The Function of Actin

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Actin, a globular protein with a molecular weight of 42-kDa, is the most abundant intracellular protein in most eukaryotes (3). It plays important roles in many essential cell biological processes. Actin exists as a monomer called G-actin and as a filament called F-actin, a linear chain of G-actin subunits (2). Two F-actin filaments intertwine to form a microfilament, one of the three major components of cytoskeleton (2). Microfilament is important for generating cell movement. One mechanism involves the assembly and disassembly of microfilaments (actin dynamics) that drives biological processes, such as cell division, cell mobility and cytokinesis (3). The other mechanism requires motor proteins that use energy from ATP to slide along a microfilament carrying organelles and vesicles with it (2). In addition, actin also plays an essential role in muscular contraction. The smallest functional unit of skeletal muscle is the sarcomere (1). A sarcomere is composed of many strands of two protein filaments, the thick and the thin filaments (1). The thick filament is made of the protein myosin, a motor protein, and the thin filament is composed of actin polymers (1). Myosin and actin work together sliding alongside each other to create contractile force of skeletal muscle (1). Actin is encoded by a highly conserved gene family. The gene sequences from algae and humans are identical at 80 percent of the positions (3). Single-celled organisms, such as yeasts, have one or two actin genes, whereas multicellular organisms have multiple actin genes (3). For instance, humans have at least six actin genes, which are divided into three classes: alpha, beta and gamma (2). Although the actin isoforms exhibit minor sequence differences, they have different functions that can not be substituted by one another in vivo (3). In general, alpha-actin isoforms (α-skeletal, α-aortic smooth, α-cardiac, and γ2-enteric smooth) are found in muscle cells and they are associated with muscle...
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