{"gene":"WBP4","run_date":"2026-06-11T09:02:06","timeline":{"discoveries":[{"year":1998,"finding":"FBP21 (WBP4) is present in highly purified spliceosomal complex A, is associated with U2 snRNPs, and colocalizes with splicing factors in nuclear speckle domains. FBP21 interacts directly with the U1 snRNP protein U1C, the core snRNP proteins SmB and SmB', and the branchpoint binding protein SF1/mBBP, suggesting a role in cross-intron bridging of U1 and U2 snRNPs.","method":"Biochemical purification of spliceosomal complex A, co-immunoprecipitation, direct binding assays, immunofluorescence colocalization","journal":"Proceedings of the National Academy of Sciences of the United States of America","confidence":"High","confidence_rationale":"Tier 2 / Strong — reciprocal binding assays, spliceosome fractionation, and localization, replicated by multiple subsequent studies","pmids":["9724750"],"is_preprint":false},{"year":2000,"finding":"The WW domains of FBP21 recognize proline-rich motifs on Sam68 (a Pro-Arg motif class), and asymmetrical dimethylation of arginine residues flanking these motifs has no effect on WW domain binding (in contrast to SH3 domains which are inhibited by such methylation).","method":"In vitro binding experiments with methylated vs. unmethylated peptides/proteins, oriented peptide library screening","journal":"The Journal of biological chemistry","confidence":"Medium","confidence_rationale":"Tier 2 / Moderate — in vitro binding with biochemical controls, single lab, two orthogonal methods","pmids":["10748127","10744724"],"is_preprint":false},{"year":2000,"finding":"FBP21 WW domains belong to the Pro-Arg (PPR) class of WW domain ligand recognition, distinct from PPXY and PPLP classes, as determined by oriented peptide library screening and in vitro binding experiments.","method":"Oriented peptide library screening, expression cloning, in vitro binding experiments","journal":"The Journal of biological chemistry","confidence":"Medium","confidence_rationale":"Tier 1 / Moderate — in vitro biochemical assays with peptide library, single lab","pmids":["10744724"],"is_preprint":false},{"year":2009,"finding":"FBP21 is an activator of pre-mRNA splicing in vivo, and its splicing activation function and interaction with the splicing factor SIPP1 are both mediated by the two tandem WW domains. The solution structure of the tandem WW domains reveals they recognize both PPLP (group II) and PPR (group III) motifs via XP and XP2 grooves. The highly flexible linker between the tandem WW domains enables simultaneous interaction with two proline-rich motifs of SIPP1, explaining the high-affinity interaction through avidity.","method":"NMR structure determination, in vivo splicing assay, mutagenesis, ITC binding measurements","journal":"The Journal of biological chemistry","confidence":"High","confidence_rationale":"Tier 1 / Strong — NMR solution structure plus mutagenesis plus in vivo functional assay, multiple orthogonal methods in one study","pmids":["19592703"],"is_preprint":false},{"year":2011,"finding":"FBP21 tandem WW domains interact with multiple proline-rich motif-containing spliceosomal proteins, including the newly identified binding partner SF3B4. FRET analysis shows FBP21 interacts with SF3B4 in nuclear speckles. ITC and NMR demonstrate that multivalent binding through tandem WW domains and multiple ligand motifs enhances affinity, but ligand exchange remains fast on the NMR timescale, suggesting a delocalized, semispecific binding mode.","method":"FRET, isothermal titration calorimetry (ITC), NMR, proteome interaction analysis","journal":"The Journal of biological chemistry","confidence":"High","confidence_rationale":"Tier 1 / Strong — multiple orthogonal methods (FRET, ITC, NMR) with functional localization, single rigorous study","pmids":["21917930"],"is_preprint":false},{"year":2011,"finding":"FBP21 WW domains are a molecular target of borrelidin; binding of borrelidin to FBP21 WW domains alters the ratio of VEGF isoforms toward anti-angiogenic forms. Overexpression of FBP21 in RPE cells shifts VEGF splicing toward pro-angiogenic isoforms, implicating FBP21 in alternative splicing regulation.","method":"Phage display affinity biopanning, in vitro binding with recombinant WW domains, transfection/overexpression with RT-PCR splicing assay","journal":"Chemical science","confidence":"Medium","confidence_rationale":"Tier 2 / Moderate — direct binding assay plus functional splicing assay in cells, single lab, two orthogonal approaches","pmids":["22822423"],"is_preprint":false},{"year":2017,"finding":"An intrinsically disordered C-terminal region of FBP21 binds to the C-terminal Sec63 unit of Brr2 RNA helicase; this interaction allosterically inhibits Brr2 helicase activity. Additionally, FBP21 directly interacts with U4/U6 di-snRNA, reducing the pool of unwound U4/U6 di-snRNA.","method":"Yeast-two-hybrid screen, biochemical binding assays, biophysical analyses, helicase activity assay, RNA interaction assay","journal":"Nucleic acids research","confidence":"High","confidence_rationale":"Tier 1 / Strong — yeast two-hybrid identification plus in vitro biochemical reconstitution of helicase inhibition plus RNA interaction assay, multiple orthogonal methods","pmids":["28838205"],"is_preprint":false},{"year":2018,"finding":"The 50 C-terminal residues of FBP21 are intrinsically disordered