Actina

Actina, or actin, is a highly conserved protein that forms the microfilaments of the cytoskeleton in eukaryotic cells. It exists as globular actin monomers (G-actin) that polymerize to form long helical filaments (F-actin). Actin filaments are dynamic, capable of rapid assembly and disassembly, and are essential for cell shape, movement, and division. In muscle cells, actin combines with myosin to produce contractile forces as part of thin filaments.

Genes encoding actin come in several isoforms with tissue-specific patterns. In humans, ACTA1 encodes skeletal muscle alpha-actin, ACTA2 encodes smooth muscle alpha-actin, ACTC1 encodes cardiac alpha-actin, ACTB encodes beta-actin, and ACTG1 encodes gamma-actin. Beta- and gamma-actins are mainly cytoplasmic and involved in general cell structure and motility, while alpha-actins provide structural components of muscle fibers.

Actin monomers bind ATP, which is hydrolyzed after polymerization. Filament ends are polarized: a fast-growing barbed (plus) end and a slower pointed (minus) end, a feature that underpins treadmilling and dynamic remodeling of the cytoskeleton. Actin dynamics are tightly regulated by a large set of actin-binding proteins, including Arp2/3 complex (branched networks), formins (elongation), profilin and thymosin beta-4 (monomer sequestration), and cofilin (filament turnover).

Functions extend from maintaining cell shape and providing mechanical support to enabling cell migration, vesicle transport, and cytokinesis. In muscle, actin filaments interact with myosin to generate force. Actin is essential for life; mutations can contribute to muscular diseases, cardiomyopathies, and actin-related pathologies. In research, actin is studied by staining (phalloidin), polymerization assays (pyrene-actin), and through analyses of G- versus F-actin.