Sfi1 to license a new round of duplication.\",\n      \"method\": \"Phosphomimetic/nonphosphorylatable mutants, SPB duplication assays, epistasis with cdc14 mutants, mass spectrometry identification of phosphosites\",\n      \"journal\": \"PLoS genetics\",\n      \"confidence\": \"High\",\n      \"confidence_rationale\": \"Tier 1-2 — phosphosite mutagenesis plus epistasis with phosphatase mutant, multiple orthogonal readouts\",\n      \"pmids\": [\"25340401\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2014,\n      \"finding\": \"In fission yeast, Sfi1 is gradually recruited to SPBs throughout the cell cycle via conserved tryptophans in its internal repeats; mutation of these tryptophans (sfi1-M46) causes Sfi1 to associate preferentially with one daughter SPB, leading to failure of new SPB assembly and monopolar spindle formation.\",\n      \"method\": \"Live-cell imaging, Trp-to-Arg mutagenesis, fluorescence microscopy of SPB segregation\",\n      \"journal\": \"Molecular biology of the cell\",\n      \"confidence\": \"High\",\n      \"confidence_rationale\": \"Tier 1-2 — mutagenesis of conserved residues with defined structural and cell-division phenotype, multiple orthogonal imaging methods\",\n      \"pmids\": [\"25031431\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2015,\n      \"finding\": \"In fission yeast, Cdc31 phosphorylation on Ser15 (Cdk1 consensus site) is required for dissociation of a significant Sfi1 pool from the bridge and timely SPB segregation at mitotic onset, indicating that Cdc31 N-terminus modulates Sfi1-Cdc31 array stability.\",\n      \"method\": \"Phosphomimetic mutagenesis of Cdc31 S15, live-cell imaging, epistasis with cdc2 (Cdk1) pathway\",\n      \"journal\": \"Journal of cell science\",\n      \"confidence\": \"Medium\",\n      \"confidence_rationale\": \"Tier 2 — phosphomimetic mutagenesis with live imaging phenotype, single lab\",\n      \"pmids\": [\"25736294\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2015,\n      \"finding\": \"Kar1 directly binds the C-terminal region of Sfi1 and anchors the SPB bridge to the nuclear envelope; PALM super-resolution microscopy localizes Kar1 to the bridge center, and kar1Δ cells show an arched bridge, while Cdc31-Cdc31 interactions provide cross-links stabilizing the Sfi1-Cdc31 arrays.\",\n      \"method\": \"Photo-activated localization microscopy (PALM), direct binding assays, kar1Δ genetic analysis, binding free energy calculations\",\n      \"journal\": \"Journal of cell biology\",\n      \"confidence\": \"High\",\n      \"confidence_rationale\": \"Tier 1-2 — direct binding assays plus super-resolution imaging plus genetic epistasis, multiple orthogonal methods\",\n      \"pmids\": [\"26076691\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2014,\n      \"finding\": \"CK2 phosphorylates human centrin 1 at T138 and human centrin 2 at T138 and S158; this phosphorylation abolishes centrin 1 binding to Sfi1 and reduces centrin 2 binding to Sfi1, as demonstrated with phosphomimetic centrin 2 (T138D-S158D).\",\n      \"method\": \"In vitro CK2 phosphorylation, isothermal titration calorimetry, phosphomimetic mutagenesis\",\n      \"journal\": \"FEBS open bio\",\n      \"confidence\": \"Medium\",\n      \"confidence_rationale\": \"Tier 1 — in vitro phosphorylation assay with binding measurement and phosphomimetics, single lab\",\n      \"pmids\": [\"24918055\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2019,\n      \"finding\": \"Mammalian SFI1 localizes to the centrosome during S phase and recruits the deubiquitylase USP9X to the centrosome, where USP9X deubiquitylates STIL to protect it from proteasomal degradation and thereby promotes centriole duplication.\",\n      \"method\": \"Co-immunoprecipitation, centrosome fractionation, depletion/rescue experiments, ubiquitylation assays, patient cell analysis (USP9X loss-of-function)\",\n      \"journal\": \"Journal of cell biology\",\n      \"confidence\": \"High\",\n      \"confidence_rationale\": \"Tier 2 — reciprocal Co-IP, fractionation, functional rescue, patient validation; multiple orthogonal methods\",\n      \"pmids\": [\"31197030\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2021,\n      \"finding\": \"The N-terminus of Sfi1 (including the first three Cdc31/centrin binding sites) interacts with SPB components Spc29 and Spc42 to trigger daughter SPB assembly; Cdc31 binding to N-Sfi1 promotes Spc29 recruitment and is essential for satellite formation, and N-Sfi1 phosphorylation has an inhibitory effect delaying daughter SPB biogenesis until G1.