{"gene":"RPS21","run_date":"2026-06-10T07:46:27","timeline":{"discoveries":[{"year":1981,"finding":"Ribosomal protein S21 is required for natural mRNA (MS2 RNA and E. coli mRNA) translation initiation but not for poly(U)/poly(A,G,U)-directed translation or AUG-directed fMet-tRNA binding. 30S subunits lacking S21 (by antibody inactivation or reconstitution) fail to bind MS2 RNA at the initiation step.","method":"In vitro reconstitution of 30S subunits lacking S21; antibody inactivation; mRNA-dependent initiation complex formation assay","journal":"European journal of biochemistry","confidence":"High","confidence_rationale":"Tier 1 / Strong — two orthogonal approaches (antibody inactivation and reconstitution) yielding consistent results in a single focused study","pmids":["7028483"],"is_preprint":false},{"year":1981,"finding":"Protein S21 is required for the 3' terminus of 16S rRNA to be accessible for intermolecular base-pairing (Shine-Dalgarno interaction). 30S subunits lacking S21 do not bind a complementary deoxyoctanucleotide to the 3' end of 16S RNA and cannot accept MS2 RNA as messenger; addition of S21 to S21-depleted subunits restores both activities.","method":"3'-end labeling of 16S RNA with [32P]pCp; RNase H/oligonucleotide probing; reconstitution of S21-depleted 30S subunits","journal":"Nucleic acids research","confidence":"High","confidence_rationale":"Tier 1 / Strong — direct biochemical reconstitution with multiple orthogonal readouts (oligonucleotide binding and mRNA acceptance), single focused study","pmids":["7232220"],"is_preprint":false},{"year":1987,"finding":"E. coli ribosomal protein S21 is cross-linked to 16S rRNA within nucleotides 693–697 (near the 690–720 loop region) and to the 3'-terminal region, establishing direct RNA-protein contact sites in the 30S subunit.","method":"Chemical cross-linking with bis-(2-chloroethyl)-methylamine followed by RNA-protein complex isolation and RNA sequencing","journal":"Nucleic acids research","confidence":"Medium","confidence_rationale":"Tier 2 / Moderate — two independent cross-linking reagent studies (PMIDs 2437528 and 2437527) identifying overlapping contact sites","pmids":["2437528","2437527"],"is_preprint":false},{"year":1987,"finding":"E. coli ribosomal protein S21 is cross-linked to the 3'-terminal dodecanucleotide of 16S rRNA, confirming proximity of S21 to the 3' end of 16S rRNA.","method":"Chemical cross-linking with methyl p-azidophenyl acetimidate followed by RNA-protein complex isolation and RNA sequencing","journal":"Nucleic acids research","confidence":"Medium","confidence_rationale":"Tier 2 / Moderate — replicated across two independent cross-linking methods","pmids":["2437527"],"is_preprint":false},{"year":1986,"finding":"E. coli ribosomal protein S21 is cross-linked to 16S rRNA at positions 723–724 and at the 3' end (positions 1531–1542) by 2-iminothiolane treatment, further defining two distinct contact sites on 16S rRNA.","method":"2-iminothiolane cross-linking with UV irradiation; RNA-protein complex isolation and RNA sequencing","journal":"Nucleic acids research","confidence":"Medium","confidence_rationale":"Tier 2 / Moderate — direct cross-linking experiment, consistent with other cross-linking studies","pmids":["3005967"],"is_preprint":false},{"year":1984,"finding":"E. coli ribosomal protein S21 binds to 50S ribosomal subunits as well as to 30S subunits, with relative binding affinities 70S > 30S > 50S. The apparent distance between a fluorescent probe on S21 and the 3' end of 16S rRNA increases from ~51 Å to ~61 Å upon addition of 50S subunits to 30S particles.","method":"Fluorescence anisotropy; glycerol gradient sedimentation; non-radiative energy transfer (FRET) measurements","journal":"Biochemistry","confidence":"Medium","confidence_rationale":"Tier 2 / Moderate — direct biophysical measurements with labeled protein, single lab","pmids":["6388639"],"is_preprint":false},{"year":1984,"finding":"Activation of 30S ribosomal subunits (by heating in 12 mM Mg2+) is associated with movement of the 3' end of 16S rRNA towards ribosomal protein S21, with an apparent distance decrease from 59 Å to 52 Å as measured by FRET.","method":"Non-radiative energy transfer (FRET) between fluorescent probes on 3'-end of 16S RNA and cysteine of S21","journal":"FEBS letters","confidence":"Medium","confidence_rationale":"Tier 2 / Moderate — direct biophysical measurement, consistent with companion study (PMID 6378638), single lab","pmids":["6378660"],"is_preprint":false},{"year":1984,"finding":"The apparent distances between S21 and the 3' end of 16S rRNA (~5.1 nm) and between S21 and S1 (~6.9 nm) in 30S subunits increase upon binding of poly(U) (mRNA analog) or 50S subunits, indicating conformational rearrangement at the decoding center upon mRNA and subunit association.","method":"Non-radiative energy transfer (FRET) with fluorescent probes on S1 cysteine, S21 cysteine, and 3' end of 16S rRNA","journal":"European journal of biochemistry","confidence":"Medium","confidence_rationale":"Tier 2 / Moderate — direct FRET measurements, multiple distance pairs, single lab","pmids":["6378636"],"is_preprint":false},{"year":1978,"finding":"E. coli ribosomal proteins S1, S11, and S21 directly participate in tRNA binding to the 30S ribosome, as unmodified versions of these proteins restore phe-tRNA binding activity to tetranitromethane-inactivated 30S particles.","method":"Chemical inactivation of 30S ribosomes with tetranitromethane followed by ribosome reconstitution with individual unmodified proteins; poly(U)-directed phe-tRNA binding assay","journal":"Nucleic acids research","confidence":"Medium","confidence_rationale":"Tier 2 / Moderate — reconstitution-based complementation assay, single lab","pmids":["25421"],"is_preprint":false},{"year":1999,"finding":"Drosophila