Actin is a highly conserved protein that polymerizes to produce filaments that form cross-linked networks in the cytoplasm of cells. Actin exists in both monomeric (G-actin) and polymeric (F-actin) forms, both forms playing key functions, such as cell motility and contraction. In addition to their role in the cytoplasmic cytoskeleton, G- and F-actin also localize in the nucleus, and regulate gene transcription and motility and repair of damaged DNA. Polymerization of globular actin (G-actin) leads to a structural filament (F-actin) in the form of a two-stranded helix (By similarity). Each actin can bind to 4 others (By similarity). Skeletal muscle, heart, gill, digestive tissue and brain (PubMed:9987040). Widespread expression throughout the brain, with highest levels in regions where neuronal proliferation is greatest (PubMed:9987040). Oxidation of Met-44 and Met-47 by MICALs (mical1, mical2 or mical3) to form methionine sulfoxide promotes actin filament depolymerization. Mical1 and mical2 produce the (R)-S-oxide form. The (R)-S-oxide form is reverted by msrb1 and msrb2, which promote actin repolymerization (By similarity). Methylation at His-73 by SETD3. Methylation stabilizes actin filaments. In vertebrates 3 main groups of actin isoforms, alpha, beta and gamma have been identified. The alpha actins are found in muscle tissues and are a major constituent of the contractile apparatus. The beta and gamma actins coexist in most cell types as components of the cytoskeleton and as mediators of internal cell motility. Belongs to the actin family.