\",\n      \"method\": \"Binding assays, phosphomimetic mutagenesis of N-Sfi1, SPB assembly assays, fluorescence microscopy\",\n      \"journal\": \"Journal of cell biology\",\n      \"confidence\": \"High\",\n      \"confidence_rationale\": \"Tier 1-2 — direct binding assays plus mutagenesis plus in vivo SPB assembly readout, single lab with multiple orthogonal approaches\",\n      \"pmids\": [\"33523111\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2022,\n      \"finding\": \"Human SFI1 is a centriolar protein that localizes to the distal end of the centriole and associates with a pool of Centrin there; depletion of SFI1 causes loss of the distal Centrin pool without affecting centriole duplication, but impairs centriolar architecture, CEP164 distribution, and CP110 removal during ciliogenesis.\",\n      \"method\": \"Expansion microscopy (U-ExM), SFI1 depletion, immunofluorescence of CEP164 and CP110, ciliogenesis assays\",\n      \"journal\": \"The EMBO journal\",\n      \"confidence\": \"High\",\n      \"confidence_rationale\": \"Tier 2 — expansion microscopy for localization, depletion with multiple defined phenotypic readouts, single lab with multiple orthogonal methods\",\n      \"pmids\": [\"36125182\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2025,\n      \"finding\": \"C2CD3 depletion destabilizes the luminal ring network composed of C2CD3/SFI1/centrin-2/CEP135/NA14 at the distal centriole, revealing that SFI1 is a component of a ~100 nm luminal ring structure that scaffolds the distal end of the centriole and contributes to appendage formation.\",\n      \"method\": \"Ultrastructure Expansion Microscopy (U-ExM), iterative U-ExM, cryo-electron tomography, C2CD3 depletion\",\n      \"journal\": \"bioRxiv\",\n      \"confidence\": \"Medium\",\n      \"confidence_rationale\": \"Tier 1-2 — cryo-ET and U-ExM structural data, depletion phenotype; preprint, single lab\",\n      \"pmids\": [\"bio_10.1101_2025.06.17.660204\"],\n      \"is_preprint\": true\n    }\n  ],\n  \"current_model\": \"SFI1 is a conserved centrin-binding scaffold protein that uses internal tryptophan-containing repeats to organize arrays of centrin molecules; in yeast it is essential for spindle pole body duplication (through antiparallel Sfi1 dimerization to form the bridge) and separation (regulated by Cdk1 phosphorylation and Cdc14 dephosphorylation), while in mammalian cells it localizes to the distal centriole, forms a luminal ring complex with centrin-2, recruits USP9X to deubiquitylate and stabilize the centriole duplication factor STIL, and is required for centriolar architecture and CP110 removal during ciliogenesis.\"\n}\n```\n\n--- ROUND 2 (R2/R3, augmented corpus) ---\n\n```json\n{\n  \"discoveries\": [\n    {\n      \"year\": 2003,\n      \"finding\": \"Yeast Sfi1p localizes to the half-bridge of the spindle pole body (SPB) and contains multiple internal repeats that each bind one molecule of the centrin Cdc31p in a 1:1 ratio, such that a single Sfi1p molecule binds multiple centrin molecules. Temperature-sensitive sfi1 mutants show a defect in SPB duplication and genetic interactions with cdc31-1, establishing Sfi1p as an essential component required for SPB duplication. Human proteins containing Sfi1 repeats also bind centrin, demonstrating conservation of this centrin-binding motif.\",\n      \"method\": \"Centrin pull-down experiments, temperature-sensitive mutant analysis, immunofluorescence localization, genetic interaction with cdc31-1\",\n      \"journal\": \"The Journal of cell biology\",\n      \"confidence\": \"High\",\n      \"confidence_rationale\": \"Tier 1-2 — direct biochemical pull-down, mutagenesis, and genetic epistasis; foundational paper with 168 citations\",\n      \"pmids\": [\"14504268\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 1999,\n      \"finding\": \"SFI1 is an essential yeast gene required for cell cycle progression through G2-M transition. Conditional sfi1 mutants arrest as doublets of undivided mother-daughter cells with a single nucleus and no mitotic spindles, indicating a role in mitotic spindle assembly. SFI1 was originally identified as a suppressor of cAMP pathway deficiency (fil1 mutant) but acts independently of the cAMP pathway.\",\n      \"method\": \"Conditional expression (galactose-inducible promoter), cell cycle arrest phenotype analysis, genetic epistasis with cAMP pathway mutants\",\n      \"journal\": \"Yeast (Chichester, England)\",\n      \"confidence\": \"Medium\",\n      \"confidence_rationale\": \"Tier 2 — clean conditional KO with defined cellular phenotype; single lab\",\n      \"pmids\": [\"10455233\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2007,\n      \"finding\": \"Novel sfi1 alleles in the C-terminal domain of Sfi1p (identified via mad1 synthetic lethal screen) cause duplicated SPBs that fail to separate (SPBs <0.3 µm apart), suggesting a role for the Sfi1p C-terminal domain in SPB splitting/bridge dissolution distinct from the earlier duplication step.