Myosin

Myosin (from Greek μυός myos, genitive to μῦς mys, muscle ' ) is a family of motor proteins in eukaryotic cells. Myosin is involved as a key component in the muscle also in the conversion of chemical energy into force and motion. Furthermore, it is involved in cooperation with other motor proteins such as kinesin and dynein much on intracellular transport of biological loads such as biological macromolecules, vesicles and organelles. In contrast to kinesin and dynein to myosin moves along actin filaments. Other cellular functions include, among others, cell motility and adhesion.

Myosin classes

First, myosin was as a motor protein of muscle fibers identified (part of the sarcomere, the contractile portions of the myofibrils ( muscle fibers )), where a bundle of myofibrils forms a muscle fiber. Later, other members of the protein family have appeared whose function is still largely unknown. For some of these unconventional myosins functions could in intracellular transport of vesicles and organelles, are identified in endocytosis or cell growth.

After phylogenetic studies (see Homology (biology) ) one divides the family of myosins in classes and subclasses. Occurring in muscle myosin part with some other non- muscle myosins to class II, which are also known as conventional myosins. All other classes are called unconventional myosins. The first discovered unconventional myosins have been grouped into class I. Newer classes of unconventional myosins are numbered consecutively (III, IV, V, ... ). Officially 18 classes of unconventional myosins were appointed, where now are known at least six additional previously named classes. Some limitations and allocations are questionable.

Protein structure

Functional Myosin consists of several chains of amino acids:

• A heavy chain (heavy chain ) and
• A different number of light chains.

Myosin II molecule is present as a so-called dimer consists of 6 sub-units ( hexamer )

• 2 heavy chains
• 4 light chains

Heavy chain ( heavy chain )

Common to all myosin heavy chains is a conserved head domain, which combines the catalytic ATPase properties and therefore motor domain is called.

At this, the neck region connects to which (, LCBD Light Chain Binding Domain) may contain a different number of binding domains for light chains. An example of this is the IQ motif common ones ( consensus sequence: IQXXXRXXXXR ) to which the protein can bind calmodulin as a light chain in the function. Depending on the class varies the number of LCBD. To that extent, coiled-coil structures are formed, there is the possibility of dimerization, so that double-headed myosins arise.

Likewise, the subsequent tail region is class-specific and pronounced, the biggest differences between the different classes. This region often contains different protein interaction domains to which the cargo to be transported can dock. Conventional myosins are also known to the tail region of these dimeric molecules tends to form filaments. In this way, myosin fibers that are part of the sarcomere form. For non- muscle myosin, it is assumed that over the domains in the tail region of the specificity of the transporter is determined.

Light chain ( light chain )

The light chains are small chains of amino acids that bind to the much larger heavy chain. The specific binding is specific to binding domains for light chains ( LCBD ).

There are several light chains, the function of one hand is purely structural (essential light chain), on the other hand, the activity of the motor domain regulates ( regulatory light chain).

This is especially pointed out the regulation of the muscle activity by Ca2 - ions, which is done via the regulatory light chain.

The movement of myosin cross- bridge cycle

The example described herein illustrates the movement of using the conventional myosin muscle myosin II, wherein the actin and myosin filaments are pushed into one another. The procedure for the unconventional myosins is equal of principle, except that a myosin dimer " wanders" here with his cargo to a actin filament along. In each case, a myosin head binds alternately, so that virtually " one foot after the other " is set before. The movement is directed, because myosin can walk along the actin filament in one direction only. Usually myosin running towards the plus end of a Aktinfilaments. An exception here so far myosin VI, which moves in the opposite direction. Different unconventional myosin classes thus assume here that movement in both directions on the actin, in contrast to kinesin and dynein on microtubules.

The motor type is described by the so-called transverse bridge cycle. Compared to the tail region of the motor domain is angled. In the absence of ATP it binds tightly to actin filaments. Binding of ATP causes dissociation from the actin filament. Then, the inclination angle of the motor domain of about 90 ° (pre -power stroke conformation) changed. By the hydrolysis of ATP to ADP P. myosin binds back to the actin filament. After delivery of P and ADP it comes to the power stroke (power stroke ), a folding back of the inclination angle to 50 ° (post -power stroke conformation). Here, because myosin is again tightly bound to the actin filament, there is a directed movement. Muscle causes the repeated passage of the transverse bridge cycle that the myosin filaments and actin filaments into each other; it leads to muscle contraction.

• Cross- bridge cycle

Phase 2 - myosin heads (yellow) cleave ATP to ADP and phosphate, and thereby lead their force beat. The bound actin is displaced in the direction of the power stroke. The indicated angle is about 50 °.

Phase 3 - myosin heads (yellow) dissolve with absorption of ATP from actin ( pink ).

Phase 4 - myosin (yellow) in the resting state. Actin ( pink ).

Provision of energy

The ATP supply in muscle is usually sufficient for five to six seconds duration load. Then sequentially creatine phosphate ( enough for about ten more seconds) and finally glucose (dextrose ) metabolized by the muscles.