RpS21 (ribosomal protein S21) is bound to native 40S ribosomal subunits in a salt-labile association and is absent from polysomes, suggesting it functions as a translation initiation factor rather than a core ribosomal protein. RpS21 interacts strongly with P40 (encoded by the stubarista/sta gene), a ribosomal peripheral protein.","method":"Sucrose gradient sedimentation of native ribosomes; polysome analysis; genetic interaction studies (double mutant analysis)","journal":"Molecular and cellular biology","confidence":"Medium","confidence_rationale":"Tier 2 / Moderate — biochemical fractionation plus genetic epistasis, single lab","pmids":["10022917"],"is_preprint":false},{"year":1999,"finding":"Human 40S ribosomal protein S21 (RPS21) directly binds LBP/p40 (laminin-binding protein precursor p40/ribosome-associated protein), as demonstrated by yeast two-hybrid screening and in vitro binding analysis. The interaction requires a broad, evolutionarily conserved region of LBP/p40.","method":"Yeast two-hybrid screening; in vitro binding assay","journal":"Biochemical and biophysical research communications","confidence":"Medium","confidence_rationale":"Tier 3 / Moderate — yeast two-hybrid plus in vitro binding, consistent with Drosophila genetic data (PMID 10022917)","pmids":["10079194"],"is_preprint":false},{"year":2003,"finding":"Yeast Rps21 (S. cerevisiae) is required for maturation of the 3' end of 18S rRNA and formation of active 40S ribosomal subunits. Disruption of both RPS21A and RPS21B genes is lethal; single disruptions reduce free 40S subunits, increase free 60S, decrease polysome size, and produce rRNA processing defects identical to those of Rps0 mutants.","method":"Gene disruption; sucrose gradient sedimentation; rRNA processing analysis by Northern blotting; tetrad dissection","journal":"Nucleic acids research","confidence":"High","confidence_rationale":"Tier 2 / Strong — multiple orthogonal methods (genetic, biochemical, rRNA processing), replicated across RPS21A and RPS21B paralogs","pmids":["14627813"],"is_preprint":false},{"year":2003,"finding":"Fission yeast (S. pombe) Rps0p physically associates with Rps21 protein; both genes are essential, and loss of either causes deficiency of 40S ribosomal subunit formation linked to insufficient 18S rRNA stability.","method":"Co-immunoprecipitation (Rps0 binding protein identification); tetrad dissection; inducible rescue strains; 40S subunit analysis","journal":"Biochemical and biophysical research communications","confidence":"Medium","confidence_rationale":"Tier 2 / Moderate — co-IP plus genetic and ribosome assembly data, single lab","pmids":["14623272"],"is_preprint":false},{"year":2023,"finding":"In yeast, rpS21/eS21 (together with rpS0/uS2 and rpS2/uS5 as the S0-cluster) is required for the initial recruitment of the pre-rRNA processing factor Nob1 and for final maturation of the central pseudoknot of 18S rRNA. Cryo-EM of S0-cluster expression mutants revealed hierarchical rRNA folding defects in cytoplasmic SSU precursors.","method":"Cryo-EM structural analysis of SSU precursors from rpS21/eS21 expression mutant yeast strains; unbiased 2'-O-methyl modification scoring","journal":"PloS one","confidence":"High","confidence_rationale":"Tier 1 / Moderate — cryo-EM structural analysis with multiple functional readouts (Nob1 recruitment, pseudoknot maturation), single lab but multiple orthogonal observations","pmids":["36996028"],"is_preprint":false},{"year":2024,"finding":"RPS21 modulates the ubiquitination and stability of GPX4 in hepatocellular carcinoma cells. Knockdown or overexpression of RPS21 significantly altered HCC cell proliferation and migration in cell lines and mouse tumor models.","method":"Lentiviral knockdown and overexpression; ubiquitination assay; GPX4 protein stability assay; mouse tumor models","journal":"Translational oncology","confidence":"Low","confidence_rationale":"Tier 3 / Weak — mechanistic claim about GPX4 ubiquitination stated but experimental details are sparse in abstract; single lab","pmids":["39546956"],"is_preprint":false},{"year":2020,"finding":"Knockdown of RPS21 in osteosarcoma cell lines suppresses cell proliferation, migration, and invasion, and inactivates the MAPK signaling pathway as shown by reduced phosphorylation of MAPK pathway proteins.","method":"siRNA knockdown; CCK-8 proliferation assay; colony formation; wound-healing and transwell invasion assays; Western blot for MAPK pathway proteins","journal":"Cancer management and research","confidence":"Low","confidence_rationale":"Tier 3 / Weak — single lab, single knockdown approach, no direct mechanistic dissection of RPS21-MAPK connection","pmids":["32612383"],"is_preprint":false},{"year":1988,"finding":"Ribosomal protein S21 of the E. coli 30S subunit is positioned by neutron scattering, completing the mapping of all 30S proteins. Its position is consistent with proximity to the 3' end of 16S rRNA.","method":"Neutron scattering distance measurements of protein pairs in the 30S subunit","journal":"Journal of molecular biology","confidence":"Medium","confidence_rationale":"Tier 1 / Moderate — neutron scattering structural mapping, single study completing the full 30S map","pmids":["3288761"],"is_preprint":false}],"current_model":"RPS21/eS21 is a small-subunit ribosomal protein (40S in eukaryotes, 30S in bacteria) that contacts the 3' end of small-subunit rRNA (16S/18S rRNA) and is required for natural mRNA translation initiation (by enabling Shine-Dalgarno base-pairing in bacteria) and for 18S rRNA 3'-end maturation and 40S subunit assembly in eukaryotes; in yeast it forms part of the S0-cluster whose assembly recruits the rRNA processing factor Nob1 and enables central pseudoknot folding, while in higher eukaryotes it physically interacts with the ribosome-associated protein LBP/p40 and its loss impairs cellular proliferation control."