\",\n      \"method\": \"Genetic screen (mad1 synthetic lethal), light and electron microscopy of SPB morphology\",\n      \"journal\": \"Molecular biology of the cell\",\n      \"confidence\": \"Medium\",\n      \"confidence_rationale\": \"Tier 2 — genetic screen plus EM structural analysis; single lab\",\n      \"pmids\": [\"17392514\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2014,\n      \"finding\": \"Cdk1 phosphorylation of Sfi1 licenses yeast centrosome (SPB) duplication to occur only once per cell cycle. Reducing Cdk1 phosphorylation by changing Sfi1 phosphorylation sites to non-phosphorylatable residues causes defects in SPB separation and inappropriate SPB reduplication during mitosis, with bipolar spindle assembly and chromosome segregation defects. The phosphatase Cdc14 opposes Cdk1 by dephosphorylating Sfi1 to activate duplication licensing.\",\n      \"method\": \"Phosphomimetic/phospho-null mutagenesis of Sfi1 phosphorylation sites, live-cell imaging, quantitative mass spectrometry of phosphorylation sites\",\n      \"journal\": \"PLoS genetics\",\n      \"confidence\": \"High\",\n      \"confidence_rationale\": \"Tier 1-2 — mutagenesis combined with functional readouts and MS identification of phosphosites; multiple orthogonal methods\",\n      \"pmids\": [\"25340401\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2014,\n      \"finding\": \"In fission yeast, Sfi1 is gradually recruited to SPBs throughout the cell cycle (not abruptly at duplication onset). Conserved tryptophan residues in the internal repeats are functionally essential: the Trp-to-Arg mutant (sfi1-M46) forms monopolar spindles, causes mitosis and cytokinesis defects, and associates preferentially with one daughter SPB during mitosis, resulting in failure of new SPB assembly in the SPB receiving insufficient Sfi1. These tryptophans are required for proper Sfi1 partitioning between daughter SPBs.\",\n      \"method\": \"Live-cell fluorescence microscopy (Sfi1-GFP), tryptophan-to-arginine mutagenesis, spindle assembly assays, electron microscopy\",\n      \"journal\": \"Molecular biology of the cell\",\n      \"confidence\": \"High\",\n      \"confidence_rationale\": \"Tier 1-2 — mutagenesis combined with live imaging and EM; multiple orthogonal methods\",\n      \"pmids\": [\"25031431\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2015,\n      \"finding\": \"In fission yeast, Sfi1 and Cdc31 form the SPB half-bridge that duplicates to promote new SPB biogenesis. Sfi1 accumulates in two distinct cell-cycle phases. Cdc31 phosphorylation on serine 15 (a Cdk1/Cdc2 consensus site) is required for dissociation of a significant pool of Sfi1 from the bridge and for timely SPB segregation at mitotic onset, indicating that Cdc31 N-terminus modulates the stability of Sfi1-Cdc31 arrays and timing of spindle bipolarity.\",\n      \"method\": \"Phosphomimetic/phospho-null mutagenesis of Cdc31 S15, quantitative live-cell fluorescence microscopy, mass spectrometry (phosphoproteomics)\",\n      \"journal\": \"Journal of cell science\",\n      \"confidence\": \"High\",\n      \"confidence_rationale\": \"Tier 1-2 — mutagenesis of phosphosite with clear functional phenotype and MS validation; multiple methods\",\n      \"pmids\": [\"25736294\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2015,\n      \"finding\": \"Kar1 (membrane-anchored SPB component) directly binds the C-terminal region of Sfi1 and localizes to the bridge center. Kar1 tethers the Sfi1-containing bridge to the nuclear envelope. In kar1Δ cells the bridge adopts an arched conformation. Sfi1 C-terminal dimerization in G1 is the licensing step for SPB duplication (bridge formation), and Cdc31 and Kar1 provide cross-links that stabilize the bridge and ensure timely SPB separation.\",\n      \"method\": \"Photo-activated localization microscopy (PALM), direct binding assays between Kar1 and Sfi1 C-terminal fragments, binding free energy calculations\",\n      \"journal\": \"The Journal of cell biology\",\n      \"confidence\": \"High\",\n      \"confidence_rationale\": \"Tier 1-2 — direct binding assays plus super-resolution microscopy and computational modeling; multiple orthogonal approaches\",\n      \"pmids\": [\"26076691\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2014,\n      \"finding\": \"CK2 phosphorylates human centrin 1 at T138 and human centrin 2 at T138 and S158. These phosphorylation events reduce or abolish centrin binding to Sfi1 (as well as to XPC and transducin β), as demonstrated by phosphomimetic centrin 2 (T138D-S158D) which completely abrogates Sfi1 binding. This places CK2 as a writer that negatively regulates centrin-Sfi1 interaction.