{"uniprot":"Q9NSB2","gene":"KRT84","name":"Keratin, type II cuticular Hb4","organism":"Homo sapiens (Human)","status":"dark","length":600,"sequence":"MSCRSYRVSSGHRVGNFSSCSAMTPQNLNRFRANSVSCWSGPGFRGLGSFGSRSVITFGSYSPRIAAVGSRPIHCGVRFGAGCGMGFGDGRGVGLGPRADSCVGLGFGAGSGIGYGFGGPGFGYRVGGVGVPAAPSITAVTVNKSLLTPLNLEIDPNAQRVKKDEKEQIKTLNNKFASFIDKVRFLEQQNKLLETKWSFLQEQKCIRSNLEPLFESYITNLRRQLEVLVSDQARLQAERNHLQDVLEGFKKKYEEEVVCRANAENEFVALKKDVDAAFMNKSDLEANVDTLTQEIDFLKTLYMEEIQLLQSHISETSVIVKMDNSRDLNLDGIIAEVKAQYEEVARRSRADAEAWYQTKYEEMQVTAGQHCDNLRNIRNEINELTRLIQRLKAEIEHAKAQRAKLEAAVAEAEQQGEATLSDAKCKLADLECALQQAKQDMARQLCEYQELMNAKLGLDIEIATYRRLLEGEESRLCEGVGPVNISVSSSRGGLVCGPEPLVAGSTLSRGGVTFSGSSSVCATSGVLASCGPSLGGARVAPATGDLLSTGTRSGSMLISEACVPSVPCPLPTQGGFSSCSGGRSSSVRFVSTTTSCRTKY","function_summary":"A type II keratin that assembles with type I keratins to build the intermediate filament network characteristic of epithelial cells. Its N-terminal head and long coiled-coil rod drive heterodimerization and filament assembly, producing mechanically resilient keratin arrays that support epidermal differentiation and tissue integrity. The protein functions through multivalent binding and filament assembly in the cytosol, integrates into keratin filament systems, and can be released within extracellular vesicles during epithelial turnover.","plain_language":"KRT84 has barely been studied. The model's best guess: A type II keratin that assembles with type I keratins to build the intermediate filament network characteristic of epithelial cells. Its N-terminal head and long coiled-coil rod drive heterodimerization and filament assembly, producing mechanically resilient keratin arrays that support epidermal differentiation and tissue integrity.","uniprot_summary":"Type II keratin.","go_terms":{"molecular_function":[{"id":"GO:0005515","label":"protein binding"}],"biological_process":[{"id":"GO:0008544","label":"epidermis development"}],"cellular_component":[{"id":"GO:0005829","label":"cytosol"},{"id":"GO:0070062","label":"extracellular exosome"},{"id":"GO:0045095","label":"keratin filament"}]},"interpro":[{"id":"IPR032444","name":"Keratin type II head","type":"domain","start":16,"end":161},{"id":"IPR039008","name":"Intermediate filament, rod domain","type":"domain","start":164,"end":476},{"id":"IPR003054","name":"Keratin, type II","type":"family","start":167,"end":175},{"id":"IPR003054","name":"Keratin, type II","type":"family","start":194,"end":206},{"id":"IPR003054","name":"Keratin, type II","type":"family","start":239,"end":252},{"id":"IPR003054","name":"Keratin, type II","type":"family","start":253,"end":272},{"id":"IPR003054","name":"Keratin, type II","type":"family","start":364,"end":381},{"id":"IPR003054","name":"Keratin, type II","type":"family","start":415,"end":430},{"id":"IPR018039","name":"Intermediate filament protein, conserved site","type":"conserved_site","start":462,"end":470}],"reasoning":"I begin with the InterPro architecture. The N-terminus carries IPR032444 (Keratin type II head, residues 16–161), a low-complexity head module that nucleates filament assembly and mediates regulatory contacts. Immediately downstream lies IPR039008 (Intermediate filament, rod domain, residues 164–476), the canonical coiled-coil shaft that drives obligate heterodimerization with type I keratins and supports higher-order filament formation. Within this rod, multiple type II keratin family signatures are embedded at characteristic positions—IPR003054 (Keratin, type II family) at 167–175, 194–206, 239–252, 253–272, 364–381, and 415–430—marking conserved type II keratin motifs that tune filament mechanics and partner specificity. Near the C-terminal end of the rod, IPR018039 (Intermediate filament protein, conserved site, residues 462–470) flags a critical assembly/conserved patch that stabilizes filament packing and end-to-end annealing. The ordered layout—type II head followed by a long coiled-coil rod punctuated by type II-defining motifs and capped by a conserved assembly site—creates a filament-forming scaffold specialized for keratin networks.\n\nThis architecture causes a molecular function centered on multivalent protein binding (GO:0005515). The head and tail regions provide flexible interaction surfaces, while the rod domain enforces high-affinity coiled-coil pairing with type I keratins and recruits accessory cytoskeletal and junctional partners. The absence of catalytic signatures and the dominance of filament-forming motifs indicate that binding and structural assembly—not enzymatic chemistry—drive activity.\n\nFrom this binding-driven assembly, the biological process emerges. Keratin filaments are core cytoskeletal elements of stratified epithelia; their assembly and crosslinking underpin epidermal differentiation and tissue integrity. By forming robust heterodimers with type I keratins and assembling into mature keratin filaments, this protein supports epidermis development (GO:0008544), providing mechanical resilience and signaling platforms that coordinate keratinocyte maturation.\n\nCellular localization follows directly from filament biogenesis and trafficking. Filament assembly and turnover occur in the cytosol (GO:0005829), where soluble keratin pools polymerize into keratin filaments (GO:0045095) that populate the cytoplasmic cytoskeleton. Keratin subunits and filament fragments are frequently detected in extracellular exosomes (GO:0070062), consistent with epithelial secretion pathways and cytoskeleton-derived cargo. The soluble-to-filament transition explains the observed distribution across cytosol and filament networks, with exosomal export reflecting epithelial vesicle traffic.\n\nMechanistically, the protein likely operates by a hierarchical assembly pathway: the type II head initiates and regulates filament nucleation; the rod domain enforces type I/type II coiled-coil heterodimerization; and conserved C-terminal motifs stabilize filament elongation and lateral alignment. This mechanism predicts interactions with type I keratins and cytolinker proteins that couple keratin networks to desmosomes and hemidesmosomes. I therefore hypothesize partnerships with type I keratin heterodimer partners and junctional adaptors (for example, desmoplakin, plectin, and plakophilins), which together anchor and remodel keratin filaments during epidermal development and epithelial stress responses.","category":"dark","control_evaluation":null,"validation_plan":null,"structure":{"source":"AlphaFold DB","pdb":"/structures/AF-Q9NSB2.pdb","alphafold":"https://alphafold.ebi.ac.uk/entry/Q9NSB2"},"links":{"uniprot":"https://www.uniprot.org/uniprotkb/Q9NSB2/entry","atlas":"/p/Q9NSB2"},"provenance":{"model":"BioReason-Pro (ESM3 esm3-sm-open-v1)","source_list":"Pharos Tdark","note":"AI prediction, not experimentally verified"}}