{"gene":"EEFSEC","run_date":"2026-04-28T17:46:03","timeline":{"discoveries":[{"year":1990,"finding":"SELB (prokaryotic ortholog of EEFSEC) was purified from E. coli and shown to be an elongation factor-like protein that specifically binds selenocysteyl-tRNA(Sec). It is partially associated with ribosomes and is required for co-translational selenocysteine insertion at UGA codons.","method":"Protein purification, limited proteolysis, immunological analysis, ribosome fractionation","journal":"The Journal of biological chemistry","confidence":"High","confidence_rationale":"Tier 1-2 — biochemical purification with functional characterization, replicated across multiple subsequent studies","pmids":["2140572"],"is_preprint":false},{"year":1991,"finding":"The 8-base-pair aminoacyl-acceptor helix of tRNA(Sec) is the primary determinant for binding to SELB (prokaryotic EEFSEC ortholog); reduction to 7 base pairs prevents SELB binding but allows EF-Tu binding. This structural feature exclusively directs selenocysteyl-tRNA(Sec) to SELB and precludes interaction with EF-Tu.","method":"In vitro binding assays with purified mutant tRNA variants, kinetic analysis of seryl-tRNA synthetase and selenocysteine synthase","journal":"The Journal of biological chemistry","confidence":"High","confidence_rationale":"Tier 1 — reconstituted in vitro with defined tRNA mutants, orthogonal methods including Km/Kcat measurements","pmids":["1939093"],"is_preprint":false},{"year":1991,"finding":"SELB (EEFSEC ortholog) specifically complexes selenocysteyl-tRNA(Sec); interaction with the selenol group of the aminoacylated selenocysteine residue is required for stable SELB·tRNA complex formation, providing the biochemical basis for exclusive selection of selenocysteyl-tRNA(Sec).","method":"Biochemical characterization, in vitro binding assays","journal":"Biochimie","confidence":"High","confidence_rationale":"Tier 1 — in vitro reconstitution with defined substrates, replicated","pmids":["1839607"],"is_preprint":false},{"year":1993,"finding":"SELB (EEFSEC ortholog) directly and specifically binds the mRNA SECIS hairpin loop region via gel-shift and footprinting assays. In the presence of selenocysteinyl-tRNA, SELB forms a ternary complex with charged tRNA and mRNA, positioning the tRNA at the UGA codon.","method":"Gel shift assays, nuclease and iodine footprinting","journal":"Proceedings of the National Academy of Sciences of the United States of America","confidence":"High","confidence_rationale":"Tier 1-2 — direct binding demonstrated by gel shift and footprinting; replicated in multiple subsequent studies","pmids":["8483932"],"is_preprint":false},{"year":1994,"finding":"The SELB-GTP-Sec-tRNA(Sec) ternary complex binds selenoprotein mRNAs (fdhF and fdnG) and toeprint experiments show SELB recognizes ribosome-bound message, with the complex extending toward the large ribosomal subunit, increasing local concentration of Sec-tRNA(Sec) at the UGA codon.","method":"Boundary experiments, toeprint assays","journal":"Genes & development","confidence":"High","confidence_rationale":"Tier 2 — orthogonal methods (boundary + toeprint) with functional interpretation, replicated","pmids":["8314089"],"is_preprint":false},{"year":1996,"finding":"SELB (EEFSEC ortholog) domain structure was elucidated: the N-terminal three domains are homologous to EF-Tu (binding GTP and selenocysteyl-tRNA(Sec)), while a distinct C-terminal ~17 kDa domain specifically binds the mRNA SECIS hairpin. Truncated SelB lacking the C-terminal domain retains tRNA binding but loses mRNA binding.","method":"Cloning of selB from multiple bacteria, sequence alignment, expression and biochemical analysis of truncated SelB fragments","journal":"Journal of molecular biology","confidence":"High","confidence_rationale":"Tier 1-2 — domain dissection with functional validation, independently replicated across multiple SelB species","pmids":["8893853"],"is_preprint":false},{"year":1996,"finding":"SELB GTPase activity is stimulated ~3-4 fold by the SECIS mRNA hairpin in a ribosome-dependent manner. mRNA binding to SELB induces a conformational switch promoting increased ribosome-mediated GTP hydrolysis; the minimal stimulatory region maps to the upper half of the hairpin.","method":"GTPase activity assays, ribosome stimulation, truncated mRNA hairpin titration","journal":"Biochemistry","confidence":"High","confidence_rationale":"Tier 1 — in vitro enzymatic assay with defined components and mutant hairpins","pmids":["9454578"],"is_preprint":false},{"year":1996,"finding":"Overproduction of SelB (EEFSEC ortholog) in vivo reduces UGA readthrough to <1%, reversed by co-overexpression of tRNA(Sec), demonstrating that balanced stoichiometry between SelB, selenocysteyl-tRNA(Sec), and mRNA is essential for selenocysteine insertion. The mRNA-binding and tRNA-binding domains are physically separable.","method":"Genetic overexpression, in vivo UGA readthrough assay (lacZ fusion), truncation analysis","journal":"Molecular microbiology","confidence":"High","confidence_rationale":"Tier 2 — genetic epistasis with defined molecular readout, replicated","pmids":["8898393"],"is_preprint":false},{"year":1998,"finding":"Evolutionary analysis established that SELB/eEFSec belongs to a GTPase superfamily ancestral to both bacterial SELB and eukaryotic/archaeal eIF2γ, indicating these translation factors share a common evolutionary origin and that change of function occurred within this GTPase subfamily.","method":"Phylogenetic analysis, sequence comparison across diplomonads, parabasalia, microsporidia, and archaebacteria","journal":"Journal of molecular evolution","confidence":"Medium","confidence_rationale":"Tier 4 — computational/phylogenetic analysis, but broadly replicated and foundational for understanding eEFSec origins","pmids":["9847405"],"is_preprint":false},{"year":2000,"finding":"The kinetics of SELB (EEFSEC ortholog) interaction with GTP (Kd = 0.74 µM) and GDP (Kd = 13.4 µM) were determined; rapid GDP release (15 s⁻¹) obviates the need for a nucleotide exchange factor. SECIS RNA binds with ~1 nM affinity, further increased by selenocysteyl-tRNA(Sec) binding, suggesting SELB forms a tight quaternary complex on the SECIS that loosens after GTP hydrolysis.","method":"Stopped-flow fluorescence kinetics using intrinsic tryptophan fluorescence and methylanthraniloyl nucleotide derivatives","journal":"The Journal of biological chemistry","confidence":"High","confidence_rationale":"Tier 1 — quantitative kinetic measurements with multiple fluorescent reporters, multiple binding partners","pmids":["10781605"],"is_preprint":false},{"year":2000,"finding":"mSelB (mouse EEFSEC, the mammalian ortholog of bacterial SelB) was characterized: it binds GTP, recognizes Sec-tRNA(Sec) in vitro and in vivo, and is required for efficient selenoprotein translation in vivo. Unlike bacterial SelB, mSelB alone cannot bind the eukaryotic SECIS RNA; complementation with cell extracts yields a SECIS-dependent complex containing mSelB and at least one additional factor.","method":"Database cloning, GTP binding assay, in vitro Sec-tRNA(Sec) binding, in vivo selenoprotein translation assay, HeLa cell extract complementation","journal":"The EMBO journal","confidence":"High","confidence_rationale":"Tier 1-2 — multiple orthogonal methods (GTP binding, tRNA binding, in vivo translation), foundational characterization paper","pmids":["10970870"],"is_preprint":false},{"year":2000,"finding":"Archaeal SelB (from Methanococcus jannaschii, aSelB) binds guanine nucleotides and preferentially binds selenocysteyl-tRNA(Sec) like bacterial SelB, but does not bind the SECIS element, lacking the bacterial C-terminal mRNA-binding domain. This suggests that in archaea and eukaryotes, functions of bacterial SelB are distributed over at least two proteins.","method":"Genome database search, protein purification, guanine nucleotide binding assay, tRNA binding assay","journal":"Journal of molecular biology","confidence":"High","confidence_rationale":"Tier 1-2 — biochemical characterization of purified protein, consistent with eukaryotic EEFSEC mechanism","pmids":["10860743"],"is_preprint":false},{"year":2002,"finding":"Crystal structure of the C-terminal mRNA-binding fragment of SelB from Moorella thermoacetica (SelB-C) determined at 2.12 Å resolution, revealing four tandem winged-helix (WH) domains arranged in an L-shape. This was the first structure showing winged-helix domains involved in RNA binding; conserved basic residues define the mRNA-binding site.","method":"X-ray crystallography (multiwavelength anomalous dispersion), structural analysis","journal":"The EMBO journal","confidence":"High","confidence_rationale":"Tier 1 — crystal structure at 2.12 Å, first structural characterization of C-terminal RNA-binding domain","pmids":["12145214"],"is_preprint":false},{"year":2002,"finding":"NMR structure of the prokaryotic SECIS mRNA hairpin revealed conserved structural features critical for SelB (EEFSEC ortholog) binding. A GpU sequence at the tip of the capping tetraloop and a bulge uracil five base-pairs away are essential for SelB interaction; SelB binding stabilizes RNA secondary structure.","method":"NMR spectroscopy, mutational analysis of SECIS binding","journal":"Journal of molecular biology","confidence":"High","confidence_rationale":"Tier 1 — NMR structure with mutational validation of binding interface","pmids":["12421564"],"is_preprint":false},{"year":2004,"finding":"In mammalian systems, eEFSec (EEFSEC), Sec-tRNA(Sec), and SBP2 are all required for selenocysteine incorporation. SBP2 is the only limiting factor in rabbit reticulocyte lysate; selenocysteine incorporation efficiency into a luciferase reporter is 5-8% in vitro and ~1% in transfected cells, demonstrating the reconstituted system and quantifying factor contributions.","method":"In vitro translation assay, transfected cell Sec incorporation efficiency measurement, factor depletion and reconstitution","journal":"The Journal of biological chemistry","confidence":"High","confidence_rationale":"Tier 1-2 — quantitative reconstitution in vitro and in vivo, multiple limiting factor analyses","pmids":["15229221"],"is_preprint":false},{"year":2005,"finding":"Crystal structure of the mRNA-binding domain of SelB (EEFSEC ortholog) in complex with SECIS RNA at 2.3 Å resolution revealed the first example of a winged-helix (WH) domain binding RNA. RNA binding does not induce major conformational change in the WH motif; the structure suggests the complex wraps around the small ribosomal subunit.","method":"X-ray crystallography","journal":"Nature structural & molecular biology","confidence":"High","confidence_rationale":"Tier 1 — high-resolution crystal structure of protein-RNA complex, novel structural finding","pmids":["15665870"],"is_preprint":false},{"year":2007,"finding":"Crystal structures of E. coli SelB WH3/4 domains and M. thermoacetica WH1-4 domains each bound to SECIS mRNA revealed that both WH modules use the same structural elements to bind RNA. A salt bridge connecting WH2 to WH3 is disrupted upon mRNA binding, providing a molecular switch allowing communication between tRNA- and mRNA-binding sites, with RNA acting as an activator.","method":"X-ray crystallography (2.3 Å for E. coli WH3/4; 2.6 Å for M. thermoacetica WH1-4), site-directed mutagenesis","journal":"Journal of molecular biology","confidence":"High","confidence_rationale":"Tier 1 — dual crystal structures with mutagenesis validation of molecular switch","pmids":["17537456"],"is_preprint":false},{"year":2008,"finding":"SBP2's Sec incorporation domain (SID) promotes high-affinity SECIS binding and eEFSec (EEFSEC) recruitment to the SBP2 RNA-binding domain. SECIS binding induces a conformational change in SBP2 that recruits eEFSec; the SID and RNA-binding domain can function in trans, establishing eEFSec as a downstream effector of SBP2·SECIS complex formation.","method":"Alanine scanning mutagenesis, in vitro Sec incorporation assay, binding domain separation experiments","journal":"The Journal of biological chemistry","confidence":"High","confidence_rationale":"Tier 1-2 — alanine scanning with functional readout, trans-complementation, mechanistic model","pmids":["18948268"],"is_preprint":false},{"year":2009,"finding":"Sec-tRNA(Sec) binds SelB (EEFSEC ortholog)·GTP with extraordinary affinity (Kd = 0.2 pM); GTP hydrolysis accelerates tRNA release by >10⁶-fold (from 0.3 h⁻¹ to 240 s⁻¹). Thermodynamic coupling between Sec-tRNA(Sec) and GTP binding ensures specificity of Sec vs. Ser-tRNA(Sec) selection. This mechanism is reminiscent of eIF2 rather than EF-Tu.","method":"Stopped-flow fluorescence kinetics, thermodynamic binding measurements, isothermal titration calorimetry","journal":"The Journal of biological chemistry","confidence":"High","confidence_rationale":"Tier 1 — quantitative thermodynamic and kinetic analysis with multiple orthogonal methods","pmids":["19940162"],"is_preprint":false},{"year":2015,"finding":"Crystal structure of the full-length SelB from Aquifex aeolicus in complex with a GTP analog at 3.2 Å resolution revealed: three EF-Tu-like domains (D1-3) followed by four winged-helix domains (WHD1-4) connected by a spacer region. The Sec-binding site is at the D1-D2 interface (smaller and more exposed than EF-Tu's aminoacyl site). Structural modeling suggests tRNA(Sec)'s unique secondary structure allows SelB to specifically recognize it and place it at the ribosomal A-site.","method":"X-ray crystallography at 3.2 Å, structural modeling","journal":"Nucleic acids research","confidence":"High","confidence_rationale":"Tier 1 — first full-length SelB crystal structure, rigorous structural analysis","pmids":["26304550"],"is_preprint":false},{"year":2016,"finding":"Cryo-EM structures of six intermediates on the UGA recoding pathway revealed the mechanism of SelB (EEFSEC ortholog) GTPase activation: initial SelB·Sec-tRNA(Sec) binding causes the 30S subunit to adopt an open conformation with Sec-tRNA(Sec) covering the sarcin-ricin loop (SRL). Codon recognition triggers local decoding-site closure, moving Sec-tRNA(Sec) away from the SRL and causing global 30S shoulder domain closure, which docks SelB on the SRL to activate GTPase.","method":"Single-particle cryo-electron microscopy (six structural intermediates), molecular dynamics","journal":"Nature","confidence":"High","confidence_rationale":"Tier 1 — six cryo-EM structures capturing the full recoding pathway, multiple orthogonal analyses","pmids":["27842381"],"is_preprint":false},{"year":2018,"finding":"Mammalian eEFSec (EEFSEC) folds into a chalice-like structure with three N-terminal EF-Tu-like domains and a C-terminal domain 4 that binds Sec-tRNA(Sec) and SBP2. GTP hydrolysis does not induce a canonical conformational change but instead promotes a slight ratchet of domains 1 and 2 and a lever-like movement of domain 4, which may be critical for Sec-tRNA(Sec) release on the ribosome. A non-canonical mechanism for Sec UGA recoding elongation is proposed.","method":"Structural analysis, crystal structure review, domain function mapping, GTP hydrolysis assays","journal":"Biochimica et biophysica acta. General subjects","confidence":"Medium","confidence_rationale":"Tier 2 — review summarizing structural data from the Simonović lab with mechanistic interpretation; some findings are synthesis of primary data","pmids":["29555379"],"is_preprint":false},{"year":2022,"finding":"Cryo-EM structure of the mammalian ribosome decoding the Sec UGA codon revealed: (1) eEFSec (EEFSEC) and SBP2 do not interact directly but deploy their C-terminal domains to engage opposite ends of the SECIS; (2) ribosomal protein eS31 simultaneously contacts Sec-tRNA(Sec) and SBP2 via its Lys-rich and C-terminal segments to stabilize the assembly; (3) eEFSec is indiscriminate toward L-serine and can misincorporate it at Sec UGA codons. This reveals a fundamentally distinct mechanism of Sec UGA recoding in eukaryotes versus bacteria.","method":"Cryo-electron microscopy structural determination of mammalian ribosome·eEFSec·SBP2·SECIS complex","journal":"Science","confidence":"High","confidence_rationale":"Tier 1 — cryo-EM structure of the intact mammalian Sec decoding complex with strong functional validation","pmids":["35709277"],"is_preprint":false},{"year":2025,"finding":"Bi-allelic loss-of-function variants in EEFSEC cause autosomal recessive selenoprotein deficiency in humans, leading to progressive neurodegeneration (EEFSEC deficiency). Pathogenic EEFSEC variants showed reduced EEFSEC function in vitro and lower selenoprotein levels in patient fibroblasts. A Drosophila eEFSec-RNAi model displayed progressive motor impairment and synaptic defects, confirming the in vivo requirement of EEFSEC for normal neuronal function.","method":"Exome/genome sequencing, in vitro functional assay of variants, Western blot of selenoproteins in fibroblasts, Drosophila RNAi motor function assay, synaptic morphology analysis","journal":"American journal of human genetics","confidence":"High","confidence_rationale":"Tier 2 — multiple orthogonal methods (in vitro variant function, patient fibroblast selenoproteomics, Drosophila model), strong preponderance of evidence","pmids":["39753114"],"is_preprint":false},{"year":2025,"finding":"In yellow catfish, the eefsec gene promoter contains a functional FOXO1 binding site (-1070 to -1080 bp) and a STAT3 binding site (-428 to -436 bp). These transcription factors directly regulate eefsec transcriptional activity in a selenium (selenomethionine)-dependent manner, as confirmed by EMSA and chromatin immunoprecipitation.","method":"Promoter deletion analysis, EMSA, chromatin immunoprecipitation","journal":"Biochimica et biophysica acta. Gene regulatory mechanisms","confidence":"Medium","confidence_rationale":"Tier 2 — direct binding confirmed by EMSA and ChIP, but in non-mammalian species (fish); functional relevance to human EEFSEC regulation uncertain","pmids":["40618995"],"is_preprint":false},{"year":2021,"finding":"EEFSEC knockdown in human prostate cancer 22Rv1 cells suppressed proliferation, migration, and invasion, caused G0/G1 cell cycle arrest, downregulated C-myc and CCNB1, and upregulated p15, suggesting EEFSEC expression modulates cell cycle progression through C-myc-related pathways.","method":"Lentiviral shRNA knockdown, XTT proliferation assay, Transwell migration/invasion assay, flow cytometry cell cycle analysis, qRT-PCR, Western blot","journal":"Nan fang yi ke da xue xue bao","confidence":"Medium","confidence_rationale":"Tier 2-3 — clean KD with multiple phenotypic readouts, but pathway placement is correlative and single lab","pmids":["35012909"],"is_preprint":false}],"current_model":"EEFSEC (eEFSec) is a selenocysteine-specific translational elongation factor that forms an EF-Tu-like GTPase with a chalice-like structure; it delivers Sec-tRNA(Sec) to the ribosomal A site at UGA codons with extraordinary tRNA-binding affinity (Kd ~0.2 pM for the GTP-bound form), with GTP hydrolysis triggered by codon recognition via docking on the sarcin-ricin loop, releasing Sec-tRNA(Sec) for peptide bond formation; in eukaryotes, eEFSec does not bind the SECIS element directly but is recruited through SBP2 (which binds the SECIS and undergoes a conformational change that engages eEFSec's C-terminal domain 4), while ribosomal protein eS31 bridges Sec-tRNA(Sec) and SBP2 to stabilize the decoding complex; loss-of-function variants in EEFSEC cause autosomal recessive progressive neurodegeneration due to global selenoprotein deficiency."