\",\n      \"method\": \"In vitro CK2 phosphorylation assay, isothermal titration calorimetry (ITC) binding measurements, phosphomimetic mutagenesis\",\n      \"journal\": \"FEBS open bio\",\n      \"confidence\": \"High\",\n      \"confidence_rationale\": \"Tier 1 — in vitro kinase assay plus ITC binding quantification and phosphomimetic mutagenesis\",\n      \"pmids\": [\"24918055\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2019,\n      \"finding\": \"Mammalian SFI1 localizes to the centrosome during S phase and interacts with the deubiquitylase USP9X. SFI1 recruits USP9X to the centrosome, where USP9X deubiquitylates STIL (a critical regulator of centriole duplication), protecting it from proteasomal degradation. Loss of USP9X (from patient mutations) reduces STIL levels. This SFI1→USP9X→STIL axis promotes centriole duplication and underlies roles of both SFI1 and USP9X in human neurodevelopment.\",\n      \"method\": \"Co-immunoprecipitation, centrosome localization experiments, ubiquitylation/deubiquitylation assays, patient cell analysis, RNAi knockdown with phenotypic readout\",\n      \"journal\": \"The Journal of cell biology\",\n      \"confidence\": \"High\",\n      \"confidence_rationale\": \"Tier 2 — reciprocal co-IP, functional deubiquitylation assay, patient cell validation; multiple orthogonal methods\",\n      \"pmids\": [\"31197030\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2021,\n      \"finding\": \"The N-terminus of Sfi1 (N-Sfi1), including the first three Cdc31-binding sites, directly interacts with SPB components Spc29 and Spc42 to trigger daughter SPB (dSPB) assembly. Cdc31 binding to N-Sfi1 promotes Spc29 recruitment and is essential for satellite formation (earliest step of dSPB biogenesis). Phosphorylation of N-Sfi1 has an inhibitory effect, delaying dSPB biogenesis until G1.\",\n      \"method\": \"Yeast two-hybrid, co-immunoprecipitation, in vivo localization, phosphomutant analysis\",\n      \"journal\": \"The Journal of cell biology\",\n      \"confidence\": \"High\",\n      \"confidence_rationale\": \"Tier 2 — multiple binding assays (Y2H and co-IP) with functional rescue and phosphomutant analysis; multiple orthogonal methods\",\n      \"pmids\": [\"33523111\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2022,\n      \"finding\": \"Human SFI1 is a centriolar protein that localizes to the distal end of the centriole, where it associates with a pool of Centrin. Both SFI1 and Centrin are recruited early during procentriole assembly. Depletion of SFI1 causes loss of the distal Centrin pool without altering centriole duplication per se; instead, the SFI1/Centrin complex is essential for centriolar architecture, CEP164 distribution, and CP110 removal during ciliogenesis.\",\n      \"method\": \"Expansion microscopy (ExM), siRNA depletion, fluorescence localization, ciliogenesis assays\",\n      \"journal\": \"The EMBO journal\",\n      \"confidence\": \"High\",\n      \"confidence_rationale\": \"Tier 2 — super-resolution expansion microscopy with functional depletion phenotypes; multiple orthogonal approaches in human cells\",\n      \"pmids\": [\"36125182\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2025,\n      \"finding\": \"C2CD3 depletion in human centrioles destabilizes a luminal ring network composed of C2CD3/SFI1/centrin-2/CEP135/NA14, revealing that SFI1 is a structural component of the luminal ring at the distal centriole. This network connects the centriolar lumen, distal microtubule cap, and appendages in an 'in-to-out' molecular hub.\",\n      \"method\": \"Ultrastructure Expansion Microscopy (U-ExM), iterative U-ExM, in situ cryo-electron tomography, siRNA depletion\",\n      \"journal\": \"bioRxiv\",\n      \"confidence\": \"Medium\",\n      \"confidence_rationale\": \"Tier 1 — cryo-ET and U-ExM structural data; preprint, not yet peer-reviewed\",\n      \"pmids\": [\"bio_10.1101_2025.06.17.660204\"],\n      \"is_preprint\": true\n    }\n  ],\n  \"current_model\": \"SFI1 is a conserved centrin-binding scaffold protein that, through its multiple internal repeat motifs, recruits centrin (Cdc31/CETN) in a 1:1 stoichiometry per repeat to form the SPB half-bridge/bridge in yeast and the distal centriolar ring structure in humans; in yeast, SPB duplication is licensed by Cdk1 phosphorylation of Sfi1 (promoting bridge dimerization and SPB separation) and counteracted by Cdc14 dephosphorylation, while CK2 phosphorylation of centrin negatively regulates centrin–Sfi1 binding; in mammalian cells, SFI1 localizes USP9X to the centrosome during S phase to deubiquitylate and stabilize STIL for centriole duplication, and the SFI1/Centrin complex at the centriole distal end is dispensable for duplication but essential for centriolar architecture, CEP164 positioning, and CP110 removal during ciliogenesis.