PFN3

Model 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.

Predicted Gene Ontology terms

1. 01

   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.

2. 02

   From 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.

3. 03

   Cellular 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.

4. 04

   Mechanistically, 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.