Book : Cell Biology
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Myosin

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Myosins are a large super-family of motor proteins that move along actin filaments, while hydrolyzing ATP to forms of mechanical energy that can be used for a variety of functions such as muscle movement and contraction. About 20 classes of myosin have been distinguished on the basis of the sequence of amino acids in their ATP-hydrolyzing motor domains. The different classes of myosin also differ in structure of their tail domains. Tail domains have various functions in different myosin classes, including dimerization and other protein-protein interactions.

Myosins comprise a superfamily of ATP-dependent motor proteins and are best known for their role in muscle contraction and their involvement in a wide range of other motility processes in eukaryotes. They are responsible for actin-based motility. The term was originally used to describe a group of similar ATPases found in the cells of both striated muscle tissue and smooth muscle tissue. Thus, although myosin was originally thought to be restricted to muscle cells, there is no single "myosin" but rather a huge superfamily of genes whose protein products share the basic properties of actin binding, ATP hydrolysis (ATPase enzyme activity), and force transduction. Virtually all eukaryotic cells contain myosin isoforms. Some isoforms have specialized functions in certain cell types (such as muscle), while other isoforms are ubiquitous. The structure and function of myosin is strongly conserved across species, to the extent that rabbit muscle myosin II will bind to actin from an amoeba.

Myosin is a common protein found in the muscles which are responsible for making the muscle contract and relax. It is a large, asymmetric molecule, and has one long tail as well as two globular heads. If dissociated, it will dissociate into six polypeptide chains. Two of them are heavy chains which are wrapped around each other to form a double helical structure, and the other four are light chains. One main characteristic of myosin is its ability to bind very specifically with actin. When myosin and actin are combined together, that makes the muscle produce force.

Myosin I

Myosin I, a ubiquitous cellular protein, functions as monomer and functions in vesicle transport. It has step size of 10 nm and has been implicated as being responsible for the adaptation response of the stereocilia in the inner ear.

 Myosin II

Myosin II (also known as conventional myosin) is the myosin type responsible for producing muscle contraction in muscle cells. In muscle cells, the long coiled-coil tails of the individual myosin molecules join together, forming the thick filaments of the sarcomere. The force-producing head domains stick out from the side of the thick filament, ready to walk along the adjacent actin-based thin filaments in response to the proper chemical signals.

Myosin III

Myosin III is a poorly understood member of the myosin family. It has been studied in vivo in the eyes of Drosophila, where it is thought to play a role in phototransduction. A human homologue gene for myosin III, MYO3A, has been uncovered through the Human Genome Project and is expressed in the retina and cochlea.

Myosin IV

Myosin IV has a single IQ motif and a tail that lacks any coiled-coil forming sequence. It has homology similar to the tail domains of Myosin VII and XV.

Myosin V

Myosin V is an unconventional myosin motor, which is processive as a dimer and has a step size of 36 nm. It translocates (walks) along actin filaments traveling towards the barbed end (+ end) of the filaments. Myosin V is involved in the transport of cargo (e.g. RNA, vesicles, organelles, mitochondria) from the center of the cell to the periphery, but has been furthermore shown to act like a dynamic tether, retaining vesicles and organelles in the actin-rich periphery of cells. A recent single molecule in vitro reconstitution study on assembling actin filaments suggests that Myosin V travels farther on newly assembling (ADP-Pi rich) F-actin, while processive runlengths are shorter on older (ADP-rich) F-actin.

Myosin VI

Myosin VI is an unconventional myosin motor, which is primarily processive as a dimer, but also acts as a nonprocessive monomer. It walks along actin filaments, travelling towards the pointed end (- end) of the filaments. Myosin VI is thought to transport endocytic vesicles into the cell.



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