\"\n}\n```","stage2_raw":"```json\n{\n  \"mechanistic_narrative\": \"SFI1 is a conserved centrin-binding scaffold protein that organizes centrin arrays through internal tryptophan-containing repeats and plays essential roles in centrosome/spindle pole body duplication, separation, and centriolar architecture. In budding yeast, antiparallel Sfi1 filaments form the spindle pole body (SPB) half-bridge: the N-terminus nucleates daughter SPB assembly by recruiting Spc29 and Spc42 in a Cdc31-dependent manner, while the C-terminus is required for bridge splitting after duplication; Cdk1 phosphorylation of Sfi1 promotes SPB separation and prevents premature reduplication, with Cdc14 dephosphorylation licensing the next duplication cycle [PMID:25340401, PMID:17392514, PMID:33523111, PMID:26076691]. In mammalian cells, SFI1 localizes to the distal centriole where it forms a luminal ring complex with centrin-2, recruits the deubiquitylase USP9X to stabilize the duplication factor STIL, and is required for proper centriolar architecture, CEP164 distribution, and CP110 removal during ciliogenesis [PMID:31197030, PMID:36125182]. Phosphorylation of centrins by CK2 negatively regulates centrin–SFI1 binding, providing an additional layer of control over SFI1-scaffolded arrays [PMID:24918055].\",\n  \"teleology\": [\n    {\n      \"year\": 1999,\n      \"claim\": \"Establishing SFI1 as essential for mitotic progression resolved the question of whether this uncharacterized gene had a cell-cycle role, showing that its depletion arrests cells at G2/M with monopolar spindles.\",\n      \"evidence\": \"Conditional depletion via galactose-inducible promoter shutdown in budding yeast, light microscopy\",\n      \"pmids\": [\"10455233\"],\n      \"confidence\": \"Medium\",\n      \"gaps\": [\"Molecular function and binding partners unknown\", \"No structural or localization data at the SPB\"]\n    },\n    {\n      \"year\": 2007,\n      \"claim\": \"Demonstrating that Sfi1 C-terminal mutants produce duplicated but unseparated SPBs established a domain-specific function in bridge splitting, distinguishing duplication from separation.\",\n      \"evidence\": \"Genetic screen for mad1-synthetic-lethal alleles, electron microscopy of SPB separation intermediates in budding yeast\",\n      \"pmids\": [\"17392514\"],\n      \"confidence\": \"Medium\",\n      \"gaps\": [\"Mechanism of bridge splitting unresolved\", \"No kinase or phosphatase regulation identified\"]\n    },\n    {\n      \"year\": 2014,\n      \"claim\": \"Identification of Sfi1 as a Cdk1 substrate whose phosphorylation state toggles SPB separation versus reduplication licensing (via Cdc14 dephosphorylation) revealed the cell-cycle control logic coupling SPB duplication to mitotic progression.\",\n      \"evidence\": \"Phosphomimetic/nonphosphorylatable mutants, mass spectrometry of phosphosites, epistasis with cdc14 mutants in budding yeast\",\n      \"pmids\": [\"25340401\"],\n      \"confidence\": \"High\",\n      \"gaps\": [\"Structural basis of how phosphorylation disrupts antiparallel Sfi1 dimerization not resolved\", \"Contribution of individual phosphosites unclear\"]\n    },\n    {\n      \"year\": 2014,\n      \"claim\": \"Showing that conserved tryptophans in Sfi1 internal repeats mediate gradual SPB recruitment in fission yeast, and that their mutation causes monopolar spindle formation, established the centrin-binding repeats as the structural unit organizing Sfi1-centrin filaments in vivo.\",\n      \"evidence\": \"Trp-to-Arg mutagenesis of internal repeats, live-cell fluorescence imaging in fission yeast\",\n      \"pmids\": [\"25031431\"],\n      \"confidence\": \"High\",\n      \"gaps\": [\"Stoichiometry and arrangement of centrin molecules per Sfi1 filament in vivo not determined\"]\n    },\n    {\n      \"year\": 2014,\n      \"claim\": \"Demonstrating that CK2 phosphorylation of human centrins abolishes or reduces their binding to Sfi1 peptides revealed a regulatory mechanism that could modulate Sfi1-centrin array stability in mammalian cells.\",\n      \"evidence\": \"In vitro CK2 phosphorylation, isothermal titration calorimetry, phosphomimetic centrin mutagenesis\",\n      \"pmids\": [\"24918055\"],\n      \"confidence\": \"Medium\",\n      \"gaps\": [\"In vivo relevance of CK2-mediated centrin phosphorylation for SFI1 function not tested\", \"Only tested with Sfi1 peptides, not full-length protein\"]\n    },\n    {\n      \"year\": 2015,\n      \"claim\": \"Super-resolution localization of Kar1 to the bridge center and demonstration that it anchors the Sfi1-Cdc31 half-bridge to the nuclear envelope, together with evidence for stabilizing Cdc31-Cdc31 cross-links, resolved the architectural organization of the SPB bridge.\",\n      \"evidence\": \"PALM super-resolution microscopy, direct binding assays, kar1Δ phenotyping in budding yeast\",\n      \"pmids\": [\"26076691\"],\n      \"confidence\": \"High\",\n      \"gaps\": [\"Dynamics of Kar1-Sfi1 interaction during bridge splitting not resolved\"]\n    },\n    {\n      \"year\": 2015,\n      \"claim\": \"Showing that Cdk1-dependent phosphorylation of Cdc31 Ser15 promotes dissociation of Sfi1 from the bridge at mitotic onset in fission yeast established centrin phosphorylation as a complementary regulatory input to direct Sfi1 phosphorylation.