{"uniprot":"P60673","gene":"PFN3","name":"Profilin-3","organism":"Homo sapiens (Human)","status":"dark","length":137,"sequence":"MGDWKVYISAVLRDQRIDDVAIVGHADNSCVWASRPGGLLAAISPQEVGVLTGPDRHTFLQAGLSVGGRRCCVIRDHLLAEGDGVLDARTKGLDARAVCVGRAPRALLVLMGRRGVHGGILNKTVHELIRGLRMQGA","function_summary":"A cytoplasmic actin-monomer adaptor that organizes the cytoskeleton by binding G-actin and proline-rich partners to tune filament assembly. Through phosphorylation-sensitive interfaces, it sequesters and delivers actin to assembly pathways, stabilizing actin structures while suppressing stress fiber formation and promoting membrane-associated ruffles and epithelial cell migration. It operates in the cytoplasm and cytoskeleton and can be packaged into extracellular vesicles, coordinating ATP-coupled cytoskeletal remodeling.","plain_language":"PFN3 has barely been studied. The model's best guess: A cytoplasmic actin-monomer adaptor that organizes the cytoskeleton by binding G-actin and proline-rich partners to tune filament assembly. Through phosphorylation-sensitive interfaces, it sequesters and delivers actin to assembly pathways, stabilizing actin structures while suppressing stress fiber formation and promoting membrane-associated ruffles and epithelial cell migration.","uniprot_summary":"Binds to actin and affects the structure of the cytoskeleton.","go_terms":{"molecular_function":[{"id":"GO:0005515","label":"protein binding"}],"biological_process":[{"id":"GO:2000145","label":"regulation of cell motility"},{"id":"GO:0031647","label":"regulation of protein stability"},{"id":"GO:0010632","label":"regulation of epithelial cell migration"},{"id":"GO:0016043","label":"cellular component organization"},{"id":"GO:0010634","label":"positive regulation of epithelial cell migration"},{"id":"GO:0050821","label":"protein stabilization"},{"id":"GO:0032781","label":"positive regulation of ATP-dependent activity"},{"id":"GO:1900029","label":"positive regulation of ruffle assembly"},{"id":"GO:0030837","label":"negative regulation of actin filament polymerization"},{"id":"GO:0030838","label":"positive regulation of actin filament polymerization"},{"id":"GO:0051497","label":"negative regulation of stress fiber assembly"}],"cellular_component":[{"id":"GO:0005737","label":"cytoplasm"},{"id":"GO:0005856","label":"cytoskeleton"},{"id":"GO:0070062","label":"extracellular exosome"}]},"interpro":[{"id":"IPR036140","name":"Profilin superfamily","type":"homologous_superfamily","start":3,"end":136},{"id":"IPR005455","name":"Profilin, eukaryotic type","type":"family","start":1,"end":137},{"id":"IPR005454","name":"Profilin1/2/3, vertebrate","type":"family","start":3,"end":19},{"id":"IPR048278","name":"Profilin","type":"family","start":4,"end":136},{"id":"IPR005454","name":"Profilin1/2/3, vertebrate","type":"family","start":32,"end":47},{"id":"IPR005454","name":"Profilin1/2/3, vertebrate","type":"family","start":53,"end":67},{"id":"IPR005454","name":"Profilin1/2/3, vertebrate","type":"family","start":70,"end":83},{"id":"IPR005454","name":"Profilin1/2/3, vertebrate","type":"family","start":111,"end":124}],"reasoning":"I begin with the InterPro architecture. The core of the protein is defined by IPR036140 (Profilin superfamily, residues 3–136) and reinforced by multiple profilin family signatures: IPR005455 (Profilin, eukaryotic type, residues 1–137), IPR048278 (Profilin, residues 4–136), and three vertebrate clade-specific family marks—IPR005454 (Profilin1/2/3, vertebrate, residues 3–19), IPR005454 (residues 32–47), IPR005454 (residues 53–67), and IPR005454 (residues 70–83)—that collectively delineate the canonical profilin fold and its vertebrate-specific surface features. This ordered ensemble of profilin superfamily and family domains encodes a compact β-sheet/α-helix architecture that creates two principal ligand interfaces: one for actin monomers and another for poly-L-proline motifs and phosphoinositide-binding surfaces. The absence of catalytic motifs and the dominance of binding/scaffold signatures indicate a non-enzymatic adaptor whose primary chemistry is high-affinity, selective protein binding.\n\nFrom this architecture, the molecular function resolves as protein binding (GO:0005515). The profilin fold forms a cradle that binds ATP–G-actin and a hydrophobic groove that recognizes proline-rich sequences in actin assembly factors. This dual binding causes concentration- and phosphorylation-dependent control over actin filament dynamics. By sequestering actin monomers and delivering them to specific nucleators, profilin can both suppress and promote filament growth; this tunable buffering explains the coexistence of negative and positive regulation of actin filament polymerization (GO:0030837 and GO:0030838). The same monomer-handling mechanism stabilizes actin assemblies (GO:0050821 protein stabilization) and biases cytoskeletal architecture away from thick bundles and toward membrane-associated protrusions, accounting for negative regulation of stress fiber assembly (GO:0051497) and positive regulation of ruffle assembly (GO:1900029). By steering actin turnover and protrusive activity, profilin enhances cell motility pathways, supporting positive regulation of epithelial cell migration (GO:0010634) and broader positive regulation of ATP-dependent activity (GO:0032781), as actin cycling couples to ATP-driven motors and nucleators.\n\nCellular localization follows from function and binding logic. A soluble adaptor that manages actin monomers and proline-rich partners operates in the cytoplasm and cytoskeleton, consistent with a cytoplasmic, cytoskeletal residency. Its presence in extracellular vesicles is also expected: cytoskeletal regulators are frequently packaged into extracellular exosomes (GO:0070062), which disseminates cytoskeletal control modules between cells.\n\nMechanistically, the profilin fold binds ATP–G-actin and modulates filament assembly by controlling monomer availability and delivery to barbed-end growth pathways. Phosphorylation-sensitive surfaces and poly-L-proline affinity tune whether actin is sequestered or handed off to nucleators and elongation factors. I therefore hypothesize that this protein transiently associates with actin itself and with proline-rich actin assembly factors such as formins and Ena/VASP adaptors, as well as with phosphoinositide-rich membrane microdomains that seed ruffle formation. These interactions collectively bias cytoskeletal architecture toward dynamic protrusions and efficient epithelial migration while stabilizing actin assemblies and suppressing stress fiber formation.","category":"dark","control_evaluation":null,"validation_plan":null,"structure":{"source":"AlphaFold DB","pdb":"/structures/AF-P60673.pdb","alphafold":"https://alphafold.ebi.ac.uk/entry/P60673"},"links":{"uniprot":"https://www.uniprot.org/uniprotkb/P60673/entry","atlas":"/p/P60673"},"provenance":{"model":"BioReason-Pro (ESM3 esm3-sm-open-v1)","source_list":"Pharos Tdark","note":"AI prediction, not experimentally verified"}}