{"uniprot":"Q12766","gene":"HMGXB3","name":"HMG domain-containing protein 3","organism":"Homo sapiens (Human)","status":"dark","length":1292,"sequence":"MDASYDGTEVTVVMEEIEEAYCYTSPGPPKKKKKYKIHGEKTKKPRSAYLLYYYDIYLKVQQELPHLPQSEINKKISESWRLLSVAERSYYLEKAKLEKEGLDPNSKLSALTAVVPDIPGFRKILPRSDYIIIPKSSLQEDRSCPQLELCVAQNQMSPKGPPLVSNTAPETVPSHAGMAEQCLAVEALAEEVGALTQSGAVQEIATSEILSQDVLLEDASLEVGESHQPYQTSLVIEETLVNGSPDLPTGSLAVPHPQVGESVSVVTVMRDSSESSSSAPATQFIMLPLPAYSVVENPTSIKLTTTYTRRGHGTCTSPGCSFTYVTRHKPPKCPTCGNFLGGKWIPKEKPAKVKVELASGVSSKGSVVKRNQQPVTTEQNSSKENASKLTLENSEAVSQLLNVAPPREVGEESEWEEVIISDAHVLVKEAPGNCGTAVTKTPVVKSGVQPEVTLGTTDNDSPGADVPTPSEGTSTSSPLPAPKKPTGADLLTPGSRAPELKGRARGKPSLLAAARPMRAILPAPVNVGRGSSMGLPRARQAFSLSDKTPSVRTCGLKPSTLKQLGQPIQQPSGPGEVKLPSGPSNRTSQVKVVEVKPDMFPPYKYSCTVTLDLGLATSRGRGKCKNPSCSYVYTNRHKPRICPSCGVNLAKDRTEKTTKAIEVSSPLPDVLNATEPLSTAQREIQRQSTLQLLRKVLQIPENESELAEVFALIHELNSSRLILSNVSEETVTIEQTSWSNYYESPSTQCLLCSSPLFKGGQNSLAGPQECWLLTASRLQTVTAQVKMCLNPHCLALHSFIDIYTGLFNVGNKLLVSLDLLFAIRNQIKLGEDPRVSINVVLKSVQEQTEKTLTSEELSQLQELLCNGYWAFECLTVRDYNDMICGICGVAPKVEMAQRSEENVLALKSVEFTWPEFLGSNEVNVEDFWATMETEVIEQVAFPASIPITKFDASVIAPFFPPLMRGAVVVNTEKDKNLDVQPVPGSGSALVRLLQEGTCKLDEIGSYSEEKLQHLLRQCGIPFGAEDSKDQLCFSLLALYESVQNGARAIRPPRHFTGGKIYKVCPHQVVCGSKYLVRGESARDHVDLLASSRHWPPVYVVDMATSVALCADLCYPELTNQMWGRNQGCFSSPTEPPVSVSCPELLDQHYTVDMTETEHSIQHPVTKTATRRIVHAGLQPNPGDPSAGHHSLALCPELAPYATILASIVDSKPNGVRQRPIAFDNATHYYLYNRLMDFLTSREIVNRQIHDIVQSCQPGEVVIRDTLYRLGVAQIKTETEEEGEEEEVAAVAE","function_summary":"A large nuclear architectural factor that uses an N-terminal DNA-bending module to engage and reshape chromatin at small nuclear RNA loci, assembling multivalent transcriptional complexes through internal cysteine-rich scaffolding. By stabilizing higher-order assemblies with polymerase-associated machinery, it promotes snRNA gene expression and interfaces with pathways used by polymerase III, operating predominantly in the nucleoplasm and nucleolar fibrillar centers to coordinate RNA biogenesis.","plain_language":"HMGXB3 has barely been studied. The model's best guess: A large nuclear architectural factor that uses an N-terminal DNA-bending module to engage and reshape chromatin at small nuclear RNA loci, assembling multivalent transcriptional complexes through internal cysteine-rich scaffolding. By stabilizing higher-order assemblies with polymerase-associated machinery, it promotes snRNA gene expression and interfaces with pathways used by polymerase III, operating predominantly in the nucleoplasm and nucleolar fibrillar centers to coordinate RNA biogenesis.","uniprot_summary":"Involved in small nuclear RNA (snRNA) gene transcription.","go_terms":{"molecular_function":[{"id":"GO:0005515","label":"protein binding"},{"id":"GO:0003677","label":"DNA binding"}],"biological_process":[{"id":"GO:0006383","label":"transcription by RNA polymerase III"},{"id":"GO:0009301","label":"snRNA transcription"}],"cellular_component":[{"id":"GO:0001650","label":"fibrillar center"},{"id":"GO:0005654","label":"nucleoplasm"}]},"interpro":[{"id":"IPR036910","name":"High mobility group box domain superfamily","type":"homologous_superfamily","start":23,"end":105},{"id":"IPR009071","name":"High mobility group box domain","type":"domain","start":40,"end":111},{"id":"IPR040648","name":"HMG domain-containing protein 3, CxC4 like cysteine cluster","type":"domain","start":735,"end":859},{"id":"IPR039598","name":"HMG domain-containing protein 3","type":"family","start":1,"end":1288}],"reasoning":"I begin with the InterPro architecture. The N-terminus carries IPR036910 (High mobility group box domain superfamily, residues 23–105) and a more specific IPR009071 (High mobility group box domain, residues 40–111). This tandem of an HMG-box superfamily signature followed by a canonical HMG-box domain defines a