\",\n      \"evidence\": \"Phosphomimetic Cdc31-S15 mutagenesis, live-cell imaging, epistasis with cdc2 pathway in fission yeast\",\n      \"pmids\": [\"25736294\"],\n      \"confidence\": \"Medium\",\n      \"gaps\": [\"Whether analogous centrin phosphorylation operates in mammalian cells is unknown\", \"Single lab observation\"]\n    },\n    {\n      \"year\": 2019,\n      \"claim\": \"Discovery that mammalian SFI1 recruits USP9X to the centrosome to deubiquitylate and stabilize STIL established a direct mechanistic link between SFI1 and centriole duplication control in human cells.\",\n      \"evidence\": \"Reciprocal co-immunoprecipitation, centrosome fractionation, ubiquitylation assays, USP9X patient cell analysis\",\n      \"pmids\": [\"31197030\"],\n      \"confidence\": \"High\",\n      \"gaps\": [\"How SFI1-USP9X interaction is regulated during the cell cycle is unknown\", \"Whether SFI1 scaffold function and USP9X recruitment are separable activities not tested\"]\n    },\n    {\n      \"year\": 2021,\n      \"claim\": \"Demonstrating that the N-terminal Cdc31-binding sites of Sfi1 recruit Spc29/Spc42 to nucleate daughter SPB assembly, with N-terminal phosphorylation delaying this until G1, resolved how daughter SPB biogenesis is initiated and temporally gated.\",\n      \"evidence\": \"Direct binding assays, phosphomimetic N-Sfi1 mutagenesis, SPB assembly and fluorescence microscopy in budding yeast\",\n      \"pmids\": [\"33523111\"],\n      \"confidence\": \"High\",\n      \"gaps\": [\"Identity of kinase(s) phosphorylating N-Sfi1 in vivo not fully established\", \"Structural model of N-Sfi1/Spc29/Spc42 nucleation complex lacking\"]\n    },\n    {\n      \"year\": 2022,\n      \"claim\": \"Expansion microscopy revealed that human SFI1 forms a distal centriole structure with centrin, is dispensable for centriole duplication per se, but is required for centriolar architecture, CEP164 organization, and CP110 removal during ciliogenesis, distinguishing its mammalian role from yeast SPB duplication.\",\n      \"evidence\": \"U-ExM, SFI1 depletion, immunofluorescence for CEP164/CP110, ciliogenesis assays in human cells\",\n      \"pmids\": [\"36125182\"],\n      \"confidence\": \"High\",\n      \"gaps\": [\"How SFI1 loss causes CP110 retention is mechanistically unresolved\", \"Whether SFI1's ciliogenesis role is centrin-dependent not tested\"]\n    },\n    {\n      \"year\": null,\n      \"claim\": \"The structural basis of the mammalian SFI1-centrin luminal ring, how it interfaces with distal appendage and ciliogenesis machinery, and whether CK2-mediated centrin regulation modulates SFI1 function in vivo remain open questions.\",\n      \"evidence\": \"\",\n      \"pmids\": [],\n      \"confidence\": \"Medium\",\n      \"gaps\": [\"No high-resolution structure of full-length mammalian SFI1 or the luminal ring complex\", \"Mechanism linking SFI1 loss to CP110 retention and appendage defects not defined\", \"In vivo significance of CK2-centrin phosphorylation for SFI1 scaffold function untested\"]\n    }\n  ],\n  \"mechanism_profile\": {\n    \"molecular_activity\": [\n      {\"term_id\": \"GO:0005198\", \"supporting_discovery_ids\": [3, 5, 8, 9, 10]},\n      {\"term_id\": \"GO:0060090\", \"supporting_discovery_ids\": [7]}\n    ],\n    \"localization\": [\n      {\"term_id\": \"GO:0005815\", \"supporting_discovery_ids\": [0, 1, 3, 5, 7, 8, 9, 10]}\n    ],\n    \"pathway\": [\n      {\"term_id\": \"R-HSA-1640170\", \"supporting_discovery_ids\": [0, 2, 3, 8]},\n      {\"term_id\": \"R-HSA-1852241\", \"supporting_discovery_ids\": [1, 8, 9, 10]}\n    ],\n    \"complexes\": [\n      \"SFI1-centrin luminal ring (distal centriole)\",\n      \"Sfi1-Cdc31 half-bridge (SPB)\"\n    ],\n    \"partners\": [\n      \"CETN2\",\n      \"USP9X\",\n      \"CDC31\",\n      \"SPC29\",\n      \"SPC42\",\n      \"KAR1\",\n      \"C2CD3\",\n      \"CEP135\"\n    ],\n    \"other_free_text\": []\n  }\n}\n```\n\n--- ROUND 2 (R2/R3, augmented corpus) ---\n\n```json\n{\n  \"mechanistic_narrative\": \"SFI1 is a conserved centrin-binding scaffold protein that organizes centrosome/spindle pole body (SPB) architecture and licenses centrosome duplication across eukaryotes. Its multiple internal repeats each bind one molecule of centrin (Cdc31/CETN2) in a 1:1 stoichiometry, forming elongated arrays that constitute the SPB half-bridge in yeast and the distal centriolar ring in human cells; Cdk1 phosphorylation of the Sfi1 C-terminus drives bridge dimerization and SPB separation, while the phosphatase Cdc14 opposes this to reset duplication licensing, and CK2 phosphorylation of centrin negatively regulates centrin–Sfi1 binding [PMID:14504268, PMID:25340401, PMID:24918055]. In budding yeast, the Sfi1 N-terminus recruits SPB components Spc29 and Spc42 to initiate daughter SPB assembly, with Cdc31 binding promoting Spc29 recruitment [PMID:33523111]. In mammalian cells, SFI1 localizes to the centriole distal end where it recruits USP9X to deubiquitylate and stabilize STIL for centriole duplication during S phase, and the SFI1/Centrin complex is dispensable for duplication per se but essential for centriolar architecture, CEP164 positioning, and CP110 removal during ciliogenesis [PMID:31197030, PMID:36125182].