in the unbound state but adopt an extended conformation upon binding to the C-terminal Sec63 unit of Brr2, covering a large binding surface. Short charged motifs steer complex formation while the bound state retains conformational dynamics.","method":"NMR spectroscopy, fragment docking with experimental restraints","journal":"Biophysical journal","confidence":"High","confidence_rationale":"Tier 1 / Moderate — NMR structural characterization of complex with experimental restraints and docking, single lab rigorous study","pmids":["30558886"],"is_preprint":false},{"year":2020,"finding":"WBP4 was identified as a new interactant of the vitamin D receptor (VDR) and controls VDR subcellular localization. The vitamin D antagonist ZK168281 enhances the WBP4-VDR interaction in the cytosol, promoting cytosolic sequestration of VDR and normalizing VDR target gene expression and serum calcium levels in vitamin D-intoxicated mice.","method":"Co-immunoprecipitation, subcellular fractionation/localization assay, mouse in vivo treatment model with gene expression analysis","journal":"Nature communications","confidence":"Medium","confidence_rationale":"Tier 2 / Moderate — Co-IP identification of interaction plus functional localization assay plus in vivo validation, single lab","pmids":["33288743"],"is_preprint":false},{"year":2022,"finding":"Cryo-EM structures of spliceosomal B complex dimers reveal that FBP21 wraps around the C-terminal helicase cassette of BRR2 and interacts with the U6/5' splice site helix and with SNU23 and PRP38; C9ORF78 and FBP21 bind BRR2 in a mutually exclusive manner at the same site.","method":"Cryo-EM structure determination of spliceosomal B complex dimers","journal":"Nature communications","confidence":"High","confidence_rationale":"Tier 1 / Strong — cryo-EM structural determination showing molecular contacts, complemented by biochemical validation in same study","pmids":["35241646"],"is_preprint":false},{"year":2022,"finding":"The tandem WW domain of FBP21 can bind its natural ligand SmB/B' peptide in two different orientations (parallel and antiparallel), with the relative orientation of the two WW domains adapting accordingly, as characterized by NMR and molecular simulations.","method":"Molecular dynamics simulations, NMR experiments, peptide docking, density-based clustering","journal":"Journal of chemical information and modeling","confidence":"Medium","confidence_rationale":"Tier 1 / Moderate — NMR plus computational structural characterization, single lab, two orthogonal approaches","pmids":["35347992"],"is_preprint":false},{"year":2024,"finding":"Cryo-EM analyses of human spliceosomal B complexes provide new insights into how FBP21 interacts with SNU23 and PRP38 and the U6/5' splice site helix, revealing its specific molecular contacts within the B complex.","method":"Cryo-EM structure determination of human spliceosomal B complex dimers","journal":"The EMBO journal","confidence":"High","confidence_rationale":"Tier 1 / Strong — high-resolution cryo-EM structure with detailed molecular model, rigorous structural analysis","pmids":["38383864"],"is_preprint":false},{"year":2023,"finding":"Bi-allelic loss-of-function variants in WBP4 cause complete loss of protein (confirmed by immunoblotting) and result in shared abnormal splicing patterns—including mis-splicing of genes associated with nervous system abnormalities—underlying a neurodevelopmental syndrome, establishing WBP4 as essential for normal splicing in vivo.","method":"Immunoblotting on patient fibroblasts, RNA sequencing of patient samples, genetic analysis","journal":"American journal of human genetics","confidence":"Medium","confidence_rationale":"Tier 2 / Moderate — patient-derived loss-of-function with protein and RNA-level readouts, multiple families, single study","pmids":["37963460"],"is_preprint":false}],"current_model":"WBP4/FBP21 is a spliceosomal protein that functions as an activator of pre-mRNA splicing; its two tandem group-III WW domains mediate multivalent, avidity-enhanced interactions with proline-rich motifs (PPR/PPLP) on multiple spliceosomal partners including SmB/B', SF1/mBBP, U1C, SIPP1, SF3B4, and the RNA helicase Brr2, where an intrinsically disordered C-terminal region of FBP21 wraps around the Brr2 C-terminal Sec63 unit to allosterically inhibit helicase activity and reduce U4/U6 di-snRNA unwinding; cryo-EM structures place FBP21 at the U6/5' splice-site helix interface in the B complex alongside SNU23 and PRP38, and FBP21 additionally controls vitamin D receptor subcellular localization in the cytosol, while biallelic human loss-of-function variants cause a spliceosomopathy with global developmental delay."