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,

actin-binding protein any of several proteins that associate with either actin monomers or actin filaments in cells and modify their properties. Many of these proteins are found in the cell cortex, an actin-rich layer just below the plasma membrane. Examples include dystropin, profilin, spectrin and ankyrin, fimbrin and a-actinin (see actinin), filamin, gelsolin, and the

actin a major protein constituent of the thin filaments of muscle and of the microfilaments found in practically all eukaryotic cells – it comprises 5–10% of the protein of such cells (see actin filament). In solutions of low ionic strength, actin is a globular 42 kDa monomer, termed G-actin. At physiological ionic strengths, G-actin polymerizes