\",\n  \"teleology\": [\n    {\n      \"year\": 1999,\n      \"claim\": \"Establishing that SFI1 is an essential gene required for progression through mitosis resolved its role beyond its original identification as a suppressor of cAMP pathway deficiency, placing it in the cell division machinery.\",\n      \"evidence\": \"Conditional sfi1 mutants in budding yeast arrest with undivided nuclei and no mitotic spindles\",\n      \"pmids\": [\"10455233\"],\n      \"confidence\": \"Medium\",\n      \"gaps\": [\"Molecular target of Sfi1 unknown at this stage\", \"No mechanism for spindle assembly failure identified\"]\n    },\n    {\n      \"year\": 2003,\n      \"claim\": \"Identification of Sfi1p as a multi-centrin scaffold at the SPB half-bridge established the molecular basis by which centrin is organized into functional arrays and explained why Sfi1 is required for SPB duplication.\",\n      \"evidence\": \"Centrin pull-down, localization to the SPB half-bridge, temperature-sensitive mutant phenotypes, and genetic interaction with cdc31-1 in budding yeast; conservation confirmed with human Sfi1 repeat–centrin binding\",\n      \"pmids\": [\"14504268\"],\n      \"confidence\": \"High\",\n      \"gaps\": [\"No structural information on the repeat–centrin interface\", \"Mechanism of SPB duplication initiation not resolved\"]\n    },\n    {\n      \"year\": 2007,\n      \"claim\": \"Demonstrating that the C-terminal domain of Sfi1 controls SPB separation (bridge splitting) rather than initial duplication revealed functionally distinct roles for different Sfi1 domains in the centrosome cycle.\",\n      \"evidence\": \"C-terminal sfi1 alleles from a mad1 synthetic-lethal screen yield duplicated but unseparated SPBs by light and electron microscopy\",\n      \"pmids\": [\"17392514\"],\n      \"confidence\": \"Medium\",\n      \"gaps\": [\"Identity of the C-terminal binding partner mediating splitting unknown\", \"Regulation of the splitting step not addressed\"]\n    },\n    {\n      \"year\": 2014,\n      \"claim\": \"Showing that Cdk1 phosphorylation of Sfi1 licenses once-per-cycle SPB duplication, counteracted by Cdc14 dephosphorylation, established a phospho-regulatory switch ensuring centrosome copy-number control.\",\n      \"evidence\": \"Phosphomimetic and phospho-null Sfi1 mutants in budding yeast combined with mass spectrometry and live-cell imaging\",\n      \"pmids\": [\"25340401\"],\n      \"confidence\": \"High\",\n      \"gaps\": [\"How phosphorylation physically blocks or permits C-terminal dimerization not structurally resolved\", \"Contribution of additional kinases not excluded\"]\n    },\n    {\n      \"year\": 2014,\n      \"claim\": \"Identification that conserved tryptophan residues in Sfi1 repeats are essential for proper Sfi1 partitioning between daughter SPBs linked Sfi1 stoichiometry to spindle bipolarity in fission yeast.\",\n      \"evidence\": \"Trp-to-Arg mutagenesis with live-cell imaging and EM in S. pombe\",\n      \"pmids\": [\"25031431\"],\n      \"confidence\": \"High\",\n      \"gaps\": [\"Whether the tryptophans mediate centrin binding or intra-Sfi1 contacts not distinguished\", \"Mechanism of asymmetric partitioning unclear\"]\n    },\n    {\n      \"year\": 2014,\n      \"claim\": \"CK2 phosphorylation of centrin at T138/S158 was shown to abolish centrin–Sfi1 binding, revealing an additional kinase-mediated layer of regulation of the Sfi1–centrin array independent of Cdk1.\",\n      \"evidence\": \"In vitro CK2 kinase assay and isothermal titration calorimetry with phosphomimetic human centrin 2\",\n      \"pmids\": [\"24918055\"],\n      \"confidence\": \"High\",\n      \"gaps\": [\"In vivo relevance of CK2-mediated centrin phosphorylation for centrosome biology not demonstrated\", \"Whether CK2 regulation is cell-cycle dependent is unknown\"]\n    },\n    {\n      \"year\": 2015,\n      \"claim\": \"Characterization of Kar1 as a direct binding partner of the Sfi1 C-terminus that tethers the bridge to the nuclear envelope, combined with evidence that C-terminal Sfi1 dimerization in G1 is the licensing step for SPB duplication, provided a structural model of bridge formation.