Actin

Actin is a helical protein that forms 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. In particular, actin can undergo constant rearrangement to produce movement. Actin filaments are also called microfilaments to distinguish them from intermediate filaments.

Actin is the most abundant protein in the typical eukaryotic cell, accounting for about 15% 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).