Structural Biochemistry/Protein function/Myosin

Introduction
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.

Structure
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.



Sarcomeres and the Sliding Filament Theory
Skeletal muscles are responsible for voluntary movement. Skeletal muscles contain many muscle fibers and these muscle fibers are actually made up of myofibrils, bundles of thick myosin filaments and thin actin filaments. Myofibrils are constructed and lined up in a chain-like formation to create what are called sarcomeres. Sarcomeres contain several regions. One region is called the A-bands and only consist of myosin filaments. The counterpart of A-bands is the I-bands that only contain actin filaments. The ends of each sarcomere are called Z discs. A middle region of each sarcomere called the H-zone only contains myosin.



According to the sliding filament theory by Andrew Huxley and Ralph Niedergerke, muscles contract when Z-discs come closer together thus shortening the sarcomeres. Actin filaments from the I-bands become very short while myosin filaments from the A-bands do not change in length. The actin filaments are actually sliding towards the H-zone and the A-bands thus creating an overlap of myosin and actin filaments. As this overlap occurs, myosin filaments are binding to the actin filaments, allowing myosin to function as the driving motor of filament sliding.



This relative movement between myosin and actin is what results in muscle contraction. The molecular basis for muscle action and contraction is explained in the next section.

Mechanism of muscle movement
This mechanism of contraction is also called "The Sliding Filament Theory."
 * ATP binding to the myosin head causes and it is in its low-energy conformation
 * The active site closes and ATP is hydrolyzed to ADP and Pi. This induces a conformational change (cocking of the head) resulting in myosin weakly binding to actin. This forms a cross-bridge.
 * Pi release results in conformational change that leads to stronger myosin binding, and the power stroke.
 * ADP dissociation leaves the myosin head tightly bound to actin.
 * Binding of a new molecule of ATP to myosin head triggers it to let go of actin and the cycle starts all over again.
 * In the absence of ATP, this state results in muscle rigidity called rigor mortis.





Different Types of the Myosin
Myosin has groups of protein that divide the motor proteins. The motor proteins are involving actin filaments that hydrolyze ATP. There are 20 different types of Myosin that already distinguished by amino acid sequence. All 20 types of Myosin have different structure following by tail domain. Because Each classes have characteristic of dimerization, and protein interactions. However, there are known classes in the Myosin. Myosin I, Myosin II, Myosin V and VI.

Myosin I

Among the proteins whose genes have been linked to deafness are several types of myosin. Myosin I appears to cross-link actin filaments to control the tension inside each stereocilium. The ratcheting activity of this myosin motor along the actin filaments may adjust the sensitivity of the hair cells to different sounds. Other types of myosin use their motor activity to redistribute cellular constituents along the length of the actin filaments.

Myosin II

MyosinII consists of six polypeptide chains: two 220-kD heavy chains and two pairs of different light chain that vary in size between 15 and 22kD, depending on their source. The N-terminal half of each individual heavy chain assumes a globular form that is stretched in one direction. Coming up next is a roughly 100 Angstrom long alpha helix stiffened by the two light chains wrapping around it. This portion of the protein acts as a lever when the muscle contracts. The C-terminal half of the heavy chain takes the form of alpha-helix that ends as a long, fibrous chain. Two of those associate and takes the form a left-handed coiled-coil motif. The overall shape of myosin is a rod 1600 Angstroms long with two globular heads.

Myosin V

MyosinV has a different structure of motor. It has a two headed motor protein which heavy chains diverge. That means actin dependent transport move to axon associated vesicle effect on a melanin. Both microtubule and actin filaments lead to the speculation and affect to the hair color. Myosin V is also a two-headed protein, but it doesn't form a thick filament like Myosin II. Myosin V acts by itself - the domain at the tail-end binds a vesicle that has pigments as its cargo. The lever region of this protein is long enough to have six light chains bound to it, giving it three times greater capacity for those light chains than Myosin II's counterpart lever arm. Under electron microscope (EM) image of Myosin V bound to F-actin filament, it is estimated that the globular heads have thirteen actin subunits between them.