\",\n      \"evidence\": \"Super-resolution PALM imaging, direct binding assays between Kar1 and Sfi1 C-terminal fragments, and bridge morphology in kar1Δ cells in budding yeast\",\n      \"pmids\": [\"26076691\"],\n      \"confidence\": \"High\",\n      \"gaps\": [\"Atomic-resolution structure of the Sfi1 C-terminal dimer interface lacking\", \"How Cdk1 phosphorylation physically disrupts the dimer not determined\"]\n    },\n    {\n      \"year\": 2015,\n      \"claim\": \"Demonstrating that Cdc31 phosphorylation on S15 promotes Sfi1 dissociation from the bridge in fission yeast established that centrin phosphorylation actively regulates bridge dynamics and timing of SPB segregation.\",\n      \"evidence\": \"Phosphomimetic/phospho-null Cdc31 S15 mutagenesis with quantitative live-cell fluorescence and mass spectrometry in S. pombe\",\n      \"pmids\": [\"25736294\"],\n      \"confidence\": \"High\",\n      \"gaps\": [\"Whether mammalian centrin phosphorylation plays an equivalent role at the centriole unknown\", \"Relationship between CK2-mediated (T138) and Cdk-mediated (S15) phosphorylation not integrated\"]\n    },\n    {\n      \"year\": 2019,\n      \"claim\": \"Discovery that mammalian SFI1 recruits the deubiquitylase USP9X to the centrosome to stabilize STIL extended Sfi1's function beyond structural scaffolding to active regulation of centriole duplication via ubiquitin signaling.\",\n      \"evidence\": \"Reciprocal co-immunoprecipitation, deubiquitylation assays, patient cell analysis, and RNAi knockdown in human cells\",\n      \"pmids\": [\"31197030\"],\n      \"confidence\": \"High\",\n      \"gaps\": [\"Domain of SFI1 that binds USP9X not mapped\", \"Whether the SFI1–USP9X axis operates in non-neural tissues not tested\"]\n    },\n    {\n      \"year\": 2021,\n      \"claim\": \"Identification of the Sfi1 N-terminus as the direct binding partner of SPB core components Spc29 and Spc42 resolved how Sfi1–Cdc31 arrays nucleate the earliest step of daughter SPB biogenesis (satellite formation), with N-terminal phosphorylation acting as an inhibitory timer.\",\n      \"evidence\": \"Yeast two-hybrid, co-immunoprecipitation, in vivo localization, and phosphomutant analysis in budding yeast\",\n      \"pmids\": [\"33523111\"],\n      \"confidence\": \"High\",\n      \"gaps\": [\"Identity of the kinase responsible for inhibitory N-terminal phosphorylation not fully resolved\", \"Stoichiometry of the N-Sfi1/Spc29/Spc42 assembly not quantified\"]\n    },\n    {\n      \"year\": 2022,\n      \"claim\": \"Super-resolution mapping of human SFI1 to the centriole distal end, and demonstration that its depletion disrupts centriolar architecture, CEP164 positioning, and ciliogenesis without blocking duplication, defined a mammalian structural role for SFI1/Centrin distinct from its duplication-licensing function.\",\n      \"evidence\": \"Expansion microscopy, siRNA depletion, and ciliogenesis assays in human cells\",\n      \"pmids\": [\"36125182\"],\n      \"confidence\": \"High\",\n      \"gaps\": [\"How SFI1/Centrin loss specifically impairs CP110 removal is mechanistically unclear\", \"Whether SFI1's structural and USP9X-recruiting functions are separable not tested\"]\n    },\n    {\n      \"year\": null,\n      \"claim\": \"Key open questions include how SFI1's structural scaffold role at the centriole distal end relates to its S-phase role in USP9X recruitment, what the atomic structure of the SFI1–centrin array looks like in situ, and whether mammalian SFI1 phosphoregulation mirrors the yeast Cdk1/Cdc14 licensing switch.\",\n      \"evidence\": \"\",\n      \"pmids\": [],\n      \"confidence\": \"High\",\n      \"gaps\": [\"No in situ or high-resolution structure of the full-length SFI1–centrin array\", \"Mammalian phosphoregulation of SFI1 during the centrosome cycle uncharacterized\", \"Relationship between centriole distal-end architecture and ciliogenesis signaling not mechanistically resolved\"]\n    }\n  ],\n  \"mechanism_profile\": {\n    \"molecular_activity\": [\n      {\"term_id\": \"GO:0005198\", \"supporting_discovery_ids\": [0, 4, 6, 10]},\n      {\"term_id\": \"GO:0060090\", \"supporting_discovery_ids\": [8, 9]}\n    ],\n    \"localization\": [\n      {\"term_id\": \"GO:0005815\", \"supporting_discovery_ids\": [0, 2, 4, 5, 6, 8, 9, 10]}\n    ],\n    \"pathway\": [\n      {\"term_id\": \"R-HSA-1640170\", \"supporting_discovery_ids\": [1, 3, 4, 5]},\n      {\"term_id\": \"R-HSA-1852241\", \"supporting_discovery_ids\": [0, 6, 9, 10]}\n    ],\n    \"complexes\": [\n      \"Sfi1-centrin (Sfi1-Cdc31) half-bridge/bridge array\"\n    ],\n    \"partners\": [\n      \"CETN2\",\n      \"CDC31\",\n      \"USP9X\",\n      \"KAR1\",\n      \"SPC29\",\n      \"SPC42\",\n      \"STIL\",\n      \"CEP164\"\n    ],\n    \"other_free_text\": []\n  }\n}\n```"}