},"narrative":{"mechanistic_narrative":"RPS21/eS21 is a small-subunit ribosomal protein that contacts the 3' end of small-subunit rRNA and is required for natural mRNA translation initiation and for small-subunit rRNA maturation and 40S/30S assembly [PMID:7028483, PMID:14627813]. In bacteria, S21 is required for translation initiation on natural mRNAs but not for poly(U)- or AUG-directed model reactions, because it renders the 3' terminus of 16S rRNA accessible for Shine-Dalgarno base-pairing with mRNA; removing S21 abolishes both 3'-end oligonucleotide binding and mRNA acceptance, and re-adding it restores them [PMID:7028483, PMID:7232220]. Cross-linking and neutron-scattering studies place S21 in direct contact with 16S rRNA at the 690–724 region and at the 3'-terminal nucleotides, positioning it at the decoding/initiation surface of the 30S subunit, where mRNA binding and subunit joining drive conformational rearrangements of this region [PMID:2437528, PMID:2437527, PMID:3005967, PMID:6378636, PMID:3288761]. In eukaryotes, RPS21 is required for 3'-end maturation of 18S rRNA and formation of active 40S subunits: in budding yeast its loss reduces free 40S subunits and disrupts rRNA processing identically to Rps0 loss, and as part of the S0-cluster (with Rps0/uS2 and Rps2/uS5) it recruits the processing factor Nob1 and enables central pseudoknot folding in cytoplasmic SSU precursors [PMID:14627813, PMID:36996028]. RPS21 physically partners with the ribosome-associated protein LBP/p40 (stubarista/Sta in Drosophila) [PMID:10022917, PMID:10079194].","teleology":[{"year":1981,"claim":"Established that S21 is functionally specialized for the initiation step on real mRNAs rather than being a generic structural subunit, by showing it is dispensable for model-template translation but essential for natural mRNA initiation.","evidence":"In vitro reconstitution and antibody inactivation of S21-deficient 30S subunits with mRNA-dependent initiation assays in E. coli","pmids":["7028483"],"confidence":"High","gaps":["Did not resolve the molecular basis by which S21 enables mRNA binding","Limited to bacterial system"]},{"year":1981,"claim":"Provided the mechanism for S21's initiation role by showing it makes the 3' end of 16S rRNA accessible for Shine-Dalgarno pairing, linking a structural rRNA effect to mRNA acceptance.","evidence":"3'-end labeling, oligonucleotide probing, and reconstitution of S21-depleted 30S subunits in E. coli","pmids":["7232220"],"confidence":"High","gaps":["Indirect inference of conformational state from oligonucleotide accessibility","No high-resolution structure"]},{"year":1986,"claim":"Mapped the physical contacts of S21 on 16S rRNA, defining where on the subunit S21 exerts its effect on the 3' end and decoding region.","evidence":"Multiple chemical cross-linking reagents (bis-chloroethyl-methylamine, azidophenyl acetimidate, 2-iminothiolane) plus neutron scattering positioning in the 30S subunit","pmids":["2437528","2437527","3005967","3288761"],"confidence":"Medium","gaps":["Cross-link sites are proximity not atomic contact maps","Does not establish causality between contacts and function"]},{"year":1984,"claim":"Showed the S21–3'-end region undergoes conformational rearrangement during activation, mRNA binding, and subunit joining, framing S21 as a dynamic sensor of initiation state.","evidence":"FRET and fluorescence anisotropy between probes on S21, S1, and the 3' end of 16S rRNA in E. coli 30S subunits","pmids":["6388639","6378660","6378636"],"confidence":"Medium","gaps":["Distance changes are apparent/ensemble measurements","Functional consequence of each conformational state not directly tested"]},{"year":1999,"claim":"Identified a conserved eukaryotic physical partner (LBP/p40, stubarista/Sta) and indicated RPS21 associates loosely with 40S subunits, extending its role beyond bacteria.","evidence":"Drosophila sucrose gradient/polysome analysis with genetic epistasis; human yeast two-hybrid and in vitro binding","pmids":["10022917","10079194"],"confidence":"Medium","gaps":["Functional consequence of the RPS21–LBP/p40 interaction not defined","Salt-labile association interpretation as initiation factor not structurally confirmed"]},{"year":2003,"claim":"Demonstrated that eukaryotic Rps21 is required for 18S rRNA 3'-end maturation and 40S subunit formation, establishing a conserved assembly/processing role parallel to its bacterial function.","evidence":"Gene disruption, sucrose gradient sedimentation, Northern rRNA processing analysis in S. cerevisiae; co-IP and genetics in S. pombe","pmids":["14627813","14623272"],"confidence":"High","gaps":["Did not resolve the assembly hierarchy or recruited factors","Mechanism of 18S 3'-end maturation contribution unspecified"]},{"year":2023,"claim":"Defined the structural mechanism of eukaryotic Rps21 in assembly by placing it in the S0-cluster that recruits Nob1 and enables central pseudoknot folding during SSU maturation.","evidence":"Cryo-EM of SSU precursors from S0-cluster expression mutants with 2'-O-methyl modification scoring in yeast","pmids":["36996028"],"confidence":"High","gaps":["Individual contribution of Rps21 versus other S0-cluster members not fully separated","Single lab"]},{"year":2024,"claim":"Proposed an extra-ribosomal disease-associated role linking RPS21 to GPX4 stability and tumor cell proliferation, beyond canonical ribosome assembly.","evidence":"Lentiviral knockdown/overexpression, ubiquitination and GPX4 stability assays, and mouse tumor models in hepatocellular carcinoma; siRNA and MAPK readouts in osteosarcoma","pmids":["39546956","32612383"],"confidence":"Low","gaps":["Mechanistic detail of RPS21-driven GPX4 ubiquitination is sparse","RPS21–MAPK link not directly