Actins

Actins are a family of highly conserved globular proteins that polymerize to form filamentous actin (F-actin), a major component of the eukaryotic cytoskeleton. In cells, actin exists as monomeric G-actin and assembles into helical, polar filaments with distinct barbed (plus) and pointed (minus) ends, enabling dynamic remodeling of the cytoskeleton.

In humans, there are six actin genes encoding different isoforms with tissue-specific expression: ACTA1 (skeletal muscle α-actin), ACTA2 (smooth muscle α-actin), ACTC1 (cardiac α-actin), ACTB (β-actin), ACTG1 (γ-actin), and ACTG2 (γ-enteric actin). Despite high sequence similarity, these isoforms have specialized roles in distinct cell types and tissues.

Biochemically, actin monomers bind ATP and add to the growing filament, with ATP hydrolysis occurring after incorporation. This ATPase activity drives dynamic turnover and treadmilling, enabling rapid cytoskeletal rearrangements crucial for cell movement and shape changes.

Actin dynamics are regulated by a diverse set of actin-binding proteins (ABPs). Nucleators such as the Arp2/3 complex and formins initiate filament formation; profilin promotes rapid elongation by delivering actin monomers; thymosin β-4 sequesters G-actin; capping proteins control filament ends; tropomyosin stabilizes filaments; cofilin promotes disassembly of aged filaments. These regulators coordinate polymerization, stabilization, branching, and turnover.

Functions of actin networks include providing mechanical support, determining cell shape, driving motility and membrane extensions, organizing organelles, enabling vesicle transport, and mediating cytokinesis. In muscle, actin filaments interact with myosin to produce contraction. Actin also participates in plant processes such as cytoplasmic streaming. Mutations in actin genes or dysregulation of ABPs can contribute to various diseases.