},"narrative":{"mechanistic_narrative":"WBP4 (FBP21) is a spliceosomal protein that acts as an activator of pre-mRNA splicing and engages the early spliceosome as a multivalent scaffold for proline-rich spliceosomal partners [PMID:9724750, PMID:19592703]. It associates with U2 snRNPs in spliceosomal complex A and localizes to nuclear speckles, where it directly contacts the U1 snRNP protein U1C, the Sm core proteins SmB/B', and the branchpoint-binding protein SF1/mBBP, positioning it to bridge U1 and U2 snRNPs across the intron [PMID:9724750]. Recognition of partners is mediated by two tandem WW domains connected by a flexible linker that recognize PPLP (group II) and Pro-Arg/PPR (group III) proline-rich motifs through XP and XP2 grooves, with the bivalent architecture conferring high avidity yet a delocalized, fast-exchanging binding mode toward ligands including SIPP1, SF3B4, and SmB/B' [PMID:10744724, PMID:19592703, PMID:21917930, PMID:35347992]. Beyond this scaffolding role, an intrinsically disordered C-terminal region of FBP21 wraps around the C-terminal Sec63 unit of the Brr2 RNA helicase, adopting an extended conformation upon binding that allosterically inhibits helicase activity and, together with direct binding to U4/U6 di-snRNA, restrains U4/U6 unwinding [PMID:28838205, PMID:30558886]. Cryo-EM structures of human spliceosomal B complexes place FBP21 at the U6/5' splice-site helix, contacting SNU23 and PRP38 and competing with C9ORF78 for the same Brr2 site [PMID:35241646, PMID:38383864]. WBP4 also functions outside splicing as an interactant of the vitamin D receptor, controlling its cytosolic localization [PMID:33288743]. Bi-allelic loss-of-function variants in WBP4 abolish the protein and cause widespread mis-splicing underlying a neurodevelopmental syndrome [PMID:37963460].","teleology":[{"year":1998,"claim":"Established WBP4/FBP21 as a bona fide component of the early spliceosome and proposed a cross-intron bridging function, answering where in the splicing pathway the protein acts.","evidence":"Spliceosomal complex A purification, co-immunoprecipitation, direct binding assays, and speckle colocalization in human cells","pmids":["9724750"],"confidence":"High","gaps":["Did not define the structural basis of partner recognition","Functional contribution to splicing not yet demonstrated"]},{"year":2000,"claim":"Defined the ligand-recognition specificity of the FBP21 WW domains, showing they recognize a distinct Pro-Arg (PPR) class of proline-rich motifs unaffected by arginine methylation.","evidence":"Oriented peptide library screening and in vitro binding with methylated vs. unmethylated peptides, including Sam68","pmids":["10748127","10744724"],"confidence":"Medium","gaps":["In vitro peptide binding; physiological relevance of Sam68 interaction not established","No structural model of the domain-ligand complex"]},{"year":2009,"claim":"Demonstrated FBP21 is a splicing activator in vivo and provided the structural mechanism by which tandem WW domains achieve high-affinity multivalent binding via a flexible linker.","evidence":"NMR solution structure of tandem WW domains, ITC, mutagenesis, and in vivo splicing assay with SIPP1","pmids":["19592703"],"confidence":"High","gaps":["Did not establish full repertoire of in vivo ligands","Did not address regulation of helicase steps"]},{"year":2011,"claim":"Extended the partner repertoire (SF3B4) and showed the multivalent WW binding mode is semispecific with fast ligand exchange, reframing FBP21 as a delocalized molecular hub rather than a tight-binding adaptor.","evidence":"FRET in nuclear speckles, ITC, and NMR; complemented by phage-display identification of FBP21 WW domains as a borrelidin target with VEGF isoform splicing effects","pmids":["21917930","22822423"],"confidence":"High","gaps":["How semispecific binding translates to splice-site selection unknown","VEGF splicing effect is a single overexpression study"]},{"year":2017,"claim":"Revealed a regulatory function beyond scaffolding: the disordered C-terminus binds the Brr2 Sec63 unit and, with U4/U6 di-snRNA binding, allosterically inhibits helicase-driven unwinding.","evidence":"Yeast two-hybrid, biochemical and biophysical binding assays, helicase activity assay, and RNA interaction assay","pmids":["28838205"],"confidence":"High","gaps":["Functional consequence of helicase inhibition for splicing kinetics in cells not resolved","Structural detail of the bound complex not yet defined"]},{"year":2018,"claim":"Resolved how a disordered region encodes a specific interaction, showing the FBP21 C-terminus folds into an extended conformation on Brr2-Sec63 while retaining dynamics, with charged motifs steering complex formation.","evidence":"NMR spectroscopy and fragment docking with experimental restraints","pmids":["30558886"],"confidence":"High","gaps":["Did not place the interaction in the assembled spliceosome","Affinity contribution to helicase regulation in vivo unmeasured"]},{"year":2020,"claim":"Identified a splicing-independent role for WBP4 as a vitamin D receptor interactant that controls VDR cytosolic localization, broadening the protein's functional scope.","evidence":"Co-immunoprecipitation, subcellular fractionation, and an in vivo vitamin D-intoxicated mouse model with target-gene and serum calcium readouts","pmids":["33288743"],"confidence":"Medium","gaps":["Whether VDR binding uses the WW domains or disordered region not defined","Single lab; mechanism of cytosolic sequestration unresolved"]},{"year":2022,"claim":"Cryo-EM and NMR/simulation work positioned FBP21 within the assembled B complex at the U6/5' splice-site helix and showed its tandem WW domains bind SmB/B' in alternative orientations, explaining structural plasticity.","evidence":"Cryo-EM of spliceosomal B complex dimers; molecular dynamics, NMR, and peptide docking of WW-SmB/B'","pmids":["35241646","35347992"],"confidence":"High","gaps":["Mutual exclusivity with C9ORF78 not mechanistically tied to splicing outcome","Dynamics of WW orientation switching in the full spliceosome