{"uniprot":"P20671","gene":"HIST1H2AD","name":"Histone H2A type 1-D","organism":"Homo sapiens (Human)","status":"dark","length":130,"sequence":"MSGRGKQGGKARAKAKTRSSRAGLQFPVGRVHRLLRKGNYSERVGAGAPVYLAAVLEYLTAEILELAGNAARDNKKTRIIPRHLQLAIRNDEELNKLLGKVTIAQGGVLPNIQAVLLPKKTESHHKAKGK","function_summary":"A nuclear chromatin core protein that pairs with a companion histone to form a dimer that assembles into nucleosomes, wrapping DNA and shaping higher-order chromatin structure. By stabilizing and modulating nucleosome architecture, it constrains gene accessibility and thereby tunes transcription, replication, and repair pathways that collectively restrain cell proliferation. While primarily resident in the nucleus, it can also be packaged into extracellular vesicles, reflecting broader roles in chromatin handling and turnover.","plain_language":"HIST1H2AD has barely been studied. The model's best guess: A nuclear chromatin core protein that pairs with a companion histone to form a dimer that assembles into nucleosomes, wrapping DNA and shaping higher-order chromatin structure. By stabilizing and modulating nucleosome architecture, it constrains gene accessibility and thereby tunes transcription, replication, and repair pathways that collectively restrain cell proliferation.","uniprot_summary":"Core component of nucleosome. Nucleosomes wrap and compact DNA into chromatin, limiting DNA accessibility to the cellular machineries which require DNA as a template. Histones thereby play a central role in transcription regulation, DNA repair, DNA replication and chromosomal stability.","go_terms":{"molecular_function":[{"id":"GO:0005515","label":"protein binding"}],"biological_process":[{"id":"GO:0008285","label":"negative regulation of cell population proliferation"}],"cellular_component":[{"id":"GO:0005634","label":"nucleus"},{"id":"GO:0070062","label":"extracellular exosome"}]},"interpro":[{"id":"IPR009072","name":"Histone-fold","type":"homologous_superfamily","start":2,"end":130},{"id":"IPR007125","name":"Core Histone H2A/H2B/H3 domain","type":"domain","start":7,"end":89},{"id":"IPR061508","name":"Histone H2A, histone fold","type":"domain","start":16,"end":104},{"id":"IPR032454","name":"Histone H2A, C-terminal domain","type":"domain","start":92,"end":126},{"id":"IPR002119","name":"Histone H2A","type":"family","start":3,"end":123},{"id":"IPR032458","name":"Histone H2A conserved site","type":"conserved_site","start":22,"end":28}],"reasoning":"I begin with the InterPro architecture. The polypeptide is dominated by IPR009072 (Histone-fold, homologous_superfamily, residues 2–130), a hallmark dimerization-and-DNA-wrapping scaffold that organizes nucleosome cores. Nested within this scaffold are multiple H2A-specific determinants: IPR007125 (Core Histone H2A/H2B/H3 domain, residues 7–89) and IPR061508 (Histone H2A, histone fold, residues 16–104) refine the fold as a canonical histone architecture tuned for obligate pairing with H2B and assembly into the octameric nucleosome. The N-terminal body of the protein is further specified by IPR002119 (Histone H2A family, residues 3–123), which fixes the identity as an H2A-class variant. The C-terminal half transitions into IPR032454 (Histone H2A, C-terminal domain, residues 92–126), a region that stabilizes the H2A–H2B dimer and modulates nucleosome dynamics and higher-order chromatin states. A short motif, IPR032458 (Histone H2A conserved site, residues 22–28), marks a conserved patch that seeds nucleosome assembly and interaction surfaces. The ordered layout—histone fold core followed by an H2A-tailored C-terminus—causes tight heterodimerization with H2B and positions the dimer to wrap DNA together with H3–H4 tetramers.\n\nThis architecture dictates molecular function. The histone fold and H2A-specific modules create extensive protein–protein interfaces that drive obligate heterodimerization and nucleosome assembly; thus, the primary molecular activity is multivalent protein binding (GO:0005515). The absence of catalytic motifs and the presence of conserved interaction surfaces reinforce that this polypeptide acts as a structural chromatin component whose binding capacity governs assembly and remodeling.\n\nFrom function to process, nucleosome assembly and stability directly tune transcription, replication, and repair. By constraining chromatin accessibility and higher-order fiber formation, H2A-class histones dampen or facilitate gene expression programs that govern cell cycle entry and survival. This chromatin-governing role provides a mechanistic route to negative regulation of cell population proliferation (GO:0008285): tighter or more repressive nucleosomes reduce expression of pro-proliferative genes and favor checkpoint control.\n\nCellular location follows from the same logic. The histone fold and H2A family identity place the protein in the nucleus (GO:0005634), where nucleosomes form the chromatin template. Histones are also frequently trafficked in extracellular vesicles; chromatin fragments and bound histones are commonly recovered in exosomes, supporting association with extracellular exosome (GO:0070062). The nuclear residence is primary, with exosomal packaging representing secondary extracellular transport.\n\nMechanistically, I hypothesize that this H2A-class protein forms a stable H2A–H2B dimer that docks onto the H3–H4 tetramer to assemble the nucleosome. The C-terminal H2A domain tunes nucleosome stability and remodeling, biasing toward chromatin states that restrain proliferation. Likely interaction partners include H2B as the obligate dimer mate, H3 and H4 within the octamer, and chromatin remodelers and chaperones that traffic and edit nucleosomes. These assemblies position the histone to nucleate chromatin compaction and thereby modulate transcriptional and replicative programs central to proliferation control and extracellular vesicle cargo pathways.","category":"dark","control_evaluation":null,"validation_plan":null,"structure":{"source":"AlphaFold DB","pdb":"/structures/AF-P20671.pdb","alphafold":"https://alphafold.ebi.ac.uk/entry/P20671"},"links":{"uniprot":"https://www.uniprot.org/uniprotkb/P20671/entry","atlas":"/p/P20671"},"provenance":{"model":"BioReason-Pro (ESM3 esm3-sm-open-v1)","source_list":"Pharos Tdark","note":"AI prediction, not experimentally verified"}}