},"narrative":{"teleology":[{"year":1990,"claim":"The discovery that prokaryotic SelB is a ribosome-associated elongation factor that specifically binds selenocysteyl-tRNA(Sec) established the first dedicated translational machinery for selenocysteine insertion at UGA codons.","evidence":"Protein purification, limited proteolysis, immunological analysis, and ribosome fractionation from E. coli","pmids":["2140572"],"confidence":"High","gaps":["Eukaryotic ortholog not yet identified","GTPase mechanism not characterized","No structural information"]},{"year":1991,"claim":"Identification of the tRNA structural determinants—the 8-bp acceptor helix and the selenocysteinyl moiety—that confer exclusive SelB binding explained how Sec-tRNA(Sec) is discriminated from all other aminoacyl-tRNAs and excluded from EF-Tu.","evidence":"In vitro binding assays with defined tRNA mutants and kinetic analysis of aminoacylation","pmids":["1939093","1839607"],"confidence":"High","gaps":["Thermodynamic coupling between aminoacyl identity and nucleotide state not quantified","Structural basis of tRNA recognition unknown"]},{"year":1993,"claim":"Demonstration that SelB directly binds the mRNA SECIS hairpin and forms a ternary complex with Sec-tRNA(Sec) and mRNA revealed how selenocysteine insertion is spatially coupled to the UGA codon in bacteria.","evidence":"Gel shift assays, nuclease and iodine footprinting, and toeprint assays on fdhF and fdnG mRNAs","pmids":["8483932","8314089"],"confidence":"High","gaps":["Unknown whether eukaryotic SelB orthologs also bind SECIS directly","Ribosomal context of the complex not visualized"]},{"year":1996,"claim":"Domain dissection and GTPase assays established that SelB's N-terminal EF-Tu-like domains bind GTP and tRNA while a distinct C-terminal domain binds SECIS mRNA, and that SECIS binding stimulates ribosome-dependent GTP hydrolysis, providing a conformational switch linking mRNA recognition to catalytic activation.","evidence":"Truncation analysis with functional binding assays, in vitro GTPase activity measurements with ribosome stimulation, and in vivo UGA readthrough quantification","pmids":["8893853","9454578","8898393"],"confidence":"High","gaps":["Atomic structure of the full-length protein not available","Mechanism of ribosome-stimulated GTPase activation unresolved"]},{"year":2000,"claim":"Quantitative kinetics revealed that SelB binds GTP preferentially over GDP with fast GDP release (obviating an exchange factor), and characterization of the mammalian ortholog mSelB/eEFSec showed it retains GTP and Sec-tRNA(Sec) binding but cannot bind the SECIS alone, establishing that eukaryotic selenocysteine incorporation requires additional trans-acting factors.","evidence":"Stopped-flow fluorescence kinetics with fluorescent nucleotide analogs for prokaryotic SelB; GTP binding, tRNA binding, in vivo translation, and HeLa extract complementation for mouse eEFSec; archaeal SelB biochemical characterization","pmids":["10781605","10970870","10860743"],"confidence":"High","gaps":["Identity of the eukaryotic factor(s) bridging eEFSec to the SECIS unknown","Eukaryotic eEFSec structure not determined"]},{"year":2002,"claim":"Crystal and NMR structures of the SelB C-terminal domain and its cognate SECIS RNA revealed a novel winged-helix RNA-binding mode and defined the critical RNA features (GpU tetraloop, bulge uracil) required for recognition, providing the first atomic view of how the mRNA-binding domain works.","evidence":"X-ray crystallography at 2.12 Å (M. thermoacetica WH domains) and NMR spectroscopy of the SECIS hairpin with mutational analysis","pmids":["12145214","12421564"],"confidence":"High","gaps":["No co-crystal of WH domain with SECIS RNA yet","Communication between mRNA-binding and tRNA-binding halves of SelB not structurally resolved"]},{"year":2005,"claim":"Co-crystal structures of the SelB WH domains bound to SECIS RNA demonstrated that RNA binding does not induce major protein conformational change but disrupts a salt bridge between WH2 and WH3, providing a molecular switch that communicates mRNA binding to the tRNA-binding N-terminal domains.","evidence":"X-ray crystallography of E. coli WH3/4 and M. thermoacetica WH1-4 bound to SECIS at 2.3–2.6 Å with site-directed mutagenesis","pmids":["15665870","17537456"],"confidence":"High","gaps":["Full-length SelB structure not available","Mechanism of signal relay to GTPase domain unresolved"]},{"year":2008,"claim":"In the eukaryotic system, SBP2 was shown to undergo a SECIS-induced conformational change that recruits eEFSec, establishing SBP2 as the obligate adaptor between the SECIS element and eEFSec and explaining why eEFSec itself cannot bind SECIS.","evidence":"Alanine scanning mutagenesis of SBP2, in vitro Sec incorporation assay, and trans-complementation of SBP2 domains","pmids":["18948268"],"confidence":"High","gaps":["Structural basis of the SBP2–eEFSec interface unknown","Whether additional factors stabilize the complex in vivo unresolved"]},{"year":2009,"claim":"Quantitative thermodynamic analysis revealed that SelB·GTP binds Sec-tRNA(Sec) with picomolar affinity (Kd ~0.2 pM) and that GTP hydrolysis accelerates tRNA release by >10⁶-fold, establishing the energetic basis for high-fidelity selenocysteine delivery and discriminating Sec- from Ser-tRNA(Sec) through thermodynamic coupling.","evidence":"Stopped-flow fluorescence kinetics and isothermal titration calorimetry with purified components","pmids":["19940162"],"confidence":"High","gaps":["Whether eukaryotic eEFSec achieves similar affinities not measured","On-ribosome kinetics of tRNA release not directly captured"]},{"year":2015,"claim":"The first full-length SelB crystal structure revealed the chalice-like seven-domain architecture (D1–D3 plus WHD1–4) and showed that the Sec-binding site at the D1–D2 interface is smaller and more exposed than EF-Tu's aminoacyl pocket, explaining selective accommodation of the unique tRNA(Sec) secondary structure.","evidence":"X-ray crystallography of A. aeolicus SelB with GTP analog at 3.2 Å and structural modeling of tRNA docking","pmids":["26304550"],"confidence":"High","gaps":["No structure of the full-length SelB–tRNA complex","Eukaryotic eEFSec full-length structure still missing"]},{"year":2016,"claim":"Cryo-EM visualization of six intermediates on the bacterial UGA recoding pathway resolved the complete GTPase activation mechanism: codon recognition causes 30S closure that repositions Sec-tRNA(Sec) away from the sarcin–ricin loop and docks SelB onto it, triggering GTP hydrolysis.","evidence":"Single-particle cryo-EM of six structural intermediates of the E. coli ribosome–SelB complex with molecular dynamics","pmids":["27842381"],"confidence":"High","gaps":["Eukaryotic ribosome recoding mechanism not yet visualized","Post-GTP-hydrolysis accommodation steps not captured"]},{"year":2022,"claim":"Cryo-EM of the mammalian ribosome decoding a Sec UGA codon revealed a fundamentally distinct eukaryotic mechanism: eEFSec and SBP2 do not contact each other directly but engage opposite ends of the SECIS, while ribosomal protein eS31 bridges Sec-tRNA(Sec) and SBP2; eEFSec was found to be indiscriminate toward L-serine, revealing a fidelity challenge in eukaryotic selenocysteine incorporation.","evidence":"Cryo-EM structural determination of the mammalian ribosome·eEFSec·SBP2·SECIS complex","pmids":["35709277"],"confidence":"High","gaps":["Mechanism preventing serine misincorporation in vivo not identified","Dynamics of SECIS delivery and factor recycling on the ribosome unknown"]},{"year":2025,"claim":"Identification of bi-allelic EEFSEC loss-of-function variants in patients with progressive neurodegeneration established EEFSEC as essential for selenoprotein homeostasis in humans and defined a new Mendelian disease of selenoprotein deficiency.","evidence":"Exome/genome sequencing, in vitro functional assay of variants, selenoprotein Western blot in patient fibroblasts, and Drosophila eEFSec-RNAi motor/synaptic phenotyping","pmids":["39753114"],"confidence":"High","gaps":["Tissue-specific consequences of partial EEFSEC loss not characterized","Genotype–phenotype correlations across variants not established","Whether neurodegeneration is driven by loss of specific selenoproteins versus global deficiency unknown"]},{"year":null,"claim":"Key open questions include: (1) how eukaryotic cells prevent eEFSec-mediated serine misincorporation at Sec UGA codons in vivo; (2) the structural basis of the SBP2–SECIS conformational change that recruits eEFSec domain 4; and (3) whether eEFSec has non-translational functions relevant to cancer cell proliferation.","evidence":"","pmids":[],"confidence":"Low","gaps":["No mechanism identified for fidelity proofreading of Sec versus Ser in eukaryotes","No high-resolution structure of the SBP2 conformational switch","Cancer cell phenotypes upon EEFSEC knockdown are correlative and from a single study"]}],"mechanism_profile":{"molecular_activity":[{"term_id":"GO:0003924","term_label":"GTPase activity","supporting_discovery_ids":[6,9,18,20,21]},{"term_id":"GO:0003723","term_label":"RNA binding","supporting_discovery_ids":[3,5,12,13,15]},{"term_id":"GO:0045182","term_label":"translation regulator activity","supporting_discovery_ids":[0,10,14,22]}],"localization":[{"term_id":"GO:0005840","term_label":"ribosome","supporting_discovery_ids":[0,4,20,22]},{"term_id":"GO:0005829","term_label":"cytosol","supporting_discovery_ids":[10,14]}],"pathway":[{"term_id":"R-HSA-392499","term_label":"Metabolism of proteins","supporting_discovery_ids":[0,10,14,22,23]}],"complexes":["eEFSec·Sec-tRNA(Sec)·GTP ternary complex","Ribosome·eEFSec·SBP2·SECIS decoding complex"],"partners":["SBP2","SEC-TRNA(SEC)","ES31","SECIS RNA"],"other_free_text":[]},"mechanistic_narrative":"EEFSEC (eEFSec) is a selenocysteine-specific translational elongation factor and GTPase that delivers Sec-tRNA(Sec) to the ribosomal A site during UGA codon recoding, enabling co-translational selenocysteine incorporation into selenoproteins [PMID:2140572, PMID:10970870]. The protein adopts a chalice-like architecture with three N-terminal EF-Tu-homologous domains that bind GTP and Sec-tRNA(Sec) with extraordinary affinity (Kd ~0.2 pM in the GTP-bound state), and a C-terminal domain 4 that engages SBP2 upon SECIS-induced conformational change; codon recognition triggers GTPase activation via docking on the sarcin–ricin loop, accelerating tRNA release by more than 10⁶-fold [PMID:19940162, PMID:27842381, PMID:29555379]. In eukaryotes, eEFSec does not directly bind the SECIS element but is recruited through SBP2, with ribosomal protein eS31 bridging Sec-tRNA(Sec) and SBP2 to stabilize the decoding complex [PMID:18948268, PMID:35709277]. Bi-allelic loss-of-function variants in EEFSEC cause autosomal recessive progressive neurodegeneration due to global selenoprotein deficiency [PMID:39753114]."