Actin and Actin Filaments

Abstract Actin is an evolutionary conserved molecule that self assembles into long polymers. These filaments form the building block of the actin cytoskeleton. This system is one of the main engines enabling cell motility processes. In muscle cells stable actin filaments participate in contraction. In nonmuscle cells, however, the filaments are more dynamic and polymerisation of actin generates the force for the formation of cellular protrusions required for cell migration. Various actin‐binding proteins orchestrate this dynamic turnover of actin filaments and thereby contribute to the different migration modes possible for nonmuscle cells. Next to its role in the cytoplasm actin also resides in the nucleus where it may regulate transcription in multiple ways. Because of its complexity, impaired regulation of the actin cytoskeleton disturbs cell function and deregulations of the actin machinery are associated with an increasing number of diseases such as myopathies and cancer. Key Concepts: Actins are conserved proteins with important functions during development and in adult life. Actin self‐assembles into structurally and kinetically polarised filaments. Numerous actin‐binding proteins regulate (dis)assembly and organise filaments in higher order structures. Dynamic actin polymerisation drives multiple types of nonmuscle cell migration via formation of specific subcellular structures. Actin cytoskeletal diseases are caused by mutations in actins or via disturbed regulation of the cytoskeleton. In addition to its cytoplasmic function in cell motility processes, actin has a nuclear role in transcription.