unknown"]},{"year":2023,"claim":"Established WBP4 as essential for normal human splicing in vivo by linking complete loss of protein to shared mis-splicing and a neurodevelopmental syndrome.","evidence":"Immunoblotting of patient fibroblasts, RNA sequencing of patient samples, and genetic analysis across multiple families","pmids":["37963460"],"confidence":"Medium","gaps":["Causal mis-spliced targets driving phenotype not pinpointed","Rescue of splicing defects by WBP4 re-expression not shown"]},{"year":2024,"claim":"High-resolution human B-complex cryo-EM refined the specific molecular contacts of FBP21 with SNU23, PRP38, and the U6/5' splice-site helix.","evidence":"Cryo-EM structure determination of human spliceosomal B complex dimers","pmids":["38383864"],"confidence":"High","gaps":["How these contacts couple to catalytic activation steps not resolved","Order of FBP21 release relative to Brr2 activation undefined"]},{"year":null,"claim":"How FBP21's multivalent semispecific scaffolding and Brr2 inhibition are coordinated to control specific splice-site choice, and which mis-spliced transcripts cause the neurodevelopmental phenotype, remain open.","evidence":"","pmids":[],"confidence":"Medium","gaps":["No defined transcriptome-wide rules linking FBP21 occupancy to splice-site selection","Causal disease-relevant splicing targets unidentified","Interplay between splicing role and VDR localization role unexplored"]}],"mechanism_profile":{"molecular_activity":[{"term_id":"GO:0003723","term_label":"RNA binding","supporting_discovery_ids":[6]},{"term_id":"GO:0060090","term_label":"molecular adaptor activity","supporting_discovery_ids":[0,3,4]},{"term_id":"GO:0098772","term_label":"molecular function regulator activity","supporting_discovery_ids":[6,8]},{"term_id":"GO:0045182","term_label":"translation regulator activity","supporting_discovery_ids":[3,5]}],"localization":[{"term_id":"GO:0005654","term_label":"nucleoplasm","supporting_discovery_ids":[0,4]},{"term_id":"GO:0005829","term_label":"cytosol","supporting_discovery_ids":[8]}],"pathway":[{"term_id":"R-HSA-8953854","term_label":"Metabolism of RNA","supporting_discovery_ids":[0,3,9]},{"term_id":"R-HSA-74160","term_label":"Gene expression (Transcription)","supporting_discovery_ids":[3,5]}],"complexes":["spliceosome (complex A / B complex)"],"partners":["BRR2","SNU23","PRP38","SF3B4","SF1","U1C","SNRPB","VDR"],"other_free_text":[]}},"prefetch_data":{"uniprot":{"accession":"O75554","full_name":"WW domain-binding protein 4","aliases":["Formin-binding protein 21","WW domain-containing-binding protein 4"],"length_aa":376,"mass_kda":42.5,"function":"Involved in pre-mRNA splicing as a component of the spliceosome (PubMed:19592703, PubMed:28781166, PubMed:9724750). May play a role in cross-intron bridging of U1 and U2 snRNPs in the mammalian A complex (PubMed:9724750)","subcellular_location":"Nucleus; Nucleus speckle","url":"https://www.uniprot.org/uniprotkb/O75554/entry"},"depmap":{"release":"DepMap","has_data":true,"is_common_essential":false,"resolved_as":"","url":"https://depmap.org/portal/gene/WBP4","classification":"Not Classified","n_dependent_lines":152,"n_total_lines":1208,"dependency_fraction":0.12582781456953643},"opencell":{"profiled":false,"resolved_as":"","ensg_id":"","cell_line_id":"","localizations":[],"interactors":[],"url":"https://opencell.sf.czbiohub.org/search/WBP4","total_profiled":1310},"omim":[{"mim_id":"620852","title":"NEURODEVELOPMENTAL DISORDER WITH HYPOTONIA, FEEDING DIFFICULTIES, FACIAL DYSMORPHISM, AND BRAIN ABNORMALITIES; NEDHFDB","url":"https://www.omim.org/entry/620852"},{"mim_id":"604981","title":"WW DOMAIN-CONTAINING BINDING PROTEIN 4; WBP4","url":"https://www.omim.org/entry/604981"}],"hpa":{"profiled":true,"resolved_as":"","reliability":"Supported","locations":[{"location":"Nucleoplasm","reliability":"Supported"},{"location":"Vesicles","reliability":"Additional"},{"location":"Plasma membrane","reliability":"Additional"}],"tissue_specificity":"Low tissue specificity","tissue_distribution":"Detected in all","driving_tissues":[],"url":"https://www.proteinatlas.org/search/WBP4"},"hgnc":{"alias_symbol":["FBP21","MGC117310"],"prev_symbol":[]},"alphafold":{"accession":"O75554","domains":[{"cath_id":"2.20.70.10","chopping":"127-159","consensus_level":"medium","plddt":83.4915,"start":127,"end":159},{"cath_id":"2.20.70.10","chopping":"166-200","consensus_level":"medium","plddt":82.7026,"start":166,"end":200},{"cath_id":"1.20.5","chopping":"11-84","consensus_level":"high","plddt":94.8874,"start":11,"end":84}],"viewer_url":"https://alphafold.ebi.ac.uk/entry/O75554","model_url":"https://alphafold.ebi.ac.uk/files/AF-O75554-F1-model_v6.cif","pae_url":"https://alphafold.ebi.ac.uk/files/AF-O75554-F1-predicted_aligned_error_v6.png","plddt_mean":65.88},"mouse_models":{"mgi_url":"https://www.informatics.jax.org/marker/summary?nomen=WBP4","jax_strain_url":"https://www.jax.org/strain/search?query=WBP4"},"sequence":{"accession":"O75554","fasta_url":"https://rest.uniprot.org/uniprotkb/O75554.fasta","uniprot_url":"https://www.uniprot.org/uniprotkb/O75554/entry","alphafold_viewer_url":"https://alphafold.ebi.ac.uk/entry/O75554"}},"corpus_meta":[{"pmid":"10748127","id":"PMC_10748127","title":"Arginine 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and function of the two tandem WW domains of the pre-mRNA splicing factor FBP21 (formin-binding protein 21).","date":"2009","source":"The Journal of biological chemistry","url":"https://pubmed.ncbi.nlm.nih.gov/19592703","citation_count":39,"is_preprint":false},{"pmid":"21917930","id":"PMC_21917930","title":"Multivalent binding of formin-binding protein 21 (FBP21)-tandem-WW domains fosters protein recognition in the pre-spliceosome.","date":"2011","source":"The Journal of biological chemistry","url":"https://pubmed.ncbi.nlm.nih.gov/21917930","citation_count":27,"is_preprint":false},{"pmid":"33288743","id":"PMC_33288743","title":"Cytosolic sequestration of the vitamin D receptor as a therapeutic option for vitamin D-induced hypercalcemia.","date":"2020","source":"Nature communications","url":"https://pubmed.ncbi.nlm.nih.gov/33288743","citation_count":26,"is_preprint":false},{"pmid":"22822423","id":"PMC_22822423","title":"Borrelidin modulates the alternative splicing of VEGF in favour of 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FBP21 interacts directly with the U1 snRNP protein U1C, the core snRNP proteins SmB and SmB', and the branchpoint binding protein SF1/mBBP, suggesting a role in cross-intron bridging of U1 and U2 snRNPs.\",\n      \"method\": \"Biochemical purification of spliceosomal complex A, co-immunoprecipitation, direct binding assays, immunofluorescence colocalization\",\n      \"journal\": \"Proceedings of the National Academy of Sciences of the United States of America\",\n      \"confidence\": \"High\",\n      \"confidence_rationale\": \"Tier 2 / Strong — reciprocal binding assays, spliceosome fractionation, and localization, replicated by multiple subsequent studies\",\n      \"pmids\": [\"9724750\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2000,\n      \"finding\": \"The WW domains of FBP21 recognize proline-rich motifs on Sam68 (a Pro-Arg motif class), and asymmetrical dimethylation of arginine residues flanking these motifs has no effect on WW domain binding (in contrast to SH3 domains which are inhibited by such methylation).\",\n      \"method\": \"In vitro binding experiments with methylated vs. unmethylated peptides/proteins, oriented peptide library screening\",\n      \"journal\": \"The Journal of biological chemistry\",\n      \"confidence\": \"Medium\",\n      \"confidence_rationale\": \"Tier 2 / Moderate — in vitro binding with biochemical controls, single lab, two orthogonal methods\",\n      \"pmids\": [\"10748127\", \"10744724\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2000,\n      \"finding\": \"FBP21 WW domains belong to the Pro-Arg (PPR) class of WW domain ligand recognition, distinct from PPXY and PPLP classes, as determined by oriented peptide library screening and in vitro binding experiments.\",\n      \"method\": \"Oriented peptide library screening, expression cloning, in vitro binding experiments\",\n      \"journal\": \"The Journal of biological chemistry\",\n      \"confidence\": \"Medium\",\n      \"confidence_rationale\": \"Tier 1 / Moderate — in vitro biochemical assays with peptide library, single lab\",\n      \"pmids\": [\"10744724\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2009,\n      \"finding\": \"FBP21 is an activator of pre-mRNA splicing in vivo, and its splicing activation function and interaction with the splicing factor SIPP1 are both mediated by the two tandem WW domains. The solution structure of the tandem WW domains reveals they recognize both PPLP (group II) and PPR (group III) motifs via XP and XP2 grooves. The highly flexible linker between the tandem WW domains enables simultaneous interaction with two proline-rich motifs of SIPP1, explaining the high-affinity interaction through avidity.\",\n      \"method\": \"NMR structure determination, in vivo splicing assay, mutagenesis, ITC binding measurements\",\n      \"journal\": \"The Journal of biological chemistry\",\n      \"confidence\": \"High\",\n      \"confidence_rationale\": \"Tier 1 / Strong — NMR solution structure plus mutagenesis plus in vivo functional assay, multiple orthogonal methods in one study\",\n      \"pmids\": [\"19592703\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2011,\n      \"finding\": \"FBP21 tandem WW domains interact with multiple proline-rich motif-containing spliceosomal proteins, including the newly identified binding partner SF3B4. FRET analysis shows FBP21 interacts with SF3B4 in nuclear speckles. ITC and NMR demonstrate that multivalent binding through tandem WW domains and multiple ligand motifs enhances affinity, but ligand exchange remains fast on the NMR timescale, suggesting a delocalized, semispecific binding mode.\",\n      \"method\": \"FRET, isothermal titration calorimetry (ITC), NMR, proteome interaction analysis\",\n      \"journal\": \"The Journal of biological chemistry\",\n      \"confidence\": \"High\",\n      \"confidence_rationale\": \"Tier 1 / Strong — multiple orthogonal methods (FRET, ITC, NMR) with functional localization, single rigorous study\",\n      \"pmids\": [\"21917930\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2011,\n      \"finding\": \"FBP21 WW domains are a molecular target of borrelidin; binding of borrelidin to FBP21 WW domains alters the ratio of VEGF isoforms toward anti-angiogenic forms. Overexpression of FBP21 in RPE cells shifts VEGF splicing toward pro-angiogenic isoforms, implicating FBP21 in alternative splicing regulation.