},"prefetch_data":{"uniprot":{"accession":"P57772","full_name":"Selenocysteine-specific elongation factor","aliases":["Elongation factor sec","Eukaryotic elongation factor, selenocysteine-tRNA-specific"],"length_aa":596,"mass_kda":65.3,"function":"Translation factor required for the incorporation of the rare amino acid selenocysteine encoded by UGA codons (PubMed:27708257, PubMed:35709277). Replaces the eRF1-eRF3-GTP ternary complex for the insertion of selenocysteine directed by the UGA codon (PubMed:27708257, PubMed:35709277). Insertion of selenocysteine at UGA codons is mediated by SECISBP2 and EEFSEC: SECISBP2 (1) specifically binds the SECIS sequence once the 80S ribosome encounters an in-frame UGA codon and (2) contacts the RPS27A/eS31 of the 40S ribosome before ribosome stalling (PubMed:35709277). (3) GTP-bound EEFSEC then delivers selenocysteinyl-tRNA(Sec) to the 80S ribosome and adopts a preaccommodated state conformation (PubMed:35709277). (4) After GTP hydrolysis, EEFSEC dissociates from the assembly, selenocysteinyl-tRNA(Sec) accommodates, and peptide bond synthesis and selenoprotein elongation occur (PubMed:35709277)","subcellular_location":"Cytoplasm; Nucleus","url":"https://www.uniprot.org/uniprotkb/P57772/entry"},"depmap":{"release":"DepMap","has_data":true,"is_common_essential":false,"resolved_as":"","url":"https://depmap.org/portal/gene/EEFSEC","classification":"Not Classified","n_dependent_lines":621,"n_total_lines":1208,"dependency_fraction":0.5140728476821192},"opencell":{"profiled":false,"resolved_as":"","ensg_id":"","cell_line_id":"","localizations":[],"interactors":[],"url":"https://opencell.sf.czbiohub.org/search/EEFSEC","total_profiled":1310},"omim":[{"mim_id":"621102","title":"NEURODEVELOPMENTAL DISORDER WITH PROGRESSIVE SPASTICITY AND BRAIN ABNORMALITIES; NEDPSB","url":"https://www.omim.org/entry/621102"},{"mim_id":"617724","title":"TSC22 DOMAIN FAMILY, MEMBER 2; 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GTP hydrolysis does not induce a canonical conformational change but instead promotes a slight ratchet of domains 1 and 2 and a lever-like movement of domain 4, proposed to be critical for release of Sec-tRNASec on the ribosome.\",\n      \"method\": \"Structural analysis and biochemical characterization of mammalian eEFSec; review integrating crystal structure data and functional studies\",\n      \"journal\": \"Biochimica et biophysica acta. General subjects\",\n      \"confidence\": \"Medium\",\n      \"confidence_rationale\": \"Tier 2 — structural and biochemical data from mammalian system, single review paper summarizing recent findings\",\n      \"pmids\": [\"29555379\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2025,\n      \"finding\": \"Bi-allelic loss-of-function variants in EEFSEC cause reduced eEFSec function in vitro, leading to lower selenoprotein levels in patient fibroblasts and progressive neurodegeneration (cerebellar hypoplasia/atrophy, spasticity, ataxia, seizures). An eEFSec-RNAi Drosophila model recapitulates progressive motor impairment with synaptic defects, establishing eEFSec as essential for selenoprotein synthesis in vivo.\",\n      \"method\": \"Patient fibroblast biochemistry (selenoprotein level measurement), in vitro functional assay of variant EEFSEC, Drosophila RNAi knockdown with motor phenotype and synaptic morphology readout\",\n      \"journal\": \"American journal of human genetics\",\n      \"confidence\": \"High\",\n      \"confidence_rationale\": \"Tier 2 — multiple orthogonal methods (patient cells, in vitro functional assay, animal model) with defined cellular and organismal phenotypes\",\n      \"pmids\": [\"39753114\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2025,\n      \"finding\": \"Compound heterozygous EEFSEC variants (p.V488Dfs*113 and p.R443P) in a patient with cerebellar atrophy are associated with elevated plasma oxidized fatty acid metabolites, consistent with impaired antioxidant selenoprotein function downstream of defective eEFSec activity.\",\n      \"method\": \"Whole exome sequencing, structural analysis of variant, plasma metabolomics\",\n      \"journal\": \"Journal of neurodevelopmental disorders\",\n      \"confidence\": \"Low\",\n      \"confidence_rationale\": \"Tier 3 — genetic and metabolomic data from a single case, no direct in vitro reconstitution of eEFSec variant activity\",\n      \"pmids\": [\"40652205\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2021,\n      \"finding\": \"EEFSEC knockdown in prostate cancer 22Rv1 cells inhibits proliferation, migration, and invasion, causes G0/G1 cell cycle arrest, and suppresses C-myc and CCNB1 expression while increasing p15, placing eEFSec upstream of cell cycle regulators in this context.\",\n      \"method\": \"Lentiviral shRNA knockdown, XTT proliferation assay, Transwell migration/invasion assay, flow cytometry cell cycle analysis, qRT-PCR/Western blot\",\n      \"journal\": \"Nan fang yi ke da xue xue bao\",\n      \"confidence\": \"Low\",\n      \"confidence_rationale\": \"Tier 3 — clean KD with cellular phenotype but no direct mechanistic pathway placement for eEFSec itself beyond selenoprotein synthesis\",\n      \"pmids\": [\"35012909\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2025,\n      \"finding\": \"In yellow catfish, the eefsec promoter contains functional FOXO1 (-1070 to -1080 bp) and STAT3 (-428 to -436 bp) binding sites that are regulated by selenium (selenomethionine), as confirmed by EMSA and chromatin immunoprecipitation, identifying transcription factors that directly regulate eefsec expression.\",\n      \"method\": \"Promoter deletion/mutation analysis, EMSA, ChIP\",\n      \"journal\": \"Biochimica et biophysica acta. Gene regulatory mechanisms\",\n      \"confidence\": \"Medium\",\n      \"confidence_rationale\": \"Tier 2 — EMSA and ChIP provide direct evidence for transcription factor binding; ortholog in teleost fish\",\n      \"pmids\": [\"40618995\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 1998,\n      \"finding\": \"Eukaryotic/archaeal eIF2γ and bacterial SelB (the prokaryotic ortholog of eEFSec) form a closely related subfamily of GTPases, indicating that eEFSec/SelB diverged from an ancient GTPase ancestor shared with eIF2γ before the last common ancestor of extant life.\",\n      \"method\": \"Phylogenetic analysis of GTPase superfamily sequences from diplomonads, parabasalia, microsporidia, and archaea\",\n      \"journal\": \"Journal of molecular evolution\",\n      \"confidence\": \"Low\",\n      \"confidence_rationale\": \"Tier 4 — computational/phylogenetic inference only\",\n      \"pmids\": [\"9847405\"],\n      \"is_preprint\": false\n    }\n  ],\n  \"current_model\": \"eEFSec (EEFSEC) is a specialized eukaryotic elongation factor that forms a chalice-like, four-domain structure: three N-terminal EF-Tu-like domains mediate GTP binding and Sec-tRNASec recognition, while the C-terminal domain 4 binds both Sec-tRNASec and the SECIS-binding protein SBP2; upon GTP hydrolysis, eEFSec undergoes a non-canonical conformational rearrangement (ratchet of domains 1–2 and lever movement of domain 4) that promotes Sec-tRNASec release at the ribosomal A site for co-translational selenocysteine insertion, and loss of eEFSec function leads to global selenoprotein deficiency and progressive neurodegeneration in humans and model organisms.\"\n}\n```\n\n--- ROUND 2 (R2/R3, augmented corpus) ---\n\nI'll classify each paper and extract discoveries only from KEEP papers.\n\n**Classification:**\n- Papers [1]-[23] (retrieved corpus, non-EEFSEC): These are about bacterial/archaeal SelB (prokaryotic ortholog). Per the rules, I need to assess if they're orthologs of mammalian EEFSEC. SelB is the prokaryotic ortholog of eEFSec/EEFSEC — same function (selenocysteine-specific elongation factor). These are KEEP as foundational ortholog papers.\n- Paper [24]: KEEP — directly about mammalian eEFSec/EEFSEC\n- Paper [25] (Hilal 2022): KEEP — cryo-EM structure of mammalian ribosome decoding Sec UGA, directly involves eEFSec\n- Paper [33] (Laugwitz 2025): KEEP — directly about human EEFSEC deficiency\n- Paper [35] (Xu 2021): KEEP — directly about human EEFSEC in prostate cancer cells\n- Paper [41] (Liu 2025): KEEP — directly about human EEFSEC variants\n- Paper [47] (Zhang 2025): KEEP — directly about eefsec promoter regulation\n- Paper [40]: EXCLUDE — \"SelB-1\" is a drug compound, not the gene\n- Paper [42]: This is a GWAS/SNP association study — no mechanistic finding, EXCLUDE from discoveries\n- Papers [44], [45], [50]: SelB variants in archaea/bacteria or directed evolution — borderline, but [45]/[50] are about engineering SelB, and [44] is about archaeal MM1309 binding Cys-tRNA — EXCLUDE as not directly EEFSEC\n- Paper [37]: SelB in CNF1 regulation — bacterial context, KEEP as ortholog\n- Paper [38]: Bioinformatic prediction — EXCLUDE (Tier 4, computational only)\n- Paper [12] (gene2pubmed, Fagegaltier 2000): KEEP — characterization of mammalian mSelB (mouse EEFSEC ortholog)\n- Paper [19] (gene2pubmed, Mehta 2004): KEEP — mammalian Sec incorporation with eEFSec\n- Paper [27] (gene2pubmed, Donovan 2008): KEEP — SBP2/eEFSec interaction\n- Other gene2pubmed papers: Most are GWAS/interactome/unrelated — EXCLUDE from discoveries unless mechanistic\n\n```json\n{\n  \"discoveries\": [\n    {\n      \"year\": 1990,\n      \"finding\": \"SELB (prokaryotic ortholog of EEFSEC) was purified from E. coli and shown to be an elongation factor-like protein that specifically binds selenocysteyl-tRNA(Sec). It is partially associated with ribosomes and is required for co-translational selenocysteine insertion at UGA codons.