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( Actin)
Actin is a globular, roughly 42-kDa protein found in all eukaryotic cells (except for nematode sperm) where it may be present at concentrations of over 100 µM. It is also one of the most highly-conserved proteins, differing by no more than 20% in species as diverse as algae and humans. It is the monomeric subunit of microfilaments, one of the three major components of the cytoskeleton, and of thin filaments, which are part of the contractile apparatus in muscle cells. Thus, actin participates in many important cellular functions, including muscle contraction, cell motility, cell division and cytokinesis, vesicle and organelle movement, cell signaling, and the establishment and maintenance of cell junctions and cell shape. Principal interactions of structural proteins at cadherin-based adherens junction. Actin filaments are linked to a-actinin and to membrane through vinculin. The head domain of vinculin associates to E-cadherin via a-, ß-, and ?-catenins. The tail domain of vinculin binds to membrane lipids and to actin filaments. The protein actin is one of the most highly conserved throughout evolution because it interacts with a large number of other proteins, with 80.2% sequence conservation at the gene level between Homo sapiens and Saccharomyces cerevisiae (a species of yeast), and 95% conservation of the primary structure of the protein product. Although most yeasts have only a single actin gene, higher eukaryotes, in general, express several isoforms of actin encoded by a family of related genes. Mammals have at least six actin isoforms coded by separate genes,[1] which are divided into three classes (alpha, beta and gamma) according to their isoelectric point. In general, alpha actins are found in muscle (a-skeletal, a-aortic smooth, a-cardiac, and ?2-enteric smooth), whereas beta and gamma isoforms are prominent in non-muscle cells (ß- and ?1-cytoplasmic). Although the amino acid sequences and in vitro properties of the isoforms are highly similar, these isoforms cannot completely substitute for one another in vivo.[2]
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