dissected","Effects may be secondary to general ribosome/translation perturbation"]},{"year":null,"claim":"How RPS21's ribosome-assembly function relates to its reported extra-ribosomal effects on protein stability and proliferation control remains unresolved.","evidence":"","pmids":[],"confidence":"Low","gaps":["No structure of the mammalian RPS21–LBP/p40 complex","No direct demonstration that GPX4/MAPK effects are independent of ribosome assembly","No mammalian in vivo loss-of-function assembly data in the corpus"]}],"mechanism_profile":{"molecular_activity":[{"term_id":"GO:0003723","term_label":"RNA binding","supporting_discovery_ids":[1,2,4]},{"term_id":"GO:0005198","term_label":"structural molecule activity","supporting_discovery_ids":[0,11,16]}],"localization":[{"term_id":"GO:0005840","term_label":"ribosome","supporting_discovery_ids":[0,9,11]},{"term_id":"GO:0005829","term_label":"cytosol","supporting_discovery_ids":[9,13]}],"pathway":[{"term_id":"R-HSA-8953854","term_label":"Metabolism of RNA","supporting_discovery_ids":[11,13]},{"term_id":"R-HSA-392499","term_label":"Metabolism of proteins","supporting_discovery_ids":[0,1]},{"term_id":"R-HSA-1852241","term_label":"Organelle biogenesis and maintenance","supporting_discovery_ids":[11,13]}],"complexes":["40S ribosomal subunit","30S ribosomal subunit","S0-cluster"],"partners":["RPS0","RPS2","NOB1","LBP/P40","RPS1","RPS11"],"other_free_text":[]}},"prefetch_data":{"uniprot":{"accession":"P63220","full_name":"Small ribosomal subunit protein eS21","aliases":["40S ribosomal protein S21"],"length_aa":83,"mass_kda":9.1,"function":"Component of the small ribosomal subunit (PubMed:23636399, PubMed:25901680, PubMed:25957688). The ribosome is a large ribonucleoprotein complex responsible for the synthesis of proteins in the cell (PubMed:23636399, PubMed:25901680, PubMed:25957688)","subcellular_location":"Cytoplasm, cytosol; Cytoplasm; Rough endoplasmic reticulum","url":"https://www.uniprot.org/uniprotkb/P63220/entry"},"depmap":{"release":"DepMap","has_data":true,"is_common_essential":true,"resolved_as":"","url":"https://depmap.org/portal/gene/RPS21","classification":"Common Essential","n_dependent_lines":1184,"n_total_lines":1208,"dependency_fraction":0.9801324503311258},"opencell":{"profiled":false,"resolved_as":"","ensg_id":"","cell_line_id":"","localizations":[],"interactors":[{"gene":"EIF2S3","stoichiometry":10.0},{"gene":"ENY2","stoichiometry":10.0},{"gene":"G3BP1","stoichiometry":10.0},{"gene":"RACK1","stoichiometry":10.0},{"gene":"RBM8A","stoichiometry":10.0},{"gene":"RPL11","stoichiometry":10.0},{"gene":"RPL4","stoichiometry":10.0},{"gene":"RPL5","stoichiometry":10.0},{"gene":"RPS16","stoichiometry":10.0},{"gene":"SRP9","stoichiometry":10.0}],"url":"https://opencell.sf.czbiohub.org/search/RPS21","total_profiled":1310},"omim":[{"mim_id":"180477","title":"RIBOSOMAL PROTEIN S21; RPS21","url":"https://www.omim.org/entry/180477"}],"hpa":{"profiled":true,"resolved_as":"","reliability":"Supported","locations":[{"location":"Endoplasmic reticulum","reliability":"Supported"},{"location":"Cytosol","reliability":"Supported"}],"tissue_specificity":"Low tissue specificity","tissue_distribution":"Detected in all","driving_tissues":[],"url":"https://www.proteinatlas.org/search/RPS21"},"hgnc":{"alias_symbol":["S21","eS21"],"prev_symbol":[]},"alphafold":{"accession":"P63220","domains":[{"cath_id":"3.30.1230.20","chopping":"15-83","consensus_level":"medium","plddt":95.7803,"start":15,"end":83}],"viewer_url":"https://alphafold.ebi.ac.uk/entry/P63220","model_url":"https://alphafold.ebi.ac.uk/files/AF-P63220-F1-model_v6.cif","pae_url":"https://alphafold.ebi.ac.uk/files/AF-P63220-F1-predicted_aligned_error_v6.png","plddt_mean":95.5},"mouse_models":{"mgi_url":"https://www.informatics.jax.org/marker/summary?nomen=RPS21","jax_strain_url":"https://www.jax.org/strain/search?query=RPS21"},"sequence":{"accession":"P63220","fasta_url":"https://rest.uniprot.org/uniprotkb/P63220.fasta","uniprot_url":"https://www.uniprot.org/uniprotkb/P63220/entry","alphafold_viewer_url":"https://alphafold.ebi.ac.uk/entry/P63220"}},"corpus_meta":[{"pmid":"6186393","id":"PMC_6186393","title":"The operon that encodes the sigma subunit of RNA polymerase also encodes ribosomal protein S21 and DNA primase in E. coli K12.","date":"1983","source":"Cell","url":"https://pubmed.ncbi.nlm.nih.gov/6186393","citation_count":237,"is_preprint":false},{"pmid":"2459389","id":"PMC_2459389","title":"Interaction of ribosomal proteins S5, S6, S11, S12, S18 and S21 with 16 S rRNA.","date":"1988","source":"Journal of molecular biology","url":"https://pubmed.ncbi.nlm.nih.gov/2459389","citation_count":94,"is_preprint":false},{"pmid":"3288761","id":"PMC_3288761","title":"Positions of S2, S13, S16, S17, S19 and S21 in the 30 S ribosomal subunit of Escherichia coli.","date":"1988","source":"Journal of molecular biology","url":"https://pubmed.ncbi.nlm.nih.gov/3288761","citation_count":88,"is_preprint":false},{"pmid":"7232220","id":"PMC_7232220","title":"Basepairing potential of the 3' terminus of 16S RNA: dependence on the functional state of the 30S subunit and the presence of protein S21.","date":"1981","source":"Nucleic acids research","url":"https://pubmed.ncbi.nlm.nih.gov/7232220","citation_count":81,"is_preprint":false},{"pmid":"925037","id":"PMC_925037","title":"Isolation of eukaryotic ribosomal proteins. Purification and characterization of the 40 S ribosomal subunit proteins Sa, Sc, S3a, S3b, S5', S9, S10, S11, S12, S14, S15, S15', S16, S17, S18, S19, S20, S21, S26, S27', and S29.","date":"1977","source":"The Journal of biological chemistry","url":"https://pubmed.ncbi.nlm.nih.gov/925037","citation_count":60,"is_preprint":false},{"pmid":"19587381","id":"PMC_19587381","title":"M-CSF elevates c-Fos and phospho-C/EBPalpha(S21) via ERK whereas G-CSF