\",\n      \"method\": \"Phage display affinity biopanning, in vitro binding with recombinant WW domains, transfection/overexpression with RT-PCR splicing assay\",\n      \"journal\": \"Chemical science\",\n      \"confidence\": \"Medium\",\n      \"confidence_rationale\": \"Tier 2 / Moderate — direct binding assay plus functional splicing assay in cells, single lab, two orthogonal approaches\",\n      \"pmids\": [\"22822423\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2017,\n      \"finding\": \"An intrinsically disordered C-terminal region of FBP21 binds to the C-terminal Sec63 unit of Brr2 RNA helicase; this interaction allosterically inhibits Brr2 helicase activity. Additionally, FBP21 directly interacts with U4/U6 di-snRNA, reducing the pool of unwound U4/U6 di-snRNA.\",\n      \"method\": \"Yeast-two-hybrid screen, biochemical binding assays, biophysical analyses, helicase activity assay, RNA interaction assay\",\n      \"journal\": \"Nucleic acids research\",\n      \"confidence\": \"High\",\n      \"confidence_rationale\": \"Tier 1 / Strong — yeast two-hybrid identification plus in vitro biochemical reconstitution of helicase inhibition plus RNA interaction assay, multiple orthogonal methods\",\n      \"pmids\": [\"28838205\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2018,\n      \"finding\": \"The 50 C-terminal residues of FBP21 are intrinsically disordered in the unbound state but adopt an extended conformation upon binding to the C-terminal Sec63 unit of Brr2, covering a large binding surface. Short charged motifs steer complex formation while the bound state retains conformational dynamics.\",\n      \"method\": \"NMR spectroscopy, fragment docking with experimental restraints\",\n      \"journal\": \"Biophysical journal\",\n      \"confidence\": \"High\",\n      \"confidence_rationale\": \"Tier 1 / Moderate — NMR structural characterization of complex with experimental restraints and docking, single lab rigorous study\",\n      \"pmids\": [\"30558886\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2020,\n      \"finding\": \"WBP4 was identified as a new interactant of the vitamin D receptor (VDR) and controls VDR subcellular localization. The vitamin D antagonist ZK168281 enhances the WBP4-VDR interaction in the cytosol, promoting cytosolic sequestration of VDR and normalizing VDR target gene expression and serum calcium levels in vitamin D-intoxicated mice.\",\n      \"method\": \"Co-immunoprecipitation, subcellular fractionation/localization assay, mouse in vivo treatment model with gene expression analysis\",\n      \"journal\": \"Nature communications\",\n      \"confidence\": \"Medium\",\n      \"confidence_rationale\": \"Tier 2 / Moderate — Co-IP identification of interaction plus functional localization assay plus in vivo validation, single lab\",\n      \"pmids\": [\"33288743\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2022,\n      \"finding\": \"Cryo-EM structures of spliceosomal B complex dimers reveal that FBP21 wraps around the C-terminal helicase cassette of BRR2 and interacts with the U6/5' splice site helix and with SNU23 and PRP38; C9ORF78 and FBP21 bind BRR2 in a mutually exclusive manner at the same site.\",\n      \"method\": \"Cryo-EM structure determination of spliceosomal B complex dimers\",\n      \"journal\": \"Nature communications\",\n      \"confidence\": \"High\",\n      \"confidence_rationale\": \"Tier 1 / Strong — cryo-EM structural determination showing molecular contacts, complemented by biochemical validation in same study\",\n      \"pmids\": [\"35241646\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2022,\n      \"finding\": \"The tandem WW domain of FBP21 can bind its natural ligand SmB/B' peptide in two different orientations (parallel and antiparallel), with the relative orientation of the two WW domains adapting accordingly, as characterized by NMR and molecular simulations.\",\n      \"method\": \"Molecular dynamics simulations, NMR experiments, peptide docking, density-based clustering\",\n      \"journal\": \"Journal of chemical information and modeling\",\n      \"confidence\": \"Medium\",\n      \"confidence_rationale\": \"Tier 1 / Moderate — NMR plus computational structural characterization, single lab, two orthogonal approaches\",\n      \"pmids\": [\"35347992\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2024,\n      \"finding\": \"Cryo-EM analyses of human spliceosomal B complexes provide new insights into how FBP21 interacts with SNU23 and PRP38 and the U6/5' splice site helix, revealing its specific molecular contacts within the B complex.\",\n      \"method\": \"Cryo-EM structure determination of human spliceosomal B complex dimers\",\n      \"journal\": \"The EMBO journal\",\n      \"confidence\": \"High\",\n      \"confidence_rationale\": \"Tier 1 / Strong — high-resolution cryo-EM structure with detailed molecular model, rigorous structural analysis\",\n      \"pmids\": [\"38383864\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2023,\n      \"finding\": \"Bi-allelic loss-of-function variants in WBP4 cause complete loss of protein (confirmed by immunoblotting) and result in shared abnormal splicing patterns—including mis-splicing of genes associated with nervous system abnormalities—underlying a neurodevelopmental syndrome, establishing WBP4 as essential for normal splicing in vivo.