\",\n      \"method\": \"Protein purification, limited proteolysis, immunological analysis, ribosome fractionation\",\n      \"journal\": \"The Journal of biological chemistry\",\n      \"confidence\": \"High\",\n      \"confidence_rationale\": \"Tier 1-2 — biochemical purification with functional characterization, replicated across multiple subsequent studies\",\n      \"pmids\": [\"2140572\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 1991,\n      \"finding\": \"The 8-base-pair aminoacyl-acceptor helix of tRNA(Sec) is the primary determinant for binding to SELB (prokaryotic EEFSEC ortholog); reduction to 7 base pairs prevents SELB binding but allows EF-Tu binding. This structural feature exclusively directs selenocysteyl-tRNA(Sec) to SELB and precludes interaction with EF-Tu.\",\n      \"method\": \"In vitro binding assays with purified mutant tRNA variants, kinetic analysis of seryl-tRNA synthetase and selenocysteine synthase\",\n      \"journal\": \"The Journal of biological chemistry\",\n      \"confidence\": \"High\",\n      \"confidence_rationale\": \"Tier 1 — reconstituted in vitro with defined tRNA mutants, orthogonal methods including Km/Kcat measurements\",\n      \"pmids\": [\"1939093\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 1991,\n      \"finding\": \"SELB (EEFSEC ortholog) specifically complexes selenocysteyl-tRNA(Sec); interaction with the selenol group of the aminoacylated selenocysteine residue is required for stable SELB·tRNA complex formation, providing the biochemical basis for exclusive selection of selenocysteyl-tRNA(Sec).\",\n      \"method\": \"Biochemical characterization, in vitro binding assays\",\n      \"journal\": \"Biochimie\",\n      \"confidence\": \"High\",\n      \"confidence_rationale\": \"Tier 1 — in vitro reconstitution with defined substrates, replicated\",\n      \"pmids\": [\"1839607\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 1993,\n      \"finding\": \"SELB (EEFSEC ortholog) directly and specifically binds the mRNA SECIS hairpin loop region via gel-shift and footprinting assays. In the presence of selenocysteinyl-tRNA, SELB forms a ternary complex with charged tRNA and mRNA, positioning the tRNA at the UGA codon.\",\n      \"method\": \"Gel shift assays, nuclease and iodine footprinting\",\n      \"journal\": \"Proceedings of the National Academy of Sciences of the United States of America\",\n      \"confidence\": \"High\",\n      \"confidence_rationale\": \"Tier 1-2 — direct binding demonstrated by gel shift and footprinting; replicated in multiple subsequent studies\",\n      \"pmids\": [\"8483932\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 1994,\n      \"finding\": \"The SELB-GTP-Sec-tRNA(Sec) ternary complex binds selenoprotein mRNAs (fdhF and fdnG) and toeprint experiments show SELB recognizes ribosome-bound message, with the complex extending toward the large ribosomal subunit, increasing local concentration of Sec-tRNA(Sec) at the UGA codon.\",\n      \"method\": \"Boundary experiments, toeprint assays\",\n      \"journal\": \"Genes & development\",\n      \"confidence\": \"High\",\n      \"confidence_rationale\": \"Tier 2 — orthogonal methods (boundary + toeprint) with functional interpretation, replicated\",\n      \"pmids\": [\"8314089\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 1996,\n      \"finding\": \"SELB (EEFSEC ortholog) domain structure was elucidated: the N-terminal three domains are homologous to EF-Tu (binding GTP and selenocysteyl-tRNA(Sec)), while a distinct C-terminal ~17 kDa domain specifically binds the mRNA SECIS hairpin. Truncated SelB lacking the C-terminal domain retains tRNA binding but loses mRNA binding.\",\n      \"method\": \"Cloning of selB from multiple bacteria, sequence alignment, expression and biochemical analysis of truncated SelB fragments\",\n      \"journal\": \"Journal of molecular biology\",\n      \"confidence\": \"High\",\n      \"confidence_rationale\": \"Tier 1-2 — domain dissection with functional validation, independently replicated across multiple SelB species\",\n      \"pmids\": [\"8893853\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 1996,\n      \"finding\": \"SELB GTPase activity is stimulated ~3-4 fold by the SECIS mRNA hairpin in a ribosome-dependent manner. mRNA binding to SELB induces a conformational switch promoting increased ribosome-mediated GTP hydrolysis; the minimal stimulatory region maps to the upper half of the hairpin.\",\n      \"method\": \"GTPase activity assays, ribosome stimulation, truncated mRNA hairpin titration\",\n      \"journal\": \"Biochemistry\",\n      \"confidence\": \"High\",\n      \"confidence_rationale\": \"Tier 1 — in vitro enzymatic assay with defined components and mutant hairpins\",\n      \"pmids\": [\"9454578\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 1996,\n      \"finding\": \"Overproduction of SelB (EEFSEC ortholog) in vivo reduces UGA readthrough to <1%, reversed by co-overexpression of tRNA(Sec), demonstrating that balanced stoichiometry between SelB, selenocysteyl-tRNA(Sec), and mRNA is essential for selenocysteine insertion. The mRNA-binding and tRNA-binding domains are physically separable.\",\n      \"method\": \"Genetic overexpression, in vivo UGA readthrough assay (lacZ fusion), truncation analysis\",\n      \"journal\": \"Molecular microbiology\",\n      \"confidence\": \"High\",\n      \"confidence_rationale\": \"Tier 2 — genetic epistasis with defined molecular readout, replicated\",\n      \"pmids\": [\"8898393\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 1998,\n      \"finding\": \"Evolutionary analysis established that SELB/eEFSec belongs to a GTPase superfamily ancestral to both bacterial SELB and eukaryotic/archaeal eIF2γ, indicating these translation factors share a common evolutionary origin and that change of function occurred within this GTPase subfamily.\",\n      \"method\": \"Phylogenetic analysis, sequence comparison across diplomonads, parabasalia, microsporidia, and archaebacteria\",\n      \"journal\": \"Journal of molecular evolution\",\n      \"confidence\": \"Medium\",\n      \"confidence_rationale\": \"Tier 4 — computational/phylogenetic analysis, but broadly replicated and foundational for understanding eEFSec origins\",\n      \"pmids\": [\"9847405\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2000,\n      \"finding\": \"The kinetics of SELB (EEFSEC ortholog) interaction with GTP (Kd = 0.74 µM) and GDP (Kd = 13.4 µM) were determined; rapid GDP release (15 s⁻¹) obviates the need for a nucleotide exchange factor. SECIS RNA binds with ~1 nM affinity, further increased by selenocysteyl-tRNA(Sec) binding, suggesting SELB forms a tight quaternary complex on the SECIS that loosens after GTP hydrolysis.\",\n      \"method\": \"Stopped-flow fluorescence kinetics using intrinsic tryptophan fluorescence and methylanthraniloyl nucleotide derivatives\",\n      \"journal\": \"The Journal of biological chemistry\",\n      \"confidence\": \"High\",\n      \"confidence_rationale\": \"Tier 1 — quantitative kinetic measurements with multiple fluorescent reporters, multiple binding partners\",\n      \"pmids\": [\"10781605\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2000,\n      \"finding\": \"mSelB (mouse EEFSEC, the mammalian ortholog of bacterial SelB) was characterized: it binds GTP, recognizes Sec-tRNA(Sec) in vitro and in vivo, and is required for efficient selenoprotein translation in vivo. Unlike bacterial SelB, mSelB alone cannot bind the eukaryotic SECIS RNA; complementation with cell extracts yields a SECIS-dependent complex containing mSelB and at least one additional factor.\",\n      \"method\": \"Database cloning, GTP binding assay, in vitro Sec-tRNA(Sec) binding, in vivo selenoprotein translation assay, HeLa cell extract complementation\",\n      \"journal\": \"The EMBO journal\",\n      \"confidence\": \"High\",\n      \"confidence_rationale\": \"Tier 1-2 — multiple orthogonal methods (GTP binding, tRNA binding, in vivo translation), foundational characterization paper\",\n      \"pmids\": [\"10970870\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2000,\n      \"finding\": \"Archaeal SelB (from Methanococcus jannaschii, aSelB) binds guanine nucleotides and preferentially binds selenocysteyl-tRNA(Sec) like bacterial SelB, but does not bind the SECIS element, lacking the bacterial C-terminal mRNA-binding domain. This suggests that in archaea and eukaryotes, functions of bacterial SelB are distributed over at least two proteins.\",\n      \"method\": \"Genome database search, protein purification, guanine nucleotide binding assay, tRNA binding assay\",\n      \"journal\": \"Journal of molecular biology\",\n      \"confidence\": \"High\",\n      \"confidence_rationale\": \"Tier 1-2 — biochemical characterization of purified protein, consistent with eukaryotic EEFSEC mechanism\",\n      \"pmids\": [\"10860743\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2002,\n      \"finding\": \"Crystal structure of the C-terminal mRNA-binding fragment of SelB from Moorella thermoacetica (SelB-C) determined at 2.12 Å resolution, revealing four tandem winged-helix (WH) domains arranged in an L-shape. This was the first structure showing winged-helix domains involved in RNA binding; conserved basic residues define the mRNA-binding site.\",\n      \"method\": \"X-ray crystallography (multiwavelength anomalous dispersion), structural analysis\",\n      \"journal\": \"The EMBO journal\",\n      \"confidence\": \"High\",\n      \"confidence_rationale\": \"Tier 1 — crystal structure at 2.12 Å, first structural characterization of C-terminal RNA-binding domain\",\n      \"pmids\": [\"12145214\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2002,\n      \"finding\": \"NMR structure of the prokaryotic SECIS mRNA hairpin revealed conserved structural features critical for SelB (EEFSEC ortholog) binding. A GpU sequence at the tip of the capping tetraloop and a bulge uracil five base-pairs away are essential for SelB interaction; SelB binding stabilizes RNA secondary structure.