{"uniprot":"A6NJW9","gene":"CD8B2","name":"Putative T-cell surface glycoprotein CD8 beta-2 chain","organism":"Homo sapiens (Human)","status":"dark","length":210,"sequence":"MRPRLWLLLAAQLTVLHGNSVLQQTPAYIKVQTNKMVMLSCEAKISLSNMCIYWLRQRQAPSSDSHHEFLTLWDSAKGTIHGEEVEQEKIAVFRDASRFILNLTSVKPEDSGIYFCMIVGSPELTFGKGTQLSVVDFLPTTAQPTKKSTLKKRVCRLPRPETQKGPLCSPVTLGLLVAGVLVLLVSLGVAMHLCCRRRRARLRFMKQFYK","function_summary":"A cell-surface glycoprotein that forms a single-pass membrane receptor with a large extracellular immunoglobulin-like ectodomain. It organizes and stabilizes signaling assemblies that drive T cell activation by clustering and tuning receptor pathways at the plasma membrane, while its endocytic itinerary through early endosomes modulates signal duration. By binding co-receptors and engaging extracellular matrix via integrin-linked adhesion, it enhances T cell receptor signaling and couples membrane cues to intracellular activation programs.","plain_language":"CD8B2 has barely been studied. The model's best guess: A cell-surface glycoprotein that forms a single-pass membrane receptor with a large extracellular immunoglobulin-like ectodomain. It organizes and stabilizes signaling assemblies that drive T cell activation by clustering and tuning receptor pathways at the plasma membrane, while its endocytic itinerary through early endosomes modulates signal duration.","uniprot_summary":"T-cell antigen and cell surface glycoprotein that interacts with both the T-cell receptor (TCR) and the beta-2-microglobulin-associated extracellular matrix. T-cell antigens transmit information to the intracellular signaling apparatus to produce the T-cell response.","go_terms":{"molecular_function":[{"id":"GO:0005515","label":"protein binding"}],"biological_process":[{"id":"GO:0042110","label":"T cell activation"},{"id":"GO:0050852","label":"T cell receptor signaling pathway"}],"cellular_component":[{"id":"GO:0005886","label":"plasma membrane"},{"id":"GO:0031901","label":"early endosome membrane"}]},"interpro":[{"id":"IPR013783","name":"Immunoglobulin-like fold","type":"homologous_superfamily","start":19,"end":143},{"id":"IPR036179","name":"Immunoglobulin-like domain superfamily","type":"homologous_superfamily","start":21,"end":136},{"id":"IPR007110","name":"Immunoglobulin-like domain","type":"domain","start":3,"end":118},{"id":"IPR013106","name":"Immunoglobulin V-set domain","type":"domain","start":24,"end":135},{"id":"IPR003599","name":"Immunoglobulin domain subtype","type":"domain","start":26,"end":135},{"id":"IPR042414","name":"T-cell surface glycoprotein CD8 beta chain","type":"family","start":1,"end":210}],"reasoning":"I begin with the InterPro architecture. The ectodomain is defined by multiple immunoglobulin signatures: IPR007110 (Immunoglobulin-like domain, residues 3–118) sits at the N-terminus and is encompassed by IPR013783 (Immunoglobulin-like fold homologous superfamily, residues 19–143) and IPR036179 (Immunoglobulin-like domain superfamily, residues 21–136). Within this scaffold, IPR013106 (Immunoglobulin V-set domain, residues 24–135) and IPR003599 (Immunoglobulin domain subtype, residues 26–135) specify a V-set Ig module that typically mediates cell–cell recognition and adhesion. The full-length classification IPR042414 (T-cell surface glycoprotein CD8 beta chain family, residues 1–210) anchors the protein’s identity and implies a type I single-pass membrane receptor with a large extracellular Ig ectodomain and a short cytoplasmic tail. The ordered layout—an N-terminal V-set Ig ectodomain followed by a transmembrane segment and cytoplasmic tail—creates a binding platform that captures ligands and co-receptors at the cell surface and couples them to intracellular signaling.\n\nThis Ig-based ectodomain causes high-affinity protein–protein interactions, establishing GO:0005515 protein binding as the molecular function. The V-set fold and CD8-family identity predict binding to other membrane proteins and extracellular matrix cues. Such binding capacity is not merely adhesive; it organizes multimeric complexes that tune receptor signaling thresholds.\n\nFrom this binding-driven molecular function, the biological process follows. The CD8-family architecture is specialized for assembling and stabilizing T cell receptor complexes and for routing signals that culminate in GO:0050852 T cell receptor signaling pathway. By stabilizing receptor clusters and recruiting co-receptors, the protein directly promotes GO:0042110 T cell activation. The Ig ectodomain’s ability to engage integrins and extracellular matrix explains how cell–matrix adhesion modulates signaling: adhesion-induced clustering increases the local density of TCR/CD3 machinery and biases downstream pathways toward productive activation.\n\nCellular localization is dictated by the topology and trafficking signals. The single-pass type I membrane configuration and extracellular Ig modules place the protein at the GO:0005886 plasma membrane, where it forms cell-surface complexes. Following activation and ligand engagement, endocytic motifs and adaptor recognition drive internalization into early endosomes, consistent with GO:0031901 early endosome membrane. This cycling between the cell surface and early endosomes is a hallmark of antigen receptor pathways and supports rapid signal modulation.\n\nMechanistically, the ectodomain’s V-set Ig module binds co-receptors and matrix-associated ligands to nucleate a signaling hub. This hub recruits and stabilizes TCR/CD3 assemblies, couples to Src-family kinases and cytoskeletal adaptors, and then undergoes endocytic down-modulation via early endosomes. I hypothesize that the cytoplasmic tail transiently engages endocytic adaptors and actin-associated scaffolds, while the ectodomain engages integrins and extracellular matrix to tune receptor avidity. Likely interaction partners include β2-microglobulin-associated complexes and MHC-presenting assemblies, integrin complexes that mediate adhesion, and endocytic machinery that governs early endosome trafficking—together forming a modular system that activates T cells and coordinates their membrane–cytoskeleton coupling.","category":"dark","control_evaluation":null,"validation_plan":null,"structure":{"source":"AlphaFold DB","pdb":"/structures/AF-A6NJW9.pdb","alphafold":"https://alphafold.ebi.ac.uk/entry/A6NJW9"},"links":{"uniprot":"https://www.uniprot.org/uniprotkb/A6NJW9/entry","atlas":"/p/A6NJW9"},"provenance":{"model":"BioReason-Pro (ESM3 esm3-sm-open-v1)","source_list":"Pharos Tdark","note":"AI prediction, not experimentally verified"}}