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Description

Actins are globular multi-functional proteins that serve as the basic building blocks of cytoskeletal microfilaments and are among the most conserved eukaryotic proteins. Six actin types exist, skeletal muscle alpha-actin (ACTA1), smooth muscle alpha-actin (ACTA2), cytoplasmic beta-actin (ACTB), cardiac muscle alpha-actin (ACTC1), cytoplasmic gamma-actin (ACTG1), and smooth muscle gamma-actin (ACTG2, a.k.a. ACTA3). Although actins show >90% overall sequence homology, isotypes do show spatial, temporal, and tissue-specific expresson patterns and only 50-60% homology is found in their 18 N-terminal residues. Cytoplasmic - and -actins are thought to be present in all cells, while the other four actin isoforms are typically found in specific adult muscle tissue types. Actins exist in a variety of structural states, depending on the specific ionic conditions or the interaction with ligand proteins. The oligomeric and polymeric forms that actin molecules assume are dependent on the distinct conformations they adopt. In additon to serving as the basic building blocks of cytoskeletal microfilaments, actins are involved in multiple nuclear functions and adopt specific conformations that are important for their association with nuclear actin-binding proteins.