\",\n      \"method\": \"NMR spectroscopy, mutational analysis of SECIS binding\",\n      \"journal\": \"Journal of molecular biology\",\n      \"confidence\": \"High\",\n      \"confidence_rationale\": \"Tier 1 — NMR structure with mutational validation of binding interface\",\n      \"pmids\": [\"12421564\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2004,\n      \"finding\": \"In mammalian systems, eEFSec (EEFSEC), Sec-tRNA(Sec), and SBP2 are all required for selenocysteine incorporation. SBP2 is the only limiting factor in rabbit reticulocyte lysate; selenocysteine incorporation efficiency into a luciferase reporter is 5-8% in vitro and ~1% in transfected cells, demonstrating the reconstituted system and quantifying factor contributions.\",\n      \"method\": \"In vitro translation assay, transfected cell Sec incorporation efficiency measurement, factor depletion and reconstitution\",\n      \"journal\": \"The Journal of biological chemistry\",\n      \"confidence\": \"High\",\n      \"confidence_rationale\": \"Tier 1-2 — quantitative reconstitution in vitro and in vivo, multiple limiting factor analyses\",\n      \"pmids\": [\"15229221\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2005,\n      \"finding\": \"Crystal structure of the mRNA-binding domain of SelB (EEFSEC ortholog) in complex with SECIS RNA at 2.3 Å resolution revealed the first example of a winged-helix (WH) domain binding RNA. RNA binding does not induce major conformational change in the WH motif; the structure suggests the complex wraps around the small ribosomal subunit.\",\n      \"method\": \"X-ray crystallography\",\n      \"journal\": \"Nature structural & molecular biology\",\n      \"confidence\": \"High\",\n      \"confidence_rationale\": \"Tier 1 — high-resolution crystal structure of protein-RNA complex, novel structural finding\",\n      \"pmids\": [\"15665870\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2007,\n      \"finding\": \"Crystal structures of E. coli SelB WH3/4 domains and M. thermoacetica WH1-4 domains each bound to SECIS mRNA revealed that both WH modules use the same structural elements to bind RNA. A salt bridge connecting WH2 to WH3 is disrupted upon mRNA binding, providing a molecular switch allowing communication between tRNA- and mRNA-binding sites, with RNA acting as an activator.\",\n      \"method\": \"X-ray crystallography (2.3 Å for E. coli WH3/4; 2.6 Å for M. thermoacetica WH1-4), site-directed mutagenesis\",\n      \"journal\": \"Journal of molecular biology\",\n      \"confidence\": \"High\",\n      \"confidence_rationale\": \"Tier 1 — dual crystal structures with mutagenesis validation of molecular switch\",\n      \"pmids\": [\"17537456\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2008,\n      \"finding\": \"SBP2's Sec incorporation domain (SID) promotes high-affinity SECIS binding and eEFSec (EEFSEC) recruitment to the SBP2 RNA-binding domain. SECIS binding induces a conformational change in SBP2 that recruits eEFSec; the SID and RNA-binding domain can function in trans, establishing eEFSec as a downstream effector of SBP2·SECIS complex formation.\",\n      \"method\": \"Alanine scanning mutagenesis, in vitro Sec incorporation assay, binding domain separation experiments\",\n      \"journal\": \"The Journal of biological chemistry\",\n      \"confidence\": \"High\",\n      \"confidence_rationale\": \"Tier 1-2 — alanine scanning with functional readout, trans-complementation, mechanistic model\",\n      \"pmids\": [\"18948268\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2009,\n      \"finding\": \"Sec-tRNA(Sec) binds SelB (EEFSEC ortholog)·GTP with extraordinary affinity (Kd = 0.2 pM); GTP hydrolysis accelerates tRNA release by >10⁶-fold (from 0.3 h⁻¹ to 240 s⁻¹). Thermodynamic coupling between Sec-tRNA(Sec) and GTP binding ensures specificity of Sec vs. Ser-tRNA(Sec) selection. This mechanism is reminiscent of eIF2 rather than EF-Tu.\",\n      \"method\": \"Stopped-flow fluorescence kinetics, thermodynamic binding measurements, isothermal titration calorimetry\",\n      \"journal\": \"The Journal of biological chemistry\",\n      \"confidence\": \"High\",\n      \"confidence_rationale\": \"Tier 1 — quantitative thermodynamic and kinetic analysis with multiple orthogonal methods\",\n      \"pmids\": [\"19940162\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2015,\n      \"finding\": \"Crystal structure of the full-length SelB from Aquifex aeolicus in complex with a GTP analog at 3.2 Å resolution revealed: three EF-Tu-like domains (D1-3) followed by four winged-helix domains (WHD1-4) connected by a spacer region. The Sec-binding site is at the D1-D2 interface (smaller and more exposed than EF-Tu's aminoacyl site). Structural modeling suggests tRNA(Sec)'s unique secondary structure allows SelB to specifically recognize it and place it at the ribosomal A-site.\",\n      \"method\": \"X-ray crystallography at 3.2 Å, structural modeling\",\n      \"journal\": \"Nucleic acids research\",\n      \"confidence\": \"High\",\n      \"confidence_rationale\": \"Tier 1 — first full-length SelB crystal structure, rigorous structural analysis\",\n      \"pmids\": [\"26304550\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2016,\n      \"finding\": \"Cryo-EM structures of six intermediates on the UGA recoding pathway revealed the mechanism of SelB (EEFSEC ortholog) GTPase activation: initial SelB·Sec-tRNA(Sec) binding causes the 30S subunit to adopt an open conformation with Sec-tRNA(Sec) covering the sarcin-ricin loop (SRL). Codon recognition triggers local decoding-site closure, moving Sec-tRNA(Sec) away from the SRL and causing global 30S shoulder domain closure, which docks SelB on the SRL to activate GTPase.\",\n      \"method\": \"Single-particle cryo-electron microscopy (six structural intermediates), molecular dynamics\",\n      \"journal\": \"Nature\",\n      \"confidence\": \"High\",\n      \"confidence_rationale\": \"Tier 1 — six cryo-EM structures capturing the full recoding pathway, multiple orthogonal analyses\",\n      \"pmids\": [\"27842381\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2018,\n      \"finding\": \"Mammalian eEFSec (EEFSEC) folds into a chalice-like structure with three N-terminal EF-Tu-like domains and a C-terminal domain 4 that binds Sec-tRNA(Sec) and SBP2. GTP hydrolysis does not induce a canonical conformational change but instead promotes a slight ratchet of domains 1 and 2 and a lever-like movement of domain 4, which may be critical for Sec-tRNA(Sec) release on the ribosome. A non-canonical mechanism for Sec UGA recoding elongation is proposed.\",\n      \"method\": \"Structural analysis, crystal structure review, domain function mapping, GTP hydrolysis assays\",\n      \"journal\": \"Biochimica et biophysica acta. General subjects\",\n      \"confidence\": \"Medium\",\n      \"confidence_rationale\": \"Tier 2 — review summarizing structural data from the Simonović lab with mechanistic interpretation; some findings are synthesis of primary data\",\n      \"pmids\": [\"29555379\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2022,\n      \"finding\": \"Cryo-EM structure of the mammalian ribosome decoding the Sec UGA codon revealed: (1) eEFSec (EEFSEC) and SBP2 do not interact directly but deploy their C-terminal domains to engage opposite ends of the SECIS; (2) ribosomal protein eS31 simultaneously contacts Sec-tRNA(Sec) and SBP2 via its Lys-rich and C-terminal segments to stabilize the assembly; (3) eEFSec is indiscriminate toward L-serine and can misincorporate it at Sec UGA codons. This reveals a fundamentally distinct mechanism of Sec UGA recoding in eukaryotes versus bacteria.\",\n      \"method\": \"Cryo-electron microscopy structural determination of mammalian ribosome·eEFSec·SBP2·SECIS complex\",\n      \"journal\": \"Science\",\n      \"confidence\": \"High\",\n      \"confidence_rationale\": \"Tier 1 — cryo-EM structure of the intact mammalian Sec decoding complex with strong functional validation\",\n      \"pmids\": [\"35709277\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2025,\n      \"finding\": \"Bi-allelic loss-of-function variants in EEFSEC cause autosomal recessive selenoprotein deficiency in humans, leading to progressive neurodegeneration (EEFSEC deficiency). Pathogenic EEFSEC variants showed reduced EEFSEC function in vitro and lower selenoprotein levels in patient fibroblasts. A Drosophila eEFSec-RNAi model displayed progressive motor impairment and synaptic defects, confirming the in vivo requirement of EEFSEC for normal neuronal function.\",\n      \"method\": \"Exome/genome sequencing, in vitro functional assay of variants, Western blot of selenoproteins in fibroblasts, Drosophila RNAi motor function assay, synaptic morphology analysis\",\n      \"journal\": \"American journal of human genetics\",\n      \"confidence\": \"High\",\n      \"confidence_rationale\": \"Tier 2 — multiple orthogonal methods (in vitro variant function, patient fibroblast selenoproteomics, Drosophila model), strong preponderance of evidence\",\n      \"pmids\": [\"39753114\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2025,\n      \"finding\": \"In yellow catfish, the eefsec gene promoter contains a functional FOXO1 binding site (-1070 to -1080 bp) and a STAT3 binding site (-428 to -436 bp). These transcription factors directly regulate eefsec transcriptional activity in a selenium (selenomethionine)-dependent manner, as confirmed by EMSA and chromatin immunoprecipitation.\",\n      \"method\": \"Promoter deletion analysis, EMSA, chromatin immunoprecipitation\",\n      \"journal\": \"Biochimica et biophysica acta. Gene regulatory mechanisms\",\n      \"confidence\": \"Medium\",\n      \"confidence_rationale\": \"Tier 2 — direct binding confirmed by EMSA and ChIP, but in non-mammalian species (fish); functional relevance to human EEFSEC regulation uncertain\",\n      \"pmids\": [\"40618995\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2021,\n      \"finding\": \"EEFSEC knockdown in human prostate cancer 22Rv1 cells suppressed proliferation, migration, and invasion, caused G0/G1 cell cycle arrest, downregulated C-myc and CCNB1, and upregulated p15, suggesting EEFSEC expression modulates cell cycle progression through C-myc-related pathways.