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Actin and Myosin

Actin Myosin

Shooting Actin Filaments in Action

Actin, the protein responsible for muscle contraction and movement of a ...

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Proteins of the contractile and motile systems

Some protein molecules can produce multisubunit structures, which can be involved in to organism motility. This motility may involve subsellular moieties, cells, or part or whole organism. One of the most important examples of such motility system is the muscular system.

All biological motion systems for production movement are using the energy released by the hydrolysis of ATP. From this point of view it is possible to say, that proteins contractile and motile systems are convert chemical energy into kinetic energy.

The main contractile system of all muscular tissue are based on the interactions of two proteins Actin and Myosin. The system of these proteins some times called actin-myosin contractile system.

Actin

Actin is one of the most condensed forms of protein, which is globular and is a monomeric subunit of microfilament. The thin filaments in actin constitute a major part of it. The formation of thin filaments involves a complex process involving the activation of G-Actin to ultimately form the ADP-bound Actin. In this case, ATP acts both as the activator and also the catalyser.

Actin rules over cell functions which include cell division, morphing of the shape of the cells, cell mobility and other contractile properties. It is a 42 kDa protein and related gene has 100 nucleotides. Functioning of Actin is generally hindered by introns. Actin filaments are linked to the membrane through vinculin.

Actin basic functions involve:
  • Giving mechanical support to cells.
  • Enabling easy movement of cellular fluids and hence enhancing cell mobility.
  • Participating in signal transmission.
  • Working upon the cytoplasm and hardening it.


Cellulites like yeasts possess only one actin gene, but higher eukaryotes have several isoforms of actin. Mammals possess six isoforms of actin which are classified as alpha, beta, or gamma depending upon their isoelectric point. Normally alpha-actins are found in muscular tissues whereas beta-actins and gamma-actins are found in non-muscular cells. Alpha-Actin 1 or ACTA 1 is one of the six identified actin isoform which are found in skeletal muscles. Actin-related myopathy may occur without any missense mutation in the ACTA1 gene, but missense mutation causes congenital disorders. Beta-actins or ACTB are categorized as one of the nonmuscular cytoskeleton actin. Gamma-Actin 1 or ACTG1 which exists as a component of the cytoskeleton is a cytoplasmic actin which is also dominant in nonmuscular cells.

Actins are further subdivided into sub-units known as globular actins or G-actins. G-actins join to form F-actins which in turn give rise to the microfilaments of the cytoskeleton. Microfilaments are helical loops which repeat at every 37 nm. The polarity of actin is determined by aligning it with myosin. Actin in combination with motor protein myosin forms the actomyosin motor fibrils which regulates muscle contraction. Actin polymerization and depolimerisation is extremely essential for cytokinesis or cell division. Nucleating factors are necessary to catalyze actin polymerization.

All non-spherical prokaryotes appear to possess genes which encode homologues of actin. It helps in maintaining cellular structure and shape. Such a homologue of actin is found among bacteria which are denoted as MreB. It is similar to that of the arrangement at the active site of the peptide sequence. MreB polymerizes to form filaments which are structurally and characteristically similar to the actin microfilaments.

Myosin

The eukaryotes seem to contain a major amount of motor proteins. These motor proteins are found to be in great coordination with the actin filaments. They are known as the Myosin. Myosin can be subdivided as Myosin 1 and Myosin 2. Myosin 1 possesses the contractile property like the actins and hence helps in muscle contraction. It also enables vesicular transportation. More of its functions are yet to be identified. Myosin 2 contains a huge proportion of Amino acids. The structure of Myosin 2 is similar to an usual Myosin molecule. They are divided as the head and the tail. The head domain combines with actin in order to initiate force, whereas in Myosin 2 this head is again subdivided into two terminals. The tail domain helps in easy communication with the cargo muscles and coordinates with other Myosin subunits.

A human genome contains more than 40 different types of myosin genes. Its high viability towards the working of cargo muscles lies in the fact that the myosins have variability towards the head domain, but the head retains its original sequence. The different genes of myosin have different shapes which in turn determine the speed at which the filaments move. The tail always speeds up to catch up with the actin filaments. The power stroke property of the myosin which characterizes the length of it's lever arm specifies the distance moved by cargo muscles. The length of the lever arm is directly proportional to the distance traversed, which means the longer the length of the lever arms the greater the movement caused by the cargo muscles. The myosins have a number of cellular functions and they are the ultimate binding force behind active transport of proteins and vesicles in the cytoplasm. Kinesis and Dyanin are a group of related motor proteins and have a significant role during mitosis and myosis.

A number of sicknesses are associated with the deficiencies of the myosins. Kidney troubles, teeth infection and respiratory suffocations are quite common. It also affects sense organs in certain cases and also results in deafness.

Myosins are subdivided into a number of families and chains. Among them, myosin 1A or MYO1A is a human gene. Here its terminal contains both ATP binding site as well as actin binding site. Its unique feature lies in brush-border which helps the head and the tail domain to interact transiently with core actin and plasma membrane respectively. Its next family is again human genome or the myosin 3A or MYO3A. They are very unconventional possessing cargo-binding site towards their tale region. It contributes to hearing in humans. Myosin 5 which is also categorized under heavy chains usually possesses an unusually longer neck. Myosin 6 supports organelle movement and also performs other vesicular functions. The epithelial functions are enhanced by Myosin 7. It is prominently expressed in a large number of mammalian tissues.

As was told previously, myosins are categorized as the heavy chains as well as the light chains. The heavy chains or MYH are functional in striated, smooth and nonmuscular cells. The light chained myosin is also a human gene like those of the heavy chained and is active in the skeletal muscles. Two transcript variants have been identified for this gene category.
published: 03 Feb 2007 (13:51)