{"uniprot":"Q99445","gene":"GML","name":"Glycosyl-phosphatidylinositol-anchored molecule-like protein","organism":"Homo sapiens (Human)","status":"dark","length":158,"sequence":"MLLFALLLAMELPLVAASATMRAQWTYSLRCHDCAVINDFNCPNIRVCPYHIRRCMTISIRINSRELLVYKNCTNNCTFVYAAEQPPEAPGKIFKTNSFYWVCCCNSMVCNAGGPTNLERDMLPDEVTEEELPEGTVRLGVSKLLLSFASIIVSNILP","function_summary":"A GPI-anchored, plasma membrane–associated LU-domain protein that uses a rigid three-finger extracellular scaffold to bind and allosterically inhibit membrane receptor complexes, notably acetylcholine receptors. By dampening receptor and adhesion signaling at the cell surface, it shifts cell-fate decisions toward checkpoint arrest and apoptosis and restrains proliferation. Its extracellular, membrane-proximal position enables high-affinity modulation of receptor assemblies that feed into p53-driven stress pathways and cell-cycle control.","plain_language":"GML has barely been studied. The model's best guess: A GPI-anchored, plasma membrane–associated LU-domain protein that uses a rigid three-finger extracellular scaffold to bind and allosterically inhibit membrane receptor complexes, notably acetylcholine receptors. By dampening receptor and adhesion signaling at the cell surface, it shifts cell-fate decisions toward checkpoint arrest and apoptosis and restrains proliferation.","uniprot_summary":"Probable function in cell adhesion.","go_terms":{"molecular_function":[{"id":"GO:0005515","label":"protein binding"},{"id":"GO:0030550","label":"acetylcholine receptor inhibitor activity"}],"biological_process":[{"id":"GO:0008285","label":"negative regulation of cell population proliferation"},{"id":"GO:0006915","label":"apoptotic process"},{"id":"GO:0030330","label":"DNA damage response, signal transduction by p53 class mediator resulting in cell cycle arrest"}],"cellular_component":[{"id":"GO:0019898","label":"extrinsic component of membrane"},{"id":"GO:0005886","label":"plasma membrane"}]},"interpro":[{"id":"IPR045860","name":"Snake toxin-like superfamily","type":"homologous_superfamily","start":22,"end":112},{"id":"IPR016054","name":"Ly-6 antigen/uPA receptor-like","type":"domain","start":29,"end":123},{"id":"IPR052874","name":"Sperm-binding zona pellucida regulatory protein","type":"family","start":1,"end":158},{"id":"IPR018363","name":"CD59 antigen, conserved site","type":"conserved_site","start":30,"end":77}],"reasoning":"I begin with the InterPro architecture. The sequence is dominated by the Ly6/uPAR module: IPR016054 (Ly-6 antigen/uPA receptor-like domain, residues 29–123) sits centrally and is flanked by IPR045860 (Snake toxin-like superfamily, residues 22–112), a three-fingered LU/three-finger fold that forms disulfide-stabilized beta-sheet modules specialized for high-affinity extracellular protein–protein interactions. This LU scaffold is reinforced by IPR018363 (CD59 antigen, conserved site, residues 30–77), marking the canonical cystine-knot-like core that rigidifies the binding surface and resists proteolysis in extracellular or membrane-proximal environments. The entire polypeptide is encompassed by IPR052874 (Sperm-binding zona pellucida regulatory protein family, residues 1–158), a family that unites LU-domain proteins functioning as membrane-tethered modulators of cell-surface receptors and adhesion systems. The ordered layout—an N-to-C LU domain with conserved CD59-like features and no catalytic motifs—dictates a non-enzymatic, extracellular binding and inhibitory role.\n\nThis LU architecture causes strong, selective protein binding as the primary molecular function, aligning with GO:0005515 protein binding. The three-finger LU fold is well known to clamp and allosterically modulate ligand-gated ion channels and other receptors; this structural logic supports GO:0030550 acetylcholine receptor inhibitor activity, because LU-domain proteins frequently attenuate nicotinic acetylcholine receptor signaling by altering gating and surface availability. The same rigid, disulfide-rich surface can sterically and electrostatically tune receptor complexes, explaining inhibitory modulation rather than enzymatic chemistry.\n\nFrom these molecular activities, the biological processes follow. LU-domain inhibitory signaling at the cell surface propagates into cell-fate pathways. By dampening proliferative receptor cascades and biasing toward checkpoint and death pathways, the protein contributes to GO:0008285 negative regulation of cell population proliferation. Its membrane-proximal inhibitory signaling and checkpoint coupling plausibly feed into GO:0006915 apoptotic process, particularly when stress pathways are engaged. The LU fold’s extracellular control over receptor and adhesion cues can intersect with genome surveillance circuits, supporting GO:0030330 DNA damage response, signal transduction by p53 class mediator resulting in cell cycle arrest. In this model, surface-derived signals modulate p53-driven arrest by curbing mitogenic inputs and promoting checkpoint thresholds.\n\nCellular localization is dictated by the LU architecture and the presence of GPI anchoring. LU-domain proteins are typically secreted or tethered to the outer leaflet of the plasma membrane via a C-terminal GPI anchor signal, positioning them as extrinsic membrane components. This matches GO:0019898 extrinsic component of membrane and GO:0005886 plasma membrane. The disulfide-rich LU fold and absence of transmembrane helices reinforce an extracellular, membrane-associated topology that optimizes receptor modulation at the cell surface.\n\nMechanistically, I hypothesize that the LU/three-finger scaffold forms a rigid extracellular clamp that binds and inhibits acetylcholine receptor assemblies and other membrane receptor complexes. By tuning receptor gating and adhesion-dependent signaling, it biases downstream pathways toward checkpoint arrest and apoptosis under stress. Likely interaction partners include nAChR subunits at the cell surface, uPAR-family receptors and co-receptors that organize adhesion and signaling microdomains, and membrane microdomain organizers that cluster GPI-anchored proteins. These assemblies provide a route from extracellular modulation to intracellular p53-mediated responses and controlled cell proliferation.","category":"dark","control_evaluation":null,"validation_plan":null,"structure":{"source":"AlphaFold DB","pdb":"/structures/AF-Q99445.pdb","alphafold":"https://alphafold.ebi.ac.uk/entry/Q99445"},"links":{"uniprot":"https://www.uniprot.org/uniprotkb/Q99445/entry","atlas":"/p/Q99445"},"provenance":{"model":"BioReason-Pro (ESM3 esm3-sm-open-v1)","source_list":"Pharos Tdark","note":"AI prediction, not experimentally verified"}}