{"uniprot":"A4QPB2","gene":"LRP5L","name":"Low-density lipoprotein receptor-related protein 5-like protein","organism":"Homo sapiens (Human)","status":"dark","length":252,"sequence":"MEGHVYWTDDEVWAIRRAYLDGSGAQTLINTKINDPDDIAVNWVARSLYWTHTGTEHIEVTCLNSTSHKILVSEDMDEPRAIALHPEMGLTYWIDWGENPEIKRANLDRQELRVLVNASLGWPNGLALDLQEGKLYWGDAKTDKIEAISVDETKRQTLLKDKLPHIFRFTLLGDFIYWTAWQHHSIKRVHKVKANRDVIIDQLPDLMGLKAVNVDKVVGTNPHADRNGGAATCASSRPTQPGLAAPSRAWNC","function_summary":"A single-pass cell-surface glycoprotein whose N-terminal beta-propeller decorated with LDL receptor repeats forms a high-avidity ectodomain that binds other signaling receptors and co-receptors. By nucleating and stabilizing receptor assemblies at the plasma membrane, it modulates receptor-driven pathways and couples extracellular ligand capture to endocytic routing. A minor trafficking pool likely moves from the membrane to the nucleus, where it can interface with transcriptional machinery that interprets receptor signals.","plain_language":"LRP5L has barely been studied. The model's best guess: A single-pass cell-surface glycoprotein whose N-terminal beta-propeller decorated with LDL receptor repeats forms a high-avidity ectodomain that binds other signaling receptors and co-receptors. By nucleating and stabilizing receptor assemblies at the plasma membrane, it modulates receptor-driven pathways and couples extracellular ligand capture to endocytic routing.","uniprot_summary":"May be involved in receptor signaling.","go_terms":{"molecular_function":[{"id":"GO:0005102","label":"signaling receptor binding"}],"biological_process":[{"id":"GO:0007166","label":"cell surface receptor signaling pathway"}],"cellular_component":[{"id":"GO:0005886","label":"plasma membrane"},{"id":"GO:0005654","label":"nucleoplasm"}]},"interpro":[{"id":"IPR011042","name":"Six-bladed beta-propeller, TolB-like","type":"homologous_superfamily","start":1,"end":245},{"id":"IPR000033","name":"LDLR class B repeat","type":"repeat","start":3,"end":155},{"id":"IPR000033","name":"LDLR class B repeat","type":"repeat","start":156,"end":196}],"reasoning":"I begin with the InterPro architecture. The N-terminal ectodomain is dominated by IPR011042 (Six-bladed beta-propeller, TolB-like homologous superfamily, residues 1–245). This fold forms a radially arranged six-blade propeller that creates a rigid, solvent-exposed platform for ligand capture and avidity-driven complex assembly. Embedded within this scaffold are tandem LDL receptor class B repeats: IPR000033 (LDLR class B repeat, residues 3–155) followed by IPR000033 (LDLR class B repeat, residues 156–196). LDLR class B repeats are calcium-stabilized beta-helix modules that frequently cooperate with beta-propellers to generate high-affinity binding surfaces for extracellular ligands and co-receptors. The ordered layout—an N-terminal six-bladed beta-propeller decorated by LDLR repeats—creates a modular ectodomain optimized for multivalent extracellular interactions rather than catalysis.\n\nThis ectodomain architecture dictates the molecular function. A beta-propeller with LDLR repeats is a classic ligand-capture and co-receptor platform that binds other signaling receptors and their accessory proteins at the cell surface. Such a platform causes signaling receptor binding, aligning with GO:0005102 signaling receptor binding. The absence of catalytic motifs and the presence of repeat-mediated binding modules reinforce a role as a scaffold/adaptor that stabilizes and organizes receptor complexes.\n\nFrom this binding function, the biological process follows. By assembling and stabilizing receptor–co-receptor assemblies, the protein modulates downstream pathways initiated at the cell surface. This assembly role propagates information inward, situating the protein in the cell surface receptor signaling pathway (GO:0007166). The ectodomain’s architecture supports ligand-induced clustering and endocytic routing, which are canonical steps that tune signaling amplitude and duration.\n\nThe cellular context is twofold. The extracellular beta-propeller and LDLR repeats imply a type I membrane topology with a luminal ectodomain and a short cytosolic tail, which places the protein at the plasma membrane (GO:0005886). The presence of a nucleoplasm annotation (GO:0005654) suggests a regulated trafficking or signaling branch: short cytosolic tails of single-pass membrane proteins often undergo endocytosis and can be escorted to the nucleus via adaptor-mediated pathways. Thus, a minor pool likely transits from the membrane into the nucleoplasm, where it may scaffold transcriptional regulators downstream of receptor signaling.\n\nMechanistically, the ectodomain’s beta-propeller and LDLR repeats create a high-avidity capture surface that binds other signaling receptors and co-receptors at the cell surface, nucleating complexes that govern receptor clustering and internalization. I hypothesize that the cytosolic tail (though not explicitly annotated here) carries short linear motifs that recruit clathrin adaptors and endocytic machinery, coupling extracellular binding to intracellular routing. In this model, the protein transiently associates with membrane-proximal signaling hubs and, in specific contexts, with nuclear effectors that interpret receptor-driven transcriptional programs.\n\nLikely interaction partners follow from this mechanism. At the membrane, it should engage other receptor families and co-receptors whose assembly benefits from beta-propeller/LDLR ectodomains—candidates include integrin-associated modules and growth factor receptor complexes. Endocytic adaptors such as AP-2 and clathrin are expected cytosolic partners that translate receptor binding into trafficking. A small nuclear subpopulation may associate with nucleoplasmic scaffolds that integrate signaling with gene regulation.","category":"dark","control_evaluation":null,"validation_plan":null,"structure":{"source":"AlphaFold DB","pdb":"/structures/AF-A4QPB2.pdb","alphafold":"https://alphafold.ebi.ac.uk/entry/A4QPB2"},"links":{"uniprot":"https://www.uniprot.org/uniprotkb/A4QPB2/entry","atlas":"/p/A4QPB2"},"provenance":{"model":"BioReason-Pro (ESM3 esm3-sm-open-v1)","source_list":"Pharos Tdark","note":"AI prediction, not experimentally verified"}}