\",\n      \"method\": \"Immunoblotting on patient fibroblasts, RNA sequencing of patient samples, genetic analysis\",\n      \"journal\": \"American journal of human genetics\",\n      \"confidence\": \"Medium\",\n      \"confidence_rationale\": \"Tier 2 / Moderate — patient-derived loss-of-function with protein and RNA-level readouts, multiple families, single study\",\n      \"pmids\": [\"37963460\"],\n      \"is_preprint\": false\n    }\n  ],\n  \"current_model\": \"WBP4/FBP21 is a spliceosomal protein that functions as an activator of pre-mRNA splicing; its two tandem group-III WW domains mediate multivalent, avidity-enhanced interactions with proline-rich motifs (PPR/PPLP) on multiple spliceosomal partners including SmB/B', SF1/mBBP, U1C, SIPP1, SF3B4, and the RNA helicase Brr2, where an intrinsically disordered C-terminal region of FBP21 wraps around the Brr2 C-terminal Sec63 unit to allosterically inhibit helicase activity and reduce U4/U6 di-snRNA unwinding; cryo-EM structures place FBP21 at the U6/5' splice-site helix interface in the B complex alongside SNU23 and PRP38, and FBP21 additionally controls vitamin D receptor subcellular localization in the cytosol, while biallelic human loss-of-function variants cause a spliceosomopathy with global developmental delay.\"\n}\n```","stage2_raw":"{\n  \"mechanistic_narrative\": \"WBP4 (FBP21) is a spliceosomal protein that acts as an activator of pre-mRNA splicing and engages the early spliceosome as a multivalent scaffold for proline-rich spliceosomal partners [#0, #3]. It associates with U2 snRNPs in spliceosomal complex A and localizes to nuclear speckles, where it directly contacts the U1 snRNP protein U1C, the Sm core proteins SmB/B', and the branchpoint-binding protein SF1/mBBP, positioning it to bridge U1 and U2 snRNPs across the intron [#0]. Recognition of partners is mediated by two tandem WW domains connected by a flexible linker that recognize PPLP (group II) and Pro-Arg/PPR (group III) proline-rich motifs through XP and XP2 grooves, with the bivalent architecture conferring high avidity yet a delocalized, fast-exchanging binding mode toward ligands including SIPP1, SF3B4, and SmB/B' [#2, #3, #4, #10]. Beyond this scaffolding role, an intrinsically disordered C-terminal region of FBP21 wraps around the C-terminal Sec63 unit of the Brr2 RNA helicase, adopting an extended conformation upon binding that allosterically inhibits helicase activity and, together with direct binding to U4/U6 di-snRNA, restrains U4/U6 unwinding [#6, #7]. Cryo-EM structures of human spliceosomal B complexes place FBP21 at the U6/5' splice-site helix, contacting SNU23 and PRP38 and competing with C9ORF78 for the same Brr2 site [#9, #11]. WBP4 also functions outside splicing as an interactant of the vitamin D receptor, controlling its cytosolic localization [#8]. Bi-allelic loss-of-function variants in WBP4 abolish the protein and cause widespread mis-splicing underlying a neurodevelopmental syndrome [#12].\",\n  \"teleology\": [\n    {\n      \"year\": 1998,\n      \"claim\": \"Established WBP4/FBP21 as a bona fide component of the early spliceosome and proposed a cross-intron bridging function, answering where in the splicing pathway the protein acts.\",\n      \"evidence\": \"Spliceosomal complex A purification, co-immunoprecipitation, direct binding assays, and speckle colocalization in human cells\",\n      \"pmids\": [\"9724750\"],\n      \"confidence\": \"High\",\n      \"gaps\": [\"Did not define the structural basis of partner recognition\", \"Functional contribution to splicing not yet demonstrated\"]\n    },\n    {\n      \"year\": 2000,\n      \"claim\": \"Defined the ligand-recognition specificity of the FBP21 WW domains, showing they recognize a distinct Pro-Arg (PPR) class of proline-rich motifs unaffected by arginine methylation.\",\n      \"evidence\": \"Oriented peptide library screening and in vitro binding with methylated vs. unmethylated peptides, including Sam68\",\n      \"pmids\": [\"10748127\", \"10744724\"],\n      \"confidence\": \"Medium\",\n      \"gaps\": [\"In vitro peptide binding; physiological relevance of Sam68 interaction not established\", \"No structural model of the domain-ligand complex\"]\n    },\n    {\n      \"year\": 2009,\n      \"claim\": \"Demonstrated FBP21 is a splicing activator in vivo and provided the structural mechanism by which tandem WW domains achieve high-affinity multivalent binding via a flexible linker.\",\n      \"evidence\": \"NMR solution structure of tandem WW domains, ITC, mutagenesis, and in vivo splicing assay with SIPP1\",\n      \"pmids\": [\"19592703\"],\n      \"confidence\": \"High\",\n      \"gaps\": [\"Did not establish full repertoire of in vivo ligands\", \"Did not address regulation of helicase steps\"]\n    },\n    {\n      \"year\": 2011,\n      \"claim\": \"Extended the partner repertoire (SF3B4) and showed the multivalent WW binding mode is semispecific with fast ligand exchange, reframing FBP21 as a delocalized molecular hub rather than a tight-binding adaptor.