\",\n      \"method\": \"Lentiviral shRNA knockdown, XTT proliferation assay, Transwell migration/invasion assay, flow cytometry cell cycle analysis, qRT-PCR, Western blot\",\n      \"journal\": \"Nan fang yi ke da xue xue bao\",\n      \"confidence\": \"Medium\",\n      \"confidence_rationale\": \"Tier 2-3 — clean KD with multiple phenotypic readouts, but pathway placement is correlative and single lab\",\n      \"pmids\": [\"35012909\"],\n      \"is_preprint\": false\n    }\n  ],\n  \"current_model\": \"EEFSEC (eEFSec) is a selenocysteine-specific translational elongation factor that forms an EF-Tu-like GTPase with a chalice-like structure; it delivers Sec-tRNA(Sec) to the ribosomal A site at UGA codons with extraordinary tRNA-binding affinity (Kd ~0.2 pM for the GTP-bound form), with GTP hydrolysis triggered by codon recognition via docking on the sarcin-ricin loop, releasing Sec-tRNA(Sec) for peptide bond formation; in eukaryotes, eEFSec does not bind the SECIS element directly but is recruited through SBP2 (which binds the SECIS and undergoes a conformational change that engages eEFSec's C-terminal domain 4), while ribosomal protein eS31 bridges Sec-tRNA(Sec) and SBP2 to stabilize the decoding complex; loss-of-function variants in EEFSEC cause autosomal recessive progressive neurodegeneration due to global selenoprotein deficiency.\"\n}\n```","stage2_raw":"```json\n{\n  \"mechanistic_narrative\": \"EEFSEC (eEFSec) is a specialized GTPase elongation factor dedicated to the co-translational incorporation of selenocysteine into selenoproteins. It adopts a chalice-like four-domain architecture in which three N-terminal EF-Tu-like domains mediate GTP binding and Sec-tRNASec recognition, while the C-terminal domain 4 contacts both Sec-tRNASec and the SECIS-binding protein SBP2; GTP hydrolysis triggers a non-canonical ratchet of domains 1–2 and lever movement of domain 4 to release Sec-tRNASec at the ribosomal A site [PMID:29555379]. Bi-allelic loss-of-function variants in EEFSEC cause global selenoprotein deficiency and progressive neurodegeneration characterized by cerebellar hypoplasia/atrophy, spasticity, ataxia, and seizures, a phenotype recapitulated by eEFSec knockdown in Drosophila [PMID:39753114].\",\n  \"teleology\": [\n    {\n      \"year\": 1998,\n      \"claim\": \"Phylogenetic analysis placed eEFSec/SelB within an ancient GTPase subfamily shared with eIF2γ, establishing that the selenocysteine-specific elongation factor diverged from a general translation-factor ancestor before the last common ancestor of extant life.\",\n      \"evidence\": \"Computational phylogenetic analysis of GTPase superfamily sequences across eukaryotes, archaea, and bacteria\",\n      \"pmids\": [\"9847405\"],\n      \"confidence\": \"Low\",\n      \"gaps\": [\n        \"Purely computational inference without biochemical validation of ancestral function\",\n        \"Does not address how eEFSec acquired specificity for Sec-tRNASec versus other aminoacyl-tRNAs\"\n      ]\n    },\n    {\n      \"year\": 2018,\n      \"claim\": \"Structural and biochemical characterization revealed that mammalian eEFSec uses a four-domain chalice-like fold and a non-canonical GTP-hydrolysis-driven conformational change — a domain 1–2 ratchet and domain 4 lever movement — to deliver Sec-tRNASec, resolving how eEFSec differs mechanistically from canonical EF-Tu.\",\n      \"evidence\": \"Crystal structure determination and biochemical characterization of mammalian eEFSec\",\n      \"pmids\": [\"29555379\"],\n      \"confidence\": \"Medium\",\n      \"gaps\": [\n        \"No ribosome-bound structure showing domain 4 lever movement during Sec-tRNASec delivery\",\n        \"Kinetics of GTP hydrolysis and Sec-tRNASec release not quantified in a reconstituted translation system\",\n        \"Precise interface between domain 4 and SBP2 not resolved at atomic level\"\n      ]\n    },\n    {\n      \"year\": 2021,\n      \"claim\": \"Knockdown of EEFSEC in prostate cancer cells demonstrated that eEFSec activity supports cell proliferation and cell cycle progression, linking selenoprotein synthesis capacity to growth regulation.\",\n      \"evidence\": \"Lentiviral shRNA knockdown in 22Rv1 cells with proliferation, migration, cell cycle, and gene expression readouts\",\n      \"pmids\": [\"35012909\"],\n      \"confidence\": \"Low\",\n      \"gaps\": [\n        \"No demonstration that the proliferative phenotype is mediated through specific selenoproteins rather than a general translation defect\",\n        \"Single cell line without rescue experiment\",\n        \"Mechanistic connection between eEFSec and C-myc/CCNB1/p15 not established\"\n      ]\n    },\n    {\n      \"year\": 2025,\n      \"claim\": \"Identification of bi-allelic EEFSEC mutations in patients with progressive neurodegeneration and selenoprotein deficiency — validated by in vitro functional assays and a Drosophila RNAi model — established eEFSec as essential for selenoprotein synthesis in vivo and defined a new Mendelian neurodegenerative disorder.\",\n      \"evidence\": \"Patient fibroblast selenoprotein quantification, in vitro eEFSec variant activity assay, Drosophila RNAi with motor and synaptic phenotyping\",\n      \"pmids\": [\"39753114\"],\n      \"confidence\": \"High\",\n      \"gaps\": [\n        \"Tissue-specific selenoprotein deficiency profiles in human brain not determined\",\n        \"Relative contribution of individual selenoproteins (e.g., GPX4, SELENOP) to the neurodegeneration phenotype unknown\",\n        \"No conditional knockout mouse model to delineate developmental versus degenerative contributions\"\n      ]\n    },\n    {\n      \"year\": 2025,\n      \"claim\": \"Identification of FOXO1 and STAT3 binding sites in the eefsec promoter that respond to selenium established a transcriptional regulatory axis for eefsec expression, revealing how selenium availability feeds back to control the selenocysteine insertion machinery.\",\n      \"evidence\": \"Promoter deletion/mutation analysis, EMSA, and ChIP in yellow catfish cells\",\n      \"pmids\": [\"40618995\"],\n      \"confidence\": \"Medium\",\n      \"gaps\": [\n        \"Demonstrated in a teleost ortholog; conservation of FOXO1/STAT3 regulation at the mammalian EEFSEC promoter not tested\",\n        \"Whether selenium-dependent transcriptional regulation of eEFSec is rate-limiting for selenoprotein synthesis is unknown\"\n      ]\n    },\n    {\n      \"year\": null,\n      \"claim\": \"The structural basis of eEFSec–ribosome interaction during selenocysteine delivery, and the mechanism by which loss of specific selenoproteins drives neurodegeneration downstream of eEFSec deficiency, remain unresolved.\",\n      \"evidence\": \"\",\n      \"pmids\": [],\n      \"confidence\": \"High\",\n      \"gaps\": [\n        \"No cryo-EM structure of eEFSec bound to the ribosome with SECIS element and SBP2\",\n        \"Identity of the critical selenoprotein(s) whose loss causes the neurodegenerative phenotype not determined\",\n        \"Quantitative translational output of eEFSec (selenoprotein hierarchy) under limiting eEFSec conditions not mapped\"\n      ]\n    }\n  ],\n  \"mechanism_profile\": {\n    \"molecular_activity\": [\n      {\"term_id\": \"GO:0003924\", \"supporting_discovery_ids\": [0, 1]},\n      {\"term_id\": \"GO:0045182\", \"supporting_discovery_ids\": [0, 1]}\n    ],\n    \"localization\": [\n      {\"term_id\": \"GO:0005829\", \"supporting_discovery_ids\": [0]}\n    ],\n    \"pathway\": [\n      {\"term_id\": \"R-HSA-392499\", \"supporting_discovery_ids\": [0, 1]}\n    ],\n    \"complexes\": [],\n    \"partners\": [\n      \"SBP2\"\n    ],\n    \"other_free_text\": []\n  }\n}\n```\n\n--- ROUND 2 (R2/R3, augmented corpus) ---\n\n```json\n{\n  \"mechanistic_narrative\": \"EEFSEC (eEFSec) is a selenocysteine-specific translational elongation factor and GTPase that delivers Sec-tRNA(Sec) to the ribosomal A site during UGA codon recoding, enabling co-translational selenocysteine incorporation into selenoproteins [PMID:2140572, PMID:10970870]. The protein adopts a chalice-like architecture with three N-terminal EF-Tu-homologous domains that bind GTP and Sec-tRNA(Sec) with extraordinary affinity (Kd ~0.2 pM in the GTP-bound state), and a C-terminal domain 4 that engages SBP2 upon SECIS-induced conformational change; codon recognition triggers GTPase activation via docking on the sarcin–ricin loop, accelerating tRNA release by more than 10⁶-fold [PMID:19940162, PMID:27842381, PMID:29555379]. In eukaryotes, eEFSec does not directly bind the SECIS element but is recruited through SBP2, with ribosomal protein eS31 bridging Sec-tRNA(Sec) and SBP2 to stabilize the decoding complex [PMID:18948268, PMID:35709277]. Bi-allelic loss-of-function variants in EEFSEC cause autosomal recessive progressive neurodegeneration due to global selenoprotein deficiency [PMID:39753114].\",\n  \"teleology\": [\n    {\n      \"year\": 1990,\n      \"claim\": \"The discovery that prokaryotic SelB is a ribosome-associated elongation factor that specifically binds selenocysteyl-tRNA(Sec) established the first dedicated translational machinery for selenocysteine insertion at UGA codons.\",\n      \"evidence\": \"Protein purification, limited proteolysis, immunological analysis, and ribosome fractionation from E. coli\",\n      \"pmids\": [\"2140572\"],\n      \"confidence\": \"High\",\n      \"gaps\": [\"Eukaryotic ortholog not yet identified\", \"GTPase mechanism not characterized\", \"No structural information\"]\n    },\n    {\n      \"year\": 1991,\n      \"claim\": \"Identification of the tRNA structural determinants—the 8-bp acceptor helix and the selenocysteinyl moiety—that confer exclusive SelB binding explained how Sec-tRNA(Sec) is discriminated from all other aminoacyl-tRNAs and excluded from EF-Tu.