stimulates SHP2 phosphorylation in marrow progenitors to contribute to myeloid lineage specification.","date":"2009","source":"Blood","url":"https://pubmed.ncbi.nlm.nih.gov/19587381","citation_count":55,"is_preprint":false},{"pmid":"2437528","id":"PMC_2437528","title":"RNA-protein cross-linking in Escherichia coli 30S ribosomal subunits; determination of sites on 16S RNA that are cross-linked to proteins S3, S4, S7, S9, S10, S11, S17, S18 and S21 by treatment with bis-(2-chloroethyl)-methylamine.","date":"1987","source":"Nucleic acids research","url":"https://pubmed.ncbi.nlm.nih.gov/2437528","citation_count":54,"is_preprint":false},{"pmid":"2437527","id":"PMC_2437527","title":"RNA-protein cross-linking in Escherichia coli 30S ribosomal subunits; 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ES2-1 Regulates Molecular Mechanisms of 17β-Estradiol Biodegradation.","date":"2024","source":"Environmental science & technology","url":"https://pubmed.ncbi.nlm.nih.gov/39636603","citation_count":6,"is_preprint":false},{"pmid":"35920767","id":"PMC_35920767","title":"Allele-specific alternative splicing of Drosophila Ribosomal protein S21 suppresses a lethal mutation in the Phosphorylated adaptor for RNA export (Phax) gene.","date":"2022","source":"G3 (Bethesda, Md.)","url":"https://pubmed.ncbi.nlm.nih.gov/35920767","citation_count":4,"is_preprint":false},{"pmid":"39546956","id":"PMC_39546956","title":"RPS21 Enhances hepatocellular carcinoma development through GPX4 stabilization.","date":"2024","source":"Translational oncology","url":"https://pubmed.ncbi.nlm.nih.gov/39546956","citation_count":4,"is_preprint":false},{"pmid":"7654221","id":"PMC_7654221","title":"The primary structures of rat ribosomal proteins: the characterization of the cDNAs for S21 and L39, corrections in the sequences of L7 and L18a, and the identification of L33.","date":"1995","source":"Biochemical and biophysical research communications","url":"https://pubmed.ncbi.nlm.nih.gov/7654221","citation_count":4,"is_preprint":false},{"pmid":"34906602","id":"PMC_34906602","title":"Probing the local secondary structure of bacteriophage S21 pinholin membrane protein using electron spin echo envelope modulation spectroscopy.","date":"2021","source":"Biochimica et biophysica acta. 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antibody inactivation; mRNA-dependent initiation complex formation assay\",\n      \"journal\": \"European journal of biochemistry\",\n      \"confidence\": \"High\",\n      \"confidence_rationale\": \"Tier 1 / Strong — two orthogonal approaches (antibody inactivation and reconstitution) yielding consistent results in a single focused study\",\n      \"pmids\": [\"7028483\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 1981,\n      \"finding\": \"Protein S21 is required for the 3' terminus of 16S rRNA to be accessible for intermolecular base-pairing (Shine-Dalgarno interaction). 30S subunits lacking S21 do not bind a complementary deoxyoctanucleotide to the 3' end of 16S RNA and cannot accept MS2 RNA as messenger; addition of S21 to S21-depleted subunits restores both activities.\",\n      \"method\": \"3'-end labeling of 16S RNA with [32P]pCp; RNase H/oligonucleotide probing; reconstitution of S21-depleted 30S subunits\",\n      \"journal\": \"Nucleic acids research\",\n      \"confidence\": \"High\",\n      \"confidence_rationale\": \"Tier 1 / Strong — direct biochemical reconstitution with multiple orthogonal readouts (oligonucleotide binding and mRNA acceptance), single focused study\",\n      \"pmids\": [\"7232220\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 1987,\n      \"finding\": \"E. coli ribosomal protein S21 is cross-linked to 16S rRNA within nucleotides 693–697 (near the 690–720 loop region) and to the 3'-terminal region, establishing direct RNA-protein contact sites in the 30S subunit.\",\n      \"method\": \"Chemical cross-linking with bis-(2-chloroethyl)-methylamine followed by RNA-protein complex isolation and RNA sequencing\",\n      \"journal\": \"Nucleic acids research\",\n      \"confidence\": \"Medium\",\n      \"confidence_rationale\": \"Tier 2 / Moderate — two independent cross-linking reagent studies (PMIDs 2437528 and 2437527) identifying overlapping contact sites\",\n      \"pmids\": [\"2437528\", \"2437527\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 1987,\n      \"finding\": \"E. coli ribosomal protein S21 is cross-linked to the 3'-terminal dodecanucleotide of 16S rRNA, confirming proximity of S21 to the 3' end of 16S rRNA.\",\n      \"method\": \"Chemical cross-linking with methyl p-azidophenyl acetimidate followed by RNA-protein complex isolation and RNA sequencing\",\n      \"journal\": \"Nucleic acids research\",\n      \"confidence\": \"Medium\",\n      \"confidence_rationale\": \"Tier 2 / Moderate — replicated across two independent cross-linking methods\",\n      \"pmids\": [\"2437527\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 1986,\n      \"finding\": \"E. coli ribosomal protein S21 is cross-linked to 16S rRNA at positions 723–724 and at the 3' end (positions 1531–1542) by 2-iminothiolane treatment, further defining two distinct contact sites on 16S rRNA.\",\n      \"method\": \"2-iminothiolane cross-linking with UV irradiation; RNA-protein complex isolation and RNA sequencing\",\n      \"journal\": \"Nucleic acids research\",\n      \"confidence\": \"Medium\",\n      \"confidence_rationale\": \"Tier 2 / Moderate — direct cross-linking experiment, consistent with other cross-linking studies\",\n      \"pmids\": [\"3005967\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 1984,\n      \"finding\": \"E. coli ribosomal protein S21 binds to 50S ribosomal subunits as well as to 30S subunits, with relative binding affinities 70S > 30S > 50S. The apparent distance between a fluorescent probe on S21 and the 3' end of 16S rRNA increases from ~51 Å to ~61 Å upon addition of 50S subunits to 30S particles.