actin filament : a two-stranded helical polymer of the protein actin. Actin filaments form the thin filaments of muscle and also the microfilaments of the cytoskeleton of eukaryotic cells. Hence they are a major component of the contractile apparatus of skeletal muscle, and one of the three types of protein filament that form the cytoskeleton, the others being microtubules and intermediate filaments. The filaments, comprising polymerized globular actin molecules, appear as flexible structures with a diameter of 5–9 nm. They are organized into a variety of linear bundles, two-dimensional networks, and three-dimensional gels. In the cytoskeleton they are most highly concentrated in the cortex of the cell just beneath the plasma membrane.
