{"uniprot":"P01880","gene":"IGHD","name":"Immunoglobulin heavy constant delta","organism":"Homo sapiens (Human)","status":"dark","length":430,"sequence":"APTKAPDVFPIISGCRHPKDNSPVVLACLITGYHPTSVTVTWYMGTQSQPQRTFPEIQRRDSYYMTSSQLSTPLQQWRQGEYKCVVQHTASKSKKEIFRWPESPKAQASSVPTAQPQAEGSLAKATTAPATTRNTGRGGEEKKKEKEKEEQEERETKTPECPSHTQPLGVYLLTPAVQDLWLRDKATFTCFVVGSDLKDAHLTWEVAGKVPTGGVEEGLLERHSNGSQSQHSRLTLPRSLWNAGTSVTCTLNHPSLPPQRLMALREPAAQAPVKLSLNLLASSDPPEAASWLLCEVSGFSPPNILLMWLEDQREVNTSGFAPARPPPQPRSTTFWAWSVLRVPAPPSPQPATYTCVVSHEDSRTLLNASRSLEVSYLAMTPLIPQSKDENSDDYTTFDDVGSLWTTLSTFVALFILTLLYSGIVTFIKVK","function_summary":"A membrane-associated immunoglobulin heavy-chain subunit that assembles a multivalent Ig-like scaffold to form the membrane-bound form of an antibody. Its tandem immunoglobulin domains and a delta-specific linker create a high-avidity platform that binds the light chain and organizes signaling upon antigen-driven clustering. This membrane receptor concentrates inflammatory signaling that elevates interleukin-1 production and is found at the cell surface of immune cells, with additional distribution via extracellular vesicles and microparticles that propagate immune signals.","plain_language":"IGHD has barely been studied. The model's best guess: A membrane-associated immunoglobulin heavy-chain subunit that assembles a multivalent Ig-like scaffold to form the membrane-bound form of an antibody. Its tandem immunoglobulin domains and a delta-specific linker create a high-avidity platform that binds the light chain and organizes signaling upon antigen-driven clustering.","uniprot_summary":"Delta chain of immunoglobulin heavy chains.","go_terms":{"molecular_function":[{"id":"GO:0005515","label":"protein binding"}],"biological_process":[{"id":"GO:0032732","label":"positive regulation of interleukin-1 production"}],"cellular_component":[{"id":"GO:0072562","label":"blood microparticle"},{"id":"GO:0070062","label":"extracellular exosome"},{"id":"GO:0005886","label":"plasma membrane"}]},"interpro":[{"id":"IPR013783","name":"Immunoglobulin-like fold","type":"homologous_superfamily","start":2,"end":107},{"id":"IPR036179","name":"Immunoglobulin-like domain superfamily","type":"homologous_superfamily","start":5,"end":98},{"id":"IPR013783","name":"Immunoglobulin-like fold","type":"homologous_superfamily","start":156,"end":261},{"id":"IPR036179","name":"Immunoglobulin-like domain superfamily","type":"homologous_superfamily","start":170,"end":260},{"id":"IPR013783","name":"Immunoglobulin-like fold","type":"homologous_superfamily","start":265,"end":375},{"id":"IPR036179","name":"Immunoglobulin-like domain superfamily","type":"homologous_superfamily","start":287,"end":371},{"id":"IPR007110","name":"Immunoglobulin-like domain","type":"domain","start":6,"end":98},{"id":"IPR003597","name":"Immunoglobulin C1-set","type":"domain","start":17,"end":94},{"id":"IPR049351","name":"Immunoglobulin delta heavy chain-like, delta linker region","type":"domain","start":106,"end":159},{"id":"IPR007110","name":"Immunoglobulin-like domain","type":"domain","start":175,"end":263},{"id":"IPR013151","name":"Immunoglobulin-like beta-sandwich domain","type":"domain","start":179,"end":260},{"id":"IPR003597","name":"Immunoglobulin C1-set","type":"domain","start":185,"end":259},{"id":"IPR007110","name":"Immunoglobulin-like domain","type":"domain","start":267,"end":373},{"id":"IPR003597","name":"Immunoglobulin C1-set","type":"domain","start":289,"end":365},{"id":"IPR050380","name":"Immune Recognition and Response Modulators","type":"family","start":2,"end":258},{"id":"IPR003006","name":"Immunoglobulin/major histocompatibility complex, conserved site","type":"conserved_site","start":353,"end":359}],"reasoning":"I begin by dissecting the InterPro architecture and its order along the sequence. The N-terminal half is dominated by immunoglobulin-like modules: IPR013783 (Immunoglobulin-like fold, residues 2–107) and IPR036179 (Immunoglobulin-like domain superfamily, residues 5–98) together with IPR007110 (Immunoglobulin-like domain, residues 6–98) and IPR003597 (Immunoglobulin C1-set, residues 17–94) define a canonical C1-set Ig-like