{"uniprot":"Q8TC99","gene":"FNDC8","name":"Fibronectin type III domain-containing protein 8","organism":"Homo sapiens (Human)","status":"dark","length":324,"sequence":"MASEALHQVGDGEEAVLKKENFNMMNALDQLPKPFSNPKSMNRTVTTKGLPLASKGNLVNFLEDDTINLLKPLPVEDSDCSSDETSISAFSSTLLNPIKLAVTQPNSSFFAGMLEGELNKLSFSPMAKNAENEDLALGPCPCPSKSQMATRGLLDLDNPELETETSSTHSESSVVVDLPDTPFIFEHTVNNSTAVISWTYALGKQPVSFYQLLLQEVAKTQENELPEAKNRPWIFNKILGTTVKLMELKPNTCYCLSVRAANTAGVGKWCKPYKFATLATDFSSFPENYPIQITVRRKEPRQKIVSIGPEEMRRLEDLEYLFPC","function_summary":"A nuclear adaptor that uses a fibronectin type III/immunoglobulin-like scaffold to assemble protein complexes that dampen canonical Wnt signaling. By stabilizing repressor assemblies on Wnt-responsive promoters and interfacing with transcription factors and chromatin-modifying machinery, it reduces transcription of target genes in the nucleus through multivalent protein binding rather than catalysis.","plain_language":"FNDC8 has barely been studied. The model's best guess: A nuclear adaptor that uses a fibronectin type III/immunoglobulin-like scaffold to assemble protein complexes that dampen canonical Wnt signaling. By stabilizing repressor assemblies on Wnt-responsive promoters and interfacing with transcription factors and chromatin-modifying machinery, it reduces transcription of target genes in the nucleus through multivalent protein binding rather than catalysis.","uniprot_summary":"Involved in the negative regulation of canonical Wnt signaling pathway.","go_terms":{"molecular_function":[{"id":"GO:0005515","label":"protein binding"}],"biological_process":[{"id":"GO:0090090","label":"negative regulation of canonical Wnt signaling pathway"}],"cellular_component":[{"id":"GO:0005634","label":"nucleus"}]},"interpro":[{"id":"IPR036116","name":"Fibronectin type III superfamily","type":"homologous_superfamily","start":166,"end":280},{"id":"IPR013783","name":"Immunoglobulin-like fold","type":"homologous_superfamily","start":174,"end":289},{"id":"IPR003961","name":"Fibronectin type III","type":"domain","start":179,"end":280}],"reasoning":"I begin with the InterPro architecture. The C-terminal half of the protein is dominated by beta-sandwich modules: IPR036116 (Fibronectin type III superfamily, residues 166–280) and IPR013783 (Immunoglobulin-like fold, residues 174–289) overlap and frame a well-defined IPR003961 Fibronectin type III domain from residues 179–280. This tandem of FN3 and Ig-like folds creates an elongated, adhesive scaffold that typically mediates protein–protein interactions in extracellular or pericellular contexts. The absence of catalytic motifs and the presence of FN3/Ig-like modules causally point to a non-enzymatic binder whose primary chemistry is multivalent protein binding, aligning with GO:0005515 protein binding as the molecular function.\n\nFN3/Ig-like modules often serve as interaction hubs that assemble signaling complexes. In a nuclear role, such beta-sandwich scaffolds can stabilize transcriptional assemblies or sequester transcription factors. The stated nuclear residency (GO:0005634 nucleus) and the soluble, non-membranous architecture together argue that this scaffold operates inside the nucleus rather than at membranes or the secretory pathway. A nuclear FN3/Ig-like platform can nucleate repressor complexes that tune pathway outputs indirectly.\n\nConnecting this binding-centric architecture to biological process, the most parsimonious route is transcriptional repression of Wnt-responsive genes. Canonical Wnt signaling culminates in nuclear complexes where beta-catenin coactivates TCF/LEF transcription. A nuclear FN3/Ig-like scaffold can inhibit this output by assembling repressors that destabilize beta-catenin’s transcriptional activity or by favoring chromatin states that reduce target gene expression. This mechanism directly supports GO:0090090 negative regulation of canonical Wnt signaling pathway.\n\nCellular localization follows naturally. The soluble, non-enzymatic binding architecture and the need to interface with transcriptional machinery place the protein in the nucleus (GO:0005634 nucleus), consistent with a role in assembling or modulating transcriptional complexes.\n\nFrom these features, I hypothesize that the protein functions as a nuclear adaptor that binds transcription factors and corepressors to attenuate Wnt target gene expression. Likely partners include beta-catenin and TCF/LEF factors at Wnt-responsive promoters, alongside corepressor assemblies such as Groucho/TLE and HDAC-containing complexes. The FN3/Ig-like fold provides the avidity and geometry needed to stabilize these assemblies, thereby enforcing pathway repression through competitive assembly and promoter-proximal sequestration.","category":"dark","control_evaluation":null,"validation_plan":null,"structure":{"source":"AlphaFold DB","pdb":"/structures/AF-Q8TC99.pdb","alphafold":"https://alphafold.ebi.ac.uk/entry/Q8TC99"},"links":{"uniprot":"https://www.uniprot.org/uniprotkb/Q8TC99/entry","atlas":"/p/Q8TC99"},"provenance":{"model":"BioReason-Pro (ESM3 esm3-sm-open-v1)","source_list":"Pharos Tdark","note":"AI prediction, not experimentally verified"}}