actin

Actin is a highly conserved, abundant eukaryotic protein that polymerizes to form microfilaments, a major component of the cytoskeleton. In cells, actin exists as globular monomers (G-actin) and as filamentous polymers (F-actin). Each actin monomer binds ATP, and after polymerization it hydrolyzes ATP, a cycle that drives filament turnover and dynamic remodeling known as treadmilling.

Actin filaments support cell shape, enable movement, and organize the cytoplasm. They lay down networks and bundles in the cell cortex, lamellipodia, filopodia, and stress fibers, and participate in processes such as vesicle transport, organelle positioning, and cytokinesis. In muscle cells, actin forms thin filaments that interact with myosin to produce contraction; this interaction is regulated by tropomyosin and the troponin complex in response to calcium.

Actin is encoded by a multigene family. In humans, the major cytoplasmic isoforms are beta- and gamma-actin, while several muscle-specific alpha-actins are expressed in skeletal and cardiac muscle. Polymerization is orchestrated by a suite of actin-binding proteins, including nucleators (Arp2/3 complex, formins), monomer regulators (profilin, thymosin beta-4), and capping or severing proteins (capping proteins, cofilin). These factors control filament nucleation, elongation, turnover, and organization, enabling precise cellular architecture and dynamics.

Actin is highly conserved across eukaryotes, with typical cytoplasmic actin polypeptides around 375 amino acids in length and about 42 kDa. Beyond the cytoplasm, actin also localizes to the nucleus, where it participates in chromatin remodeling and transcription, though nuclear actin dynamics are less well understood. Alterations in actin expression or regulation are associated with a range of diseases, including muscle disorders and cancer metastasis.