domain that forms the membrane-proximal constant region of an antibody heavy chain. Immediately downstream sits IPR049351 (Immunoglobulin delta heavy chain-like, delta linker region, residues 106–159), a hallmark flexible linker unique to delta-class heavy chains that tunes spacing and signaling geometry. The central region continues with a second Ig-like module: IPR013783 (Immunoglobulin-like fold, residues 156–261) accompanied by IPR036179 (Immunoglobulin-like domain superfamily, residues 170–260), IPR007110 (Immunoglobulin-like domain, residues 175–263), IPR013151 (Immunoglobulin-like beta-sandwich domain, residues 179–260), and IPR003597 (Immunoglobulin C1-set, residues 185–259), establishing a tandem C1-set scaffold. The C-terminal third repeats this architecture with IPR013783 (Immunoglobulin-like fold, residues 265–375), IPR036179 (Immunoglobulin-like domain superfamily, residues 287–371), IPR007110 (Immunoglobulin-like domain, residues 267–373), and IPR003597 (Immunoglobulin C1-set, residues 289–365), producing a tripartite Ig-like array typical of membrane-bound immunoglobulin heavy chains. A conserved motif near the C-terminus, IPR003006 (Immunoglobulin/major histocompatibility complex, conserved site, residues 353–359), supports structural integrity and assembly. The breadth of Ig-like scaffolding and the delta-specific linker collectively specify a membrane-bound antibody heavy chain that signals via antigen-triggered clustering rather than intrinsic catalysis.\n\nThis architecture dictates molecular function as multivalent protein binding (GO:0005515). The tandem Ig-like domains and the delta linker create a rigid-but-adaptable platform that binds the immunoglobulin light chain and assembles higher-order complexes with signaling adaptors. The absence of catalytic motifs and the presence of multiple Ig-like beta-sandwich folds focus the function on avidity-driven assembly and signal initiation.\n\nFrom this binding-centric mechanism, the biological process emerges. Membrane-bound immunoglobulin complexes initiate immune signaling that elevates inflammatory mediators. The delta-chain architecture is specialized for antigen-driven clustering that triggers pathways culminating in cytokine production. Such signaling can drive positive regulation of interleukin-1 production (GO:0032732), consistent with antibody-receptor pathways that amplify innate inflammatory responses.\n\nCellular localization follows directly from the heavy-chain scaffold and its role in antigen recognition. The Ig-like array and delta linker are hallmarks of the membrane-associated immunoglobulin form, placing the protein at the plasma membrane (GO:0005886) as a single-pass membrane receptor. Biosynthesis and secretion routes, together with immune cell shedding and vesicular export, explain its presence in extracellular exosomes (GO:0070062) and blood microparticles (GO:0072562). These vesicular compartments disseminate immunoglobulin complexes and their fragments, extending signaling beyond the originating cell.\n\nMechanistically, the tandem Ig-like domains and the delta-specific linker cause antigen-induced clustering that recruits and organizes signaling modules at the membrane. I hypothesize that the complex nucleates assembly with the immunoglobulin light chain and membrane-proximal signaling machinery, forming higher-order arrays that couple to Src-family kinases and Syk to drive downstream pathways that enhance IL-1 production. Likely interaction partners include the immunoglobulin light chain, membrane-proximal adaptors and kinases that read out receptor clustering, and trafficking systems that package membrane Ig into exosomes and microparticles for extracellular signaling.","category":"dark","control_evaluation":null,"validation_plan":null,"structure":{"source":"AlphaFold DB","pdb":"/structures/AF-P01880.pdb","alphafold":"https://alphafold.ebi.ac.uk/entry/P01880"},"links":{"uniprot":"https://www.uniprot.org/uniprotkb/P01880/entry","atlas":"/p/P01880"},"provenance":{"model":"BioReason-Pro (ESM3 esm3-sm-open-v1)","source_list":"Pharos Tdark","note":"AI prediction, not experimentally verified"}}