microfilaments

Microfilaments, also known as actin filaments, are long, thin polymers of actin that form a central part of the cytoskeleton in most eukaryotic cells. They have a diameter of about 7 nanometers and are built from actin monomers (G-actin) that assemble into two-stranded helical filaments (F-actin). F-actin is polar, with a fast-growing barbed (+) end and a slower pointed (-) end, enabling dynamic assembly and disassembly, a process known as treadmilling.

In cells, microfilaments organize into networks and bundles that support the plasma membrane, define cell shape, and generate forces for movement. They are abundant in the cortex, lamellipodia, filopodia, and microvilli, where polymerization at the leading edge pushes the membrane forward during migration. They form the contractile ring during cytokinesis and cooperate with myosin motors to produce contractile forces in non-muscle cells.

In muscle cells, actin filaments are the thin filaments that, together with myosin, drive contraction within sarcomeres. Microfilaments also serve as tracks for myosin-based transport of organelles and vesicles and participate in endocytosis and exocytosis, providing force for membrane remodeling.

Actin polymerization and organization are regulated by a diverse set of actin-binding proteins, including Arp2/3 complex for branched networks, formins for linear filaments, profilin and thymosin beta-4 for monomer dynamics, cofilin for turnover, gelsolin for severing, tropomyosin for filament stabilization in muscle, and cross-linkers such as alpha-actinin, fimbrin, and fascin for network architecture.