ACTIN

Abstract

Eukaryotic cells contain three distinct cytoskeletal filament systems, including actin, that exhibit very different assembly properties, supramolecular architectures, dynamic behavior, and mechanical properties. Actin, which is involved in a plethora of functions, is the most abundant protein in most cell types and is impressively conserved across species. The highest concentrations of actin (about 20% of total protein) are found as stable microfilament systems assembled within myofibrillar contractile structures in striated muscles. In addition to its specialized role in muscle contraction, actin is present in all muscle and nonmuscle cells, where it plays a variety of roles thanks to its ability to assemble and disassemble depending on cell requirements. Actin plays an important role in maintaining cell structure and function by conferring mechanical strength and enabling intracellular contraction and/or tension. In nonmuscle cells, microfilaments are involved in cell motility and cytokinesis. The dynamics of the actin cytoskeleton are maintained by two factors: (1) the ability of actin to undergo reversible transformation from the monomeric state (G-actin) to the polymeric state (F-actin) and (2) the interaction of actin with actin-binding proteins (ABPs), that can inhibit or stimulate actin polymerization, sever the polymers, cross-link actin filaments into bundles or in filamentous three-dimensional networks, and bind them to cell membranes. Considering the multiple cellular functions of actin, alterations in the organization of microfilaments will result in disorganized cell arrangement and orientation, uncontrolled cell growth, and abnormal responses to the environment. Higher vertebrates express six different highly conserved actin isoforms. Over the last decades, numerous studies have tried to elucidate the specific expressions, localizations, regulations, properties, and functions of the different isoactins. The understanding of their specific underlying mechanisms would be of major relevance not only for fundamental research but also for clinical applications, since modulations of actin isoforms are directly or indirectly correlated with severe pathologies.