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Actin is a globular protein that can polymerise to form microfilaments. It is an essential element of the cytoskeleton of a biological cell, and is the component of the cytoskeletal system that allows movement of cells and cellular processes; actin filaments also participate in muscle contraction.

Actin exists in two forms: G-actin (monomeric globular actin); and F-actin (polymeric fibrous actin), the form involved in muscle contraction. Actin filaments are called microfilaments to distinguish them from intermediate filaments.

Actin is typically the most abundant protein in a eukaryotic cell, accounting for about 15% of the total protein in some cell types. Actin is composed of four domains with a large cleft that almost bisects the molecule. This cleft forms both a divalent cation and nucleotide binding site. Actin is highly conserved (the amino-acid sequence of actin from Acanthamoeba (a small soil amoeba) is 95% identical to vertebrate isoforms of actin), and it forms a huge variety of structure in cells in concert with many different actin binding proteins (there are between 60 and 100 different actin binding proteins). The ADF/cofilin group of actin-binding proteins are particularly complex in their relationship with actin, and can either stimulate polymerisation or depolymerisation, depending on the conditions (particularly pH).