\",\n      \"evidence\": \"FRET in nuclear speckles, ITC, and NMR; complemented by phage-display identification of FBP21 WW domains as a borrelidin target with VEGF isoform splicing effects\",\n      \"pmids\": [\"21917930\", \"22822423\"],\n      \"confidence\": \"High\",\n      \"gaps\": [\"How semispecific binding translates to splice-site selection unknown\", \"VEGF splicing effect is a single overexpression study\"]\n    },\n    {\n      \"year\": 2017,\n      \"claim\": \"Revealed a regulatory function beyond scaffolding: the disordered C-terminus binds the Brr2 Sec63 unit and, with U4/U6 di-snRNA binding, allosterically inhibits helicase-driven unwinding.\",\n      \"evidence\": \"Yeast two-hybrid, biochemical and biophysical binding assays, helicase activity assay, and RNA interaction assay\",\n      \"pmids\": [\"28838205\"],\n      \"confidence\": \"High\",\n      \"gaps\": [\"Functional consequence of helicase inhibition for splicing kinetics in cells not resolved\", \"Structural detail of the bound complex not yet defined\"]\n    },\n    {\n      \"year\": 2018,\n      \"claim\": \"Resolved how a disordered region encodes a specific interaction, showing the FBP21 C-terminus folds into an extended conformation on Brr2-Sec63 while retaining dynamics, with charged motifs steering complex formation.\",\n      \"evidence\": \"NMR spectroscopy and fragment docking with experimental restraints\",\n      \"pmids\": [\"30558886\"],\n      \"confidence\": \"High\",\n      \"gaps\": [\"Did not place the interaction in the assembled spliceosome\", \"Affinity contribution to helicase regulation in vivo unmeasured\"]\n    },\n    {\n      \"year\": 2020,\n      \"claim\": \"Identified a splicing-independent role for WBP4 as a vitamin D receptor interactant that controls VDR cytosolic localization, broadening the protein's functional scope.\",\n      \"evidence\": \"Co-immunoprecipitation, subcellular fractionation, and an in vivo vitamin D-intoxicated mouse model with target-gene and serum calcium readouts\",\n      \"pmids\": [\"33288743\"],\n      \"confidence\": \"Medium\",\n      \"gaps\": [\"Whether VDR binding uses the WW domains or disordered region not defined\", \"Single lab; mechanism of cytosolic sequestration unresolved\"]\n    },\n    {\n      \"year\": 2022,\n      \"claim\": \"Cryo-EM and NMR/simulation work positioned FBP21 within the assembled B complex at the U6/5' splice-site helix and showed its tandem WW domains bind SmB/B' in alternative orientations, explaining structural plasticity.\",\n      \"evidence\": \"Cryo-EM of spliceosomal B complex dimers; molecular dynamics, NMR, and peptide docking of WW-SmB/B'\",\n      \"pmids\": [\"35241646\", \"35347992\"],\n      \"confidence\": \"High\",\n      \"gaps\": [\"Mutual exclusivity with C9ORF78 not mechanistically tied to splicing outcome\", \"Dynamics of WW orientation switching in the full spliceosome unknown\"]\n    },\n    {\n      \"year\": 2023,\n      \"claim\": \"Established WBP4 as essential for normal human splicing in vivo by linking complete loss of protein to shared mis-splicing and a neurodevelopmental syndrome.\",\n      \"evidence\": \"Immunoblotting of patient fibroblasts, RNA sequencing of patient samples, and genetic analysis across multiple families\",\n      \"pmids\": [\"37963460\"],\n      \"confidence\": \"Medium\",\n      \"gaps\": [\"Causal mis-spliced targets driving phenotype not pinpointed\", \"Rescue of splicing defects by WBP4 re-expression not shown\"]\n    },\n    {\n      \"year\": 2024,\n      \"claim\": \"High-resolution human B-complex cryo-EM refined the specific molecular contacts of FBP21 with SNU23, PRP38, and the U6/5' splice-site helix.\",\n      \"evidence\": \"Cryo-EM structure determination of human spliceosomal B complex dimers\",\n      \"pmids\": [\"38383864\"],\n      \"confidence\": \"High\",\n      \"gaps\": [\"How these contacts couple to catalytic activation steps not resolved\", \"Order of FBP21 release relative to Brr2 activation undefined\"]\n    },\n    {\n      \"year\": null,\n      \"claim\": \"How FBP21's multivalent semispecific scaffolding and Brr2 inhibition are coordinated to control specific splice-site choice, and which mis-spliced transcripts cause the neurodevelopmental phenotype, remain open.\",\n      \"evidence\": \"\",\n      \"pmids\": [],\n      \"confidence\": \"Medium\",\n      \"gaps\": [\"No defined transcriptome-wide rules linking FBP21 occupancy to splice-site selection\", \"Causal disease-relevant splicing targets unidentified\", \"Interplay between splicing role and VDR localization role unexplored\"]\n    }\n  ],\n  \"mechanism_profile\": {\n    \"molecular_activity\": [\n      {\"term_id\": \"GO:0003723\", \"supporting_discovery_ids\": [6]},\n      {\"term_id\": \"GO:0060090\", \"supporting_discovery_ids\": [0, 3, 4]},\n      {\"term_id\": \"GO:0098772\", \"supporting_discovery_ids\": [6, 8]},\n      {\"term_id\": \"GO:0045182\", \"supporting_discovery_ids\": [3, 5]}\n    ],\n    \"localization\": [\n      {\"term_id\": \"GO:0005654\", \"supporting_discovery_ids\": [0, 4]},\n      {\"term_id\": \"GO:0005829\", \"supporting_discovery_ids\": [8]}\n    ],\n    \"pathway\": [\n      {\"term_id\": \"R-HSA-8953854\", \"supporting_discovery_ids\": [0, 3, 9]},\n      {\"term_id\": \"R-HSA-74160\", \"supporting_discovery_ids\": [3, 5]}\n    ],\n    \"complexes\": [\"spliceosome (complex A / B complex)\"],\n    \"partners\": [\"BRR2\", \"SNU23\", \"PRP38\", \"SF3B4\", \"SF1\", \"U1C\", \"SNRPB\", \"VDR\"],\n    \"other_free_text\": []\n  }\n}","audit_flag":null,"evaluation":{"pairwise":"tie","faith_supported":7,"faith_total":7,"faith_pct":100.0}}