{"uniprot":"Q8IVN8","gene":"SBSPON","name":"Somatomedin-B and thrombospondin type-1 domain-containing protein","organism":"Homo sapiens (Human)","status":"dark","length":264,"sequence":"MRTLWMALCALSRLWPGAQAGCAEAGRCCPGRDPACFARGWRLDRVYGTCFCDQACRFTGDCCFDYDRACPARPCFVGEWSPWSGCADQCKPTTRVRRRSVQQEPQNGGAPCPPLEERAGCLEYSTPQGQDCGHTYVPAFITTSAFNKERTRQATSPHWSTHTEDAGYCMEFKTESLTPHCALENWPLTRWMQYLREGYTVCVDCQPPAMNSVSLRCSGDGLDSDGNQTLHWQAIGNPRCQGTWKKVRRVDQCSCPAVHSFIFI","function_summary":"A secreted extracellular matrix glycoprotein that uses an N-terminal somatomedin B module and a thrombospondin type-1 repeat to build a multivalent adhesion scaffold. By anchoring to matrix components and cell-surface receptors, it organizes local signaling environments that temper hemostatic pathways and favor balanced coagulation–fibrinolysis dynamics. Its disulfide-rich architecture and spondin-like C-terminus stabilize extracellular residency and avid protein binding, enabling it to modulate vascular and hemolysis processes within collagen-rich matrices.","plain_language":"SBSPON has barely been studied. The model's best guess: A secreted extracellular matrix glycoprotein that uses an N-terminal somatomedin B module and a thrombospondin type-1 repeat to build a multivalent adhesion scaffold. By anchoring to matrix components and cell-surface receptors, it organizes local signaling environments that temper hemostatic pathways and favor balanced coagulation–fibrinolysis dynamics.","uniprot_summary":"Could be involved in hemolysis and coagulation processes.","go_terms":{"molecular_function":[{"id":"GO:0005515","label":"protein binding"}],"biological_process":[{"id":"GO:0030195","label":"negative regulation of blood coagulation"}],"cellular_component":[{"id":"GO:0062023","label":"collagen-containing extracellular matrix"}]},"interpro":[{"id":"IPR036024","name":"Somatomedin B-like domain superfamily","type":"homologous_superfamily","start":35,"end":76},{"id":"IPR036383","name":"Thrombospondin type-1 repeat superfamily","type":"homologous_superfamily","start":72,"end":122},{"id":"IPR001212","name":"Somatomedin B domain","type":"domain","start":24,"end":75},{"id":"IPR044004","name":"Spondin-like TSP1 domain","type":"domain","start":75,"end":122},{"id":"IPR056801","name":"SBSPON-like, C-terminal","type":"domain","start":148,"end":257},{"id":"IPR039942","name":"Somatomedin-B and thrombospondin type-1 domain-containing protein","type":"family","start":4,"end":263},{"id":"IPR000884","name":"Thrombospondin type-1 (TSP1) repeat","type":"repeat","start":74,"end":133}],"reasoning":"I begin with the domain architecture. The N-terminal third contains IPR001212 (Somatomedin B domain, residues 24–75), which is encompassed by IPR036024 (Somatomedin B-like domain superfamily, residues 35–76). Somatomedin B (SB) modules are compact, disulfide-rich extracellular folds that mediate high-affinity protein–protein interactions and often anchor secreted matrix proteins to cell-surface receptors and protease systems. Immediately downstream lies a thrombospondin type-1 module: IPR044004 (Spondin-like TSP1 domain, residues 75–122) sits within IPR036383 (Thrombospondin type-1 repeat superfamily, residues 72–122) and is further supported by IPR000884 (Thrombospondin type-1 repeat, residues 74–133). TSP1 repeats are canonical extracellular adhesion/redistribution modules that bind glycosaminoglycans and cell-surface receptors, frequently shaping angiogenic and hemostatic cues. The C-terminal half is defined by IPR056801 (SBSPON-like, C-terminal, residues 148–257), a spondin-family tail that typically stabilizes matrix association and oligomerization. The entire polypeptide is captured by IPR039942 (Somatomedin-B and thrombospondin type-1 domain-containing protein family, residues 4–263), confirming a modular extracellular adhesion scaffold. The ordered layout—SB domain followed by a TSP1 repeat and a spondin-like C-terminus—creates a multivalent extracellular platform specialized for avid binding rather than catalysis.\n\nThis architecture dictates molecular function as protein binding (GO:0005515). The SB and TSP1 modules together generate high-avidity interfaces for extracellular matrix components, cell-surface receptors, and soluble mediators. The absence of catalytic motifs and the presence of multiple adhesive modules causally enforce a role as a binding hub that organizes and modulates extracellular signaling.\n\nFrom this binding-centric function, the biological process emerges. TSP1 modules and SB domains are frequently deployed in vascular and hemostatic contexts; their multivalent matrix anchoring and receptor engagement enable suppression of procoagulant signaling and promotion of fibrinolytic balance. By sequestering or spatially modulating coagulation/hemolysis factors at the extracellular matrix, this scaffold can dampen thrombotic propagation, aligning with negative regulation of blood coagulation (GO:0030195). The spondin-like C-terminus further stabilizes matrix localization, reinforcing this anti-coagulation role through persistent extracellular residency.\n\nCellular location follows directly. The disulfide-rich SB and TSP1 modules are archetypal secreted extracellular folds, and the spondin-like tail supports matrix association. This composition necessitates secretion and extracellular matrix residency, placing the protein in the collagen-containing extracellular matrix (GO:0062023). The experimentally observed secreted status is mechanistically explained by the requirement for disulfide formation and extracellular folding of SB/TSP1 modules.\n\nMechanistically, I hypothesize that the protein assembles a matrix-tethered complex that binds glycosaminoglycans and cell-surface receptors to locally titrate coagulation and hemolysis pathways. Likely partners include integrins and heparan sulfate proteoglycans via the TSP1 repeat; vitronectin- and uPAR-like receptor systems via the SB domain; and matrix constituents such as collagens and fibronectin. By concentrating fibrinolytic and anticoagulant cues in defined extracellular niches, the scaffold attenuates thrombus formation and tunes vascular homeostasis.","category":"dark","control_evaluation":null,"validation_plan":null,"structure":{"source":"AlphaFold DB","pdb":"/structures/AF-Q8IVN8.pdb","alphafold":"https://alphafold.ebi.ac.uk/entry/Q8IVN8"},"links":{"uniprot":"https://www.uniprot.org/uniprotkb/Q8IVN8/entry","atlas":"/p/Q8IVN8"},"provenance":{"model":"BioReason-Pro (ESM3 esm3-sm-open-v1)","source_list":"Pharos Tdark","note":"AI prediction, not experimentally verified"}}