Myosin ATPase Activity

Myosin motor protein structure[Edit]

Myosin is a motor protein that uses the energy from ATP hydrolysis to move along actin filaments. Many myosin isoforms have been found in eukaryotes (See Figure below), which differ in the type of heavy and light chains they are composed of. All myosins are composed of a diverse 'tail' domain at their carboxy terminus and an evolutionarily conserved globular 'head' domain at their amino terminus.

Figure 1. The myosin superfamily of motor proteins: All myosins share a motor domain on their heavy chains at the amino-terminus (the 'head' domain), but they differ considerably at their carboxy-terminus (the 'tail' domain). A few myosin types also have an amino-terminal extension. The number of light chains varies considerably between myosin types and certain myosins exist as dimers. Myosins that form dimers have two motor domains, and the number of light chains can influence the "lever arm" length between the myosin heads - this regulates the length of the myosin 'powerstroke' and the distance the myosin can travel along the actin filament in a single round of ATP hydrolysis (see also 'myosin powerstroke').
The diverse 'tails' of different myosin isoforms bind specific substrates or cargo, whilst their conserved 'heads' contain sites for ATP binding [1], F-actin binding and force generation (i.e. motor domains) (reviewed in [23]). 

All myosins bind to actin filaments via a globular 'head' domain located at the end of the heavy chains. Actin binding to this region increases the ATPase activity of myosins (reviewed in [2][4]. Some myosins have a single heavy chain and contact actin filaments at only one site, while other myosin isoforms have two heavy chains and contact actin filaments at two sites. Myosin II is the only family member that can form polymeric assemblies[3]) (See "thick filaments" below).

The number of light chains influences the length of the "lever arm" or "neck region" and therefore the "step size" of different myosin types [5]. Myosin V contains more light chains relative to myosin II and so myosin V moves in larger steps along actin filaments after an equivalent round of ATP hydrolysis (reviewed in [6]). 

Myosin motors move along actin filaments in defined directions. With the exception of myosin VI, which moves towards the pointed end, all myosins move towards the barbed end. Most actin filaments have the barbed end directed towards the plasma membrane and the pointed end towards the interior. This arrangement allows certain myosins (e.g. myosin V) to function primarily for cargo export, while myosin VI acts as the major motor protein for import. Myosin II is commonly associated with retraction fibers and retrograde actin flow at the pointed end of actin filaments. All non-muscle cells use contractile bundles containing myosin II to generate forces that promote the assembly of actin filaments.

Although most myosins function as motor proteins in the cytoplasm, some species of myosin are localized to, and function in, the nucleus. Nuclear Myosin I (NMI) myosin II, myosin V, myosin VI, myosin XVIB and myosin XVIIIB have all been found in the nucleus [789], with NMI being the most extensively studied. 

The Myosin 'Powerstroke Mechanism'[Edit]

Each myosin motor protein possesses ATPase activity and functions in a cyclical manner that couples ATP binding and hydrolysis to a conformational change in the protein. This process is known as the ‘powerstroke cycle’ (reviewed in [10, 11, 2]) and is outlined in the steps below using myosin II as an example. This is further described in the Figure below. 

The direction in which the actin filament will be moved is dictated by the structural orientation of myosin in relation to the filament. A complete round of ATP hydrolysis produces a single 'step' or movement of myosin along the actin filament. This process is regulated by changes in the concentration of intracellular free calcium (reviewed in [12]). The steps involved are detailed below:

Figure 2. The "power stroke" mechanism for myosin movement along actin filaments
Step 1: At the end of the previous round of movement and the start of the next cycle, the myosin head lacks a bound ATP and it is attached to the actin filament in a very short-lived conformation known as the 'rigor conformation'.

Step 2: ATP binding to the myosin head domain induces a small conformational shift in the actin-binding site that reduces its affinity for actin and causes the myosin head to release the actin filament.

Step 3: ATP binding also causes a large conformational shift in the 'lever arm' of myosin that bends the myosin head into a position further along the filament. ATP is then hydrolysed, leaving the inorganic phosphate and ADP bound to myosin.

Step 4: The myosin head makes weak contact with the actin filament and a slight conformational change occurs on myosin that promotes the release of the inorganic phosphate.

Step 5: The release of inorganic phosphate reinforces the binding interaction between myosin and actin and subsequently triggers the 'power stroke'. The power stroke is the key force-generating step used by myosin motor proteins. Forces are generated on the actin filament as the myosin protein reverts back to its original conformation.

Step 6: As myosin regains its original conformation, the ADP is released, but the myosin head remains tightly bound to the filament at a new position from where it started, thereby bringing the cycle back to the beginning.

Video: The "power stroke" mechanism for myosin movement along actin filaments. [Video uploaded to YouTube by caveman29 and created by www.encognitive.com.]
Updated on: Mon, 20 Oct 2014 09:43:06 GMT