\",\n      \"method\": \"Fluorescence anisotropy; glycerol gradient sedimentation; non-radiative energy transfer (FRET) measurements\",\n      \"journal\": \"Biochemistry\",\n      \"confidence\": \"Medium\",\n      \"confidence_rationale\": \"Tier 2 / Moderate — direct biophysical measurements with labeled protein, single lab\",\n      \"pmids\": [\"6388639\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 1984,\n      \"finding\": \"Activation of 30S ribosomal subunits (by heating in 12 mM Mg2+) is associated with movement of the 3' end of 16S rRNA towards ribosomal protein S21, with an apparent distance decrease from 59 Å to 52 Å as measured by FRET.\",\n      \"method\": \"Non-radiative energy transfer (FRET) between fluorescent probes on 3'-end of 16S RNA and cysteine of S21\",\n      \"journal\": \"FEBS letters\",\n      \"confidence\": \"Medium\",\n      \"confidence_rationale\": \"Tier 2 / Moderate — direct biophysical measurement, consistent with companion study (PMID 6378638), single lab\",\n      \"pmids\": [\"6378660\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 1984,\n      \"finding\": \"The apparent distances between S21 and the 3' end of 16S rRNA (~5.1 nm) and between S21 and S1 (~6.9 nm) in 30S subunits increase upon binding of poly(U) (mRNA analog) or 50S subunits, indicating conformational rearrangement at the decoding center upon mRNA and subunit association.\",\n      \"method\": \"Non-radiative energy transfer (FRET) with fluorescent probes on S1 cysteine, S21 cysteine, and 3' end of 16S rRNA\",\n      \"journal\": \"European journal of biochemistry\",\n      \"confidence\": \"Medium\",\n      \"confidence_rationale\": \"Tier 2 / Moderate — direct FRET measurements, multiple distance pairs, single lab\",\n      \"pmids\": [\"6378636\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 1978,\n      \"finding\": \"E. coli ribosomal proteins S1, S11, and S21 directly participate in tRNA binding to the 30S ribosome, as unmodified versions of these proteins restore phe-tRNA binding activity to tetranitromethane-inactivated 30S particles.\",\n      \"method\": \"Chemical inactivation of 30S ribosomes with tetranitromethane followed by ribosome reconstitution with individual unmodified proteins; poly(U)-directed phe-tRNA binding assay\",\n      \"journal\": \"Nucleic acids research\",\n      \"confidence\": \"Medium\",\n      \"confidence_rationale\": \"Tier 2 / Moderate — reconstitution-based complementation assay, single lab\",\n      \"pmids\": [\"25421\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 1999,\n      \"finding\": \"Drosophila RpS21 (ribosomal protein S21) is bound to native 40S ribosomal subunits in a salt-labile association and is absent from polysomes, suggesting it functions as a translation initiation factor rather than a core ribosomal protein. RpS21 interacts strongly with P40 (encoded by the stubarista/sta gene), a ribosomal peripheral protein.\",\n      \"method\": \"Sucrose gradient sedimentation of native ribosomes; polysome analysis; genetic interaction studies (double mutant analysis)\",\n      \"journal\": \"Molecular and cellular biology\",\n      \"confidence\": \"Medium\",\n      \"confidence_rationale\": \"Tier 2 / Moderate — biochemical fractionation plus genetic epistasis, single lab\",\n      \"pmids\": [\"10022917\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 1999,\n      \"finding\": \"Human 40S ribosomal protein S21 (RPS21) directly binds LBP/p40 (laminin-binding protein precursor p40/ribosome-associated protein), as demonstrated by yeast two-hybrid screening and in vitro binding analysis. The interaction requires a broad, evolutionarily conserved region of LBP/p40.\",\n      \"method\": \"Yeast two-hybrid screening; in vitro binding assay\",\n      \"journal\": \"Biochemical and biophysical research communications\",\n      \"confidence\": \"Medium\",\n      \"confidence_rationale\": \"Tier 3 / Moderate — yeast two-hybrid plus in vitro binding, consistent with Drosophila genetic data (PMID 10022917)\",\n      \"pmids\": [\"10079194\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2003,\n      \"finding\": \"Yeast Rps21 (S. cerevisiae) is required for maturation of the 3' end of 18S rRNA and formation of active 40S ribosomal subunits. Disruption of both RPS21A and RPS21B genes is lethal; single disruptions reduce free 40S subunits, increase free 60S, decrease polysome size, and produce rRNA processing defects identical to those of Rps0 mutants.\",\n      \"method\": \"Gene disruption; sucrose gradient sedimentation; rRNA processing analysis by Northern blotting; tetrad dissection\",\n      \"journal\": \"Nucleic acids research\",\n      \"confidence\": \"High\",\n      \"confidence_rationale\": \"Tier 2 / Strong — multiple orthogonal methods (genetic, biochemical, rRNA processing), replicated across RPS21A and RPS21B paralogs\",\n      \"pmids\": [\"14627813\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2003,\n      \"finding\": \"Fission yeast (S. pombe) Rps0p physically associates with Rps21 protein; both genes are essential, and loss of either causes deficiency of 40S ribosomal subunit formation linked to insufficient 18S rRNA stability.\",\n      \"method\": \"Co-immunoprecipitation (Rps0 binding protein identification); tetrad dissection; inducible rescue strains; 40S subunit analysis\",\n      \"journal\": \"Biochemical and biophysical research communications\",\n      \"confidence\": \"Medium\",\n      \"confidence_rationale\": \"Tier 2 / Moderate — co-IP plus genetic and ribosome assembly data, single lab\",\n      \"pmids\": [\"14623272\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2023,\n      \"finding\": \"In yeast, rpS21/eS21 (together with rpS0/uS2 and rpS2/uS5 as the S0-cluster) is required for the initial recruitment of the pre-rRNA processing factor Nob1 and for final maturation of the central pseudoknot of 18S rRNA. Cryo-EM of S0-cluster expression mutants revealed hierarchical rRNA folding defects in cytoplasmic SSU precursors.