Factin

Factin is a hypothetical cytoskeletal protein described in fictional biology as a regulator of actin filament assembly in eukaryotic cells. In the imagined model, Factin promotes nucleation of filament formation and modulates the stability and branching of actin networks at the cell cortex. The protein is named for its association with F-actin and is typically portrayed as a small, acidic molecule of about 20 to 25 kilodaltons.

Structural features include a central helical core flanked by flexible termini, enabling simultaneous binding to actin monomers, profilin, and components of the Arp2/3 complex. In simulations, Factin concentrates at the leading edge of migrating cells and at sites of cytokinesis, where it collaborates with other regulators to shape filament dynamics.

Regulation is described in the fictional literature as controlled by phosphorylation at conserved serine residues and by calcium-dependent conformational changes. Membrane association is proposed through a lipid-binding motif that anchors Factin to the inner plasma membrane, aligning filaments with directional cues.

Genetic and evolutionary notes indicate that Factin is encoded by a gene named FCTN in the model organism Factinia curiosa and has tentative orthologs in several hypothetical eukaryotes, though real-world genomes contain no confirmed equivalents.

Because Factin is a fictional construct, there is no experimental evidence for its existence. It is used primarily as a teaching example to illustrate how actin-regulating proteins might function and to test computational models of cytoskeletal dynamics.