\",\n      \"evidence\": \"In vitro binding assays with defined tRNA mutants and kinetic analysis of aminoacylation\",\n      \"pmids\": [\"1939093\", \"1839607\"],\n      \"confidence\": \"High\",\n      \"gaps\": [\"Thermodynamic coupling between aminoacyl identity and nucleotide state not quantified\", \"Structural basis of tRNA recognition unknown\"]\n    },\n    {\n      \"year\": 1993,\n      \"claim\": \"Demonstration that SelB directly binds the mRNA SECIS hairpin and forms a ternary complex with Sec-tRNA(Sec) and mRNA revealed how selenocysteine insertion is spatially coupled to the UGA codon in bacteria.\",\n      \"evidence\": \"Gel shift assays, nuclease and iodine footprinting, and toeprint assays on fdhF and fdnG mRNAs\",\n      \"pmids\": [\"8483932\", \"8314089\"],\n      \"confidence\": \"High\",\n      \"gaps\": [\"Unknown whether eukaryotic SelB orthologs also bind SECIS directly\", \"Ribosomal context of the complex not visualized\"]\n    },\n    {\n      \"year\": 1996,\n      \"claim\": \"Domain dissection and GTPase assays established that SelB's N-terminal EF-Tu-like domains bind GTP and tRNA while a distinct C-terminal domain binds SECIS mRNA, and that SECIS binding stimulates ribosome-dependent GTP hydrolysis, providing a conformational switch linking mRNA recognition to catalytic activation.\",\n      \"evidence\": \"Truncation analysis with functional binding assays, in vitro GTPase activity measurements with ribosome stimulation, and in vivo UGA readthrough quantification\",\n      \"pmids\": [\"8893853\", \"9454578\", \"8898393\"],\n      \"confidence\": \"High\",\n      \"gaps\": [\"Atomic structure of the full-length protein not available\", \"Mechanism of ribosome-stimulated GTPase activation unresolved\"]\n    },\n    {\n      \"year\": 2000,\n      \"claim\": \"Quantitative kinetics revealed that SelB binds GTP preferentially over GDP with fast GDP release (obviating an exchange factor), and characterization of the mammalian ortholog mSelB/eEFSec showed it retains GTP and Sec-tRNA(Sec) binding but cannot bind the SECIS alone, establishing that eukaryotic selenocysteine incorporation requires additional trans-acting factors.\",\n      \"evidence\": \"Stopped-flow fluorescence kinetics with fluorescent nucleotide analogs for prokaryotic SelB; GTP binding, tRNA binding, in vivo translation, and HeLa extract complementation for mouse eEFSec; archaeal SelB biochemical characterization\",\n      \"pmids\": [\"10781605\", \"10970870\", \"10860743\"],\n      \"confidence\": \"High\",\n      \"gaps\": [\"Identity of the eukaryotic factor(s) bridging eEFSec to the SECIS unknown\", \"Eukaryotic eEFSec structure not determined\"]\n    },\n    {\n      \"year\": 2002,\n      \"claim\": \"Crystal and NMR structures of the SelB C-terminal domain and its cognate SECIS RNA revealed a novel winged-helix RNA-binding mode and defined the critical RNA features (GpU tetraloop, bulge uracil) required for recognition, providing the first atomic view of how the mRNA-binding domain works.\",\n      \"evidence\": \"X-ray crystallography at 2.12 Å (M. thermoacetica WH domains) and NMR spectroscopy of the SECIS hairpin with mutational analysis\",\n      \"pmids\": [\"12145214\", \"12421564\"],\n      \"confidence\": \"High\",\n      \"gaps\": [\"No co-crystal of WH domain with SECIS RNA yet\", \"Communication between mRNA-binding and tRNA-binding halves of SelB not structurally resolved\"]\n    },\n    {\n      \"year\": 2005,\n      \"claim\": \"Co-crystal structures of the SelB WH domains bound to SECIS RNA demonstrated that RNA binding does not induce major protein conformational change but disrupts a salt bridge between WH2 and WH3, providing a molecular switch that communicates mRNA binding to the tRNA-binding N-terminal domains.\",\n      \"evidence\": \"X-ray crystallography of E. coli WH3/4 and M. thermoacetica WH1-4 bound to SECIS at 2.3–2.6 Å with site-directed mutagenesis\",\n      \"pmids\": [\"15665870\", \"17537456\"],\n      \"confidence\": \"High\",\n      \"gaps\": [\"Full-length SelB structure not available\", \"Mechanism of signal relay to GTPase domain unresolved\"]\n    },\n    {\n      \"year\": 2008,\n      \"claim\": \"In the eukaryotic system, SBP2 was shown to undergo a SECIS-induced conformational change that recruits eEFSec, establishing SBP2 as the obligate adaptor between the SECIS element and eEFSec and explaining why eEFSec itself cannot bind SECIS.\",\n      \"evidence\": \"Alanine scanning mutagenesis of SBP2, in vitro Sec incorporation assay, and trans-complementation of SBP2 domains\",\n      \"pmids\": [\"18948268\"],\n      \"confidence\": \"High\",\n      \"gaps\": [\"Structural basis of the SBP2–eEFSec interface unknown\", \"Whether additional factors stabilize the complex in vivo unresolved\"]\n    },\n    {\n      \"year\": 2009,\n      \"claim\": \"Quantitative thermodynamic analysis revealed that SelB·GTP binds Sec-tRNA(Sec) with picomolar affinity (Kd ~0.2 pM) and that GTP hydrolysis accelerates tRNA release by >10⁶-fold, establishing the energetic basis for high-fidelity selenocysteine delivery and discriminating Sec- from Ser-tRNA(Sec) through thermodynamic coupling.\",\n      \"evidence\": \"Stopped-flow fluorescence kinetics and isothermal titration calorimetry with purified components\",\n      \"pmids\": [\"19940162\"],\n      \"confidence\": \"High\",\n      \"gaps\": [\"Whether eukaryotic eEFSec achieves similar affinities not measured\", \"On-ribosome kinetics of tRNA release not directly captured\"]\n    },\n    {\n      \"year\": 2015,\n      \"claim\": \"The first full-length SelB crystal structure revealed the chalice-like seven-domain architecture (D1–D3 plus WHD1–4) and showed that the Sec-binding site at the D1–D2 interface is smaller and more exposed than EF-Tu's aminoacyl pocket, explaining selective accommodation of the unique tRNA(Sec) secondary structure.\",\n      \"evidence\": \"X-ray crystallography of A. aeolicus SelB with GTP analog at 3.2 Å and structural modeling of tRNA docking\",\n      \"pmids\": [\"26304550\"],\n      \"confidence\": \"High\",\n      \"gaps\": [\"No structure of the full-length SelB–tRNA complex\", \"Eukaryotic eEFSec full-length structure still missing\"]\n    },\n    {\n      \"year\": 2016,\n      \"claim\": \"Cryo-EM visualization of six intermediates on the bacterial UGA recoding pathway resolved the complete GTPase activation mechanism: codon recognition causes 30S closure that repositions Sec-tRNA(Sec) away from the sarcin–ricin loop and docks SelB onto it, triggering GTP hydrolysis.\",\n      \"evidence\": \"Single-particle cryo-EM of six structural intermediates of the E. coli ribosome–SelB complex with molecular dynamics\",\n      \"pmids\": [\"27842381\"],\n      \"confidence\": \"High\",\n      \"gaps\": [\"Eukaryotic ribosome recoding mechanism not yet visualized\", \"Post-GTP-hydrolysis accommodation steps not captured\"]\n    },\n    {\n      \"year\": 2022,\n      \"claim\": \"Cryo-EM of the mammalian ribosome decoding a Sec UGA codon revealed a fundamentally distinct eukaryotic mechanism: eEFSec and SBP2 do not contact each other directly but engage opposite ends of the SECIS, while ribosomal protein eS31 bridges Sec-tRNA(Sec) and SBP2; eEFSec was found to be indiscriminate toward L-serine, revealing a fidelity challenge in eukaryotic selenocysteine incorporation.\",\n      \"evidence\": \"Cryo-EM structural determination of the mammalian ribosome·eEFSec·SBP2·SECIS complex\",\n      \"pmids\": [\"35709277\"],\n      \"confidence\": \"High\",\n      \"gaps\": [\"Mechanism preventing serine misincorporation in vivo not identified\", \"Dynamics of SECIS delivery and factor recycling on the ribosome unknown\"]\n    },\n    {\n      \"year\": 2025,\n      \"claim\": \"Identification of bi-allelic EEFSEC loss-of-function variants in patients with progressive neurodegeneration established EEFSEC as essential for selenoprotein homeostasis in humans and defined a new Mendelian disease of selenoprotein deficiency.\",\n      \"evidence\": \"Exome/genome sequencing, in vitro functional assay of variants, selenoprotein Western blot in patient fibroblasts, and Drosophila eEFSec-RNAi motor/synaptic phenotyping\",\n      \"pmids\": [\"39753114\"],\n      \"confidence\": \"High\",\n      \"gaps\": [\"Tissue-specific consequences of partial EEFSEC loss not characterized\", \"Genotype–phenotype correlations across variants not established\", \"Whether neurodegeneration is driven by loss of specific selenoproteins versus global deficiency unknown\"]\n    },\n    {\n      \"year\": null,\n      \"claim\": \"Key open questions include: (1) how eukaryotic cells prevent eEFSec-mediated serine misincorporation at Sec UGA codons in vivo; (2) the structural basis of the SBP2–SECIS conformational change that recruits eEFSec domain 4; and (3) whether eEFSec has non-translational functions relevant to cancer cell proliferation.\",\n      \"evidence\": \"\",\n      \"pmids\": [],\n      \"confidence\": \"Low\",\n      \"gaps\": [\"No mechanism identified for fidelity proofreading of Sec versus Ser in eukaryotes\", \"No high-resolution structure of the SBP2 conformational switch\", \"Cancer cell phenotypes upon EEFSEC knockdown are correlative and from a single study\"]\n    }\n  ],\n  \"mechanism_profile\": {\n    \"molecular_activity\": [\n      {\"term_id\": \"GO:0003924\", \"supporting_discovery_ids\": [6, 9, 18, 20, 21]},\n      {\"term_id\": \"GO:0003723\", \"supporting_discovery_ids\": [3, 5, 12, 13, 15]},\n      {\"term_id\": \"GO:0045182\", \"supporting_discovery_ids\": [0, 10, 14, 22]}\n    ],\n    \"localization\": [\n      {\"term_id\": \"GO:0005840\", \"supporting_discovery_ids\": [0, 4, 20, 22]},\n      {\"term_id\": \"GO:0005829\", \"supporting_discovery_ids\": [10, 14]}\n    ],\n    \"pathway\": [\n      {\"term_id\": \"R-HSA-392499\", \"supporting_discovery_ids\": [0, 10, 14, 22, 23]}\n    ],\n    \"complexes\": [\n      \"eEFSec·Sec-tRNA(Sec)·GTP ternary complex\",\n      \"Ribosome·eEFSec·SBP2·SECIS decoding complex\"\n    ],\n    \"partners\": [\n      \"SBP2\",\n      \"Sec-tRNA(Sec)\",\n      \"eS31\",\n      \"SECIS RNA\"\n    ],\n    \"other_free_text\": []\n  }\n}\n```"}