\",\n      \"method\": \"Cryo-EM structural analysis of SSU precursors from rpS21/eS21 expression mutant yeast strains; unbiased 2'-O-methyl modification scoring\",\n      \"journal\": \"PloS one\",\n      \"confidence\": \"High\",\n      \"confidence_rationale\": \"Tier 1 / Moderate — cryo-EM structural analysis with multiple functional readouts (Nob1 recruitment, pseudoknot maturation), single lab but multiple orthogonal observations\",\n      \"pmids\": [\"36996028\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2024,\n      \"finding\": \"RPS21 modulates the ubiquitination and stability of GPX4 in hepatocellular carcinoma cells. Knockdown or overexpression of RPS21 significantly altered HCC cell proliferation and migration in cell lines and mouse tumor models.\",\n      \"method\": \"Lentiviral knockdown and overexpression; ubiquitination assay; GPX4 protein stability assay; mouse tumor models\",\n      \"journal\": \"Translational oncology\",\n      \"confidence\": \"Low\",\n      \"confidence_rationale\": \"Tier 3 / Weak — mechanistic claim about GPX4 ubiquitination stated but experimental details are sparse in abstract; single lab\",\n      \"pmids\": [\"39546956\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2020,\n      \"finding\": \"Knockdown of RPS21 in osteosarcoma cell lines suppresses cell proliferation, migration, and invasion, and inactivates the MAPK signaling pathway as shown by reduced phosphorylation of MAPK pathway proteins.\",\n      \"method\": \"siRNA knockdown; CCK-8 proliferation assay; colony formation; wound-healing and transwell invasion assays; Western blot for MAPK pathway proteins\",\n      \"journal\": \"Cancer management and research\",\n      \"confidence\": \"Low\",\n      \"confidence_rationale\": \"Tier 3 / Weak — single lab, single knockdown approach, no direct mechanistic dissection of RPS21-MAPK connection\",\n      \"pmids\": [\"32612383\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 1988,\n      \"finding\": \"Ribosomal protein S21 of the E. coli 30S subunit is positioned by neutron scattering, completing the mapping of all 30S proteins. Its position is consistent with proximity to the 3' end of 16S rRNA.\",\n      \"method\": \"Neutron scattering distance measurements of protein pairs in the 30S subunit\",\n      \"journal\": \"Journal of molecular biology\",\n      \"confidence\": \"Medium\",\n      \"confidence_rationale\": \"Tier 1 / Moderate — neutron scattering structural mapping, single study completing the full 30S map\",\n      \"pmids\": [\"3288761\"],\n      \"is_preprint\": false\n    }\n  ],\n  \"current_model\": \"RPS21/eS21 is a small-subunit ribosomal protein (40S in eukaryotes, 30S in bacteria) that contacts the 3' end of small-subunit rRNA (16S/18S rRNA) and is required for natural mRNA translation initiation (by enabling Shine-Dalgarno base-pairing in bacteria) and for 18S rRNA 3'-end maturation and 40S subunit assembly in eukaryotes; in yeast it forms part of the S0-cluster whose assembly recruits the rRNA processing factor Nob1 and enables central pseudoknot folding, while in higher eukaryotes it physically interacts with the ribosome-associated protein LBP/p40 and its loss impairs cellular proliferation control.\"\n}\n```","stage2_raw":"{\n  \"mechanistic_narrative\": \"RPS21/eS21 is a small-subunit ribosomal protein that contacts the 3' end of small-subunit rRNA and is required for natural mRNA translation initiation and for small-subunit rRNA maturation and 40S/30S assembly [#0, #11]. In bacteria, S21 is required for translation initiation on natural mRNAs but not for poly(U)- or AUG-directed model reactions, because it renders the 3' terminus of 16S rRNA accessible for Shine-Dalgarno base-pairing with mRNA; removing S21 abolishes both 3'-end oligonucleotide binding and mRNA acceptance, and re-adding it restores them [#0, #1]. Cross-linking and neutron-scattering studies place S21 in direct contact with 16S rRNA at the 690\\u2013724 region and at the 3'-terminal nucleotides, positioning it at the decoding/initiation surface of the 30S subunit, where mRNA binding and subunit joining drive conformational rearrangements of this region [#2, #4, #7, #16]. In eukaryotes, RPS21 is required for 3'-end maturation of 18S rRNA and formation of active 40S subunits: in budding yeast its loss reduces free 40S subunits and disrupts rRNA processing identically to Rps0 loss, and as part of the S0-cluster (with Rps0/uS2 and Rps2/uS5) it recruits the processing factor Nob1 and enables central pseudoknot folding in cytoplasmic SSU precursors [#11, #13]. RPS21 physically partners with the ribosome-associated protein LBP/p40 (stubarista/Sta in Drosophila) [#9, #10].\",\n  \"teleology\": [\n    {\n      \"year\": 1981,\n      \"claim\": \"Established that S21 is functionally specialized for the initiation step on real mRNAs rather than being a generic structural subunit, by showing it is dispensable for model-template translation but essential for natural mRNA initiation.\",\n      \"evidence\": \"In vitro reconstitution and antibody inactivation of S21-deficient 30S subunits with mRNA-dependent initiation assays in E. coli\",\n      \"pmids\": [\"7028483\"],\n      \"confidence\": \"High\",\n      \"gaps\": [\"Did not resolve the molecular basis by which S21 enables mRNA binding\", \"Limited to bacterial system\"]\n    },\n    {\n      \"year\": 1981,\n      \"claim\": \"Provided the mechanism for S21's initiation role by showing it makes the 3' end of 16S rRNA accessible for Shine-Dalgarno pairing, linking a structural rRNA effect to mRNA acceptance.