Myosin ATPase Activity
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 . 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) (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 . 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 ).
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 [7, 8, 9], with NMI being the most extensively studied.
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 ). The steps involved are detailed below:
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.]