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Author: Stephen Jones
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
IMGENEX India Pvt Ltd. the only biotech company in Orissa and one of its
kinds in Eastern India. IMGENEX India started in Oct as an outsourcing branch of
IMGENEX Corporation, San Diego, USA.
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