\",\n      \"evidence\": \"3'-end labeling, oligonucleotide probing, and reconstitution of S21-depleted 30S subunits in E. coli\",\n      \"pmids\": [\"7232220\"],\n      \"confidence\": \"High\",\n      \"gaps\": [\"Indirect inference of conformational state from oligonucleotide accessibility\", \"No high-resolution structure\"]\n    },\n    {\n      \"year\": 1986,\n      \"claim\": \"Mapped the physical contacts of S21 on 16S rRNA, defining where on the subunit S21 exerts its effect on the 3' end and decoding region.\",\n      \"evidence\": \"Multiple chemical cross-linking reagents (bis-chloroethyl-methylamine, azidophenyl acetimidate, 2-iminothiolane) plus neutron scattering positioning in the 30S subunit\",\n      \"pmids\": [\"2437528\", \"2437527\", \"3005967\", \"3288761\"],\n      \"confidence\": \"Medium\",\n      \"gaps\": [\"Cross-link sites are proximity not atomic contact maps\", \"Does not establish causality between contacts and function\"]\n    },\n    {\n      \"year\": 1984,\n      \"claim\": \"Showed the S21\\u20133'-end region undergoes conformational rearrangement during activation, mRNA binding, and subunit joining, framing S21 as a dynamic sensor of initiation state.\",\n      \"evidence\": \"FRET and fluorescence anisotropy between probes on S21, S1, and the 3' end of 16S rRNA in E. coli 30S subunits\",\n      \"pmids\": [\"6388639\", \"6378660\", \"6378636\"],\n      \"confidence\": \"Medium\",\n      \"gaps\": [\"Distance changes are apparent/ensemble measurements\", \"Functional consequence of each conformational state not directly tested\"]\n    },\n    {\n      \"year\": 1999,\n      \"claim\": \"Identified a conserved eukaryotic physical partner (LBP/p40, stubarista/Sta) and indicated RPS21 associates loosely with 40S subunits, extending its role beyond bacteria.\",\n      \"evidence\": \"Drosophila sucrose gradient/polysome analysis with genetic epistasis; human yeast two-hybrid and in vitro binding\",\n      \"pmids\": [\"10022917\", \"10079194\"],\n      \"confidence\": \"Medium\",\n      \"gaps\": [\"Functional consequence of the RPS21\\u2013LBP/p40 interaction not defined\", \"Salt-labile association interpretation as initiation factor not structurally confirmed\"]\n    },\n    {\n      \"year\": 2003,\n      \"claim\": \"Demonstrated that eukaryotic Rps21 is required for 18S rRNA 3'-end maturation and 40S subunit formation, establishing a conserved assembly/processing role parallel to its bacterial function.\",\n      \"evidence\": \"Gene disruption, sucrose gradient sedimentation, Northern rRNA processing analysis in S. cerevisiae; co-IP and genetics in S. pombe\",\n      \"pmids\": [\"14627813\", \"14623272\"],\n      \"confidence\": \"High\",\n      \"gaps\": [\"Did not resolve the assembly hierarchy or recruited factors\", \"Mechanism of 18S 3'-end maturation contribution unspecified\"]\n    },\n    {\n      \"year\": 2023,\n      \"claim\": \"Defined the structural mechanism of eukaryotic Rps21 in assembly by placing it in the S0-cluster that recruits Nob1 and enables central pseudoknot folding during SSU maturation.\",\n      \"evidence\": \"Cryo-EM of SSU precursors from S0-cluster expression mutants with 2'-O-methyl modification scoring in yeast\",\n      \"pmids\": [\"36996028\"],\n      \"confidence\": \"High\",\n      \"gaps\": [\"Individual contribution of Rps21 versus other S0-cluster members not fully separated\", \"Single lab\"]\n    },\n    {\n      \"year\": 2024,\n      \"claim\": \"Proposed an extra-ribosomal disease-associated role linking RPS21 to GPX4 stability and tumor cell proliferation, beyond canonical ribosome assembly.\",\n      \"evidence\": \"Lentiviral knockdown/overexpression, ubiquitination and GPX4 stability assays, and mouse tumor models in hepatocellular carcinoma; siRNA and MAPK readouts in osteosarcoma\",\n      \"pmids\": [\"39546956\", \"32612383\"],\n      \"confidence\": \"Low\",\n      \"gaps\": [\"Mechanistic detail of RPS21-driven GPX4 ubiquitination is sparse\", \"RPS21\\u2013MAPK link not directly dissected\", \"Effects may be secondary to general ribosome/translation perturbation\"]\n    },\n    {\n      \"year\": null,\n      \"claim\": \"How RPS21's ribosome-assembly function relates to its reported extra-ribosomal effects on protein stability and proliferation control remains unresolved.\",\n      \"evidence\": \"\",\n      \"pmids\": [],\n      \"confidence\": \"Low\",\n      \"gaps\": [\"No structure of the mammalian RPS21\\u2013LBP/p40 complex\", \"No direct demonstration that GPX4/MAPK effects are independent of ribosome assembly\", \"No mammalian in vivo loss-of-function assembly data in the corpus\"]\n    }\n  ],\n  \"mechanism_profile\": {\n    \"molecular_activity\": [\n      {\"term_id\": \"GO:0003723\", \"supporting_discovery_ids\": [1, 2, 4]},\n      {\"term_id\": \"GO:0005198\", \"supporting_discovery_ids\": [0, 11, 16]}\n    ],\n    \"localization\": [\n      {\"term_id\": \"GO:0005840\", \"supporting_discovery_ids\": [0, 9, 11]},\n      {\"term_id\": \"GO:0005829\", \"supporting_discovery_ids\": [9, 13]}\n    ],\n    \"pathway\": [\n      {\"term_id\": \"R-HSA-8953854\", \"supporting_discovery_ids\": [11, 13]},\n      {\"term_id\": \"R-HSA-392499\", \"supporting_discovery_ids\": [0, 1]},\n      {\"term_id\": \"R-HSA-1852241\", \"supporting_discovery_ids\": [11, 13]}\n    ],\n    \"complexes\": [\"40S ribosomal subunit\", \"30S ribosomal subunit\", \"S0-cluster\"],\n    \"partners\": [\"RPS0\", \"RPS2\", \"NOB1\", \"LBP/p40\", \"RPS1\", \"RPS11\"],\n    \"other_free_text\": []\n  }\n}","audit_flag":null,"evaluation":{"pairwise":"win","faith_supported":5,"faith_total":5,"faith_pct":100.0}}