Aktín

Actin is a highly conserved protein that forms microfilaments, a major component of the cytoskeleton in eukaryotic cells. It exists as globular actin (G-actin) and filamentous actin (F-actin). F-actin filaments are polar, with a fast-growing barbed (plus) end and a slower pointed (minus) end. In Turkish, aktín refers to this protein.

Actin monomers are about 42 kilodaltons in size, consisting of roughly 375 amino acids. In vertebrates, actin exists as several isoforms: alpha-actins in muscle (ACTA1 for skeletal muscle, ACTA2 for smooth muscle) and beta- and gamma-actin in non-muscle cells (ACTB and ACTG1), along with additional tissue-specific variants. Despite high sequence similarity, different isoforms show distinct expression patterns and functional specializations.

Polymerization occurs when G-actin binds ATP and assembles into F-actin in a head-to-tail arrangement. ATP is hydrolyzed after incorporation into the filament, and actin dynamics are tightly regulated by a large set of actin-binding proteins. Formins and the Arp2/3 complex promote nucleation and branching of filaments; profilin delivers actin monomers to growing filaments; thymosin beta-4 sequesters G-actin; cofilin accelerates disassembly; tropomyosin stabilizes filaments; capping proteins regulate filament ends; and severing by proteins such as gelsolin influences filament turnover.

Actin filaments provide mechanical support, determine cell shape, enable movement and morphogenesis, and drive processes such as cytokinesis and muscle contraction through interaction with myosin. They form the cortical cytoskeleton, participate in endocytosis and vesicle trafficking, and contribute to organelle positioning.

Actin is also present in the nucleus, where it participates in chromatin remodeling and transcription. Mutations or dysregulation of actin isoforms are associated with various diseases, including muscle disorders and cardiomyopathies; ACTA1 mutations can cause congenital myopathies, and ACTA2 variants are linked to vascular diseases. Actin is highly conserved across eukaryotes.