Actin filament

Microfilaments, also called actin filaments, are filamentous structures in the cytoplasm of eukaryotic cells and form part of the cytoskeleton. They are primarily composed of polymers of actin, but in cells are modified by and interact with numerous other proteins. Microfilaments are usually about 7 nm in diameter and composed of two strands of actin. Microfilament functions include cytokinesis, amoeboid movement and cell motility in general, changes in cell shape, endocytosis and exocytosis, cell contractility and mechanical stability. Microfilaments are flexible and relatively strong, resisting buckling by multi-piconewton compressive forces and filament fracture by nanonewton tensile forces. In inducing cell motility, one end of the actin filament elongates while the other end contracts, presumably by myosin II molecular motors. Additionally, they function as part of actomyosin-driven contractile molecular motors, wherein the thin filaments serve as tensile platforms for myosin's ATP-dependent pulling action in muscle contraction and pseudopod advancement. Microfilaments have a tough, flexible framework which helps the cell in movement.

Actin and microfilament-mediated processes were subject of research since long time ago. Engelmann (1879) suggested that many kinds of movement observed in plants and protozoa like cytoplasmic streaming and amoeboid movement were in fact a primitive version of the movements of muscle contraction.

...
Wikipedia