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Actin is a ubiquitous protein involved in the formation of filaments that are major components of the cytoskeleton. 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. It is the most abundant protein in the typical eukaryotic cell, accounting for about 15% in some cell types. The protein is highly conserved, and forms a huge variety of structure in cells in concert with a huge numbers of actin binding proteins. The actin filaments interact with myosin to produce a sliding effect, which is the basis of muscular contraction and many aspects of cell motility, including cytokinesis.

The individual subunits of actin are known as globular actin (G-actin) that assembles into long filamentous polymers called F-actin. Two parallel F-actin strands twist around each other in a helical formation, giving rise to microfilaments of the cytoskeleton. Microfilaments measure approximately 7 nm in diameter with a loop of the helix repeating every 37nm. Each actin protomer binds one molecule of ATP and has one high affinity site for either calcium or magnesium ions, as well as several low affinity sites. It exists as a monomer in low salt concentrations, but filaments form rapidly as salt concentration rises, with the consequent hydrolysis of ATP. Actin from many sources forms a tight complex with deoxyribonuclease (DNase I) although the significance of this is still unknown.

The formation of this complex results in the inhibition of DNase I activity, and actin loses its ability to polymerise. It has been shown that an ATPase domain of actin shares similarity with ATPase domains of hexokinase and hsp70 proteins. In vertebrates there are three groups of actin isoforms: alpha, beta and gamma. The alpha actins are found in muscle tissues and are a major constituent of the contractile apparatus. The beta and gamma actins co-exist in most cell types as components of the cytoskeleton and as mediators of internal cell motility. MreB, a major component of the bacterial cytoskeleton, exhibits high structural homology to its eukaryotic counterpart actin. Further it has been suggested that members of the Rho family of small guanosine triphosphatases have emerged as key regulators of the actin cytoskeleton, and through their interaction with multiple target proteins, they ensure coordinated control of other cellular activities such as gene transcription and adhesion.

Reference:
1. Actin isoforms. Curr Opin Cell Biol. 1993 Feb;5(1):48-55 Herman IM
2. The assembly of MreB, a prokaryotic homolog of actin. The assembly of MreB, a prokaryotic homolog of actin. J Biol Chem. 2005 Jan 28;280(4):2628-35. Epub 2004 Nov 16
3. Rho GTPases and the Actin Cytoskeleton Science 23 January 1998:
Vol. 279. no. 5350, pp. 509 – 514 Alan Hall

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