DNA-architectural module that binds and bends DNA, often recognizing non-B-form DNA and stabilizing nucleoprotein assemblies. The full-length sequence is encompassed by IPR039598 (HMG domain-containing protein 3 family, residues 1–1288), indicating a very large, low-complexity, transcription-associated scaffold with multiple regulatory regions. In the distal region, IPR040648 (HMG domain-containing protein 3, CxC4-like cysteine cluster, residues 735–859) specifies a cysteine-rich module that coordinates metal ions and forms a structured or oligomeric platform. This ordered layout—an N-terminal HMG DNA-bending core followed by an internal cysteine-cluster scaffold within a giant family-specific backbone—causally equips the protein to bind DNA and assemble large transcriptional complexes.\n\nFrom this architecture, the molecular functions follow directly. The HMG-box causes high-affinity DNA association and bending, supporting GO:0003677 DNA binding. The extensive family scaffold and cysteine-cluster platform favor multivalent assembly with other regulators and enzymes, consistent with GO:0005515 protein binding. Together, these features define a DNA-architectural hub that nucleates transcription machinery rather than catalyzing chemistry itself.\n\nThe biological processes emerge from how this architecture is used. HMG-box-driven DNA bending and scaffold-mediated complex assembly are hallmarks of small nuclear RNA gene control. By stabilizing chromatin loops and recruiting polymerase-associated factors, the protein drives snRNA gene expression, aligning with GO:0009301 snRNA transcription. Its ability to coordinate polymerase-associated assemblies and nucleolar transcription factors also supports participation in RNA polymerase III pathways, consistent with GO:0006383 transcription by RNA polymerase III. The same architectural principles that bend DNA and recruit cofactors explain how it can operate at both snRNA and tRNA/snRNA-related transcriptional nodes.\n\nCellular localization is dictated by where such assemblies form. The nucleoplasm is the principal compartment for snRNA gene transcription and assembly of RNA polymerase II complexes, matching GO:0005654 nucleoplasm. The presence of a nucleolar transcriptional role and polymerase III activity points to the nucleolus, particularly the fibrillar center where rDNA and associated transcription machineries concentrate, supporting GO:0001650 fibrillar center. The experimentally observed nuclear residency is therefore a necessary consequence of its chromatin-architectural function.\n\nMechanistically, I hypothesize that the N-terminal HMG-box binds and bends target DNA at snRNA loci, while the internal CxC4-like cysteine cluster stabilizes higher-order assemblies that recruit polymerase-associated factors and chromatin remodelers. This architecture enables cooperative assembly with small nuclear RNA transcription complexes and polymerase III-associated proteins, explaining its dual role in snRNA and polymerase III pathways. Likely interaction partners include components of the snRNA transcription apparatus and nucleolar transcription hubs—such as small nuclear RNA- and snRNA-specific transcription factors and polymerase-associated scaffolds—forming a modular, DNA-bending platform that orchestrates RNA biogenesis within the nucleus and fibrillar centers.","category":"dark","control_evaluation":null,"validation_plan":null,"structure":{"source":"AlphaFold DB","pdb":"/structures/AF-Q12766.pdb","alphafold":"https://alphafold.ebi.ac.uk/entry/Q12766"},"links":{"uniprot":"https://www.uniprot.org/uniprotkb/Q12766/entry","atlas":"/p/Q12766"},"provenance":{"model":"BioReason-Pro (ESM3 esm3-sm-open-v1)","source_list":"Pharos Tdark","note":"AI prediction, not experimentally verified"}}